U.S. patent application number 10/234488 was filed with the patent office on 2003-04-03 for photovoltaic device and method for preparing the same.
Invention is credited to Miteva, Tzenka, Nelles, Gabriele, Yasuda, Akio.
Application Number | 20030062082 10/234488 |
Document ID | / |
Family ID | 8178540 |
Filed Date | 2003-04-03 |
United States Patent
Application |
20030062082 |
Kind Code |
A1 |
Miteva, Tzenka ; et
al. |
April 3, 2003 |
Photovoltaic device and method for preparing the same
Abstract
The present invention relates to a photovoltaic device,
especially a hybrid solar cells, comprising at least one layer
comprising evaporated fluoride and/or acetate; and to a method for
preparing the same.
Inventors: |
Miteva, Tzenka; (Stuttgart,
DE) ; Nelles, Gabriele; (Stuttgart, DE) ;
Yasuda, Akio; (Stuttgart, DE) |
Correspondence
Address: |
FROMMER LAWRENCE & HAUG LLP
745 FIFTH AVENUE
NEW YORK
NY
10151
US
|
Family ID: |
8178540 |
Appl. No.: |
10/234488 |
Filed: |
September 3, 2002 |
Current U.S.
Class: |
136/263 ;
136/256; 438/82 |
Current CPC
Class: |
Y02E 10/549 20130101;
Y02E 10/542 20130101; H01L 51/0086 20130101; H01L 51/441 20130101;
H01L 51/4253 20130101; H01L 51/002 20130101; H01L 51/424 20130101;
H01L 51/0059 20130101; H01L 51/006 20130101; H01L 51/4226
20130101 |
Class at
Publication: |
136/263 ;
136/256; 438/82 |
International
Class: |
H01L 031/00 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 4, 2001 |
EP |
01 121 178.6 |
Claims
1. Photovoltaic device having at least one layer comprising
evaporated fluoride and/or acetate.
2. Photovoltaic device according to claim 1, further having a solid
conjugated semiconductor comprising a hole transport material,
wherein the hole transport material is mixed with a dopant.
3. Photovoltaic device according to claim 1 or 2, further
comprising a blend or bilayer structure of conductive organic
and/or polymer materials, wherein one component is a p-type
conductor and the other one is a n-type conductor.
4. Photovoltaic device according to any of the preceding claims,
wherein the evaporated fluoride is an alkali or alkaline earth
metal fluoride.
5. Photovoltaic device according to any of the preceding claims,
wherein the evaporated acetate is an alkali metal acetate.
6. Photovoltaic device according to any of the preceding claims,
further comprising a semi-conductor oxide layer sensitized with a
dye, preferably a ruthenium complex dye.
7. Photovoltaic device according to any of the preceding claims,
wherein the evaporated layer containing fluoride and/or acetates is
evaporated on top of the semiconductor oxide layer and/or on top of
a layer of the hole transport material and/or an top of a
transparent conductive oxide electrode.
8. Photovoltaic device according to claim 7, wherein the
semiconductor oxide layer is titanium dioxide.
9. Photovoltaic device according to any of the preceding claims,
wherein the evaporated layer has a thickness of about 0.5 to about
30 nm, preferably about 0.5 to about 15 nm.
10. Photovoltaic device according to any of the preceding claims,
wherein fluorides and/or acetates evaporated on different layers of
the device have different counter cations.
11. Photovoltaic device according to claim 10, wherein the
evaporated layer which is evaporated on the semiconductor oxide
layer comprises lithium fluoride having a thickness of about 5 nm,
and the evaporated layer which is evaporated on the hole transport
material comprises cesium fluoride having a thickness of about 15
nm.
12. Photovoltaic device according to any of the preceding claims,
wherein the hole transport material is represented by formula (I)
4wherein R in each occurrence is dependently selected from hexyl
and ethylhexyl within the wt % ratio of hexyl:ethylhexyl being
about 40:about 60, or represented by formula (II) 5or represented
by formula (III) 6
13. Photovoltaic device according to any of the preceding claims
wherein the semiconductor oxide layer is porous.
