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From PCBM-Polymer to Low-cost and Thermally Stable C60/C70-Polymer Solar Cells: the Role of Molecular Structure, Crystallinity and Morphology Control Yalong Xu, Xiaodong Huang, Jianyu Yuan, and Wanli Ma ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b05795 • Publication Date (Web): 26 Jun 2018 Downloaded from http://pubs.acs.org on June 28, 2018

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ACS Applied Materials & Interfaces

From PCBM-Polymer to Low-cost and Thermally Stable C60/C70-Polymer Solar Cells: the Role of Molecular Structure, Crystallinity and Morphology Control Yalong Xu,† Xiaodong Huang,† Jianyu Yuan,†* Wanli Ma†* †

Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Institute of

Functional Nano & Soft Materials (FUNSOM), Soochow University, Suzhou 215123, China. Corresponding Author: [email protected] (J. Y.); [email protected] (W. M.). KEYWORDS：polymer/neat-fullerene blend; thermally stable; morphology; crystallinity; photovoltaic.

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ABSTRACT: The adoption of neat-fullerene (C60, C70) in polymer solar cells offers opportunities to develop cost-effective and thermally-stable devices. Here, through rational sidechain

engineering

of

low

optical-gap

polymer

Poly

(benzodithiophene-furan-

diketopyrrolopyrrole)s (PBDs), we demonstrated for the first time a polymer/C70 blend exhibited higher efficiency (best 6.1%) compare to their polymer/[70]PCBM (best 5.7%) counterparts, and the best efficiency is at the front of efficient polymer/neat-fullerene solar cells. More importantly, we firstly demonstrated the morphology optimization methodology for solution-processed polymer/neat-fullerene blends in order to reduce the strong crystallization and aggregation of neat-fullerene molecules. In comparison with previous work, these results can provide not only material design strategy but also fundamental difference between polymer/neatfullerene and polymer/PCBM blend morphology, which allow us better understanding of how to choose proper materials and optimize blend morphology in polymer/neat-fullerene based device to deliver higher photovoltaic performance.


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1. Introduction Fullerenes have attracted substantial interests for application in solution-processed bulk heterojunction (BHJ) organic solar cells (OSCs).1-4 The fullerene derivatives phenyl-C61/71butyric acid methyl esters (PCBMs), containing fullerene bulky ball and solubilizing side-chains, are dominated as electron acceptors in previous organic photovoltaics (OPVs).5-7 However, conventional acceptor PCBMs present issues for wide-spread application, including limited absorption, complicated purification process, inherent tendency to aggregate and relatively fixed structure and properties.8-11 Quite recently, high-efficiency OSCs using non-fullerene acceptors significantly outperform their PCBMs counterparts.12-17 With respect to recently developed highperforming non-fullerene acceptors,18-25 fullerene molecules provide features isotropic charge transport properties, like high electron affinity and dielectric constant, and sufficient exciton diffusion length.26-27 Therefore, future use of polymer/fullerene active layer may be proved to be advantageous if cost, stability and efficiency issues can be well balanced. Most high-efficiency polymer solar cells (PSCs) devices are fabricated by blending a donor polymer with PCBMs or recently developed non-fullerene acceptors.12-25 PCEs up to 13-14 % can be achieved by further optimizing the donor/acceptor blend morphology.12-25,28-33 Despite these significant achievements in the community, materials cost and device stability need to be further considered for the development of OPVs before reaching the threshold for large-scale commercialization.34-36 In the prospect of an industrial manufacturing, the costs of OSCs fabrication are mainly decided by the material costs,37 which are strongly dependent on the synthetic steps of the organic materials.38-39 Therefore, the adoption of PCBM as the electron acceptor is not desirable. Multiple steps are required to synthesize these derivatives from neat C60 or C70, resulting in increased materials cost as well as higher environmental impact (see


