Ideality Factor Mapping of Back-Contact Perovskite Solar Cells

The efficiency of back-contact perovskite solar cells has steadily increased over the past few years and now exceeds 11%, with interest in these devices shifting from proof-of-concept to viable technology. In order to make further improvements in the efficiency of these devices it is necessary to understand the cause of the low fill factor, low open-circuit voltage (V-OC), and severe hysteresis. Here a time-dependent Suns-V-oc and Suns-photoluminescence (PL) analysis are performed to monitor the transient ideality factor spatially. Two sets of quasi-interdigitated back-contact perovskite solar cells are studied; cells with and without a mesoporous TiO2 layer. Maps of the PL intensity and ideality factor resemble the periodic structure of the back-contact electrodes and the transient behavior exhibit distinct features such as a temporary variation in the periodicity of the modulation, spatial phase shifting, and phase offsets. It is shown that the presence of the mesoporous layer greatly reduces recombination, increasing the V-OC by 0.12 V. Coupled 2D time-dependent drift-diffusion simulations allow the experimental results to be modeled, and replicate the key features observed experimentally. They reveal that non-uniform ion distribution along the transport layer interfaces can drastically alter the PL intensity and ideality factor throughout the device.