Investigation on voltage loss in organic triplet photovoltaic devices based on Ir complexes

Abstract

Voltage losses in singlet material-based organic photovoltaic devices (OPVs) have been intensively studied, whereas, only a few investigations on triplet material-based OPVs (T-OPVs) are reported. To investigate the voltage loss in T-OPVs, two homoleptic iridium(III) complexes based on extended π-conjugated benzo[g]phthalazine ligands, Ir(Ftbpa)₃ and Ir(FOtbpa)₃, are synthesized as sole electron donors. T-OPVs are fabricated by mixing two donors with phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) as an electron acceptor. Insertion of oxygen-bridges as flexible inert δ-spacers in Ir(FOtbpa)₃ has slightly elevated both the lowest unoccupied molecular orbital and the highest occupied molecular orbital levels compared to those of Ir(Ftbpa)₃, which results in a lower charge transfer (CT) state energy (E_CT) for Ir(FOtbpa)₃-based devices. However, a higher V_oc (0.88 V) is observed for Ir(FOtbpa)₃-based devices than those of Ir(Ftbpa)₃ (0.80 V). To understand the above result, the morphologies of the two blend films are studied, which excludes the influence of morphology. Furthermore, radiative and non-radiative recombination in two devices is quantitatively investigated, which suggests that a higher V_oc can be attributed to reduced radiative and non-radiative recombination loss for the Ir(FOtbpa)₃-based devices.