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Table S1, SI). In this regard, using un-functionalized fullerenes like C60 or C70 would be more advantageous. Gratifyingly, recent studies37 including our latest report40-42 have shown that the solubility of pristine fullerene C60 or C70 can be good enough in regular organic solvent like odichlorobenzene (o-DCB) and halogen-free solvent 1,2,4-trimethylbenzene (TMB). Especially for C70, the solubility can be further enhanced to 20~30 mg/mL in these processing solvents (see Table S2, SI).37,40-42 Neat fullerene acceptors like C60 and C70 have been seldom adopted in the solutionprocessed BHJ solar cells. Due to their low solubility and strong tendency to crystallize, the use of neat fullerenes will induce strong aggregation and poor film morphology. Therefore, previous work focuses on BHJ PSCs using neat fullerenes as acceptors are scarce. Early efforts have been dedicated on P3HT/C60 or P3HT/C70 based devices,43-45 with improved stability by incorporating C60,45 while the best efficiencies for pure P3HT/C60 or P3HT/C70 devices are around 2.0%. Due to the achievements in donor-acceptor(D-A) copolymers, devices using pristine fullerene C70 as acceptor have been reported with PCE ranging between 4.0 and 5.0 %.40-42, 46-49 Very recently, Müller and co-workers reported highest PCE of 6.5 % using low bandgap polymer PTB7 as the donor and neat C60/C70 mixture as the acceptor.50 Compared to the readily soluble [70]PCBM, neat C70 exhibits higher molar extinction coefficient49, 51 in the wavelength region of 350-650 nm (as shown in Figure 1), which will enhance the sunlight absorption of the solar cell device. However, current solution-processed devices adopting pristine fullerene are significantly lower compared to those with more soluble PCBMs using the same donor polymer. All these results highlight the importance and urgency of designing donor materials that can be processed with neat fullerenes and even output better performance than PCBMs. Moreover, the stability issuse should also be addressed for practical application. In this


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communication, we have demonstrated higher short circuit current density (Jsc) (~17.0 mA/cm2) and PCE (6.10%) in polymer/C70 solar cell devices by rational donor polymer design. The designed donor polymer can facilitate the formation of uniform and ordered polymer/neat-C70 blend films. In addition, we have shown that well control of the donor polymer crystallinity is an effective method to fabricate high-performance and thermally stable polymer/neat-fullerene PSCs.

2. Results and Discussion The present polymer Poly (benzodithiophene-furan-diketopyrrolopyrrole)s (PBDs) were synthesized as described in Scheme 1. PBDs were synthesized via Stille cross coupling polymerization between distannylated alkylthienyl chains modified benzodithiophene (BDT) monomers and dibrominated furan diketopyrrolopyrrole (DPP) monomer similar to our previous report.52-53 The synthesis detail is described in the experimental section. The application of 2D conjugated side chains can further improve the coplanarity of the target polymer as well as lead to an enhanced absorption, deeper highest occupied molecular orbital (HOMO) level and higher charge mobilities, thereby theoretically can improve the fill factor (FF) and PCEs.28-33 The length and type (e.g., linear, branched) of the flexible side chains attached to the thiophene block could further adjust the crystallinity, molecular packing, and photovoltaic performance of the materials.53 Herein, the alkylthienyl chains with different linear chain length were prepared using the same or similar method as we reported.53 Similar to our previous report,53 the modification of solubilizing side-chains can further optimize the molecular structure and improve intermolecular packing, and all of them show relative high polymerization yields around 85%. The total five target polymers exhibit sufficient solubility in commonly used solvents such as o-DCB and chloroform (CF). The average molecular weight and polydispersity (PDI) were characterized


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through using gel permeation chromatography (GPC) in chloroform at 40 °C with standard as reference, with details listed in experimental section (Figure S1). Systematical characterizations of polymer PBDs were carried out, emphasizing on the effect of alkyl side-chain on material properties. The optical properties of PBDs solutions were studied by using UV-vis-NIR absorption spectroscopy (Figure S2). PBD4 and PBD6 with short side-chains exhibit enhanced absorption 700 nm in solutions, which may be attributed to the solubility induced aggregation in the diluted solutions. The calculated optical band-gap (Eg) judging from the thin film absorption edge films is 1.45 eV for all the polymers (Figure S3). As shown in Figure S4, The electrical properties of PBDs were further investigated by performing cyclic voltammetry measurements. From these results, we discover that the increase of side-chain length can result in up-shift of the polymer HOMO energy levels. PBD4 and PBD6 with short side-chains exhibit deeper HOMO energy level. According to our previous work,53 the steric hindrance from longer alkyl side chains could lead to more twisted backbones and slightly altered conjugation length, which may account for the change of energy levels. The solar cell performance of polymer PBDs were investigated by using a BHJ configuration by incorporating PCBM or C60/C70 as the acceptor. As shown in Figure 1a, solar cell device with a architecture of ITO/PEDOT-PSS(40 nm)/ polymer:fullerene/LiF(0.6 nm)/Al(100 nm) was adopted and all devices were operated under N2 atmosphere. For the solar cell device optimization, we systemically adjust the processing solvents, additives, D/A blend ratios, electron acceptors (Tables S3-S10). We find that C70 based device exhibits improved compared to that in C60 ones, which may be due to the improved solubility and absorption of C70. The optimal active blend films are prepared via spin-coating process from PBDs/PC71BM and PBDs/C70 blends solutions in o-DCB with a total solid concentration of 20 mg/mL.


