Structural, chemical and magnetic properties of pristine and Fe- doped kesterites for photovoltaic application before and after thermal annealing

Kesterites, Cu2ZnSnS4 (Copper-Zinc-Tin Sulfide, CZTS) are a class of light-absorbing materials employed in sustainable thin-film solar cells due to the lack of toxic elements and critical raw materials. However, they are affected by the presence of secondary phases, antisite defects and vacancies, which degrade their efficiency. Here, we foster the combined use of conventional techniques to obtain a comprehensive structural and chemical characterization of these materials (X-Ray Diffraction, micro-Raman scattering, Diffuse Reflectance spectroscopy and Scanning Transmission Electron Microscopy), with magnetometry and Electron paramagnetic resonance (EPR) spectroscopy, to identify the nature of their paramagnetic centers. As a case study, we undertook a synthetic approach incorporating iron as a substitute for zinc into the composition. CZTS and Cu2(Zn1-xFex)SnS4 (Copper-Zinc-Iron-Tin Sulfide, CZFTS) nanocrystals were synthesized by hot-injection method and then thermally treated at 450°C, a process typically implemented to improve the performance in optoelectronic devices. The structural analysis evidenced that while the raw CZTS and CZFTS nanocrystals do not show any secondary phase, a minority phase is present in the annealed samples. The complementary distribution of the metal ions in both CZTS and CZFTS was also assessed. Magnetometry and EPR spectroscopy pointed out the formation of paramagnetic vacancies following the high temperature treatment and the successful doping with Iron. Remarkably, this work highlighted the unique capability of EPR spectroscopy to detect paramagnetic defects in kesterite nanocrystals, which are otherwise challenging to identify using other techniques. The exploitation of EPR can thus unveil new opportunities to optimize materials features for optoelectronics and photovoltaics.