Perovskite ionics - elucidating degradation mechanisms in perovskite solar cells via device modelling and iodine chemistry

We study recombination pathways in halide perovskite solar cells using a semiconductor device model that includes iodide diffusion and iodide reactions. We stress the device at a bias of 0.9 V, close to the maximum power point, and compare blocking layers impermeable to ions and those that allow ionic transport. For both cases, we examine the impact of incorporating iodide reactions. By comparing stress under dark and one sun excitation conditions, we find that the photoexcited charges enable the redistribution of iodide, which in the case of permeable blocking layers results in the deterioration of the power conversion efficiency and the appearance of hysteresis. If all the reactions are reversible and there is no loss of iodine, their main effect is the introduction of mobile recombination centres. Introducing reactive electrodes that react with and immobilise the iodide results in a rapid loss of performance on the time scale of an hour, which is only slightly accelerated under illumination. We also find that under light excitation, there is a generation of iodine molecules (I₂). If the I₂ is allowed to leave the device, this constitutes a slow (1000s of hours), but non-reversible degradation pathway. Encouragingly, a judicial choice of the electron-blocking layer's energy levels could suppress this degradation path.