Article pubs.acs.org/Macromolecules
2D/1A Strategy to Regulate Film Morphology for Efficient and Stable Nonfullerene Organic Solar Cells Xiaoyu Liu, Jialin Wang, Jiajun Peng, and Ziqi Liang* Department of Materials Science, Fudan University, Shanghai 200433, China S Supporting Information *
ABSTRACT: Recently, the ternary blend method has been successfully applied to nonfullerene organic solar cells (OSCs) and enhanced the device performance by utilizing complementary optical absorption. Here we demonstrate the two polymer donors and one small-molecule acceptor (i.e., 2D/1A) strategy to finely regulate the blend film morphology in fullerene-free OSCs. One crystalline polymer donor, PffBT4T−2OD, can act as an effective morphology regulator for a benchmark blend of PTB7−Th and ITIC, leading to appropriate phase-separated morphology, suppressed charge recombination, efficient charge transport and high carrier mobility. The resulting solvent additive- and annealing-free fabricated bulkheterojunction OSCs show the best power conversion efficiency (PCE) of 8.22% with a significant increase of fill factor compared to their binary counterparts. Importantly, such ternary OSCs when processed under ambient condition retain excellent device performance with a PCE of 7.57%, indicative of good air-stability. manifested the synergetic effects of improving film morphology and increasing optical absorption.22−28 Such a strategy has also been extended to fullerene-free OSCs, mostly comprising one donor and two acceptors (namely, 1D/2A).29−33 For instance, Sun et al. exploited a combination of two nonfullerene acceptors (namely, SdiPBI−Se and ITIC−Th) and a polymer donor (e.g., PDBT−T1) with complementary light absorption in ternary OSCs and acquired significant enhancement of shortcircuit current density (JSC).32 Recently, we have introduced ITIC into PTB7−Th:PDI binary system to enhance optical absorption and suppress PDI aggregates simultaneously in ternary blend.33 On the other hand, for the system of two donors and one acceptor (i.e., 2D/1A), only two notable reports existed to the best of our knowledge.34,35 Both works not only yielded complementary light absorption but also utilized the effect of Förster resonance energy transfer (FRET) to further enhance optical absorption, which resulted in a significant improvement ofJSC yet little variation of both opencircuit voltage (VOC) and fill factor (FF). In addition to varying light absorption, regulation of film morphology is effective to boost device performance by achieving high FF, which has yet to be explored. Meanwhile, considering the striking difference between spherical fullerenes and planar nonfullerene molecules, further studies are required to dictate the nanostructured morphology of nonfullerene ternary blend films. Herein, we aim to construct such a 2D/1A nonfullerene ternary blend in which both two polymer donors possess similar light absorption range, enabling a facile and
1. INTRODUCTION Solution-processed organic solar cells (OSCs) hold good prospect for large-area roll-to-roll production due to their advantages such as low cost, light weight, and flexibility.1−4 In the system of binary bulk-heterojunction (BHJ), the power conversion efficiencies (PCEs) of single-junction OSCs based on the π-conjugated polymer donor (D):fullerene acceptor (A) blend have now exceeded 11%.5,6 However, fullerene acceptors possess inherent drawbacks including fixed energy levels, rigid molecular structures, weak absorption in the visible light region, and thermal instability, which largely limit the performance improvement and future solar module applications.7,8 By overcoming these limitations of spherical fullerene acceptors, the development of planar π-structural nonfullerene acceptors therefore plays a critical role in advancing OSCs, which have recently attracted intensive attention.9−21 In early attempts, Zhan et al. reported a novel nonfullerene acceptor 3,9-bis(2-methylene(3-(1,1-dicyanomethylene)indanone)-5,5,11,11-tetrakis(4-hexylphenyl)dithieno[2,3d:2′,3′-d’]-s-indaceno[1,2-b:5,6-b′]dithiophene (ITIC), and when it was blended with a paradigm two-dimensional polymer donor poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2b:4,5-b′]dithiophene-co-3-fluorothieno[3,4-b]thiophene-2-carboxylate] (PTB7−Th) (Figure 1a) to fabricate BHJ OSCs, the resulting devices yielded an PCE of 6.80%.9 In subsequent much effort, Hou et al. developed a novel polymer donor, PBDB−T−SF and a ITIC derivative, IT−4F for OSCs and achieved the record PCE over 13% in single-junction devices, which outperformed fullerene-based OSCs.21 As a promising alternative to binary cells, ternary blend OSCs via incorporation of the third component have been recently well studied in polymer:fullerene systems and © XXXX American Chemical Society
Received: July 16, 2017 Revised: August 10, 2017
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DOI: 10.1021/acs.macromol.7b01509 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
Figure 1. (a) Chemical structures and (b) schematic energy levels of PTB7−Th,9 PffBT4T−2OD,42 and ITIC,9 respectively.
