U.S. patent application number 14/563387 was filed with the patent office on 2016-01-14 for scalable, highly transparent paper with microsized fiber.
This patent application is currently assigned to UNIVERSITY OF MARYLAND AT COLLEGE PARK. The applicant listed for this patent is UNIVERSITY OF MARYLAND AT COLLEGE PARK. Invention is credited to Zhiqiang Fang, Liangbing HU, Hongli Zhu.
Application Number | 20160010279 14/563387 |
Document ID | / |
Family ID | 55067156 |
Filed Date | 2016-01-14 |
United States Patent
Application |
20160010279 |
Kind Code |
A1 |
HU; Liangbing ; et
al. |
January 14, 2016 |
SCALABLE, HIGHLY TRANSPARENT PAPER WITH MICROSIZED FIBER
Abstract
Solar cell substrates require high optical transparency, but
also prefer high optical haze to increase the light scattering and
consequently the absorption in the active materials. Unfortunately
there is a tradeoff between these optical properties, which is
exemplified by common transparent paper substrates exhibiting a
transparency of about 90% yet a low optical haze (<20%). In this
work we introduce a novel transparent paper made of wood fibers
that display both ultra-high optical transparency (.about.96%) and
ultra-high haze (.about.60%), thus delivering an optimal substrate
design for solar cell devices. Compared to previously demonstrated
nanopaper composed of wood-based cellulose nanofibers, our novel
transparent paper has better dual performance in transmittance and
haze, but also is fabricated at a much lower cost. This
high-performance, low-cost transparent paper is a potentially
revolutionary material that may influence a new generation of
environmentally friendly printed electronics.
Inventors: |
HU; Liangbing; (Hyattsville,
MD) ; Fang; Zhiqiang; (Hyattsville, MD) ; Zhu;
Hongli; (College Park, MD) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
UNIVERSITY OF MARYLAND AT COLLEGE PARK |
College Park |
MD |
US |
|
|
Assignee: |
UNIVERSITY OF MARYLAND AT COLLEGE
PARK
College Park
MD
|
Family ID: |
55067156 |
Appl. No.: |
14/563387 |
Filed: |
December 8, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61912923 |
Dec 6, 2013 |
|
|
|
Current U.S.
Class: |
136/252 ;
162/181.2 |
Current CPC
Class: |
H01L 31/048 20130101;
D21H 17/07 20130101; D21H 17/66 20130101; H01L 31/04 20130101; H01L
31/02168 20130101; Y02E 10/549 20130101; Y02B 10/10 20130101; H01L
51/0097 20130101; H01L 51/44 20130101 |
International
Class: |
D21H 17/07 20060101
D21H017/07; H01L 31/04 20060101 H01L031/04; D21H 17/66 20060101
D21H017/66 |
Goverment Interests
GRANT INFORMATION
[0002] This invention was made with government support under Grant
No. FA95501310143 awarded by AFOSR. The United States government
has certain rights in the invention.
Claims
1. A transparent paper comprising: an optical transparency of at
least 96%; and an optical haze of at least 60%.
2. The transparent paper of claim 1 further comprising; a cell
wall; a plurality of wood fibers in the cell wall, wherein the
plurality of wood fibers comprise a primary layer and a secondary
layer, the primary layer having a thickness of 0.1-0.2 .mu.m and
the secondary layer having a thickness of about 1.0-5.5 .mu.m.
3. The transparent paper of claim 2 wherein the wood fibers
comprise a plurality of microfibrils, the plurality of microfibrils
being randomly oriented in the primary layer and helically wound in
a fiber axis of the secondary layer.
4. The transparent paper of claim 2 wherein the wood fibers have a
diameter of 5 nm to 20 nm, a tensile strength of about 105 MPa, and
a toughness of about 1.88 J/M.sup.3.
5. A method of making transparent paper comprising the steps of:
(a) treating bleached sulfate softwood pulp with a TEMPO-Oxidized
system comprising; (i) dispersing 5 g of wood fibers into 1% pulp
with deionized water to make wood pulp; (ii) adding TEMPO and
sodium bromide (NaBr) separately into the wood pulp; (iii) stifling
the mixture continuously to form a uniform suspension; (iv)
titrating the uniform suspension with sodium hypochlorite (NaClo);
(b) filtrating the uniform suspension; and (c) fabricating a
transparent paper from the filtrated uniform suspension.
6. The method of making transparent paper of claim 1 wherein the
bleached sulfate softwood pulp is extracted from the southern
yellow pine without beating or refining.
7. A solar cell application comprising: a laminated transparent
paper, the laminated transparent paper comprising an optical
transparency of at least 96% and an optical haze of at least
60%.
8. The solar cell application of claim 1, wherein the laminated
transparent paper further comprises: a cell wall; a plurality of
wood fibers in the cell wall, wherein the plurality of wood fibers
comprise a primary layer and a secondary layer, the primary layer
having a thickness of 0.1-0.2 .mu.m and the secondary layer having
a thickness of about 1.0-5.5 .mu.m.
9. The solar cell application of claim 8, wherein the wood fibers
comprise a plurality of microfibrils, the plurality of microfibrils
being randomly oriented in the primary layer and helically wound in
a fiber axis of the secondary layer.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of co-pending U.S.
Provisional Application Ser. No. 61/912,923 filed Dec. 6, 2013, the
entire contents of which are incorporated herein by reference.
BACKGROUND
[0003] Traditional ways to produce transparent paper involve
fiber-based and sheet processing techniques. Fiber-based methods
use overbeaten wood pulp, while sheet processing requires coating,
impregnating, supercalendering, or chemical immersion to produce
transparent paper. These methods consume large amounts of energy or
rely on petroleum-based materials to produce paper with no more
than 80% transmittance. Since Herrick and Turbak successfully
separated nanofibers from wood pulp using a mechanical process in a
high pressure homogenizer in 1983, cellulose nanofibers have
attracted great attention because they can be used to manufacture
transparent paper for printed electronics, optoelectronic devices,
and also for packaging. Related art transparent paper is made of
NFCs (nanofibrillated cellulose) which involves a fabrication
process that is too time and energy consuming to be practical for
commercial applications.
