Se-Containing inks for the formation of CuInSe2 films without gas-phase selenization

To make thin film photovoltaics (TFPV) more competitive with classic Si-based devices, research is focusing on the production of absorber layers. Printable inks containing nanocrystal (NC) precursors are explored, since a variety of NCs can be synthesized in apolar media. When printing photovoltaic absorber layers of NCs such as CIGS, the formation of a dense thin film is required, which is however hampered by the presence of the organic ligands. Our research focusses on CuInS2 NCs, for which the original steric stabilizers are exchanged for inorganic moieties containing sulfide or selenide species. This leads to an ink stabilized by charge that, apart from the solvent, doesn’t introduce unwanted components in the film. In CIGS processing, crystal growth is typically promoted by a gas-phase selenization step after film deposition. However, this introduces an additional process step that involves working with toxic gasses in a closed atmosphere. We show that introducing Se-containing moieties to stabilize CuInS2 dispersions enables us to enhance NC sintering and transformation, without the need for a selenization step. Two inks were prepared: both containing (N2H5)2Se capped CIS NCs and one of them containing Se NPs as well. Upon thermal annealing under He atmosphere phase transformation from CIS to CISe was observed with in situ XRD, resulting in a close-to pure CISe phase in both cases. Finally, we address the prospects of using this approach to form CIGS absorber layers for TFPV