14. Photovoltaic device according to claim 13, wherein the
semiconductor oxide layer comprises nanoparticles, preferably
nanoparticles of TiO.sub.2.
15. Method for preparing of a photovoltaic device having a solid
conjugated semiconductor, preferably a device according to any of
the claims 1 to 14, comprising evaporating at least one layer
containing fluoride and/or acetate on at least one layer of the
device.
16. Method according to claim 15, additionally comprising the steps
of: (i) mixing a hole transport material with dopant; and (ii)
applying the mixture to a semiconductor oxide layer.
17. Method according to claim 15 or 16, wherein the at least one
layer is evaporated on top of a dye sensitized semiconductor oxide
layer and/or on top of a layer of the hole transport material
and/or on top of a transparent conductive electrode.
18. Method according to any of claim 15 to 17, wherein the method
further comprises at least one of the following steps: providing a
semiconductor oxide layer, applying said mixture to said
semiconductor oxide layer; and connecting electrodes to said
semiconductor oxide layer and to said mixture.
19. Photovoltaic device according to any of the claims 1 to 14,
wherein said photovoltaic device is a solar cell.
20. Photovoltaic device cell according to claim 19, wherein the
solar cell is a solid state hybrid solar cell.
Description
[0001] The present invention relates to a photovoltaic device,
especially a solar cell, and methods for the preparation of such
devices.
[0002] Since the demonstration of crystalline silicon p/n junction
solar cell in 1954 by Chapin et al with a reported efficiency of
6%, there was a dramatic increase in the efficiencies of such cells
as a result of improvements in current, significant increase in
voltage and splitting the sunlight among solar cells of different
bandgaps. The higher voltages resulted directly from increasing the
densities of minority carriers generated by absorbed sunlight. By
reducing the minority carrier recombination rate, trapping light in
active layers and by increasing the intensity of light with
concentration optics, efficiencies as high as 25-30% have been
reported for two band-gap single crystal laboratory cells like
AlGaAs/GaAs. Thin film multijunction, multi-band-gap cells using
hydrogenated amorphous silicon or polycrystalline alloys exhibit up
to 15% laboratory efficiency. The efficiencies of commercial power
systems in the field remain in the range of 3 to 12%.
[0003] As an alternative a dye sensitized semiconductor-electrolyte
solar cell was developed by Grtzel et al consisting of titanium
dioxide nanoparticles with a ruthenium complex adsorbed on the
surface of an iodine-iodide electrolyte as disclosed in WO91/16719.
The ruthenium complex acts as a sensitizer, which absorbs light and
injects an electron into titanium dioxide; the dye is then
regenerated by electron transfer from the iodine-iodide redox
couple. The advantage of such a solar cell results from the fact
that no crystalline semiconductors have to be used anymore while
already providing conversion efficiencies of light into electrical
energy of up to 12% (O'Reagan, B. et al; Nature (1991), 353, page
737).
[0004] However, replacement of the liquid electrolyte with solid
charge transport material has been found important due to practical
applications. Solid-state dye sensitized solar cells on nanoporous
film of TiO.sub.2 are a significant area of research for chemists,
physicists and material scientists. These researches on solar cells
became very important due to its low costs and the easiness of
fabrication.
[0005] In the field of dye sensitized solid state solar cells,
Hagen et al, Synethic Metals 89, 1997, 215, reports for the first
time the concept of a new type of solid-state dye sensitized solar
cell using organic hole transport material (HTM), which was further
improved by Bach et al, Nature 398, 1998, 583, to obtain an overall
conversion efficiency of 0.74%. The basic structure of the cell
consists of a nanoporous TiO.sub.2 layer coated on a conducting
glass substrate, covered with a compact TiO.sub.2 layer. Dye was
absorbed by the nanoporous layer and the HTM along with dopant and
salt was coated over the dye. The additives, salt and dopant (tris
(4-bromophenyl)ammoniumyl hexachloroantimonate
(N(PhBr).sub.3SbCl.sub.6) increased the efficiency.
