D–A–D–π–D–A–D type diketopyrrolopyrrole based small molecule electron donors for bulk heterojunction organic solar cells

Abstract

Two organic small molecules based on diketopyrrolopyrrole (DPP) units having a D–A–D–π–D–A–D structure denoted as DPP-DPP and DPPTDPP were synthesized. Their optical and electrochemical properties relevant to organic solar cells were investigated. The wider optical absorption coverage from 450–800 nm, the highest occupied molecular orbital (HOMO) (−5.23 eV and −5.34 eV for DPP-DPP and DPPTDPP, respectively) and the lowest unoccupied molecular orbital (LUMO) (−3.47 and −3.45 eV for DPP-DPP and DPPTDPP, respectively) make these small molecules suitable as donors for bulk heterojunction organic solar cells. The bulk heterojunction (BHJ) organic solar cells based on an active layer consists of a blend of these small molecules as donors and PC₇₁BM as an acceptor with an optimized weight ratio of 1 : 2 cast from chloroform (CF) showed overall power conversion efficiencies (PCEs) of 1.98% (with J_sc = 5.38 mA cm⁻², V_oc = 0.84 V and FF = 0.42) and 1.85% (with J_sc = 4.56 mA cm⁻², V_oc = 0.96 V and FF = 0.42) for DPP-DPP and DPPTDPP respectively. The relatively high V_oc value based on the DPPTDPP based device has been attributed to the deeper HOMO of DPPTDPP compared to DPP-DPP. The optimized DPP-DPP : PC₇₁BM (1 : 2) and DPPTDPP : PC₇₁BM (1 : 2) active layers were subjected to two step annealing (TSA), i.e. thermal annealing and subsequent solvent vapor annealing and the corresponding BHJ organic solar cells showed a PCE of 5.28% (J_sc = 11.53 mA cm⁻², V_oc = 0.79 V and FF = 0.58) and 5.52% (J_sc = 10.84 mA cm⁻², V_oc = 0.91 V and FF = 0.56), respectively. The enhancement in PCE is mainly due to the improvement in J_sc and FF, related to light absorption in an active layer, a better nanoscale morphology, and an increase in the crystalline nature of the active layer and balanced charge transport, induced by the TSA treatment.