To date, the stability of perovskite-based photodiodes under reverse bias has primarily been investigated in the context of solar cell applications. This condition commonly arises during partial shading, where a shaded cell is forced to conduct the current generated by its unshaded neighbors. The choice of electron transport layer (ETL) has been shown to play a crucial role in the breaking down mechanism. Unlike solar cells—where reverse biasing falls outside the normal operating range—photodetectors are designed to operate in this regime to achieve optimal signal-to-noise ratio and faster carrier extraction. However, the effects of prolonged reverse biasing on the performance of perovskite-based photodetectors remain largely unexplored, with most studies limiting their operating range to relatively safe levels, typically up to -0.5 V.

In this work, we investigate the impact of the ETL on the performance of all-inorganic p-i-n perovskite-based photodiodes (PePDs), fabricated exclusively through physical vapor deposition methods to ensure high thermal stability and scalable production. Specifically, we demonstrate that the use of a fullerene–metal oxide bilayer, along with careful tuning of their respective thicknesses, not only enhances reverse-bias stability, but also reduces performance variability and improves carrier extraction speed. The optimized photodiode exhibits a dark current below 0.1 μA/cm² even after hour-long biasing at -2 V, along with a >70% improvement in extraction speed—promising sub-μs rise times for further scaled-down pixels. These results pave the way for the development of reliable, all-evaporated perovskite-based imagers integrated atop silicon read-out circuits, offering ultra-high-speed performance and compatibility with fabrication processes that require a high thermal budget.

Furthermore, we utilize novel characterization techniques to gain insights into the mechanisms associated with the observed performance improvements. For instance, the enhancement in reverse-bias stability is attributed to a reduction in defect states at the perovskite/ETL interface, as revealed through a combination of transient photocurrent measurements with various excitation wavelengths and transfer-matrix algorithm simulations. Simultaneously, the improvements in response speed are further investigated through capacitance spectroscopy of both the photodiode stack itself and a simpler metal-oxide-semiconductor structure. Ultimately, the enhanced response speed is attributed to an extension of the depletion width into the ETL and a corresponding reduction of the equivalent RC constant.

The presented enhancements in the reliability and speed of all-inorganic, vacuum-deposited PePDs, coupled with the comprehensive understanding of their multifaceted performance in the reverse bias regime open new pathways for the development of CMOS-compatible perovskite photodetectors.