Small-molecular iridium complex based organic solar cells with improved photovoltaic performance through device optimization

Abstract

Small-molecular iridium complex based organic solar cells (OSCs) show inferior power conversion efficiencies (PCEs) to those of pure organic/polymer analogues. To further improve the performance of such OSCs, we reported a bilayer device structure, which was fabricated by sequentially spin-coating a p-type polymer semiconductor (poly[4,4′-bis(2-butyloctoxycarbonyl-[2,2′-bithiophene]-5,5-diyl)-alt-(2,2′bithiophene-5,5′diyl)]) (PDCBT) layer and a bulk-heterojunction (BHJ) layer with the cyclometalated Ir complex (TBzIr) as the donor and PC₇₁BM as the acceptor. Compared to the original TBzIr:PC₇₁BM BHJ device, the bilayer PDCBT/TBzIr:PC₇₁BM structure exhibited an identical high open circuit voltage of 0.92 V, and increased both short circuit current from 9.25 to 11.14 mA cm⁻² and fill factor from 0.46 to 0.61. The p-type PDCBT layer was inserted to afford increased light absorption, assist the upper BHJ blends to form optimized morphologies, and provide supplementary donor–acceptor interfaces to facilitate exciton dissociation. Therefore, the PCE could be significantly improved from 3.91% for TBzIr:PC₇₁BM to 6.17% for PDCBT/TBzIr:PC₇₁BM. To the best of our knowledge, this is the highest efficiency ever reported for small-molecular Ir complex based organic solar cells.