Theoretical investigation on cyano-substituted isoindigo based copolymer for organic photovoltaics

摘要

A theoretical study is reported for designing π-conjugated small molecules composed of (E)-1, 2-Di(thiophen-2-yl) ethene (TVT) donor and -CN substituted isoindigo (2CNIID) acceptor unit. We investigate the effect cyano (-CN) substitution in the isoindigo (IID) acceptor unit on the optoelectronic and photovoltaic properties. The ground state and excited state properties of the designed oligomers are evaluated via density functional theory (DFT) and time-dependent density functional theory (TD-DFT) respectively. The results reveal that -CN substitution in the acceptor unit leads to deeper HOMO, LUMO levels, and narrow HOMO-LUMO energy gap. A deep HOMO level leads to higher open circuit voltage (V_(oc)) and the higher LUMO level leads to low driving force energy (ΔE). Moreover, the red-shifted broad absorption spectrum is also observed after tuning the acceptor unit strength with -CN substitution. The power conversion efficiencies (PCEs) of the designed oligomers are predicted by Scharber model. The results show that -CN substituted isoindigo based D-A copolymer has higher PCE than the bare TVT-IID copolymer.