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The atomic structure and electronic properties of aluminum (Al)-related defect complexes in silicon (Si) are investigated using first-principles calculations. Individual substitutional Al (AlSi), interstitial Al (Ali) and their possible complex configurations with oxygen (O) atoms are studied. We find a unique stable complex configuration consisting of an Ali and an oxygen dimer, Ali-2Oi, which introduces deep levels in the band gap of Si. The formation energies of the Ali-2Oi complexes could be lower than that of individual Ali atoms under oxygen-rich conditions. The formation of Ali-2Oi complexes may explain the experimental observation that the coexistence of Al and O results in reduced carrier lifetime in Si wafers. [ABSTRACT FROM AUTHOR]

Through first-principle density functional theory (DFT) calculations, the atomic structure and electronic properties of intrinsic and passivated Σ3 (114) grain boundaries (GBs) in Cu2ZnSnSe4 (CZTSe) are studied. Intrinsic GBs in CZTSe create localized deep states within the band gap and thus act as Shockley-Read-Hall recombination centers, which are detrimental to cell performance. Defects, such as ZnSn (Zn atoms on Sn sites), Na+i (interstitial Na ions), and OSe (O atoms on Se sites), prefer to segregate into GBs in CZTSe. The segregation of these defects at GBs exhibit two beneficial effects: 1) eliminating the deep gap states via wrong bonds breaking or weakening at GBs, making GBs electrically benign; and 2) creating hole barriers and electron sinkers, promoting effective charge separation at GBs. The results suggest a unique chemical approach for engineering GBs in CZTSe to achieve improved cell performance. [ABSTRACT FROM AUTHOR]

Abstract: Aberration corrected scanning transmission electron microscopy (STEM) has been used to determine the structures of a variety of dislocation cores in CdTe, including 30° and 90° Shockley partial dislocations, positive and negative Frank sessile partial dislocations, and steps on twin boundaries. Structure models have been constructed from the images and electrical activity has been investigated with density functional calculations. An integrated electron energy loss spectroscopy, cathodoluminescence and electron beam induced current system has been designed and built to probe electrical and optical properties of individual defects. The first STEM-cathodoluminescence result shows strong impurity segregation between the CdTe and the glass. The correlation between the scanning electron microscopy-electron beam induced current and electron backscatter diffraction maps proves that the grain structures and boundaries dominate the electrical activity. After heat treatment in CdCl2, Cl is found to segregate to the grain boundaries, and they show higher efficiency than the bulk material. [Copyright &y& Elsevier]