[0004] Some related art techniques are used to liberate nanofibers.
These techniques include mechanical treatments and acid hydrolysis.
Mechanical treatment techniques are currently considered efficient
ways to isolate nanofibers from the cell wall of a wood fiber.
However, solely mechanical processes consume large amounts of
energy and insufficiently liberate the nanofibers while damaging
the microfibril structures in the process. Pretreatments,
therefore, are conducted before conducting mechanical
disintegration in order to effectively separate the fibers and
minimize the damage to the nanofiber structures.
[0005] TEMPO-mediated oxidation is proven to be an efficient way to
weaken the interfibrillar hydrogen bonds that facilitate the
disintegration of wood fibers into individualized nanofibers yet
maintain a high yield of solid material. Nanopaper made of
nanofibers can attain a transmittance of over 80%, yet this type of
transparent paper takes a longer time to fabricate and has a very
low haze.
[0006] Solar cell substrates require high optical transparency, but
also prefer high optical haze to increase the light scattering and
consequently the absorption in the active materials. Common
transparent paper substrates generally possess only one of these
optical properties, which is exemplified by common transparent
paper substrates exhibiting a transparency of about 90% yet a low
optical haze of <20%.
[0007] Substrates play a key role as to the foundation for
optoelectronic devices. Mechanical strength, optical transparency,
and maximum processing temperature, are among the critical
properties of these substrates that determine its eligibility for
various applications. The optoelectronic device industry
predominantly utilizes glass substrates and plastic substrates for
flexible electronics; however, recent reports demonstrate
transparent nanopaper based on renewable cellulose nanofibers that
may replace plastic substrates in many electronic and
optoelectronic devices. Nanopaper is entirely more environmentally
friendly than plastic substrates due to its composition of natural
materials; meanwhile it introduces new functionalities due to NFCs'
fibrous structure.
[0008] The maximum transparency among all current reports on glass,
plastic, and nanopaper substrates is about 90%, but with a very low
optical haze (<20%). Optical haze quantifies the percent of the
transmitted light that diffusely scatters, which is preferable in
solar cell applications. Optical transparency and haze are
inversely proportional values in various papers. Trace paper has a
high optical haze of over 50%, but a transparency of less than 80%;
whereas plastic has a transparency of about 90%, but with an
optical haze of less than 1%. Related art Nanopaper based on NFCs
has the highest reported optical haze among transparent substrates
due to its nanoporous structure, yet it is still a relatively low
value.
[0009] Although optical haze is a property preferably maximized in
transparent substrates integrated into solar devices, other
optoelectronic devices require different levels of light
scattering; for example, displays and touch screens need high
clarity and low optical haze. Current commercial substrates are
best suited for displays, but are not optimized for solar cell
devices because of the low optical haze. Various materials such as
SiO.sub.2 nanoparticles or silver nanowires are reported to
effectively increase light absorption and consequentially the
short-circuit current by enhancing the path of light through the
active solar layer with increased diffuse light scattering. The
light scattering induced by these nanostructures is limited,
however, and incorporating these materials requires additional
steps that add cost to the solar cells devices.
[0010] There is a need in the market for a paper with high
transparency and high optical haze.
SUMMARY OF THE INVENTION
[0011] The inventors of the present application have developed a
method of making a transparent paper based on wood fibers, which
has an ultra-high optical transparency (.about.96%) and
simultaneously an ultra-high optical haze (.about.60%). The primary
wood fibers are processed by using a TEMPO/NaBr/NaClO oxidization
system to introduce carboxyl groups into the cellulose. This
process weakens the hydrogen bonds between the cellulose fibrils,
and causes the wood fibers to swell up and collapse resulting in a
high packing density and excellent optical properties. The
advantages of this invention is that it exhibits a dramatic dual
improvement on the optical transmittance and optical haze; and it
is formed from much less energy intensive processes that enable low
cost paper devices. The optical properties allow a simple
light-management strategy for improving solar cell performances.
This is demonstrated with an organic solar cell by simply
laminating a piece of such transparent paper, and observed its
power conversion efficiency (PCE) increased from 5.34% to 5.88%.
Transparent paper with an optical transmittance of .about.96% and
transmittance haze of .about.60% is most suitable for solar cell
applications.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] The above and/or other aspects of the present invention will
be more apparent by describing in detail exemplary embodiments
thereof, with reference to the accompanying drawings in which:
[0013] FIG. 1 (a) shows a Hierarchical structure of a tree and the
conversion to elementary fibrils.
[0014] FIG. 1 (b) shows a related art regular paper.
[0015] FIG. 1 (c) shows transparent paper made of TEMPO-oxidized
wood fibers.
[0016] FIG. 1(d) shows molecular structure of cellulose.
[0017] FIG. 1 (e) shows TEMPO-oxidized cellulose with carboxyl
groups in the C6 position.
[0018] FIG. 2 (a) shows the morphology of original bleached sulfate
wood fibers under an optical microscope. The inset is a 0.25 wt %
original bleached sulfate wood pulp suspension.
[0019] FIG. 2(b) shows the morphology of TEMPO-oxidized wood fibers
under an optical microscope. Inset is a 0.25 wt % TEMPO-oxidized
wood fiber suspension.
[0020] FIG. 2(c) shows SEM images of unzipped TEMPO-oxidized wood
fibers.
[0021] FIG. 2(d) shows nanofibers on the cell wall of
TEMPO-oxidized wood fiber.
[0022] FIG. 2 (e) shows a digital image of transparent paper
produced from TEMPO-oxidized wood fibers with a diameter of 20
cm.
