Hole-selective NiO:Cu contact for NiO/Si heterojunction solar cells

Heterojunction architectures become a major trend in the development of high power conversion efficiency (PCE) c-Si solar cell in recent. However, NiO as a prevailing material in other application is not favored in c-Si solar cells. In this study, a novel solar cell architecture using nonstoichiometric p-type NiO thin films as a hole-selective, dopant-free contact to n-type crystalline silicon (n-Si) is successfully fabricated. We achieved a power conversion efficiency (PCE) of 4.3% in the pure NiO/n-Si heterojunction solar cell, which is the first case to be reported. Further Cu-incorporation enhanced the work function and conductivity of NiO films (NiO:Cu, for short), increasing built-in potential and favoring hole transport in the NiO:Cu/n-Si heterojunction, which results in a record PCE of 9.1% among all bulk-like NiO/Si heterojunction solar cells. Analysis of the spectra of Cu 2p and Cu LMM Auger lines, measured by X-ray photoelectron spectroscopy (XPS), shows that the fabricated NiO:Cu film is a mixture of metallic Cu, Cu2O and CuO multi-phase coexistence. Among these species, the proper Cu incorporation contributes the improvement of acceptor concentration due to the dominant phase of Cu+, but the excessive Cu incorporation creates defect states within the band gap, increasing recombination and deteriorating interface quality, which leads to S-shape current density-voltage (J-V) characteristics.