Vladimir Dyakonov: Charge carrier recombination and electronic trap states in perovskite solar cells

Hybrid perovskite solar cells are progressing very fast, showing extraordinary performance exceeding 20% therefore competing with inorganic thin-film technologies. However, there is a lack of fundamental understanding of the photovoltaic properties and working principles of this class of solar cells. For example, anomalous current-voltage hysteresis is often observed in perovskite solar cells, being either an interface phenomenon or related to ion diffusion in the bulk. Other interesting issues are polarization effects either related to the reorientation of organic molecules or due to trapped charges or migration of charged species.

Here, we present our studies on the charge carrier recombination in vapour deposited planar methylammonium lead iodide perovskite (MAPbI3) solar cells. [1] In the open-circuit voltage decay, we observed two very different time domains in the transients. [2] On short time scales, we observed a voltage drop very similar to the reference polymer-fullerene solar cells. A second, much slower decay on longer time scales was observed only in perovskite solar cells. Interestingly, in perovskite devices, the recombination dynamics at all timescales were found to be dependent on the preconditioning of the devices by light illumination prior to measuring. We will discuss the potential origins of the voltage transients. To address the possible influence of electronic traps on the devices performance and to identify the energy levels of such states, we performed thermally stimulated current (TSC) measurements on solution processed MAPbI3 solar cells. [3] To separate between bulk and interfacial traps, we varied the device configuration using different transport layers in normal and inverted device geometry and also studied pure perovskite layers. We observed several peaks in the TSC measurements. Whereas the current peaks at low temperature are indicative of very shallow traps near the conduction or valence bands, the peak at high temperature is assigned to deep traps in the band gap of the perovskite. Finally, we observed an asymmetric peak at around 160 K, which we assigned to the structural phase transition of the perovskite crystal from the orthorhombic to the tetragonal crystal structure. The origin of these TSC peaks and their impact on the performance of the solar cells will be discussed in detail.