Investigation on voltage loss in organic triplet photovoltaic devices based on Ir(III) complexes
Voltage losses in singlet material-based organic photovoltaic devices (OPVs) are intensively studied, whereas, there is little investigation on triplet material-based OPVs (T-OPVs). 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 mixed with phenyl-C71-butyric acid methyl ester (PC71BM) as an electron acceptor. Insertion of oxygen-bridge 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 that of Ir(Ftbpa)3 and a lower charge transfer (CT) state energy (ECT) is determined for Ir(FOtbpa)3-based devices. However, a higher Voc (0.88 V) is obtained for Ir(FOtbpa)3-based devices compared to Ir(Ftbpa)3-based devices. Similar morphology for blend films indicate that the difference in Voc is not due to morphology. Radiative and non-radiative recombination are further quantified for both devices, indicating that for the Ir(FOtbpa)3-based devices both radiative and non-radiative recombination loss are reduced, which leads to a higher Voc.