Non-fullerene polymer solar cells based on a new polythiophene derivative as donor

Abstract

With the development of non-fullerene small-molecule acceptors, non-fullerene polymer solar cells (PSCs) have garnered increased attention due to their high performance. While photons are absorbed and converted to free charge carriers in the active layer, the donor and acceptor materials both play a critical role in determining the performance of PSCs. Among the various conjugated-polymer donor materials, polythiophene (PT) derivatives such as poly(3-hexylthiophene), have attracted considerable interest due to their high hole mobility and simple synthesis. However, there are limited studies on the applications of PT derivatives in non-fullerene PSCs. Fabrication of highly efficient nonfullerene PSCs utilizing PT derivatives as the donor is a challenging topic. In this study, a new PT derivative, poly[5,5′-4,4′- bis(2-butyloctylsulphanyl)-2,2′-bithiophene-alt-5,5′-4,4′-difluoro-2,2′-bithiophene] (PBSBT-2F), with alkylthio groups and fluorination was synthesized for use as the donor in non-fullerene PSC applications. The absorption spectra, electrochemical properties, molecular packing, and photovoltaic properties of PBSBT-2F were investigated and compared with those of poly(3-hexylthiophene) (P3HT). The polymer exhibited a wide bandgap of 1.82 eV, a deep highest occupied molecular orbital (HOMO) of −5.02 eV, and an ordered molecular packing structure. Following this observation, PSCs based on a blend of PBSBT-2F as the donor and 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone)-5,5,11,11- tetrakis(4-hexylphenyl)-dithieno-[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]dithiophene (ITIC) as the acceptor were fabricated. The absorption spectra were collected and the energy levels were found to be well matched. These devices exhibited a power conversion efficiency (PCE) of 6.7% with an open-circuit voltage (VOC) of 0.75 V, a short-circuit current density (JSC) of 13.5 mA·cm-2, and a fill factor (FF) of 66.6%. These properties were superior to those of P3HT (1.2%) under the optimal conditions. This result indicates that PBSBT-2F is a promising donor material for non-fullerene PSCs.