U.S. patent application number 09/930771 was filed with the patent office on 2002-02-14 for organic diodes with switchable photosensitivity useful in photodetectors.
Invention is credited to Cao, Yong, Yu, Gang.
Application Number | 20020017612 09/930771 |
Document ID | / |
Family ID | 22113190 |
Filed Date | 2002-02-14 |
United States Patent
Application |
20020017612 |
Kind Code |
A1 |
Yu, Gang ; et al. |
February 14, 2002 |
Organic diodes with switchable photosensitivity useful in
photodetectors
Abstract
Organic photodetectors with switchable photosensitivity are
achieved using organic photoactive layers in
electrode/organic/electrode structures. The photosensitivity can be
switched on and off by the biasing voltage across the detectors.
The photocurrent can be probed with a read-out circuit in the loop.
These photodetectors can be arranged in linear arrays or in
two-dimensional matrices that function as high performance, linear
or two-dimensional image sensors. These image sensors can achieve
fill color or selected color detection capability.
Inventors: |
Yu, Gang; (Santa Barbara,
CA) ; Cao, Yong; (Goleta, CA) |
Correspondence
Address: |
E I DU PONT DE NEMOURS AND COMPANY
LEGAL DEPARTMENT - PATENTS
1007 MARKET STREET
WILMINGTON
DE
19898
US
|
Family ID: |
22113190 |
Appl. No.: |
09/930771 |
Filed: |
August 16, 2001 |
Related U.S. Patent Documents
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Application
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Filing Date |
Patent Number |
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09930771 |
Aug 16, 2001 |
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09241660 |
Feb 2, 1999 |
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6303943 |
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60073346 |
Feb 2, 1998 |
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Current U.S.
Class: |
250/370.11 ;
257/40; 257/E27.133 |
Current CPC
Class: |
B82Y 10/00 20130101;
H01L 51/0046 20130101; H01L 27/307 20130101; H01L 51/0036 20130101;
H01L 51/004 20130101; H01L 27/14643 20130101; H01L 51/0034
20130101; H01L 51/0038 20130101; H01L 51/424 20130101; H01L 51/0037
20130101; H01L 51/0078 20130101; H01L 51/005 20130101; H01L 51/4253
20130101; H01L 51/0081 20130101; H01L 51/0591 20130101; H01L
51/0035 20130101; H01L 51/0039 20130101; H01L 51/0077 20130101;
H01L 51/0041 20130101; Y02E 10/549 20130101; H01L 51/4206
20130101 |
Class at
Publication: |
250/370.11 ;
257/40 |
International
Class: |
G01T 001/24 |
Goverment Interests
[0002] This invention was made partially with Government support
under Grant No. SBIR Ph I DMI 9660975 awarded by the National
Science Foundation. Accordingly, the Government has certain rights
to this invention.
Claims
What is claimed is:
1. A switchable organic photodetector capable of producing a
photocurrent in response to light impinging thereupon comprising a
photodiode and a variable voltage source, said photodiode having a
built-in potential and comprising: a first electrode; a photoactive
organic layer disposed on said first electrode; and a second
electrode disposed on said photoactive organic layer; and said
voltage source adapted to selectively apply a switching voltage
across said first electrode and said second electrode, said
switching voltage imparting a photosensitivity above 1 mA/W at a
preselected operating bias and near-zero photosensitivity at a
cut-off bias substantially equivalent in magnitude to said built-in
potential.
2. A photodiode detector of claim 1 wherein the operating bias is
an operating reverse bias.
3. A photodiode detector of claim 1 wherein the operating bias is
an operating forward bias.
4. A read-out circuit comprising an organic photodiode detector of
claim 1 and means for detecting the photocurrent, wherein the
operating bias is in the range of 1-15 V and represents an ON state
of the photodiode, said detector having a photosensitivity above 1
mA/Watt in said ON state, and wherein the cut-off bias represents
an OFF state of the photodiode equivalent to zero photoresponse at
an output of the read-out circuit.
5. The read out circuit of claim 4 wherein the ON and OFF states
provide a digital read out.
6. A photodiode array comprising a plurality of photodiode
detectors of claim 1 said detectors having their photodiodes
arranged in an array, each of said photodiodes being selectively
addressable as a pixel of said array.
7. The photodiode array of claim 6, wherein said array comprises at
least one row of photodiodes and at least one column of
photodiodes, each row having associated therewith a common anode,
each column having associated therewith a common cathode, the first
electrode of each photodiode of a row being connected to said
common anode, the second electrode of each photodiode of a column
being connected to said common cathode, said voltage source adapted
to apply said switching voltage across at least one common anode
and at least one common cathode to thereby selectively activate at
least one pixel of said array.
8. The photodiode array of claim 7, comprising means for applying
said switching voltage across a plurality of common anodes and at
least one common cathode to thereby selectively activate at least
one column of pixels of said array.
9. The photodiode array of claim 7, comprising means for applying
said switching voltage across a plurality of common cathodes and at
least one common anode to thereby selectively activate at least one
row of pixels of said array.
10. A scannable array of voltage-switchable organic photodiodes
each having a built-in potential and a predetermined
photosensitivity range, said array comprising: a support substrate;
a first electrode layer comprising at least one linear electrode
disposed on said support substrate along a first direction; a
photoactive organic layer disposed on said linear electrode; a
second electrode layer comprising a plurality of linear electrodes
disposed on said photoactive layer along a second direction
transverse to said first direction; and a voltage source adapted to
apply a switching voltage across at least one electrode of said
first electrode layer and at least one electrode of said second
electrode layer, said switching voltage thereby imparting to at
least one selected photodiode a photosensitivity above 1 mA/W at an
operating reverse bias and near-zero photosensitivity at a cut-off
bias substantially equivalent in magnitude to said built-in
potential.
11. A method of selectively detecting light incident on an array of
voltage-switchable organic photodiode detectors, said array
comprising a plurality of photodiodes arranged in a row and column
matrix, each photodiode having a built in potential and adapted to
generate an output in response to incident radiation, each
photodiode comprising a first electrode, a photoactive organic
layer disposed on said first electrode, and a second electrode
disposed on said photoactive layer, the first electrode of each
photodiode in a row being electrically connected to a common anode,
the second electrode of each photodiode in a column being
electrically connected to a common cathode, said method comprising:
sequentially activating a selected column of photodiodes by;
applying an operating bias voltage across the common cathode
associated with said selected column and all the common anodes,
said operating bias voltage imparting to each photodiode of the
selected column a photosensitivity above 1 mA/W; applying a cut-off
voltage across remaining cathodes and all the anodes, said cut-off
voltage being equivalent in magnitude to said built-in potential
and imparting to the photodiodes of all columns other than the
selected column near-zero photosensitivity; and sequentially
reading out the generated output of the selected column of
photodiodes.
12. A method of selectively detecting light incident on an array of
voltage-switchable organic photodiode detectors, said array
comprising a plurality of photodiodes arranged in a row and column
matrix, each photodiode having a built in potential and adapted to
generate an output in response to incident radiation, each
photodiode comprising a first electrode, a photoactive organic
layer disposed on said first electrode, and a second electrode
disposed on said photoactive layer, the first electrode of each
photodiode in a row being electrically connected to a common anode,
the second electrode of each photodiode in a column being
electrically connected to a common cathode, said method comprising:
sequentially activating a selected row of photodiodes by; applying
an operating bias voltage across the common anode associated with
said selected row and all the common cathodes, said operating bias
voltage imparting to each photodiode of the selected row a
photosensitivity above 1 mA/W; applying a cut-off voltage across
remaining anodes and all the cathodes, said cut-off voltage being
equivalent in magnitude to said built-in potential and imparting to
the photodiodes of all rows other than the selected row near-zero
photosensitivity; and sequentially reading out the generated output
of the selected row of photodiodes.
13. An organic photodiode detector comprising a photodiode and a
voltage source, said photodiode having a built-in potential and a
prescribed photosensitivity range in response to incident
radiation, said photodiode comprising: a first electrode; a
photoactive organic layer disposed on said first electrode; a
second electrode disposed on said photoactive organic layer; and
said voltage source adapted to apply an operating biasing voltage
across said first electrode and said second electrode, said biasing
voltage operating to vary said prescribed photosensitivity
range.
14. The organic photodiode detector of claim 13, wherein the
photosensitivity of said photodiode is above 1 mA/W at an operating
bias of said voltage source and is at a near-zero level at a
cut-off bias substantially equivalent in magnitude to said built-in
potential, said voltage source being switchable between said
operating bias and said cut-off bias.
15. The organic photodiode detector of claim 13, additionally
comprising a support substrate upon which the first electrode is
disposed wherein said support substrate and said first electrode
are substantially transparent to the incident radiation.
16. The organic photodiode detector of claim 13, wherein said
photoactive organic layer is comprised of a semiconducting
conjugated polymer.
17. The organic photodiode detector of claim 16, wherein said
semiconducting conjugated polymer is selected from:
poly(phenylenevinylene), and its derivatives; polythiophene, and
its derivatives; poly(thiophene vinylene), and its derivatives;
polyacetylene, and its derivatives; polyisothianaphene, and its
derivatives; polypyrrole, and its derivative;
poly(2,5-thienylenevinylene- ), and its derivatives;
poly(p-phenylene), and its derivatives; polyflourene, and its
derivatives; polycarbazole, and its derivatives;
poly(1,6-heptadiyne), and its derivatives; polyquinolene, and its
derivatives; and polyaniline, and its derivatives.
18. The organic photodiode detector of claim 16, wherein said
semiconducting conjugated polymer is the donor of a donor/acceptor
polyblend, said acceptor being selected from poly(cyanophenylene
vinylene), fullerene molecules including C.sub.60 and functional
derivates thereof, PCBM and PCBCR.
19. The organic photodiode detector of claim 16 wherein said
semiconducting conjugated polymer is the donor of a donor/acceptor
polyblend, said acceptor being selected from an organic
photoreceptor molecule or an electron transport molecule.
20. The organic photodiode detector of claim 13, wherein said
photoactive organic layer comprises a material selected from a
polymer/polymer polyblend, a polymer/(organic molecule) polyblend,
and organic molecules, organometallic molecules, oligomers or
molecular blends selected from: anthracene and its derivatives,
tetracene and its derivatives, phthalocyanine and its derivatives,
pinacyanol and its derivatives, fullerene C.sub.60 and its
derivatives, thiophene and its derivatives, phenylene and its
derivatives, oxadiazole and its derivatives, PBD and its
derivatives, Alq.sub.3 and other metal-chelate (M-L.sub.3) type
organometallic molecules, 6T/C.sub.60 and blends comprising their
derivatives, 6T/pinacyanol and blends comprising their derivatives,
phthalocyanine/o-chloranil and blends comprising their derivatives,
anthracene/C.sub.60 and blends comprising their derivatives, and
anthracene/o-chloranil and blends comprising their derivatives.
21. The organic photodiode detector of claim 13, wherein said
photoactive organic layer is arranged in a semiconducting
heterojunction structure having at least one set of donor and
acceptor regions disposed therein.
22. The organic photodiode detector of claim 13, wherein said
photoactive organic layer comprises optically inert organic
additives and/or optically inert inorganic nanoparticles.
23. The organic photodiode detector of claim 13, wherein at least
one of said first and second electrodes comprises conducting
polymer.
24. The organic photodiode detector of claim 13, additionally
comprising an optical filter layer adapted to restrict transmission
of incident radiation to a predetermined wavelength range.
25. The organic photodiode detector of claim 24, wherein the
predetermined wavelength range is selected to permit a spectral
response which follows that of the human eye.
26. A photodiode array comprising a pluraty of photodiode detectors
of claim 13, said detectors having thereon photodiode arranged in
an array, each of said photodiodes being selectively addressable as
a pixel of said array which said pixels including pixels for
detecting radiation in the red range, pixels for detecting
radiation in the green range, and pixels for detecting radiation in
the blue range.
27. The organic photodiode detector of claim 13, additionally
comprising a scintillating over layer, said scintillation over
layer emitting photons in response to incident high energy ionized
particles, said photons being detected by the organic
photodetector.
28. The organic photodiode detector of claim 27, wherein said
ionized particles are selected from high energy photons, electrons,
characteristic of X-rays, beta particles and gamma radiation.
29. The organic photodiode detector of claim 13, additionally
comprising an organic sensing layer that generates mobile electrons
and holes in response to incident high energy ionized
particles.
30. The scannable array of claim 10, wherein the support substrate
is made of insulating or semiconducting material and embedded with
integrated driving and readout circuits.
31. The scannable array of claim 30, wherein the integrated driving
circuit comprises a column or row selection circuit.
32. The scannable array of claim 30, wherein the readout circuit
comprises current integrators or current-voltage converters.
33. The photodiode array of claim 7, additionally comprising a
coating of black matrix in the space between the pixels.
34. The organic photodetector of claim 13, additionally comprising
an optical mirror placed to form a microcavity optical etalon
device which possesses selective response at resonant
wavelengths.
35. The organic photodetector of claim 13, additionally comprising
two optical mirrors placed outside to form a microcavity device
(optical etalon) which possesses selective response at resonant
wavelengths.
