Realization of Intrinsically Stretchable Organic Solar Cells Enabled by

Subscriber access provided by the Henry Madden Library | California State University, Fresno

Surfaces, Interfaces, and Applications

Realization of Intrinsically Stretchable Organic Solar Cells Enabled by Charge-Extraction Layer and Photoactive Material Engineering Yun-Ting Hsieh, Jung Yao Chen, Seijiro Fukuta, Po-Chen Lin, Tomoya Higashihara, Chu-Chen Chueh, and Wen-Chang Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 04 Jun 2018 Downloaded from http://pubs.acs.org on June 4, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 27 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

ACS Applied Materials & Interfaces

Realization of Intrinsically Stretchable Organic Solar Cells Enabled by Charge-Extraction Layer and Photoactive Material Engineering Yun-Ting Hsieh,a Jung-Yao Chen,a Seijiro Fukuta,b Po-Chen Lin,a Tomoya Higashihara,b Chu-Chen Chueh,a,c* and Wen-Chang Chena,c* a

Department of Chemical Engineering, National Taiwan University, Taipei, 10617 Taiwan.

b

Department of Organic Materials Science, Graduate School of Organic Materials Science, Yamagata University, 4-3-16 Jo-nan, Yonezawa, Yamagata 99208519, Japan

c

Advanced Research Center of Green Materials Science & Technology, Taipei 10617, Taiwan

*Corresponding authors. E-mail: [email protected]; [email protected] KEYWORDS: stretchability, charge-extraction layer, organic solar cells, interfacial materials, non-fullerene acceptors

ABSTRACT. The rapid development of wearable electronic devices has prompted a strong demand to develop stretchable organic solar cells (OSCs) to serve as the advanced powering systems. However, to realize an intrinsically stretchable OSC is challenging since it requires all the constituent layers to possess certain elastic properties. It thus necessitates a combined engineering of charge-transporting layers and photoactive materials. Herein, we first describe a stretchable electron-extraction layer (EEL) using a blend of PFN and nitrile butadiene rubber (NBR, Nipol®1072). This hybrid PFN/NBR layer exhibits a much lower Derjaguin-

1 ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 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

Muller-Toporov (DMT) modulus (0.45 GPa) than the value (1.25 GPa) of the pristine PFN and could withstand a high strain (60% strain) without showing any cracks. Moreover, besides enriching the stretchability of PFN, the terminal carboxyl groups of NBR can ionize PFN to promote its solution processability in polar solvent and to ensure the interfacial dipole formation at the corresponding interface in the device, as evidenced by the FT-IR and UPS analyses. By further coupling the replacement of PCBM with non-fullerene acceptors owning better mechanical stretchability in the photoactive layer, OSCs with improved intrinsically stretchability and performance were demonstrated. An all-polymer OSC can exhibit a power conversion efficiency (PCE) of 2.82% after 10% stretching, surpassing the PCBM-based device that can only withstand 5% strain.

Introduction The development of a portable powering system has become an essential task nowadays in order to catch the rapid progress of wearable electronic devices. Considering the lightweight and printable features of organic semiconductors, stretchable organic solar cells (OSCs) have been considered as one of the most promising system among the power supply techniques developed to date and increasing research efforts in both material and device levels have been tremendously devoted to this direction in recent years.1-4 At present, the most important progress for the development of stretchable OSCs lies in the aspect of device engineering.5 The recent invention of using mechanical buckling scaffold to fabricate the device has been proven as the most effective way to largely increase its overall stretchability because the preformed wrinkles in the complete device effectively relieve the stretching and compressive strain.6-8 However, despite this important progress, it is still an indirect way to increase device’s stretchability rather than a direct revolution of an intrinsically stretchable device. In addition, such buckling architecture might incur an unsatisfactory long-term stability since the active area will become more wrinkled when 2 ACS Paragon Plus Environment

Page 2 of 27

Page 3 of 27 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


undergoing more compression-stretching cycles. This will cause the shrinkage of active area and thus induce the change of photoactive layer’s morphology.8 In this regard, to realize an intrinsically stretchable OSC, it is necessary to return to the basic material innovation since each constituent layer in the stratified device must possess certain elastic properties.9-12 It thus necessitates a combined material engineering covering both charge-transporting layers (CTLs) and the photoactive materials. Currently, most of the reported works are mainly focusing on improving the stretchability of photoactive layer and significant achievement has been made.13-14 The major breakthrough in ameliorating the ductility of organic bulk-heterojunction (BHJ) layers is traced to the recently prosperous development of the non-fullerene acceptors (NFAs) since the fullerene-based BHJ system has been proven to possess a brittle nature, leading to easy fracture under strain.3 In contrast, the newly developed NFA that generally owns nonsphere geometry can have better miscibility with the component polymer donor and thus possess a less tendency of aggregation, resulting in a more ductile nature of its derived BHJ system.3, 15-16 Compared to the photoactive materials, the studies pertaining to the development of stretchable CTLs compatible to OSCs are still rarely reported, especially for the electronextraction layer (EEL).17-18 CTLs has been documented to play critical roles in optimizing the performance and stability of OSCs, which can simultaneously minimize the resistance and enhance the charge selectivity at the BHJ/electrode interfaces.19-20 In general, the most commonly used EELs in OSCs are based on inorganic transition metal oxides (TMOs), like ZnO and TiO2, due to their superior ambient stability, optical transparency, and suitable work functions.21-26 However, such inorganic-based EELs are usually crystalline and thus possess a brittle nature. In this respect, the organic counterparts are considered to be a more promising system for the development of stretchable EELs.



