Imperfections are not 0 K: free energy of point defects in crystals

Defects determine many important properties and applications of materials, ranging from doping in semiconductors, to conductivity in mixed ionic-electronic conductors used in batteries, to active sites in catalysts. The theoretical description of defect formation in crystals has evolved substantially over the past century. Advances in supercomputing hardware, and the integration of new computational techniques such as machine learning, provide an opportunity to model longer length and time-scales than previously possible. In this Tutorial Review, we cover the description of free energies for defect formation at finite temperatures, including configurational (structural, electronic, spin) and vibrational terms. We discuss challenges in accounting for metastable defect configurations, progress such as machine learning force fields and thermodynamic integration to directly access entropic contributions, and bottlenecks in going beyond the dilute limit of defect formation. Such developments are necessary to support a new era of accurate defect predictions in computational materials chemistry.

Is It Possible To Develop Complex S-Se Graded Band Gap Profiles in Kesterite-Based Solar Cells?

This work presents the development of a novel chalcogenization process for the fabrication of Cu₂ZnSn(S,Se)₄ (CZTSSe or kesterite)-based solar cells that enable the generation of sharp graded anionic compositional profiles with high S content at the top and low S content at the bottom. This is achieved through the optimization of the annealing parameters including the study of several sulfur sources with different predicted reactivities (elemental S, thiourea, SnS, and SeS₂). As a result, depending on the sulfur source employed, devices with superficially localized maximum sulfur content between 50 and 20% within the charge depletion zone and between 10 and 30% toward the bulk material are obtained. This complex graded structure is confirmed and characterized by combining multiwavelength depth-resolved Raman spectroscopy measurements together with in-depth Auger electron spectroscopy and X-ray fluorescence. In addition, the devices fabricated with such graded band gap absorbers are shown to be fully functional with conversion efficiencies around 9% and with improved V_OC deficit values that correlate with the presence of a gradient. These results represent one step forward toward anionic band gap grading in kesterite solar cells.

Advanced Raman Spectroscopy of Methylammonium Lead Iodide: Development of a Non-destructive Characterisation Methodology

In recent years, there has been an impressively fast technological progress in the development of highly efficient lead halide perovskite solar cells. However, the stability of perovskite films and respective solar cells is still an open point of concern and calls for advanced characterization methods. In this work, we identify appropriate measurement conditions for a meaningful analysis of spin-coated absorber-grade perovskite thin films based on methylammonium (MA) lead iodide (MAPbI3) by Raman spectroscopy. The material under investigation and its derivates is the most commonly used for high efficiency devices in the literatures and has yielded working solar cell devices with efficiencies around 10% in our laboratory. We report highly detailed Raman spectra obtained with excitation at 532 nm and 633 nm and their deconvolution taking advantage of the simultaneous fitting of spectra obtained with varying excitation wavelengths. Finally, we propose a fast and contactless methodology based on Raman to probe composition variations and/or degradation of these perovskite thin films and discuss the potential of the presented technique as quality control and degradation monitoring tool in other organic-inorganic perovskite materials and complete solar cell devices.

Role of S and Se atoms on the microstructural properties of kesterite Cu2ZnSn(SxSe1−x)4 thin film solar cells

Grazing incidence X-ray diffraction of kesterite shows a strong dependence of microstructural properties (domain size, microstrain) on the anion composition.

Raman scattering quantitative assessment of the anion composition ratio in Zn(O,S) layers for Cd-free chalcogenide-based solar cells

This work reports the use of Raman scattering for the chemical characterization of Zn(O,S) layers that are being developed as a Cd-free alternative for the buffer layer in advanced chalcogenide solar cells.