Synthesis and photophysical properties of semiconductor molecules D1-A-D2-A-D1-type structure based on derivatives of quinoxaline and dithienosilole for organics solar cells

Abstract

A novel small molecule with D1-A-D2-A-D1 structure denoted as DTS(QxHT2)2 based on quinoxaline acceptor and dithienosilone donor units was synthesized and its optical and electrochemical properties were investigated. The thin film of DTS(QxHT2)2 showed a broad absorption profile covering the solar spectrum from 350 nm to 780 nm with an optical bandgap of 1.63 eV. The energy levels estimated from the cyclic voltammetry indicate that this small molecule is suitable as donor along with PC71BM as acceptor for the fabrication solution processed bulk heterojunction solar cells for efficient exciton dissociation and high open circuit voltage. The organic solar cells based on optimized DTS(QxHT2)2:PC71BM active layers processed with chloroform and DIO/CF showed overall power conversion efficiency of 3.16% and 6.30%, respectively. The higher power conversion efficiency of the solar cell based on the DIO/CF processed active layer is attributed to enhanced short circuit photocurrent and fill factor may be related to better phase separation between donor and acceptor in the active layer and more balanced charge transport, induced by the solvent additive. The power conversion efficiency of the organic solar cell was further improved up to 7.81% based on active layer processed with solvent additive, using CuSCN as hole transport layer instead of PEDOT:PSS and mainly attributed to increased fill factor and open circuit voltage due the formation of better Ohmic contact between the active layer and the CuSCN layer.