[0023] FIG. 3 (a) shows a graph of the total optical transmittance
vs. wavelength measured with an integrating sphere setup.
[0024] FIG. 3 (b) shows a graph of the transmission haze vs.
wavelength.
[0025] FIG. 3 (c) shows the transmission haze of transparent paper
with varying thicknesses at 550 nm.
[0026] FIG. 3 (d) shows optical transmission haze vs. transmittance
for different substrates at 550 nm. Glass and PET plastics are in
the green area, in which are suitable for displays due to their low
haze and high transparency; transparent paper developed embodied by
this invention are located in the cyan area is the most suitable
for solar cells.
[0027] FIG. 4 (a) shows an SEM image of a regular paper.
[0028] FIG. 4(b) shows an SEM image of a transparent paper.
[0029] FIG. 4(c) Shows a graph of the tensile strength of
transparent paper according an embodiment of the invention and
regular paper.
[0030] FIG. 4(d) shows MD simulation model of inter-sliding of two
related art wood fibers.
[0031] FIG. 4 (e) shows MD simulation of two TEMPO-oxidized wood
fibers.
[0032] FIG. 4 (f) shows a graph of variation of potential energy of
the model systems as a function of relative sliding displacement
for FIGS. 4(d) and (e).
[0033] FIG. 5 (a) shows schematics and images of
cellulose-deposited silicon slab. Top left: a schematic structure
of wood fibers deposited on a silicon slab by Meyer rod coating;
top right: a schematic of a transparent paper attached silicon slab
by lamination; bottom left: TEMPO-oxidized wood fibers deposited on
a silicon slab; bottom right: transparent paper with a thickness of
33 .mu.m attached on a silicon slab.
[0034] FIG. 5(b) shows the effective refractive index profiles of
the interfaces between air and silicon slab.
[0035] FIG. 5(c) shows the effective refractive index profiles of
33 .mu.m cellulose-deposited on a silicon slab.
[0036] FIG. 5(d) shows a schematic diagram of transparent paper and
its light scattering behavior.
[0037] FIG. 5(e) shows a scattering angular distribution with an
arbitrary y-axis unit for transparent paper, the maximum scattering
angle is 34.degree..
[0038] FIG. 5(f) shows a photo to show the light scattering effect
of transparent paper when a laser with a diameter of 0.4 cm passes
though transparent paper.
[0039] FIG. 5(g) shows the light absorption of transparent paper
laminated on a silicon slab.
[0040] FIG. 6 (a) shows the structure of the OPV device with
transparent paper attached on the opposite glass side.
[0041] FIG. 6(b) shows the dependence of the photocurrent of the
OPV device with or without transparent paper on the light incident
angle (defined as the angle between the incident light and the
normal direction of the substrate), W and W/O represent OPV device
with and without transparent paper, respectively
[0042] FIG. 6(c) Angular distribution of the light caused by the
haze effect of the transparent paper, where the light was incident
at different angle; (d) Comparison of the I-V curves of the OPV
device illuminated by diffused light (13 mW/cm.sup.2).
[0043] FIG. S1 (a) shows the length distribution of original wood
fibers
[0044] FIG. S1 (b) shows the length distribution of TEMPO-oxidized
wood fibers.
[0045] FIG. S1 (c) shows the width distribution of original wood
fibers.
[0046] FIG. S1 (d) shows the width distribution of TEMPO-oxidized
wood fibers. The y-axis unit for (c) and (d) is .mu.m.
[0047] FIG. S2 (a) shows optical microscopic picture of
TEMPO-oxidized wood fiber. SEM images of crushed wood fiber
[0048] FIG. S2 (b) shows SEM images of crushed wood fiber.
[0049] FIG. S2 (c) shows SEM images of unzipped wood fiber
[0050] FIG. S2 (d) shows nanofibers on the primary layer of cell
wall after TEMPO treatment.
[0051] FIG. S3 shows the total optical transmittance of transparent
paper with varying thicknesses.
[0052] FIG. S4 (a) shows the side view MD simulation model of
inter-sliding of two original cellulose fibers.
[0053] FIG. S4 (b) shows the side view MD simulation model of two
TEMPO-oxidized cellulose flakes.
[0054] FIG. S4 (c) pulling force needed to drive the sliding as a
function of relative sliding displacement for both cases.
[0055] FIG. S5 shows scattering angular distribution with an
arbitrary y-axis unit for transparent paper and nanopaper
[0056] FIG. S6 (a) shows the images to show the light scattering
effect of PET when a green laser with a diameter of 0.4 cm strikes
on them.
[0057] FIG. S6 (b) shows the images to show the light scattering
effect of glass when a green laser with a diameter of 0.4 cm
strikes on them.
[0058] FIG. S6 (c) shows the images to show the light scattering
effect of the disclosed transparent paper when a green laser with a
diameter of 0.4 cm strikes on them. The disclosed transparent paper
obviously scatters light much more than PET and glass
substrates.
[0059] FIG. S7 shows the light absorption of TEMPO-oxidized wood
fibers deposited on a silicon slab.
[0060] FIG. S8 shows the molecular structure of PCDTBT (a) and
PC70BM (b) used in OPV devices.
DETAILED DESCRIPTION OF THE INVENTION
[0061] The invention relates to a method of making a transparent
paper substrate made of earth-abundant wood fibers that
simultaneously achieves an ultra-high transmittance (.about.96%)
and ultra-high optical haze (.about.60%), and its optimal
application on the solar cells with a PCE enhancement of 10% by a
simple lamination process. The modified wood pulp with high
fragment content and fewer hollow structures lead to a higher
packing density, which dramatically increases both the optical
transmittance and mechanical strength of our transparent paper
compared to regular paper. The transparent paper demonstrates a
much higher optical transmittance than nanopaper made of nanoscale
fibers while using much less energy and time to process paper with
a similar thickness. Such low-cost, highly transparent, and high
haze paper can be utilized as an excellent film to enhance
light-trapping properties for photovoltaic applications such as
solar panel, solar roof, or solar windows. Transparent paper is
made of mesoscale fibers. The primary fibers have an average
diameter of .about.26 .mu.m.
