Synergetic Transparent Electrode Architecture for Efficient

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Synergetic Transparent Electrode Architecture for Efficient Nonfullerene Flexible Organic Solar Cells with >12% Efficiency Yue-Xing Zhang, Jin Fang, Wei Li, Yang Shen, Jing-De Chen, Yanqing Li, Hongwei Gu, Sara Pelivani, Maojie Zhang, Yongfang Li, and Jian-Xin Tang ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.9b00970 • Publication Date (Web): 20 Mar 2019 Downloaded from http://pubs.acs.org on March 20, 2019

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ACS Nano

Synergetic Transparent Electrode Architecture for Efficient Nonfullerene Flexible Organic Solar Cells with >12% Efficiency

Yue-Xing Zhang,1 Jin Fang,2 Wei Li,1 Yang Shen,1 Jing-De Chen,1 Yanqing Li,1,* Hongwei Gu,2 Sara Pelivani,3 Maojie Zhang,2,* Yongfang Li,2 and Jian-Xin Tang1,*

1

Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow University, Suzhou 215123, Jiangsu China

2 College

of Chemistry, Chemical Engineering and Materials Science, Soochow

University, Suzhou 215123, Jiangsu, China 3

School of Physics, Trinity College Dublin, The University of Dublin, Dublin 2, Ireland

* Corresponding authors. E-mail addresses: [email protected] (Y.Q. Li), [email protected] (M. Zhang), [email protected] (J.X. Tang)

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ABSTRACT Flexible organic solar cells (OSCs) are considered one key component in wearable, intelligent electronics due to the unique capacity for highly flexible renewable energy sources. However, it is urgently required to enhance their efficiency as it is far inferior to that of their conventional, glass-based counterparts. To boost the performance of flexible OSCs on plastic substrates, we here present a synergetic transparent electrode structure, which combines the electrically conductive silver nanowires, the sol-gel-derived ZnO planarization layer, and the imprinted light-trapping nanostructures. This synergetic composite electrode exhibits the good properties in terms of optical transparency, electrical conductivity, mechanical flexibility, and low-temperature processability. As a result, the single-junction nonfullerene-based flexible OSCs achieve a power conversion efficiency exceeding 12% due to the synergetic interplay between broadband light trapping and suppressed charge recombination loss. Moreover, these flexible OSCs are repeatedly bendable in both inward and outward bending directions, retaining over 60% of the initial efficiency after 1000 cycles of the bending test at a 3.0 mm radius. These results convey a clear depiction of the practicality of flexible OSCs in a variety of highperformance flexible applications.

KEYWORDS: organic solar cell; nonfullerene solar cell; flexible transparent electrode; silver nanowires; light manipulation

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Flexible organic solar cells (OSCs) have attracted considerable attention for their use in foldable or roll-type applications that call for light weight and mechanical flexibility to be integrated with soft materials and curvilinear surfaces.1-6 However, significant efforts in improving various aspects of the devices are required for practical applications. An important task involving highly flexible OSCs is to find an alternative flexible transparent electrode (FTE), that demonstrates superior optical, electrical, and mechanical properties. Indium-tin-oxide (ITO) is typically used as a transparent electrode due to its high electrical conductivity and optical transmission. However, its brittle nature under repeated bending conditions is not compatible with wearable or foldable electronics.7 To circumvent this problem, several kinds of FTEs have been proposed to replace ITO in flexible OSCs, such as carbon-based materials (e.g., graphene, nanotube, and conductive polymers),7-10 metallic nanostructures (e.g., nanowires, mesh, metal/dielectric multilayer),11-17 and hybrid composite electrodes.18-20 Amongst them, metal nanowires, particularly silver nanowires (AgNWs), hold great promise as an effective FTE for flexible OSCs due to their superior mechanical flexibility, electrical conductivity, optical transparency, and scalable solution processability.11-15 Regardless of the high mechanical flexibility mentioned above, the efficiency of these state-of-the-art flexible OSCs is still lags behind that of their rigid glass-based counterparts. To date, most of the best power conversion efficiencies (PCEs) were achieved on rigid glass substrates due to rapid progress in material synthesis and device engineering.7-24 For instance, the PCEs of single-junction OSCs have recently exceeded 15%.25,26 However, full realization of such a potential is challenging for flexible OSCs on plastics.27 Further enhancement in the efficiency is thus regarded as a matter of urgency for flexible OSCs when concerning the viability of their applications.