Among the organic EELs exploited thus far, the PFN-based derivatives have attracted the most attention and have been widely used in OSCs to improve the device performance.27-30 On one hand, it has been demonstrated to effectively facilitate the charge-extraction efficiency at the TMO/BHJ interface.31-33 On the other hand, it can also tune the work functions of the ITO/metal electrodes through its polar side-chains containing amine groups.34-35 The nitrogen-containing side-chains possessing high electronegativity and electron-donating property can modulate the surface charges of electrodes to tune the interfacial dipoles to enable the energy level alignment at the BHJ/metal interface.28, 36 Given these advantages, we thus focus to develop stretchable EELs based on PFN. It is worthwhile to note that although the PFN is solution-processable, it requires adding a small amount of acetic acid in its precursor solution to enhance the solubility in alcohol-based solvents.27-30 It is because the acetic acid can protonate the nitrogen-containing side-chains to increase its ionic feature and thus the solubility in polar solvents. As considering this limitation together with the fulfillment of the stretchability, we herein blend nitrile butadiene rubber (NBR, Nipol®1072) (denoted as NBR hereafter) with PFN, aiming to develop a stretchable EEL for the realization of stretchable OSCs (Figure 1). NBR is a well-known rubber possessing superior elastic property; therefore, incorporating NBR into PFN can straightforwardly raise its overall stretchability. Further, it is revealed that the constituent carboxyl groups of NBR can ionize PFN to enhance its solubility in alcohol-based solvents while maintaining its work-function tuning capability, as exemplified by the FT-IR and UPS analyses. Consequently, the hybrid PFN/NBR layer exhibits a much reduced DerjaguinMuller-Toporov (DMT) modulus (0.45 GPa) compared to the pristine PFN (1.25 GPa) and can withstand a high strain (60% strain) without showing any cracks. Finally, by further replacing the PCBM in the active layer with non-fullerene acceptors (NFAs) owning better mechanical stretchability, OSCs with improved stretchability were


Page 4 of 27

Page 5 of 27 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


successfully demonstrated. The PC71BM-based device using the PFN/NBR EEL can yield a much improved power conversion efficiency (PCE) of 3.21% after stretching to 5% compared the control device without using it. However, limited by the brittle nature of PC71BM, the maximum stretching that can be applied to this device is restrained to 5%. After replacing PC71BM with NFAs, a n-type small molecule (ITIC) and an n-type polymer (P(NDI2HD-T)), the derived devices using the PFN/NBR EEL showed an improved ductility and the PTB7Th:P(NDI2HD-T) OSC can exhibit an encouraging PCE of 2.82% after 10% stretching.

Results and Discussion Characteristics of the hybrid PFN/NBR film As mentioned earlier, PFN can dissolve in methanol in the presence of a small amount of acetic acid since the carboxyl group of acetic acid can protonate its amine groups. We thus speculate the carboxyl groups in NBR might ionize the neutral PFN polymer, providing a similar function as acetic acid did, to increase its solubility in the alcohol-based solvents. To verify this, the FT-IR spectra of PFN, NBR, and PFN/NBR blend were conducted and compared in Figure 2. As shown, a broad band centered around 3200 cm-1 was clearly observed in NBR’s spectrum, representing the O-H stretch of its carboxyl group (Figure 2b). Whereas, this broad band disappeared in PFN/NBR’s spectrum, indicating the dissociation of the carboxyl groups in NBR to –COO- group. Meanwhile, the C=O stretch located at 1734 cm-1 and 1698 cm-1 in NBR’s spectrum also vanished in PFN/NBR’s spectrum (Figure 2c). Note that the C=O characteristic located at 1698 cm-1 in NBR’s spectrum originates from the dimer formation between the neighboring carboxylic acids through the hydrogen bonding. Such dimerization weakens the C=O stretching characteristic, leading to a split and shift into a lower frequency. Thus, the disappearance of this characteristic in PFN/NBR’s spectrum suggests the dissociation of the dimers, substantiating the ionization of carboxyl groups into – COO- group. These results clearly certified the chemical interactions between PFN and NBR 5 ACS Paragon Plus Environment


and it thus can be presumed that the carboxyl groups of NBR can protonate the amine groups (NHR3+) of PFN after blending them together. The PFN EEL has been widely proven to form the interfacial dipole at the organic/metal interface and thus tune the work functions of TMO-based interlayers or metal electrodes.28-29 To evaluate this capability of the hybrid PFN/NBR blend, ultraviolet photoelectron spectroscopy (UPS) was conducted to study the work-function shifting of the ITO and metal surfaces. As depicted in Figure 3a, after depositing PFN, NBR, and PFN/NBR blend on the ITO surface, its pristine work function (5.22 eV) was reduced to 5.02 eV, 4.97 eV, and 4.92 eV, respectively (Table S1). The largest work-function shifting observed in the PFN/NBR sample affirms its capability of forming interfacial dipole at the corresponding interface due to formation of NHR3+ group in PFN. Figure 3b and 3c showed the work-function tuning capability of PFN/NBR blend on metal surfaces. As seen, the pristine work functions of Au (5.11 eV) and Ag (4.89 eV) were respectively reduced to 4.19 eV and 4.34 eV after the coating of thin PFN/NBR film (Table S1). Such large shifting again demonstrated the well work-function-modifying capability of the PFN/NBR blend. The morphology of the PFN, NBR, and PFN/NBR films were then investigated by the atomic force microscopy (AFM). As shown in Figure S1, the pristine ITO exhibited an average root-mean-square roughness of 8.90 ± 0.70 nm while the PFN-, NBR-, and PFN/NBR-coated ITO samples displayed the value of 7.33 ± 0.28 nm, 9.98 ± 0.30 nm, and 7.54 ± 0.26 nm, respectively. The smooth surface observed in both topographic and phase images of PFN/NBR film suggest the well miscibility between them, which is beneficial for device applications. The optical transparency of PFN, NBR, and PFN/NBR blend coated on the glass substrate was compared in Figure S2, wherein the thickness of the polymer layer was all fixed at ~10 nm for a fair comparison. As shown, owing to the high transparency of NBR in the Vis-NIR wavelengths, the hybrid blend possessed a similar optical transparency


Page 6 of 27

Page 7 of 27 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


to the pristine PFN. Therefore, it can be concluded that the incorporation of NBR into PFN does not increase the optical losses in its derived device. This, together with its superior work-function tuning capability, ensures its eligibility to serve as an efficient EEL for OSCs. The main purpose to blend PFN with NBR herein is to increase its overall ductility because the NBR can act as the soft components in the blend to ease the external strain. To confirm this, the mechanical properties were then examined by the PeakForce Quantitative NanoMechanical to characterize the DMT modulus of the thin films. Presented in Figure 4 is the distribution histogram of the modulus of the PFN, NBR, and PFN/NBR films, wherein the DMT modulus of them is 1.25, 0.016, and 0.45 GPa, respectively. The modulus of the PFN/NBR film is apparently much lower than that of the PFN film, supporting its improved stretchability. In principle, the stretched-out state for a polymer under a stress is not an equilibrium thermodynamics state; therefore, the polymer will restore to its original state once the stress is removed. However, this recovery from the stretched-out state is closely related to the interchain interactions of polymers. Generally, the van der Waals’ force dominates the interchain interactions of polymers, like PFN, which is not high enough to induce the recovery from the stretched-out state. In such case, the stress will engender the breakdown of polymer networks to result in the cracks since the van der Waals’ force between the polymer chains has been broken and the mass of the polymer chains was shifted to cause irreversible deformation. Whereas, the polymers with more intense interchain interactions, like NBR, can inhibit the mass shift of the polymer chains. The polymer chains will be pulled back to original place once the stress is removed, resulting in the elastic property. To further confirm this, the optical microscopy images of the PFN/NBR film under various strains were recorded and compared with the pristine PFN film. Note that the studied films were spin-coated on PDMS without pre-strain. As displayed in Figure 5a-c, the PFN film yielded conspicuous cracks once the tensile strain was applied and the cracks became



