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Roll-to-roll Printing of Meter-scale Composite Transparent Electrodes with Optimized Mechanical and Optical Properties for Photoelectronics Xiangchuan Meng, Xiaotian Hu, Xia Yang, Jingping Yin, Qingxia Wang, Liqiang Huang, Zoukangning Yu, Ting Hu, Licheng Tan, Weihua Zhou, and Yiwang Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b00093 • Publication Date (Web): 19 Feb 2018 Downloaded from http://pubs.acs.org on February 20, 2018
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ACS Applied Materials & Interfaces
Roll-to-roll Printing of Meter-scale Composite Transparent Electrodes with Optimized Mechanical and Optical Properties for Photoelectronics
Xiangchuan Meng,1,2 Xiaotian Hu,1,2 Xia Yang,1 Jingping Yin,1 Qingxia Wang,1 1
Liqiang Huang, Zoukangning Yu,1 Ting Hu,
1,2
Licheng Tan,1,2 Weihua Zhou*,1,2
Yiwang Chen*1,2 X. Meng, X. Hu, X. Yang, J. Yin, Q. Wang, L. Huang, Z. Yu, Dr. T. Hu, Prof. L. Tan, Prof. W. Zhou and Prof. Y. Chen 1
School of Materials Science and Engineering, Nanchang University, 999 Xuefu Avenue, Nanchang 330031, China X. Meng, X. Hu, Dr. T. Hu, Prof. L. Tan, Prof. W. Zhou and Prof. Y. Chen
2
Jiangxi Provincial Key Laboratory of New Energy Chemistry/Institute of Polymers, Nanchang University, 999 Xuefu Avenue, Nanchang 330031, China Corresponding author. Tel.: +86 791 83968703; fax: +86 791 83969561. E-mail:
[email protected] (Y. Chen);
[email protected] (W. Zhou).
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ABSTRACT Flexible transparent electrodes (FTEs) are an indispensable component for flexible optoelectronic devices. In this work, the meter-scale composite transparent electrodes (CTEs)
composing
of
poly(3,4-ethylenedioxythiophene):polystyrene
sulfonate
(PEDOT:PSS) and Ag-grid/PET with optimized mechanical and optical properties are demonstrated by slot-die roll-to-roll technique with solution printing method under a low cost (15~20 $ for per square meter), via control of the viscosity and surface energy of PEDOT:PSS ink as well as the printing parameters. The CTEs show excellent flexibility remaining 98% of the pristine value after bending 2000 times under various bending situation and the square resistance (Rs) of CTEs can be reduced to 4.5~5.0 Ω/□ with an appropriate transmittance. Moreover, the optical performances such as haze, extinction coefficient and refractive index are investigated as compared with ITO/PET, which are potential for the inexpensive optoelectronic flexible devices. The CTEs could be successfully employed in polymer solar cells (PSCs) with different area, showing of a maximal power conversion efficiency (PCE) of 8.08%. KEYWORDS: transparent electrodes, roll-to-roll technology, bending resistance, multiple optical characterization, polymer solar cells
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1. INTRODUCTION Flexible transparent electrodes (FTEs) as a necessary component have been widely used in diverse photoelectric devices. For FTEs, conductivity, mechanical toughness and light transmittance are the essential properties1-3. For practical application, low cost is also required. Besides, the preparation of large area FTEs should be achieved by printing methods, especially by roll-to-roll (R2R) technology with solution processing4-10. Currently, tin indium oxide (ITO) transparent electrode is the broadest used transparent conductive film in laboratories and factories. However, several shortcomings, such as brittle film, scarcity of indium elements and the complex preparation process make it unsuitable for roll-to-roll technology and limit its further commercial applications11-14.
Alternatives of ITO electrode have been reported in previous literatures, such as metal grid or nanowire, graphene, carbon nanotube and conductive polymer transparent electrodes9, 10, 12, 15-19. However, each of these electrode has its own disadvantages which do not match with roll-to-roll printing. Metal grid or nanowire electrode are easily peeled off from the flexible substrates. Graphene electrode has good overall performance, but the poor doping stability limits its performance to be further improved. Carbon nanotube transparent electrode itself has impressive conductivity, however, the large resistance between nanotubes is difficult to optimize. Besides, conductive polymer is also a viable choice for flexible electrode. Unfortunately, it is limited by low pristine conductivity17. Thus, all of these electrodes are not perfectly suitable for roll-to-roll technology to product large FTEs20-23.
