Inkjet-printed flexible kesterite solar cells

Among the emerging inorganic thin-film photovoltaic technologies, kesterite Cu2ZnSn(S,Se)4 (CZTSSe) is one of the most promising material thanks to its earth-abundant constituents, p-type semiconductor properties, a high absorption coefficient and a direct tunable bandgap of 1.0-1.5 eV, according to the [S]/([S]+[Se]) ratio, which allows for an efficient harvest of an abundant portion of the solar spectrum. This material can be easily grown as a thin film, both on rigid and flexible substrates, through different chemical and physical processes. Solution-based methods, relying on the deposition of multiple layers to reach the desired thickness of the film, are currently considered the best choice. The most common are spin-coating, blade-coating, spray pyrolysis, and dip-coating. In particular, spin-coating led to the current record efficiency of η = 15.1%. However, the small-area limitations of this methodology and the waste of a non-negligible amount of precursor solution still hinder its scalability and broad applicability. Drop-on-demand inkjet printing is a promising, fully automated solution-based technique that is appealing from an industrial point of view because it's suitable for large areas and roll-to-roll processes. In this work, the inkjet printing method has been applied to fabricate flexible CZTSSe thin-film solar cells for the first time. A novel 2-methoxyethanol (MOE) -based precursor solution has been properly optimized, together with the surface engineering of the flexible molybdenum foil substrate, to allow inkjet printing of the CZTSSe thin-film absorber. The obtained results show the first example of a working inkjet-printed flexible kesterite CZTSSe solar cell, overcoming the performance of its corresponding spin-coated counterpart, relying on a more homogenous and compact absorber material. With inkjet printing technology, a significant reduction of preparation cost and better raw materials utilization has been possible, proving it as the most reliable path for a sustainable industrial scale-up of flexible CZTSSe solar cells.