Nanoscale Insight into the p-n Junction of Alkali-Incorporated Cu(In,Ga)Se2 Solar Cells

The effects of alkali diffusion and post-deposition treatment in three-stage processed Cu(In,Ga)Se₂ solar cells are examined by using atom probe tomography and electrical property measurements. Cells, for which the substrate was treated at 650°C to induce alkali diffusion from the substrate prior to absorber deposition, exhibited high open-circuit voltage (758 mV) and efficiency (18.2%) and also exhibited a 50 to 100-nm-thick ordered vacancy compound layer at the metallurgical junction. Surprisingly, these high-temperature samples exhibited higher concentrations of K at the junction (1.8 at.%) than post-deposition treatment samples (0.4 at.%). A model that uses Ga/(Ga + In) and Cu/(Ga + In) profiles to predict bandgaps (±17.9 meV) of 22 Cu(In,Ga)Se₂ solar cells reported in literature was discussed and ultimately used to predict band properties at the nanoscale by using atom probe tomography data. The high-temperature samples exhibited a greater drop in the valence band maximum (200 meV) due to a lower Cu/(Ga + In) ratio than the post-deposition treatment samples. There was an anticorrelation of K concentrations and Cu/(Ga + In) ratios for all samples, regardless of processing conditions. Changes in elemental profiles at the active junctions correlate well with the electrical behaviour of these devices.