Solar Cells
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CdTe is one of the most promising photovoltaic materials due to its near optimum band gap and a high absorption coefficient. However, the efficiencies of both champion poly-crystalline CdTe photovoltaic cells as well as production line modules are still significantly below the theoretical Shockley-Queisser limit of ~30%. In recent years, it has become increasingly clear that minority carrier life-time and doping activation impose a fundamental limit to improving the efficiency of poly-CdTe solar-cell devices. Reduction of non-radiative recombination at grain boundaries is the key to improving the efficiency of polycrystalline CdTe-based solar cells. Atomistic-level characterization, including scanning transmission electron microscopy(STEM) and first principles density functional theory (DFT) modeling, is crucial in developing a fundamental understanding of how grain boundaries affect the solar cells’ efficiency.
We examine grain boundaries in poly-crystalline CdTe solar-cell samples and compare the interfacial atomic and electronic structures with model-system CdTe grain boundaries using ultra-high vacuum bonded bi-crystals. Atomic-resolution characterization is carried out in the JEOL ARM200CF aberration-corrected scanning transmission electron microscope (STEM) using high-angle annular dark field (HAADF) and annular bright field (ABF) imaging. Electronic defect structures are studied using first-principles density functional theory (DFT) calculations. These calculations are performed on structural models based on atomic-resolution STEM images. Using DFT, we predict several dopants that can passivate defects states in grain boundaries to increase the efficiency of CdTe solar cells.