36. The organic photodetector of claim 13, wherein a buffer layer
is inserted between an electrode and the photoactive organic
layer.
37. The organic photodetector arrays of claim 7, wherein the
switching voltage varies among pixels so that inhomogeneity of
photosensitivity can be compensated with external bias.
38. The organic photodetector array of claim 7, wherein the
switching voltage varies among pixels following a defined pattern
so that a sensing array with designed photosensitivity pattern is
achieved for specific applications such as image procession.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Application No. 60/073,346, filed Feb. 2, 1998, which application
is incorporated herein by reference in its entirety.
BACKGROUND OF THE INVENTION
[0003] 1. Field of the Invention
[0004] The present invention relates to organic, polymer-based
photodiodes and to their use in one and two dimensional image
sensors. In more preferred embodiments, it concerns organic
polymer-based photodiodes which are voltage switchable and which
may be arrayed as image sensors in the form of a column-row (x-y)
passively addressable matrix, where the x-y addressable organic
image sensors (image arrays) have fall-color or selected-color
detection capability, or as linear photodiode arrays.
[0005] 2. State of the Art
[0006] The development of image array photodetectors has a
relatively long history in the solid state device industry. Early
approaches to imaging technology included devices based on thermal
effects in solid state materials. These were followed by high
sensitivity image arrays and matrices based on photodiodes and
charge-coupling devices ("CCDs") made with inorganic
semiconductors. These arrays can be simple linear (or "one
dimensional") arrays which scan an image or they can be two
dimensional, like the image.
[0007] Photodiodes made with inorganic semiconductors, such as
silicon, represent a class of high quantum yield, photosensitive
devices. They have been used broadly in visible light detection
applications in the past decades. However, they characteristically
present a flat current-voltage response, which makes it difficult
to use them in fabricating high pixel density, x-y
matrix-addressable passive image sensors. An "x-y" matrix is a two
dimensional array with a first set of electrodes perpendicular to a
second set of electrodes. When passive devices such as resistors,
diodes or liquid crystal cells are used as the pixel elements at
the intersection points, the matrix is often called a "passive"
matrix in contrast to an "active" matrix in which active devices,
such as transistors, are used to control the turn-on for each
pixel.
[0008] To effectively address an individual pixel from the column
and row electrodes in a two dimensional passive matrix, the pixel
elements must exhibit strongly nonlinear current-voltage ("I-V")
characteristics or an I-V dependence with a threshold voltage. This
requirement provides the foundation for using light-emitting diodes
or liquid crystal cells to construct passive x-y addressable
displays. However, since the photoresponse of inorganic photodiodes
is voltage-independent in reverse bias, photodiodes made with
inorganic semiconductor crystals are not practical for use in high
pixel density, passive image sensors--there is too much cross-talk
between pixels. To avoid cross-talk, existing two dimensional
photodiode arrays made with inorganic photodiodes must be
fabricated with each pixel wired up individually, a laborious and
costly procedure. In the case of such individual connections, the
number of input/output leads is proportional to the number of the
pixels. The number of pixels in commercial two dimensional
photodiode arrays is therefore limited to .ltoreq.16.times.16=256
due to the difficulties in manufacturing and in making inter-board
connections. Representative commercial photodiode arrays include
the Siemens KOM2108 5.times.5 photodiode array, and the Hamamatsu
S3805 16.times.16 Si photodiode array.
[0009] The development of charge-coupled devices ("CCDs") provided
an additional approach toward high pixel density, two-dimensional
image sensors. CCD arrays are integrated devices. They are
different than x-y addressable matrix arrays. The operating
principle of CCDs involves serial transfer of charges from pixel to
pixel. These interpixel transfers occur repeatedly and result in
the charge migrating, eventually, to the edge of the array for
read-out. These devices employ super-large integrating circuit
("SLIC") technology and require an extremely high level of
perfection during their fabrication. This makes CCD arrays costly
(.about.$10.sup.3-10.sup.4 for a CCD of 0.75" -1" size) and limits
commercial CCD products to sub-inch dimensions.
[0010] The thin film transistor ("TFT") technology on glass or
quartz substrates, which was developed originally for the needs of
liquid crystal displays, can provide active-matrix substrates for
fabricating large size, x-y addressable image sensors. A large
size, full color image sensor made with amorphous silicon (a-Si)
p-i-n photocells on a-Si TFT panels was demonstrated recently [R.
A. Street, J. Wu, R. Weisfield, S. E. Nelson and P. Nylen, Spring
Meeting of Materials Research Society, San Francisco, Apr. 17-21
(1995); J. Yorkston et al., Mat. Res. Soc. Sym. Proc. 116, 258
(1992); R. A. Street, Bulletin of Materials Research Society
11(17), 20 (1992); L. E. Antonuk and R. A. Street, U.S. Pat. No.
5,262,649 (1993); R. A. Street, U.S. Pat. No 5,164,809 (1992)].
Independently, a parallel effort on small size, active-pixel
photosensors based on CMOS technology on silicon wafers has been
re-activated following developments in the CMOS technology which
provide submicron resolution [For a review of recent progress, see:
Eric J. Lemer, Laser Focus World 32(12) 54, 1996]. This CMOS
technology allows the photocells to be integrated with the driver
and the timing circuits so that a mono-chip image camera can be
realized.
[0011] CCDs, a-Si TFTs, and active-pixel CMOS image sensors
represent the existing/emerging technologies for solid state image
sensors. However, because of the costly processes involved in
fabrication of these sophisticated devices, their applications are
severely limited. Furthermore, the use of SLIC technologies in the
fabrication processes limits the CCDs and the active-pixel CMOS
sensors to sub-inch device dimensions.
[0012] Photodiodes made with organic semiconductors represent a
novel class of photosensors with promising process advantages.
Although there were early reports, in the 1980s, of fabricating
photodiodes with organic molecules and conjugated polymers,
relatively small photoresponse was observed [for an review of early
work on organic photodiodes, see: G. A. Chamberlain, Solar Cells 8,
47 (1983)]. In the 1990s, there has been progress using conjugated
polymers as the active materials; see for example the following
reports on the photoresponse in poly(phenylene vinylene), PPV, and
its derivatives,: S. Karg, W. Riess, V. Dyakonov, M. Schwoerer,
Synth. Metals 54, 427 (1993); H. Antoniadis, B. R. Hsieh, M. A.
Abkowitz, S. A. Jenekhe, M. Stolka, Synth. Metals 64, 265 (1994);
G. Yu, C. Zhang, A. J. Heeger, Appl. Phys. Lett. 64,1540 (1994); R.
N. Marks, J. J. M. Halls, D. D. D. C. Bradley, R. H. Frield, A. B.
Holmes, J. Phys.: Condens. Matter 6,1379 (1994); R. H. Friend, A.
B. Homes, D. D. C. Bradley, R. N. Marks, U.S. Pat. No. 5,523,555
(1996)].
[0013] The photosensitivity in organic semiconductors can be
enhanced by excited-state charge transfer; for example, by
sensitizing the semiconducting polymer with acceptors such as
C.sub.60 or its derivatives [N. S. Saricifici and A. J. Heeger,
U.S. Pat. No. 5,331,183 (Jul. 19, 1994); N. S. Saricifici and A. J.
Heeger, U.S. Pat. No. 5,454,880 (Oct. 3, 1995); N. S. Sariciftci,
L. Smilowitz, A. J. Heeger and F. Wudl, Science 258, 1474 (1992);
L. Smilowitz, N. S. Sariciftci, R. Wu, C. Gettinger, A. J. Heeger
and F. Wudl, Phys. Rev. B 47, 13835 (1993); N. S. Saricifici and A.
J. Heeger, Intern. J. Mod. Phys. B 8, 237 (1994)]. Photoinduced
charge transfer prevents early time recombination and stabilizes
the charge separation, thereby enhancing the carrier quantum yield
for subsequent collection [B. Kraabel, C. H. Lee, D. McBranch, D.
Moses, N. S. Sariciftci and A. J. Heeger, Chem. Phys. Lett. 213,
389 (1993); B. Kraabel, D. McBranch, N. S. Sariciftci, D. Moses and
A. J. Heeger, Phys. Rev. B 50,18543 (1994); C. H. Lee, G. Yu, D.
Moses, K. Pakbaz, C. Zhang, N. S. Sariciftci, A. J. Heeger and F.
Wudl, Phys. Rev. B. 48, 15425 (1993)]. By using charge transfer
blends as the photosensitive materials in photodiodes, external
photosensitivity of 0.2-0.3 A/Watt and external quantum yields of
50-80% el/ph have been achieved at 430 nm at low reverse bias
voltages [G. Yu, J. Gao, J. C. Hummelen, F. Wudl and A. J. Heeger,
Science 270, 1789 (1995); G. Yu and A. J. Heeger, J. Appl. Phys.
78, 4510 (1995); J. J. M. Halls, C. A. Walsh, N. C. Greenham, E. A.
Marseglia, R. H. Frield, S. C. Moratti and A. B. Holmes, Nature
376, 498 (1995)]. At the same wavelength, the photosensitivity of
the UV-enhanced silicon photodiodes is .about.0.2 A/Watt,
independent of bias voltage [S. M. Sze, Physics of Semiconductor
Devices (Wiley, New York, 1981) Part 5]. Thus, the photosensitivity
of thin film photodiodes made with polymer charge transfer blends
is comparable to that of photodiodes made with inorganic
semiconducting crystals. In addition to their high
photosensitivity, these organic photodiodes show large dynamic
range; relatively flat photosensitivity has been reported from 100
mW/cm.sup.2 down to nW/cm.sup.2; i.e., over eight orders of
magnitude [G. Yu, H. Pakbaz and A. J. Heeger, Appl. Phys. Lett. 64,
3422 (1994); G. Yu, J. Gao, J. C. Hummelen, F. Wudl and A. J.
Heeger, Science 270, 1789 (1995); G. Yu and A. J. Heeger, J. Appl.
Phys. 78, 4510 (1995)]. The polymer photodetectors can be operated
at room temperature, and the photosensitivity is relatively
insensitive to the operating temperature, dropping by only a factor
of 2 from room temperature to 80 K [G. Yu, K. Pakbaz and A. J.
Heeger, Appl. Phys. Lett. 64, 3422 (1994)].
[0014] As is the case for polymer light-emitting devices [G.
Gustafsson, Y. Cao, G. M. Treacy, F. Klavetter, N. Colaneri, and A.
J. Heeger, Nature 357, 477 (1992); A. J. Heeger and J. Long, Optics
& Photonics News, Aug. 1996, p.24], high sensitivity polymer
photodetectors can be fabricated in large areas by processing from
solution at room temperature. They can be made in unusual shapes
(e.g. on a hemisphere to couple with an optical component or an
optical system), or they can be made in flexible or foldable forms.
The processing advantages also enable one to fabricate the
photosensors directly onto optical fibers. Similarly, polymer
photodiodes can be hybridized with optical devices or electronic
devices, such as an integrated circuits on a silicon wafer. These
unique features make polymer photodiodes special for many novel
applications.
SUMMARY OF THE INVENTION
[0015] Recent progress in our group has demonstrated that the
photosensitivity in organic photodiodes can be enhanced by applying
a reverse bias. It was further found that the photosensitivity
increases with reverse bias voltage, with the increase being
independent of incident light intensity [G. Yu, C. Zhang and A. J.
Heeger, Appl. Phys. Lett. 64, 1540 (1994); A. J. Heeger and G. Yu,
U.S. Pat. No. 5,504,323 (1996)]. This work showed a
photosensitivity of .about.90 mA/Watt in
poly(2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene)
("MEH-PPV")-based thin film devices, such as ITO/MEH-PPV/Ca thin
film devices, at 10 V reverse bias (430 nm), corresponding to a
quantum efficiency of >20% el/ph. In photodiodes fabricated with
poly(3-octylthiophene), photosensitivity over 0.3 A/Watt was
observed over most of visible spectral range at -15 V bias [G. Yu,
H. Pakbaz and A. J. Heeger, Appl. Phys. Lett. 64, 3422 (1994)].
[0016] We have now found that this variable photosensitivity
enables on-off voltage-switchable photosensors. At a reverse bias,
typically in the range of 2-15 V, the photodiode can be switched on
with photosensitivity of 30-300 mA/W. The photosensitivity at a
voltage close to the internal (built-in) potential is several
orders of magnitude lower, equivalent to zero at the output of a
digital read-out circuit. This near zero state can thus be defined
as the off state of the photodiode.
[0017] These voltage-switchable, organic photodiodes can serve as
individual pixels in passive diode arrays. These arrays can be in
the form of x-y addressable arrays with anodes connected via row
(column) electrodes and cathodes connected via column (row)
electrodes. Every pixel can be selected, and the information
(intensity of the incident light) at each pixel can be read out
without crosstalk. Alternatively, the voltage-switchable, organic
photodiodes can be arrayed in a linear manner.
[0018] These arrays can utilize the processing advantages
associated with the fabrication of organic diode structures from
soluble, semiconducting, conjugated polymers (and/or their
precursor polymers). Layers of these materials can be cast from
solution to enable the fabrication of large active areas, onto
substrates with desired shapes. This also enables active areas to
be in flexible form. These photoactive materials can be patterned
onto an optically uniform substrate by means of photolithography,
microcontact printing, shadow masking and the like. In a preferred
embodiment for the visible region of the spectrum, the substrate is
opaque for .lambda.<400 nm so that the pixels are insensitive to
UV radiation.