As is known, the Ag-grid/PET and PEDOT:PSS ink have realized commercialization. In Ag-grid/PET film, the height difference between Ag-grid and PET substrate could even reach to larger than 700 nm, which is obviously unsuitable to serve as the electrode using for OPVs with active layer thickness of 100-200 nm. Similarly, due to the limitation of PEDOT:PSS conductivity, the pristine PEDOT:PSS ink could not be using as the electrode for OPVs devices either. Thus, the height difference between ACS Paragon Plus Environment
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Ag-grid and PET substrate could largely be minimized and the overall conductivity of films will be more homogeneous in addition to optimized performance, via the combination of doped PEDOT:PSS and Ag-grid/PET. Moreover, the meter-scale transparent electrode by printing of PEDOT:PSS onto Ag-grid/PET film through roll-to-roll technique has not been reported previously according to our knowledge.
In
this
work,
a
composite
transparent
poly(3,4-ethylenedioxythiophene):polystyrene
electrode sulfonate
(CTEs)
composing
(PEDOT:PSS)
of and
Ag-grid/PET with good comprehensive performance is demonstrated by slot-die roll-to-roll technology under a low cost (15~20 $ for per square meter). Poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) film was printed onto the Ag-grid/PET electrode. Various additives (ethanol, ethylene glycol and glycerine) are selected to modulate viscosity of PEDOT:PSS ink, realizing the control of surface energy of PEDOT:PSS ink. By this way, the CTEs shows excellent flexibility remaining 98% of the pristine value after bending 2000 times under various bending situation. Meanwhile, the haze, extinction and refractivity of CTEs have been investigated, as compared with ITO/PET electrode. The square resistance (Rs) of CTEs can be reduced to 4.5~5.0 Ω/□ with an appropriate transmittance. Finally, polymer solar cells (PSCs) with multiple areas are prepared by these FTEs, proposing a promising potential for commercial flexible transparent electrodes in the near future. 2. RESULTS AND DISCUSSION Figure 1a, b and Figure S1 show the schematic diagrams of roll-to-roll instrument and fabricating process of CTEs. The printing process mainly includes four steps: (I) Ultrasonic cleaning the Ag-grid electrode, followed by making the corona treatment at low-intensity to control a suitable surface energy for the flexible substrate. (II) Printing PEDOT:PSS ink with various additives via slot-die R2R technology, as shown in Figure 1a, b. In order to completely cover Ag-grid electrode to afford homogeneous conductive films, the film thickness is controlled by tape speed, injection speed, viscosity and surface energy of PEDOT:PSS ink. The sectional
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diagram of CTEs reflects the coverage of Ag-grid by PEDOT:PSS (Figure 1c). (III) After printing, the CTEs films are annealed at 120 °C for 15 min. (IV) Dipping of films into methanol to remove residual PSS. The FTEs are defined as Ag-grid/PEDOT:PSS. The CTEs can be printed successfully for a meter-scale area as shown in Figure 1d and it is worth nothing that the prime cost of CTEs is only 15-20 $ for per square meter (15 $ for Ag-grid/PET, 0.25 $ for PEDOT:PSS, 0.5 $ for other additives and solvents). The price is much cheaper than that of the commercial ITO/PET which is always over 50 $ for per square meter24.
In the printing process, the ink performance and printing parameters affect the quality of conductive films. Figure 2 shows the schematic diagram of printing effect under different ratio of injection speed and tape speed (mL/m), die spacing (µm) and viscosity of PEDOT:PSS ink. The best printing conditions can be confirmed by this way. In this work, the printing parameters are chosen as that the injection speed is 0.1 mL/min, tape speed is 0.1 m/min, die spacing is 300 µm, let-off tension and releasing tension are all 45 N and the annealing temperature is 120 oC. The electrical properties of CTEs with various additives (ethanol, ethylene glycol (EG) and glycerol) and different volume fractions (3, 5, 8 vt%) are revealed in Figure 3a. It is easy to find that the CTEs with 5 or 8 vt% EG show of superior conductivity. The sheet resistance (Rs) is evenly distributed and can be maintained at 4.5 Ω/□ under randomizing 200 points on CTEs. It is well known that PEDOT:PSS is a weak acid tending to adsorb water. These features lead to poor stability25. However, the Rs of CTEs only changes 2 ~ 3 Ω/□ under ambient environment with 35% humidity for 20 days in this way. For the FTEs, the transmittance of films is also an important factor. In general, transmittance will be affected by the transparent electrode types and thickness of films. In this work, the film thickness can be controlled by injection speed, tape speed, die spacing and viscosity of PEDOT:PSS ink. The printing parameters have been determined according to the above processing conditions. In order to regulate the viscosity of PEDOT:PSS ink, various additives (ethanol, ethylene glycol (EG) and glycerol) with different volume fractions (3, 5, 8 vt%) are added into the ink and the ACS Paragon Plus Environment
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specific viscosity values are shown in Table S1. The transmittance spectra of PET, ITO/PET, Ag-grid/PET and PEDOT:PSS/Ag-grid/PET (Figure 3b and Figure S2) also confirms the relationship between viscosity, thickness and transmittance. The transmittance of CTEs with different additives and volume fraction is 71~75% at 550 nm which is higher than ITO/PET electrode before 450 nm wavelength. As for balancing the conductivity with transmittance of CTEs, the CTEs with 5 or 8 vt% EG as additives are chosen for the further optoelectronic applications. Finally, so as to judge the performance of CTEs objectively, merit figures are calculated as shown in Figure 3c. The calculation formula of merit figures can be expressed as26: 188.5 σOp ( λ ) T ( λ) = 1 + Rs σ DC
−2
where T(λ) is transmittance at fixed wavelength, Rs is sheet resistance of CTEs and the merit figures is the reciprocal of σOp(λ)/σDC. The performance of FTEs should be better if the merit figure is higher. The highest merit figure could be observed for the CTEs with 5 or 8 vt% EG (Table S3) and this value is superior to that reported in other literatures as shown in Figure 3c. Besides, the ambient stability of CTEs is checked under 45% relative humidity and room temperature as illustrated in Figure 3d, and it is found that the conductivity only improve 2 ~ 3 Ω/□ for 20 days which is
also lower than that of ITO-PET transparent electrodes.
