Depth-Profiling of High Efficiency Kesterite Solar Cells: The Role of Co-Alloying

The kesterite (Cu₂ZnSn(S,Se)₄, CZTSSe) absorber family is composed of low-toxicity, earth-abundant elements, and offers an unmatched stability which aligns with low-cost applications such as building-integrated and indoor photovoltaics. Moreover, it has recently experienced a breakthrough thanks to the research centered on solution-based precursors, showcasing a remarkable efficiency of 13% in 2022, which has now been further improved to 14.9%. This new paradigm for the Kesterite community has been possible through the smart design of solution chemistry in organic polar solvents yielding amorphous kesterite films, together with optimized thermal annealings for direct phase conversion from precursor to absorber, and the implementation of beneficial low-temperature heterojunction heat treatments. In addition, the alloying with isovalent elements (Ag, Cd, and Ge) has been shown to be crucial to achieve record efficiencies, suppressing lattice defects by improving the synthesis pathway.<br/>A thorough understanding of the actual role of isovalent element alloying could enable the synthesis of high quality kesterite absorber materials without the need to use expensive, toxic and scarce elements during the synthesis. In this work, the role of additional elements will be studied by comparing the device performance of absorbers without alloying, with Ag alloying and with Ag,Cd co-alloying. Thus, demonstrating the beneficial role of these alloying strategies on the optoelectronic properties of the devices. Furthermore, the effects on the grain growth mechanism of Ag and Ag,Cd co-alloying will be thoroughly investigated by secondary electron microscopy (SEM), X-ray diffraction (XRD) and Raman spectroscopy combined in break-off experiment at different temperatures. Finally, to shed more light on the performance improvement, depth gradient ion-milled samples will be fully analyzed by combining micro-Raman (325, 532 and 785 nm excitation wavelengths), micro-photoluminescence (PL) and X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS), thereby showing the actual role of Ag and Ag,Cd alloying in high efficiency kesterite absorbers. In particular, the distribution of secondary phases, grain quality, defect concentration, sulfur content and carbon residues will be assessed by micro-Raman spectroscopy. Also, the in-depth compositional homogeneity as well as Sn and Cu oxidation states will be characterized by XPS, revealing insights on the deep-defect density (Sn, lifetime related) and the shallow defect density (Cu, doping related). Moreover, the combination of XPS, UPS and PL will allow reconstructing the in-depth band diagram, revealing key insights on the charge carrier transport of high quality kesterite materials.<br/>Overall, the present study will reveal the most relevant factors induced by the Ag and Cd incorporation in the lattice of high efficiency kesterite absorbers, paving the way for further improvements without requiring the use of expensive, scarce and toxic elements.