Theoretical investigation of FAPbSnGeX3 efficiency.

The use of hybrid lead halide perovskites as light absorbers in photovoltaic cells have gained large interest due to their optoelectronic properties and high efficiency. However, most hybrid perovskites contain toxic lead which has a negative impact on the environment. In this work, we systematically study the structural, electronic, and optical properties of lower lead halide perovskites FAPb0.5Sn0.25Ge0.25X3 (X = I, Br, Cl), as well as discussing their photovoltaic performance (open circuit voltage (V oc), the short circuit current density (J sc), and the power conversion efficiency (η)) using density functional theory (DFT), and we compare these with FAPbX3 (X = I, Br, Cl) frameworks. The compounds show a suitable band gap for photovoltaic applications, in which iodine has a lower gap value compared to chlorine. It is noteworthy that we found that lead doping by both germanium and tin in the FAPb0.5Sn0.25Ge0.25X3 (X = I, Br, Cl) materials significantly improves the adsorption coefficient and the stability of these systems compared to the FAPbX3 (X = I, Br, Cl) systems. The calculated Jsc shows a monotonical decrease from FAPb0.5Sn0.25Ge0.25I3 to FAPbCl3, which represents the lowest Jsc. Results reveal that FAPb0.5Sn0.25Ge0.25Cl3 demonstrates promising potential for photovoltaic application as it shows the highest efficiency. This study can help reduce the toxicity of hybrid lead halide perovskites and also raises their experimental power conversion efficiency.

Strain and size combined effects on the GaN band structure: VEELS and DFT study.

A nanoscale study of combined strain/size effects has been performed using monochromated valence electron energy-loss spectroscopy and density functional theory (DFT) calculations to locally explore the valence and conduction bands of a strained 2 nm GaN quantum well inserted between two fully relaxed AlN thick layers. Two main electronic transitions from the valence to the conduction band were experimentally detected and interpreted. The first transition was shown to be a collective oscillation (or plasmon), which was significantly blue-shifted in energy mainly due to the widening of the valence-band top-part. The second, however, had a single-particle character, that is: Ga-3d → Ga-4p, and was weakly affected by strain and size. In addition, our DFT calculations showed that strain and size can be adjusted separately to tune the GaN band-gap energy.