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To maximize the light harvesting efficiency in the OSCs, the control over film morphology and interface engineering is the commonly used method. However, a quantitative analysis of the energy losses in OSCs indicated one inherent limit of these devices to be insufficient absorption, which contributes to 20-30% of the energy loss due to the use of an ultrathin photoactive layer.28 Recently, the manipulation of the incident light with periodic or random structures has been regarded as an efficient route to capturing more solar radiation in the ultrathin photoactive layers for enhancing the photocurrent generation. In this regard, various methods have been proposed to manipulate the incident light by increasing the internal light scattering, reducing the external antireflection, or exciting the surface plasmon effect in the photoactive layers.2935

For instance, the introduction of biomimetic nanostructures (like moth-eye

nanostructures) into OSCs has been demonstrate as an effective strategy for light manipulation, which is capable of enhancing the light absorption and thereby the larger photocurrent due to plasmonic resonance effect or light scattering.30,32 However, the light manipulation schemes should be compatible with flexible devices, enabling a substantial increase in the efficiency of flexible OSCs.36 Particularly, the direct nanopatterning of the active layer may inevitably increase the difficulty in the morphology control and induce the negative effect on the phase separation during the film formation. According to previous reports,37-40 the robust film property of sol-gel-derived ZnO layers can serve as a perfect “pattern acceptor”, which can definitely avoid the film destruction when spincoating the active layers. In this work, we explore an efficient FTE architecture that is fully compatible with flexible OSCs on plastic substrates. The key feature of this FTE is based on a synergetic interplay of an AgNWs-based network for electrical conduction and a moth-eye

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nanopatterned ZnO layer for broadband light trapping (Figure 1a-b). This combination leads to the enhanced light harvesting efficiency while still retaining the form-factor advantages of AgNWs, such as conductivity and transparency. As a result, nonfullerenebased single-junction flexible OSCs on polyethylene terephthalate (PET) yield optimal PCE values of 10.91 % and 12.02 % for the PBDB-T:ITIC and PM6:IT-4F-based photoactive layers (see the chemical structures in Figure 1c and the full names of these materials in METHODS). These results are the highest values amongst those using metal nanowires as a FTE. Furthermore, flexible OSCs with this FTE reveal a high resistance to flexural strain under repeated bending testing. Detailed analysis of the bending attenuation mechanism is also conducted for the entire device configuration.

RESULTS AND DISCUSSION Figure 2a displays an atomic force microscopy (AFM) image of the solutionprocessed AgNWs on PET substrate. The AgNWs show a diameter of ~20 nm and the length typically of 20-30 μm. The spin-coating and heating treatment of the AgNWs solutions were repeated twice to improve the uniformity of randomly distributed AgNWs (see METHODS for further details). It is known that there is a close inverse correlation between sheet resistance (Rs) and optical transmittance of AgNWs-based films, which are highly dependent on the film fabrication conditions.11-14,41 After a thorough comparison (Supporting Figure S1), an averaged optical transmittance of ~93 % (from 400 nm to 800 nm) and an averaged Rs of ~41 Ohm sq-1 are obtained for the bare AgNWs film at an optimal condition of the diluted AgNWs by ethanol in volume (1:1.8, v/v). However, the electrical conductivity of these AgNWs films is not good enough to replace ITO (its Rs is usually 15 Ohm sq-1). As reported previously,42 the high Rs of bare

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AgNWs was ascribed to the insulating surface ligands of polyvinylpyrrolidone (PVP), which inevitably wrapped the AgNWs during the synthetic process and resulted in high junction contact resistance at AgNWs-PVP-AgNWs interfaces. Accordingly, sodium borohydride (NaBH4) solution treatment was imposed on the AgNWs to remove the residual PVP,42 leading to a decrease in Rs of AgNWs films (~28 Ohm sq-1) without sacrificing their light transmittance (Supporting Figure S1b).

