Efficient and Mechanically Robust Ultraflexible Organic Solar Cells Based on Mixed Acceptors

Article

Efficient and Mechanically Robust Ultraflexible Organic Solar Cells Based on Mixed Acceptors Wenchao Huang, Zhi Jiang, Kenjiro Fukuda, Xuechen Jiao, Christopher R. McNeill, Tomoyuki Yokota, Takao Someya [email protected] (K.F.) [email protected] (T.S.)

HIGHLIGHTS A facile strategy to simultaneously improve efficiency and mechanical stability An efficiency of 13% achieved in 3mm-thick ultraflexible organic solar cells A 97% efficiency retention after 1,000 cycles of a bending test An 89% efficiency retention after 1,000 cycles of a compressionstretching test

A simple strategy to simultaneously improve power conversion efficiency (PCE) and mechanical stability of ultraflexible organic solar cells is reported. By using a fullerene/non-fullerene mixed acceptor, 3-mm-thick ultraflexible organic solar cells achieve a PCE of 13% (a certified value of 12.3%) with 97% PCE retention after 1,000 bending cycles and 89% PCE retention after 1,000 compression-stretching cycles.

Huang et al., Joule 4, 1–14 January 15, 2020 ª 2019 Elsevier Inc. https://doi.org/10.1016/j.joule.2019.10.007

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Article

Efficient and Mechanically Robust Ultraflexible Organic Solar Cells Based on Mixed Acceptors Wenchao Huang,1,2,3,4,6 Zhi Jiang,1,3,6 Kenjiro Fukuda,2,3,* Xuechen Jiao,4,5 Christopher R. McNeill,4 Tomoyuki Yokota,1 and Takao Someya1,2,3,7,*

SUMMARY

Context & Scale

Flexible organic solar cells (OSCs) with high power conversion efficiency (PCE) and excellent mechanical properties are considered a promising power source for wearable electronic devices. However, simultaneously achieving high efficiency and robust mechanical stability is still challenging because highly crystalline or aggregated microstructures that are thought to be critical for enabling efficient device operation render the active layer brittle. In this study, we demonstrate 3-mm-thick ultraflexible OSCs by utilizing a mixed fullerene/ non-fullerene acceptor that can achieve an efficiency of 13% (certified value of 12.3%) with 97% retention in the PCE after 1,000 bending cycles (bending radius of 0.5 mm). In addition, although ultraflexible OSCs cannot survive under the intrinsic tensile test with a large strain, they exhibit excellent mechanical behavior under the cyclic compression-stretching test via the formation of a buckling device structure, yielding an 89% retention in the PCE after 1,000 cycles (45% compression and bending radius of 10 mm). A facile approach introducing a small amount of high-electron-mobility fullerene acceptor into a non-fullerene binary blend enhances charge transport, improves exciton separation, and optimizes the blend morphology with more amorphous regions, thus producing a more efficient and mechanically robust device.

Ultraflexible organic solar cells (OSCs) are considered a promising power source for wearable electronic systems owing to their robust mechanical properties, low cost, and light weight. The proper selection of active layer materials and the optimization of morphology are two key factors to simultaneously achieve high efficiency and robust mechanical stability in ultraflexible OSCs. Here, a facile approach that employs a fullerene/non-fullerene mixed acceptor is reported to enhance charge separation and transport and to optimize the morphology of the active layer with more amorphous regions. By using this strategy, 3-mm-thick ultraflexible OSCs achieve a power conversion efficiency (PCE) of 13% with a certified value of 12.3%. In addition, ultraflexible OSCs exhibit a 97% retention in the PCE after 1,000 cycles of a bending test with a bending radius of 0.5 mm and an 89% retention in the PCE after 1,000 cycles of a compression-stretching test with a bending radius of 10 mm.

INTRODUCTION Ultralightweight and ultraflexible photovoltaic devices that can be conformably adhered to complex curved surfaces are considered as a promising power source for wearable electronic systems powering different functional devices such as sensors, actuators, and displays.1–4 Among various photovoltaic technologies, organic solar cells (OSCs) based on p-conjugated organic semiconductors are the most suitable candidate owing to their mechanical flexibility, low cost, light weight, and ease of large-area coating via solution processing.5–7 Since the introduction of the bulk-heterojunction (BHJ) structure in 1995,8,9 impressive progress in the field of organic photovoltaics has been realized through the development of novel materials and the optimization of active layer morphology. Indeed, the power conversion efficiency (PCE) of single-junction OSCs has surpassed beyond 16%.10–14 Because of their excellent mechanical flexibility, OSCs fabricated on flexible substrates have attracted significant attention.15 Device PCE and mechanical properties are two key parameters for flexible OSCs. Several strategies have