[0062] The highly transparent paper has a high haze based on
TEMPO-oxidized micro-sized wood fibers, plus an efficient and
economic approach to improve the light absorption of a silicon slab
is presented by applying a layer of TEMPO-treated wood fibers or by
laminating a piece of highly transparent paper onto the surface.
This approach to produce highly transparent paper with high haze
using micro-sized wood fibers has the potential to be scaled up to
industrial manufacturing levels, which is crucial for commercial
applications. The wood fibers are processed by using a
TEMPO/NaBr/NaClO oxidization system to introduce carboxyl groups
into the C6 positions of the cellulose. This process weakens the
bonds between the cellulose fibrils and causes the wood fibers to
swell up. The oxidized wood fibers are then fabricated into highly
transparent paper. The transparent paper requires less time to
fabricate than nanopaper due to the use of micro-sized wood fibers,
and it achieves both higher transmittance and higher haze. The
treated wood fibers and fabricated transparent paper are applied on
the surface of a silicon slab by coating and lamination,
separately. A significant enhancement in the light absorption of a
silicon slab is observed for both methods.
[0063] Wood fibers extracted from trees by chemical processes and
mechanical treatments are the main building blocks of paper and
consist of millions of microfibrils (nanofibers) with a diameter
ranging from 5 nm to 20 nm mainly distributing in the S2 layer of
cell wall. The primary wood fibers are processed by using a
TEMPO/NaBr/NaClO oxidization system to introduce carboxyl groups
into the cellulose.
[0064] Natural biomaterials are renewable and environmentally
friendly materials that encourage the development of a sustainable
human society. Cellulose is the most abundant renewable organic
polymer on the earth that is primarily extracted from plants and
composed of repeating anhydrogluclose links through
.beta.-1,4-glucosidic bonds. The TEMPO/NaBr/NaClO oxidation system
weakens the hydrogen bonds between the cellulose fibrils, and
causes the wood fibers to swell up and collapse resulting in a high
packing density and excellent optical properties.
[0065] About 30-40 individual linear cellulose chains are assembled
together into elementary fibrils 1.5-3.5 nm wide, and these
elementary fibrils are hierarchically structured into a macroscopic
structure, such as microfibrils (10-30 nm) or microfibrillar bands
(.about.100 nm)..sup.[3] Microfibrillar bands are organized into
the cell wall of wood fiber..sup.[4] Wood fibers have a slender,
hollow, and hierarchical structure that is approximately 10-50 lam
wide and several millimeters long. These properties enable the
paper to have its three dimensional structure, tailored optical
properties, and tunable porosity. The structure of cellulose
includes hydroxyl, ether, carbon-carbon, and carbon-hydrogen bonds
that do not absorb light in the visible range;.sup.[2]
consequently, pure cellulose is colorless. Although wood fibers
consist of 85-95% cellulose after digesting and bleaching, the
fibers' hollow structure prevents optical transparency due to light
scattering that occurs in the interfacial area between the dense
cell walls and the air present within the micro-sized cavities.
Paper made of wood fibers also appears opaque due to the light
scattering behavior from the porosity of the wood fiber
network.
[0066] Transmission haze refers to the percentage of light
diffusely scattered through a transparent surface from the total
light transmitted. Higher transmission haze improves the light
absorption efficiency of solar cells from the increased path of
light transmitted into the active layer, resulting in an enhanced
short circuit current density
[0067] Optical haze quantifies the percent of the transmitted light
that diffusely scatters, which is preferable in solar cell
applications.
[0068] FIG. 1a. is a hierarchical structure of a tree. A schematic
of cellulose and paper before and after TEMPO-mediated oxidation is
portrayed. As shown in FIG. 1, the TEMPO/NaBr/NaClO system was used
to modify the surface properties of the pristine wood fibers by
selectively oxidizing the C6 hydroxyl groups of glucose (left
bottom in FIG. 1b) into carboxyl groups under aqueous conditions
(right bottom in FIG. 1c). The repulsive force resulting from
additional higher negative charges at the surface of the nanofibers
loosens the interfibrillar hydrogen bonds between the cellulose
nanofibers resulting in the fiber cell walls are significantly open
and crush. Regular paper with micro-sized wood fibers has limited
optical transparency due to the many micro-cavities existing within
the porous structure that cause light scattering (top left in FIG.
1b). Eliminating these pores is the primary direction to improve
the optical transmittance of paper. Many approaches based on the
above mechanism are used to produce transparent paper involving
fiber-based and sheet processing techniques. Regular paper is a
porous structure composed of untreated wood fibers with an average
width of .about.27 .mu.m (top left in FIG. 1b); however paper made
from TEMPO-oxidized wood fibers with an average width of
approximately 26 .mu.m displays a more densely packed configuration
(top left FIG. 1c). The morphology of wood fibers plays a
significant role in producing highly transparent paper, hence fiber
morphological analysis of TEMPO-treated wood fibers was conducted
for explaining the high packing density of transparent paper made
from TEMPO-oxidized micro-sized wood fibers.
[0069] FIG. 2 portrays the morphology of original bleached sulfate
wood fibers under an optical microscope. FIGS. 2a and 2b portray
the significant morphological changes in the dimensions of the wood
fibers before and after TEMPO treatment was conducted for 10 h at a
stirring speed of 1000 rpm. Compared to the original fibers, the
TEMPO-oxidized fibers swelled such that the width of the fibers
expanded while the length decreased. FIGS. 2c and S2 indicate that
most fibers are cleaved and unzipped in the axial direction, and
the degree of polymerization of the cellulose decreases. FIGS. 2d
and S2d show the configuration of cellulose nanofibers on the cell
wall of wood fibers revealing portion of cellulose nanofiber were
removed from the primary layer of cell wall during the TEMPO
treatment due to weak interfibrillar hydrogen bonds. As seen in
Table 1, the average length of the wood fibers dramatically
decreased from 1.98 mm to 0.71 mm after the TEMPO treatment, and
there was a slight reduction in the average width and an enormous
increase in fines from 5.90% to 18.68%.