[0019] The photoactive layer employed in these switchable
photodiodes is made up of organic materials. These take numerous
forms. They can be conjugated semiconducting polymers or polymer
blends. For donor-acceptor blends with polymeric donors, the
acceptor can be a polymer, macromolecule, oligomer or small
molecule (monomers). Alternatively, molecular donor/polymeric
acceptor systems also work well. The higher molecular weight
component in many cases provides mechanical strength and prevents
phase changes. The donor-acceptor blends can also be made with
small molecule donors and acceptors that are well known in the art.
Examples of the molecular and oligomeric donors include anthracene
and its derivatives, pinacyanol and its derivatives thiophene
oligomers (such as sexithiophene.6T, and octylthiophene, 8T) and
their derivatives and the like, phenyl oligomers (such as
sexiphenyl or octylphenyl) and the like. Examples of molecular
acceptors include fullerenes (such as C.sub.60 and their functional
derivatives), Alq.sub.3-type organometallic molecules and the like.
In addition, one can employ multiple layers of organic
semiconducting materials in donor/acceptor heterojunction or
quantum-well configurations.
[0020] The organic image sensors enabled by this invention can have
mono-color or multi-color detection capability. In these image
sensors color (optical wavelength) selection can be achieved by
combining a suitable color filter panel with the organic image
sensors and image sensor arrays already described. If desired, the
color filter panel can serve as a substrate upon which the image
sensor is carried. The detection wavelength of the organic image
sensors can also be selected by using resonant cavity device
structures as demonstrated in the examples of the invention. The
organic image sensor arrays with the ability to select specific
wavelengths can be used for spectrographic applications (such as
flat-panel spectrometers).
[0021] In addition, embodiments of the present invention provide
organic image sensors with full-color detection capability. In
these organic image sensors, a filter panel is made up of red,
green and blue color filters which are patterned in a format
corresponding to the format of a photodiode array. The panel of
patterned filters and the patterned photodiode array are coupled
(and coordinated) such that a colored image sensor is formed. The
patterned color filter panel can be used directly as the substrate
of the image sensor.
[0022] Full-color detectivity, is also achieved when red, green and
blue colors are detected by three of these photodiodes with
spectral response cut-off at 500 nm, 600 nm and 700 nm,
respectively. Differentiation operations in the read-out circuit
extract the red (600-700 nm), green (500-600 nm) and blue (400-500
nm) signals.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] This invention will be further described with reference
being made to the drawings in which:
[0024] FIG. 1 is a cross-sectional schematic view of a
voltage-switchable photodiode of this invention 10 assembled into a
circuit. The photocurrent can be read out by a current meter or a
read-out device inserted in the loop;
[0025] FIG. 2 is a cross-sectional schematic view of a
voltage-switchable photodiode 20 in reversed configuration, in
which the reversed configuration refers the structure with the
transparent electrode contacted with the free surface of the active
layer;
[0026] FIG. 3 is an exploded schematic view of a 2D image sensor 30
made of an x-y addressable, passive matrix of voltage-switchable
photodiodes;
[0027] FIG. 4 is an exploded schematic view of a full-color image
sensor 40 made with an x-y addressable photodiode matrix coupled to
a color filter panel;
[0028] FIG. 5 is an exploded schematic view of a full-color image
sensor 50 made with an x-y addressable photodiode matrix of which
each full-color pixel is made of three photosensitive materials
having differing long-wavelength cut-offs such as at 700 nm, 600 nm
and 500 nm;
[0029] FIG. 6 is a graph of the photocurrent as a function of bias
voltage in a ITO/MEH-PPV/Ca device;
[0030] FIG. 7 is a graph of the transmission characteristics of
PANI-CSA and PEDT-PSSA conducting polymer electrodes; also shown is
the visual response, V(.lambda.), of human eye.
[0031] FIG. 8 is a graph of the photocurrent (circles) and the dark
current (solid line) of a ITO/MEH-PPV:PCBM/Al photodiode. The
photocurrent was taken under white light of intensity .about.10
mW/cm.sup.2.
[0032] FIG. 9 is a graph of the current-voltage characteristics of
an ITO/P3OT/Au photodiode in the dark (circles), and illuminated
under .about.10 mW/cm at 633 nm (squares);
[0033] FIG. 10 is a graph of the current-voltage characteristics
measured between a row electrode and a column electrode from a
7.times.40 photodiode matrix in the dark (lines) and under room
light illumination (circles);
[0034] FIG. 11 is a schematic representation of the driving scheme
for a 7.times.40 photodiode matrix. It will be described in terms
of ITO/MEH-PPV:PCBM/Ag switchable photodiodes;
[0035] FIG. 12 is a graph of the photoresponse of a
voltage-switchable photodiode made with P3OT;
[0036] FIG. 13A is a graph of the photoresponse of
voltage-switchable photodiodes with spectral response simulating
that of human eye, V(.lambda.);
[0037] FIG. 13B is a graph of the transmittance of the
long-wavelength-pass filter and the visual response, V(.lambda.)
corresponding to FIG. 13A;
[0038] FIG. 14 is a graph of the spectral response of a solar blind
UV detector operating at -2V. The photoresponse of the
MEH-PPV:C.sub.60 photodiode on ITO/glass substrate and the
photoresponse of an UV-enhanced Si photodiode are plotted for
comparison;
[0039] FIG. 15A is a graph of the response of a PTV photodiode;
[0040] FIG. 15B is a graph of the photoresponse of R, G, B
photosensors made of PTV photodiodes coupled with a color-filter
panel;
[0041] FIG. 15C is a graph of the transmittance of the color
filters used in the generation of the data graphed in FIGS. 15A and
15B;
[0042] FIG. 16A is a graph of normalized spectral response of
photodiodes made with PPV (open squares), PDHPV (open circles), and
PTV (solid circles);
[0043] FIG. 16B is a graph of red, green and blue color detection
derived from the diode responses in FIG. 16A;
[0044] FIG. 17 is a graph showing I-V response of a photodiode made
with PPV in the dark and under illumination;
[0045] FIG. 18 is a graph showing I-V response of a photodiode with
a donor/acceptor heterojunction structure in the dark and under
illumination;
[0046] FIG. 19 is a graph of the dark (solid circles) and
photocurrents (circles) of a P3HT photodiode under 8 mW/cm.sup.2
broad band white light (400-700 nm);
[0047] FIGS. 20A and 20B are cross-sectional schematic views of
linear photodiode arrays made with organic semiconductors;
[0048] FIG. 21 is a sketch of the circuit used to drive the organic
photodiode array;
[0049] FIGS. 22A-D show images achieved by a P30T linear photodiode
array of 100 pixels over a 2.5 inch length. FIG. 22A is a red color
image; FIG. 22B is a green color image; FIG. 22C is a blue color
image; and FIG. 22D is a full-color image recovered by superposing
the red, green and blue color images of FIGS. 22A-C;
[0050] FIG. 23 is a graph of an optical beam analyzer made with a
1.times.102 polymer photodiode array;
[0051] FIG. 24 is a graph of the angular distribution of the light
emission from a GaP LED measured with a flexible linear photodiode
array;
[0052] FIG. 25 is a schematic view of a spectrographer made of P30T
photodiode array;
[0053] FIG. 26 is a graph of the transmission spectra of a PPV film
measured with the spectrographer of FIG. 25.
[0054] FIG. 27 is a graph of the spectral response of an organic
photosensor in microcavity (optic etalon) structure.
DETAILED DESCRIPTION OF THE INVENTION
[0055] This invention provides high sensitivity photodiodes with
voltage-switchable photosensitivity; the photosensitivity can be
switched on and off by the application of selected voltages,
thereby reducing cross-talk between pixels in an array of such
voltage-switchable photodiodes to acceptable levels. These
switchable photosensors enable the fabrication of either one- or
two-dimensional (2D), passive image sensors with column-row (x-y)
addressability. The voltage-switchable photodetector is constructed
in a metal-semiconductor-metal (M-S-M) thin film structure in which
an organic film such as a film of semiconducting polymer or a
polymer blend is used as the photoactive material. Selected-color
or multi-color detection in the visible and near UV can be achieved
by coupling the image sensor to an optical filter(s). Fabrication
processes for red, green and blue (RGB) and full-color image
sensors are described by coupling the x-y addressable polymer diode
matrix or linear array with a RGB color filter panel, or by
fabricating photodiodes with cut-off of the photoresponse at 500
nm, 600 nm and 700 nm, respectively, onto optically uniform
substrates, or by fabricating the photodiodes in microcavity
structures with defined spectral responses in the red, green and
blue regions.
[0056] Voltage-switchable photodiodes make possible 2D image
sensors. Using such photodiodes as the sensing elements in a
column-row matrix, a 2D x-y addressable, passive image sensor can
be constructed which operates without crosstalk. Because of the
strong voltage dependence of the photosensitivity, a column of
pixels in the 2D photodiode matrix can be selected and turned on
with proper voltage bias, leaving the rest of the pixels on other
rows insensitive to the incident light. With this type of
operation, the physical M row, N column 2D matrix is reduced to N
isolated linear diode arrays each with M elements; said isolated
linear diode arrays are free from the crosstalk which originates
from finite resistance between devices on different columns. With
such 2D, passive photodiode arrays, an image can be read out with a
pulse train scanning through each column of the matrix. Since the
number of contact electrodes are reduced to N+M in the x-y
addressable matrix, compared to N.times.M in the case of individual
connection, large size, high pixel density, 2D image arrays become
practical (comparable to the high pixel density display arrays made
with LCD technology). For example, for a 1000 by 1000 pixel array,
the present invention reduces the number or required electrodes by
500 times. The polymer image sensor matrix thus provides a unique
approach to fabricating large size, low cost, high pixel density,
2D image sensing arrays with a room temperature manufacturing
process.
[0057] In addition to being used as the sensing elements in x-y
addressable, 2D passive photodiode matrices, these
voltage-switchable organic photosensors can also be used to
construct linear photodiode arrays. As shown in examples disclosed
in this invention, the ratio of
I.sub.ph(V.sub.on)/I.sub.ph(V.sub.off) can be more than
3.times.10.sup.7 under photoexcitation of a few mW/cm.sup.2. The
large I.sub.ph(V.sub.on)/I.sub.dark(V.sub.on) ratio
(>1.3.times.10.sup.5) allows the collection of image data with
gray scale resolution of more than 12 bits (12 bit has 4096 gray
levels). Linear photodiode arrays made with these materials can be
used for high image quality (over 18 bits), full-page color digital
image scanners. Contrary to active image sensors, no analog
switches are needed to drive these arrays. A digital shift register
or a BCD decoder can be used for pixel selection.
[0058] The device structure of the linear photodiode array is shown
in FIG. 19. Transparent glass or PET films can be used as the
substrates. Opaque materials such as silicon wafers can also be
used as the substrate material. In this case, the light is incident
onto the free surface side as shown in FIG. 19B. When organic PET
films are used as substrates, the linear diode array can be made in
flexible form. Optical devices with curved surface can also be used
as the substrate for these diode arrays; i.e., the linear diode
array can be coupled to and integrated with other optical devices
in a desired optical arrangement and with a desired optical
wavefront.
[0059] Linear photodiode arrays can be made in the configurations
similar to that shown in FIG. 3 with one row and n columns or with
one column and n rows. The cross sectional views of two typical
device structures are shown in FIG. 19. The substrates can be
transparent or opaque. In a preferred configuration (FIG. 19A), the
linear photodiode arrays (210) can be fabricated onto a transparent
glass substrate (214) with patterned ITO (211) or other transparent
electrode materials (such as conducting polymer electrodes, thin
metal films, metal/conducting polymer bilayer electrodes dielectric
film/ITO or metal film/dielectric film bilayer electrodes). The
process of ITO patterning is well known in the existing art, and
has been used broadly in LCD technologies. The deposition of the
organic layer (212) can be achieved by spin casting, drop casting,
printing, electrochemical synthesis or vapor deposition. The back
electrode, in the form of a narrow bar shape (213), can be vacuum
deposited with a simple shadow mask or patterned by means of
photolithography. In most applications (especially for larger pixel
sizes), no patterning of the sensing material is necessary. This
sensing array can be mounted onto a print circuit (PC) board with a
driving circuit. Several existing connection techniques (such as
card-edge connectors, zebra connectors, bonding tapes, wire
bonding, soldering bumper etc.) can be used for interboard
connection. The drive circuits can also be arranged (surrounding
the sensor array) onto the same substrate. This is especially
preferred in arrays with a high pixel density (e.g., >80
pixels/inch). In these cases, the IC chips can be bonded to the
glass substrate, and the electrical connections can be achieved via
soldering, one-dimensional conducting epoxy or other existing
connection technologies.