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been proposed to improve these two aspects. First, various low-temperatureprocessed novel electrodes (such as conducting polymers, carbon-based materials, and silver nanowires) and polymeric interfacial layers (such as PEDOT:PSS, PFN, and PEIE) have been developed to enhance the mechanical properties and facilitate charge extraction.16–18 Next, proper active layer selection and morphology control have been identified as important in determining the device efficiency and mechanical stability.19,20 A pure crystalline phase is thought to be necessary for efficient charge transport, while a mixed phase is thought to play an essential role in facilitating exciton separation.21,22 Optimization of the relative volume fractions of the pure phase and mixed phase is necessary for achieving a high efficiency. Moreover, highly aggregated or crystalline materials such as fullerene derivatives exhibit higher stiffness; therefore, using more ductile materials or increasing the amorphous region in the blend can facilitate the dissipation of mechanical stress in flexible devices for robust mechanical stability.20,23–26 With the rapid development of non-fullerene acceptors, state-of-the-art flexible OSCs have achieved an uncertified efficiency of 12.5% on polyethylene terephthalate (PET) substrates with a PCE retention of 90% after 2,000 bending cycles with a millimeter-level bending radius.27 Recently, to achieve better conformability, OSCs with reduced thickness (less than 10 mm) have been fabricated on ultrathin parylene or transparent polyimide substrates.28,29 Stretchability can be achieved by transferring the ultraflexible device to a pre-stretched elastomer. Compared with the conventional bending test, OSCs subjected to cyclic compression stretching are found to exhibit a significantly accelerated degradation in device performance.17 The poor mechanical stability under compression is owing to the large strain (with a micrometer-level buckling bending radius) under compression. The best-performing mechanically durable ultraflexible OSCs under a bending radius of 10 mm exhibit a PCE retention of 74% after 500 cycles of compression.17 However, the realization of ultraflexible OSCs that can simultaneously achieve high efficiency and excellent mechanical properties especially under large strain is still challenging. A major obstacle is that compared with fullerene acceptors, nonfullerene acceptors usually exhibit relatively low electron mobilities; therefore, highly crystalline phases with enhanced intermolecular p-p stacking are required for efficient charge transport.5,12 Improving the crystallinity—a popular method to enhance charge carrier transport—results in unfavorable phase separation and increased brittleness, which is detrimental to exciton separation and the mechanical properties of an active layer film.25,30 1Department

In this study, we demonstrate a 3 mm ultraflexible ternary OSC employing a mixed fullerene/non-fullerene acceptor with carefully controlled morphology that can achieve a certified PCE of 12.3% and demonstrate excellent mechanical properties under both bending and compression tests. To the best of our knowledge, this is the highest certified PCE for flexible or ultraflexible OSCs. In addition to achieving a high PCE in both small-area (13% for 0.04 cm2) and large-area devices (11.6% for 1 cm2), this ultraflexible OSC exhibits excellent mechanical properties with a PCE retention of 97% after 1,000 cycles of bending test (with a millimeter-level bending radius) and PCE retention of 89% after 1,000 cycles of compression-stretching test (with a micrometer-level bending radius), significantly outperforming previous high-efficiency flexible devices.27,28,31 This improved device efficiency and mechanical stability are achieved by adding a small amount of a fullerene-based acceptor into a polymer/non-fullerene system based on the thermally stable

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of Electrical Engineering and Information Systems, the University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan

2Center

for Emergent Matter Science, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan

3Thin-Film

Device Laboratory, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan

4Materials

Science and Engineering, Monash University, Clayton, VIC 3168, Australia

5Australian

Synchrotron, ANSTO, 800 Blackburn Rd, Clayton, VIC 3168, Australia

6These 7Lead

authors contributed equally

Contact

*Correspondence: [email protected] (K.F.), [email protected] (T.S.) https://doi.org/10.1016/j.joule.2019.10.007

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polymer donor PBDTTT-OFT32 and the low bandgap acceptor IEICO-4F.33,34 Fullerene-based acceptors are typically considered to present negative effects on mechanical properties owing to the formation of stiff aggregates.23 From this study, we found that the addition of a proper amount of PC71BM molecules slightly disrupted the crystallization of IEICO-4F, and PC71BM molecules were located in the amorphous region without the aggregation formation, thus facilitating exciton separation and improving film ductility simultaneously. Although more of the amorphous phase is observed in the ternary blend, the electron mobility in the ternary blend does not show any decrease owing to the intrinsic high mobility of PC71BM. Using a mixed fullerene/non-fullerene acceptor provides a new pathway for realizing mechanically robust, high-efficiency, and flexible OSCs for wearable devices.

