High photo-current in solution processed organic solar cells based on a porphyrin core A-π-D-π-A as electron donor material

Abstract

Two new conjugated acceptor-donor-acceptor (A-π-D-π-A) molecules with a porphyrin core linked by ethynylene bridges to two thiophene (1a) or thienylenevinylenethiophene (1b) units and both capped by N-ethylrhodanine have been synthesized. These compounds were used as the main electron donor moieties for bulk heterojunction small molecule organic solar cells (BHJ-SMOSC). The optimized devices, with PC71BM as the main electron acceptor molecule, show remarkable short circuit currents, up to 13.2 mA/cm2, an open circuit voltage of around 0.85 V, and power conversion efficiencies up to 4.3% under 100 W/cm2. The External Quantum Efficiency (EQE), Atomic Force Microscopy (AFM), hole mobility, Photo-Induced Charge Extraction (PICE) and Photo-Induced Transient Photo-Voltage (PIT-PV) were analyzed in devices based on 1a and 1b in order to account for differences in the final performance of the two molecules. The PIT-PV decays showed slower recombination kinetics for devices fabricated with 1b. Moreover, the EQE was greater for 1b and this is ascribed to the better nanomorphology, which allows better charge collection before carrier recombination takes place.