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Relevant PCEs under AM 1.5G illumination are listed in Figure 2a and Table 1. An interesting initial observation is that devices based on as-cast PBDs/C70 blends operate better with steady PCE between 4.5% and 5.0% for all the polymers (Figure S5, Table S5). However, the PCEs of as-cast PBDs/[70]PCBM devices exhibit quite different values in a wide range from 1% to 4%. In these PCBM based devices, the polymers with longer side-chains show relatively lower PCEs. This observation indicates that devices with neat fullerenes are less sensitive to the polymer sidechain structures under as-cast processing conditions, which is important for realizing reproducible photovoltaic performance. As shown in Figure S5 and Table S8, significant improvements are observed for PBDs/PC71BM devices upon the introduction of additive 1,8diiodooctane (DIO)54-56 to the processing solvent, resulting highest PCEs of 5.21%, 5.43%, 4.00%, 5.06% and 4.95% for PBD4, PBD6, PBD8, PBD10 and PBD12, respectively. However, the solvent additive DIO hardly improves the PCE of PBDs/C70 based devices (Table S7), which may due to the strong tendency of pristine fullerenes to crystallize during solvent evaporation, which already results in self-contained BHJ blend morphology.44-47 Similar trend was observed in previous reports, polymer/pristine fullerene based solar cell devices can output optimized PCE without any processing additive treatment.40-42 In this work, we further incorporate a solid additive PDMS to enhance the viscosity of the polymer/C70 blend solutions, which can increase the active layer thickness for improved photon absorption (Table S10). As shown in Figure 2a, PBDs/C70 devices with the introduction of PDMS exhibit highest PCEs of 5.45%, 6.10%, 5.12%, 5.01% and 4.95% for PBD4, PBD6, PBD8, PBD10 and PBD12, respectively. The optimal PCE of 6.1% is comparable to the recently reported highest PCE of 6.4% for solution-processed polymer/pristine fullerenes solar cells.50 In addition, for the first


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time, we reported efficient polymer/C70 solar cells which can demonstrate higher PCE than that of polymer/[70]PCBM ones with the same donor polymer. We further evaluate the performance of highly-crystalline polymer P3HT and semi-crystalline PBDTTPD (P8)

57-58

under the same processing conditions, in order to reveal the impact of

polymer crystallinity on the compatibility between polymer and pristine fullerenes. The 2D grazing incidence wide-angle X-ray scattering (2D-GIWAXS) patterns of polymer films are shown in Figure S6. In solution-processed polymer/pristine fullerene blends, even the morphology control is not yet fully understood; the amorphous donor polymers (i.e. PTB7, PCDTBT) have previously been reported to output reasonable solar cell performance when blending with pristine fullerene.49 In this contribution, the polymer crystallinity was thought to be an important factor for engineering the morphology of polymer/pristine fullerene BHJ blend. Current density-voltage (J-V) characteristics of solar cell devices using on conventional P3HT, P8 and PBD6 with different electron acceptors under AM 1.5G 100 mW cm−2 irradiation are shown in Figure 2b and Table 1. Apparently, all C60/C70 based devices exhibited decreased open-circuit voltage Voc. The lower Voc compared to that in PCBMs based devices may be partially attributed to the down-shift LUMO energy levels of neat fullerene, as shown in Figure 1c.40-49 The P8/C60 and PBD6/C70 solar cell devices exhibit slight lower FF values relative to those in PCBM based devices. However, more crystalline P3HT/C60 devices exhibit unexpectedly S shape J-V curve, resulting in a much lower FF of 0.46. Meanwhile, we observe decreased Jsc for P8/ C60 and P3HT/C60 based devices despite the enhancement of light absorption by using neat fullerene (Figure 1d). The change of Jsc strongly depends on the type of donor polymer. For the highly crystalline polymer P3HT, we observe a huge decrease of Jsc (5.1 mA/cm2) for C60 based devices compared to that of PCBM based ones (9.0 mA/cm2). In


