All-Polymer Solar Cells with 9.4% Efficiency from Naphthalene Diimide-Biselenophene Copolymer Acceptor

We report herein the development of high performance all-polymer solar cells (all-PSCs) based on a naphthalene diimide-biselenophene copolymer acceptor bearing 2-octyldodecyl side chains (PNDIBS). Inverted all-PSCs with up to 9.4% power conversion efficiency, high photocurrent (18.32 mA/cm(2)), and remarkably low optical band-gap energy loss (0.53 eV) were achieved by blending high molecular weight (M-n = 53.7 kDa) PNDIBS with a donor polymer PBDB-T that has a complementary absorption spectrum. In contrast, the corresponding lower M-n (28.4 kDa) PNDIBS devices had a maximum efficiency of 7.9%. The enhanced performance of the high-M-n devices is correlated with improved face-on molecular orientation, enhanced electron mobility, and reduced domain sizes of PNDIBS and balanced charge transport in the blend active layers. These findings demonstrate that PNDIBS acceptor polymer is promising for developing highly efficient all-PSCs while providing insights on the critical roles of size of side chains, molecular weight, and choice of the donor polymer component paired with the acceptor polymer.