Mode of action of the third component in ternary organic photovoltaic blend PBDB-T/ITIC:PC₇₀BM revealed by EPR spectroscopy

Light-induced EPR (LEPR) and out-of-phase electron spin echo (ESE) techniques were applied to trace the difference of charge generation and recombination processes between highly efficient ternary OPV blend PBDB-T/ITIC:PC₇₀BM and the corresponding binary blends PBDB-T/ITIC and PBDB-T/PC₇₀BM. The average charge trap depth measured from LEPR decay traces decreases in the series PBDB-T/ITIC, PBDB-T/PC₇₀BM, PBDB-T/ITIC:PC₇₀BM. As it is evident from the out-of-phase ESE, the initial electron-hole distance within the charge-transfer state upon its formation increases in this series. The former effect facilitates charge transport, in accordance with increase of the electron mobility for the ternary blend revealed previously. The latter effect leads to reduced geminate recombination. Both effects are suggested to improve the power conversion efficiency of the ternary blends. Presumably, they originate from modification of the PBDB-T/ITIC blend morphology by addition of small amount of PC₇₀BM.