An Updated Approach to Quantify Mobile Ions in Perovskite Solar Cells Based on Capacitance Transients

Even though the efficiency of perovskite solar cells has increased significantly in recent years, their long-term stability is still poor, hindering commercial applications. One of the main reasons for this poor stability is mobile ionic carriers, which can migrate within the perovskite crystal lattice and accumulate at the adjacent charge transport layers, reducing the extraction efficiency of electronic carriers. To compare strategies to mitigate ion migration, reliable ways of quantifying the density, mobility, and activation energy of mobile ions are necessary. Here, we propose an updated way of characterizing mobile ions based on capacitance transients. In capacitance transients, we generally measure the modulation of the electronic capacitance due to mobile ions drifting through the perovskite. In our updated approach, we first approximate the time-dependent ionic carrier, electronic carrier, and potential distribution within a perovskite solar cell after applying a voltage pulse. Subsequently, we calculate the capacitance from these distributions using a small-signal approximation of the drift-diffusion equations, resulting in capacitance transients. By fitting capacitance transients generated from drift-diffusion simulations, we show that an accurate extraction of the density, mobility, and activation energy of mobile ions within the perovskite is possible. Lastly, we apply the proposed model to measured capacitance transients of p-i-n perovskite solar cells and approximate their ion density, ionic mobility, and activation energy.