TABLE-US-00001 TABLE 1 Dimension of wood fibers before and after
TEMPO oxidization Average length Average width Fine content (mm)
(.mu.m) (%) Pristine fibers 1.98 27.25 5.90 TEMPO-oxidized fibers
0.71 25.79 18.68
[0070] FIGS. 2a and 2b indicate how TEMPO-oxidized wood pulp with a
concentration of 0.25% (by weight) shows a more homogeneous and
transparent appearance than the original wood pulp with the same
consistency. FIG. 2e shows how increased fines, reduced fiber
lengths, and crushed and unzipped TEMPO-oxidized fibers tend to
form denser fiber network during fabrication that perpetuates high
optical transmittance. The "transparent paper" seen in FIG. 2
refers to paper produced from the TEMPO-oxidized wood fibers and it
exhibits an excellent transmittance.
[0071] Table 1 portrays a highly transparent paper with high haze
that was fabricated with obtained TEMPO-oxidized micro-sized wood
fibers by vacuum filtration showing a considerable reduction of
filtration time and energy.
[0072] The filtration time for transparent paper with a thickness
of 50 .mu.m is generally less than 1 hour, however, it will take at
least 8 hours to filter a piece of nanopaper with a similar
thickness using 5.about.30 nm wide TEMPO-oxidized nanofibers under
the same conditions. The total light transmittance of transparent
paper, nanopaper, and PET (polyethylene terephthalate) is compared
in FIG. 3a and the basic information of the two types of paper is
shown in Table S1.
TABLE-US-00002 TABLE S1 Mass and thickness of paper Mass
(g/m.sup.2) Thickness (.mu.m) Nanopaper 33 32 Transparent paper 50
44
[0073] According to this data, transparent paper has the highest
optical transmittance compared to nanopaper and PET.
[0074] FIG. 3 shows optical properties of our transparent paper,
nanopaper, and PET. Highly transparent paper with high haze was
fabricated with obtained TEMPO-oxidized micro-sized wood fibers by
vacuum filtration showing a considerable reduction of filtration
time and energy. The filtration time for transparent paper with a
thickness of 50 .mu.m is generally less than 1 hour, however, it
will take at least 8 hours to filter a piece of nanopaper with a
similar thickness using 5.about.30 nm wide TEMPO-oxidized
nanofibers under the same conditions. The total light transmittance
of transparent paper, nanopaper, and PET (polyethylene
terephthalate) is compared in FIG. 3a and the basic information of
the two types of paper is shown in Table S1.
[0075] According to this data, transparent paper has the highest
optical transmittance compared to nanopaper and PET.
[0076] FIG. 3b. depicts the wavelength vs. transmission haze as
plotted. Transmission haze is an important optical property for
optoelectronic devices, and refers to the percentage of light
diffusely scattered through a transparent surface from the total
light transmitted. For the transparent paper in this work, a
transmission haze over 50% is demonstrated while maintaining a
transmittance of over 90%. Additionally, the transmission haze and
the optical transmittance of transparent paper are also determined
by the paper thickness. Higher transmission haze improves the light
absorption efficiency of solar cells from the increased path of
light transmitted into the active layer, resulting in an enhanced
short circuit current density.
[0077] FIG. 3c shows how the transmission haze tends to increase
with an increase in paper thickness while the optical transmittance
increases slightly with a decrease in paper thickness (FIG. S3 and
Table S2).
TABLE-US-00003 TABLE S2 Mass and thickness of transparent paper
with oxidized wood fibers Mass (g/m.sup.2) Thickness (.mu.m) 1 81
69 2 63 50 3 37 33 4 20 20
[0078] It is critical to combine the optical haze and transmittance
for substrates toward different applications. The performance of
optical transmittance vs. wavelength of substrates has been widely
investigated; but the optical haze is largely ignored as most
substrates have a much lower optical haze (<1%).
[0079] As shown in FIG. 3d, a high clarity for substrates is
crucial for displays. Glass and plastic substrates all meet this
requirement. Recently developed nanopaper has an optical haze of
15-20%, which is too high for display applications, but it is more
suitable for solar cells. Note some outdoor displays also requires
substrates with a high haze to avoid glare effect in sunlight. All
these substrates have an optical transmittance of .about.90%. Our
transparent paper has an optical transmittance of .about.96% and
transmittance haze of .about.60%, which is the most suitable
substrate for solar cell applications.
[0080] FIGS. 4a and 4b show SEM images taken to study the
morphology of regular paper and transparent paper. These images
were taken to further explore why our transparent paper exhibit the
highest transmittance. Although the cylindrical wood fibers within
the regular paper collapse during pressing and drying, there are
plenty of cavities that form throughout the network of micro-sized
fibers causing enhanced light scattering behavior due to the
refractive index mismatch between cellulose (1.5) and air (1.0). In
FIG. 4b, a homogenous and more conformal surface is observed due to
the collapse of the TEMPO-oxidized wood fibers. There is a
significant amount of small fragments in the pulp that fill in the
voids within the paper (see the insert in FIG. 4b). This causes
less light scattering to occur within the TEMPO treated paper
allowing more light to pass through it.
[0081] A possible explanation for the transparent paper
demonstrating a higher optical transmittance than nanopaper could
be that the cell wall of the wood fibers are comprised of a primary
and secondary layer with thicknesses of approximately 0.1-0.2 .mu.m
and 1-5.5 .mu.m, respectively. The microfibrils are randomly
oriented in the primary layer whereas the microfibrils in the
secondary layer are helically wound around the fiber axis (see FIG.
