Determining the role of polymer molecular weight for high-performance all-polymer solar cells: Its effect on polymer aggregation and phase separation

The molecular weight of a conjugated polymer is one of the key factors determining the electrical, morphological, and mechanical properties as well as its solubility in organic solvents and miscibility with other polymers. In this study, a series of semicrystalline poly[(2,5-bis(2-hexyldecyloxy)phenylene)-alt-(5,6-difluoro-4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole)] (PPDT2FBT) polymers with different number-average molecular weights (M_ns) (PPDT2FBT_L, M_n = 12 kg/mol; PPDT2FBT_M, M_n= 24 kg/mol; PPDT2FBT_H, M_n= 40 kg/mol) were synthesized, and their photovoltaic properties as electron donors for all-polymer solar cells (all-PSCs) with poly[[N,N-bis(2-octyldodecyl)-napthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5-(2,2-bithiophene)] (P(NDI2OD-T2)) acceptor were studied. The M_n effect of PPDT2FBT on the structural, morphological, electrical, and photovoltaic properties was systematically investigated. In particular, tuning the M_n induced dramatic effects on the aggregation behaviors of the polymers and their bulk heterojunction morphology of all-PSCs, which was thoroughly examined by grazing incident X-ray scattering, resonant soft X-ray scattering, and other microscopy measurements. High M_n PPDT2FBT_H promoted a strong "face-on" geometry in the blend film, suppressed the formation of an excessively large crystalline domain, and facilitated molecularly intermixed phases with P(NDI2OD-T2). Therefore, the optimized all-PSCs based on PPDT2FBT_H/P(NDI2OD-T2) showed substantially higher hole and electron mobilities than those of PPDT2FBT_L/P(NDI2OD-T2), leading to a power conversion efficiency exceeding 5%, which is one of the highest values for all-PSCs reported thus far.