Origin of the High Donor-Acceptor Composition Tolerance in Device Performance and Mechanical Robustness of All-Polymer Solar Cells

High tolerance regarding photovoltaic performance in terms of donor:acceptor (D:A) composition ratio is reported for all-polymer solar cells (all-PSCs), which is a crucial advantage in producing large-scale devices with high reproducibility. To understand the origin of high D:A ratio tolerance in all-PSCs, we investigate the molecular weight (MW) effects of the P(NDI2OD-T2) polymer acceptor (P_A) on photovoltaic and mechanical robustness of PBDB-T:P(NDI2OD-T2) all-PSCs. Also, we compare the all-PSCs with other types of PSCs consisting of the same polymer donor but using small molecule acceptors (SMAs) including ITIC and PC₇₁BM. We observe that the D:A ratio tolerances of both the photovoltaic and mechanical properties are highly dependent on the P_A MW and the acceptor material types. For example, at a high D:A ratio of 15:1, all-PSCs using high MW P_A (number-average molecular weight (M_n) = 97 kg mol^-1) exhibit 13 times higher normalized power conversion efficiency (PCE) than all-PSCs using low MW P_A (M_n = 11 kg mol^-1), and 20 times higher than ITIC-based PSCs. In addition, the electron mobilities in all-PSCs based on high MW P_A are well-maintained even at very high D:A ratio, whereas the electron mobilities in low MW P_A all-PSCs and SMA-based PSCs decrease by 3-and 4-orders of magnitude, respectively, when the D:A ratio increases from 1:1 to 15:1. Thus, we suggest that the formation of tie molecules and chain entanglements by long polymer chains bridging adjacent crystalline domains is the main origin of excellent D:A tolerance in both mechanical robustness and photovoltaic performance. This work provides an important material design guideline for the reproducible production of flexible and stretchable all-PSCs.