Influence of the -CN substitution position on the performance of dicyanodistyrylbenzene-based polymer solar cells

作者：He, Baitian; Yin, Qingwu; Xie, Boming; Zhang, Jie^*; Xie, Ruihao; Hu, Zhicheng; Peng, Xiaobin; Huang, Fei^*; Cao, Yong

来源：Polymer Chemistry, 2020, 11(9): 1653-1662.

DOI：10.1039/c9py01781j

• FIELD-EFFECT TRANSISTORS
• CONJUGATED POLYMERS
• EFFICIENCY
• UNIT
• ORIENTATIONS
• DERIVATIVES
• ACCEPTORS
• VOLTAGE
• DESIGN
• LAYER

Molecular engineering of polymer donors is imperative to improve the photovoltaic performance of polymer solar cells (PSCs). Here, we report that the substitution position of the -CN group in dicyanodistyrylbenzene (DCB) moiety-based copolymers greatly affected the molecular packing and photovoltaic performance of PSCs. Four novel copolymers based on DCB units as electron donors and 5,6-difluorobenzo[c][1,2,5]thiadiazole (DFBT) or naphtho[1,2-c:5,6-c ']bis[1,2,5]thiadiazole (NT) units as electron acceptors were designed and synthesized to investigate the effects of altering the -CN substitution position. Density functional theory (DFT) calculations showed that the P-o-derivative copolymers, i.e., those with outer substitution positions, possessed a more planar conjugated backbone than the P-i-derivative copolymers, i.e., those with inner substitution positions, which enhanced the absorption coefficient and provided higher charge mobility. Moreover, two-dimensional (2D) grazing incidence wide-angle X-ray scattering (GIWAXS) patterns showed clearly that the P-o-derivative blends exhibited strong face-on pi-pi stacking. This ordered polymer packing facilitated charge transport in the vertical direction. In contrast, the P-i-derivative blends were prone to edge-on lamellar stacking. Combining each copolymer with a small molecular acceptor (ITIC-4F), an optimum power conversion efficiency (PCE) of 10% was achieved for the P-o-derivative blend devices, whereas the devices based on the P-i-derivative copolymers exhibited a PCE of only 6.67%. Our comparative research indicates that changing the functional group substitution position could affect molecular packing and molecular properties. This approach provides a path toward the molecular design of more materials for high-performance PSCs.