[0060] As demonstrated in the examples herein, the spectral
response of the polymer image sensors can cover the entire visible
spectrum with relatively flat response. A portion of the visible
spectrum can also be selected with a band-pass or low-pass optical
filter. Multi-color detection in the visible and the near UV can be
achieved by coupling the image sensor with a color-filter panel. A
fabrication process for full-color image sensors is described with
the x-y addressable polymer diode matrix and a RGB (red, green,
blue) color filter panel. A similar fabrication process can be
employed to prepare a linear photodiode array.
[0061] Definitions and Device Structures
[0062] In this description of preferred embodiments and in the
claims, reference will be made to several defined terms. One group
of terms concerns the structure of the voltage-switchable
photodiode. A cross-sectional view of the voltage-switchable
photodiode is shown in FIG. 1. The voltage-switchable photodiode 10
is constructed using the metal-semiconductor-metal (M-S-M) thin
film device configuration. Specifically, the device 10
includes:
[0063] A "photoactive layer" (layer 12) comprised of organic,
semiconducting material(s), such as a conjugated polymer, a polymer
blend, a polymer/molecule polyblend, a layer of organic molecule or
molecular blends; or a multilayer structure combining the above
materials;
[0064] Two "contact electrodes" (layers 11, 13) which serve as the
anode and cathode of the photodiodes to extract electrons and
holes, respectively, from the photoactive layer. One of the
electrodes (layer 11 in FIG. 1) is made transparent or
semitransparent in the spectral range of interest to allow the
incident light 18 to be absorbed in the active layer (12).
[0065] The "anode" electrode is defined as a conducting material
with higher work function than the "cathode" material.
[0066] This same relationship of electrodes 11 and 13 to active
layer 12 and light source 18 (or 18') is found in devices 10, 20,
30 40 and 50 as depicted in FIGS. 1, 2, 3, 4, and 5,
respectively.
[0067] As shown in FIGS. 1 and 2, electrodes 11 and 13 are
connected to bias voltage source 15 via lines 17 and 17',
respectively. Detector 16 (that represents a current meter or a
read out device) is wired in series into this circuit to measure
the photoresponse generated in the photodiode in response to light
18. This same circuit would be employed in all of the devices (10,
20, 30, 40 and 50) depicted in FIGS. 1-5.
[0068] The devices may also include an optional substrate or
support 14, as shown in FIGS. 1-5. This is a solid, rigid or
flexible layer designed to provide robustness to the diodes and/or
to the matrix array of diodes. When light is incident from the
substrate side, the substrate should be transparent or
semitransparent in the spectral range of interest. Glass, polymer
sheets or flexible plastic films are substrates commonly used. Wide
band semiconductor wafers (such as SiC, SiN) which are transparent
below their optical gaps can also be used in some applications. In
these cases, a thin, doped region can also serve as the contact
electrode 11.
[0069] Devices with the inverted geometry shown in FIG. 2 are also
useful in applications. In this configuration, light 18 is incident
from the "back" electrode side, and optically opaque materials can
be used as the substrate material. For example, by using an
inorganic semiconductor wafer (such as silicon) as the substrate
14, and by doping the semiconductor to "conductive" levels (as
defined in the following), the wafer can serve both as the
substrate 14 and the contact electrode 11. The inverted structure
offers the advantage of integrating the photosensor with
driving/read-out circuitry built directly onto the inorganic
semiconductor substrate (using integrated circuit technology).
[0070] The incident light 18 (or 18') is defined generally so as to
include wavelengths in visible (400-700 mn), wavelengths in the
ultraviolet (200-400 nm), wavelength in the vacuum UV (<200 mn),
and wavelengths in the infrared (700-2000 nm).
[0071] Several layers are designated as "transparent" or
"semi-transparent". These terms are used to refer to the property
of a material which transmits a substantial portion of the incident
light incident on it. The term "transparent" is used to describe a
substrate with transmittance over 50% and the term
"semi-transparent" is used to describe a substrate with
transmittance between 50% to 5%.
[0072] A "conductive" layer or material has a conductivity
typically larger than 0.1 S/cm. A semiconducting material has
conductivity of from 10.sup.-14 to 10.sup.-1 S/cm.
[0073] The "positive" (or "negative") bias refers to the cases when
higher potential is applied to the anode electrode (cathode
electrode). When values of negative voltage are referred to, as in
the case of the reverse bias voltages applied to obtain enhanced
photosensitivity, the relative values will be stated in terms of
absolute values; that is, for example, a -10 V (reverse) bias is
greater than a -5 V (reverse) bias.
[0074] The structure of the x-y addressable, passive photodiode
matrix (2D image sensor 30) is depicted in FIG. 3. Shown in FIG. 4
is the structure of a full-color image sensor 40 made with the x-y
addressable photodiode matrix. In these devices, the anode and
cathode electrodes 11', 13' are typically patterned into rows and
columns perpendicular to one another. Patterning of the photoactive
layer 13 is not necessary for pixels with sufficient space between
adjacent electrodes. Each intersection of the row and column
electrodes defines a photosensitive element (pixel) with device
structure similar to that shown in FIG. 1 or FIG. 2. The widths of
the row and column electrodes 11', 13' define the active area of
each pixel.
[0075] A matrix of color filters 19 (each pixel of the color filter
is comprised of red, green and blue color filters 19') is coupled
with the photodiode panel. A separate sheet of color filters
similar to that used for color-LCD displays [For a review, see: M.
Tani and T. Sugiura, Proceeding of SID, Orlando, Fla. (1994)] can
be used for this purpose. In a more preferred embodiment, the
color-filter panel can be coated directly onto the substrate for
the photodiode matrix. The set of transparent electrodes 11 (for
example, made of indium-tin-oxide, ITO) can be fabricated over the
color filter coating. In this configuration, high pixel densities
with micron-size feature size can be achieved.
[0076] A coating of "black" material (opaque in the spectral range
of interest) in the area between each sensing pixel can be placed
in front of the photodetector plane, forming a "black matrix". This
coating is helpful in some situations to further reduce cross-talk
between neighbor pixels in devices with an unpatterned photoactive
organic layer. Black matrices have been used in CRT monitors and
other flat panel displays to increase display contrast, and are
well known in the display industry. The patterning of the "black
matrix" can be achieved with standard photolithography, stamp,
ink-jet or screen printing techniques.
[0077] Full-color detection can be achieved with an alternative
approach 50 as shown in FIG. 5. In this approach, each full-color
pixel 12' comprises three photodiodes 12R, 12G and 12B with long
wavelength cut-offs at 700, 600 and 500 nm, respectively. These
photodiodes are made of three photosensitive materials in the
defined areas on the substrate. The patterning of the active layers
can be achieved by photolithography, screen printing, shadow
masking and the like. The correct red, green and blue color
information can be obtained by differentiation of the signals (in
the read-out circuit) from the three sub-pixels, 12R, 12G and 12B,
as demonstrated in the examples of this invention. An optically
uniform material is used as the substrate which is transparent in
the visible and opaque in UV.
[0078] The color selection can also be achieved by combining the
device structure shown in FIG. 4 with that shown in FIG. 5. For
instance, with the photosensing material in the photodiode defining
part of the spectral response, the optical filter placed in front
fine-tunes the response desired. Example 15 utilizes this approach
for a photosensor simulating the response of the human eye.
[0079] The Photoactive Layer
[0080] The photoactive layer 12 in the voltage-switchable
photodiodes is made of a thin sheet of organic semiconducting
material. The active layer can comprise one or more semiconducting,
conjugated polymers, alone or in combination with non-conjugated
materials, one or more organic molecules, or oligomers. The active
layer can be a blend of two or more conjugated polymers with
similar or different electron affinities and different electronic
energy gaps. The active layer can be a blend of two or more organic
molecules with similar or different electron affinities and
different electronic energy gaps. The active layer can be a blend
of conjugated polymers and organic molecules with similar or
different electron affinities and different energy gaps. The latter
offers specific advantages in that the different electron
affinities of the components can lead to photoinduced charge
transfer and charge separation; a phenomenon which enhances the
photosensitivity [N. S. Sariciftci and A. J. Heeger, U.S. Pat. No.
5,333,183 (Jul. 19, 1994); N. S. Sariciftci and A. J. Heeger, U.S.
Pat. No. 5,454,880 (Oct. 3, 1995); N. S. Sariciftci, L. Smilowitz,
A. J. Heeger and F. Wudl, Science 258, 1474 (1992); L. Smilowitz,
N. S. Sariciftci, R. Wu, C. Gettinger, A. J. Heeger and F. Wudl,
Phys. Rev. B 47,13835 (1993); N. S. Sariciftci and A. J. Heeger,
Intern. J. Mod. Phys. B 8, 237 (1994)]. The active layer can also
be a series of heterojunctions utilizing layers of organic
materials or blends as indicated above.
[0081] The thin films of organic molecules, oligomers and molecular
blends can be fabricated with thermal evaporation, chemical vapor
deposition (CVD) and so on. Thin films of conjugated polymers,
polymer/polymer blends, polymer/oligomer and polymer/molecule
blends can often be fabricated by casting directly from solution in
common solvents or using similar fluid phase processing. When
polymers or polyblends are used as the active layer, the devices
can be fabricated onto flexible substrates yielding unique,
mechanically flexible photo sensors.
[0082] Examples of typical semiconducting conjugated polymers
include, but are not limited to, polyacetylene, ("PA"), and its
derivatives; polyisothianaphene and its derivatives; polythiophene,
("PT"), and its derivatives; polypyrrole, ("PPr"), and its
derivatives; poly(2,5-thienylenevinylene), ("PTV"), and its
derivatives; poly(p-phenylene), ("PPP"), and its derivatives;
polyflourene, ("PF"), and its derivatives; poly(phenylene
vinylene), ("PPV"), and its derivatives; polycarbazole and its
derivatives; poly(1,6-heptadiyne); polyisothianaphene and its
derivatives; polyquinolene and semiconducting polyanilines (i.e.
leucoemeraldine and/or the emeraldine base form). Representative
polyaniline materials are described in U.S. Pat. No. 5,196,144
which is incorporated herein by reference. Of these materials,
those which exhibit solubility in organic solvents are preferred
because of their processing advantages.
[0083] Examples of PPV derivatives which are soluble in common
organic solvents include
poly(2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene),
("MEH-PPV") [F. Wudl, P. -M. Allemand, G. Srdanov, Z. Ni and D.
McBranch, in Materials for Nonlinear Optics: Chemical Perspectives,
edited by S. R. Marder, J. E. Sohn and G. D. Stucky (The American
Chemical Society, Washington D.C., 1991), p. 683.],
poly(2-butyl-5-(2-ethyl-hexyl)-1,4-phenylenevinylene), ("BuEH-PPV")
[M. A. Andersson, G. Yu, A. J. Heeger, Synth. Metals 85, 1275
(1997)], poly(2,5-bis(cholestanoxy)-1,4-phenylenevinylene),
("BCHA-PPV") [see U.S. patent application Ser. No. 07/800,555,
incorporated herein by reference] and the like. Examples of soluble
PTs include poly(3-alkylthiophenes), ("P3AT"), wherein the alkyl
side chains contain more than 4 carbons, such as from 5 to 30
carbons.
[0084] Organic image sensors can be fabricated using donor/acceptor
polyblends as the photoactive layer. These polyblends can be blends
of semiconducting polymer/polymer, or blends of semiconducting
polymer with suitable organic molecules and/or organometallic
molecules. Examples for the donor of the donor/acceptor polyblends
include but are not limited to the conjugated polymers just
mentioned, that is PPV, PT, PTV, and poly(phenylene), and their
soluble derivatives. Examples for the acceptors of the
donor/acceptor polyblends include but are not limited to
poly(cyanaophenylenevinylene) ("CN-PPV"), fullerene molecules such
as C.sub.60 and its functional derivatives, and organic molecules
and organometallic molecules used heretofore in the art for
photoreceptors or electron transport layers.
[0085] One can also produce photoactive layers using two
semiconducting organic layers in a donor/acceptor heterojunction
(i.e., bilayer) structure or alternation layer structures. In these
structures, the donor layer is typically a conjugated polymer layer
and the acceptor layer is made up of poly(cyanaophenylenevinylene)
("CN-PPV"), fullerene molecules such as C.sub.60 and its functional
derivatives (such as PCBM and PCBCR), or organic molecules used
heretofore in the art for photoreceptors and electron transport
layers. Examples of this heterojunction layer structure for a
photoactive layer include but are not limited to, PPV/C.sub.60,
MEH-PPV/C.sub.60, PT/C.sub.60, P3AT/C.sub.60, PTV/C.sub.60 and so
on.
[0086] The active layer can also be made of wide band polymers such
as poly-N-vinylcarbazole ("PVK") doped with dye molecule(s) to
enhance photosensitivity in the visible spectral range. In these
cases, the wide band organic serves as both host binder as well as
hole (or electron) transport material. Examples include, but are
not limited to, PVK/o-chloranil, PVK/rhodamine B and PVK/coronene
and the like.
