Transparent Conductive Nanofiber Paper for Foldable Solar Cells

Optically transparent nanofiber paper containing silver nanowires showed high electrical conductivity and maintained the high transparency, and low weight of the original transparent nanofiber paper. We demonstrated some procedures of optically transparent and electrically conductive cellulose nanofiber paper for lightweight and portable electronic devices. The nanofiber paper enhanced high conductivity without any post treatments such as heating or mechanical pressing, when cellulose nanofiber dispersions were dropped on a silver nanowire thin layer. The transparent conductive nanofiber paper showed high electrical durability in repeated folding tests, due to dual advantages of the hydrophilic affinity between cellulose and silver nanowires, and the entanglement between cellulose nanofibers and silver nanowires. Their optical transparency and electrical conductivity were as high as those of ITO glass. Therefore, using this conductive transparent paper, organic solar cells were produced that achieved a power conversion of 3.2%, which was as high as that of ITO-based solar cells.

Optically transparent and electrically conductive nanofiber paper. Optically transparent and electrically conductive nanofiber paper was fabricated using the three methods of heating, mechanical pressing, and deposition via dropping (Fig. 1d). In the heating and mechanical pressing methods, the optically transparent nanofiber papers were prepared before the deposition of the silver nanowires. The fabrication of the transparent nanofiber paper was achieved as follows: The nanofiber dispersions were dropped onto a silicon wafer and were then oven-dried at 50 °C for 1 day. After drying, an optically transparent nanofiber paper with a thickness of 15-20 μ m was obtained 18 .
(1) Heating method (Fig. 1d): A 0.3 wt.% silver nanowire/ethanol suspension was bar-coated onto the transparent nanofiber paper, and then air-dried for 3-5 min. The air-dried silver nanowires on the nanofiber papers were heated at 150 °C for 30 min in air. (2) Mechanical pressing method (Fig. 1d): A 0.3 wt.% silver nanowire/ethanol suspension was bar-coated onto the transparent nanofiber papers, and was then air-dried for 3-5 min. The air-dried silver nanowire networks on the nanofiber papers were mechanically pressed at 2 MPa and 20 °C for 20 s. Using a polyethylene terephthalate (PET) film as a transparent substrate, as shown in Fig. 2c, air-dried silver nanowire networks were pressed at 10 MPa and 20 °C for 20 s. (3) Dropping method (Fig. 1d): A 0.3 wt.% silver nanowire/water suspension was cast on a silicon wafer, and then air-dried. A 0.7 wt.% cellulose nanofiber/water dispersion was cast over the dried silver nanowire layer on a silicon wafer, and then air-dried at 50 °C for 12-24 hours. After drying, the nanofiber paper was removed from the silicon wafer. The obtained optically transparent nanofiber paper with a silver nanowire layer was 15-20 μ m thick. (a) Traditional white paper (left), transparent nanofiber paper (center), and transparent conductive nanofiber paper (right). (b) Optical transmittance of transparent nanofiber paper (solid line), and transparent conductive nanofiber paper (dotted line). (c) Optical transmittance of silver nanowire layers fabricated on the transparent nanofiber paper using a heating method (dotted line), pressing (gray line), and dropping (red line). (d)Transparent conductive nanofiber paper produced using the heating method (upper), pressing method (middle), and dropping method (lower). The cellulose nanofiber dispersion is shown as blue, and the silver nanowire suspension is shown as black. Folding tests on the transparent conductive films with silver nanowires. Silver nanowire patterns on nanofiber paper (produced using a dropping method), silver nanowire patterns on PVA films (produced using a dropping method), and silver nanowire patterns on PET substrates (produced using a heating method) were subjected to the folding tests. The pattern was 3 mm wide and 50 mm long, and the thickness of all of the substrates was 50 μ m. The samples were folded across the center, to − 180° (silver patterns inside), and were then repeatedly passed through rollers with a gap of approximately 100 μ m. The electrical resistance was measured using a two-point probe method (34410A, Agilent).

Organic solar cells.
Organic solar cells were fabricated on nanofiber papers with silver nanowires, and on conventional ITO glass. Optically transparent and electrically conductive nanofiber papers were fabricated with silver nanowires using a pressing method. Before deposition of active layer and transparent anode, conductive nanofiber paper was laminated on glass substrate using a double-sided tape. The transparent anodes were coated with a layer of poly(3,4-ethylenedioxythiophene) doped with polystyrene sulfonate (PEDOT: PSS) using spin-coating applied at 500 r/min for 5 s, and at 3000 r/min for 60 s.  chlorobenzene solution was spin-coated (at 1500 r/min, for 60 s) on top of the PEDOT: PSS coating on the transparent anode, and a 60 nm Al cathode was vacuum evaporated at 10-5 Torr. The active device area was 3 × 3 mm 2 .
Characterization. The total light transmittance spectrum of the nanofiber paper was measured at wavelengths from 200 to 800 nm, using a UV-visible spectrometer with an integrating sphere (U-3900, Hitachi High-Tech. Corp.). The haze was measured using a haze meter (HZ-V3, Suga Test Instruments Co., Ltd.). The sheet resistance was measured using the four-point probe method (MCP-T610 Loresta type, Mitsubishi Chemical Analytech Co., Ltd.).