Figure 1. a) Schematic of a flexible OSC with moth-eye nanopatterned AgNWs/ZnO FTE on PET substrate. b) A photograph of the fabricated cell. c) Chemical structures of PBDB-T, ITIC, PM6, and IT-4F.

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Figure 2. a-c) AFM images of a) bare AgNWs, b) AgNWs/2-ZnO, and c) AgNWs/3-ZnO on PET substrates. Scale bar: 500 nm. The RMS roughnesses are 8.4 nm, 5.4 nm, and 4.6 nm, respectively. d) Total optical transmittance spectra and haze values of ITO and various AgNWs-based electrodes. All transmittances are inclusive of the glass and PET substrates.

Besides, the rough surface of solution-processed AgNWs is detrimental to the interfacial contact with the overlayers in OSCs because of the possible interelectrode shorting induced by the protuberant nodes of AgNWs.43 One facile method to alleviate the surface roughness of the AgNWs film includes the planarization with a relatively thick conductive polymer or metal oxide layer, forming the AgNWs composite electrodes with a smooth surface and high electrical conductivity.11,13,14,44 Therefore, the sol-gel-

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derived zinc oxide (ZnO) has been extensively adopted as a planarization layer due to its high electron mobility, wide optical bandgap, suitable interface energetics, and lowtemperature solution processability.45,46 Here, the sol-gel-derived ZnO layers were repeatedly spun on AgNWs for the smoothening of their extremely coarse surface. Figure 2 and Supporting Figure S2 show the AFM images and scanning electron microscopy (SEM) images of the AgNWs coated with double and triple ZnO layers, which are hereafter referred to as 2-ZnO and 3-ZnO, respectively. Bare AgNWs (Figure 2a) are randomly distributed and overlapped to each other, producing many pitch points with a root-mean-square (RMS) roughness of ~8.4 nm. On the contrary, the AgNWs appear with a more nebulous morphology after the coating of 2-ZnO (~80 nm) and 3-ZnO (~120 nm) (Figures 2b and 2c), showing a decrease in the RMS roughness, declining down to 5.4 nm and 4.6 nm, respectively. In addition, the sol-gel-derived ZnO layer can easily penetrate into the porous AgNWs network to fill up the empty spaces and tighten their connections. This will be beneficial for the carrier collection through these extra charge transfer channels.11 Figure 2d displays the optical transmittance spectra for the AgNWs-based electrodes on PET substrates. For comparative purposes, the optical property of the ITO-coated glass substrate is shown. It is noted that the AgNWs/2-ZnO and AgNWs/3-ZnO FTEs display almost the same optical transmittance (>80%) over entire visible wavelength range, inferring the light insensitivity for the ZnO layer thickness. Moreover, the haze values of different electrodes were obtained by measuring the specular and diffuse transmission. It is presented in Figure 2d that both AgNWs/2-ZnO and AgNWs/3-ZnO electrodes exhibit a higher haze value than that of the ITO-glass substrate in the spectral range of 450-600 nm. The transmittance haze corresponds to the amount of light

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scattering relative to the total transmitted light. The higher haze values indicate a stronger light scattering capability of this AgNWs-based electrode, implying a potential of enhancing the light trapping in the OSCs.47

Figure 3. a-c) SEM images of a) AgNWs/2-ZnO, b) AgNWs/3-ZnO, and c) PBDBT:ITIC active layer patterned with moth-eye nanostructures. d-f) AFM topography morphologies and g-i) relevant surface profiles of the corresponding films.

