An effective heteroatom-substituted strategy on photovoltaic properties of D(A-Ar)2 small molecules for efficient organic solar cells

Abstract To research the effects of different kinds of terminals on the solar cell performance, in this paper, carbazole (Cz), 2-octylthiophene (T) and 2-octylhiothiophene (TS) units are picked as end-capping units, with a pyran-bridged indacenodithiophene (IDTP) as central core and two fluorinated benzothiadiazole (DFBT) as acceptors, three π-conjugated small molecules (SMs) with D(A-Ar) 2 framework, namely IDTP(DFBT-Cz) 2 , IDTP(DFBT-T) 2 and IDTP(DFBT-TS) 2 , respectively, were designed and synthesized for application as donor materials in solution-processed small molecular organic solar cells (SM-OSCs). Encouragingly, after the tetrahydrofuran (THF) solvent vapor annealing (SVA) process, the device based on IDTP(DFBT-TS) 2 exhibited outstanding efficiencies up to 7.33%, which is 1.71 times equal to the 4.29% PCE of the alkyl terminal substitution IDTP(DFBT-T) 2 , and 1.45 times equal to the 5.06% PCE of the carbazolyl terminal substitution IDTP(DFBT-Cz) 2 , which is mainly attributed from its better molecular planarity, higher hole mobility, and better morphologies of blend films in comparison with the other two SMs. To our best knowledge, this work with the IDTP core in SM donor materials is reported for the first time, compared to the IDT core, the IDTP core proved to effectively improve molecular crystallinity and fill factor (FF). The results reported here clearly demonstrate that the alkylthio substitution terminal-group engineering is an effective strategy to further improve the photovoltaic performance of the molecules.