RESULTS AND DISCUSSION Device Performance and Device Physics The molecular structures and energy levels of the materials used in the ultraflexible OSCs (PBDTTT-OFT, IEICO-4F, and PC71BM) are shown in Figures 1A and 1B. The three components show complementary absorption spectra that fully cover the whole visible region and extend to the near infrared (NIR) region (Figures 1C and S1). OSCs were first fabricated on supporting glass substrates with an inverted device architecture of parylene (1 mm in thickness)/Su8/ indium tin oxide (ITO)/ZnO/active layer/MoO3/Ag (Figure 1D). Subsequently, after being encapsulated with another layer of 1-mm-thick parylene as a protection layer, the ultraflexible OSCs were delaminated from the supporting glass substrate, see Figure S2. The total thickness of our device is less than 3 mm; thus, the device demonstrates excellent flexibility (Figure 1E). The active layer is sandwiched between two layers of parylene to minimize strain in the active layer under mechanical deformation.35 Figure 1F presents the current density-voltage (J-V) characteristics measured from the best-performing devices based on different active layers (efficiency histogram shown in Figure S3), and Table 1 summarizes the corresponding photovoltaic parameters. Devices that are fabricated based on PBDTTT-OFT:IEICO-4F and PBDTTT-OFT:PC71BM binary blends demonstrate the highest efficiencies of 11.9% and 9.5%, respectively. The PBDTTT-OFT:IEICO-4F device exhibits a shortcircuit current (Jsc) of 24.7 mA/cm2, an open-circuit voltage (Voc) of 0.71 V, and a fill factor (FF) of 0.68. The substitution of 20 wt % IEICO-4F with PC71BM results in the most significant improvement in device performance (Figure S4). The highest PCE of 13.0% is achieved for devices based on the PBDTTT-OFT:IEICO4F:PC71BM (1:1.2:0.3) ternary blend. The Jsc, Voc, and FF of the optimized ternary system increased to 26.1 mA/cm2, 0.72 V, and 0.69, respectively. Devices fabricated on the ultraflexible substrate are comparable to those fabricated on the rigid substrate (Figure S5). The champion ternary device was certified by the Japan Electrical Safety & Environment Technology Laboratories (JET) with a PCE of 12.3%, as shown in Figure S6. To the best of our knowledge, this is the highest certified efficiency for a flexible or ultraflexible OSC. External quantum efficiency (EQE) spectra of binary and ternary devices (Figure 2A) are compared to understand the mechanism behind the increased Jsc in the ternary device. The PBDTTT-OFT:IEICO-4F binary device exhibits a wide EQE spectrum covering the region between 400 and 980 nm. Moreover, the addition of 20 wt % PC71BM to substitute IEICO-4F provides an enhancement in EQE across the

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Figure 1. Ultraflexible Organic Solar Cells (A) Chemical structures of PBDTTT-OFT, IEICO-4F, and PC71 BM. (B) Energy-level diagrams of PBDTTT-OFT, IEICO-4F, and PC71 BM. (C) Ultraviolet-visible (UV-vis) absorption spectra of neat PBDTTT-OFT, IEICO-4F, and PC 71 BM films. (D) Schematic of the ultraflexible organic solar cell. (E) Photograph of the ultraflexible organic solar cell. The free-standing device is wrapped over a syringe needle. (F) Current density-voltage (J-V) characteristics of best-performing ultraflexible organic solar cells with different active layers. The blue, red, and black lines represent ultraflexible organic solar cells fabricated based the on PBDTTT-OFT:IEICO-4F (1:1.5) binary blend, PBDTTT-OFT:IEICO4F:PC 71 BM (1:1.2:0.3) ternary blend, and PBDTTT-OFE:PC 71 BM (1:1.5) binary blend, respectively.

entire absorption region. Integrating the measured EQE spectra with the AM1.5G spectrum, the obtained values of Jsc for the binary and ternary cells confirm the trends exhibited when measured under a solar simulator with a comparable increase in short-circuit current. It is noteworthy that the most significant increase in EQE is observed for the region between 700 and 800 nm, which is primarily attributed to the absorption of PBDTTT-OFT. Steady-state photoluminescence (PL) measurement results confirm that PL from the polymer is more strongly quenched in the ternary blend than in the binary blend (Figure S7). Therefore, the addition of PC71BM facilitates the dissociation of excitons photogenerated in PBDTTT-OFT.36,37

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Table 1. Photovoltaic Parameters of Best-Performing Organic Solar Cells Active Layer

Voc (V)

Jsc (mA/cm2)

Fill Factor

Efficiency (%)

PBDTTT-OFT:IEICO-4F

0.71

24.7

0.68

11.9 (11.6 G 0.4)

PBDTTT-OFT:IEICO-4F:PC71BM

0.72

26.1

0.69

13.0 (12.5 G 0.5)

PBDTTT-OFT: PC71BM

0.77

17.7

0.69

9.5 (9.2 G 0.4)

The average efficiency is shown in parentheses.

