Simultaneous enhancement of performance and insensitivity to active layer thickness for OPVs by functionalizing π-spacer's side chain

In the design of high performance conjugated copolymers, π-spacer plays one of the most important roles in finely manipulating photovoltaic properties of the polymers. However, up to now, little attention has been paid to functionalize the π-spacer. In this work, a novel D–A conjugated copolymer PBTI3T-S, with 3-(decylthio)thiophene as the π-spacer, is designed and synthesized. The introduction of a modified π-spacer has little influence on the absorption spectrum and band gap of the polymer. However, it is found that the modified π-spacer could create a noncovalent attractive interaction between neighboring moieties, resulting in good planarity and decreased π–π stacking distance in comparison to its analogue PBTI3T with decyl substitution on the π-spacers. More interestingly, the face-on population of crystallite orientation is significantly enhanced. Thus, the hole mobility of the PBTI3T-S/PC71BM blend is 1.29 × 10−2 cm2 V−1 s−1, which is one order of magnitude higher than that of the PBTI3T/PC71BM blend (1.15 × 10−3 cm2 V−1 s−1). The PBTI3T-S device provides a higher PCE of 7.14%, while the optimized PBTI3T device provides 6.51%. Impressively, the PCEs of the PBTI3T-S device can remain above 7% without substantial loss as the active layer thickness increases up to ∼270 nm, while the PCE reduces to 5.70% for the PBTI3T. These results demonstrate that the introduction of an alkylthio chain modified π-spacer would be an effective way to further improve the device performance and provide a guideline for molecular engineering towards the application of the roll to roll printing technique.