It is well known that sodium at grain boundaries (GBs) increases the photovoltaic efficiencies of CuInSe 2 and Cu 2 ZnSnS 4 significantly. However, the mechanism of how sodium influences the GBs is still unknown. Based on the recently proposed self-passivation rule, it is found that the dangling bonds in the GBs can completely be saturated through doping the Na, thus GB states are successfully passivated. It is shown that the Na can easily incorporate into the GB with very low formation energy. Although Cu can also passivate the GB states, it requires a copper rich condition which, however, suppresses the formation of copper vacancies in the bulk and thus decreases the concentration of hole carriers, so copper passivation is practically not as beneficial as sodium. The present work reveals the mechanism about how the Na enhances the photovoltaic performance through passivating the dangling bonds in the GBs of chalcogenide semiconductors, and sheds light on how to passivate dangling bonds in GBs with alterative processes.
Due to its high chemical stability and high power conversion efficiency as a solar cell absorber, the inorganic halide perovskite, CsPbI3, is considered one of the most promising competitors to its hybrid organic-inorganic counterpart, CH3NH3PbI3. However, the phase transition from the photoactive black phase to the inactive yellow phase is a remarkable limitation that harms long-term phase stability. In particular, the phase transitions follow different pathways as the temperature increases and/or decreases, a phenomenon that is anomalous and remains poorly understood. In this study, we systematically calculated the temperature-dependent free energy of CsPbI3 in different crystal phases (α, β, γ, δ) by considering the phonon contribution to the Gibbs free energy. It is found that the free energy results from calculations that include harmonic phonons cannot reproduce experimental observations. Alternatively, we utilized the renormalized phonon quasiparticle approach to derive the free energies of different CsPbI3 phases at finite temperatures. Based on these calculated free energies, whose derivations included the anharmonic effect, we observed phase-transition processes consistent with experimental results. The analysis of the temperature effect on the phonon frequencies further demonstrated that anharmonic effects in the CsPbI3 had a significant influence on its phase transitions.
Grain boundaries (GBs) are significant in determining the electrical properties of polycrystalline semiconductors. However, the electronic structures and passivation mechanisms of polycrystalline semiconductors remain poorly understood. In this study, we systematically investigated the Σ3 (112) GB properties of several typical zinc-blende semiconductors via first-principles density functional calculations. We found significant differences of Σ3 (112) GB structures and properties between IV/III and V types, where dangling atoms formed new covalent bonds, and II–VI/I–VII types, where dangling atoms formed no new bonds. These different bonding configurations lead to different origins of defect states at GBs. We successfully designed a targeted doping approach to passivate such defect states for different types of semiconductors. We demonstrated the validity of the proposed approach in Σ3 (112) GB of the zinc-blende semiconductors. This work elucidates the defect states at GBs in common zinc-blende semiconductors, rationalizes diverse post-treatment approaches reported in previous experiments, and provides general guidance for defect passivation at the GBs of polycrystalline semiconductors.
By applying the locally optimal rotation method to deal with the lowest eigenvalue of a Hessian matrix, we have efficiently incorporated the hyperdynamics method into the ab initio scheme. In the present method, we only need to calculate the first derivative of the potential and several more force calls in each molecular dynamics (MD) step, which makes hyperdynamics simulation applicable in ab initio MD simulations. With this implementation, we are able to simulate defect diffusion in silicon with boost factors up to 105. We utilized both direct MD and the hyperdynamics method to investigate diffusion of lithium atoms and silicon vacancies in silicon. We identified the complex diffusion process. The obtained diffusion coefficients of Li atoms and Si vacancies are in good agreement with the direct MD results.
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