Impedance spectroscopy of high performance bulk-heterojunction solar cells: detecting signature of low bandgap states

The advancement of organic photovoltaics was acceleration by the development of novel push-pull polymers and of non-fullerene acceptors. Simultaneously, the level of impurities and structural defects during the material synthesis were decreased substantially. Further improvement depends on small amounts of imperfections leading to subgap energy states that may act as recombination centers, or as traps for charge carriers. The trapped change may modify the internal electric field, and hence the collection efficiency of photovoltaic devices. We have studied the properties of temperature dependence of subgap energy states using impedance spectroscopy in dark and under visible light illumination, under increasing electric field. The impedance spectra detect states whose occupancy changes as result of the applied oscillating voltage. Our results show increase of the impedance response up to ten time when charge carriers are generated in the devices, e.g. temperature excitation, injection by external bias, and light illumination. The observed trends suggest the presence of trap-and-release processes, which are strongly affected by temperature, and external bias, which is consistent with the disordered nature of the bulk-heterojunction films. The use of different interfacial layers has little effect on these observations, which confirms that these are intrinsic properties of the bulk-heterojunction films.