Unraveling the origin of improved photovoltaic performance in acceptor–acceptor-structured perylene-diimide-based polymeric acceptors through partially fluorinating benzo[c][1,2,5]thiadiazole

Abstract

Recently, perylene diimide (PDI)-based all-polymer solar cells (All-PSCs) have gained increasing attention due to molecular structural diversity, facile chemical modification, and mechanical and morphological stability. However, acceptor–acceptor (A–A)-structured PDI-based polymeric acceptors (PAs) exhibit inferior photovoltaic performance to donor–acceptor type and/or fused-PDI-based ones. Herein, two A–A type thermo- and photo-stable PAs, namely, PPDI-DTBT and PPDI-DTFBT, by choosing a long 2-octyldodecyl-substituted PDI unit and 4,7-dithienylbenzo[c][1,2,5]thiadiazole (DTBT) and/or partially fluorinated 4,7-dithienyl-5-fluorobenzo[c][1,2,5]thiadiazole (DTFBT) subunits to construct a polymer backbone, were developed. Both of them possess a complementary absorption profile and a similar E_LUMO to donor PTB7-Th. Not only a slightly descended LUMO energy level, better molecular planarity and aggregation, and reduced exciton binding energy (E_b), but also enhanced exciton dissociation efficiency, more closed and ordered stacking, higher and balanced charge mobility and a desired microstructural morphology structure were achieved in partially fluorinated PPDI-DTFBT. Moreover, femtosecond transient absorption (fs-TA) spectra suggested faster and more efficient exciton dissociation and transfer characteristics in the PTB7-Th:PPDI-DTFBT system. These changes enabled the PPDI-DTFBT-based device to acquire a PCE of 6.04%, which was 51.76% higher than that (3.98%) of its counterpart. This increase was mainly profited from 32.70% increased J_SC from 11.10 to 14.73 mA cm⁻² and 13.92% enhanced FF from 46.54% to 53.02%. The current results demonstrate that precisely incorporating fluorine into an A–A type PDI-based polymer backbone can synergistically adjust the molecular aggregation and improve the photophysical process, with the aim of gaining an inspirational device efficiency in all-PSCs.