Atomic Resolution Characterization of CdTe Interfaces

Reduction of a non-radiative recombination at grain boundaries is believed to be the key to improve the efficiency of polycrystalline CdTe-based solar cells. The present study employs bi-crystal interfaces to establish the structure-property relationships of atomic grain boundary defects in CdTe. Structural and chemical configurations of the interfaces were characterized using atomic-column-resolved X-ray spectroscopy and high-angle annular dark-field imaging in an aberration-corrected scanning transmission electron microscope. To assist the interpretation of images auxiliary quantum diffusion and spectral method numerical techniques were also developed and applied to the multislice image simulation code. First-principles calculations showed formation of electronic mid-gap states that enhance charge carrier recombination at the interfacial defects. Two-photon absorption charge carrier lifetime measurements were performed on the bi-crystals and analyzed with a custom-built numerical simulations code to quantify the recombination. The results indicated that grain boundaries dominate the lifetimes in polycrystalline CdTe solar cells. First-principles doping investigations suggested that incorporation of P and Cl in CdTe can partially mitigate the adverse effects which would then lead to improvement in the solar cell efficiency. Further studies to test these predictions are needed. As a result of the research on grain boundaries a novel 2D atomic structure was stabilized using CdTe bicrystals. Structural analysis elucidated several factors that are responsible for formation of the monolayer. Band-structure calculations also indicated relativistic momentum dispersion and a possibility of topologically non-trivial order. The findings suggest that suitably chosen interfacial planes can be used as templates to stabilize new atomic configurations via wafer bonding.