A′–A–π–D–π–A–A′ extended small-molecule photovoltaic donor based on fluorene-diketopyrrolopyrrole with an end-group fluorination effect

Abstract

Three novel small-molecule donors (SMDs), namely Flu(DPP)₂, Flu(DPPsF)₂ and Flu(DPPdF)₂, were designed and synthesized to comprehensively investigate the synergistic effect of extended conjugation and end-group fluorination on photoelectric properties. Firstly, acceptor-π-donor-π-acceptor (A–π–D–π–A) small molecule Flu(DPP)₂ was constructed with alkylated fluorene, ethynyl functionalized thiophene, and diketopyrrolopyrrole (DPP) as the donor, π-bridge bond and acceptor units, respectively. Subsequently, two novel A′–A–π–D–π–A–A′ extended small-molecules Flu(DPPsF)₂ and Flu(DPPdF)₂ were constructed by introducing monofluoro and difluoro substituted phenyl groups at the end group of Flu(DPP)₂. Compared with Flu(DPP)₂, Flu(DPPsF)₂ and Flu(DPPdF)₂ show broader absorption. Moreover, due to the introduction of a fluorine substituted phenyl group, the band gap becomes narrower, and difluoro-terminal substituted Flu(DPPdF)₂ shows a narrower electrochemical band gap (E^CV_g) than monofluoro-terminal substituted Flu(DPPsF)₂. More importantly, bulk heterojunction (BHJ) organic photovoltaic (OPV) devices were prepared using Flu(DPP)₂, Flu(DPPsF)₂ and Flu(DPPdF)₂ as donors mixed with PC₆₁BM, respectively. As a result, the devices based on Flu(DPPsF)₂ and Flu(DPPdF)₂ achieve higher power conversion efficiency (PCE) than those based on Flu(DPP)₂, which are 4.39% and 4.74%, respectively. This work shows that the synergistic strategy with A′–A–π–D–π–A–A′ extended molecular and end-group fluorination provides crucial guidance for the design of novel photovoltaic materials with outstanding photoelectric properties.