Ionic–Electronic Ambipolar Transport in Metal\nHalide Perovskites: Can Electronic Conductivity Limit Ionic Diffusion?

Abstract

Ambipolar transport describes the\nnonequilibrium, coupled motion\nof positively and negatively charged particles to ensure that internal\nelectric fields remain small. It is commonly invoked in the semiconductor\ncommunity where the motion of excess electrons and holes drift and\ndiffuse together. However, the concept of ambipolar transport is not\nlimited to semiconductor physics. Materials scientists working on\nion conducting ceramics understand ambipolar transport dictates the\ncoupled diffusion of ions and the rate is limited by the ion with\nthe lowest diffusion coefficient. In this Perspective, we review a\nthird application of ambipolar transport relevant to mixed ionic–electronic\nconducting materials for which the motion of ions is expected to be\ncoupled to electronic carriers. In this unique situation, the ambipolar\ndiffusion model has been successful at explaining the photoenhanced\ndiffusion of metal ions in chalcogenide glasses and other properties\nof materials. Recent examples of photoenhanced phenomena in metal\nhalide perovskites are discussed and indicate that mixed ionic–electronic\nambipolar transport is similarly important for a deep understanding\nof these emerging materials.