Effect of Side-Chain Engineering of Bithienylbenzodithiophene-<i>alt</i>-fluorobenzotriazole-Based Copolymers on the Thermal Stability and Photovoltaic Performance of Polymer Solar Cells

Side-chain engineering of conjugated polymer donor materials is an important way for improving photovoltaic performances of polymer solar cells (PSCs). On the basis of the polymer J61 synthesized in our group, here, we design and synthesize three new 2D-conjugated polymers J62, J63, and J64 with different types of side chains to further investigate the effect of side chain on their physicochemical and photovoltaic properties. With the narrow bandgap n-type organic semiconductor (n-OS) ITIC as acceptor, the optimized PSCs based on polymer donor of J62 with linear octyl, J63 with linear unsaturated hexylene, and J64 with cyclohexane side chains display power conversion efficiency (PCE) of 10.81%, 8.13%, and 8.59%, respectively. After thermal treatment at 200 °C for 2 h on the active layer,the PCE of the PSC based on J63 still keeps 92% of the original value, which verifies that the cross-linking of the polymer can improve the thermal stability of PSCs. Morphological studies show that the active layer based on J63 displays strong lamellar packing with RMS 1.26, and the active layer based on J64 shows little phase separation with RMS 0.65. The RMS of the active layer based on J62 is 0.900, and the size of phase separation is between that of J63 and J64, which indicates the excessive high lamellar packing or low phase separation is harmful to the performance of PSCs. These results indicate that the side-chain engineering is an effective way to adjust the aggregation of polymers and the morphology of blend films, which are key factors to influence the performance of PSCs