The competition between charge extraction dynamics and recombination after exciton separation affects device performance significantly. Charge recombination behavior has been investigated by measuring light-intensity-dependent Jsc and Voc, as shown in Figures 2B and 2C. A linear proportionality of Jsc with respect to incident light intensity indicates a constant EQE with different intensities. In Figure 2B, both PBDTTT-OFT:IEICO-4F binary and PBDTTT-OFT:IEICO-4F:PC71BM ternary devices show a near-linear power law dependence of Jsc upon light intensity ðJsc f Ia Þ, with a values of 0.985 for the PBDTTT-OFT:IEICO-4F binary device and 0.998 for the ternary device. The a value in the ternary device is closer to 1, suggesting slightly less bimolecular recombination in these devices compared with PBDTTTOFT:IEICO-4F binary devices. Furthermore, the slope of Voc against light intensity is calculated from Figure 2C. The slope increases from 1.09 kT/q in the ternary device to 1.32 kT/q in the PBDTTT-OFT:IEICO-4F binary device, suggesting that charge trapping effects are more significant in the PBDTTT-OFT:IEICO-4F binary device.38 The difference in charge extraction dynamics in these two devices have been studied using transient photocurrent (TPC) analysis (Figure 2D). The photocurrent decay time fitted with exponential decay in the PBDTTT-OFT:IEICO-4F binary and ternary devices are 0.23 and 0.19 ms, respectively. Coupled with the observation of enhanced EQE, the faster photocurrent decay time in the ternary device indicates that charge extraction in the ternary device is more efficient than that in the PBDTTT-OFT:IEICO-4F binary device. As the device architecture and film thickness of the active layer are the same, the faster decay time in the ternary device is attributed to superior charge carrier transport and charge extraction in the ternary active layer owing to the addition of an additional fullerene acceptor component. The lifetime of the perturbed charge carriers under different background sun intensities are calculated using transient photovoltage (TPV) measurements (Figure 2E). The TPV profiles (Figure S8) fitted using monoexponential decays suggest that bimolecular recombination is the dominating mechanism in an open-circuit condition. The ternary devices contain longer-lived charge carriers than the PBDTTT-OFT:IEICO4F binary devices, thus yielding a lower bimolecular recombination rate in the ternary device. From these device physics studies, the introduction of the third component PC71BM facilitates exciton separation in addition to improving charge extraction and suppressing bimolecular recombination, thereby resulting in enhanced Jsc and FF in the ternary blend. Morphological Investigation In OSCs, morphology control of the active layer is important for improving the PCE of the device and mechanical properties of the active layer. In this study, the effects of the additional acceptor component on both lateral and vertical morphologies of the active layer have been comprehensively investigated with a combination of atomic force microscopy (AFM), in-depth grazing incidence wide angle X-ray scattering (GIWAXS), and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. The AFM results (Figures 3A and 3B) confirm the lack of coarse phase separation in both binary and ternary systems, suggesting nano-scale phase separation as confirmed by the PL quenching measurements. The ternary device exhibits a

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Figure 2. Device Physics of Binary and Ternary Organic Solar Cells (A) External quantum efficiency (EQE) spectra. (B) Short-circuit current as a function of light intensity. (C) Open-circuit voltage as a function of light intensity. (D) Transient photocurrent (TPC) decay measurement (E) Transient photovoltage (TPV) decay time as a function of light intensity. The blue and red lines represent the ultraflexible organic solar cells fabricated based on the PBDTTT-OFT:IEICO-4F (1:1.5) binary blend and PBDTTT-OFT:IEICO-4F:PC 71 BM (1:1.2:0.3) ternary blend, respectively.

smoother surface topography (root mean square roughness [Rq] =1.7 nm) compared to the binary device (Rq = 2.7 nm), which could be owing to a lower crystallinity in the ternary film. Transmission electron microscopy (TEM) results (Figure S9) demonstrate that the introduction of the third component PC71BM can further refine the PBDTTT-OFT:IEICO-4F phase separation and facilitate the charge separation at the donor/acceptor interface. The molecular packing of the polymer PBDTTT-OFT and non-fullerene acceptor IEICO-4F in binary and ternary blends on ultraflexible substrates has been further studied using two-dimensional (2D) GIWAXS (Figures 3C and 3D). Their corresponding line profiles along the out-of-plane (OOP) and in-plane (IP) directions are presented in Figures 3E–3H. Two scattering peaks located at q = 1.6 and 1.8 A˚1 along the OOP direction correspond to p-p reflections of PBDTTT-OFT and IEICO-4F, respectively, and another two scattering peaks located at q = 0.27 and 0.31 A˚1 along the IP direction correspond to lamellar stackings of PBDTTT-OFT and IEICO-4F, respectively.32,39 The scattering peak located at q = 1.0 A˚1 originates from the substrate (Figures S10 and S11). From the GIWAXS data, both the polymer PBDTTT-OFT and non-fullerene IEICO-4F exhibit a predominately face-on stacking configuration, which is regarded as more favorable for charge transport across the active layer. The non-fullerene IEICO-4F exhibits a significantly higher and narrower scattering peak than polymer PBDTTT-OFT in the binary blend. IEICO-4F exhibits a relatively planar structure; thus, the strong p-p interaction promotes the formation