Results and Discussion
Traditional paper-which is typically fabricated using 15-50-μ m-wide cellulose pulp fibers-is white and opaque because the cavities between the fibers produce light scattering (Fig. 1a). In contrast, the nanofiber paper produced using 15-nm-wide cellulose fibers exhibited high optical transparency (Fig. 1a), because the densely packed cellulose nanofibers did not produce light scattering either inside the paper or at its surfaces 5 . The nanofiber paper showed a high total transmittance of 91.4% at a wavelength of 600 nm (Fig. 1b), a value as high as theoretically predicted values 18 . However, the optically transparent nanofiber paper does not itself have any electrical conductivity; therefore, high electrical conductivity was achieved here in the optically transparent nanofiber paper via the deposition of a silver nanowire thin film (Fig. 1c).
Silver nanowires synthesized using the polyol process have diameters in the range 50-100 nm, and are surrounded by the insulating polymer PVP 1,19 . When these silver nanowires are deposited on transparent substrates, they maintain the high optical transparency of the substrate. However, as-deposited networks of these silver nanowires do not display high conductivity, because the surface PVP prevents electrical contact between the silver nanowires. To increase their conductivity, silver nanowires on transparent substrates should be heated to above 150 °C 1,19 . This temperature is too high for commonly used plastic substrates, but such high temperatures do not damage the transparent nanofiber paper. After the silver nanowires were deposited on the nanofiber paper and heated at 150 °C for 30 min (Fig. 1d), the thin layer of silver nanowires exhibited a low sheet resistance of 39 Ω /square, and a high optical transmittance of 91.0% at 600 nm (Fig. 1c). Mechanical pressing at room temperature can also be used to enhance the conductivity of silver nanowire networks 19 . Transparent nanofiber paper is a high-strength material, because it consists of nanofibers that have high mechanical strengths of 1.6-3 GPa 20 . The silver-nanowire-coated nanofiber paper was subjected to mechanical pressing to further enhance the conductivity of the silver nanowire networks (Fig. 1d). In our previous study, silver nanowires deposited on PET films or glass substrates were exposed to pressures greater than 10 MPa to obtain low sheet resistances of less than 50 Ω /square 19 . Here, exposure to just 2 MPa yielded a low sheet resistance of 43 Ω /square in the thin layer of silver nanowires, with a high optical transmittance of 92.8% (Fig. 1c). Because of the high thermal and mechanical durability of the cellulose nanofiber paper, these transparent and conductive nanofiber papers exhibited sheet resistance and optical transmittance values as good as those of ITO glass.
These two types of transparent and conductive nanofiber paper were fabricated using the following steps: making the transparent nanofiber paper; depositing the silver nanowire suspensions; and performing post treatments consisting of heating or mechanical pressing (Fig. 1d). As an alternative to these time-and labor-intensive processes, we also developed a simple procedure that did not require any post treatment (Fig. 1d). First, silver nanowire suspensions were deposited on the silicon wafer drying plate. Next, cellulose nanofiber dispersions were dropped on the dried silver nanowire layer. These samples were dried, and the transparent nanofiber paper was obtained by peeling the sample off the plate. As mentioned above, the as-deposited silver nanowire layer did not have a high conductivity. When a cellulose nanofiber/water dispersion (99.3 wt% water, 0.7 wt% nanofibers) was dried, the final volume of the dispersion was less than 1% of the original volume (after the water had evaporated). During the drying process, the drop dimensions decreased only in thickness; the spreading area was maintained. This anisotropic shrinkage had a mechanical pressing effect, thus increasing the number of electrical contacts between the silver nanowires, as Zhu et al. has also suggested 21 . As a result, the obtained silver nanowire layer displayed a maximum transmittance of 94.4% at a wavelength of 600 nm (Fig. 1c), and a minimum sheet resistance of 17 Ω /square. As a result, the transparent nanofiber paper had an electrical conductivity that was as high as that of ITO glass, without any loss in the high optical transparency (Fig. 1b).
