Studies and Optimization of All-Polymer Solar Cells based on Naphthalene Diimide-Biselenophene Copolymer Acceptors

Abstract

Enormous progress has been made in the field of organic photovoltaics (OPVs) for future energy applications in the last two decades. Studies and understanding of fullerene-based polymer solar cells have led to a power conversion efficiency (PCE) of over 12%. However, such fullerene-based OPVs are not stable in air, among other disadvantages. This thesis mainly focuses on studies all-polymer solar cells (all-PCSs) based on naphthalene diimide-biselenophene copolymer acceptors and aims to better understand the relationship between photovoltaic properties, blend morphology, and device processing variables. In this study, we found that optimal thermal annealing and use of a processing additive facilitated favorable blend morphology and enhanced photovoltaic properties in an active layer composed of a high molecular weight (Mn) acceptor polymer, PNDIBS, and a donor polymer, PBDB-T, with complementary absorption bands. We observed a PCE of over 9% in optimized PBDB-T:PNDIBS blend devices. In addition, all-PCSs based on donor PBDB-T and random copolymer acceptors, BSSx (x=10, 20, and 50), were found to have improved morphology, due to the fine tuned bulk crystallinity of the random copolymers. Finally, we explored solution-processed electron transport layers (ETLs) as a way to engineer the cathode interface of the all-PCSs and found that ETLs doped with alkali metal salt can also further enhance the device performance due to the better charge extraction at the cathode.