Intrinsic burn-in efficiency loss of small-molecule organic photovoltaic cells due to exciton-induced trap formation

The intrinsic degradation mechanisms leading to the initial burn-in deterioration in power conversion efficiency in small-molecule-based organic photovoltaics (OPVs) are studied. Specifically, we examine degradation in archetype boron subphthalocyanine chloride/fullerene OPVs in the absence of atmospheric contaminants such as water and oxygen. During the initial burn-in period (<20 h), planar OPVs employing C₆₀ as the acceptor exhibits a rapid decrease in efficiency that is primarily due to a reduction in photocurrent contributed by excitons generated in C₆₀, as observed by the changes in the spectrally-resolved external quantum efficiency. We develop an analytical model that ascribes the decrease in power conversion efficiency with aging to an energetically-driven increase in the density of exciton-induced quenching sites that hinder exciton diffusion to the donor-acceptor interface. This mechanism is mitigated by employing a C₇₀ acceptor, or a mixed donor-acceptor active layer where excitons are rapidly dissociated following photogeneration, thereby significantly reducing their lifetime and density.