lager and denser as the strain increases, revealing its hard and brittle nature. In striking contrast, no clear cracks were observed in the PFN/NBR film even under a 60 % strain (insert in Figure 5f), in well congruence with the above analysis (Figure 4). As hypothesized, the soft NBR distributed in the PFN matrix eases the external stress and thus enables the entire film to remain intact under strain.

Photovoltaic performance of the PFN/NBR-derived device We next examine the effectiveness of PFN/NBR film as an EEL by first fabricating the inverted-type solar cell device consisting of ITO/PFN:NBR/polymer:PC71BM/MoO3/Ag, wherein representative polymer, PTB7-Th, was employed as the donor materials. In parallel, the reference devices using EELs other than the PFN/NBR blend were also fabricated for comparison. Note that the optimized composition of the hybrid PFN/NBR film was made with a 1:1 weight ratio of PFN to NBR (at a total concentration of 0.15 mg/ml in DMSO). Table S2 listed the detailed photovoltaic performance of the device using the hybrid PFN/NBR films consisting of different weight ratios of PFN to NBR. As seen, when the blend ratio of NBR in the hybrid film was between 30 wt% to 50 wt%, there was no apparent influence on the resultant PCE. However, the performance of the derived device was apparently dropped as the blending amount of NBR exceeds 50 wt%, possibly owing to its insulating nature. Nevertheless, the more of NBR is blended in the hybrid PFN/BRN film, the better elastic property of the film is expected. Therefore, we herein use the PFN/NBR (1:1) hybrid film as the optimized condition for further device fabrication. Figure 6 presented their current density-voltage (J-V) characteristic measured under AM 1.5G illumination at 100 mW cm-2 and the detailed photovoltaic performance was summarized in Table 1. As expected, the control device without using any EEL showed a poor PCE of 2.53±0.16% with a low open-circuit (Voc) of 0.36±0.02 V, a short-circuit (Jsc)


Page 8 of 27

Page 9 of 27 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


of 14.39±0.23 mA cm-2, and a fill factor (FF) of 0.48±0.02. In contrast, the device using the PFN/NBR EEL exhibited an improved Voc of 0.73 ± 0.01 V along with enhanced Jsc (15.95±0.26 mA cm-2) and FF (0.56±0.01), boosting PCE to 6.59±0.21%. However, it still showed a lower performance compared to the regular PFN-based device processed with the acetic acid. It could be attributed to the inactive electronic nature of NBR, which slightly increase the series resistance in device, leading to a reduced FF. Based on the above results, it therefore can be concluded that the PFN/NBR layer can enable the energy level alignment by tuning the interfacial dipole at the associated interfaces, revealing the effective incorporation of NBR into PFN. As the energy level diagram portrayed in Figure 7a, the reference inverted device possesses a large electron-injecting energy offset of 1.3 eV at the ITO/PC71BM interface. The unfavorable energy level alignment incurs potential loss and hampers the electron collection at the ITO cathode, thus leading to the poor device parameters. However, after introducing the PFN/NBR EEL, an interfacial dipole is formed to reduce the work function of ITO and to enable the energy level alignment at the interface between ITO and PC71BM (Figure 7b). Such alignment can promote the charge transport across the interface and result in enhanced photovoltaic performance, especially the Voc.19-20 To demonstrate the good solution-processability of PFN/NBR in device fabrication, the conventional-type

OSC

with

a

configuration

of

ITO/PEDOT:PSS/PTB7-

Th:PC71BM/PFN:NBR/Al were next fabricated in parallel with the reference device (ITO/PEDOT:PSS/PTB7-Th:PC71BM/Al) for comparison. As shown in Figure S3, the reference device showed a poor PCE of 2.77±0.48% with a low Voc of 0.55±0.04 V, a Jsc of 12.19±0.37 mA cm-2, and a FF of 0.41±0.03 whereas the device using the PFN/NBR EEL exhibited a much higher PCE of 6.49±0.33% with a much increased Voc of 0.80±0.01 V, a Jsc of 15.27±0.62 mA cm-2, and a FF of 0.53±0.01. Note that the slightly lower PCE 9 ACS Paragon Plus Environment


(6.49 ± 0.33%) observed in the conventional architecture than the inverted configuration (PCE: 6.59±0.21%) might stem from lower interfacial quality at the BHJ/EEL interface since the processing of PFN/NBR layer might dissolve the PCBM to a certain degree and adversely influence the BHJ morphology. Presented in Figure 7c-d is the energy level diagrams of the fabricated conventional devices. Similar to the previous case, the PFN/NBR layer can also effectively tune the work function of metal electrode through the formation of interfacial dipole to mitigate the energy barrier at the PC71BM/metal interface. The enhancements shown in both inverted and conventional device architectures clearly corroborate the effectiveness of the hybrid PFN/NBR layer to serve as an efficient EEL for OSCs.