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addition, the semi-crystalline polymer P8 exhibits a moderate Jsc decrease (from 11.0 to 9.1 mA/cm2). In contrast, devices based on amorphous polymer PBD6 show a notably enhancement in Jsc (from 12.8 to 16.9 mA/cm2). Similar change of Jsc has been also observed in our previous polymer/C70 work and other reports using amorphous donor polymers.40-49 The combination of pure fullerene and donor polymer with weak crystallinity evidently shows synergistic effect on the improvement of device current. However, the origin of the lower Jsc in polymer/pristine fullerene devices for P8 and P3HT is not totally understood. To confirm the effect of pristine fullerene on device photo-current, the absorption, extracting from reflection spectrum (R) by 100 %–R, and external quantum efficiency (EQE) of optimized PBD6/[70]PCBM and PBD6/C70 devices have been characterized, with the results shown in Figure 2c. Owing to the narrow optical gap of the donor polymers, PBDs based solar cell devices exhibit high and steady EQE values in a wide range of 350 to 800 nm. In addition, the EQE values of PBD6/C70 device exhibit ~30% enhancement relative to PBD6/[70]PCBM device in the whole range. The Jsc calculated by integrating the EQE curve fits well (within 5% error) with the Jsc obtained from the J-V curves. Besides, the internal quantum efficiency (IQE) of both solar cell devices was also measured for further comparison. According to previous report,59 the solar cell IQE is defined as the percentage of absorbed photons that are successfully converted to current. Here, the IQE of the optimized devices have been calculated based the EQE spectra (Figure 2c) and the absorption spectra (Figure S7), with the corresponding IQE shown in Figure 2c. First, we find that the device absorption of C70 based cell is improved compared to that of [70]PCBM ones, which is consistent with that the higher neat-C70 have molar extinction coefficient in the wavelength region of 350-650 nm. In addition, PBD6/C70 device exhibit more pronounced IQE values than that of PBD6/[70]PCBM device between 350 and 650 nm. Thus the


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photon conversion efficiency in PBD6/C70 device is higher than that in PBD/[70]PCBM ones. The higher IQE value indicates favorable blend morphology existing in our PBD6/C70 system for exciton generation and dissociation, which may be the major factor for the improved device performance. In addition, we also evaluate the thermal stability of solar cell devices adopting [70]PCBM or neat-C70, all the devices were continuously annealed on a hotplate for 10 mins at different temperatures (80, 100, 120, 140, 160, 180 and 200 oC), As shown in Figure 2d, PBD6/C70 cell exhibits improved stability compared to that of PBD6/[70]PCBM. The device can maintain ~50% PCE after annealing at 200 oC for 10 mins, while PBD6/[70]PCBM based device can only maintain ~20% of the original PCE. PCBM-based polymer solar cells have poor thermal stability because of macro-phase separation from large PCBM crystallites under thermal stress.60-62 Although previous studies indicated that bulky organic moieties on PCBM can sterically interfere with the formation of the PCBM aggregation,63 most polymer/C60 or polymer/C70 blend films were not optimized due to the unmatched donor-acceptor crystallinity. Here, through rational molecular design, we obtained optimal polymer/C70 blends without macro-phase separation, which should be a main factor for the improved thermal stability. Considering the vast effect of blend morphology on the solar cell performance, we then studied the corresponding BHJ blend morphology. Phase transitions and changes in crystallinity of polymer/fullerene are measured by differential scanning calorimetry (DSC). Figure 3a shows the heat flow (HF) traces of P3HT and PBD6 based blends with different electron acceptor. In the first cooling cycle of P3HT based blends, crystallization of P3HT is observed as an peak with shoulder at 156 oC and 190 oC for P3HT/PCBM and P3HT/C60, respectively. In the subsequent heating, there is a melt crystallization peak of fullerene at 191 oC and 220 oC for P3HT/PCBM and P3HT/C60, respectively. In comparison with the P3HT/PCBM blend, both the melt


