Charge photogeneration and Recombination in Ternary Organic Photovoltaic Blend PCDTBT/PC60BM/ICBA Studied by EPR Spectroscopy

Abstract

Using the stationary and pulsed EPR methods, the ternary composite PCDTBT and two fullerene acceptors PCDTBT/PC₆₀BM/ICBA 1:1:1, as well as the corresponding binary composites PCDTBT/PC₆₀BM 1:2 and PCDTBT/ICBA 1:2, were studied at a temperature of 80 K. Modeling these spectra allows us to estimate the contributions of PC₆₀BM and ICBA to the light-induced EPR signal of the PCDTBT/PC₆₀BM/ICBA ternary composite as 0.7:0.3. The absence of new lines in the EPR spectrum of the ternary composite, in comparison with the corresponding binary ones, means that the mechanism of the molecular alloy of PC₆₀BM and ICBA, as previously assumed, is not operative in this system, and the most probable scenario is the existence of two parallel heterojunctions PCDTBT/PC₆₀BM and PCDTBT/ICBA. This conclusion is confirmed by modeling the decay curves of the light-induced EPR upon turning off the light, as well as the out-of-phase electron spin echo from the charge transfer state (the main intermediate of the photoelectric conversion) in these composites. It is noteworthy that in the ternary composite with the same fullerene acceptors, but with a different polymer donor (P3HT), the molecular alloy mechanism of two acceptors is realized (Angmo et al. in J Mater Chem C 3: 5541–5548, 2015). It is likely that the polymer donor has a decisive influence on the morphology and electron-transport properties of such ternary composites. It should be noted that the methods of light-induced EPR and out-of-phase ESE were used for the first time to study ternary donor–acceptor composites.