Photovoltaic performance of the stretchable device using PFN/NBR EEL Provided the well ductility of PFN/NBR film, we next utilize it to fabricate stretchable OSCs based on the device configuration shown in Figure 1d, for which 3M VHB 4905 elastomeric tape was employed as the substrate, and modified PEDOT:PSS and EGaIn with decent ductility were employed as the respective electrodes. The J-V characteristics of the fabricated stretchable devices were presented in Figure S4 and the relevant photovoltaic parameters were summarized in Table 2. In addition, Figure S5 illustrated the stretchable test of the fabricated device and the stretchable tests of the above-mentioned films were conducted in a similar way. As shown, the control device without using the PFN/NBR EEL yielded a PCE of 2.63±0.84% with a Voc of 0.69±0.01 V, a Jsc of 11.05±2.64 mA cm-2, and a FF of 0.34±0.04, while the PFN/NBR-based device showed an improved PCE of 3.41±0.28% with a higher Voc of 0.75±0.02 V as observed in previous cases. Whereas, after stretching to 5%, the PCE of the control device was quickly dropped to 1.87±0.62% mainly owing to the decreased Jsc and FF. In contrast, the PFN/NBR-based device retained a similar PCE of 3.21±0.02%,


Page 10 of 27

Page 11 of 27 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


demonstrating an improved stretchability. However, the maximum strain that can be applied to the PTB7-Th:PC71BM device was restrained to 5%. External strain exceeding 5 % will fracture the fabricated device, which was attributed to the poor ductility of the fullerenebased BHJs as discussed earlier. To address this, we thus first replace PC71BM with a non-fullerene-based oligomer (ITIC) as portrayed in Figure 1a. Although the overall stretchability of the derive device can be improved to 10% strain by replacing the PC71BM with ITIC, the performance of the device using a PFN/NBR EEL did not improve compared to the device without using EEL. Such decrease might be because the DMSO will partially dissolve the ITIC and degrade the BHJ morphology. Therefore, a recently developed n-type polymer (P(NDI2HD-T))37-38 was next used to replace PC71BM. As shown in Figure 8a, the maximum strain that can be applied to the device was similarly increased to 10%, representing a double enhancement in stretchability. Especially, the reference PTB7-Th:P(NDI2HD-T) device exhibited a PCE of 2.52±0.67% with a Voc of 0.74±0.02 V, a Jsc of 11.77±2.66 mA cm-2, and an FF of 0.29 ± 0.01, while its commensurate device using the PFN/NBR EEL showed a Voc of 0.76±0.02 V, a Jsc of 13.13±1.37 mA cm-2, and an FF of 0.31±0.08, corresponding to a much higher PCE of 3.00±0.51%. More intriguingly, after stretching to 10%, the device using the PFN/NBR EEL can retain a PCE of 2.82±0.84%, showing negligible decrease in performance. Note that the low FF observed in the stretchable OSCs might originate from the high series resistance due to the low conductivity of bottom PEDOT:PSS electrode and the contact resistance among each layer. Also, at this stage, the fabricated stretchable devices just showed mediocre stretching durability, which can only sustain a few stretching-relaxing cycles. We suspect it is due to the poor adhesion of the constituent interfaces under stretching, especially for the EEL/EGaIn or BHJ/EGaIn interfaces. Further device optimization is necessary and related device engineering is in progress.



Notably, the reference stretchable device using the pristine PFN EEL was also fabricated for fair comparison and the results were shown in Figure S6 and Table S3. As shown, owing to the limited strain (5%), the PTB7-Th:PC71BM device using the PFN EEL yielded a comparable performance to the device using PFN/NBR EEL. However, for the PTB7Th:P(NDI2HD-T) case that sustained a higher strain to 10%, the advantage of PFN/NBR EEL over PFN EEL is better reflected, for which higher PCE can be reserved after stretching. This reveals the merit of PFN/NBR EEL to maintain high performance of device under a large strain. To scrutinize the improved stretchability of the non-fullerene-based BHJ blend, we examined the optical microscopy image of the PTB7-Th:P(NDI2HD-T) film under different strain. As shown in Figure 8a, no clear cracks were observed in the film even under 10% strain, in contrast to the fullerene-based film, revealing its superior ductility. Presented in Figure 8b-d is the distribution histogram of the DMT modulus of PTB7-Th:PC71BM, PTB7Th:ITIC, and PTB7-Th:P(NDI2HD-T) films. As shown, there was peak deconvolution of the modulus in the PTB7-Th:PC71BM film. It accordingly suggests the existence of two different mechanical properties, for which the peaks in the lower modulus can be contributed from PTB7-Th while PC71BM contributes to the peaks in the higher modulus. When the stress was applied, the area owning lower modulus would transfer the strain to the area owning higher modulus, resulting in the focalization of strain. Once the external stain exceeded the limit of the film, the cracks were thus formed. Different to the PTB7-Th:PC71BM film, two peaks became very close to appear nearly one distribution in the PTB7-Th:P(NDI2HD-T) film. This indicates that the strain can be homogeneously distributed in the film once an external strain is applied and thus can better ease the stress to avoid the crack formation.

Conclusion


Page 12 of 27

Page 13 of 27 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


In summary, we describe a novel stretchable EEL consisting of PFN/NBR that can function as an efficient interlayer to realize efficient OSCs. This PFN/NBR film possesses high transmittance in the Vis-NIR wavelengths and exhibits a much lower Derjaguin-MullerToporov (DMT) modulus (0.45 GPa) than the value (1.25 GPa) of the pristine PFN. It thus can withstand a high strain (60% strain) without showing any cracks. Moreover, besides enriching the stretchability of PFN, the terminal carboxyl groups of NBR can ionize PFN to enhance its solution processability in the polar solvents and to ensure the interfacial dipole formation at the corresponding interface in the device, as is evident from the FT-IR and UPS analyses. By further coupling the replacement of PCBM with NFAs, a n-type small molecule (ITIC) and an n-type polymer (P(NDI2HD-T)), owning better mechanical stretchability in the photoactive layer, OSCs with improved stretchability were successfully demonstrated. Impressively, an all-polymer, PTB7-Th:P(NDI2HD-T), OSC can exhibit a PCE of 2.82% after 10% stretching, surpassing the PCBM-based device that can only withstand 5% strain. Overall, the demonstrated stretchable EEL combing the photoactive material engineering provides a universal strategy for the development of efficient intrinsically stretchable OSCs.

