A dithienobenzothiadiazole-quaterthiophene wide bandgap polymer enables non-fullerene based polymer solar cells with over 15% efficiency
Graphical abstract
Introduction
Bulk-heterojunction (BHJ) polymer solar cells (PSCs) have attracted great attention as a promising energy conversion technology because of low-cost solution processing, potential flexibility and semi-transparency, and compatibility with high throughput roll-to-roll printing [[1], [2], [3], [4]]. The BHJ PSCs, whose photoactive layer is composed of a blend of bicontinuous and interpenetrating donor and acceptor, can maximize interfacial area between the donor and the acceptor [5,6]. For a long time, fullerene derivatives, such as PC61BM and PC71BM, are the typical acceptors for high performance BHJ PSCs [3,7,8]. Due to limited light absorption of the fullerene acceptors, polymer donors are required to contribute the majority of light absorption [9,10] such that narrow bandgap polymer donors are beneficial for the efficient PSCs if compared with some wide bandgap (WBG) polymer donors [[11], [12], [13], [14], [15]]. In the past few years, considerable efforts have been devoted to develop small molecular non-fullerene acceptors (NFAs) [[16], [17], [18], [19]], and some of them, eg., Y6 [20] and L8-BO [21] (please see the chemical structures in Fig. S1), belong to narrow bandgap NFAs that can show strong absorptions reaching near-infrared region. This greatly revives the WBG polymer donors because of easy establishment of complementary absorption with a WBG polymer donor and a narrow bandgap NFA [[22], [23], [24], [25], [26], [27]]. Recent breakthrough of single-junction BHJ PSCs with power conversion efficiencies (PCEs) over 18% are based the WBG polymer donors such as PM6, PM7, and D18 (Fig. S1), well demonstrating the importance of the WBG polymer donors [21,28,29].
The PM6, PM7, and D18 belong to 2-dimensional benzodithiophene-based polymers, whose backbones are composed by an electron-donating 2-dimensional benzodithiophene (BDT), an electron-deficient benzodithiophene-4,8-dione (BDD) or dithienobenzothiadiazole (DTfBT), and two thiophene π-bridges. Considering the structural complexity of the PM6, PM7, and D18, it is of great interest to develop new molecular designs towards simple and efficient WBG polymer donors. Polymer PTQ10, with an optical bandgaps (Eg) of 1.95 eV and a repeating unit based on electron-deficient difluoroquinoxaline and electron-donating thiophene, is such an example [30]. The PTQ10:Y6 BHJ active layers have shown notable PCEs more than 16% [31]. A WBG structural design based on an electron-deficient moiety and an oligothiophene is also plausible for D-A type polymer donors, which can decrease the relative amount of a high-cost electron-deficient unit in a polymer [32,33]. It should be noted that the structural design is extremely beneficial for achieving high hole mobility (μh) polymers [34,35]. However, there are two challenges for the resulting polymer donors. The first challenge comes from the control of the Eg. The polymers with too strong electron-deficient moieties would show Eg values below 1.7 eV, somewhat deviated from WBG polymers. For examples, quinoxalinoquinoxaline (NQx)-quaterthiophene (4T) [36], BDD-pentathiophene (5T) [37], difuorobenzothiadiazole (FBT)-4T [11,13], and diketopyrrolopyrrole (DPP)-4T [38] based polymers possess Eg of 1.66, 1.63, 1.61, and ∼1.2 eV, respectively. Another one is related to their photovoltaic performances in NFA PSCs. Except the FBT-4T based polymers [[39], [40], [41]], it is not easy for these narrow bandgap polymer donors to match well with NFAs to achieve PCEs more than 11% (a benchmark of top PCEs of fullerene-based PSCs). Even for the resulting WBG polymer donors (arbitrarily with Eg ≥ 1.75 eV), their NFA PSCs only showed limited success. Thieno [3,4-c]pyrrole-4,6-dione (TPD)-bithiophene (2T), TPD-terthiophene (3T) [42], phthalimide-2T-FBT-2T [43], difluorophthalimide-2T-FBT-2T, DTfBT-3T [44], dithienobenzoxadiazole (DTfBO)-3T [45], DTfBO-4T [45,46], and difluorobenzotriazole-5T [47] based WBG polymer donors show Eg values of between 1.75 and 1.91 eV, however, the achieved PCEs for their NFA based PSCs are between 7% and 13.31%.
Herein, we report the synthesis of an alternating WBG polymer PDTfBT-4T derived from the DTfBT and 4T (Fig. 1a) and its high performance NFA based PSCs. To the best of our knowledge, it is the first time that a PDTfBT-4T polymer is employed as the polymer donor for a PSC. Polymer PDTfBT-4T possesses an Eg of 1.93 eV, comparable to those of fluorinated or esterified polythiophenes [48,49]. NFA Y14 (Fig. 1a), with a close skeleton to that of Y6 but containing a central benzotriazole-embedded fused core and end mono-fluorination, is selected to construct the NFA based active layers. The PDTfBT-4T:Y14 based PSCs showed an open-circuit voltage (Voc) of 0.83 V, a short-circuit current density (Jsc) of 25.42 mA cm−2, and a fill factor (FF) of 72.0%, corresponding to a PCE of 15.19%. This is the highest efficiency among WBG polymer donors based on an electron-deficient unit and an oligothiophene, which is also significantly higher than those (PCEs up to 13.65%) of the fluorinated or esterified polythiophenes [48]. Our results suggest that the simple and low-cost structural design based on an electron-deficient unit and an oligothiophene is promising to achieve powerful WBG polymer donors for efficient NFA based PSCs.
Section snippets
Results and discussion
The synthetic route of PDTfBT-4T is shown in Fig. 1b, and the detailed synthetic procedures and structure characterizations are provided in Supporting Information. Long branched alkyl side chains (decyltetradecyl) are attached on the two flanked thiophenes of the DTfBT unit so as to provide the polymer with enough solubility in the processing of an optoelectronic device [35]. The Stille coupling reaction of 2-stannyl-4-decyltetradecylthiophene with DTfBT-2Br afforded compound 1 in a satisfying
Conclusions
In summary, a WBG polymer PDTfBT-4T derived from the DTfBT and 4T was synthesized as the polymer donor for the photovoltaic application. The UV absorption spectrum of the PDTfBT-4T film shows a main peak at 554 nm and an absorption edge at 642 nm, corresponding to a large Eg of 1.93 eV. Complementary absorption and sufficiently energetic offsets can be achieved between the WBG polymer and a small Eg NFA Y14. In the optimized device, the PDTfBT-4T:Y14 active layer gives a Voc of 0.83 V, showing
CRediT authorship contribution statement
Zesheng Zhang: Material synthesis, Data curation, Writing – original draft, preparation. Feilong Pan: Device characterization. Mei Luo: Material synthesis. Dong Yuan: Material synthesis. Haizhen Liu: Device characterization. Qin Liu: Investigation. Zhuhao Wu: Material synthesis. Lianjie Zhang: Investigation. Junwu Chen: Conceptualization, Supervision, Funding acquisition.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
Financial supports by National Natural Science Foundation of China (51521002, U1401244, and 51673070), National Key Research and Development Program of China (2019YFA0705900), and Basic and Applied Basic Research Major Program of Guangdong Province (2019B030302007) are gratefully acknowledged.
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