Emerging photovoltaic technologies are commonly processed using solution deposition methods. Layers deposited in this way often suffer from non-uniformities in thickness and composition, resulting in locally varied cell performance. Such inhomogeneities and defects can be visualized by various imaging techniques, such as photo- and electroluminescence (PL/EL), dark and illuminated lock-in thermography, as well as optical imaging. Such non-uniformities can be challenging for upscaling emerging solar cells, as they result in performance losses and may lead to local hot-spots, which are the origin of degradation.

In this contribution, we first present EL images of carbon-based perovskite solar cells with a mesoporous layer stack which exhibit locally varying temporal evolutions. The scan-rate dependent current-voltage characteristics of perovskite solar cells and the temporal evolution of the EL signal are generally associated with the presence of mobile ions in the cell.[1] To analyse the transient EL images, we set up a device model in the drift-diffusion simulation software Setfos which quantitatively reproduces a set of steady-state and transient measurements. By employing this model, we are able to attribute the inhomogeneities in EL intensity to spatially varying ion densities. We further show how the mobile ion density influences the reverse bias breakdown behaviour in perovskite solar cells due to the strongly varying potential at layer interfaces, which facilitates tunnelling current.[2] Reverse bias conditions are imposed on shaded sub cells in modules or can be induced by current mismatch situations in monolithically stacked tandem devices. Non-uniformities caused by a spatially varying ion density and consequently reverse bias breakdown voltages result in strongly varying (reverse) bias potentials, inducing current (and temperature) hot-spots.[2]

The effect of reverse bias stressing on perovskite solar cells is further assessed by a combination of characterization techniques. To this end, cells are stressed below their breakdown voltage while the EL or PL signal is recorded. During regular intermittent measurements, current-voltage scans, impedance measurements, as well as (forward) electroluminescence images are taken to assess the underlying degradation mechanisms induced by reverse bias conditions.