Effect of BaSi 2 template growth duration on the generation of defects and performance of p-BaSi 2 /n-Si heterojunction solar cells

Abstract We investigated the effect of BaSi 2 template growth duration ( t RDE = 0–20 min) on the defect generation and performance of p-BaSi 2 /n-Si heterojunction solar cells. The p-BaSi 2 layer grown by molecular beam epitaxy (MBE) was 15 nm thick with a hole concentration of 2 × 10 18 cm −3 . The conversion efficiency η increased for films grown at long t RDE , owing to improvements of the open-circuit voltage ( V OC ) and fill factor ( FF ), reaching a maximum of η = 8.9% at t RDE = 7.5 min. However, η decreased at longer and shorter t RDE owing to lower V OC and FF . Using deep-level transient spectroscopy, we detected a hole trap level 190 meV above the valence band maximum for the sample grown without the template ( t RDE = 0 min). An electron trap level 106 meV below the conduction band minimum was detected for a sample grown with t RDE = 20 min. The trap densities for both films were (1–2) × 10 13 cm −3 . The former originated from the diffusion of Ba into the n-Si region; the latter originated from defects in the template layer. The crystalline qualities of the template and MBE-grown layers were discussed. The root-mean-square surface roughness of the template reached a minimum of 0.51 nm at t RDE = 7.5 min. The a -axis orientation of p-BaSi 2 thin films degraded as t RDE exceeded 10 min. In terms of p-BaSi 2 crystalline quality and solar cell performance, the optimum t RDE was determined to be 7.5 min, corresponding to approximately 4 nm in thickness.