To further study the optical properties of CTEs, haze of ITO/PET, ITO/glass and CTEs with various additives of different volume fraction were measured by integrating sphere method. It is worth noting that haze and transmittance are uncorrelated, some materials can remain high haze and transmittance. For the FTEs applying in OPVs, high haze increases the light scattering and consequent absorption in active layer of OPVs27. The factors influencing plastic film haze mainly has two aspects: one is the raw materials, the other is the processing conditions such as different processing procedure under the same technology. The calculation formula of haze can be expressed as:
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T T H = 1 − 2 ×100% T3 T4
where T1 is the scattered luminous flux of films, T2 is the incident luminous flux of films, T3 is the scattered luminous flux of air, T4 is the incident luminous flux of air, and H is the haze. By this way, the haze of various FTEs is shown in Table 1. The haze of commercial ITO/glass is 1.47%, which is high enough for the glass substrate. In this work, the haze of CTEs with various additives of different volume fraction is 0.72 ~ 1.99% and it can reach higher using glycerin as additive. However, taking into account of transmittance and conductivity of FTEs, the CTEs with 5 or 8 vt% EG are the most promising, and the haze is 1.53% or 1.31% under these conditions.
Besides, the extinction coefficient and refractive index are also investigated in this work. The extinction coefficient and refractivity with various additives of different volume fraction is shown in Figure 4. Extinction coefficient reflects absorption of incident light by the films, and the higher extinction coefficient may reduce FTEs performance28. In Figure 4a-c, it is clearly noticed that the extinction coefficient values decreases gradually with increase of additive volume fraction, and the best values are observed for CTEs with 5 or 8 vt% EG. Meanwhile, the refractive index demonstrates light transmission through films. In general, the refractive index of zinc oxide is 2.008~2.029. In Figure 4d-f, it is easy to observe that the refractive index of CTEs becomes higher with increase of additive volume fraction. The matched refractive index between CTEs and zinc oxide indicates of more light penetrating films, so the refractivity shows an opposite law as compared with extinction coefficient. The refractivity value increases with the increase of additive volume fraction, which is consistent to the result of transmittance spectra.
The traditional metal oxide transparent electrode (ITO) is crisp and it will produce many cracks on film surface under low stress11-14. This feature is unsuitable to research and commercialization of flexible large-area optoelectronic devices. In contrast, Ag-grid, PEDOT:PSS films and PET all have good flexibility. The ACS Paragon Plus Environment
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conductivity and scanning electron microscope (SEM) images of CTEs and ITO/PET after bending 2000 times with different bending radius (R=1, 2.5 and 5 mm) are shown in Figure 5a-f and Figure S3. As expected, the conductivity of ITO shows poor mechanical stability after bending 50 times and the ITO surface also exhibits many cracks with different degrees. However, the minor reduction in conductivity of CTEs after bending demonstrates excellent flexibility, which remains 98% of its pristine value after bending 2000 times with bending radius (R=1, 2.5 and 5 mm). Meanwhile, the surface of CTEs is smooth and flat, and Ag-grid remains unchanged. In addition, the Young’s modulus of CTEs and ITO with or without active layer via force-separation curves are shown in Figure 5g-l. As expected, the Young’s modulus of the ITO film is 2.16 GPa due to its poor mechanical character. The films of PET, PET-PEDOT:PSS, PET-PEDOT:PSS with 5% EG and PET-PEDOT:PSS with 5% EG covered by active layer exhibit Young’s modulus of 65 MPa, 243 MPa, 164 MPa and 172 MPa, respectively. The Young’s modulus of these films are much lower than ITO electrode, indicating of excellent bending property for the CTEs.