Experimental Section Material: Nitrile Butadiene Rubber (NBR) (Nipol®1072) was received from Golden Gakun Industrial Co., LTD. Poly[(9,9-bis(3'-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt2,7-(9,9–dioctylfluorene)]

(PFN),

Poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-

b;4,5-b']dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2carboxylate-2-6-diyl)]

(PTB7-Th)

and

3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-

indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2’,3’-d’]-s-indaceno[1,2-b:5,6b’]dithiophene (ITIC) were purchased from Luminescence Technology Corp. The weightaverage molecular weight (Mw) of PFN, PTB7-Th, and P(NDI2HD-T) is > 10,000, > 50,000, and > 221,500, respectively, as measured by gel permeation chromatography (GPC). 13 ACS Paragon Plus Environment


Anhydrous solvents (methanol, tetrahydrofuran (THF), chlorobenzene (CB),

Page 14 of 27

1,2-

dichlorobenzene (DCB), chloroform (CF)), 1,8-diiodooctane (DIO, 97%), [6,6]-Phenyl C71 butyric acid methyl ester (PC71BM, 99.5%), and Molybdenum(VI) oxide (MoO3) were purchased

from

Sigma-Aldrich.

Poly(3,4-ethylenedioxythiophene):polystyrenesulfonate

(PEDOT:PSS, CLEVIOS™ P VP AI 4083) is used as the hole-transporting layer in the conventional device. The mixture of PEDOT:PSS (Heraeus Clevios™ PH1000), Dimethyl sulfoxide (DMSO), and Zonyl fluorosurfactant (Zonyl FS-300) is employed as the bottom electrode for the stretchable device. Device fabrication: The patterned ITO-coated glass substrates (obtained from Lumtec, (5Ω/□)) were first cleaned with detergent and ultra-sonicated in detergent, DI water, acetone, and isopropyl alcohol for 15 min, respectively, followed by the oxygen plasma treatment for 15 min. The PFN/NBR solution with a concentration of 0.2 mg/mL was prepared in THF and stirred overnight. The weight proportion of PFN to NBR was 1 to 1. This precursor solution was filtered with a 0.45 μm PTFE filter and then spin-coated on the ITO-coated substrates at 5000 rpm for 1 min. For the deposition of active layer, the PTB7-Th:PC71BM solution (1:1.5 w/w, polymer concentration of 14 mg/ml in DCB with 3% DIO) was filtered with a 0.45 μm PTFE filter and spin-coated onto the PFN/NBR layer at 1100 rpm for 90 s. Finally, 8 nm of MoO3 and 150 nm of Ag were thermally evaporated under < 10-6 torr through a shadow mask to define the top electrode. The active area of the device is 4 mm2. The schematic illustration of the device configuration was shown in Figure 1b. For the fabrication of the conventional device (Figure 1c), PEDOT:PSS (CLEVIOS™ P VP AI 4083) filtered by a 0.45-µm PTFE filter was first spin-coated on the ITO-coated substrate at 3500 rpm for 70s in air, followed by annealing at 140 °C for 15 min in air. The PTB7-Th:PC71BM film was spin-coated at 1100 rpm for 90 s and then vacuumed for 5 min to remove residual solvent. Afterward, the PFN/NBR solution (1:1 w/w, total concentration of


Page 15 of 27 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


0.15 mg/ml in DMSO) was sequentially spin-coated at 5000 rpm for 2 min. The purpose of using DMSO is to prevent to degrade the underlying photoactive layer. Finally, 100 nm of Al were thermally evaporated under < 10-6 torr through a shadow mask to complete the top electrode. The active area of the device is 4 mm2. To fabricate the stretchable device shown in Figure 1d, a PDMS template (sylgard 184 Dow Corning) was first prepared at a ratio of 15:1 (base: crosslinker, w/w) and cured for 12hr at 80 °C. Afterward, the PDMS substrate with appropriate size was cut and attached onto the rigid glass slide for supporter. The 3M elastomer (3M VHB 4905) was adhered to the PDMS and used as received (i.e., without any treatment). PEDOT:PSS (Heraeus Clevios™ PH1000) containing 5 vol% of DMSO and 10 vol% of Zonyl fluorosurfactant was prepared and stirred overnight. The modified PH1000 solution was filtered by a 0.45-µm PTFE and spin-coated onto the 3M elastomer at 1000 rpm in air, followed by dry at 100 °C for 30 min in air. The solution of PTB7-Th:ITIC (1:1.3 w/w, total concentration of 25 mg/ml in CB) and PTB7Th:P(NDI2HD-T) (1.5:1 w/w, total concentration of 10 mg/ml in a mixed solvent of CF and DIO (99:1 vol %)) were filtered through a 0.45-µm PTFE filter and then spin-coated at 1000 rpm and 1500 rpm for 90 s respectively onto the modified PH1000 layer in a glove box. Sequentially, the PFN/NBR solution was spin-coated onto the active layer at 5000 rpm for 2 min. Finally, EGaIn was then deposited on the device to define the cathodes. The active area depends on the size of EGaIn on top of the active layer. Characterization: FT-IR spectrum was used to analyze the interaction between PFN and NBR by PerkinElmer FT-IR Spectrometer (Spectrum Two). UPS data were measured by using an Electron Spectroscopy for Chemical Analysis (ESCA) ULVAC-PHI PHI 5000 Versaprobe II with a He I (21.22 eV) light source and the film thickness of the sample is ~10 nm. Transmittance spectra were recorded using the UV-HITACHI U-4100 UV-Vis spectrophotometer. The morphology of film surfaces were examined with a nano-scope 3D



Controller atomic force microscopy (AFM, Digital Instruments (Santa Barbara, CA)) operated in the tapping mode at room temperature. The samples for DMT modulus tests were fabricated by spin-coating studied films onto a Si wafer (1 cm × 1 cm), and the samples were measured by PeakForce Quantitative NanoMechanical model by atomic force microscopic imaging. The current-voltage (J-V) measurement of the solar cell device was recorded by a computer-controlled Keithley 2400 source measurement unit (SMU) with a Peccell solar simulator under the illumination of AM 1.5G, 100 mW cm-2. The illumination intensity was calibrated by a standard Si photodiode detector with KG-5 filter.

ASSOCIATED CONTENT Supporting Information. Additional information of UPS analysis, AFM images, optical transmittance, and J-V characteristics supplied as supporting information. This material is available free of charge via the Internet at http://pubs.acs.org

AUTHOR INFORMATION Corresponding Author [email protected] (C.-C. Chueh); [email protected] (W.-C. Chen)

ACKNOWLEDGMENT This work was financially supported by the “Advanced Research Center of Green Materials Science and Technology” from The Featured Area Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education (107L9006) and the Ministry of Science and Technology in Taiwan (MOST 106-2218-E-002-021-MY2 & 107-3017-F-002-001).

