Indole and triisopropyl phenyl as capping units for a diketopyrrolopyrrole (DPP) acceptor central unit: an efficient D–A–D type small molecule for organic solar cells

Abstract

Two simple D–A–D structured small molecules based on diketopyrrolopyrrole as central acceptor unit and different terminal units (indole and 2,4,6-triisopropylphenyl for CSDPP1 and CSDPP3 respectively) with broad absorption band and suitable energy levels were synthesized. The new DPP molecules are fully characterized and their photovoltaic properties were investigated using them as donor components along with PC₇₀BM as acceptor in the bulk heterojunction (BHJ) active layer. The organic BHJ solar cells based on CSDPP1:PC₇₀BM and CSDPP3:PC₇₀BM (1 : 1 D–A ratio) cast from chloroform solvent showed moderate power conversion efficiency of about 3.26% and 2.42%, respectively. The higher power conversion efficiency (PCE) of organic solar cells based on CSDPP1 as donor as compared to CSDPP3 has been attributed to the higher hole mobility and absorption coefficient and low optical band gap of CSDPP1. The PCE of the thermally annealed device based on CSDPP1:PC₇₀BM has been improved up to 4.96% attributed to the better film morphology and increased absorption of the BHJ active layer. We attribute the increase in absorption intensity upon thermal annealing to the stronger electrical polarization in the BHJ active layer resulting from morphological changes i.e. crystallinity and molecular aggregation. Finally, the capacitance–voltage measurements indicates that the increasing local electrical polarizations upon thermal annealing can lead to an enhancement of the charge generation from the excitons at donor–acceptor interfaces and their transportation to the respective electrodes in the BHJ device, leading to better short circuit photocurrents and fill factors.