[0087] The photoactive layer can employ organic molecules,
oligomers or molecular blends. In this embodiment, the
photosensitive material can be fabricated into thin films by
chemical vapor deposition, molecular epitaxy or other known
film-deposition technologies. Examples of suitable materials
include but are not limited to include anthracene and its
derivatives, tetracene and its derivatives, phthalocyanine and its
derivatives, pinacyanol and its derivatives, fullerene ("C.sub.60")
and its derivatives, thiophene oligomers (such as sixethiophene
"6T" and octithiophene "8T") and their derivatives phenyl oligomers
(such as sixephenyl "6P" or octiphenyl "8P") and their derivatives,
aluminum chelate (Alq3) and other metal-chelate molecules (m-q3),
PBD, spiro-PBD, oxadiazole and its derivatives and blends such as
6T/C.sub.60, 6P/C.sub.60, 6P/PBD, 6P/Alq3, 6T/pinacyanol,
phthalocyanine/o-chloranil, anthracene/C.sub.60,
anthracene/o-chloranil. For the photoactive layer containing more
than two types of molecules, the organic layer can be in a blend
form, in bilayer form or in multiple alternate layer forms.
[0088] In some embodiments, the active layer comprises one or more
organic additives (which are optically non-active) to modify and to
improve the device performance. Examples of the additive molecules
include anionic surfactants such as ether sulfates with a common
structure,
R(OCH.sub.2CH.sub.2).sub.nOSO.sub.3.sup.-M.sup.+
[0089] wherein R represents alkyl alkyllaryl,
[0090] M.sup.+ represents proton, metal or ammonium counterion,
[0091] n is moles of ethylene oxide typically n=2-40).
[0092] Application of such anionic surfactants as additives for
improving the performance of polymer light-emitting diodes has been
demonstrated by Y. Cao [U.S. patent application, Ser. No.
08/888,316, which is incorporated by reference].
[0093] Other types of additives include solid state electrolytes or
organic salts. Examples include poly(ethylene oxide), lithium
trifluoromethanesulfonate, or their blends, tetrabutylammonium
dodecylbenzenesulfonate and the like. Application of such
electrolyte to luminescent polymers and invention of new type of
light-emitting devices have been demonstrated in U.S. Pat. Nos.
5,682,043 and 5,677,546.
[0094] In cases where the active layer is made of organic blends
with two or more phases with different electron affinities and
optical energy gaps, nanoscale phase separation commonly occurs,
and heterojunctions form at the interfacial area. The phase(s) with
higher electron affinity acts as an electron acceptor(s) while the
phases with lower electron affinity (or lower ionization energy
serves as an electron donor(s). These organic blends form a class
of charge-transfer materials, and enable the photo-initiated charge
separation process defined by the following steps [N. S. Sariciftci
and A. J. Heeger, Intern. J. Mod. Phys. B 8, 237 (1994)]:
[0095] Step 1: D+A".sup.1,3D*+A, (excitation on D);
[0096] Step 2: .sup.1,3D*+A".sup.1,3(D-A)*, (excitation delocalized
on D-A complex);
[0097] Step 3: .sup.1,3(D-A)*".sup.1,3(D.sup.d+-A.sup.d-)*, (charge
transfer initiated);
[0098] Step 4:
.sup.1,3(D.sup.d+-A.sup.d-)*".sup.1,3(D.sup.+.degree.-A.sup-
.-.degree.), (ion radical pair formed);
[0099] Step 5:
.sup.1,3(D.sup.+.degree.-A.sup.-.degree.)"D.sup.+.degree.+A-
.sup.-.degree., (charge separation)
[0100] where (D) denotes the organic donor and (A) denotes the
organic acceptor; 1,3 denote singlet or triplet excited states,
respectively.
[0101] Typical thickness of the active layer range from a few
hundred .ANG.ngstrom units to a few thousand .ANG.ngstrom units;
i.e., 100-5000 .ANG.(1 .ANG.ngstrom unit=10.sup.-8 cm). Although
the active film thicknesses are not critical, device performance
can typically be improved by using thinner films with optical
densities of less than two in the spectral region of interest.
[0102] Electrodes
[0103] As shown in FIGS. 1 and 2, the organic photodiodes of this
invention are constructed in an M-S-M structure, in which the
organic photoactive layer is bounded on two sides with conductive
contact electrodes. In the configuration shown in FIG. 1, a
transparent substrate 14 and a transparent electrode 11 are used as
one contact electrode. Indium-tin-oxides ("ITO") can be used as the
electrode 11. Other transparent electrode materials include
aluminum doped zinc oxides ("AZO"), aluminum doped tin-oxides
("ATO"), tin-oxides and the like. These conducting coatings are
made of doped metal-oxide compounds which are transparent from near
UV to mid-infrared.
[0104] The electrode 11 can also be made with other doped inorganic
compounds or alloys. These compounds can be doped into metallic (or
near metallic) form by varying the composition of the elements
involved, the valance of the elements or the morphology of the
films. These semiconducting or metallic compounds are known in the
art and are well documented (e.g., N. F. Mott, Metal-Insulating
Transitions, 2nd edition (Taylor & Francis, London, 1990); N.
F. Mott and E. A. Davis, Electronic Processes in Non-crystalline
Materials (Claredon, Oxford, 1979)]. Examples of such compounds
include the cuprate materials which possess superconductivity at
low temperatures (so-called high temperature superconductors).
[0105] The electrode 11 in FIG. 1 (or 13 in FIG. 2) can be formed
of a conductive polymer such as polyaniline in the emeraldine salt
form prepared using the counterion-induced processability
technology disclosed in U.S. Pat. No. 5,232,631 and in Appl. Phys.
Lett. 60, 2711 (1992) or other suitable techniques. The polyaniline
film which serves as the electrode can be cast from solution with
high uniformity at room temperature. The organic conducting
electrodes in combination with polymer substrates and organic
active layers allow these photosensors be fabricated in fully
flexible form. Other conductive polymers can be used for the
transparent or semitransparent electrode (11 in FIG. 1 or 13 in
FIG. 2) include polyethylene dioxythiophene polystyrene sulfonate,
("PEDT/PSS") [Y. Cao, G. Yu, C. Zhang, R. Menon and A. J. Heeger,
Synth. Metals, E, 171 (1997)], poly(pyrrole) or its function
derivatives doped with dodecylbenzene sulfonic acid ("DBSA") or
other acid [J. Gao, A. J. Heeger, J. Y. Lee and C. Y. Kim, Synth.
Metals 82, 221 (1996)] and the like.
[0106] A thin semitransparent layer of metals (such as Au, Ag, Al,
In etc.) can also be used as the electrode 11 in FIG. 1 and 13 in
FIG. 2. Typical thicknesses for this semitransparent metal
electrode are in the range of 50-1000 .ANG., with optical
transmittance between 80% and 1%. A proper dielectric coating
(often in the form of multilayer dielectric stacks) can enhance the
transparency in the spectral range of interest [For examples, see
S. M. Sze, Physics of Semiconductor Devices (John Wiley & Sons,
New York, 1981) Chapter 13].
[0107] A transparent electrode can also be made from
metal/conducting polymer, conducting polymer/metal/conducting
polymer or dielectric layer/metal/conducting polymer structures.
The transmission properties of these composite electrodes are
improved relative to that of a single metal layer of the same
thickness.
[0108] A metal layer with low optical transmittance can also be
used as the electrode 11 for some applications in which spectral
response at certain wavelengths is of interest. The
photosensitivity can be enhanced by fabricating the device in a
micro-cavity structure where the two metal electrodes 11 and 13 act
also as optical mirrors. Light resonance between the two electrodes
enhances the photosensitivity at certain wavelengths and results in
selective spectral response, similar to that seen in optical
microcavity (optical etalon) devices.
[0109] The "back" electrode 13 in FIG. 1 (and 11 in FIG. 2) is
typically made of a metal, such as Ca, Sm, Y, Mg, Al, In, Cu, Ag,
Au and so on. Metal alloys can also be used as the electrode
materials. These metal electrodes can be fabricated by, for
example, thermal evaporation, electron beam evaporation,
sputtering, chemical vapor deposition, melting process or other
technologies. The thickness of the electrode 13 in FIG. 1 (and 11
in FIG. 2) is not critical and can be from hundreds of
.ANG.ngstroms to hundreds of microns or thicker. The thickness can
be controlled to achieve a desired surface conductivity.
[0110] When desired, for example, for a photodiode with detectivity
on both front and back side, the transparent and semi-transparent
materials described above can also be used as the "back" electrode
13 in FIG. 1 (and 11 in FIG. 2).
[0111] The patterning of the row and column electrodes shown in
FIG. 3 and FIG. 4 can be achieved by standard patterning
technologies well-known in semiconductor industry such as shadow
masking, photolithographing, silk-screen printing or stamp
(microcontact) printing etc. These methods are well known to those
knowledgeable of the art of display and image sensor
technologies.
[0112] To improve the device performance (for example, device
lifetime, operation speed etc.), a buffer layer comprising
conducting polymers or blends containing them can be inserted in
between the electrode 11 (or 13) and the photoactive layer. The
conductivity of the buffer layer can be chosen from a broad range
(between that of the pure conducting polymer and the photoactive
material). Conductivity of the buffer layer is changed by
processing conditions of the conducting polymer (counter-ion,
solvent, concentration etc.) and the composition ratio of the
blend. In certain situations the thickness of the buffer layer also
affects the spectral response of the photosensor.
[0113] Color Filter Coating
[0114] In some applications, multicolor detection or selected color
detection are of interest. These can be achieved by properly
selecting the material for the photoactive layer along with
coupling the photosensor with a color filter coating.
[0115] One type of application is a photosensor with selected
spectral response, for example, from 500 to 600 nm. One effective
approach is taking an organic photodiode with low energy cut-off at
600 nm (for example, a photodiode made with MEH-PPV), and placing a
long-wavelength, low pass optical filter (with cut-off at 500 nm)
in front. The spectral response of semiconducting oligomers and
polymers can be controlled by modifying the side chain or main
chain structures. For example, by varying the side chain of the PPV
system, the optical gap can be tuned from 500 nm to 700 nm. An
alternative approach to achieving bandpass selection is to place a
bandpass optical filter in front of an organic photodiode with
wider spectral response.
[0116] In photoimaging applications, full-color detection is
frequently of interest. This can be achieved by splitting a sensor
element (pixel) into three subpixels with response to red (600-700
nm), green (500-600 nm) and blue (400-500 nm) (R, G, B) spectral
regions (as shown in FIG. 4) respectively, similar to that commonly
used in liquid crystal display (LCD) color-display
technologies.
[0117] A simple but effective approach to full-color image sensors
is sketched in FIG. 4. In this approach, the photodiode matrix is
made of single sheet of active layer without patterning. The active
areas are defined by the row and column electrodes. The spectral
response of these organic photodiodes should cover the entire
visible region (400-700 nm). Color selection is achieved by the
color filter panel in front of the transparent electrodes. There
are many organic materials or blends with photoresponse covering
the entire visible spectrum. Examples include PT derivatives such
as "P3AT" [G. Yu, et al., Phys. Rev. B42, 3004 (1990), "POPT",
poly(3-(4-octylphenyl)thiophene) [M. R. Andersson, D. Selese, H.
Jarvinen, T. Hjertberg, 0. Inganas, 0. Wennerstrom and J. E.
Osterholm, Macromolecules 27, 6503 (1994)], PTV and its derivatives
and the like.
[0118] Several color filter techniques have been developed and have
been used broadly in color displays made with liquid crystal
technologies, including dyeing, pigment-dispersed, printing and
electro-deposition [M. Tani and T. Sugiura, Digest of SID 94
(Orlando, Fla.)]. Another approach uses multilayer dielectric
coating based on optical interference. Because of better stability,
pigment dispersion has become the major process used in large-scale
manufacturing. Color filter panels with designed patterns, often in
arrangements in triangular, striped (similar to that shown in FIG.
4), or diagonal mosaics, with transparent electrode coating (such
as ITO) are existing art and are commercially available to the
display industry. This type of substrate can be used in the
fabrication of full-color image sensors shown in FIG. 4.
[0119] The photodetectors provided by the present invention can be
adapted to respond to various types of ionized particles in
addition to photons, themselves. This can be accomplished by
incorporating in the photodetector structure a scintillating
material adapted to emit photons in response to ionized particles.
This material can be present in admixture with the active layer, it
can be present as a separate layer or it can be present as part of
the substrate or the transparent electrode. In one example this
scintillating material is a phosphor, present, for example as a
phosphor layer.
[0120] Examples of ionized particles which may be detected with
devices of this structure are high energy photons, electrons,
X-rays and ionized particles are characteristic of X-rays, beta
particles and ionized particles are characteristic of gamma
radiation.
[0121] Applications of the Invention
[0122] The invention of voltage-switchable organic photodiodes
provides the foundation for fabrication of large size, low cost 2D
image sensors based on x-y addressable passive diode matrices. This
type of photodiode shows high photosensitivity (typically in the
range of 30-300 mA/W), quantum efficiency (even over 100%
electrons/photon at given reverse biases) and virtually zero
response at a bias voltage close to the built-in potential. Thus, a
row of pixels in a column-row matrix of such photodiodes can be
selected by setting the selected row at reverse bias and the pixels
on the other row biased at a voltage close to the built-in
potential. In this way, crosstalk from pixels in different rows is
eliminated. The image information at the pixels in the selected row
can be read-out correctly in both the serial mode or the parallel
mode. The information on the pixels in the other rows can be
read-out in sequence or in selected fashion by setting the row of
interest to reverse bias. The x-y addressable organic photodiode
matrices provide a new type of 2D image sensor which can be made in
large size, with low fabrication cost, onto substrates in desired
shape or flexible, and hybridizable with other optical or
electronic devices.