The light manipulation nanostructures were further introduced to increase the trapping of the incident light in the photoactive layers for the absorption enhancement, since these sub-wavelength structures could benefit the light harvesting over the entire solar spectral range.48 By adopting a soft nanoimprint lithography technique, the quasiperiodic moth-eye nanostructures were replicated onto the ZnO layer through the imposition of compressive stress onto the polydimethylsiloxane (PDMS) mold (see

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METHODS for further details of the fabrication process). Figure 3a-f display the SEM and AFM images of the patterned 2-ZnO, an additional ZnO layer coated on patterned 2ZnO, and subsequently spin-coated photoactive layer, respectively. The corrugated morphologies of these layers indicate an effective transfer of the moth-eye nanostructures to the ZnO surface. Evidently, the moth-eye nanostructures are well maintained by an approximate 110 nm-thick photoactive layer, with the exception of the gradual smoothing of the groove depth. Figure 3g-i display that the depth profile of the patterned 2-ZnO exhibits an quasi-period of ~460 nm and an average groove depth of ~60 nm, while the groove depths of the patterned 3-ZnO and photoactive layer are gradually decreased to ~40 nm and ~15 nm. To assess the feasibility of the patterned AgNWs/ZnO FTEs, flexible OSCs were constructed with a device architecture of PET/AgNWs/ZnO/photoactive layer/MoO3/Ag (Figure 1a), where MoO3 and Ag were utilized as a hole extraction layer and anode. Figure 4a-b plot the current density-voltage (J-V) curves of flexible OSCs using PBDBT:ITIC and PM6:IT-4F blends as a photoactive layer, respectively, under 100 mW cm-2 air mass 1.5 global (AM 1.5G) illumination. The rigid device on ITO-glass substrate was also fabricated as a reference. Table 1 lists the photovoltaic parameters of all the devices deduced from the J-V characteristics. It is worth noting that the use of the AgNWs/3-ZnO electrode results in a substantial increase in short-circuit current density (JSC), fill factor (FF), and PCE as compared to the case of AgNWs/2-ZnO. However, the open-circuit voltage (VOC) is kept at the similar value. For example, the JSC and FF are increased to 16.89 mA cm-2 and 68.11% for the PBDB-T:ITIC-based flexible OSC on AgNWs/3ZnO, while those of the PM6:IT-4F-based flexible OSC were optimized to 19.50 mA cm2

and 66.30%. The resulting PCEs were enhanced to 10.24% and 10.99%, respectively. In

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addition, the device performance of flexible OSCs on AgNWs/3-ZnO is comparable to the rigid device fabricated on the ITO-glass substrate.

Figure 4. a) J-V curves of PBDB-T:ITIC-based OSCs. b) J-V curves of PM6:IT-4Fbased OSCs. c) EQE spectra (solid symbols) and integrated JSC (open symbols) of PBDBT:ITIC-based OSCs. d) EQE spectra (solid symbols) and integrated JSC (open symbols) of PM6:IT-4F-based OSCs. e) Total light transmittance spectra (solid symbols) and haze

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values (open symbols) of flat and nanopatterned electrodes. f) Absorption spectra and relative enhancement of flat and nanopatterned electrodes.

Table 1. Detailed photovoltaic parameters of flexible OSCs on PET as a comparison with the rigid devices on ITO-glass substrates. Active layer

VOC

JSC

FF

PCEb)

[V]

[mA cm-2]

[%]

[%]

ITO-glass

0.91

16.50

68.33

10.28 (9.82)

AgNWs/2-ZnO

0.89

16.13

60.57

8.69 (8.41)

AgNWs/3-ZnO

0.89

16.89

68.11

10.24 (10.02)

AgNWs/3-ZnOME a)

0.90

18.67

65.32

10.91 (10.41)

ITO-glass

0.85

19.86

66.64

11.25 (11.03)

AgNWs/2-ZnO

0.85

17.73

62.86

9.47 (9.01)

AgNWs/3-ZnO

0.85

19.50

66.30

10.99 (10.34)

AgNWs/3-ZnOME a)

0.84

22.26

64.66

12.02 (11.69)

Electrode

PBDB-T:ITIC

PM6:IT-4F

a)

The ZnO layers were patterned with moth-eye nanostructures.

b)

The data in parenthesis were the average PCE values obtained from 20 devices.

