Jinglong Guo1 Fatih Sen3 Luhua Wang2 Seungjin Nam2 Moon Kim2 Maria Chan3 Robert Klie1

1, UIC, Chicago, Illinois, United States
3, Argonne National Laboratory, Lemont, Illinois, United States
2, The University of Texas at Dallas, Richardson, Texas, United States

CdTe is one of the most promising photovoltaic materials due to its direct band-gap and a high absorption coefficient. However, the practical efficiencies of poly-crystalline CdTe photovoltaic cells are still significantly below the theoretical limit. Reduction of non-radiative recombination at grain boundaries is believed to be the key to improve the efficiency of polycrystalline CdTe-based solar cells. Atomic-scale characterization of grain boundaries is crucial in developing a fundamental understanding how grain boundaries effect the efficiency. However, due to the small grain sizes, separating bulk and grain boundary contribution to the solar cell efficiency is nearly impossible.
In this work, atomic-resolution scanning transmission electron microscopy (STEM) is combined with in-situ heating experiments of poly-crystalline CdTe solar cells and CdTe bi-crystals to simulate PV cell aging on the atomic and electronic structures of the grain boundaries. More specifically, we use aberration-corrected electron microscope to identify gain boundaries in poly-crystalline solar cell. CdTe bi-crystals are synthesized to create a specific grain boundary model system of poly-crystalline solar cells. Using in-situ heating experiments, we measure how atomic structures of grain boundaries vary during the PV cell aging. Based on the atomic-resolution characterization, structural models are built to use for first- principles density functional theory (DFT) calculations to understand how grain boundaries and PV cell aging effect efficiencies of CdTe photovoltaic cells.