1a). Although oxidization effectively weakens the interfibrillar
hydrogen bonds between the microfibrils and shortens the fiber
length, it only happens within the non-crystalline region and/or on
the crystal surfaces of the microfibrils. As a result, the parallel
arrangement of microfibrils in the secondary layer is preserved
within the cell wall of the wood fibers, giving rise to a higher
stacking density (1.14 g/cm3) compared to nanopaper (1.03 g/cm3)
made of randomly arranged microfibrils. Transparent paper made of
micro-sized fibers, therefore, has better optical transmittance yet
consumes much less energy and time for fabrication.
[0082] FIG. 4c compares the stress-strain curves of regular paper
and transparent paper. The mechanical properties of paper (e.g.,
toughness, strength) are important for various applications. To
test the mechanical properties of the paper, tensile tests of both
the transparent paper TEMPO-oxidized wood fibers and regular paper
were conducted using the Tinius Olsen H25KT universal testing
machine. The comparison shows that the transparent paper is both
much stronger (with a tensile strength of .about.105 MPa) and much
tougher (with a toughness of .about.1.88 J/M.sup.3) than the
regular paper (with a tensile strength of .about.8 MPa and a
toughness of .about.0.15 J/M.sup.3).
[0083] Such substantial improvements of the mechanical properties
of the transparent paper (.about.13-fold stronger and
.about.12-fold tougher) find their origin in the enhanced contact
area in between nanoscale building blocks of the paper due to
TEMPO-treatment, whose effect is twofold: unzipping and cleaving
the originally hollow cellulose fibers not only exposes their inner
surface to neighboring fibers, but also leads to ribbon-like
cellulose flakes and fragments that facilitate higher packing
density and more overlapping between neighboring fibers.
[0084] The rich hydroxyl groups of the cellulose surface allow
facile formation of strong hydrogen bonds. The
inter-cellulose-flake bonding in TEMPO-oxidized transparent paper
is expected to be consequently much stronger than the
inter-cellulose-fiber bonding in regular paper, the physical origin
of the substantial improvements in both strength and toughness.
[0085] FIGS. 4d and 4e and FIG. S4a, S4b portray molecular dynamic
(MD) simulations of scaled-down models for both TEMPO-oxidized
fibers and original wood fibers with roughly comparable size. The
simulation is based on simplified fiber with uniform dimension, but
fiber morphology, fines content, kink index, pigments within paper
are some properties that may impact the mechanical strength of
paper. The inter-flake (and inter-fiber) sliding and the
representative molecular-scale deformation mechanism that leads to
the final mechanical failure of the paper were simulated.
[0086] FIG. 4f compares the variation of potential energy as a
function of sliding displacement for both cases. The zig-zag nature
of the curve denotes the cascade stick-slip events due to breaking
and reforming of hydrogen bonds in between two cellulose flakes (or
fibers) under sliding displacement. The accumulated energy
dissipation, calculated by the sum of all local energy barriers,
represents the energy to fracture the neighboring flakes/fibers
(i.e., toughness). Comparison in FIG. 4f reveals that the energy
needed to separate two flat cellulose flakes is more than 14 times
higher than that in the case of two cellulose fibers (.about.536
kcal/mol vs. .about.38 kcal/mol), which clearly explains the huge
increase in fracture toughness due to TEMPO treatment.
[0087] FIG. S4c portrays the resultant force variation as a
function of sliding displacement for both cases. The average
pulling force necessary to slide the TEMPO-oxidized cellulose
flakes (.about.-284 kcal/mol/.ANG.) is much larger than that in the
original cellulose fiber case (-66 kcal/mole/.ANG.). Considering
the effective reduction of the cross-sectional area from the hollow
cellulose fibers to TEMPO-treated flat flakes, the mechanical
strength of TEMPO-oxidized cellulose paper (largest force that can
sustain divided per unit cross-section area) is expected to be even
higher than that of regular paper, as revealed by the tensile test
results.
[0088] FIG. 5a shows schematics and images of cellulose-deposited
silicon, where the left plots refer to the TEMPO-oxidized wood
fiber deposited silicon, and the right diagrams represent
transparent paper laminated silicon. Paper with ultra-high
transmittance and high transmission haze has potential applications
in optoelectronic devices. The light scattering effect of
transparent paper can improve the path of light travelling through
the active layers of thin film solar cells resulting in an enhanced
light absorption. To verify the assumption, TEMPO-oxidized wood
fibers are directly coated onto the surface of a silicon slab and
transparent paper laminated onto the surface of silicon using NFC
as a binder to analyze any resulting enhancement of light
absorption in the silicon.
[0089] There are three possible mechanisms to achieve increased
light absorption in the active layer: (1) the index of transparent
paper is between the values for the Si substrate and air, which can
effectively decrease the index contrast and lower the reflection
for light entering from air to Si (compare FIGS. 5b and 5c); (2) a
large light forward scattering effect of transparent paper, which
can increase the path length of light in the Si layer (as shown in
FIG. 5d); (3) a ultra-high optical transparency, up to 96%, of our
transparent paper. These effects make transparent paper
fundamentally better than plastic substrates for thin film solar
cells.
[0090] As shown in the schematic FIG. 5d, the direct incident light
is scattered as it propagates through the transparent paper,
generating a high transmission haze. To quantitatively explain the
light scattering effect of transparent paper, an optical setup
consisting of a rotating light detector was applied to measure the
angular distribution of transmitted light. Light passing through
transparent paper exhibits high diffuse scattering with an expected
inverse Gaussian-like pattern (FIG. 5e). The angle is defined
whereas the incident light is perpendicular to the surface of
transparent paper as 90.degree. and the scattering angle range is
defined as the transmitted light at angles with an intensity larger
than 5% of the peak transmission intensity at 90.degree..
