Tuning the Bandgap Without Compromising Efficiency: Ambient Solution Processing of Ge-Alloyed (ag,cu) 2zn(sn,ge)(s,se) 4 Kesterite Thin-Film Solar Cells

The photovoltaic sector currently experiences a growing demand for deeply-integrated technologies in agriculture, buildings, vehicles, among others. Kesterite compounds provide a versatile solution through their tunable bandgap via isovalent alloying. However, this should be done while preserving material opto-electronic quality and device performance, which is an unresolved challenge. Indeed, tuning the bandgap of kesterites must ensure low crystalline disorder to counteract the associated open-circuit voltage (Voc) losses when used in solar cells. To achieve this goal, this work introduces a simple molecular ink recipe realized in ambient air to process singlephase high-quality Ag,Gealloyed kesterite absorbers. By varying the Ge content from 0 to 100 %, the absorber bandgap covers the range from 1.15 to 1.5 eV, in which the Urbach energy remains close to 20 meV and promisingly low Voc deficits are observed. In particular, the champion efficiency of 12.1 % obtained with 40 % Ge breaks the record for Ge-alloyed kesterites with a 1.2 eV bandgap by remarkably reaching 62 % of the ShockleyQueisser limit for Voc. This study demonstrates a promising strategy to solve the issue of large band tailing and low opto-electronic quality in widebandgap kesterites while establishing a foundation for solution-processed kesterite solar cells with tunable bandgap and promising performance.