When compared to those of the flat devices, the patterned AgNWs/3-ZnO with the moth-eye nanostructures (referred to as AgNWs/3-ZnOME) induces the further increase in JSC accompanied with negligible difference in VOC and FF. As a result, flexible OSCs on AgNWs/3-ZnOME yield the maximum PCEs of 10.91% and 12.02% for the PBDBT:ITIC- and PM6:IT-4F photoactive layers, which are the highest values amongst previously reported flexible OSCs.27 Given that the same batch processing was used for each series of the photoactive layers, it is inferred that the enhanced JSC and PCE are

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related to the improved photocurrent generation in the photoactive layers caused by the incorporation of the moth-eye nanostructures. To divulge the reason of the JSC enhancement, external quantum efficiency (EQE) spectra of flexible OSCs on AgNWs/3-ZnO and AgNWs/3-ZnOME were measured. It is evident in Figure 4c and 4d that flexible OSCs on AgNWs/3-ZnOME exhibit the enhanced EQEs relative to the flat devices over whole visible wavelength region. The integrated JSC values that were deduced from the EQE spectra are 17.75 mA cm-2 and 21.10 mA cm-2 for the PBDB-T:ITIC- and PM6:IT-4F-based flexible OSCs on AgNWs/3-ZnOME, respectively. These values are larger than those of AgNWs/3-ZnO (e.g., 16.14 mA cm-2 for PBDB-T:ITIC-based device and 18.62 mA cm-2 for PM6:IT-4F-based device). It is also noted that the integrated JSC values agree quite well with those directly measured from the J-V curves. These results clearly show that the use of the moth-eye nanostructures can improve the device performance due to broadband light harvesting enhancement. To further clarify the origin of the EQE enhancements, the optical properties of these electrodes are compared in Figure 4e. Note that the AgNWs/3-ZnOME electrode shows the increase in light scattering over a wide range from 400 to 850 nm, while its optical transmittance resembles that of the AgNWs/3-ZnO electrode. The absorption spectra of these OSCs were obtained from the transmittance (T) and reflectance (R) spectra by (1-R-T). Figure 4f clearly shows a broadband absorption enhancement for the use of the AgNWs/3-ZnOME electrode. It is thus inferred that the AgNWs/3-ZnOME electrode can strengthen the light trapping effect inside the photoactive layer through the enhanced light scattering.

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To gain more insight into the optical effects of the AgNWs/3-ZnOME electrode on device performance, the light behaviors in the organic photoactive layers were simulated with the finite-difference-time-domain (FDTD) method (see METHODS for further details). Figure 5 presents the cross-section near-field profiles of transverse electric (TE) polarized light at 550 and 800 nm in flexible OSCs with and without the moth-eye nanostructures. Supporting Figure S3 shows the cases of transverse magnetic (TM) polarized light. The simulation results provide an intuitionistic image on the performance improvement of flexible OSCs on AgNWs/3-ZnOME. Compared to the flat device, the more intensive field distributions can be observed in the patterned photoactive layers for either TE or TM polarized light. According to previous literature reports,32,33,49,50 the corrugated metal back electrode can excite the surface plasmonic effect at the patterned interfaces, contributing to the simultaneous increase in light scattering and diffusive reflection. As a consequence, the trapping and absorption of the incident light inside the photoactive layers can be boosted for flexible OSCs on the AgNWs/3-ZnOME electrode due to the nanopattern-induced light trapping enhancement. To estimate the flexural mechanical properties of these flexible OSCs, bending tests were performed by repeatedly bending the devices in two directions at a radius of curvature (RC) of 3.0 mm on a custom-made cyclic bending tester (Figures 6a and 6b). Figures 6c-6f illustrate a plot of the variation of normalized photovoltaic parameters of flexible OSCs on AgNWs/3-ZnO and AgNWs/3-ZnOME as a function of the bending cycles. It is shown that flexible OSCs on AgNWs/3-ZnO and AgNWs/3-ZnOME exhibit a similar performance degradation. For example, the steady VOC value and a slight decrease in JSC are observed when increasing the bending cycles for two bending directions.