REFERENCES


Page 16 of 27

Page 17 of 27 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


(1) White, M. S.; Kaltenbrunner, M.; Głowacki, E. D.; Gutnichenko, K.; Kettlgruber, G.; Graz, I.; Aazou, S.; Ulbricht, C.; Egbe, D. A. M.; Miron, M. C.; Major, Z.; Scharber, M. C.; Sekitani, T.; Someya, T.; Bauer, S.; Sariciftci, N. S. Ultrathin, Highly Flexible and Stretchable PLEDs. Nat. Photonics 2013, 7, 811-816. (2) Kaltenbrunner, M.; Adam, G.; Glowacki, E. D.; Drack, M.; Schwodiauer, R.; Leonat, L.; Apaydin, D. H.; Groiss, H.; Scharber, M. C.; White, M. S.; Sariciftci, N. S.; Bauer, S. Flexible High Power-Per-Weight Perovskite Solar Cells with Chromium Oxide-metal Contacts for Improved Stability in Air. Nat. Mater. 2015, 14, 1032-1039. (3) Kim, T.; Kim, J. H.; Kang, T. E.; Lee, C.; Kang, H.; Shin, M.; Wang, C.; Ma, B.; Jeong, U.; Kim, T. S.; Kim, B. J. Flexible, Highly Efficient All-Polymer Solar Cells. Nat. Commun 2015, 6, 8547. (4) Jinno, H.; Fukuda, K.; Xu, X.; Park, S.; Suzuki, Y.; Koizumi, M.; Yokota, T.; Osaka, I.; Takimiya, K.; Someya, T. Stretchable and Waterproof Elastomer-Coated Organic Photovoltaics for Washable Electronic Textile Applications. Nat. Energy 2017, 2, 780-785. (5) Root, S. E.; Savagatrup, S.; Printz, A. D.; Rodriquez, D.; Lipomi, D. J. Mechanical Properties of Organic Semiconductors for Stretchable, Highly Flexible, and Mechanically Robust Electronics. Chem. Rev. 2017, 117, 6467-6499. (6) Kaltenbrunner, M.; White, M. S.; Glowacki, E. D.; Sekitani, T.; Someya, T.; Sariciftci, N. S.; Bauer, S. Ultrathin and Lightweight Organic Solar Cells with High Flexibility. Nat. Commun 2012, 3, 770. (7) Chen, C.-P.; Chiang, C.-Y.; Yu, Y.-Y.; Hsiao, Y.-S.; Chen, W.-C. High-performance, Robust, Stretchable Organic Photovoltaics Using Commercially Available Tape as a Deformable Substrate. Sol. Energy Mater. Sol. Cells 2017, 165, 111-118. (8) Hsieh, Y.-T.; Chen, J.-Y.; Shih, C.-C.; Chueh, C.-C.; Chen, W.-C. Mechanically Robust, Stretchable Organic Solar Cells Via Buckle-on-Elastomer Strategy. Org. Electron. 2018, 53, 339-345. (9) Lipomi, D. J.; Bao, Z. Stretchable, Elastic Materials and Devices for Solar Energy Conversion. Energy Environ. Sci. 2011, 4, 3314. (10) O’Connor, B.; Chan, E. P.; Chan, C.; Conrad, B. R.; Richter, L. J.; Kline, R. J.; Heeney, M.; McCulloch, I.; Soles, C. L.; DeLongchamp, D. M. Correlations Between Mechanical and Electrical Properties of Polythiophenes. ACS Nano 2010, 4, 7538-7544. (11) Awartani, O.; Lemanski, B. I.; Ro, H. W.; Richter, L. J.; DeLongchamp, D. M.; O'Connor, B. T. Correlating Stiffness, Ductility, and Morphology of Polymer:Fullerene Films for Solar Cell Applications. Adv. Energy Mater. 2013, 3, 399-406. (12) Savagatrup, S.; Printz, A. D.; O’Connor, T. F.; Zaretski, A. V.; Lipomi, D. J. Molecularly Stretchable Electronics. Chem. Mater. 2014, 26, 3028-3041. (13) Lipomi, D. J.; Chong, H.; Vosgueritchian, M.; Mei, J.; Bao, Z. Toward Mechanically Robust and Intrinsically Stretchable Organic Solar Cells: Evolution of Photovoltaic Properties with Tensile Strain. Sol. Energy Mater. Sol. Cells 2012, 107, 355-365. (14) Savagatrup, S.; Printz, A. D.; Rodriquez, D.; Lipomi, D. J. Best of Both Worlds: Conjugated Polymers Exhibiting Good Photovoltaic Behavior and High Tensile Elasticity. Macromolecules 2014, 47, 1981-1992. (15) Kim, W.; Choi, J.; Kim, J.-H.; Kim, T.; Lee, C.; Lee, S.; Kim, M.; Kim, B. J.; Kim, T.-S. Comparative Study of the Mechanical Properties of All-Polymer and Fullerene-Polymer Solar Cells: The Importance of Polymer Acceptors for High Fracture Resistance. Chem. Mater. 2018, 30, 2102-2111. (16) Lee, W.; Kim, J.-H.; Kim, T.; Kim, S.; Lee, C.; Kim, J.-S.; Ahn, H.; Kim, T.-S.; Kim, B. J. Mechanically Robust and High-Performance Ternary Solar Cells Combining the Merits of All-Polymer and Fullerene Blends. J. Mater. Chem. A 2018, 6, 4494-4503.