[0006] A problem with organic devices having a solid conjugated
semiconductor is that all interfaces are sources for energy
potential losses, for example by introducing serial resistances.
Lots of efforts are done to modify the interfaces, for example in
solar cells. Desired effects of such modifications are to avoid
diffusion of atoms from the back-electrode material into the layer
system, to enhance charge carrier transfer or to block it, to fill
pinholes to avoid undesired recombination, and to influence the
workfunction of electrodes in the desired direction, etc.
[0007] A major factor limiting the energy conversion efficiencies
in such devices is the low photo-voltage, wherein charge
recombination at the TiO.sub.2-electrolyte interface plays a
significant role.
[0008] Interface modifications that were done, e.g. in hybrid solar
cells, are for example: Nb.sub.2O.sub.5 coating of TiO.sub.2 porous
layer to match the energy levels, as disclosed in Guo, P. et al.,
Thin Solid Films 351, 1999, 290; introducing the adsorption of
benzoic acid derivatives to improve the charge injection in
heterojunctions for dye sensitized solid state solar cells, as
disclosed in Kruger et al., Advanced Materials 12, 2000, 447.
Further, in Kelly et al., Langmuir 1999, 15, 7047 dye sensitized
liquid solar cells with a cation-controlled interfacial charge
injection are disclosed, proposing a model in which Li.sup.+
adsorption stabilizes TiO.sub.2 acceptor states resulting in
energetically more favorable interfacial electron transfer. Huang
et al., J. Phys. Chem. B 1997, 101, 2576 discloses dye sensitized
liquid solar cells where the dye coated TiO.sub.2 was treated with
pyridine derivatives improving the efficiency remarkably, since
recombination was blocked.
[0009] Small molecules like derivatives of benzoic acid or pyridine
adsorb to TiO.sub.2 and block the free interface, which results in
a reduced recombination, as described above. However, these
adsorption processes are wet-chemical processes that are not so
easy to control, since physisorption might take place in addition
to the desired chemisorption, which will give a thicker layer at
the interface which might block the electron-transfer completely. A
respective intermediate layer is most often introduced by
spin-coating, trop-casting, self-assembly or electro
deposition.
[0010] It is therefore an object of the present invention to
overcome the drawbacks of the prior art, especially to provide a
photovoltaic device having an increased stability compared to the
respective devices known in the prior art and decreasing energy
potential losses on interfaces between layers of such devices in a
controllable manner.
[0011] A further object of the present invention is to provide a
method for preparation of a device showing photovoltaic
characteristic, more particularly of a device exhibiting the
favorable characteristics as defined above.
[0012] The first object of the invention is solved by a
photovoltaic device comprising at least one layer containing
evaporated fluoride and/or acetate.
[0013] In a preferred embodiment the evaporated fluoride is an
alkali or alkaline earth metal fluoride.
[0014] In another preferred embodiment the evaporated acetate is an
alkali metal acetate.
[0015] It is also preferred that the device further comprises a
semiconductor oxide layer sensitized with a dye, preferably a
ruthenium complex dye.
[0016] Further it is preferred that the evaporated layer containing
fluoride and/or acetates is evaporated on top of the semiconductor
oxide layer and/or on top of a layer of the hole transport material
and/or on top of a transparent conductive electrode.
[0017] It is even more preferred that the semiconductor oxide layer
is titanium dioxide.
[0018] In a further embodiment of the invention the evaporated
layer has a thickness of about 0.5 to about 30 nm, preferably about
0.5 to about 15 nm.
[0019] Moreover, it is possible that fluorides and/or acetates
evaporated on different layers of the device have different counter
cations.
[0020] It is still preferred that the evaporated layer which is
evaporated on the semiconductor oxide layer comprises lithium
fluoride having a thickness of about 5 nm, and the evaporated layer
which is evaporated on the hole transport material comprises cesium
fluoride having a thickness of about 15 nm.
