Solution-processed Ge(ii)-based chalcogenide thin films with tunable bandgaps for photovoltaics

Abstract

Solution processes have been widely used to construct chalcogenide-based thin-film optoelectronic and electronic devices that combine high performance with low-cost manufacturing. However, Ge(II)-based chalcogenide thin films possessing great potential for optoelectronic devices have not been reported using solution-based processes; this is mainly attributed to the easy oxidation of intermediate Ge(II) to Ge(IV) in the precursor solution. Here we report solution-processed deposition of Ge(II)-based chalcogenide thin films in the case of GeSe and GeS films by introducing hypophosphorous acid as a suitable reducing agent and strong acid. This enables the generation of Ge(II) from low-cost and stable GeO₂ powders while suppressing the oxidation of Ge(II) to Ge(IV) in the precursor solution. We further show that such solution processes can also be used to deposit GeSe_1−xS_x alloy films with continuously tunable bandgaps ranging from 1.71 eV (GeS) to 1.14 eV (GeSe) by adjusting the atomic ratio of S- to Se-precursors in solution, thus allowing the realization of optimal-bandgap single-junction photovoltaic devices and multi-junction devices.