As for the application of FTEs in OPVs, the surface morphology is also an important factor influencing the OPVs performance29-31. The corresponding atomic force microscopy (AFM) height images of PEDOT:PSS films (Figure S4) are measured to compare surface roughness (Rs) with various additives of different volume fraction. In general, the Rs of ITO/glass and PET is 7.30 nm and 8.45 nm, respectively. However, after printing PEDOT:PSS, the Rs decreases obviously. The Rs is 3.55 nm and 3.59 nm for the CTEs with 5 or 8 vt% EG, which is suitable for OPVs32, 33, 34. Besides, the thickness difference between Ag-grid and PEDOT:PSS films is also worth of attention, due to the fact that excessive thickness difference could destroy the morphology or device structure of active layer in OPVs. The thickness difference is shown in Figure 6a and Table S2 and the corresponding scanning electron microscope (SEM) images are revealed in Figure S5.
To authenticate the performance of CTEs, the large-area flexible OPVs are fabricated ACS Paragon Plus Environment
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with
the
structure
of
CTEs/ZnO/Active
layer
(Poly({4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl}{3-fluoro -2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl}):[6,6]-phenyl-C71-butyric acid
methyl
ester
(PTB7:PC71BM)
and
Poly([2,6
′
-4,8-di(5-ethylhexylthienyl)benzo[1,2-b;3,3-b]dithiophene]{3-fluoro-2[(2-ethylhexyl) carbonyl]thieno[3,4-b]thiophenediyl})(PTB7-Th):PC71BM)/MoO3/Ag, as shown in Figure 6c. In order to guarantee the high reproducibility and reduce error for flexible
OPVs35, the effective area is 0.04, 0.20, 1.00, 2.00 and 4.00 cm2. The current density-voltage (J-V) curves of OPVs are tested under one standard sun using a solar simulator with an Air Mass 1.5 global (AM 1.5G) and irradiation intensity of 100 mW cm2 as illustrated in Figure 6d. The OPVs based on CTEs and PTB7-Th:PC71BM show a record PCE of 8.08% with an open circuit voltage (Voc) of 0.78 V, a short current density (Jsc) of 17.38 mA·cm-2 and a fill factor (FF) of 59.51% at an effective area of 0.04 cm-2. For the OPVs with other effective areas (0.20, 1.00, 2.00 and 4.00 cm2), the PCE are determined to be 7.32%, 5.31%, 3.91% and 2.52%, respectively. Besides, the PCE of OPVs with PTB7:PC71BM as active layer is 6.14%. The specific device parameters are shown in Table 2 and the corresponding J-V curves with different effective areas are illustrated in Figure 6e-h. The inserts show photographs of OPVs with different areas. These performances are even superior to the devices with traditional ITO/PET electrode. In fact, the Ag-grid itself occupies the area of OPVs 36, which will decrease surface area with which the active layer contacts the bracket of hexagon Ag-grid electrodes and restricts the quantity of charge recombination sites that may be introduced through bracket partitioning. In order to more objectively measure the photoelectric conversion efficiency of solar cells, the scaled-PCE is calculated by dividing the PCE by microcell area fraction (fMA)36. The fMA can be calculated as (2 × apothem)2/dper2, where apothem is the distance between hexagon grid center and bracket (0.925 mm), dper is the distance between two hexagon grid centers (2 mm), so the fMA of OPVs with bracket width of 150 µm is calculated to be 0.855. By this way, the PCE of OPVs with different areas (0.04, 0.20, 1.00, 2.00 and 4.00 cm2) can be changed from 8.08%, 7.32%, 5.31%, 3.91% and 2.52% to ACS Paragon Plus Environment
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9.45%, 8.56%, 6.26%, 4.57% and 2.95%. Remarkably, the annealing temperature of ZnO precursor is reduced from 220 °C to 150 °C when preparing the flexible OPVs and the device performance does not show of significant changes. 3. CONCLUSIONS
In summary, we have successfully fabricated a composite transparent electrode (CTEs) composing of poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) and Ag-grid/PET via the slot-die R2R technology. The conductive polymer (PEDOT:PSS) is printed on Ag-grid electrode by regulating the viscosity and surface energy of PEDOT:PSS ink as well as the printing parameters. By this way, the Rs of CTEs can be maintained at 4.5~5.0 Ω/□ with a considerable transmittance in ambient stability for 20 days. Meanwhile, the CTEs demonstrate significant mechanical flexibility under various bending radius with bending 2000 times. The extinction coefficient, refractivity and haze of CTEs are measured to compare with commercial ITO-PET. In terms of preparing costs, the CTEs is only 15 ~ 20 $ for per square meter, which is about one quarter of ITO-PET. The CTEs can be used as transparent electrodes in flexible OPVs, revealing a considerable PCE of 8.08%. The CTEs demonstrate a worthy learning method for high performance and low cost photoelectric devices.