[0123] Specific advantages of this invention over the prior art
include the following:
[0124] (i) Organic photosensors with switchable photosensitivity.
High photosensitivity can be switched on (typically in the range of
30-300 mA/W) at a selected reverse biasing voltage. The
photosensitivity can be switched off effectively when the diode is
biased externally at a voltage closed to that corresponding to the
internal potential.
[0125] (ii) 2D, x-y addressable, passive image sensors fabricated
with the organic photodiodes with switchable photosensitivity.
Crosstalk-free read-out can be achieved with these passive image
sensors by means of proper electronic pulse sequences.
[0126] (iii) Multi-color detection and full-color image sensing can
be achieved by coupling the image matrix with a color filter panel
or by fabricating the image sensor matrix directly onto a color
filter panel.
[0127] (iv) Organic photodetector arrays in combination with other
known advantages which characterize devices made with organic
materials such as soluble conjugated polymers (ease in fabrication
into large areas and desired shapes on rigid or flexible
substrates, room temperature processing, ease in hybridization with
optical, electro-optical, opto-electric or electric devices) offer
promise for large size, low cost, high pixel density, 1D or 2D
image sensors for use in office automation, in industrial
automation, in biomedical devices and in consumer electronics.
EXAMPLES
Example 1
[0128] Voltage-switchable photodiodes were fabricated by
evaporating a 5000 .ANG. calcium contact (13) on the front of a
thin MEH-PPV film 12 which was spin-cast from solution onto a
ITO/glass substrate 14. The glass substrate had been previously
partially coated with a contact layer 11 of indium-tin-oxide (ITO).
The active area of each device was 0.1 cm.sup.2. The MEH-PPV film
was cast from a 0.5% (11 mg/2 ml) xylene solution at room
temperature. Details on the synthesis of MEH-PPV can be found in
literature [F. Wudl, P. M. Allemand, G. Srdanov, Z. Ni, and D.
McBranch, in Materials for Nonlinear Optics:Chemical Perspectives,
Ed. S. R. Marder, J. E. Sohn and G. D. Stucky (American Chemical
Society, Washington, D.C., 1991) p. 683]. The thickness of the
active layer was adjusted by varying the concentration of the
solution, by varying the spin speed of the spinner head and by
applying multiple coating layers.
[0129] Electrical data were obtained with a Keithley 236
Source-Measure Unit. The excitation source was a tungsten-halogen
lamp filtered with a bandpass filter (center wavelength of 430 nm,
bandwidth of 100 nm) and collimated to form a homogeneous 5
mm.times.10 mm area of illumination. The maximum optical power at
the sample was 20 mW/cm.sup.2 as measured by a calibrated power
meter. A set of neutral density filters were used for measurements
of intensity dependence.
[0130] FIG. 6 shows the magnitude of the photocurrent (absolute
value) as a function of bias voltage under 20 mW/cm.sup.2
illumination at 430 nm. The photocurrent at 1.5 V bias was
.about.3.times.10.sup.-8 A/cm.sup.2,increasing to 9.times.10.sup.-4
A/cm.sup.2 at -10 V reverse bias corresponding to a
photosensitivity of 45 mA/W and a quantum efficiency of 13% el/ph.
The ratio of the photosensitivity between the two bias voltages was
3.times.10.sup.4, thus the photosensitivity at 1.5 V bias was
practically zero in the read-out circuit. This degree of difference
enabled an analog-to-digital (A/D) convertor of 8-12 bit
resolution.
[0131] The photoresponse increased nearly linearly with light
intensity (I.sup.0.921-1) over the entire range measured from
nW/cm.sup.2 to tens mW/cm.sup.2. No signature of saturation was
observed at 20 mW/cm.sup.2 (the highest light intensity in the
measurement).
[0132] Other metals such as Al, In, Cu, Ag and the like were also
used for the counterelectrode 13 (see FIG. 1) which is the cathode
in these devices. Similar photosensitivities to that shown in FIG.
5 were observed in all these devices. The off-state voltage which
balanced off the internal potential of the photodiode varied with
the work function of the metal; the off-state voltage is determined
by the work function difference between the metal cathode and the
ITO anode. Table 1 lists the off-state voltage found for MEH-PPV
photodiodes with several metal electrodes.
[0133] This experiment was also repeated with a thin layer (0.5-20
nm) of one of the metals listed in Table 1 with a thick Al layer
deposited on top as a current conducting layer. The device
performance is similar to that discussed above, with the off-state
voltage mainly determined by the thin metal layer at the
interface.
[0134] Devices were also fabricated with other photoactive organic
materials, including P3AT, POPT, PTV, PPV, BuEH-PPV, BUHP-PPV,
C.sub.60, 6T, 6P, spiro-6P, Alq.sub.3, anthracene and
phthalocyanine. Similar results to those shown in FIG. 6 were
observed.
[0135] This example demonstrates that high photosensitivity can be
achieved with MEH-PPV organic photodiodes under reverse bias. The
desired photosensitivity can be achieved at a given reverse bias.
The photosensitivity can be switched off at a proper bias voltage
which is dependent on the electrode materials selected. As shown in
Table 1, air stable metals with work functions over 4 V can be used
for the electrodes in organic photodiodes. This example also
demostrates that the off-state voltage is determined by the work
function of the electrode close to the interface area. This example
also demonstrates the broad dynamic range of the polymer
photodiodes, a dynamic range which is sufficient to enable image
detection with multi-grey levels.
1TABLE 1 Off-state voltage in ITO/MEH-PPV/metal photodiodes Metal
cathode Ca Sm Yb Al In Ag Cu V.sub.off(V) 1.5 1.5 1.5 1.1 0.9 0.7
0.4
Example 2
[0136] Devices of Example 1 were fabricated onto flexible ITO/PET
substrates. The thickness of the PET sheets used as substrates was
5-7 mils (125-175 .mu.m). Similar device performance was
observed.
[0137] This example demonstrates that the voltage-switchable
organic photodiodes can be fabricated in a thin structure, in
flexible form, or in a desired shape to meet the special needs in
specific applications.
Example 3
[0138] Devices of Example 1 were fabricated on glass and PET
substrates. In these devices, the ITO anode 11 was replaced with
organic conducting coatings or with ITO overcoated with conducting
organic films. PANI-CSA and PEDT/PSS were used as the organic
electrode. The PANI-CSA layers were spin-cast from m-cresol
solution [details about preparation of the PANI solution and
PANI-CSA film have been disclosed in U.S. Pat. No. 5,232,631]. The
PEDT-PSS films were cast from an aqueous dispersion (1.3% W/W)
which was supplied by Bayer [Bayer trial product, TP AI 4071],
details about the synthesis can be found in the literature [G.
Heywang and F. Jonas, Adv. Materials 4, 116 (1992)]. The cast films
were then baked at 50-85.degree. C. for several hours in a vacuum
oven or in a N.sub.2 dry box. In the case of PEDT/PSS, the films
were finally baked at a temperature over 100.degree. C. for several
minutes to complete the drying process. The thickness of the
conducting polymer electrodes was controlled from a few hundred
Angstroms to a few thousand Angstroms.
[0139] The optical transmission spectra of the polymer anode
electrodes are shown in FIG. 7, including data from PANI-CSA and
PEDT-PSS. Also shown in FIG. 7 is the spectral response of the
normal human eye, V(.lambda.). The data indicate that these organic
conducting electrodes can be used for photosensors for applications
in visible spectral range. Moreover, the PEDT-PSS electrodes can
also be used for ultraviolet (250-400 mn) and for near infrared.
Thus, the polymer electrodes can be used in photosensors with
full-color (white color or R, G, B three color) detection.
[0140] In addition to the electrodes made with PANI-CSA or PEDT-PSS
alone, devices were fabricated with ITO/PANI-CSA and ITO/PEDT-PSS
bilayer electrodes. In these cases, the polymer electrodes were
cast in a thin layer (thickness of a few hundred Angstrom units) to
maximize the optical transmission. Organic light-emitting devices
with bilayer electrodes have demonstrated improved device
performance such as carrier injection and device stability.
Examples are shown in U.S. patent applications Ser. Nos. 08/205,519
and 08/609,113.
[0141] The photosensitivities of the devices with organic anode
electrodes or bilayer electrodes were similar to those shown in
FIG. 6; i.e. tens of mA/W at reverse bias voltage in the -5 to
about -10 V range.
[0142] This example demonstrates that conducting polymer materials
can be used as the transparent electrodes of the photodiodes and
image sensors. These plastic electrode materials provide the
opportunity to fabricate organic photosensors in flexible or
foldable forms. This example also demonstrates that the polymer
electrode can be inserted between a metal-oxide transparent
electrode (such as ITO) and the active layer to modify the
interfacial properties and the device performance.
Example 4
[0143] Devices of Example 1 were repeated. A thin buffer layer was
inserted between the ITO and the MEH-PPV layers to reduce the
leakage current through pinhole imperfections in the active layer.
The materials used for the buffer layer were PAZ, TPD (prepared via
chemical vapor deposition) and PVK (cast from cyclohexanone
solution). The thickness of the buffer layer was 100-500 .ANG.. The
photoresponse of these devices was similar to that shown in FIG. 6.
However, in these devices, the dark current (which is caused
frequently from microshorts due to pinholes in the active layer)
was reduced in magnitude. In these short-free devices, a photon
flux as small as 1 nW/cm.sup.2 was detected under direct current
operation. The off-state voltage was 1.6.about.1.7 V in these
devices, slightly higher than in the devices in Example 1.
[0144] This example demonstrates that a buffer layer can be
inserted between the active layer and the contact electrode(s) to
reduce device shorts and to improve the device response to weak
light. This buffer layer can be made of organic molecules via
chemical vapor deposition or polymeric materials through wet
casting processes.
Example 5
[0145] Devices of Example 1 were repeated. The active material,
MEH-PPV, was blended with an anionic surfactant Li-CO436 in molar
ratios of 0, 1, 5, 10 and 20%. The Li-CO436 was synthesized by a
substitution reaction from Alipal CO436 (ammonium salt nonylphenoxy
ether sulfate) supplied by Phone-Poulenc Co. [Y. Cao, U.S. patent
application Ser. No. 08/888,316]. Al was used as the cathode. The
photosensitivity was enhanced in the devices with blended Li-CO436.
For example, the photocurrent increased by a factor of 2 in a
device made with MEH-PPV:Li-OC436 (10 wt %) compared with a similar
device made without the Li-OC436. Moreover, the off-state voltage
shifted from 1.1 V in ITO/MEH-PPV/Al devices (see Example 1) to 1.5
V in the ITO/MEH-PPV:Li-OC436 (20%)/Al devices. Similar effects
were also observed in devices having an ITO/MEH-PPV/Li-OC436/Al
structure. The off-state voltage increases from 1.1 V to 1.6 V.
[0146] Devices of Example 1 were also fabricated with LiF,
Li.sub.2O or BaO layers (1-30 nm) inserted between the MEH-PPV and
the Al cathode. Similar enhancement of the short circuit current
and the off-state voltage was observed.
[0147] Devices of Example 1 were also fabricated with a TiO.sub.2
layer (1-30 nm) inserted between the MEH-PPV and the Al cathode,
and with TiO.sub.2 nanoparticles dispersed in the MEH-PPV film
(forming a phase separated MEH-PPV:TiO.sub.2 blend film. Similar
results to those obtained with ITO/MEH-PPV/BaO/Al were
observed.
[0148] This example demonstrates that organic additives can be
added to the active layer or inserted between the active player and
the contact electrode to modify the device performance including
photosensitivity and off-state voltage. This example also
demonstrates that a layer of inorganic dielectric or semiconducting
compounds can be inserted between the active layer and the contact
electrode to modify device performance, including photosensitivity
and off-state voltage. The inorganic dielectric or semiconducting
compounds can also be made in nanoparticle form and blended with
the organic photosensing materials.
Example 6
[0149] Voltage-switchable photodiodes were fabricated in the
structure of ITO/MEH-PPV:PCBM/metal, similar to that shown in FIG.
1. The PCBM (a C.sub.60 derivative) served as an acceptor in a
donor-acceptor pair with the MEH-PPV acting as donor. The active
area of these devices was .about.0.1 cm.sup.2. The blend solution
was prepared by mixing 0.8% MEH-PPV and 2% PCBM/xylene solutions
with 2:1 weight ratio. The solution was clear, uniform, and was
processable at room temperature. Solutions were stored in a N.sub.2
box for over 1.5 years and no aggregation or phase separation were
observed. The active layer was spin-cast from the solution at
1000-2000 rpm. Typical film thicknesses were in the range of
1000.about.2000 .ANG.. Ca, Al, Ag, Cu, and Au were used as the
counter electrode 13. In each case, the film was deposited by
vacuum evaporation with thickness of 1000-5000 .ANG.. In another
experiment, the concentration of the acceptor PCBM was varied from
0 to 1:1 molecular ratio. Higher on state photosensitivity and
lower on-state operation voltage were observed in devices with
higher concentrations.
