RF magnetron sputtered NiO<sub><italic>x</italic></sub> and NiO<sub><italic>x</italic></sub>/c-Si single-side heterojunction solar cells

NiO_x is a p-type semiconductor with wide-band gap (3.6–4.0 eV), and the energy band structure at NiO_x/c-Si(n) interface is suitable for c-Si(n) heterojunction solar cell. On the one hand, the big conduction band offset forms a reflection barrier of electron that reduces the interface electron concentration and interface recombination. On the other hand, the valance band offset will change with the deposition conditions, which lower the hole transport barrier and make the hole transport smoothly. Therefore, NiO_x is a promising hole-selective contact layer for c-Si(n) heterojunction solar cell. In this work, in order to simplify the study, we fabricate single-side heterojunction solar cell with Al-grid/ITO/NiO_x/SiO_x/c-Si(n)/SiO_x/Al-electrode structure. The n-type c-Si(100) wafers with resistivity of 1–10 Ω cm and thickness of 200 μm were firstly dipped in KOH solution (25wt%) at 80°C for 4 min to remove the surface sawing damage, then standardized RCA1 procedure and DHF (6wt%) dip were adopted to remove the contaminants and native oxide presented on the wafer surface. The SiO_x layer was grown in 5wt% H₂O₂ at 80°C for 20 min. Afterward, a NiO_x thin film with thickness of 10 nm was deposited on the wafer front side using RF magnetron sputtering, and a 100 nm thick ITO layer was deposited on NiO_x by DC magnetron sputtering. Then Al-grid on the front and Al-electrode on the back were deposited by electron-beam evaporation. Finally, completed NiO_x/c-Si(n) single-side heterojunction solar cells were characterized by light/dark J-V (SAN EI XEC 500M2 solar simulator, KEYTHLEY 2400 source-meter). We investigate the optical, electronical properties and energy band of NiO_x thin films by changing the sputtering conditions, analyze the carrier transport mechanisms and the interface recombination mechanisms of NiO_x/c-Si heterojunction. And we find that the valance band offset is the key factor that affects cell performance. The increase of the valance band offset leads to the increase of the series resistance (R_s) of the cell, thus reducing the fill factor (FF) and power conversion efficiency (PCE) of the cell (cell Sample B, E and A). When the valance band offset is high and almost the same, the lower the resistivity of NiO_x thin film is, the lower the series resistance (R_s) of the cell becomes, which contributes to the higher FF and the higher PCE of the cell. This is because the lower resistivity of NiO_x thin film narrows the width of the blocking barrier of the hole and increases the tunneling current (cell Sample A and B). In addition, the experiment and the simulation results (AFORS-HET) show that there are two ways to enhance the cell performance. One is to reduce the valence band offset and interface defects of NiO_x/c-Si heterojunction so as to lower the interface recombination. The other one is to increase the doping density in NiO_x that increase the built-in electric field. Therefore, the passivation of NiO_x/c-Si(n) interface, the improvement of acceptor concentration of NiO_x emitter, the optimization of back surface field and ITO/NiO_x contact are the main directions for further research to achieve high efficiency NiO_x/c-Si heterojunction solar cells.