4. EXPERIMENTAL SECTION 4.1 Materials
PTB7-Th was purchased from 1-Material Chemscitech Inc. PC71BM was purchased from Nano-C Inc. Aqueous PEDOT:PSS solution (CLEVIOSTM PH1000) was purchased from Heraeus. Ethanol, ethylene glycol, glycerine, chlorobenzene and 1,8-diiodooctane were supplied by Sigma Aldrich. The Ag-grid transparent electrodes were supplied by InfinityPV Aps.
4.2 Fabrication of PEDOT:PSS/Ag-grid transparent electrodes
The large area flexible composite transparent electrodes were fabricated by roll-to-roll
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coating technology. The roll-to-roll coating was achieved by the GTB150B-0602E multifunctional coating machine from Shenzhen Shining Automation Equipment Co. Ltd. (China). The tension of take-up roll and unwinding roll are all 45N. The width of Ag-grid/substrate is 10 cm with four hexagon Ag-grid stripes. The separation distance of Ag-grid is 2 mm and the length of Ag-grid side is 2 mm. The Ag-grid/PET substrates were cleaned by sonication in acetone, deionized water and isopropanol, respectively. Subsequently, the substrate was dried by 75 °C thermal annealing. The PEDOT:PSS
with various additives of different volume fraction was coated on
Ag-grid/PET substrate via slot-die technology with different coating parameter (belt speed, die spacing and feed rate) to control the quality of conductive film. After 120 °C thermal annealing, the transparent electrodes were dipped into methanol solution to remove the residual PSS, followed by annealing at 120 °C for 15 min.
4.3 Fabrication of flexible PSCs
The device structure of flexible PSCs is CTEs/ZnO/Active layer/MoO3/Ag. Before the
device
fabrication,
flexible
transparent
electrodes
were
treated
with
ultraviolet-ozone plasma for 3s. The ZnO thin films were obtained by spin-coating with a speed of 1500 r.p.m for 40 s, following by annealing at 150 ℃ for 50 min. Then CTEs/ZnO was transferred to glove box to spin-coat the active layer of PSCs with a speed of 800 r.p.m for 30 s and 1000 r.p.m for 30 s (PTB7-Th:PC71BM solutions at weight ratio of 1:1.7 with polymer concentration of 10 mg mL−1, the volume of DIO is 3 vol%). Finally, 7 nm MoO3 and 90 nm Ag were deposited by vacuum evaporation with a vacuum of 6×10-4 Torr.
4.4 Characterizations
The sheet resistances (Rs) of composite transparent electrodes were measured by the four point probe setup with a source measurement unit (Keithley 2400) that can provide more accurate data. Then, Rs can be calculated by the equation: Rs=πV/(Iln2) =4.53V/I. The transmittance curves were provided by UV-vis spectroscopy (Perkin Elmer Lambda 750). Atomic force microscopy (AFM) images were measured by ACS Paragon Plus Environment
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MultiMode 8-HR (Bruker). The bending performance of transparent electrode was measured by an accurate drawframe (CL-01A Stepper Motor Controllers). The haze of CTEs was measured by the integrating sphere method. The refractivity can be tested by ellipsometer. Scanning electron microscope (SEM) images were taken by FEI XL30 Sirion SEM equipped with an energy dispersive spectrometer. For the flexible PSCs, the J-V curves were measured by Keithley 2400 source meter (Abet Solar Simulator Sun2000) under AM 1.5G light source (100 mW cm-2).
ASSOCIATED CONTENT Supporting Information
The photographs of R2R instrument, transmittance spectra, SEM images of ITO-PET under different bending radius, AFM topography images, SEM images of PET film and CTEs with various additives, the contact angle measurements, the viscosity of the conductive ink and the thickness difference between Ag-grid and PEDOT:PSS are presented in Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at www.pubs.org.
Corresponding Author
*Tel.: +86 791 83968703; fax: +86 791 83969561. E-mail:
[email protected] (Y. Chen);
[email protected] (W. Zhou)
ACKNOWLEDGMENT
M. X and H. X. contributed equally to this work. Y. C. thanks for support from the National Natural Science Foundation of China (NSFC) (51673091) and the National Science Fund for Distinguished Young Scholars (51425304). W. Z. thanks for the support from the National Natural Science Foundation of China (NSFC) (21764009). L. T. thanks for the support from the National Natural Science Foundation of China
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(NSFC) (51672121). We also thank for support from Natural Science Foundation of Jiangxi Province (20161BBH80044, 20161BCB24004 and 20161ACB20020).