Polymer solutions could also be dropped on the silver nanowire networks 22,23 . When a PVA solution was dropped on the silver nanowires, the obtained film showed high optical transparency and high electrical conductivity. However, because the PVA solution penetrated between the silver nanowires (Fig. 2a), the transparent and conductive PVA film had a high sheet resistance of 297 Ω /square at 95% transmittance. In contrast, when cellulose nanofiber dispersions were cast on the silver nanowires, the silver nanowires remained on the surface of the cellulose nanofiber networks (Fig. 2b). Because the cellulose nanofibers were more than several dozen micrometers in length, they could not penetrate between the silver nanowires, which defined cavities with dimensions smaller than a few micrometers. As a result, the transparent and conductive cellulose nanofiber paper exhibited a low sheet resistance of 148 Ω /square, less than half of the sheet resistance of the transparent and conductive PVA film (297 Ω /square) at a transmittance of 95%. The dropped transparent and conductive nanofiber paper exhibited high electrical durability in repeated folding tests (Fig. 2c). Because the silver nanowires were surrounded by hydrophilic PVP, there was a low adhesion strength between the silver nanowires and the hydrophobic polymer substrates. When a hydrophobic PET film with silver nanowire layers was folded four times, the conductivity was lost, because of the removal of the silver nanowire layer from the PET film. However, the silver nanowire layers on the hydrophilic PVA film maintained their conductivity after five folding cycles, as a result of the good affinity between the PVA substrate and the hydrophilic PVP. Notably, the silver nanowire layers on the transparent nanofiber paper maintained their high conductivity even after twenty folding cycles. The high electrical durability of the nanofiber paper did not result only from the high affinity between the PVP on the silver nanowires and the cellulosic nanofiber paper. Careful observations of the conductive nanofiber paper showed that the silver nanowires were entangled in the cellulose nanofibers ( Fig. 2b). Therefore, the high adhesion strength against folding was enhanced by the dual advantages of the hydrophilic affinity between the PVP and the cellulose, and the entanglement between the silver nanowires and the cellulose nanofibers.
This procedure produced not only high electrical durability, but also transparent conductive patterns. When the silver nanowire inks were printed on a drying plate, and then the peeling off of the nanofiber paper (Fig. 1d), transparent and conductive patterns were fabricated on the nanofiber paper without the use of any etching processes. The transparent silver nanowire patterns on the nanofiber paper could be used to illuminate LED lights, as a result of their high electrical conductivity (Fig. 3a). The LED lights could still be illuminated under folding, and after recovery to the original flat form, because of the high foldability of the devices (Fig. 3a).
The transparent nanofiber paper was used to fabricate paper solar cells, via the printing of organic solar cell components on the transparent conductive nanofiber paper. We fabricated organic solar cells based on ITO glass with an active layer of P3HT/PCBM, their short current density was 7.89 mA/cm 2 , and their power conversion efficiency was 3.1% (Fig. 3b). In the paper solar cells, we used the transparent nanofiber paper instead of glass, and silver nanowires instead of ITO electrodes. In previous studies of paper solar cells, the measured power conversion efficiency was less than one-tenth, or half, than that of ITO-based solar cells, even when the same active solar layer was used 13,24 . Our transparent conductive nanofiber paper had optical transparency and electrical conductivity values as high as those of ITO glass, as mentioned above. In our study, the nanofiber paper consisted of native cellulose fibers, which have high chemical durability. Therefore, they maintained their high optical transparency and high electrical conductivity after coating with acid PEDOT:PSS and P3HT/PCBM chlorobenzene solutions. Moreover, the conductive nanofiber paper did not have dimensional change such as wrinkle and shrinkage during a coating process, since it was laminated on glass substrate using a double-sided tape. As a result, our paper solar cell achieved a power conversion efficiency of 3.2%, as high as that of ITO-based solar cells, and a short current density of 9.58 mA/cm 2 (Fig. 3b). Moreover, we found that the nanofiber paper solar cells exhibited power conversion under folding, and after folding. The nanofiber solar cells could therefore supply electric power everywhere, while (and after) being carried in a pocket or bag (Fig. 3c).

Conclusions
In conclusion, we reported optically transparent conductive paper produced using cellulose nanofibers and silver nanowires. The optical transparency and electrical conductivity of the optically transparent conductive paper were as high as those of ITO glass. Paper solar cells were fabricated using the transparent conductive paper; these paper solar cells exhibited a high power conversion efficiency of 3.2%, equal to that of ITO glass-based solar cells. Because of the high affinity and high degree of entanglement between the cellulose nanofibers and the silver nanowires, the nanofiber paper maintained its high conductivity-and the paper solar cells still generated electrical power-under folding, and after folding. Moreover, transparent conductive patterns were successfully formed on the nanofiber paper via the printing of silver nanowires. We believe that this highly transparent conductive nanofiber paper will play an important role in future portable electronics.