Exciton-induced degradation of photocurrent in small-molecule organic solar cells

The reliability of organic photovoltaic cells (OPVs) has become a focus of research. In this work, the intrinsic degradation mechanism of archetypal subphthalocyanine/fullerene OPVs in the absence of water and oxygen is studied. We focus on the initial burn-in period (\textless 10h) during which there is no significant change in fill factor or open-circuit voltage, suggesting stable interfacial and bulk morphology. In planar OPVs employing C$_{\mathrm{60}}$ as the acceptor, the efficiency drop is primarily due to a reduction of photocurrent contributed by C$_{\mathrm{60}}$, as observed in the spectrally-resolved external quantum efficiency (EQE). The current loss occurs after the cell is illuminated in the C$_{\mathrm{60}}$ absorption range, regardless of intensity and proportional to the total number of C$_{\mathrm{60}}$-absorbed photons. The degradation over time is modeled as due to an increasing density of exciton-induced quenching sites that hinder exciton diffusion to the donor-acceptor interface. Experimentally, we find this mechanism can be effectively mitigated by employing a mixed donor-acceptor active layer where excitons are rapidly dissociated and the steady-state exciton density is reduced. The trap formation rate and exciton dynamics will be discussed in detail. Degradation of different OPV systems will be compared.