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The presence of hydrogen in solar cells based on chalcopyrite CuInSe 2 (CIS) absorbers has been linked with improvements in structural properties and cell performance but also with detrimental reliability issues. A major concern is to understand how hydrogen interacts with the absorber-buffer CIS/CdS heterojunction which is the main building block of a typical thin-film solar cell, with CdS most commonly used as the buffer layer. The present study reports calculations based on density-functional theory that examine the segregation propensity of single hydrogen impurities at the interfacial region of the CIS/CdS heterojunction. Two distinct interface variants of the heterojunction were constructed by joining the polar {112} crystalline planes of the absorber (CIS) and buffer (CdS) lattices. Ordered point defects comprising copper vacancies and cation antisites were created to stabilize the {112} facets. The calculations provide detailed information on the type of configurations that hydrogen impurities can form locally at the CIS/CdS interfaces and their defect association with the stabilizing point defects. Essential aspects of the local electronic structure such as the electron spatial localization and the position of the defect-induced levels were also determined. [ABSTRACT FROM AUTHOR]

Owing to their strong interaction with carriers, early 3d-row (Ti, V, and Cr) and 4d-row (Zr, Nb, and Mo) transition metals (TMs) are undesired contaminants in solar- and electronic-grade Si. The increasing stringent control of contamination levels is urging an accurate picture of their electronic structure. In the present work, the electrical levels and migration energies of these TMs are determined by means of standard density-functional theory (DFT) and a rotationally invariant formulation of DFT+U. The latter approach improves on the treatment of electronic correlations at the TM sites and relies on on-site Hubbard Coulomb and Hund's exchange parameters U and J , respectively. These are calculated self-consistently from linear-response theory without fitting to experimental data. The effect of correlation was found more pronounced for Ti and V, with a strong impact on the location of their electrical levels. In most cases, the agreement with the experimental data is satisfactory allowing the identification of the type and character of the levels. For Cr and Mo in particular, the results resolve longstanding controversies concerning the type and position of the levels. The obtained migration barriers display moderate charge-state and correlation dependency. High barriers were found for all metals studied, with the exception of Cr, confirming them as slow diffusers in silicon among the whole TM family. [ABSTRACT FROM AUTHOR]

Secondary ion mass spectroscopy (SIMS) and deep level transient spectroscopy measurements were carried out on Czochralski (Cz)- and float-zone-grown (FZ) Si crystals, which were implanted with Ti ions and annealed in the temperature range 600-900 °C. The electrical behavior of Ti atoms is found to be different in Cz- and FZ-Si annealed at 650 °C, although the Ti SIMS profiles are similar. It is argued that interstitial Ti atoms (Tii) in FZ-Si crystals interact with implantation-induced vacancies and take a substitutional position (Tis). No energy levels which can be assigned to Tis have been detected in this work or in previous experimental literature. However, previous calculations suggest that Tis is a deep acceptor in Si. We show from density functional calculations that by taking proper account of interactions within the d-shell of the Ti impurity the electronic structure of Tis has no levels in the band gap. The calculations show that Tii is more energetically favorable than Tis and that Tii binds more strongly to the silicon vacancy than interstitial oxygen does, explaining the observed differences between FZ- and Cz-irradiated materials. [ABSTRACT FROM AUTHOR]

The electronic structure of hydrogen impurity in Lu2O3 was studied by first-principles calculations and muonium spectroscopy. The computational scheme was based on two methods which are well suited to treat defect calculations in f-electron systems: first, a semilocal functional of conventional density-functional theory (DFT) and secondly a DFT+U approach which accounts for the on-site correlation of the 4f electrons via an effective Hubbard-type interaction. Three different types of stable configurations were found for hydrogen depending upon its charge state. In its negatively charged and neutral states, hydrogen favors interstitial configurations residing either at the unoccupied sites of the oxygen sublattice or at the empty cube centers surrounded by the lanthanide ions. In contrast, the positively charged state stabilized only as a bond configuration, where hydrogen binds to oxygen ions. Overall, the results between the two methods agree in the ordering of the formation energies of the different impurity configurations, though within DFT+U the charge-transition (electrical) levels are found at Fermi-level positions with higher energies. Both methods predict that hydrogen is an amphoteric defect in Lu2O3 if the lowest-energy configurations are used to obtain the charge-transition, thermodynamic levels. The calculations of hyperfine constants for the neutral interstitial configurations show a predominantly isotropic hyperfine interaction with two distinct values of 926 MHz and 1061 MHz for the Fermi-contact term originating from the two corresponding interstitial positions of hydrogen in the lattice. These high values are consistent with the muonium spectroscopy measurements which also reveal a strongly isotropic hyperfine signature for the neutral muonium fraction with a magnitude slightly larger (1130 MHz) from the ab initio results (after scaling with the magnetic moments of the respective nuclei). [ABSTRACT FROM AUTHOR]