Molecular-ink route to 13.0% efficient low-bandgap CuIn(S,Se)2 and 14.7% efficient Cu(In,Ga)(S,Se)2 solar cells

Abstract

Solar cells based on Cu(In,Ga)(S,Se)2 absorber layers have achieved the highest conversion efficiency amongst several thin film solar technologies but have so far struggled to translate their high performance into cost advantages on a module level. The use of printing processes has the potential to close that gap, provided high efficiency levels can be maintained and benign reagents for the material synthesis are employed. Additionally, low bandgap absorbers such as CuInSe2 are a highly attractive material for bottom cells in tandem devices, as their bandgap is an excellent match to some of the best performing hybrid perovskites. Here we present an ink composed of molecular complexes formed from metal chlorides, thiourea, and dimethyl sulfoxide and a fabrication route for low-bandgap CuIn(S,Se)2 absorbers whose device efficiency exceeds the performance of all previous non-vacuum methods and approaches device parameters of conventional cells formed by vacuum co-evaporation. The formation of complexes was found critical to control oxidation states and loss of metals during processing and tailor the composition of the absorber. These findings may pave the way for all-printed tandem solar modules and dramatically reduced cost of solar electricity. A stable dimethyl sulfoxide (DMSO)-based ink containing a copper–thiourea–chloride complex and an indium–DMSO–chloride complex leads to 13.0% efficient CuIn(S,Se)2 (CIS) solar cells, which is a record for solution processed CIS. The formation of these complexes was found critical to control oxidation states and loss of metals during processing and to tailor the final composition of the absorber.