REFERENCES
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Roll-to-Roll Encapsulation of Metal Nanowires between Graphene and Plastic Substrate for High-Performance Flexible Transparent Electrodes. Nano Lett. 2015, 15, 4206-4213. (7) Hu, X.; Chen, L.; Zhang, Y.; Hu, Q.; Yang, J.; Chen, Y., Large-Scale Flexible and Highly Conductive Carbon Transparent Electrodes via Roll-to-Roll Process and Its High Performance Lab-Scale Indium Tin Oxide-Free Polymer Solar Cells. Chem. Mater. 2014, 26, 6293-6302. (8) Søndergaard, R. R.; Hösel, M.; Krebs, F. C., Roll-to-Roll fabrication of large area functional organic materials. J. Polym. Sci. Part B: Polym. Phys. 2013, 51, 16-34. (9) Ning, J.; Han, L.; Jin, M.; Qiu, X.; Shen, Y.; Liang, J.; Zhang, X.; Wang, B.; Li, X.; Zhi, L., A Facile Reduction Method for Roll-to-Roll Production of High Performance Graphene-Based Transparent Conductive Films. Adv. Mater. 2017, 29, 1605028. (10) Wang, B. Y.; Lee, E. S.; Lim, D. S.; Kang, H. W.; Oh, Y. J., Roll-to-roll slot die production of 300 mm large area silver nanowire mesh films for flexible transparent electrodes. RSC Adv. 2017, 7, 7540-7546. (11) Kim, D. J.; Shin, H. I.; Ko, E. H.; Kim, K. H.; Kim, T. W.; Kim, H. K., Roll-to-roll slot-die coating of 400 mm wide, flexible, transparent Ag nanowire films for flexible touch screen panels. Sci. Rep. 2016, 6, 34322. (12) Yang, Y.; Wang, J. L.; Liu, L.; Wang, Z. H.; Liu, J. W.; Yu, S. H., A room-temperature environmentally friendly solution process to assemble silver nanowire architectures for flexible transparent electrodes. Nanoscale 2017, 9,
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52-55. (13) Park, J.; Hyun, B. G.; An, B. W.; Im, H. G.; Park, Y. G.; Jang, J.; Park, J. U.; Bae, B. S., Transparent Conductive Films with High Performance and Reliability Using Hybrid Structures of Continuous Metal Nanofiber Networks for Flexible Optoelectronics. ACS Appl. Mater. Interfaces 2017, 9, 20299-20305. (14) Bi, Y. G.; Feng, J.; Ji, J. H.; Chen, Y.; Liu, Y. S.; Liu, Y. F.; Zhang, X. L.; Sun, H. B., Ultrathin and ultrasmooth Au films as transparent electrodes in ITO-free organic light-emitting devices. Nanoscale 2016, 8, 10010-10015. (15) Schneider, J.; Rohner, P.; Thureja, D.; Schmid, M.; Galliker, P.; Poulikakos, D., Electrohydrodynamic NanoDrip Printing of High Aspect Ratio Metal Grid Transparent Electrodes. Adv. Funct. Mater. 2016, 26, 833-840. (16) Du, J. H.; Jin, H.; Zhang, Z. K.; Zhang, D. D.; Jia, S.; Ma, L. P.; Ren, W. C.; Cheng, H. M.; Burn, P. L., Efficient organic photovoltaic cells on a single layer graphene transparent conductive electrode using MoOx as an interfacial layer. Nanoscale 2017, 9, 251-257. (17) Yu, L. P.; Shearer, C.; Shapter, J., Recent Development of Carbon Nanotube Transparent Conductive Films. Chem. Rev. 2016, 116, 13413-13453. (18) Zhang, X.; Wu, J.; Wang, J.; Zhang, J.; Yang, Q.; Fu, Y.; Xie, Z., Highly conductive PEDOT:PSS transparent electrode prepared by a post-spin-rinsing method for efficient ITO-free polymer solar cells. Sol. Energy Mater. Sol. Cells 2016, 144, 143-149.
(19) Vosgueritchian, M.; Lipomi, D. J.; Bao, Z., Highly Conductive and Transparent
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PEDOT:PSS Films with a Fluorosurfactant for Stretchable and Flexible Transparent Electrodes. Adv. Funct. Mater. 2012, 22, 421-428. (20) Kim, A.; Won, Y.; Woo, K.; Kim, C. H.; Moon, J., Highly Transparent Low Resistance ZnO/Ag Nanowire/ZnO Composite Electrode for Thin Film Solar Cells. ACS Nano 2013, 7, 1081-1091. (21) Hösel, M.; Søndergaard, R. R.; Jørgensen, M.; Krebs, F. C., Fast Inline Roll-to-Roll Printing for Indium-Tin-Oxide-Free Polymer Solar Cells Using Automatic Registration. Energy Technol. 2013, 1, 102-107. (22) Emmott, C. J. M.; Urbina, A.; Nelson, J., Environmental and economic assessment of ITO-free electrodes for organic solar cells. Sol. Energy Mater. Sol. Cells 2012, 97, 14-21.
