Atomic-scale study of aging effects in poly-crystalline CdTe solar cell*

CdTe is a widely-used photovoltaic (PV) material due to its near optimum band gap and ease of low-costing manufacturing. However, practical efficiencies are well below the theoretical Shockley efficiency limit.One factor limiting the efficiency is recombination at grain boundaries. Due to the variety of grain boundary structures in poly-crystalline CdTe, understanding grain boundary contribution to the solar cell efficiency is very challenging. As such, an atomic-scale understanding of grain boundaries becomes crucial in improving efficiency.
In this work, we investigate the effects of PV cell aging on the atomic structure of grain boundaries in poly-CdTe and model CdTe bi-crystals by combining atomic-resolution scanning transmission electron microscopy (STEM) with in-situ heating experiments. Based on the STEM characterization, atomic structure models of grain boundaries are constructed and used to calculate the electronic structure by first-principles density functional theory (DFT). The results of this work enabled to understand how grain boundaries and PV cell aging affect efficiencies of CdTe photovoltaic cells.