[0021] In another embodiment it is preferred that the hole
transport material is represented by formula (I) 1
[0022] wherein R in each occurrence is dependently selected from
hexyl and ethylhexyl within the wt % ratio of hexyl:ethylhexyl
being about 40:about 60, or represented by formula (II) 2
[0023] or represented by formula (III) 3
[0024] Further it is preferred that the semiconductor oxide layer
is porous.
[0025] In another embodiment the semiconductor oxide layer
comprises nanoparticles, preferably nanoparticles of TiO.sub.2.
[0026] The second object of the invention is solved by a method for
preparing of a photovoltaic device, preferably a device according
to the invention, comprising the step of evaporating at least one
layer containing fluoride and/or acetate on at least one layer of
the photovoltaic device.
[0027] The method preferably comprises the additional steps of:
[0028] (i) mixing the hole transport material with dopant;
[0029] (ii) applying the mixture to a semiconductor oxide
layer;
[0030] whereas these steps are preferably conducted before
conducting the above mentioned step of
[0031] (iii) evaporating at least one layer containing fluoride
and/or acetate on at least one layer of the photovoltaic
device.
[0032] In a preferred embodiment at least one layer comprising
fluoride and/or acetate is evaporated on top of a dye sensitized
semiconductor oxide layer and/or on top of a layer of the hole
transport material and/or on top of a transparent conductive
electrode.
[0033] Also preferred is an embodiment, wherein the method further
comprises at least one of the following steps:
[0034] providing a semiconductor oxide layer,
[0035] applying said mixture to said semiconductor oxide layer,
and
[0036] connecting electrodes to said semiconductor oxide layer and
to said mixture.
[0037] The object of the invention is also solved by a solar cell
according to claim 18 and especially to a solar cell comprising an
organic and/or polymer blend, and/or organic and/or polymeric
semiconductor bilayer structure containing solar cells.
[0038] With respect to the organic and/or polymer blend, and/or
organic and/or polymeric semiconductor bilayer structure containing
solar cells it is especially referred to the patent application EP
1 028 475 (application number 99102473.8-2214) of the same
applicant and especially the structures described therein, as well
as to Shaheen et al., Applied Physics Letters 78 (2001), 841,
Brabec et al., Advanced Functional Materials, 11 (2001), 15 and
Schmidt-Mende et al., Science 294 (2001), 5532.
[0039] It is preferred that the solar cell is an organic or
polymeric solid state hybrid solar cell.
[0040] It was surprisingly found that devices according to the
present invention show higher energy conversion efficiencies than
the ones without fluoride and/or acetate layer. In particular, open
circuit voltage V.sub.OC and fill factor FF were increased, which
yield to the higher efficiency. Further, evaporation of an
additional layer is a simpler technique than a wet-chemical
process.
[0041] The use of efficient light emitting diodes with alkaline and
alkaline earth metal fluoride Al cathode has been already
disclosed, e.g. the first time by Hung et al., Applied Physical
Letters, 70 (1997), 152, and by Yang et al., Applied Physical
Letters 79, 2001, 563. Further disclosures may be found in, for
example, Ganzorig et al., Materials Science and Engineering B85
(2000) 140, and Brown et al., Applied Physics Letters, 77 (2000)
3096.
[0042] The origin of the effect that devices having fluoride and/or
acetate layers show increased efficiency might be explained,
without being bonded to theory, by a better shielding of the
TiO.sub.2 from the hole transport material layer which reduces the
recombination on one side. Applying the aligned dipole mechanism
theory, the effect may be explained by an increased charge carrier
density close to the interface but also inside the hole transport
material due to dissociation and diffusion of the cations.
[0043] As was found by the inventors, various alkali or alkaline
earth metal fluorides and alkali metal acetates, respectively, may
be chosen in the device according to the present invention. It was
found that lithium fluoride works better at the TiO.sub.2/HTM
interface and cesium fluoride at the HTM/Au interface, possibly
because of the higher tendency for dissociation of cesium fluoride
than of lithium fluoride.