(23) Gao, T.; Li, Z.; Huang, P. S.; Shenoy, G. J.; Parobek, D.; Tan, S.; Lee, J. K.; Liu, H.; Leu, P. W., Hierarchical Graphene/Metal Grid Structures for Stable, Flexible Transparent Conductors. Acs Nano 2015, 9, 5440-5446. (24) Azzopardi, B.; Emmott, C. J. M.; Urbina, A.; Krebs, F. C.; Mutale, J.; Nelson, J., Economic assessment of solar electricity production from organic-based photovoltaic modules in a domestic environment. Energy Environ. Sci. 2011, 4, 3741-3753. (25) Kim, Y. H.; Sachse, C.; Machala, M. L.; May, C.; Müller-Meskamp, L.; Leo, K., Highly Conductive PEDOT:PSS Electrode with Optimized Solvent and Thermal Post-Treatment for ITO-Free Organic Solar Cells. Adv. Funct. Mater. 2011, 21, 1076-1081. (26) Hu, X.; Chen, L.; Tan, L.; Ji, T.; Zhang, Y.; Zhang, L.; Zhang, D.; Chen, Y., In
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situ polymerization of ethylenedioxythiophene from sulfonated carbon nanotube templates: toward high efficiency ITO-free solar cells. J. Mater. Chem. A 2016, 4, 6645-6652. (27) Fang, Z.; Zhu, H.; Yuan, Y.; Ha, D.; Zhu, S.; Preston, C.; Chen, Q.; Li, Y.; Han, X.; Lee, S.; Chen, G.; Li, T.; Munday, J.; Huang, J.; Hu, L., Novel Nanostructured Paper with Ultrahigh Transparency and Ultrahigh Haze for Solar Cells. Nano Lett. 2014, 14, 765-773.
(28) Rowell, M. W.; McGehee, M. D., Transparent electrode requirements for thin film solar cell modules. Energy Environ. Sci. 2011, 4, 131-134. (29) Kim, I.; Kwak, S. W.; Ju, Y.; Park, G. Y.; Lee, T. M.; Jang, Y.; Choi, Y. M.; Kang, D., Roll-offset printed transparent conducting electrode for organic solar cells. Thin Solid Films 2015, 580, 21-28. (30) Mo, L.; Ran, J.; Fang, Y.; Zhai, Q.; Li, L., Flexible transparent conductive films combining flexographic printed silver grids with CNT coating. Nanotechnology 2016, 27, 065202. (31) Hösel, M.; Søndergaard, R. R.; Jørgensen, M.; Krebs, F. C., Fast Inline Roll-to-Roll Printing for Indium-Tin-Oxide-Free Polymer Solar Cells Using Automatic Registration. Energy Technol. 2013, 1, 102-107. (32) Kim, S. M.; Walker, B.; Seo, J. H.; Kang, S. J., Hybrid Transparent Conductive Films of Multilayer Graphene and Metal Grid for Organic Photovoltaics. Jap. J. Appl. Phys. 2013, 52, 125103. (33) Shin, K.; Park, J.; Lee, C., A 250-mm-width, flexible, and continuous roll-to-roll
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slot-die coated carbon nanotube/silver nanowire film fabrication and a study on the effect of anti-reflective overcoat. Thin Solid Films 2016, 598, 95-102. (34) Li, Y.; Xu, G.; Cui, C.; Li, Y., Flexible and Semitransparent Organic Solar Cells. Adv. Energy Mater. 2018, DOI:10.1002/aenm.201701791 (35) Mao, L.; Chen, Q.; Li, Y.; Li, Y.; Cai, J.; Su, W.; Bai, S.; Jin, Y.; Ma, C. Q.; Cui, Z.; Chen, L., Nano Energy 2014, 10, 259-267 (36) Watson, B. L.; Rolston, N.; Printz, A. D.; Dauskardt, R. H., Scaffold-reinforced perovskite compound solar cells. Energy Environ. Sci. 2017, 10, 2500-2508.
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Figure 1. (a) The schematic illustration of roll-to-roll instrument and (b) structure of composite transparent electrodes (CTEs). (c) Sectional view of CTEs, and (d) the photograph of CTEs in length of 5 m before and after rolling.
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Figure 2. Schematic illustration of printing effect.
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Figure 3. (a) Sheet resistance distribution histogram of CTEs and (b) transmittance spectra of PET, PET/Ag-grid, PET/Ag-grid/PH1000 with various additives of ethanol, ethylene glycol and glycerine at different volume fractions. (c) The comparison of CTEs performance in other references and (d) sheet resistance of CTEs in air versus time.
