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

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^-2and 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.