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Figure 3. Morphological Characterization of Ultraflexible Organic Solar Cells Atomic force microscopy (AFM) surface topography of the (A) PBDTTT-OFT:IEICO-4F (1:1.5) binary blend and (B) PBDTTT-OFT:IEICO-4F:PC 71 BM (1:1.2:0.3) ternary blend. 2D grazing-incidence wide angle X-ray scattering (GIWAXS) patterns of the (C) binary blend and (D) ternary blend are shown. 1D out-of-plane (OOP) GIWAXS line profiles of the (E) binary blend and (F) ternary blend and 1D in-plane (IP) GIWAXS line profiles of the (G) binary blend and (H) ternary blend are extracted from the 2D scattering pattern taken at different X-ray incident angles. The X-ray incident angle varies from 0.1  (below critical angle, red line) to 0.3  (above critical angle, purple line). Near-edge X-ray absorption fine structure (NEXAFS) spectroscopy of the top surface of the (I) binary blend and (J) ternary blend was performed. The chemical composition on the top surface is determined by fitting the blend spectrum with a linear combination of neat spectra.

of pure crystalline regions with a large crystallite size. Although the higher crystallinity could facilitate charge transport through the pure IEICO-4F phase, it may cause the active layer to be more brittle and also lead to poorer exciton separation. With the introduction of PC71BM into the PBDTTT-OFT:IEICO-4F binary blend, both PBDTTT-OFT and IEICO-4F still exhibit a face-on configuration, but the crystallinity of IEICO-4F is significantly weakened. As shown in GIWAXS patterns measured at the critical angle (Figure S12), IEICO-4F exhibits about 30% decrease in its crystallinity as the third component PC71BM is added (Table S1). However, this decreased crystallinity does not negatively affect the electron mobility in the ternary blend, as measured from the space-charge-limited current (SCLC) (Table S2). Indeed, the electron mobility is actually found to increase from 3.7 3 104 cm2/Vs in the binary blend to 5.2 3 104 cm2/Vs in the ternary blend. The intrinsic high electron mobility of PC71BM thus compensates the decreased crystallinity in IEICO-4F. Furthermore, as 20 wt % of IEICO is replaced by PC71BM, there is no PC71BM scattering peak observed in the GIWAXS pattern, indicating no aggregation of PC71BM in the PBDTTT-OFT:IEICO-4F:PC71BM (1:1.2:0.3) ternary blend. The main reason for the

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lack of pure PC71BM aggregation is the low fraction of PC71BM in the ternary blend (the pure aggregation of PC71BM observed as more PC71BM molecules are introduced in the ternary blend (1:1:0.5), see Figure S13). As the fraction of PC71BM is lower than a threshold concentration, most of the PC71BM molecules are dispersed into the amorphous region of the polymer matrix instead of the formation of brittle pure PC71BM aggregates,40 which plays an important role in improving mechanical properties of the active layer. The crystallization of low-crystalline polymer PBDTTTOFT is slightly improved in the ternary blend, thus improving hole transport through the polymer donor phase (hole mobility increases from 1.2 3 104 cm2/Vs in the binary blend to 3.9 3 104 cm2/Vs in the ternary blend) leading to more balanced electron and hole mobilities. For both the binary and ternary layers, as the X-ray incident angle is varied from below (surface sensitive scattering) to above (bulk sensitive scattering) the critical angle of the active layer, the peak ratio of IEICO-4F to PBDTTT-OFT is found to increase significantly. This observation suggests that an enrichment in PBDTTT-OFT occurred at the top surface of the active layer. The chemical composition of the top surface has been quantitatively investigated with NEXAFS (Figures 3I and 3J). By fitting the blend NEXAFS spectra (Figure S14) with a linear combination of neat spectra (Table S3), a surface composition of 72.2 wt % PBDTTT-OFT is determined for the top surface of the binary blend. This value decreases slightly to 66.8 wt % PBDTTT-OFT for the ternary blend; however, the surface composition is still dominated by PBDTTT-OFT and should support effective hole extraction from the top electrode. Mechanical Stability and Large-Area Ultraflexible OSCs Wearable devices are subject to a wide range of mechanical stimuli such as stretching-compression and bending during daily use. Ultraflexible OSCs based on the ternary blend exhibit negligible degradation in device performance after delamination from a glass supporting substrate (Figure S15). Once delaminated, the freestanding 3-mm OSCs are subsequently transferred onto a 200% pre-stretched adhesive elastomer, and then the device is compressed by releasing the elastomer to examine the device performance under cyclic uniaxial mechanical deformation. As the pre-stretched elastomer is compressed to its original position, the formation of a buckling structure is observed in the ultraflexible OSCs with a bending radius of 10 mm (Figure 4A), and with the buckling structure, the device can be either under tensile bending or compressive bending (Figure S16). During the compression where the strain in the elastomer changed from 200% to 0% (corresponding to a real compression rate of 45%), the device performance indicated a continuous decrease, primarily attributed to the reduced Jsc owing to the area change in the active layer (Figures 4B and S17). The Voc and FF exhibit negligible changes during compression. After 600 compression-stretching cycles with a real compression rate of 45%, the device still maintains 92.5% of its initial efficiency (Figures 4C and S18). The efficiency retention slightly decreases to 89.2% when the cycle number is further increased to 1,000. The degradation is primarily attributed to the decrease in Jsc and FF. Voc exhibits negligible change. In order to understand the device degradation mechanism under the cyclic compression-stretching test, we firstly test the durability of the ITO electrode. As shown in Figures S19A and S19B, the resistance of ITO under the cyclic compression-stretching test is measured with a neutral plane structure design, which has a similar device structure with our ultraflexible OSCs. We find that the resistance of ITO shows a slight increase from 77.4 to 87.7 Ohm after 1,000 cycles. We also investigate the ternary-blend

