Structure evolution from D-A-D type small molecule toward D-A-D-A-D type oligomer for high-efficiency photovoltaic donor materials

Abstract

In order to study the influence of structure evolution on properties, a D-A-D-A-D-type oligomer of 5BDTBDD and its D-A-D type small molecule of 3BDTBDD were designed and synthesized, which consist of electron-accepting (A) unit of benzo [1,2-c:4,5-c'] dithiophene-4,8-dione (BDD) and electron-donating (D) unit of 4,8-di (6-ethylhexylthiophen-2-yl)benzo [1,2-b:4,5-b']dithiophene (BDT). The effect of structure evolution on crystallinity, absorption, mobility, morphology and photovoltaic properties was primarily investigated. It is found that, by simply inserting a D-A repeat unit, 5BDTBDD shows more improved crystallization, absorption, mobility and morphology than 3BDTBDD. As a result, 5BDTBDD exhibits better photovoltaic properties than 3BDTBDD in their non-fullerene organic solar cells using 9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis (4-hexylphenyl)-di thieno [2,3-d:2′,3′-d']-s-indaceno [1,2-b:5,6-b'] dithiophene (ITIC) as acceptor material. An increasing power conversion efficiency of 7.89% is obtained in the 5BDTBDD:ITIC cells, which is 1.8 times higher than that value (4.33%) in the 3BDTBDD:ITIC cells. It indicates that structure evolution from D-A-D type small molecule toward D-A-D-A-D type oligomer is an efficient strategy to achieve high-efficiency donor materials in organic solar cells.