[0044] Besides the hole transport materials already disclosed in
the application, other compounds are as well suitable and may
comprise linear as well as branched or starburst structures and
polymers carrying long alkoxy groups as sidechains or in the
backbone. Such hole transport materials are in principle disclosed
in EP 0 901 175 A2, the disclosure of which is incorporated herein
by reference.
[0045] Other possible hole transport materials are, e.g. described
in the WO 98/48433, DE 19704031.4 and DE 19735270.7. The latter two
references disclose TDAB for application in organic LEDs. It is to
be noted that any of the known TDAB may be--further--derivatized
such as by using substitutions such as alkoxy, alkyl, silyl at the
end-standing phenyl rings which could be in p-, m- and opposition
mono-, bi-, or tri-substituted. As indicated already above the
guidelines disclosed herein apply not only to single organic hole
transport materials but also to mixtures thereof.
[0046] As dopant all agents may be used which are suitable to be
used in organic devices and which are known to a person skilled in
the art. A preferable dopant is, for example, oxidized hole
transport material and doping agents disclosed in EP 111 493.3, the
disclosure of which is incorporated herein by reference.
[0047] Dyes which can be used for sensitizing a semiconductor oxide
layer are known in the art such as EP 0 887 817 A2 the disclosure
of which is incorporated herein by reference. Among the dyes to be
used are also Ru(II) dyes.
[0048] The dyes used to sensitize the semiconductor oxide layer may
be attached thereto by chemisorption, adsorption or by any other
suitable ways.
[0049] The semiconductor oxide layer used in the inventive device
is preferably a nanoparticulate one. The material can be a metal
oxide and more preferably an oxide of the transition metals or of
the elements of the third main group, the fourth, fifth and sixth
subgroup of the periodic system. These and any other suitable
materials are known to those skilled in the art and are, e.g.
disclosed in EP 0 333 641 A1, the disclosure of which is
incorporated herein by reference.
[0050] The semiconductor oxide layer material may exhibit a porous
structure. Due to this porosity the surface area is increased which
allows for a bigger amount of sensitizing dye to be immobilized on
the semiconductor oxide layer and thus for an increased performance
of the device. Additionally, the rough surface allows the trapping
of light which is reflected from the surface and directed to
neighbouring surface which in turn increases the yield of the
light.
[0051] The method for the manufacture of a photoelectric conversion
device according to the present invention can exemplary be
summarized as follows.
[0052] I. Structuring of TCO (Transparent Conductive Oxide Layer)
Substrates
[0053] II. Cleaning of TCO Substrates
[0054] a. Ultrasonic cleaning 15 minutes in an aqueous surfactant
at ca. 70.degree. C.
[0055] b. Rinse thoroughly with ultrapure water and dry in air
[0056] c. Ultrasonic rinsing with ultrapure water 15 min at ca.
70.degree. C.
[0057] d. Ultrasonic cleaning 15 minutes in pure isopropanol at ca.
70.degree. C.
[0058] e. Blow dry with nitrogen
[0059] III. Preparation of Blocking Layer
[0060] a. Making polycrystalline TiO.sub.2 by spray pyrolysis of
titanium acetylacetonate solution;
[0061] b. Temper film at 500.degree. C.
[0062] IV. Preparation of Nanoporous TiO.sub.2 Semiconductor Oxide
Layer
[0063] a. Screen printing: use a TiO.sub.2 paste with a screen
structured with the desired geometry (thickness depends on screen
mesh); resulting standard thickness is about 3 .mu.m; doctor
blading is an alternative technique to make porous TiO.sub.2
layer
[0064] b. Sintering of film
[0065] 1. Heat the substrates up to 85.degree. C. for 30 minutes to
dry the film
[0066] 2. Sinter at 450.degree. C. for 1/2 hour, ideally under
oxygen flow, otherwise in air
[0067] 3. Let sample cool down slowly to avoid cracking
[0068] V. Dyeing of Nanocrystalline TiO.sub.2 Film
[0069] a. Prepare a solution of dye in ethanol, concentration ca.
