Configuration dependent photovoltaic properties of cyclometalated heavy metal complexes for organic solar cells

Cyclometalated heavy metal complexes as a new class of photovoltaic materials have attracted specific attention owing to their triplet feature, which is expected to be beneficial for longer exciton diffusion lengths and more efficient exciton dissociation in organic solar cells (OSCs). In this work, based on the organic linear ligand of 2-(5''-tert-butyl-[2,2':5',2''-terthiophen]-5-yl)benzo[d]thiazole (tTBz), three cyclometalated heavy metal complexes including square-planar heteroleptic Pt(ii) complex tTBzPt, octahedral heteroleptic Ir(iii) complex tTBzIr and octahedral homoleptic tTBz3Ir are designed as electron donor materials for OSCs. Notably, the influences of molecular spatial configuration on the optoelectronic properties and photovoltaic performances are systematically investigated. tTBz and tTBzPt show only fluorescence emission with lifetime <1 ns, while tTBzIr and tTBz3Ir exhibit a phosphorescent triplet lifetime of 176 and 276 ns, respectively. The power conversion efficiencies (PCEs) follow the order of tTBz3Ir > tTBzIr > tTBzPt > tTBz, with values from 5.71, 3.73, 1.08 to ~0% for PCBM and 7.97, 6.75, 1.40 to 0.47% for Y6 acceptor based devices, respectively. The more significant 3D geometry of tTBz3Ir demonstrates the best photovoltaic performance owing to comprehensive factors of enhanced absorption, extended exciton lifetime, increased charge transport and optimized film morphologies. Our work not only promoted the PCE of triplet cyclometalated heavy-metal complexes from previously reported <4% to a higher level of ~8%, but also illustrates the significance of structural geometries for the design of new organic photovoltaic materials.