Titania as Buffer Layer for Cd-Free Kesterite Solar Cells.

Pure sulfide kesterite (Cu2ZnSnS4) is one of the most promising emerging photovoltaic technologies thanks to its excellent absorption coefficient, cost-effectiveness, and environmental sustainability. However, record efficiencies are not exceeding 11% due to several issues, such as absorber defects or a nonoptimal band alignment with the toxic but conventionally used CdS buffer layer. To get rid of it, several efforts have been made in the past few years. Among recent theoretical works, TiO2 has been suggested as a suitable buffer layer due to its optical and electrical properties, giving extremely promising results in device simulation. However, there are few experimental examples combining TiO2 with kesterite, and they generally show very modest performances. In this Letter, we report on the preliminary and promising results of our experimental procedure for the production of Cd-free kesterite photovoltaic devices featuring ALD-TiO2 as a buffer layer, leading to efficiencies comparable with our CZTS/CdS reference devices.

Band-Gap Tuning Induced by Germanium Introduction in Solution-Processed Kesterite Thin Films.

In the last few decades, the attention of scientific community has been driven toward the research on renewable energies. In particular, the photovoltaic (PV) thin-film technology has been widely explored to provide suitable candidates as top cells for tandem architectures, with the purpose of enhancing current PV efficiencies. One of the most studied absorbers, made of earth-abundant elements, is kesterite Cu2ZnSnS4 (CZTS), showing a high absorption coefficient and a band gap around 1.4-1.5 eV. In particular, thanks to the ease of band-gap tuning by partial/total substitution of one or more of its elements, the high-band-gap kesterite derivatives have drawn a lot of attention aiming to find the perfect partner as a top absorber to couple with silicon in tandem solar cells (especially in a four-terminal architecture). In this work, we report the effects of the substitution of tin with different amounts of germanium in CZTS-based solar cells produced with an extremely simple sol-gel process, demonstrating how it is possible to fine-tune the band gap of the absorber and change its chemical-physical properties in this way. The precursor solution was directly drop-cast onto the substrate and spread with the aid of a film applicator, followed by a few minutes of gelation and annealing in an inert atmosphere. The desired crystalline phase was obtained without the aid of external sulfur sources as the precursor solution contained thiourea as well as metal acetates responsible for the in situ coordination and thus the correct networking of the metal centers. The addition of KCl in dopant amounts to the precursor solution allowed the formation of well-grown compact grains and enhanced the material quality. The materials obtained with the optimized procedure were characterized in depth through different techniques, and they showed very good properties in terms of purity, compactness, and grain size. Moreover, solar-cell prototypes were produced and measured, exhibiting poor charge extraction due to heavy back-contact sulfurization as studied in depth and experimentally demonstrated through Kelvin probe force microscopy.