Nitrogen-Doped Cu2O Thin Films for Photovoltaic Applications

Cuprous oxide (Cu₂O) is a p-type semiconductor with high optical absorption and a direct bandgap of about 2.1 eV, making it an attractive material for photovoltaic applications. For a high-performance photovoltaic device, the formation of low-resistivity contacts on Cu₂O thin films is a prerequisite, which can be achieved by, for instance, nitrogen doping of Cu₂O in order to increase the carrier concentration. In this work, nitrogen-doped p-type Cu₂O thin films were prepared on quartz substrates by magnetron sputter deposition. By adding N₂ gas during the deposition process, a nitrogen concentration of up to 2.3 × 10²¹ atoms/cm³ in the Cu₂O thin films was achieved, as determined from secondary ion mass spectroscopy measurements. The effect of nitrogen doping on the structural, optical, and electrical properties of the Cu₂O thin films was investigated. X-ray diffraction measurements suggest a preservation of the Cu₂O phase for the nitrogen doped thin films, whereas spectrophotometric measurements show that the optical properties were not significantly altered by incorporation of nitrogen into the Cu₂O matrix. A significant conductivity enhancement was achieved for the nitrogen-doped Cu₂O thin films, based on Hall effect measurements, i.e., the hole concentration was increased from 4 × 10¹⁵ to 3 × 10¹⁹ cm⁻³ and the resistivity was reduced from 190 to 1.9 Ω⋅cm by adding nitrogen to the Cu₂O thin films.

Metal Oxide Thin-Film Heterojunctions for Photovoltaic Applications

Silicon-based tandem solar cells incorporating low-cost, abundant, and non-toxic metal oxide materials can increase the conversion efficiency of silicon solar cells beyond their conventional limitations with obvious economic and environmental benefits. In this work, the electrical characteristics of a metal oxide thin-film heterojunction solar cell based on a cuprous oxide (Cu₂O) absorber layer were investigated. Highly Al-doped n-type ZnO (AZO) and undoped p-type Cu₂O thin films were prepared on quartz substrates by magnetron sputter deposition. The electrical and optical properties of these thin films were determined from Hall effect measurements and spectroscopic ellipsometry. After annealing the Cu₂O film at 900 °C, the majority carrier (hole) mobility and the resistivity were measured at 50 cm²/V·s and 200 Ω·cm, respectively. Numerical modeling was carried out to investigate the effect of band alignment and interface defects on the electrical characteristics of the AZO/Cu₂O heterojunction. The analysis suggests that the incorporation of a buffer layer can enhance the performance of the heterojunction solar cell as a result of reduced conduction band offset.

Improving carrier transport in Cu2O thin films by rapid thermal annealing

Cuprous oxide (Cu2O) is a promising material for large scale photovoltaic applications. The efficiencies of thin film structures are, however, currently lower than those for structures based on Cu2O sheets, possibly due to their poorer transport properties. This study shows that post-deposition rapid thermal annealing (RTA) of Cu2O films is an effective approach for improving carrier transport in films prepared by reactive magnetron sputtering. The as-deposited Cu2O films were poly-crystalline, p-type, with weak near band edge (NBE) emission in photoluminescence spectra, a grain size of ~100 nm and a hole mobility of 2-18 cm2 V-1 s-1. Subsequent RTA (3 min) at a pressure of 50 Pa and temperatures of 600-1000 °C enhanced the NBE by 2-3 orders of magnitude, evidencing improved crystalline quality and reduction of non-radiative carrier recombination. Both grain size and hole mobility were increased considerably upon RTA, reaching values above 1 µm and up to 58 cm2 V-1 s-1, respectively, for films annealed at 900-1000 °C. These films also exhibited a resistivity of ~50-200 Ω cm, a hole concentration of ~1015 cm-3 at room temperature, and a transmittance above 80%.