(17) Lee, B. H.; Jung, I. H.; Woo, H. Y.; Shim, H.-K.; Kim, G.; Lee, K. Multi-Charged Conjugated Polyelectrolytes as a Versatile Work Function Modifier for Organic Electronic Devices. Adv. Funct. Mater. 2014, 24, 1100-1108. (18) Cao, F.-Y.; Lai, Y.-Y.; Chen, Y.-L.; Cheng, Y.-J. Self-assembled Tri-, Tetra- and Pentaethylene Glycols as Easy, Expedited and Universal Interfacial Cathode-Modifiers for Inverted Polymer Solar Cells. J. Mater. Chem. A 2016, 4, 8707-8715. (19) Yip, H.-L.; Jen, A. K. Y. Recent Advances in Solution-Processed Interfacial Materials for Efficient and Stable Polymer Solar Cells. Energy Environ. Sci. 2012, 5, 5994. (20) Chueh, C.-C.; Li, C.-Z.; Jen, A. K. Y. Recent Progress and Perspective in SolutionProcessed Interfacial Materials for Efficient and Stable Polymer and Organometal Perovskite Solar Cells. Energy Environ. Sci. 2015, 8, 1160-1189. (21) Li, C.-Y.; Wen, T.-C.; Lee, T.-H.; Guo, T.-F.; Huang, J.-C.-A.; Lin, Y.-C.; Hsu, Y.-J. An Inverted Polymer Photovoltaic Cell with Increased Air Stability Obtained by Employing Novel Hole/Electron Collecting Layers. J. Mater. Chem. 2009, 19, 1643. (22) Znaidi, L. Sol-Gel-Deposited ZnO Thin Films: A Review. Mater. Sci. Eng., B 2010, 174, 18-30. (23) Huang, J.-H.; Wei, H.-Y.; Huang, K.-C.; Chen, C.-L.; Wang, R.-R.; Chen, F.-C.; Ho, K.C.; Chu, C.-W. Using A Low Temperature Crystallization Process to Prepare Anatase TiO2 Buffer Layers for Air-Stable Inverted Polymer Solar Cells. Energy Environ. Sci. 2010, 3, 654. (24) de Bruyn, P.; Moet, D. J. D.; Blom, P. W. M. A Facile Route to Inverted Polymer Solar Cells Using a Precursor Based Zinc Oxide Electron Transport Layer. Org. Electron. 2010, 11, 1419-1422. (25) Sun, Y.; Seo, J. H.; Takacs, C. J.; Seifter, J.; Heeger, A. J. Inverted Polymer Solar Cells Integrated with A Low-Temperature-Annealed Sol-Gel-Derived ZnO Film as An Electron Transport Layer. Adv. Mater. 2011, 23, 1679-1683. (26) Grossiord, N.; de Bruyn, P.; Moet, D. J. D.; Andriessen, R.; Blom, P. W. M. Characterization of Precursor-Based ZnO Transport Layers in Inverted Polymer Solar Cells. J. Mater. Chem. C 2014, 2, 8761-8767. (27) Huang, F.; Wu, H.; Cao, Y. Water/Alcohol Soluble Conjugated Polymers as Highly Efficient Electron Transporting/Injection Layer in Optoelectronic Devices. Chem. Soc. Rev. 2010, 39, 2500-2521. (28) He, Z.; Zhong, C.; Su, S.; Xu, M.; Wu, H.; Cao, Y. Enhanced Power-Conversion Efficiency in Polymer Solar Cells Using An Inverted Device Structure. Nat. Photonics 2012, 6, 593-597. (29) van Reenen, S.; Kouijzer, S.; Janssen, R. A. J.; Wienk, M. M.; Kemerink, M. Origin of Work Function Modification by Ionic and Amine-Based Interface Layers. Adv. Mater. Interfaces 2014, 1, 1400189. (30) Zhang, K.; Gao, K.; Xia, R.; Wu, Z.; Sun, C.; Cao, J.; Qian, L.; Li, W.; Liu, S.; Huang, F.; Peng, X.; Ding, L.; Yip, H.-L.; Cao, Y. High-Performance Polymer Tandem Solar Cells Employing a New n-Type Conjugated Polymer as an Interconnecting Layer. Adv. Mater. 2016, 28, 4817-4823. (31) Hau, S. K.; Yip, H.-L.; Ma, H.; Jen, A. K. Y. High Performance Ambient Processed Inverted Polymer Solar Cells Through Interfacial Modification with a Fullerene SelfAssembled Monolayer. Appl. Phys. Lett. 2008, 93, 233304. (32) Hau, S. K.; Cheng, Y.-J.; Yip, H.-L.; Zhang, Y.; Ma, H.; Jen, A. K. Y. Effect of Chemical Modification of Fullerene-Based Self-Assembled Monolayers on the Performance of Inverted Polymer Solar Cells. ACS Appl. Mat. Interfaces 2010, 2, 1892-1902. (33) Cowan, S. R.; Schulz, P.; Giordano, A. J.; Garcia, A.; MacLeod, B. A.; Marder, S. R.; Kahn, A.; Ginley, D. S.; Ratcliff, E. L.; Olson, D. C. Chemically Controlled Reversible and Irreversible Extraction Barriers Via Stable Interface Modification of Zinc Oxide Electron 18 ACS Paragon Plus Environment

Page 18 of 27

Page 19 of 27 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


Collection Layer in Polycarbazole-based Organic Solar Cells. Adv. Funct. Mater. 2014, 24, 4671-4680. (34) Zhou, Y.; Fuentes-Hernandez, C.; Shim, J.; Meyer, J.; Giordano, A. J.; Li, H.; Winget, P.; Papadopoulos, T.; Cheun, H.; Kim, J.; Fenoll, M.; Dindar, A.; Haske, W.; Najafabadi, E.; Khan, T. M.; Sojoudi, H.; Barlow, S.; Graham, S.; Brédas, J.-L.; Marder, S. R.; Kahn, A.; Kippelen, B. A Universal Method to Produce Low-Work Function Electrodes for Organic Electronics. SCIENCE 2012, 336, 327-332. (35) Woo, S.; Hyun Kim, W.; Kim, H.; Yi, Y.; Lyu, H.-K.; Kim, Y. 8.9% Single-Stack Inverted Polymer Solar Cells with Electron-Rich Polymer Nanolayer-Modified Inorganic Electron-Collecting Buffer Layers. Adv. Energy Mater. 2014, 4, 1301692. (36) Wu, H.; Huang, F.; Mo, Y.; Yang, W.; Wang, D.; Peng, J.; Cao, Y. Efficient Electron Injection from a Bilayer Cathode Consisting of Aluminum and Alcohol-/Water-Soluble Conjugated Polymers. Adv. Mater. 2004, 16, 1826-1830. (37) Changyeon, L.; Hyunbum, K.; Wonho, L.; Taesu, K.; KiffiHyun, K.; Young, W. H.; Cheng, W.; J., K. B. HighffiPerformance AllffiPolymer Solar Cells Via SideffiChain Engineering of the Polymer Acceptor: The Importance of the Polymer Packing Structure and the Nanoscale Blend Morphology. Adv. Mater. 2015, 27, 2466-2471. (38) Lee, C.; Giridhar, T.; Choi, J.; Kim, S.; Kim, Y.; Kim, T.; Lee, W.; Cho, H.-H.; Wang, C.; Ade, H.; Kim, B. J. Importance of 2D Conjugated Side Chains of BenzodithiopheneBased Polymers in Controlling Polymer Packing, Interfacial Ordering, and Composition Variations of All-Polymer Solar Cells. Chem. Mater. 2017, 29, 9407-9415.



