Revealing frontier energy levels in blended mixed-halide perovskite thin films with electrochemistry*

Devices made from thin films of halide perovskites are advancing due to their potential in photovoltaic and optoelectronic applications, largely attributed to their energy level tunability, which can be achieved by modifying the chemical or morphological composition. Measuring the frontier energy levels of functional perovskite thin films—the valence and conduction bands—is essential for designing and tuning the electrical and optical properties of perovskite devices. However, as these thin films dissolve in organic and aqueous solutions, measuring the frontier energy levels of as-cast films typically requires sophisticated vacuum instrumentation and high-intensity excitation. Here, we measure a series of mixed-halide perovskite frontier energy levels in blended thin films with electrochemistry, utilizing a hydrofluoroether electrolyte (HFE) that preserves the perovskite structure. We prepare thin films of CsPbBr_xCl_yI_z perovskites in red, yellow, green, and blue colors, together with a polyelectrolyte and a salt additive, and quantify their frontier energy levels in a HFE electrolyte using square wave voltammetry (SWV) for enhanced sensitivity to electronic and faradaic processes. Scanning electron microscopy reveals that these perovskite thin films retain their underlying microstructure after exposure to the electrolyte and subsequent electrochemical measurements. This approach enables the repeatable and quantitative determination of various perovskite energy levels while preserving the structure of perovskite solid-state films with a cost-effective, low-power benchtop technique.