Over 13% Efficient, Ambient Air-Processed CuIn(S,Se)2 Solar Cells via Compositional Engineering of Molecular Inks

A dimethylformamide (DMF) and thiourea (TU)-based ink deposition route is used to fabricate narrow bandgap (& AP;1.0 eV) CuIn(S,Se)(2) (CISSe) films with Cu-poor ([Cu]/[In] = 0.85), stoichiometric ([Cu]/[In] = 1.0), and Cu-rich ([Cu]/[In] = 1.15) compositions for photovoltaic applications. Characterization of KCN- or (NH4)(2)S-treated Cu-rich absorber films using X-ray diffraction and scanning electron microscopy confirms the removal of copper-selenide phases from the film surface, while electron backscatter diffraction measurements and depth-dependent energy-dispersive X-ray spectroscopy indicate remnant copper-selenides in the absorber layer bulk. Contrary to best practice for vacuum-processed cells, optimum [Cu]/[In] ratios appear to be stoichiometric, rather than Cu-poor, in DMF-TU-based CISSe devices. Accordingly, stoichiometric film compositions yield large-grained (& AP;2 & mu;m) absorber layers with smooth absorber surfaces (root mean square roughness <20 nm) and active area device efficiencies of 13.2% (without antireflective coating). Notably, these devices reach 70.0% of the Shockley-Queisser limit open-circuit voltage (i.e., 526 mV at E-g of 1.01 eV), which is among the highest for ink-based CISSe devices.

Automated summary

A deposition route using dimethylformamide (DMF) and thiourea (TU) is utilized to fabricate CuIn(S,Se)(2) (CISSe) films with narrow bandgap and different Cu/In ratios for photovoltaic applications. The characterization of the films indicates the removal of copper-selenide phases from the film surface, but remnant copper-selenides still exist in the absorber layer bulk. Contrary to vacuum-processed cells, stoichiometric film compositions yield high-efficiency devices with large-grained absorber layers and smooth surfaces.