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Contrary to this, the PCE decay (Figure 6f) is mainly determined by the decline of the FF (Figure 6e).

Figure 5. Simulated cross-section near-field profiles in a flat device structure for TE polarized light at a) 550 nm and b) 800 nm. Near-field profiles in moth-eye nanopatterned device for TE polarized light at c) 550 nm and d) 800 nm.

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Figure 6. Flexural mechanical properties of flexible OSCs without and with moth-eye nanopatterns. a) Schematics of inward and outward bending tests. b) Photographs of a flexible OSC bent at Rc = 3 mm on a cyclic bending tester. c-f) Relative decays of c) VOC, d) JSC, e) FF, and f) PCE as a function of the bending cycles.

To understand the origin of the mechanical degradation, the changes of Rs and film morphologies under the repeated bending were characterized for various functional

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layers. Figure 7a shows the ratios of Rs values relative to the initial ones (R0) with respect to the repeated bending cycles at a RC of 3.0 mm. There is negligible change in Rs for the AgNWs-based electrodes due to their superior mechanical flexibility. However, it is noteworthy that the AgNWs/3-ZnO electrode can withstand the good flexural strain after 1000 bending cycles in both inward and outward directions, since the oxide layers (e.g., ZnO) are usually regarded as brittle and easy to fracture. Furthermore, the presence of moth-eye nanostructures has negligible influence on the flexibility of the AgNWs/3-ZnOME electrode after the repeated bending (Supporting Figure S4). However, a drastic rise in R/R0 can be observed for the Ag electrode bent in the inward direction, which is significantly different from that of the outward bending test. SEM images of various layers in Figure 7b reveal that hair cracks are rarely observed in either the AgNWs/3-ZnO electrode or the photoactive layer after 1000 cycles, regardless of the bending directions. In contrast, more discernible cracks appeared on the top Ag electrode after the repeated bending, leading to a significant degree of film disconnection. Interestingly, there were only a few incomplete cracks that materialized on the Ag surface in the outward bending situation. Such a difference can be ascribed to the larger compressional force taking place on the Ag electrode when subject to the inward bending condition. These results provide direct evidence of the Rs changes observed in Figure 7a. As a consequence, the decay in FF and PCE observed in Figure 6 originates from the bending-induced geometrical deformation of the top Ag electrode and the Rs increase of the devices under the repeated bending, resulting in the degenerated charge transport and the decline in FF. Further research is needed to improve the flexibility of Ag film-based electrode for flexible OSCs by considering the adhesion between Ag film and underlying MoO3 layer, or the deposition methods for Ag films. Nevertheless, the proposed 17


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AgNWs/3-ZnOME structure offers the necessary feasibility as a FTE for flexible OSCs, which can remain intact and operate well under the repeated bending conditions. In addition, it should be mentioned that flexible OSCs with flat and patterned AgNWs exhibited the almost identical decay in PCE when they were stored in the same environment (Supporting Figure S5).

Figure 7. a) The relative resistance changes of various layers as a function of the bending cycles for inward and outward bending tests. b) SEM images of AgNWs/3-ZnO, PBDBT:ITIC layers and Ag electrodes after 1000 bending cycles in either inward or outward direction.

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CONCLUSIONS In conclusion, an ideal AgNWs-based FTE architecture has been fabricated for the demonstration of high-efficiency flexible OSCs. A substantial increase in light harvesting efficiency is achieved due to a synergetic interplay between the AgNWs-based network for electrical conduction and a nanopatterned ZnO layer for broadband light trapping. During the collective optimization process of this electrode structure, flexible OSCs yield optimal PCE values of 10.91 % and 12.02 % for PBDB-T:ITIC and PM6:IT-4F photoactive layers, respectively. Moreover, this AgNWs-based electrode retains its high resistance to flexural strain under the repeated bending testing. In light of the level of efficiency and bendability exhibited, we anticipate that this flexible electrode architecture could stimulate a more emotive desire for high-performance, flexible OSCs to be utilized within commercially viable photovoltaic applications. Moreover, this AgNWs-based FTE architecture can also be applied to other optoelectronic devices such as organic and perovskite light-emitting diodes.

