Ternary polymer solar cells based on wide bandgap and narrow bandgap non-fullerene acceptors with an efficiency of 16.40 % and a low energy loss of 0.53 eV

Abstract

Polymer solar cells (PSCs) are based on a ternary bulk heterojunction (BHJ) active layer consisting of the conjugated polymer PTB7-Th as a donor and two non-fullerene acceptors, that is, a wide bandgap acceptor based on perylene-diimide (PDI) unit, TPhEPDI-4, and narrow bandgap A-D-A acceptor denoted as BThIND-Cl. The lowest unoccupied molecular orbital offset between donor and acceptor materials used for the binary PTB7-Th:BThIND-Cl BHJ is about 0.34 eV, so there is still scope to narrow this offset and further reduce the energy loss through the incorporation of a second acceptor in a ternary active layer of PSCs. Interestingly, when TPhEPDI-4 is used as the second acceptor, there is an increase in open-circuit voltage (VOC) for the PSCs based on the PTB7-Th:TPhEPDI-4:BThIND-Cl ternary system owing to the creation of a cascade LUMO energy level alignment by TPhEPDI-4. After adjusting the weight ratios in the PTB7-Th:TPhEPDI-4:BThIND-Cl ternary BHJ layer to 1: 0.2:1.0 and subsequent solvent vapor annealing, we have achieved a power conversion efficiency of 16.40% (an energy loss of 0.53 eV and a VOC of 0.97 V) which is higher than that for binary PTB7-Th:BThIND-Cl (1:1.2) (14.18% with energy loss and an VOC of 0.58 eV and 0.91 V).