Figure 2. (a) Optical absorption and (b) photoluminescence spectra of binary and ternary blend films in comparison to neat films. Symbols: D1 = PTB7−Th, D2 = PffBT4T−2OD, and A = ITIC.
precise control of the influences of morphology variations on the device performance. Note that the ideal blend morphology should have bicontinuous interpenetrating network (IPN) structures,36,37 containing three distinct regions(i) highpurity crystalline phases of polymer donor for rapid hole transport, (ii) pure acceptor aggregates for fast electron transport, and (iii) mixed domains of D and A for efficient exciton splitting. We therefore chose a high-crystallinity polymer donor poly[(5,6-difluoro-2,1,3-benzothiadiazol-4,7-diyl)-alt-(3,3‴di(2-octyldodecyl)-2,2′;5′,2″;5″,2‴-quaterthiophen-5,5‴-diyl)] (PffBT4T−2OD)38 (Figure 1a) as a morphology regulator for the benchmark binary photovoltaic system of PTB7−Th and ITIC. Given that both PTB7−Th and ITIC are amorphous molecules, they are thermodynamically miscible in the blend and hence a lack of individual phase of PTB7−Th or ITIC will aggravate charge recombination and hinder carrier transport. Thus, by incorporation of PffBT4T−2OD with adjusting its weight percentage, an optimal blend film morphology is attained with phase-separated yet well-balanced crystalline and amorphous phases. Also, as indicated in Figure 1b, PffBT4T−2OD can facilitate charge transfer owing to the formed cascade energy level alignment39−41 between PTB7− Th and ITIC. Consequently, the ternary OSCs based on PTB7−Th:PffBT4T−2OD:ITIC (0.8:0.2:1.5, w/w/w) blend are fabricated without any solvent additive or postannealing treatment and afford the best PCEs of 8.22% in nitrogen and 7.57% in air, respectively. Remarkably, these devices largely
outperform both PTB7−Th:ITIC (6.48%) and PffBT4T− 2OD:ITIC (4.51%) binary counterparts.
2. EXPERIMENTAL SECTION Materials and Characterization. PTB7−Th, PffBT4T−2OD, and ITIC were obtained from Solarmer Ltd., and the other chemicals and solvents were purchased from Sigma−Aldrich and J&K Scientific, Ltd. (China). The optical absorption spectra, photoluminescence spectra, GIWAXS patterns, and TEM images were measured according to the procedures shown in our previous report.46 Device Fabrication and Measurements. Patterned indium tin oxide (ITO) substrates (around 12 Ω/square) were cleaned sequentially in an ultrasonic solvent bath of soapy water, deionized water, acetone, and isopropyl alcohol for 20 min, respectively. Then, the ITO substrates were dried with N2 flow and further treated in an ozone reactor for 20 min. Next, the ZnO precursor solution was spincoated onto the cleaned ITO substrates at 5000 rpm for 30 s and baked in air at 180 °C for 60 min to form a thin ZnO layer of ca. 30 nm. After being cooled to room temperature, the ZnO-coated substrates were transferred to a N2-filled glovebox. Then, PTB7− Th:ITIC and PTB7−Th:PffBT4T−2OD:ITIC in chlorobenzene solution and PffBT4T−2OD:ITIC in o-dichlorobenzene solution, with a total concentration of 15 mg/mL, were spin-coated onto hot substrates at 1000 to 1200 rpm for 60 s, yielding the active layer of ∼100 nm thick with an area of 0.04 cm2. Note that the processing temperature of chlorobenzene and o-dichlorobenzene solution are 60 and 120 °C, respectively. Finally, MoO3 (10 nm) and Al (100 nm) layers were deposited by thermal evaporation under a pressure of 1 V and High Efficiency > 10% in Fullerene-Free Polymer Solar Cells via Energy Driver. Adv. Mater. 2017, 29, 1605216. (32) Liu, T.; Guo, Y.; Yi, Y.; Huo, L.; Xue, X.; Sun, X.; Fu, H.; Xiong, W.; Meng, D.; Wang, Z.; Liu, F.; Russell, T. P.; Sun, Y. Ternary Organic Solar Cells Based on Two Compatible Nonfullerene Acceptors with Power Conversion Efficiency > 10%. Adv. Mater. 2016, 28, 10008−10015. (33) Wang, J.; Peng, J.; Liu, X.; Liang, Z. Efficient and Stable Ternary Organic Solar Cells Based on Two Planar Nonfullerene Acceptors with Tunable Crystallinity and Phase Miscibility. ACS Appl. Mater. Interfaces 2017, 9, 20704−20710. (34) Zhong, L.; Gao, L.; Bin, H.; Hu, Q.; Zhang, Z.-G.; Liu, F.; Russell, T. P.; Zhang, Z.; Li, Y. High Efficiency Ternary Nonfullerene Polymer Solar Cells with Two Polymer Donors and an Organic Semiconductor Acceptor. Adv. Energy Mater. 2017, 7, 1602215. (35) Bi, P.; Zheng, F.; Yang, X.; Niu, M.; Feng, L.; Qin, W.; Hao, X. Dual Förster Resonance Energy Transfer Effects in Non-Fullerene Ternary Organic Solar Cells with the Third Component Embedded in the Donor and Acceptor. J. Mater. Chem. A 2017, 5, 12120−12130. (36) Yang, Y. M.; Chen, W.; Dou, L.; Chang, W.-H.; Duan, H.-S.; Bob, B.; Li, G.; Yang, Y. High-Performance Multiple-Donor Bulk Heterojunction Solar Cells. Nat. Photonics 2015, 9, 190−198.
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DOI: 10.1021/acs.macromol.7b01509 Macromolecules XXXX, XXX, XXX−XXX