METHODS AgNWs-based Electrode and Patterning. The original AgNWs (GU’S NEW MATERIAL, a diameter of ~20 nm, a length of 20-30 μm) were dispersed in ethanol (5 mg mL-1), and further diluted by ethanol in volume so as to improve their optical and electrical properties. The diluted AgNWs dispersion was spin-coated at 1500 rpm for 20 s on UV-treated PET substrate, which was followed by a heating treatment at 100 °C for 5 min. Analogously, the coating of AgNWs dispersion was repeated once more to improve

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the uniformity of the randomly distributed AgNWs. To further increase the electrical conductivity of the AgNWs-based electrode, the AgNWs-coated PET substrates were immersed in NaBH4 solution (0.5 M NaBH4 dissolved in the mixed solution of deionized water and ethanol, 1:1 in volume), followed by consecutive rinsing with ethanol and drying in air. The sol-gel-derived ZnO precursor solution was prepared by dissolving zinc acetate dehydrate (Sigma-Aldrich, 99.0%, 220 mg) in the mixture of 2-methoxyethanol (TCL, 99.0 %, 2 mL) and ethanolamine (Aladdin, 99.0 %, 61 μL) with the stirring for 12 h for the hydrolysis reaction under the ambient conditions. The ZnO precursor solution was spin-coated on the AgNWs-coated PET substrate at 3000 rpm for 40 s, and annealed at 150 °C for 10 mins to form a dense ZnO film. To effectively planarize the AgNWs network, the spin-coating procedures of ZnO were repeated two or three times. The patterning of moth-eye nanostructures on AgNWs-based FTE was conducted by soft nanoimprint lithography, where the PDMS mold was prepared as previous reports.32,51 In order to transfer the moth-eye nanostructures to the ZnO surface, the PDMS mold was placed on the unannealed sol-gel-derived ZnO layer under continuous pressure for 40 s, and then a post-annealing process at 150 °C for 10 s was followed.

OSC Fabrication. OSCs were fabricated on AgNWs-coated PET and ITO-glass substrates, respectively. The ITO-glass substrates with a Rs of 17 Ω sq-1 were successively ultrasonic-cleaned using acetone, ethanol, and deionized water for 15 mins, which were then dried in an oven at 110 °C. The new cleaned ITO-glass substrates were further treated with UV ozone for 15 min before the device fabrication. The sol-gelderived ZnO layer was formed on the ITO under the same conditions as mentioned above. All the resulting samples were transferred into a nitrogen-filled glove box for

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depositing the photoactive layers. The main materials used in the photoactive layers have the following acronyms: PBDB-T: poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b’]dithiophene))-alt-(5,5-(1’,3’-di-2-thienyl-5’,7’-bis(2ethylhexyl)benzo[1’,2’-c:4’,5’-c’]dithiophene-4,8-dione))].

ITIC:

2,2′-[[6,6,12,12-

Tetrakis(4-hexylphenyl)-6,12-dihydrodithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6b′]dithiophene-2,8-diyl]

bis[methylidyne(3-oxo-1H-indene-2,1(3H)-diylidene)]]

