3 years ago

Tuning Energy Levels without Negatively Affecting Morphology: A Promising Approach to Achieving Optimal Energetic Match and Efficient Nonfullerene Polymer Solar Cells

Tuning Energy Levels without Negatively Affecting Morphology: A Promising Approach to Achieving Optimal Energetic Match and Efficient Nonfullerene Polymer Solar Cells
Wei Ma, Zhengke Li, Joshua Yuk Lin Lai, Tingxuan Ma, He Yan, Jianquan Zhang, Jing Liu, Guofang Yang, Kui Jiang
One advantage of nonfullerene polymer solar cells (PSCs) is that they can yield high open-circuit voltage (VOC) despite their relatively low optical bandgaps. To maximize the VOC of PSCs, it is important to fine-tune the energy level offset between the donor and acceptor materials, but in a way not negatively affecting the morphology of the donor:acceptor (D:A) blends. Here, an effective material design rationale based on a family of D–A1–D–A2 terthiophene (T3) donor polymers is reported, which allows for the effective tuning of energy levels but without any negative impacts on the morphology of the blend. Based on a T3 donor unit combined with difluorobenzothiadiazole (ffBT) and difluorobenzoxadiazole (ffBX) acceptor units, three donor polymers are developed with highly similar morphological properties. This is particularly surprising considering that the corresponding quaterthiophene polymers based on ffBT and ffBX exhibit dramatic differences in their solubility and morphological properties. With the fine-tuning of energy levels, the T3 polymers yield nonfullerene PSCs with a high efficiency of 9.0% for one case and with a remarkably low energy loss (0.53 V) for another polymer. This work will facilitate the development of efficient nonfullerene PSCs with optimal energy levels and favorable morphology properties. A terthiophene-based donor polymer PffBTBX-T3 with the D–A1–D–A2 structure is demonstrated and compared with the other two D–A-type polymers PffBT-T3 and PffBX-T3. While the energy levels of three polymers are fine-tuned, their optical and morphological pro­perties are not dramatically changed. The optimal energetic match and well-controlled morphology enable 9.0% effi­cient nonfullerene devices based on PffBTBX-T3 and ITIC-Th.

Publisher URL: http://onlinelibrary.wiley.com/resolve/doi

DOI: 10.1002/aenm.201602119

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