Small molecule based N-phenyl carbazole substituted diketopyrrolopyrroles as donors for solution-processed bulk heterojunction organic solar cells

Abstract

We report two acetylene-bridged small molecules DPP5 and DPP6 with low HOMO–LUMO gaps as donors along with PC₇₁BM as an acceptor for the fabrication of solution-processed bulk heterojunction solar cells. After the optimization, i.e. weight ratio of donor to acceptor and surface treatment of the active layer, we achieved overall power conversion efficiencies up to 4.65% (J_sc = 8.19 mA cm⁻², V_oc = 0.98 V and FF = 0.58) and 5.73% (J_sc = 9.58 mA cm⁻², V_oc = 0.98 V and FF = 0.61), for DPP5:PC₇₁BM and DPP6:PC₇₁BM respectively, which are superior to those for the devices based on as-cast active layers. The significant change in the power conversion efficiency is attributed to the improvement in nanoscale morphology, balanced charge transport and charge collection efficiency, induced through the surface treatment.