Formation of NiO films by reactive sputtering and application to BaSi2 heterojunction solar cells as hole-selective interlayer material

Semiconducting BaSi2 has attractive features for thin-film solar cell applications. In this study, we investigated the potential of NiO as a hole transport layer in NiO/BaSi2 heterojunction solar cells both by simulation and by experiment. To find deposition conditions to form NiO layers, a NiO target was sputtered on glass substrates under various O2-to-Ar gas flow ratios. The hole concentration of the NiO films was controlled in the range 1017–1021 cm−3 mainly by the substrate temperature during deposition. After that, NiO/BaSi2 heterojunction solar cells were designed using a one-dimensional simulation software (AFORS-HET v2.5). The conversion efficiency exceeded 16% for 400 nm thick n-BaSi2 absorption layers. We actually formed NiO/BaSi2 heterojunction solar cells on glass substrates by radio-frequency sputtering, and demonstrated that the carriers photogenerated in the BaSi2 films contributed to the internal quantum efficiency spectrum at wavelengths shorter than approximately 900 nm, corresponding to the band gap of BaSi2.