Electrochemical reactions under reverse bias create additional mobile ions that enable hole tunneling in metal halide perovskite diodes

TL;DR

This study reveals that reverse bias in metal-halide perovskite diodes causes a significant increase in mobile ion concentrations, enabling hole tunneling and breakdown, with implications for device stability and design.

Contribution

It demonstrates that mobile ion concentrations can increase over 100 times within minutes under reverse bias, explaining breakdown mechanisms and the importance of uniform HTLs.

Findings

Mobile ion concentrations increase over 100× within 3 minutes of reverse bias.

Iodide oxidation and iodine vacancy creation are key to increased mobile ions.

Thick, uniform HTLs improve reverse-bias stability and delay breakdown.

Abstract

Gradual reverse-bias breakdown in metal-halide perovskite diodes and solar cells is thought to originate from hole tunneling through steep bands in an ionic depletion region near the electron transport layer after positively charged iodine vacancies accumulate near the hole-transport layer (HTL). However, typical reported mobile ion concentrations near $1\times10^{17}$ cm$^{-3}$ are too small to quantitatively explain significant tunneling current densities and (Zener) breakdown observed near $-5$ V. Here, we show that inferred mobile ion concentrations increase by more than 100$\times$, to over $1\times10^{18}$ cm$^{-3}$, within just three minutes of reverse bias at $-6.0$ V in p-i-n perovskite diodes. We attribute the increase in mobile ion concentration to iodide oxidation and the resulting iodine vacancy creation which must be balanced by reduction reactions near the HTL. Thin and sub-optimal HTL coverage leads to direct contact between the transparent conducting electrode and perovskite and facilitates electron transfer and reduction, enabling the creation of even larger inferred mobile ion concentrations ($\sim1\times10^{19}$ cm$^{-3}$) and leading to faster degradation under reverse bias. This explains previous work that showed increased breakdown voltages and improved reverse-bias stability by implementing thick, uniform HTLs.