[0150] FIG. 8 shows the I-V characteristics of an
ITO/MEH-PPV:PCBCR/Al device in the dark and under light
illumination. The thickness of the blend film was .about.2000
.ANG.. The dark current saturated at .about.1 nA/cm.sup.2 below 3 V
and then increased superlinearly at high bias voltages
(>E.sub.g/e). Zener tunneling can account for this effect. The
photocurrent was measured. The photocurrent at 0.65 V was
.about.1.times.10.sup.-7 A/cm.sup.2, increasing to 5.times.10.sup.4
A/cm.sup.2 at -10 V bias. The on-off ratio was .about.500. Devices
with thinner blend films showed improved photosensitivity and
higher on-off ratio. Similar photosensitivity was also observed in
devices fabricated with other metals or metal alloys as the counter
electrodes. These included Ag, Cu, Ca, Sm, Pb, Mg, LiAl, MgAg,
BaAl.
[0151] Other organic molecules were used as the photoacceptor,
including C.sub.60. Other mixtures were prepared using the C.sub.60
derivative, PCBM with different solvents. Higher photosensitivity
was observed from MEH-PPV:PCBM processed from 1,2-dichlorobenzene
solution. The photosensitivity reached 0.2 A/W at 430 nm when
biased at -2 V.
[0152] This example demonstrates that the photosensitivity can be
further improved by blending a donor polymer with a molecular
acceptor such as C.sub.60, PCBM, PCBCR. High photosensitivity can
be achieved at relatively low bias and low field (.about.10.sup.5
V/cm). This example also demonstrates that the photosensitivity can
be switched to nearly zero when bias the device at a voltage
balancing the internal built-in potential (.about.0.65 V for Al
cathode). The data in this example show that, due to its low dark
current level, the polymer photodiode can be used to detect weak
light down to intensity level of tens of nW/cm.sup.2. Thus, the
polymer photodiodes have a dynamic range spanning more than six
orders of magnitude, from nW/cm.sup.2 to 100 mW/cm.sup.2.
Example 7
[0153] Devices similar to those of Example 6 were fabricated with
glass/ITO and PET/ITO substrates in 4.5 cm.times.4 cm (18 cm.sup.2)
and in 3.8 cm.times.6.4 cm (24.3 cm.sup.2) using a fabrication
process similar to that of Example 6. I-V characteristics similar
to that shown in FIG. 8 were observed. The photodiodes made with
flexible PET substrates were bent into circular shapes any without
change in their photosensitivity.
[0154] This example demonstrates that the high sensitivity,
voltage-switchable photosensors can be fabricated in large sizes.
With flexible PET substrates, the photosensors can be bent into
desired shapes for special needs in optics, physics and biomedical
fields.
Example 8
[0155] Devices similar to those of Example 6 were repeated with the
active layer made of MEH-PPV:CN-PPV, a polyblend with two polymers
as the donor and acceptor phases. Ca and ITO were used as the
cathode and anode electrodes, respectively. The molecular ratio
between this donor and the acceptor was varied from pure MEH-PPV
(1:0) to pure CN-PPV (0:1). Similar I-V characteristics to those
shown in FIG. 8 were observed in devices with intermediate
molecular ratios. The off-state voltage shifted to .about.1.2 V as
anticipated from the change in the potential barrier between the
donor and acceptor phases.
Example 9
[0156] Devices similar to those of Example 6 were repeated with the
active layer made of sexithiophene (6T):PCBCR, a blend with two
organic molecules as the donor and acceptor phases. Similar I-V
characteristics were obtained to those shown in FIG. 8.
[0157] Devices were fabricated in the form of ITO/6P/C.sub.60/Al,
ITO/6P/t-Bu-PBD/Al. The photoactive layer comprised two types of
organic molecules in heterojunction form, made by thermal
evaporation. Similar I-V characteristics to those shown in FIG. 8
were observed.
[0158] Examples 8 and 9 demonstrate that the active layer of the
voltage-switchable photodiodes can be organic molecules arranged in
bilayer or multilayer structures, a blend of organic molecules, or
a blend of conjugated polymers, in addition to a polymer/molecule
blend as demonstrated in example 6. The data in these examples
along with that in the Example 1 also demonstrate that, for a given
cathode such as Ca, the off-state voltage varies with the
electronic structure of the active material.
Example 10
[0159] Voltage-switchable organic photodiodes was fabricated with
P3OT as the active layer in an ITO/P3OT/Au structure. The I-V
characteristics in the dark and under light illumination are shown
in FIG. 9. Since the work function of Au is higher than ITO, the Au
electrode serves as the anode in these devices. Positive bias was
defined such that a higher potential was applied to Au electrode.
Light was incident from the cathode (ITO) electrode. In this
experiment, a He--Ne laser at 633 nm was used as the illumination
source with a photon density of 10 mW/cm.sup.2.
[0160] The built-in potential in this photodiode was reduced to
nearly zero volts. Thus, the off-state of the photodiode was
shifted to close to zero volts. The photocurrent at -12 V was 1
mA/cm.sup.2, which was 10.sup.4 times higher than that at zero
bias. Values of the ratio I.sub.ph(-12V)/I.sub.ph(0) in excess of
1.5.times.10.sup.5 have been realized in similar devices. The
photosensitivity at 633 nm was .about.100 mA/W, corresponding to a
quantum efficiency of .about.20% ph/el. The dark current in the
test range was below 5.times.10.sup.-7 A/cm.sup.2. The
photocurrent/dark current ratio was greater than 1000 over a broad
bias range (-4.about.-12 V).
[0161] This example demonstrates that the off-state of the
photodiode can be varied by proper selection of the active material
and the electrode materials. This voltage can be set to a voltage
close to zero volts. A photodiode matrix fabricated with this type
of photodiode can be driven by pulse trains with mono-polarity,
thus simplifying the driving circuitry. The large on/off switching
ratio and the large photocurrent/darkcurrent ratio permit the
photodiodes to be used in the fabrication of x-y addressable
passive matrices with high pixel density and with multiple-gray
levels.
Example 11
[0162] Two-dimensional, photodiode matrices were fabricated with
seven rows and 40 columns. Pixel size was 0.7 mm.times.0.7 mm. The
space between the row electrodes and the column electrodes was 1.27
mm (0.05"). The total active area was .about.2".times.0.35".
Typical I-V characteristics from a pixel are shown in FIG. 10.
White light from a fluorescent lamp on the ceiling of the lab was
used as the illumination source with intensity of .about.tens of
.mu.W/cm.sup.2. This is much weaker than the light intensity used
in document scanners.
[0163] This example demonstrates that pixelated photodiode matrices
can be fabricated without shorts and without crosstalk. This
example also demonstrates that these devices can be used for
applications with light intensities equal to or much less than a
microwatt/cm.sup.2. Thus, polymer photodiode matrices are practical
for image applications under relatively weak light conditions.
Example 12
[0164] A scanning scheme for the photodiode matrix was developed
(see FIG. 11). Due to the strong voltage dependence of the
photosensitivity, a column of pixels in the 2D photodiode matrix
could be selected and turned on with proper voltage bias, leaving
the pixels in the adjacent rows insensitive to the incident light.
Under such operation, the physical M row, N column 2D matrix is
reduced to N isolated M element linear diode arrays which are free
from crosstalk between columns. This is reminiscent of the concept
that is used in solving a 2D integral by dimension reduction,
.intg.f(x,y)dxdy=.intg.g(x)dx.intg.h(y)dy. With such 2D, passive
photodiode arrays, an image can be read out with a pulse train
scanning through each column of the matrix.
[0165] FIG. 11 shows a instantaneous "snap-shot" of the voltage
distribution in a 7.times.40 photodiode matrix. At a specific time
t, all the pixels were biased at +0.7 V except the pixels in column
1. The pixels in column 1 were all biased at -10 V so as to achieve
high photosensitivity (tens-hundreds of mA/Watt). The information
at each of the pixels in column 1 was read-out in both parallel
(with N channel converting circuits and A/D converters) or serial
(with N channel analog switches) sequences. Pixels in other columns
were selected by switching the column bias from +0.7 V to -10 V in
sequence. A digital shift register was used for the column
selection.
[0166] To simplify the driver circuit, it was preferable that the
photosensor can be switched on and off between 0 V and a reverse
bias voltage (-2 to -10 V). Such a mono-polarity,
voltage-switchable photodiode was demonstrated with ITO/P3OT/Au, as
shown in Example 10.
Example 13
[0167] An image of multi-gray levels was selected, the image was
scanned with the 7.times.40 photodiode matrix following the
scanning scheme discussed in Example 12. The original image and the
readout image were recorded photographically. The readout image
reproduced the original image with excellent fidelity.
[0168] This example demonstrates that the voltage-switchable
photodiodes can be used as the pixel elements of a column-row
matrix (as shown in FIG. 3). The photodiodes at each pixel can be
addressed effectively from the column and row electrodes. Image
information with multiple gray-levels can be read-out without
distortion.
Example 14
[0169] Devices similar to those of Example 10 were fabricated and
their spectral response was measured at a reverse bias of -15V. The
data are shown in FIG. 12. In contrast to the significant
sensitivity decrease at short wavelength in conventional inorganic
photodiodes, the P3OT photodiodes exhibited relatively flat
response for wavelengths shorter than 630 nm; the apparent decrease
in sensitivity below 350 nm was mainly due to the transmission
cut-off of the ITO coated glass substrate. For -15 V bias, the
sensitivity at 540 nm reached 0.35 A/W (a quantum yield of
.about.80% el/ph), the same value as obtained with UV-enhanced Si
diodes. Similar photosensitivity values persisted into the UV
region below 400 nm. In some devices, quantum efficiency of over
100% el/ph (140.about.180% el/ph) was observed under reverse
bias.
[0170] Devices were also fabricated in the form of
ITO/P3HT/P3HT:PCBM/A1. White light was used as the illumination
source. Quantum efficiency of over 100% electrons/photon was
observed. The highest value observed was .about.1100%
electrons/photon. A gain mechanism may play a role in these
multilayer devices.
[0171] This example demonstrates high photosensitivity organic
photodetectors with response covering, simultaneously, the near UV
and the entire visible spectra. This example also demonstrates that
organic photodetectors in the metal/organic/metal sandwich
structure can have quantum efficiency over 100% electrons/photon;
i.e., possesses a gain mechanism.
Example 15
[0172] Voltage-switchable photodiodes were fabricated to achieve a
response similar to the visual response of the human eye,
V(.lambda.). The devices were fabricated by coating a
long-wavelength-pass filter onto the front panel of the glass
substrates of devices, similar to those shown in Example 15. The
coating material in this example was a layer of PPV which was
converted from its precursor film at 230.degree. C. The
photoresponses of the devices with and without the filter are shown
in FIG. 13A. The visual response of the human eye, V(.lambda.) (see
FIG. 13B), and the transmittance of the PPV optical filter are
shown for comparison. The photoresponse of the P3OT diode closely
coincided with V(.lambda.) for wavelengths longer than 560 nm,
while the optical transmittance of the PPV filter followed
V(.lambda.) over a broad range between 450 nm and 550 nm.
[0173] This example demonstrates a polymer photodetector with
visual response essentially equivalent to V(.lambda.), which is of
great interest in optical engineering and biophysical/biomedical
applications.
Example 16
[0174] Solar-blind UV detectors were fabricated with polyblend
MEH-PPV:C.sub.60. ITO and Al were used as anode and cathode
materials. The devices were fabricated on an UV bandpass filter
purchased from Melles Griot Inc. (product No. 03 FCG 177). FIG. 14
shows the spectral response of the UV detector operating at -2V.
The spectral response of the MEH-PPV:C.sub.60 photodiode on
ITO/glass substrate and the response of an UV-enhanced Si
photodiode are plotted for comparison. The data show that the
polymer UV detector was sensitive to UV radiation between 300-400
nm with photosensitivity of .about.150 mA/W, comparable to that of
UV-enhanced silicon photodiode. The data also show that the
photoresponse of the MEH-PPV photodiode was suppressed (over
10.sup.3 times) by the optical bandpass filter.
[0175] This example demonstrates that high sensitivity, solar-blind
UV detectors can be fabricated by integrating voltage-switchable
organic photodiodes with UV pass optical filters.
Example 17
[0176] Example 14 was repeated except that the active layer was a
thin PTV layer. The spectral response of a PTV photodiode is shown
in FIG. 15A, which covers the range from 300 to 700 nm; i.e.,
spanning the entire visible range. Selected color detection was
achieved by inserting a bandpass filter or a long wavelength filter
in front of the detectors. FIG. 15B shows the responses of a
blue-color pixel, a green-color pixel and a red-color pixel made
with a panel of color filters and an array of PTV photodiodes. The
transmittance of the corresponding R, G, B color filters is shown
in FIG. 15C.
[0177] This example demonstrates that by coupling the polymer image
sensor with a panel of color filters, R, G, B color recognition can
be achieved with a panel of polymer photodiode matrix with response
covering entire visible spectrum.