[0091] Our transparent paper delivers a maximum scattering angle of
34.degree.. Moreover, the distribution of light transmitted through
the transparent paper demonstrated in this work is quite different
from nanopaper (as shown in FIG. S5), since the transmitted light
has a much narrower angular distribution. The light scattering
effect is also visualized in FIG. 5f (the distance between the
transparent paper and target is about 30 cm). A laser with a
wavelength of 532 nm and a beam diameter of 0.4 cm passes through
transparent paper and forms a larger illuminated circular area on
the surface of the target with a diameter of over 18.5 cm. The same
experiment was also applied to glass and PET to illustrate the
light scattering effect, and the results are presented in FIG. S6.
Since the transmission haze of PET and glass is lower than 1%, the
transmitted light is scattered only slightly as visualized by a
smaller illuminated area on the target behind the transparent
paper.
[0092] FIG. 5g illustrates the light absorption of transparent
paper laminated on a silicon slab. The data on the light absorption
of TEMPO-oxidized wood fibers deposited on silicon is very similar
and is shown in FIG. S7. Compared to a bare silicon slab, there is
enhanced light trapping in all the prepared samples by
approximately 10-18% from 400 to 1000 nm. These results show that:
(1) both TEMPO-oxidized wood fibers and transparent paper can
enhance the broadband absorption efficiency of the silicon slab;
(2) transparent paper or TEMPO-treated wood fibers can be applied
to a silicon slab with a simple coating, dipping, or lamination
that depends on the specific application desired.
[0093] FIG. 6a illustrates that the light harvesting of the OPV
device can also be improved by simply attaching the transparent
paper to the glass side of the OPV sample. On the opposite side,
OPV device with a structure of indium tin oxide
(ITO)/poly(3,4-ethylenedioxythiophene)-polystyrenesulfonic acid
(PEDOT-PSS)/Poly[N-9''-hepta-decanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-th-
ienyl-2',1',3'-benzothiadiazole)] and [6,6]-phenyl-C71 butyric acid
methyl ester (PCDTBT:PC70BM, 90 nm)/calcium (Ca)/aluminum (Al) has
been previously fabricated, and the molecular structures of the
photoactive materials for the OPV device is indicated in FIG.
S8.
[0094] It is expected that the haze effect of the transparent paper
causes incident angle dependent the photocurrent response. To
verify this, the photocurrents of the devices under illumination
from different incident angle were measured by illuminating the
devices with parallel white light and rotating the devices
gradually. The measured incident angle dependent photocurrents are
shown in FIG. 6b. Here the incident angle is defined as the angle
between the incident light and the normal direction of the
substrate.
[0095] The photocurrent has been normalized to the values obtained
from the control device (without transparent paper) with light
incident to the normal direction. The photocurrent of the device
with transparent paper was about 3% less than that of the control
device at the normal incident direction, most likely due to the
roughly 90% diffusive transmittance of the transparent paper.
Interestingly, the photocurrents of the device with transparent
paper exceed that of the control device at a larger incident angle
above 7.degree.. A large photocurrent improvement of over 15% were
observed in an incident angle range of 60.degree..about.87.degree..
The improved photocurrent should be correlated with the reduced
reflection of the light at glass surface and a broadened angular
distribution of the redirected incident light caused by the
transparent paper, as shown in FIG. 6c. Similar antireflection
effects have been observed in solar cells with microstructure
arrays or a textured surface. The increased light harvesting at
oblique incidence indicates that the device with transparent paper
can collect the ambient light more efficiently.
[0096] FIG. 6d compares the PCE of the device illuminated by
diffused light with an intensity of 13 mW/cm2 which further
demonstrates the improved ambient light harvesting by the
transparent paper. The PCE of the PCDTBT: PCBM device with
transparent paper was increased from 5.34% to 5.88% due to the
increased the photocurrent by 10% (FIG. 6c). The performance
improvement is attributed to better light harvesting from the
diffused light since the I-V curves were obtained from the same OPV
device upon attaching or after peeling off the transparent
paper.
[0097] FIG. 6d further indicates the molecular structure of
photoactive materials used in this OPV device. The increased
ambient light harvesting by the transparent paper is particularly
desirable for many photovoltaic applications, such as in
application that cannot use mechanical light tracking systems to
compensate for shift in the incident sunlight throughout the day
like solar roofs, solar windows, and solar panels working in cloudy
days where the sunlight is strongly scattered by the
atmosphere.
Experimental
[0098] Bleached sulfate softwood pulp extracted from the southern
yellow pine without beating or refining was treated with
TEMPO-oxidized system. 5 g of wood fibers were dispersed into 1%
pulp with deionized water, TEMPO and sodium bromide (NaBr) were
then separately added into the wood pulp with doses of 10 wt % and
1.6 wt % based on oven-dry wood fibers, and the mixtures were
finally stirred continuously for 10 min at 700 rpm to form a
uniform suspension. 35 mL of sodium hypochlorite (NaClO) with a
concentration of 12.5 wt % was titrated into the abovementioned
suspension. The reaction time was monitored and the pH of the
reaction system was kept constant at 10.5. The reaction lasted
approximately 3-4 hours; however, the mixture was continuously
stirred at 700 rpm for an additional 4 hours to ensure adequate
reaction of the wood fibers. The dimension and morphology of the
wood pulp before and after oxidization was tested using a
KajaaniFS300 fiber analyzer and an optical spectroscope (OLYMPUS
BX51). NFC (nanofibrillated cellulose) with a diameter of
approximately 5-30 nm was extracted from the abovementioned
TEMPO-oxidized wood fiber solution by homogenization with a
microfluidizer.
[0099] The treated pulp was diluted to approximately 0.2 wt % in
solution with deionized water. This diluted pulp was then used to
fabricate transparent paper by a filtration method using a 20 cm
filter membrane (0.65 .mu.m DVPP, Millipore, U.S.A). The resulting
wet film was placed between two stacks of regular paper and dried
at room temperature. The optical properties of the paper were
measured using a UV-Vis Spectrometer Lambda 35 containing an
integrating sphere (PerkInElmer, USA).