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crystallization peak of P3HT and fullerene in P3HT/C60 blend shift to higher temperatures with a sharper shape, which implies that both donor and acceptor phase is in an improved crystalline state relative to that in P3HT/PCBM ones.64-65 Similar trend can be also found in PBD6/fullerene blends. Due to the amorphous properties of donor polymer PBD6, no transpiration peak was observed in the first cooling cycle of both blend films. However, melt crystallization of fullerene is observed as an exothermic peak with shoulder at 211 oC and 214 oC for PBD6/[70]PCBM and PBD6/C70, respectively. Again, the crystallization peak of fullerene in PBD6/C70 blend slightly shift to higher temperatures with a sharper shape, suggesting that the acceptor C70 phase is in an improved crystalline state compared to that of PCBM in PBD6/[70]PCBM blend. In order to confirm these results, we further conduct grazing incidence x-rays diffraction (GIXD) measurements of these blends (Figure S8). As shown in Figure 3b, we can observe that peak coming from P3HT at 3.9 nm-1 in both cases of P3HT/[60]PCBM and P3HT/C60. The full width at half maximum (FWHM) of the peak turned out to be 0.55 nm-1 and 0.45 nm-1 for PBD6/[70]PCBM and PBD6/C70 blend, respectively. Therefore, according to Scherrer equation,66 we can further estimated the correlation length value of fullerene crystallites to be 3.65 nm and 4.44 nm for C60 and [60]PCBM respectively. More importantly, we can observe an enhanced broad peak at 14 nm-1 which is the scattering signatures of neat-C60 in P3HT/C60 blend film.67 In contrast, the crystallization of PCBM in P3HT/[60]PCBM blend is relatively hindered. In the case of PBD6/fullerene, we can also observe the apparent peak coming from PBD6 at 8.27 nm-1 in PBD6/C70 film, indicating improved ordered structure in polymer/neatfullerene blend. In contrast, we do not observe the polymer peak in PBD6/[70]PCBM film, suggesting the disordered state of the polymer domains. This discrepancy may be attributed to the higher electron mobility of neat fullerenes. Neat fullerene can move out of the polymer


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matrix more easily, which in turn promotes the crystallization of the polymer. Altogether, the DSC and GIXD characterization for the first time clearly reveals the crystallinity change of polymers and fullerenes during transition when using both PCBM and neat-C60/C70 as the electron acceptors. We found that: 1. the crystallinity of both donor polymers and fullerenes phase are significantly improved in polymer:C60/C70 system; 2. the polymer:C60/C70 BHJ blend morphology depends greatly on the crystallinity of the donor polymer. To confirm the changes in crystallinity or long range order of polymers and fullerenes during spin-casting, thorough morphology investigations including atomic force microscopy (AFM), transmission electron microscopy (TEM) and high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) were finally carried out to well understand the morphology evolution when using neat-C60/C70 as an alternative electron acceptor. As shown in Figure 4a-d, compared to the optimized P3HT/[60]PCBM blend film, there are huge branches and aggregations in the blend film of P3HT/C60. Similar trend can be also found in the AFM height images (Figure S9), the P3HT/C60 blend film exhibits significantly enhanced surface roughness. According to previous report,68 the domains in HAADF-STEM images containing elements with larger atomic numbers show a lighter image, which is contrary to the conventional TEM image. Hence, in Figure 4b, the dark branches and aggregations represent the carbon-rich areas, corresponding to fullerene-rich domains. The formation of long range order of neat-fullerene at the film surface provides insight into why neat fullerenes decrease the device performance. In contrast, for PBD6, after replacing PCBM with C70 under the same D/A blend weight ratio, judged from both AFM (Figure S9) and TEM (Figure 4) morphology characterization, the blend exhibit similar microstructure feature and phase separation compared to the PBD6/[70]PCBM blend film, which may explain the comparable PBD6/C70 solar cell


