Earth abundant kesterite copper-zinc-tin-sulfide–selenide (CZTS–Se) is considered as cost-effective material for next generation solar cells. However, current CZTS–Se solar cells have much lower efficiency than CIGS solar cells. Rapid progress in achieving the target efficiency in CZTS–Se solar cells is hindered by the narrow phase stability of the quaternary phase, Cu₂ZnSn(S_xSe_1−x)₄, and the existence of other competitive and complex secondary phases and defects. This resulted in structural inhomogeneity, local fluctuation of open circuit voltage and high carrier recombination that finally lead to poor device performance and repeatability issues. The higher performance of off-stoichiometric CZTS materials, copper-poor and zinc-rich, and their inherent association with secondary phases and defects force the scientific community to investigate them together. This work aims to provide a comprehensive review for optimum growth conditions to achieve efficient kesterite CZTS–Se material under different conditions, complementary characterization techniques to detect unwanted phases, defects and defect-complexes and various approaches to reduce the secondary phases, defects and defect-complexes for higher performance in CZTS–Se solar cells. Understanding and addressing the structural inhomogeneity, control growth and material characterization are expected to yield closer performance parity between CZTS–Se and CIGS solar cells.

Nitrogen dioxide (NO₂), a hazardous gas with acidic nature, is continuously being liberated in the atmosphere due to human activity. The NO₂ sensors based on traditional materials have limitations of high-temperature requirements, slow recovery, and performance degradation under harsh environmental conditions. These limitations of traditional materials are forcing the scientific community to discover future alternative NO₂ sensitive materials. Molybdenum disulfide (MoS₂) has emerged as a potential candidate for developing next-generation NO₂ gas sensors. MoS₂ has a large surface area for NO₂ molecules adsorption with controllable morphologies, facile integration with other materials and compatibility with internet of things (IoT) devices. The aim of this review is to provide a detailed overview of the fabrication of MoS₂ chemiresistance sensors in terms of devices (resistor and transistor), layer thickness, morphology control, defect tailoring, heterostructure, metal nanoparticle doping, and through light illumination. Moreover, the experimental and theoretical aspects used in designing MoS₂-based NO₂ sensors are also discussed extensively. Finally, the review concludes the challenges and future perspectives to further enhance the gas-sensing performance of MoS₂. Understanding and addressing these issues are expected to yield the development of highly reliable and industry standard chemiresistance NO₂ gas sensors for environmental monitoring.

The article presents a strategic review of secondary phases, defects and defect-complexes in kesterite CZTSSe solar cells responsible for performance gap compared to CIGS solar cells.