bis[propanedinitrile]. PM6: poly[(2,6-(4,8-bis(4-fluoro-5-(2-ethylhexyl)- thiophen-2-yl)benzo[1,2-b:4,5-b’]dithiophene))-alt-(5,5-(1’,3’-di-2-thienyl-5’,7’-bis(2ethylhexyl)benzo[1’,2’-c:4’,5’-c’]dithiophene-4,8-dione))]. IT-4F: 3,9-bis(2-methylene((3-(1,1-dicyanomethylene)-6,7-difluoro)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)dithieno[2,3-d:2’,3’-d’]-s-indaceno[1,2-b:5,6-b’]dithiophene. PBDB-T and ITIC were purchased from Solarmer Materials Inc, Beijing, and were not tampered with before use. PM6 and IT-4F were synthesized in our lab as reported previously.52 The solutions of the photoactive layers were prepared by dissolving PBDB-T:ITIC (1:1 by weight, 14.4 mg mL-1) in chlorobenzene (CB) with 0.8 vol % of 1,8-diiodooctane (DIO) additives, or by dissolving PM6:IT-4F (1:1 by weight, 20 mg mL-1) in a mixture of CB and DIO (99.5:0.5 by volume). The photoactive layers were spin-coated onto ZnO at 1000 rpm for 1 min, and thermally annealed at 100 °C for 3 mins (corresponding to a nominal thickness of ~110 nm). Finally, the samples were transferred into an interconnected evaporation chamber with a high vacuum of 2 × 10-6 Torr. To complete the device fabrication, a MoO3 layer (8 nm) and a Ag film (100 nm) were thermally deposited with a shadow mask to form the anode for hole extraction. The active area was determined to be 0.0725 cm-2. To guarantee the consistent results, flat and patterned devices were fabricated in the same batch for the deposition of the photoactive layers and the MoO3/Ag anode. 21


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Film and Device Characterizations. Sheet resistance was measured by a ST-2258A multifunction digital four-probe tester. Absorption, transmittance and reflection spectra were recorded through a UV/vis/near-IR spectrometer (Perkin Elmer, Lambda 950), with an integrating-sphere accessory. Surface morphologies were characterized by atomic force microscopy (AFM, Veeco MultiMode V) in the tapping mode, as well as the scanning electron microscopy (SEM, Zeiss, Gemini 500). The layer thickness, frequencydependent refractive index (n), and extinction coefficient (k) values were obtained by an alpha-SE™ Spectroscopic Ellipsometry, with an angle of incidence at 70°. The photovoltaic characteristics of flexible OSCs were recorded with a programmable Keithley 2612 source measurement unit under AM 1.5G illumination from a solar simulator (Oriel model 91160, 100 mW cm-2) at room temperature in air. The light intensity of the solar simulator was calibrated by a standard silicon solar cell. The EQE spectra were measured with a photo-modulation spectroscopic setup (Solar Cell Scan 100, Zolix Instruments Co. Ltd.), and the power-density calibration was performed by a traceable silicon photodiode.

Optical Simulation. Near-field distributions of TE and TM polarized incident light in the OSCs were simulated based on the FDTD method (Lumerical FDTD Solutions 8.7.3) in the combination with the in-house generated codes. The period and depth of the patterned nanostructures were referred to the date determined from the AFM images. The thicknesses, n, and k values of different layers were determined by ellipsometry. In the simulation, the AgNWs network was ignored so as to have an intuitive investigation on the effect of nanopatterns on light harvesting. All the related optical simulations were

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conducted with the same modeling parameters and the monitor was positioned in the same place in the OSCs as to conduct a reasonable analysis.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org. Optical and electrical properties of AgNWs, SEM images of AgNWs and AgNWs/ZnO layers, Calculated field distributions for TM polarized light, the changes of the electrode sheet resistance under bending bests, and the stability measurements.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (Y.Q. Li), [email protected] (M. Zhang), [email protected] (J.X. Tang)

ACKNOWLEDGMENTS This work is financially supported by the National Natural Science Foundation of China (Nos. 91633301, 61520106012, 61722404, 51873138, 51573120), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), Collaborative Innovation Center of Suzhou Nano Science & Technology.

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Materials for High Performance Non-Fullerene Polymer Solar Cells with 13.5% Efficiency. Sci. China Chem. 2018, 61, 531-537.

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Synergetic Transparent Electrode Architecture for Efficient

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