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Figure 4. Mechanical Properties of Ultraflexible Organic Solar Cells (A) Photograph of the ultraflexible ternary organic solar cell under a cyclic compression-stretching test. The images demonstrate the transition of the ultraflexible organic solar cell from an initial state (left) to a compressed state (right). The real compression rate is 45%. (B) J-V characteristics of ultraflexible ternary solar cells under different compression rates. (C) Efficiency of the ultraflexible organic ternary solar cell with an area of 0.04 cm 2 as a function of cycle number under the cyclic compression-stretching test. (D) Schematics of the bending test setup and the photograph of the ultraflexible organic solar cell under bending test. The bending radius is 0.5 mm. (E) Efficiency of the ultraflexible organic ternary solar cell with an area of 0.04 cm2 as a function of cycle number under the cyclic bending test. (F) J-V characteristic of the best-performing ultraflexible ternary organic solar cell with a large area of 1 cm 2 . (G) Efficiency of the ultraflexible organic ternary solar cell with an area of 1 cm 2 as a function of cycle number under the cyclic compression-stretching test (inset: the images of the large-area ultraflexible organic solar cell under cyclic compression-stretching test).

morphological change before and after the cyclic compression-stretching test (Figure S20), GIWAXS results indicate that the blend film after 1,000 cycles test still largely remains its initial morphology, but a slight decrease in the intensity of the IEICO-4F p-p scattering peak is still observed. Combining several factors mentioned above, the series resistance of the device increases from 4.8 to 5.6 Ohm/cm2 and shunt resistance of the device decreases from 374.8 to 289.7 Ohm/cm2 after 1,000 cycles (Figures S19C and S19D).