Example 18
[0178] Red, green and blue (R, G, B) color detection were achieved
following the approach shown in FIG. 5. The materials used for the
active layers were PPV with a long wavelength cut-off at 500 nm;
poly(dihexyloxy phenylene vinylene), "PDHPV", with a long
wavelength cut-off at 600 nm; and PTV with long wavelength cut-off
at 700 nm. Films were cast from solutions in their precursor forms
with thickness between 1000 .ANG.-3000 .ANG.. Conversion to the
conjugated forms was carried out at temperatures between
150-230.degree. C. The conjugated films formed in this way were
insoluble to organic solvents. Thus, patterning of these materials
on a single substrate in dot or strip shapes can be achieved with
standard photolithography, screen printing and the like. The
normalized photoresponse of these photodiodes is shown in FIG. 16A.
An ITO/glass substrate was used in this experiment which is
optically transparent in visible and opaque in UV.
[0179] Red and green selective color detection were achieved by
differentiation of the signals from these photodiodes (this
operation can be done in the read-out circuit). The differential
responses of these photodiodes are shown in FIG. 16B. Red color
detection (with response between 600-700 nm) was achieved by
subtracting the signal from the PTV photodiode from the signal from
the PDHPV photodiode. Green color detection (with response between
500-600 nm) was achieved by subtraction of the PDHPV signal from
the PPV signal. The blue color detection was obtained by PPV
photodiode directly.
[0180] This example demonstrates that R, G, B selected color
detection and full-color image sensors can be achieved by
patterning three photosensitive materials on a substrate with
uniform optical characteristics.
Example 19
[0181] Voltage-switchable photodiodes were fabricated with the
conjugated polymer poly(p-phenyl vinylene), PPV as photoactive
material. The PPV films were spin-cast onto ITO substrates from a
nonconjugated precursor solution and then converted to conjugated
form by heating at 200-230.degree. C. for 3 hours. Al was used as
the back electrode. The active area was .about.0.15 cm.sup.2. The
I-V characteristics of this photodiode in the dark and under
illumination are shown in FIG. 17. The photocurrent/darkcurrent
ratio is in the range of 10.sup.4 for white light illumination of a
few mW/cm.sub.2. Relatively low dark current was observed in
forward bias as compared to that observed in photodiodes of, for
example, in Example 1. This allows photodetection in both forward
bias and reverse bias as shown in FIG. 17. The photosensitivity can
be switched on and off by varying the external biasing voltage. For
example, under white (or UV) light illumination, the photocurrent
at +5V or -5V is 2000 times higher than that at +0.95V (or
0.3V).
[0182] This example demonstrates that the photodiode can be
switched on by applying a forward bias (beyond the vicinity of the
voltage corresponding the off state) or a reverse bias. Photodiodes
operable in both switch polarities are useful in certain circuit
designs and applications.
Example 20
[0183] Voltage-switchable photodetectors were fabricated which had
a heterojunction structure as their active layers, they had an
ITO/donor layer/acceptor layer/metal structure. The materials used
for the donor layer were MEH-PPV and PPV. The material used for the
acceptor layer were C.sub.60, laid down by physical vapor
deposition and PCBM and PCBCR laid down by drop casting or spin
casting. A data set for a MEH-PPV/C.sub.60 photodiode is shown in
FIG. 18.
[0184] Multiple junctions were observed in these devices. A
build-up potential of .about.-0.5V (forward bias was assigned as
the positive bias to ITO) was seen in the I-V curve taken in dark.
The other junction was revealed when the devices were illuminated.
The overall effective barrier is -0.15V (changed sign). The
photocurrent/darkcurrent ratio was 10.sup.4 over a broad bias
range. Voltage-switchable photosensitivity was seen in both forward
and reverse bias. For instance, the on/off ratio of the
photocurrent is .about.10.sup.3 between +2V and +0.15V bias.
[0185] This example demonstrates that voltage-switchable
photodetectors can be fabricated in heterojunction form with two
(or more) organic semiconductors with different electronic
structures. The photosensitive mode can be achieved in both forward
and reverse biases in these devices.
Example 21
[0186] Voltage-switchable photosensors was fabricated in the
configuration shown in FIG. 1. Glass with patterned ITO was used as
the substrate. The size of each test pixel was .about.0.1 cm.sup.2.
The sensing material used was poly(3-hexyl thiophene), P3HT, which
was spin cast at room temperature from a 2.5 wt % solution in
toluene. Similar to the spectral response of P3OT (see Example 10),
the photoresponse of P3HT sensor covers the entire visible and near
UV spectral region such that red, green and blue full-color
recognition can be achieved by color filtering techniques.
[0187] FIG. 19 shows the photo- and dark currents from a P3HT
device with 3150 .ANG. film thickness. The data were taken with
white light illumination of 8 mW/cm.sup.2 (between 400 nm and 700
nm) and with monochromatic light (600 nm at 1.1 mW/cm.sup.2). In
the dark, the reverse current saturates at low field region and
then increases with the biasing voltage, to
.about.2.times.10.sup.-5 mA/cm.sup.2 at -25 V bias. The forward
current increases exponentially under forward bias (for voltages
>1 V), reaching .about.1 mA/cm.sup.2 at 3 V bias. The
exponential forward current covers more than 5 orders of magnitude
in the voltage range from 1-2 V. The rectification ratio at 2 V is
over 10.sup.4. Strong photosensitivity was observed in reverse
bias. The photocurrent at -25 V reaches 5.33 mA/cm.sup.2 under 8
mW/cm.sup.2, white light illumination. This number corresponds to a
photoresponsivity of in excess of 0.5 A/W, corresponding to a
quantum efficiency larger than 100% electrons/photon. A high
I.sub.ph(V.sub.on)/I.sub.ph(V.sub.off) switching ratio was also
achieved in this devices: under 8 mW/cm.sup.2,
I.sub.ph(-25V)/I.sub.ph(0.- 5)is .about.4.times.10.sup.7. This
switching ratio is equal to or even better than the switching ratio
of TFT-based photosensors made with inorganic semiconductors
(10.sup.4-10.sup.7).These organic photodiodes also exhibit a high
I.sub.ph(V)/I.sub.dark(V) ratio. The I.sub.ph/I.sub.dark at -25 V
is .about.4.times.10.sup.5 for 8 mW/cm.sup.2 white light
illumination, which implies that more than 18 bits
(2.6.times.10.sup.5) gray levels can be resolved for image
applications.
[0188] The high switching ratio implies that for an x-y addressable
2D photodiode matrix of 400.times.390 pixels (refer to FIG. 3 of
the 2D patent), more than 256 gray levels can be resolved. Adopting
quad-matrix design (four sub-matrices arranged in each quadrants),
more than 1000.times.625 pixels are possible with the same
resolution. This pixel density is even better than the SVGA
standard. The drive circuit for these photodiode matrices is
simplified; digital shift registers and BCD digital decoders can be
used.
[0189] These photosensors can also be used to fabricate high pixel
density linear photodiode arrays. Since only the pixel at the node
contributes to the pixel dark current, there is no restriction on
the number of pixels. Hence, the gray level of the sensor array can
be as high as 2.sup.18=3.times.10.sup.5. These results suggest that
the organic photosensor arrays constructed from ITO/P3HT/Al can be
used for high quality image sensing. Moreover, the driver circuits
for column selection are simplified considerably and digital shift
registers or digital decoders can be used directly.
[0190] This example demonstrates an organic photosensor with high
switching ratio and high I.sub.ph/I.sub.dark ratio. The
photosensitivity of such photosensors covers the entire visible
spectral range. These sensors are especially suitable for
constructing linear photodiode arrays and 2D photodiode matrices
for high quality image sensing applications.
Example 22
[0191] Linear photodiode arrays were fabricated with 102 sensing
elements, each made with P3OT as the semiconducting polymer. Two
typical structures of the photodiode arrays are shown in FIG. 20A
and FIG. 20B. The pixel size was .about.0.635 mm.times.0.635 mm.
The length of the total sensing area was .about.2.5", longer than
any linear photodiode array commercially available. A full-color
linear scanner was constructed with a sensing circuit shown in FIG.
21, no analog switching elements (such as field effect transistors)
were used in this driver. The read out circuit was digitized into 8
bit with 256 gray levels. Red, green and blue color filters were
mounted on a panel and was switched in front of the linear diode
array when collecting the corresponding images. The linear
photodiode array was mounted on a computer controlled translation
stage for the image scanning. A full-color image taken with this
scanner is shown in FIG. 22D. It was recovered by a superposition
of the red, green and blue color images (FIGS. 22(a, b, c)) taken
separately. The image quality was similar to that achieved with a
commercial color scanner in the same pixel format (40 dpi) with
so-called "multi-million ( 256.sup.3) colors" format.
[0192] Linear photodiode arrays were also fabricated in 40 dpi and
50 dpi forms with total pixels of 200 and 240. The total sensing
length is close to 5". The arrays were used for image sensing
experiments. Large size (5".times.11"), high quality (8-10 bit),
full-color image sensing was demonstrated.
[0193] This example demonstrates that organic photodiode arrays can
be used for large size image sensing applications with full-color
capability and with multiple gray levels.
Example 23
[0194] The linear photodiode arrays demonstrated in Example 22 were
also used for visible-blind UV sensing. In this experiment, a
visible blocking, UV pass filter was placed in front of the array.
The UV image generated with UV ink was projected onto the sensor.
The UV image was read out with the organic photodiode array.
[0195] This example demonstrates that visible-blind UV sensors can
be achieved with organic photosensors, and that image in UV
spectral region can be detected.
Example 24
[0196] Linear photodiode arrays were fabricated in the same
configuration as that of Example 22 (1.times.102 pixels, 40
pixels/in). One of the sensor arrays was used as an optical beam
analyzer to test the optical field distribution a laser beam. The
intensity distribution of the testing optical field is shown in
FIG. 23. This example demonstrates that the polymer photodiode
array can be used to detect spatial distribution of an optical
beam. This function is of broad applications in industrial
automation.
Example 25
[0197] Another 1.times.102 linear photodiode array was fabricated
on PET substrate (7 mil in thickness). The flexible sensor array
was arranged in a semicircular shape. A point light source from a
green light emitting diode was placed at the center of the circle,
and the angular distribution of the light intensity was tested with
the curved sensor array. The result is shown in FIG. 24.
[0198] This example demonstrates that the polymer linear photodiode
arrays can be fabricated onto flexible substrates or on curved
substrates to fit into an optical apparatus or to probe the spatial
distribution of an optical field. The fabrication process and the
thin film architecture of the polymer photodiode arrays also allow
them to be integrated with electronic drivers on a silicon wafer or
integrated with an adapted optical component.
Example 26
[0199] A P3OT photodiode array was used as the detector of an
UV-visible spectrometer for transmission measurement. The setup is
shown in FIG. 25. A transmission spectrum of a thin film of
poly(p-phenylene vinylene), PPV, was measured with the polymer
linear photodiode arrays. The result is shown in FIG. 26.
[0200] This example demonstrates that the organic photodiode arrays
can be used for spectrographic applications.
Example 27
[0201] Voltage-switchable photosensors were fabricated in a
metal(1)/P3HT/metal(2) sandwich structure. In one case, metal(1)
was Au and metal(2) was Al. The thickness of the Au layer was
varied from 20 nm to 80 nm and the optical transmission of the Au
layer was varied from 50% to .about.1%. The optical reflection of
the Au layer varies correspondingly. The thickness of the Al layer
was more than 100 nm, so that its reflectance was almost 100%. Such
a metal/organic layer/metal structure forms an optical microcavity
(optical etalon) device in the spectral region where the optical
absorption of the organic layer is relatively low. Such a
microcavity structure possesses optical resonance at selected
wavelengths. The center wavelength and the bandwidth of the sensing
profile can be adjusted by changing the reflection of the metal
electrode, by the absorption coefficient, the dielectric constant
and the thickness of the organic layer. FIG. 27 shows the spectral
response of such device.
[0202] Microcavity devices were also made in the "reverse"
structure similar to that shown in FIG. 2; i.e., with light
incident onto the free surface electrode (13). The devices were
made in both configurations: glass/Au(100 nm)/MEH-PPV/Ag(50 nm) and
glass/Ag(100 nm)/MEH-PPV/Au(50 nm). In these devices, Au acts as
the anode and Ag as the cathode. Selective spectral response was
observed in both structures. These results demonstrate the
flexibility of fabricating the wavelength selective sensors on
either transparent substrates or opaque substrates. These results
also demonstrate that the devices can be designed so that the light
is incident onto either the anode or cathode electrode.
[0203] Wavelength selective photosensors were also fabricated with
substrates containing an optical stack (sometimes called DBR,
Defractive Bragg Reflector). The transmission of the DBR was
.about.2%. The photosensors were fabricated as follows:
glass/DBR/ITO/MEH-PPV:PCBM/Al. Wavelength selective spectral
response was observed with .about.2 nm bandwidth.
[0204] This example also demonstrates that the organic photosensors
can be constructed with wavelength selectivity of narrow bandwidth.
Building such a photodiode array or 2D matrix in which each pixel
has a different sensing profile forms a flat-panel spectrometer.
These kinds of devices have great potential for image sensing,
spectrographic, biophysical and biomedical applications.
* * * * *