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 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 cyclometalated Ir complex (TBzIr) as donor and PC71BM as acceptor. Compared to the original TBzIr:PC71BM BHJ device, the bilayer PDCBT/ TBzIr:PC71BM structure exhibited identical high open circuit voltages of 0.92 V, both increased 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 is inserted to afford extra light absorption, assist the upper BHJ blends to form optimized morphologies, as well as 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 our best knowledge, this is the highest efficiency ever reported for small-molecular Ir complex based organic solar cells.