Ionic accumulation as a diagnostic tool in perovskite solar cells: characterising band alignment with rapid voltage pulses.

Despite record-breaking devices, interfaces in perovskite solar cells are still poorly understood, inhibiting further progress. The mixed ionic-electronic nature of lead-halide perovskites results in compositional variations at the interfaces, depending on the history of externally applied biases. This makes it difficult to measure critical parameters accurately, for example, the band energy alignment of charge extraction layers. As a result, the field often resorts to a lengthy trial-and-error process to optimise these interfaces. While techniques to measure interfacial energy level alignment exist, they are typically carried out in a vacuum and on incomplete cells, hence values may not reflect those found in complete device stacks. To address this, w e have developed a pulsed measurement technique that can characterise the electrostatic potential energy drop across the perovskite layer in a functioning device. O ur method reconstructs the JV curve for a range of stabilisation biases which hold the ion distribution "static" during subsequent rapid voltage pulses. W e observe two different regimes: at low applied biases, the reconstructed JV curve is "s-shaped", whereas, at high applied biases, typical diode-shaped curves are returned. Using drift-diffusion simulations, w e demonstrate that the intersection of the two regimes changes based on the band offsets at the transport layer interfaces. This approach effectively allows measurements of interfacial energy level alignment in a complete device under illumination and without the need for expensive vacuum equipment. This article is protected by copyright. All rights reserved.