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3500 3000 2500 2000
1500 350 400 450 500 550 600 650 700 750 800
(c) 4500 4000 3500
2500 2000 1500 350 400 450 500 550 600 650 700 750 800
Wavelength (nm)
(e) 1.65
3% Ethanol 5% Ethanol 8% Ethanol
1.60 1.55 1.50 350 400 450 500 550 600 650 700 750 800
Wavelength (nm)
1.65
4500 4000 3500
3% Glycerine 5% Glycerine 8% Glycerine
3000 2500 2000 1500 350 400 450 500 550 600 650 700 750 800
Wavelength (nm)
(f)
1.70
Refractivity
1.70
3% Ethylene glycol 5% Ethylene glycol 8% Ethylene glycol
3000
Wavelength (nm)
(d)
Extinction coefficient
4000
3% Ethanol 5% Ethanol 8% Ethanol
1.70
3% Ethylene glycol 5% Ethylene glycol 8% Ethylene glycol
1.60 1.55
1.65
Refractivity
4500
Extinction coefficient
(b)
Extinction coefficient
(a)
Refractivity
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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1.60 1.55
3% Glycerine 5% Glycerine 8% Glycerine
1.50 1.45 1.40 1.35
1.50 350 400 450 500 550 600 650 700 750 800
Wavelength (nm)
1.30 350 400 450 500 550 600 650 700 750 800
Wavelength (nm)
Figure 4. The extinction coefficient and refractivity of CTEs with various additives of (a, d) ethanol, (b, e) ethylene glycol, (c, f) glycerine at different volume fractions.
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Figure 5. The sheet resistance for CTEs and ITO-PET under bending radius of (a) 1 mm, (b) 2.5 mm and (c) 5 mm versus bending times. The corresponding SEM images of CTEs after bending for 2000 times under bending radius of (d) 1 mm, (e) 2.5 mm and (f) 5 mm. Limit deflection curves of (g) ITO, (h) ITO covered by active layer, (i) PET, (j) CTEs, (k) CTEs with ethylene glycol, and (l) CTEs with ethylene glycol covered by active layer.
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Figure 6. (a) Thick difference between the Ag-grid and PEDOT:PSS PH1000 with various additives at different volume fractions. (b) The photograph of light bulb
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experiment, the length of CTEs is 1.50 FT. (c) Schematic illustration of solar cell geometry. (d) J-V curves of solar cells based on CTEs and ITO-glass transparent electrode covered by different active layers. J-V curves of PTB7-Th:PC71BM solar cells based on CTEs with different effective area of (e) 0.20 cm2, (f) 1.00 cm2, (g) 2.00 cm2, (h) 4.00 cm2, respectively.
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Table 1. The haze of PET, ITO/glass, ITO/PET and CTEs with different additives and volume fraction. Type
Haze (%)
PET ITO-PET ITO-glass 3% Ethanol 5% Ethanol 8% Ethanol 3% EG 5% EG 8% EG 3% Glycerin 5% Glycerin 8% Glycerin W/O additive
0.82 3.83 1.47 0.72 0.85 0.91 1.41 1.53 1.31 1.45 1.54 1.99 2.00
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Table 2. Photovoltaic performance of devices based on CTEs and ITO-PET transparent electrode. Scaled Area Active layer
Voc
Jsc
FF
PCE
(V)
(mA/cm2)
(%)
(%)
FTEs (cm2)
-PCE (%)
0.04
ITO-PET
0.78 ± 0.01
15.66 ± 0.41
61.8 ± 1.6
7.54 ± 0.09
-
0.04
CTEs
0.78 ± 0.01
17.38 ± 0.37
59.5 ± 1.4
8.08 ± 0.11
9.45
0.20
CTEs
0.78 ± 0.01
16.38 ± 0.35
57.2 ± 0.9
7.32 ± 0.10
8.56
1.00
CTEs
0.78 ± 0.02
14.30 ± 0.47
47.5 ± 1.8
5.31 ± 0.09
6.26
2.00
CTEs
0.77 ± 0.02
14.05 ± 0.52
35.8 ± 1.6
3.91 ± 0.17
4.57
4.00
CTEs
0.73 ± 0.03
10.03 ± 0.48
31.8 ± 1.1
2.52 ± 0.16
2.95
0.04
ITO-PET
0.72 ± 0.00
15.84 ± 0.15
63.5 ± 0.7
7.22 ± 0.06
-
0.04
CTEs
0.71 ± 0.01
16.89 ± 0.28
50.4 ± 1.1
6.14 ± 0.12
7.23
PTB7-Th:PC71BM
PTB7:PC71BM
*The statistical results of 60 cells obtained from five batches.
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Graphical Abstract
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