Figure 1. (a) Chemical structures of PFN, NBR, PTB7-Th, PC71BM, ITIC, and P(NDI2HDT). Schematic diagram of the fabricated (b) inverted structure, (c) conventional structure, and (d) stretchable devices with its real device photo attached onto the finger.


Page 20 of 27

Page 21 of 27 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


Figure 2. The FT-IR spectra of PFN, NBR, and PFN/NBR blend shown in the wavenumber interval between (a) 1500-3500 cm-1, (b) 2600-3400 cm-1, and (c) 1600-2200 cm-1.



(a) 7

3.5x10

ITO ITO/PFN ITO/PFN:NBR ITO/NBR

7

3.0x10

7

Intensity

2.5x10

7

2.0x10

7

1.5x10

7

1.0x10

6

5.0x10

0.0

14

13

12

11

10

9

8

Binding energy (eV)

Intensity

(b) 2.4x10

7

2.0x10

7

1.6x10

7

1.2x10

7

8.0x10

6

4.0x10

6

Au Au/PFN:NBR

0.0

14

13

12

11

10

9

8

7

6

Binding energy (eV)

(c)

Intensity

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

Page 22 of 27

2.0x10

7

1.6x10

7

1.2x10

7

8.0x10

6

4.0x10

6

0.0

Ag Ag/PFN:NBR

14

13

12

11

10

9

8

Binding energy (eV)

Figure 3. The photoemission cutoff obtained via UPS for the (a) ITO, ITO/PFN, ITO/NBR, and ITO/PFN:NBR blend, (b) Au and Au/PFN:NBR blend, and (c) Ag and Ag/PFN:NBR blend samples on the glass substrates.


Page 23 of 27

40

Frequency (%)

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


NBR

30 PFN/NBR

20

PFN

10 0

0.0

0.3

0.6

0.9

1.2

1.5

1.8

DMT modulus(GPa)

Figure 4. Distribution histogram of the DMT modulus of the PFN, NBR, and PFN/NBR films.

Figure 5. The optical microscopy images of the pristine (a-c) PFN and (d-f) PFN/NBR films subjected to various strains ((a,d) 0 %, (b,e) 20 %, and (c,f) 40 %) on the PDMS substrate without pre-strain treatment. The PFN/NBR film under 60 % strain was shown in the inset of 23 ACS Paragon Plus Environment


(f).

5

-2

Current density (mA cm )

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

Page 24 of 27

0

Without EEL PFN with acetic acid PFN PFN/NBR NBR

-5 -10 -15 0.0

0.2

0.4

0.6

0.8

Voltage (V) Figure 6. The current density-voltage (J-V) characteristics of the studied inverted PTB7Th:PC71BM devices using different EELs.

Table 1. Photovoltaic performance of the studied PTB7-Th:PC71BM devices using different EELs. EEL

Voc (V)

Jsc (mA cm-2)

FF (%)

PCE (%)

Inverted structure Without EEL

0.36±0.02

14.39±0.23

0.48±0.02

2.53±0.16

PFN with acetic acid

0.77±0.01

15.58±0.22

0.64±0.01

7.64±0.04

PFN

0.61±0.01

13.56±0.31

0.54±0.02

4.41±0.13

PFN/NBR

0.73±0.01

15.95±0.26

0.56±0.01

6.59±0.21

NBR

0.55±0.01

14.68±0.50

0.54±0.02

4.29±0.26

Conventional structure Without EEL

0.55±0.04

12.19±0.37

0.41±0.03

2.77±0.48

PFN/NBR

0.80±0.01

15.27±0.62

0.53±0.01

6.49±0.33


Page 25 of 27 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


Figure 7. The energy level diagrams of the device: inverted structure (a) ITO/PC71BM:PTB7-Th/MoO3/Ag and (b) ITO/PFN:NBR blend/PC71BM:PTB7Th/MoO3/Ag, conventional structure (c) ITO/PEDOT:PSS/PTB7-Th:PC71BM/Al and (d) ITO/PEDOT:PSS/PTB7-Th:PC71BM/PFN:NBR blend/Al.



Page 26 of 27

Table 2. Photovoltaic performance of studied intrinsically stretchable devices different BHJ systems fabricated. Jsc (mA cmAcceptor EEL Strain Voc (V) FF 2 ) 0% 0.69±0.01 11.05±2.64 0.34±0.04 Without EEL 5% 0.31±0.01 0.69±0.03 8.95±3.04 PC71BM 0% 0.75±0.02 13.50±1.11 0.34±0.01 PFN/NBR 5% 0.77±0.01 12.39±0.08 0.34±0.01

comprising PCE (%) 2.63±0.84 1.87±0.62 3.41±0.28 3.21±0.02

Without EEL

0% 10%

0.70±0.01 0.70±0.05

10.48±0.83 8.11±1.83

0.28±0.04 2.05±0.42 0.31±0.01 1.74±0.36

PFN/NBR

0% 10%

0.74±0.03 0.73±0.01

9.24±0.83 7.69±1.05

0.25±0.05 1.66±0.23 0.25±0.01 1.38±0.19

Without EEL

0% 10%

0.74±0.02 0.73±0.01

11.77±2.66 10.09±0.21

0.29±0.01 2.52±0.67 0.28±0.01 2.05±0.16

PFN/NBR

0% 10%

0.76±0.02 0.73±0.01

13.13±1.37 14.00±1.69

0.31±0.08 3.00±0.51 0.27±0.04 2.82±0.84

ITIC

P(NDI2HD-T)

Figure 8. (a) The optical microscopy images of PTB7-Th:P(NDI2HD-T) film subjected to various strains on the 3M elastomeric substrate without pre-strain treatment. Distribution histogram of DMT modulus of (b) PTB7-Th:PC71BM, (c) PTB7-Th:ITIC, and (d) PTB7Th:P(NDI2HD-T) films.


Page 27 of 27 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


TOC


Realization of Intrinsically Stretchable Organic Solar Cells Enabled by

Recommend Documents