Increase in efficiency on using selenophene instead of thiophene in π-bridges for D-π-DPP-π-D organic solar cells

Abstract

A new D–π–A–π–D small molecule (VC7) based on a selenophene-flanked diketopyrrolopyrrole core and two Zn-porphyrin peripheral moieties was designed and synthesized. The optical and electrochemical properties of VC7 were investigated and compared with those of VC6, which has the same chemical structure but with thiophene units in the π-bridge rather than selenophene. VC7 exhibited extended absorption spectra in the near infrared region of the solar spectrum and showed improved hole mobility with a high HOMO energy level. After optimization of the active layers, VC7:PC₇₁BM organic solar cells (OSCs) showed a significantly higher power conversion efficiency of 9.24% (J_SC = 15.98 mA cm⁻², V_OC = 0.89 V and FF = 0.66) in comparison to VC6:PC₇₁BM (PCE of 7.27%, J_SC = 14.31 mA cm⁻², V_OC = 0.82 V and FF = 0.62); this result represents a 27% increase in efficiency. Our results show that the higher J_SC value for the VC7-based OSC is due to a combination of the more extended absorption profile, higher hole mobility, more balanced charge transport and high exciton dissociation probability.