As defects frequently govern the properties of crystalline solids, the precise microscopic knowledge of defect atomic structure is of fundamental importance. We report a new class of point defects in single-layer transition metal dichalcogenides that can be created through 60° rotations of metal–chalcogen bonds in the trigonal prismatic lattice, with the simplest among them being a three-fold symmetric trefoil-like defect. The defects, which are inherently related to the crystal symmetry of transition metal dichalcogenides, can expand through sequential bond rotations, as evident from in situ scanning transmission electron microscopy experiments, and eventually form larger linear defects consisting of aligned 8–5–5–8 membered rings. First-principles calculations provide insights into the evolution of rotational defects and show that they give rise to p-type doping and local magnetic moments, but weakly affect mechanical characteristics of transition metal dichalcogenides. Thus, controllable introduction of rotational defects can be used to engineer the properties of these materials.

The efficiency of Cu­(In,Ga)­Se2 (CIGS)-based solar cells can be markedly improved by controlled introduction of alkali metal (AM) atoms using post-deposition treatment (PDT) after CIGS growth. Previous studies have indicated that AM atoms may act as impurities or agglomerate into secondary phases. To enable further progress, understanding of atomic level processes responsible for these improvements is required. To this end, we have investigated theoretically the effects of the AM elements Li, Na, K, Rb, and Cs on the properties of the parent material CuInSe2. First, the effects of the AM impurities in CuInSe2 have been investigated in terms of formation energies, charge transition levels, and migration energy barriers. We found that AM atoms preferentially substitute for Cu atoms at the neutral charge state. Under In-poor conditions, AM atoms at the In site also show low formation energies and are acceptors. The migration energy barriers show that the interstitial diffusion mechanism may be relevant only for Li, Na, and K, whereas all the AM atoms can diffuse with the help of Cu vacancies. The competition between these two mechanisms strongly depends on the concentration of Cu vacancies. We also discuss how AM atoms can contribute to increasing Cu-depleted regions. Second, AM atoms can form secondary phases with Se and In atoms. We suggest a mechanism for the secondary phase formation following the PDT process. On the basis of the calculated reaction enthalpies and migration considerations, we find that mixed phases are more likely in the case of LiInSe2 and NaInSe2, whereas formation of secondary phases is expected for KInSe2, RbInSe2, and CsInSe2. We discuss our findings in the light of experimental results obtained for AM treatments. The secondary phases have large energy band gaps and improve the morphology of the buffer surface by enabling a favorable band alignment, which can improve the electrical properties of the device. Moreover, they can also passivate the surface by forming a diffusion barrier. Overall, our work points to different roles played by the light and heavy AM atoms and suggests that both types may be needed to maximize their benefits on the solar cell performance.

Chalcopyrite solar cells achieve efficiencies above 23%. The latest improvements are due to post‐deposition treatments (PDT) with heavy alkalis. This study provides a comprehensive description of the effect of PDT on the chemical and electronic structure of surface and bulk of Cu(In,Ga)Se2. Chemical changes at the surface appear similar, independent of absorber or alkali. However, the effect on the surface electronic structure differs with absorber or type of treatment, although the improvement of the solar cell efficiency is the same. Thus, changes at the surface cannot be the only effect of the PDT treatment. The main effect of PDT with heavy alkalis concerns bulk recombination. The reduction in bulk recombination goes along with a reduced density of electronic tail states. Improvements in open‐circuit voltage appear together with reduced band bending at grain boundaries. Heavy alkalis accumulate at grain boundaries and are not detected in the grains. This behavior is understood by the energetics of the formation of single‐phase Cu‐alkali compounds. Thus, the efficiency improvement with heavy alkali PDT can be attributed to reduced band bending at grain boundaries, which reduces tail states and nonradiative recombination and is caused by accumulation of heavy alkalis at grain boundaries.

Birefringence is an inherent optical property of anisotropic materials introduced by the anisotropic confinement in their crystal structures. It enables manipulation of light propagation properties (e.g., phase velocity, reflection, and refraction) for various photonic and optoelectronic applications, including waveplates and liquid crystal displays. Two-dimensional (2D) layered materials with high anisotropy are currently gaining an increasing interest for polarization-integrated nanodevice applications, which advances the research on birefringent materials. In this article, we investigate the optical birefringence of three anisotropic 2D layered materials (black phosphorus (BP), rhenium disulfide (ReS2), and rhenium diselenide (ReSe2)). We demonstrate that the birefringence in BP (~0.245) is ~ 6 times larger than that of ReS2 (~0.037) and ReSe2 (~0.047) at 520 nm and is comparable to the current state of the art bulk materials (e.g., CaCO3). We use these 2D materials to fabricate atomically-thin optical waveplates and investigate their performance. In particular, for BP, we observe a polarization-plane rotation of ~0.05° per atomic layer at 520 nm. Our results show that the relatively large birefringence of anisotropic 2D layered materials can enable accurate manipulation of light polarization with atomically controlled device thickness for various applications where integrated, nanoscale polarization-controllers are required.

Grain boundaries in 2D materials can have marked influence on the material properties. The effects can be not only detrimental, but also beneficial in transition metal dichalcogenides (TMDs), so that controlling the density and type of the boundaries in these systems should be important for engineering their properties. However, this is often possibly only during the growth stage. Molybdenum and tungsten dichalcogenides feature a particular set of 60° mirror twin boundaries, which are reported to occur upon merging of the growing flakes, to appear during growth to accommodate for the nonstoichiometry of the sample, or to be produced a posteriori by electron irradiation or thermal annealing. Furthermore, different preparation conditions lead to different atomic structure of the boundary, which consequently exhibit different electronic properties. This has obviously garnered interest for the ability to control grain boundary types and densities. In this progress report, the recent experimental and theoretical work related to the characterization of mirror twin boundaries is reviewed. A consistent set of formation energies for the mirror twin boundaries is provided, which then allows a coherent picture on the formation mechanisms under different conditions to be drawn. Finally, the electronic structure of these boundaries is analyzed and their potential applications are discussed.