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The mechanical properties of the ternary blend are found to be superior to those of the PBDTTT-OFT:IEICO-4F and PBDTTT-OFT:PC71BM binary devices (Figure S21). PC71BM-based devices typically demonstrate poorer mechanical performances than non-fullerene-based devices because the non-fullerene acceptor is more ductile and exhibits enhanced molecular interactions with polymer donors.23 However, the introduction of a small amount of PC71BM disrupts the crystallization of IEICO-4F and avoids the formation of large IEICO-4F crystallites and PC71BM aggregates, thus resulting in a slight enhancement in mechanical performance (with 10% less degradation). The above phenomenon is consistent with previous studies. The pure donor film, composed of PBDTTT-EFT with a Young’s modulus of less than 0.8 GPa, showed better mechanical properties than the blend film, composed of PBDTTT-EFT:PC71BM (weight ratio: 1:1.5) with a Young’s modulus of 2.6 GPa.23 By optimizing the content of PC71BM or/and increasing the content of another ductile content in the ternary blend, the mechanical property can be further improved with a lower Young’s modulus of less than 0.4 Gpa.23,41 Therefore, improved mechanical properties are achieved in optimized devices based on the ternary blend, together with a high efficiency, significantly outperforming previously reported devices.28,31 We also test the intrinsic stretchability of this ultraflexible OSC. The device cannot be directly stretched over 5% (Figure S22) because the parylene substrate, ITO, and the electron transport layer ZnO are not intrinsically stretchable. The active layer shows some stretchability such that there are no cracks under 7% tensile strain. We can start to observe the formation of cracks when the tensile strain further increases to over 10%, and the cracks become significantly larger at 20% tensile strain (Figure S23). In our buckling structure device, the maximum local tensile strain exerted on the active layer is less than 7% based on our calculation as shown in Figure S24. There are no cracks observed in both neat polymer and blend films under such strain (Figure S25). Therefore, there is no significant damage on the active layer during the cyclic compression-stretching test when a buckling structure is used. In addition to the compression-stretching stability, the stability of the devices subject to the bending test was examined. Compared with the compression test, ultraflexible OSCs subject to bending tests experience a bending radius of 0.5 mm, which is several orders of magnitude larger than the buckling bending radius in the compression test (Figure 4D). The ultraflexible OSC exhibits a robust performance under 1,000 cycles in the bending test, achieving a PCE retention of over 97% (Figures 4E and S26), which is significantly better than that of the device fabricated on flexible substrates with a thickness of 100 mm.27 Scalability is crucial in practical applications. To demonstrate the potential for scalability, we fabricated ultraflexible large-area OSCs with an active area of 1 cm2 (Figure 4F), achieving a PCE of 11.6% with a Jsc of 26.5 mA/cm2, Voc of 0.72 V, and FF of 0.61. Compression-stretching testing was also performed for the large-area OSCs. Although the large-area OSC exhibits slightly faster degradation than the smallarea device, after 1,000 cycles of the compression-stretching test, the large-area device still exhibits a PCE retention of 83% (Figures 4G and S27). Conclusions We have demonstrated a 3-mm-thick ultraflexible ternary OSC based on a fullerene/ non-fullerene mixed acceptor combining a high PCE of 13% (12.3% certified) with excellent mechanical stability under both compression and bending. To the best of our knowledge, this is the highest certified efficiency for an ultraflexible OSC,

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with these cells exhibiting the highest mechanical stability under compression. The use of a fullerene/non-fullerene mixed acceptor with carefully controlled morphology combines the advantages of both the fullerene acceptor (high mobility and efficient exciton dissociation) and non-fullerene acceptor (good mechanical properties and higher absorption coefficient). The strategy of using a mixed fullerene/non-fullerene acceptor opens up a new approach to realize highly efficient and mechanically robust ultraflexible OSCs, representing a significant step forward toward the application of ultraflexible solar cells as a power source for next-generation wearable electronics. In the ultraflexible OSCs, the reduction of crystallization through the introduction of the third component is an effective way to improve the ductility of the active layer, leading to a more robust device under mechanical deformation because currently most of the non-fullerene acceptors need a high crystallinity to enhance their high mobility. In the future, the design of amorphous non-fullerene acceptors with a high electron mobility could be another approach for further improving the efficiency and mechanical properties of ultraflexible or stretchable OSCs. In addition to optimization of crystallinity, molecule orientation and phase purity are also of importance. Although the effects of molecule orientation and phase purity on device performance have been well studied, how these two aspects affect the mechanical properties is still largely unexplored. Most of the organic semiconductor thin films exhibit a preferential orientation with either face-on or edge-on configuration, which leads to an anisotropic mechanical property. Careful control of the orientation of the polymer crystallite could also help improve device mechanical properties as well. The phase purity is another critical factor that will influence the mechanical properties. High phase purity is usually associated with a sharp donor/acceptor interface, while low phase purity is usually associated with a smooth and highly mixed interface. How to maintain a good donor/acceptor interface under mechanical stress through optimization of phase purity also needs to be considered in future studies.

EXPERIMENTAL PROCEDURES Materials Polymer PBDTTT-OFT was synthesized by Toray Inc. IEICO-4F and PC71BM were supplied from 1-Materials and Nano-C, respectively. All anhydrous solvents were purchased from Wako Pure Chemical Ltd. Device Fabrication Ultraflexible organic solar cells were fabricated based on the device architecture of parylene/Su-8/ITO/ZnO/active layer/MoO3/Ag. Fluoropolymer-coated glass substrates were prepared by spin-coating the polymer solution (Novec 1700/7100, 3 M company) on a precleaned glass at 2,000 rpm for 60 s. The 1-mm-thick parylene film was subsequently deposited on the fluoropolymer-coated glass substrates using chemical vapor deposition (diX-SR Daisan Kasei Company), followed by thermal annealing at 180 C for 1 h in a glove box. A 500-nm-thick film of Su-8 (MicroChem company) was spin coated on the parylene film to reduce the surface roughness and subsequently annealed at 180 C for 30 min in nitrogen atmosphere. A 100nm indium tin oxide (ITO) film was sputtered and subsequently patterned using photolithography and wet etching. For the large-area device, the thickness of the ITO film was increased from 100 to 180 nm, leading to the reduced sheet resistance from 56 to 28 Ohm/sq. The electron transport layer, ZnO with a thickness of 30 nm, was fabricated using the sol-gel process. The ZnO precursor solution was prepared by dissolving 1,098 mg of