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device performance. Since the neat fullerenes and polymers can all be well dissolved, solubility can’t account for the evidently different morphology between PBD6/C70 and P3HT/C60 blend films. The excessively aggregated film of P3HT/C60 may be due to the strong crystallization trend of both P3HT and C60, as seen by the rapid appearance of the diffraction peak at 3.9 nm−1 for P3HT during spin-coating and the large branches of the fullerene clusters in the blend film. Fortunately, for donor polymer PBD6, its amorphous nature limits the formation of highly aggregated polymer clusters, as well as restrains the sizes of neat fullerenes clusters. Note that the aggregation of neat fullerenes in turn can slightly promote the crystallization of PBD6, as confirmed by the GIXD characterization. As a result, the moderately crystallized C70 and more ordered PBD6 can improve the carrier transport in the blend film without largely reducing the exciton dissociation efficiency. As shown in Figure 4c-4d, we noticed that the polymer P8 with moderate crystallinity shows a phase segregation extent between highly crystalline P3HT and amorphous PBD6. The optimized P8/PCBM blend exhibits fine phase separation domains with local formation of crystalline polymer fibrils, which is useful for efficient charge transport. Neat fullerenes exhibit quicker crystallization during spin-coating process and result in enhanced polymer and fullerene crystallization. Therefore, we observed that a number of enlarged fullerene-rich clusters appearing in P8/C60 blends. The DSC, GIXD and morphology results shown in Figure 3 and Figure 4a-4f can be interpreted by the morphology change illustrated in Figure 4g. When blended with a highly crystalline donor polymer like P3HT, the neat fullerene is more likely to aggregate compared with PCBM and meanwhile further enhance the crystallization of the donor polymer, leading to suppressed charge separation efficiency and discontinuous charge transport resulting from huge boundary resistance. In addition, the large aggregations in the film may


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induce leakage current and increased recombination, which partially account for the S shaped JV curves of the P3HT/C60 cell. Conversely, adopting preferred amorphous donor polymer like PBD6, the crystallization of the neat fullerenes are greatly hindered while the order of the donor polymer is slightly enhanced (Figure 4g). It have been reported that the interfacial tension determines the morphology and its stability,69 we hypothesis that the adjustment of the donor polymer crystallinity will further affect the interfacial tension between polymer and fullerene molecules. Pristine fullerene exhibits enhanced crystallization relative to PCBM, according to previous report,30 the charge transfer and excitons dissociation is greatly affected by the domain purity at the interfaces. Thus, PBD6/C70 may facilitate the exciton dissociation and charge transport, further improving the device performance of solution-processed BHJ solar cells. Similar trend can be found in the electron-only devices based these blends, as shown in Figure S10, the electron mobilities of the optimized PBD6/C70 is 3.7*10−4 cm2 V−1 s−1, which is higher than that of the optimal PBD6/PCBM blend (1.6*10−4 cm2 V−1 s−1). Hence, well selection of donor polymer with desired crystallinity plays a critical role in deciding the photovoltaic properties of solution-processed polymer/neat-fullerene solar cell. It is also worth commenting on the high efficiency of the PBD6/C70 cell compared to that of PBD6/PCBM ones. To highlight the relatively high and improved PCE of PBD/C70, we provide a plot of polymer crystallinity vs PCE in Figure 5 for a series of reported polymers used in solution-processed solar cell devices adopting C60/C70 as the electron acceptors.40-50,

70-71

Figure 5 exhibits that the PBD6/C70 device is excellent in terms of the PCE drop when switching acceptor from PCBM to C60/C70. We note that polymer crystallinity-PCE disregards the effect of the fullerenes and only describes the effect of donor polymer. Most of successful solution-processed polymer/neat-fullerene devices were based on relatively amorphous donor


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polymers, and most of the polymers exhibit lower PCE compared to PCBM-based devices under similar conditions. PBD has the improved performance when adopting C70 as the acceptor, and achieved PCE is one of the highest in neat fullerene based devices.

3. Conclusions In conclusion, solution-processed polymer/neat-C70 OSCs employing a series of novel narrow optical gap polymer PBDs with improved efficiency relative PCBM have been successfully demonstrated. A best PCE of 6.1% with improved thermal stability have been achieved for such type of OSCs based on a tailored polymer PBD6 with modified side-chains. In comparison with the highly crystalline donor polymer P3HT and semi-crystalline donor polymer P8, the high device efficiency using PBDs can partially be attributed to the well control of donor crystallinity which leads to improved blend morphology than the polymer/PCBM system. Furthermore, jointly using DSC, GIXD and electrical microscopy characterization techniques allowed us better understanding how to optimize blend morphology in solution processed polymer/neat-fullerene BHJ blend to deliver higher photovoltaic performance. While we recognize that these findings are based on a specific polymer BHJ system, the overall morphology optimization methodology may become a general approach to improve the film morphology in solution-processed polymer/neat-C70 OSCs.