5.times.10.sup.-4 M
[0070] b. Put the ca. 80.degree. C. warm substrates into the dye
solution.
[0071] c. Let them sit in the dye-solution at room temperature in
the dark for about 8 hours or overnight.
[0072] d. Remove from dye solution, rinse with ethanol and let dry
several hours or overnight in the dark.
[0073] VI. Evaporating LiF (ca. 5 nm).
[0074] VII. Deposition of Hole Transport Material (HTM)
[0075] a. Prepare a solution of HTM. Current "standard conditions"
are:
[0076] Solvent: chlorobenzene (plus ca. 10% acetonitrile from
dopant solution)
[0077] HTM: concentration (5-60 mg/substrate)
[0078] Dopant: oxidized HTM (ca. 0.2 mol % of hole conductor
concentration, to be added from a solution in acetonitrile)
[0079] Salt: Li((CF.sub.3SO.sub.2).sub.2N), (ca. 9 mol %)
[0080] b. Spin-coat the solution onto the film using the following
parameters
[0081] c. Let the samples dry at least several hours in air or
preferably overnight
[0082] VIII. Evaporating CsF (ca. 15 nm).
[0083] IX. Deposition of Counterelectrode
[0084] a. Evaporate the counterelectrode on top (currently Au)
[0085] As is understood changes may be done in that method without
departing from the scope of protection.
[0086] The invention is now further illustrated by the accompanying
figures from which further embodiments, features and advantages may
be taken and where
[0087] FIG. 1 shows an embodiment of a basic design of an inventive
photovoltaic device, namely the hybrid solar cell, described
above;
[0088] FIG. 2 shows the I/V curve of the first type of solar cell
according to the present invention and having 5 nm LiF at
TiO.sub.2/dye interface and 15 nm CsF at HTM/back electrode
interface evaporated.
[0089] FIG. 3 shows an embodiment of a basic design of an inventive
photovoltaic device, comprising an organic and/or polymer blend,
and/or organic and/or polymeric semiconductor bilayer
structure.;
[0090] As shown in FIG. 1 a solar cell according to the present
invention is built of a substrate, followed by a FTO layer, a
blocking TiO.sub.2 layer, dye-sensitized TiO.sub.2 with a fluoride
layer, hole transport material (HTM), a second fluoride layer, and
a gold (Au) layer.
[0091] To demonstrate the improved efficiency of an inventive
device having the features of claim 1 of the present invention,
particularly an evaporated layer of 5 nm LiF at TiO.sub.2/dye
interface and an evaporated layer of 15 nm CsF at
HTM/back-electrode interface, respectively, the I/V curve for that
device is shown in FIG. 2. The parameters according to this curve
are listed in table 1.
1 TABLE 1 J.sub.sc[mA/cm.sup.2] V.sub.oc[mV] FF [%] .eta. [%]
CsF(15 nm)/Au 1.25 535 65 0.7 TiO.sub.2/LiF (5 nm) 1.52 562 43 0.6
TiO.sub.2/LiF (5 nm)//CsF 1.99 476 64 1.0 CsF (15 nm)/Au
[0092] As a result, the combination of LiF and CsF yields to an
efficiency of 1% at 100 mW/cm.sup.2.
[0093] FIG. 3 shows an embodiment of a basic design of an inventive
photovoltaic device, comprising a substrate, a TCO-layer, a
counter-electrode, especially an Al electrode as well as two
fluoride layers, enclosing a blend or bilayer of p- or n-type
organic/or polymeric semiconductors.
[0094] The features of the present invention disclosed in the
description, the claims and/or the drawings may both separately and
in any combination thereof be material for realizing the invention
in various forms thereof.
* * * * *