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zinc acetate dehydrate and 312 mL of ethanolamine in 10 mL of 2-methoxyethanol. The ZnO layers were spin coated at 3,000 rpm for 1 min, followed by annealing at 180 C for 30 min. The active layers were spun from a cosolvent of chlorobenzene (97 vol %) and 1-chloronaphthalene (3 vol %). The ratio of polymer donor PBDTTTOFT and acceptor was fixed at 1:1.5. MoO3 (7.5 nm) and Ag (100 nm) were consecutively evaporated thermally through a shadow mask. External wirings were fabricated by thermally evaporating 100 nm Au onto a 12.5-mm polyimide substrate and were subsequently connected to the top electrodes of the device using an anisotropic conductive film tape (ECATT 9703, 3M company). Devices were encapsulated by depositing another 1 mm of the parylene layer. Device Characterization The current density-voltage (J-V) curve of the solar cells was obtained using the Keithley 2400 source meter under a solar simulator. Light intensity was adjusted using neutral density filters and then calibrated by a reference silicon diode. Transient photocurrent and photovoltage measurements were performed following our previous method.42 Light perturbation was provided using a Kingbright L-7104VGC-H green LED (525 nm wavelength) driven by a function generator (Agilent 33533A). The transient response was measured using a digital oscilloscope (Agilent Technologies Infiniivision DSO-X 3032A). In transient photovoltage measurements, the device was connected with a termination of 1 MU to achieve an open-circuit condition, while in photocurrent measurements, the device was connected with a termination of 50 U to achieve a short-circuit condition. For the cyclic compression-stretching test, the free-standing ultraflexible device was transferred onto a pre-stretched acrylic elastomer (VHB Y-4905J, 3M company). Compressing and stretching were achieved using a homemade screw machine. For the bending test, ultraflexible OSCs were transferred to the adhesive elastomer, and the elastomer was subsequently transferred onto a 75-mm-thick polyimide film. J-V curves of ultraflexible solar cells under mechanical test were measured under ambient atmosphere. Microstructure Characterization GIWAXS measurements were conducted at the Australian Synchrotron on the SAXS/ WAXS beamline.43 All samples were subjected to the same procedure as device fabrication. The samples were irradiated by a 10-KeV X-ray beam with different X-ray incident angles varying from 0.1 (below critical angle) to 0.3 (above critical angle). A Dectris Pilatus 1M detector was used to collect two-dimensional (2D) scattering patterns. The X-ray exposure time was 1 s and no film damage was observed. NEXAFS spectra at the carbon edge were acquired at the Australian Synchrotron on the soft X-ray beamline.44 A nearly perfect linearly polarized X-ray beam was used, and the total electron yield (TEY) was measured from the drain current flowing to the sample under X-ray illumination. The detected signal was normalized by a ‘‘stable monitor method,’’45 and NEXAFS spectra were normalized by setting the preedge (280 eV) to 0 and the intensity of 320 eV to 1. The interfacial chemical composition was calculated by fitting the NEXAFS spectrum of blend films with a linear combination of the NEXAFS spectrum of neat films acquired at 55 .46

SUPPLEMENTAL INFORMATION Supplemental Information can be found online at https://doi.org/10.1016/j.joule. 2019.10.007.

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ACKNOWLEDGMENTS This work was financially supported by the Japan Science and Technology Agency (JST) A-STEP grant number AS3015021R and JST ACCEL grant number JPMJMI17F1. The authors thank Toray Industries Inc. for supplying the polymer material (PBDTTT-OFT). The authors also thank Dr. N. Chandrasekaran for his assistance in TPV and TPC measurements and discussions. W.H. acknowledges the ACAP fellowship supported by the Australian Government through the Australian Renewable Energy Agency (ARENA). Z.J. was supported by the Junior Research Associate (JRA) program in RIKEN and a Doctoral Student Special Incentives Program: the SEUT RA program at the Graduate School of Engineering, the University of Tokyo. GIWAXS and NEXAFS experiments were performed at SAXS/WAXS and soft X-ray beamlines, respectively, at the Australian Synchrotron, part of ANSTO.

AUTHOR CONTRIBUTIONS W.H., K.F., and T.S. conceived and designed the research. W.H. and Z.J. fabricated the ultraflexible organic solar cells and performed device characterization. X.J. and C.R.M. performed the synchrotron-based morphological investigations. Z.J. and W.H. conducted the mechanical test. W.H., Z.J., K.F., T.Y., and T.S. analyzed and interpreted the data and prepared the manuscript with comments from all co-authors.

DECLARATION OF INTERESTS The authors declare no competing interests. Received: June 24, 2019 Revised: August 29, 2019 Accepted: October 17, 2019 Published: November 13, 2019

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