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Scheme 1. The preparation of polymer PBDs.

Figure 1. a) Device structure of the polymer solar cells studied in this work; b) molecular structure of [60]PCBM, [70]PCBM, C60 and C70; c) energy levels of the solar cell device; d) UV-vis absorption spectra of [60]PCBM, [70]PCBM, C60 and C70 in ODCB (0.1 mg/mL) solutions.


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Figure 2. Optimal PCE values of polymer/fullerene solar cell devices with or without processing additives (a); J−V curves of the polymer/fullerene solar cell devices based on P3HT, P8 and PBD6 (b); EQE and IQE curves of the optimized PBD6/[70]PCBM and PBD6/C70 solar cells (c); normalized PCEs of optimized PBD6/[70]PCBM and PBD6/C70 solar cells as a function postannealing temperature (d).


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Figure 3. a) DSC thermograms showing HF for P3HT/fullerene and PBD6/fullerene blends (the first cooling and the second heating); b) GIXD measurements of P3HT/fullerene and PBD6/fullerene blends prepared according to the optimal conditions.


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Figure 4. Bright-field TEM images of optimal P3HT/PCBM (a), P3HT/C60 (b), P8/PCBM (c), P8/C60 (d), PBD6/PCBM (e), PBD6/C70 (f) BHJ blend, the corresponding drift corrected HAADF-STEM images are shown with red fringe, the scale bar for the inserted images is 100 nm; schematic depiction of the BHJ film morphology blend neat-fullerene with different donor polymers (g).


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9 8

Solid Symbol:

PCBM Solar Cells

C60/C70 Solar Cells

9 Our work

3

P8

4

2

5 4 3

PTB7

5

PBSF PBD6

6 PEDOTNDIF PCDTBT

6

2

1 0

8 7

PCE Limit of 6%

P3HT

PCE(%) PCBM

7

Open Symbol:

PCE(%) C60/C70

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1 Highly Crystalline

Semi-crystalline

Amorphous

0

Figure 5. Polymer crystallinity and PCE distributions of reported high performance solution processable polymer/neat-fullerene solar cells.

Table 1. Optimized photovoltaic parameters of solar cell devices adopting on PTP8, P8 and PBD6 as donor, PCBMs or pristine fullerenes as electron acceptor.

a

BHJ blend

Thickness (nm)

Voc (V)

Jsc (mA/cm2)

FF

PCE(Avg.)a (%)

P3HT/[60]PCBM

120

0.62

8.9

0.65

3.6 (3.4)

P3HT/C60

105

0.53

5.1

0.46

1.3 (1.2)

P8/[60]PCBM

110

0.90

11.0

0.63

6.2 (6.0)

P8/C60

100

0.75

9.1

0.56

3.8 (3.6)

PBD6/[70]PCBM

95

0.73

12.8

0.61

5.7 (5.5)

PBD6/C70 90 0.63 Average values are over 6 devices.

16.9

0.57

6.1 (5.9)


20

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ASSOCIATED CONTENT Supporting Information. The preparation of polymer PBDs. The absorption spectra and Cyclic voltammetry curves of polymer PBDs, absorption spectra of PBD6/[70]PCBM and PBD6/C70 blend film, 2D GIWAXS patterns of neat P8 and PBD6 solid film, and AFM height images of BHJ blend films. The cost comparison of fullerenes, ITIC and naphthalene n-type derivatives, and solubility of neat fullerene in conventional organic solvent. Device optimization, device performance with different blend weight ratios and thermal annealing, w/wo additive effect. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author [email protected] (J. Yuan) [email protected] (W. Ma) Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes Any additional relevant notes should be placed here. ACKNOWLEDGMENT This work was supported by the Natural Science Foundation of Jiangsu Province of China (BK20170337), the National Natural Science Foundation of China (Grant No.


21


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51761145013 and 61674111), and “111” projects. The author thanks the Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University. We also acknowledge the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

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[71]Tada, K., Characterization of Polymer Bulk Heterojunction Photocell with Unmodified C70 Prepared with Halogen-free Solvent for Indoor Light Harvesting. Org. Electron., 2016, 30, 289-295.

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