[0100] 600 .mu.L wood fiber dispersion with a consistency of 1 wt %
was coated onto a 1 cm2 silicon slab and dried at room temperature.
To measure the optical properties of this sample, we built a custom
optical setup. A xenon light source was used with a monochromator
to select specific wavelengths from 400 nm to 1000 nm with a 10 nm
step size. By comparing the amount of light entering the
integrating sphere to the amount of light exiting the integrating
sphere, the total absorption was measured. Two separate
measurements are made: one baseline measurement with no sample in
place to calibrate the system and a second measurement with the
sample. By considering the difference between these two
measurements, the absorptivity of the sample was calculated.
[0101] For the device fabrication, a 30 nm thick PEDOT: PSS layer
was fabricated on a cleaned ITO/glass substrate by spin-coating
with a rotating speed of 3,500 rpm. The spun PEDOT: PSS film was
then baked at 130.degree. C. for 15 min. PCDTBT: PC70BM dissolved
in 1,2-dichlorobenzene with a blending ratio of 1:2 (by weight) was
used for the spin-coating of photoactive layer. The active layer
obtained by spin-coating with a rotating speed of 2400 rpm for 20 s
has a thickness of approximately 90 nm. Then the Ca/Al bilayer
cathode was thermally deposited in succession. When attaching the
transparent paper on the glass surface, for a better light coupling
from the transparent paper to the glass, as well as strong
adhesion, a cross-linked polymer (ethoxylated bisphenol A
dimethacrylate mixed with 1 wt %
2,2-dimethoxy-2-phenylacetophenone56) was formed between the
transparent paper and the glass substrate.
[0102] Distribution of Wood Fibers Before and after TEMPO
Treatment
[0103] Fiber analyzer FS300 was used to investigate the
distribution of fiber length and width before and after TEMPO
treatment. The length distribution of original wood fibers is
uniform (FIG. S1a), yet the length distribution of TEMPO-treated
wood fibers tends to concentrate in the range of 0.about.1.0 mm,
which indicates wood fibers are cracked into short fibers during
the treatment (FIG. S1b).
[0104] FIG. S1c illustrates the width distribution of original wood
fibers, after TEMPO system treatment, the percentage of wood fibers
in the width range of 16.0.about.32.0 .mu.m decreased from 74.7% to
22.9%, yet the amount of wood fibers in the width range of
8.0.about.16.0 .mu.m is 6-fold more than that of original wood
fibers (FIG. S1c and d).
[0105] Wood Fibers after TEMPO Treatment
[0106] The morphological changes of wood fibers were clearly
observed in FIG. S2a. After TEMPO treatment, the length of wood
fibers becomes short and the cell walls of the fibers were cracked
into small fragments. FIG. S2b and S2c show the wood fiber unzipped
and cleaved in the axial direction that can improve the density of
paper. Table S1 shows the grammage and thickness of various paper
substrates for measurement of transmittance and transmission
haze.
[0107] Transmittance and Haze of Paper with Different Thickness
[0108] Thickness of paper affects the transmission haze and
transmittance of our transparent paper. As the thickness increases,
the transmittance decreases due to an increase in to light
scattering within the paper occurred (FIG. S3a). Meanwhile an
increase in transmission haze was observed in FIG. S3b. Table S2
demonstrates the basic weight and thickness of transparent
paper.
[0109] Mechanical Modelling
[0110] To reveal the origin of the enhanced mechanical properties,
we conducted molecular dynamics simulations on scaled-down models
for both TEMPO-oxidized fiber and original wood fiber with roughly
comparable size. Our full atomistic simulation study employs the
ReaxFF potential and simulation is carried out using Large-scale
Atomic/Molecular Massively Parallel Simulator (LAMMPS). ReaxFF
force field was developed via first principle and is also able to
account for various non-bonded interactions such as van der Waals
and coulombic types, and particularly important and convenient for
the present study, it has an explicit expression for hydrogen
bonds.
[0111] FIG. 4d and FIG. S4a describes the atomistic model used to
study the interaction between two original cellulose fibers. Each
fiber takes a tubular structure and has the same axial length (7.8
nm) as the TEMPO-oxidized flakes in FIG. 4e and FIG. S4b but with a
tube diameter of around 6 nm. FIG. 4e and FIG. S4b shows the
atomistic model used to study the interaction between two
TEMPO-oxidized cellulose flakes. Each flake has three layers and
each layer consists of 6 TEMPO-oxidized cellulose chains. The
initial stacking of those chains follows the crystalline
parameters. The global size for each TEMPO-oxidized fiber is around
7.8 nm.times.3.9 nm.times.2 nm (7.8 nm is along axial direction).
To reduce the computational expense, we model the two contacting
halves of the neighboring hollow fibers and each half tubular fiber
consists of 24 cellulose chains (with 16 chains on the outer
surface). The simulations were subjected to a microcanonical (NVT)
ensemble, carried out at a temperature of 5K, for the purpose of
suppressing thermal noise to clearly reveal the fine feature of the
hydrogen bonding stick-slip event. The time step is set to 0.5
femtoseconds (fs). The system is free to evolve for 50000 time
steps until the right end of the top flake/fiber is assigned a
constant axial velocity of 0.001 .ANG./fs, pulling the top
flake/fiber to slide on the bottom flake/fiber, the left end of
which is hold in position. The energy data points were sampled on
every 200 time steps while the force data points were sampled on
every 1000 time steps.
[0112] Although a few embodiments have been shown and described, it
would be appreciated by those skilled in the art that changes may
be made in this embodiment without departing from the principles
and spirit of the invention, the scope of which is defined in the
claims and their equivalents.
* * * * *