Comparison of different types of interfacial oxides on hole-selective p+-poly-Si passivated contacts for high-efficiency c-Si solar cells

We present a systematic study of highly boron (B)-doped poly-silicon (p+-poly-Si) and ultrathin silicon oxide (SiOx) bi-layer structure, also named as p-TOPCon, as the hole-selective passivated contact on n-type c-Si wafer, where the SiOx layer is made with three methods of hot nitric acid oxidation SiOx (NAOS-SiOx), plasma-assisted nitrous-oxide (N2O) gas oxidation (PANO-SiOx), and thermal oxidation (Thermal-SiOx). We demonstrate that the SiOx has a strong influence on the passivation quality. The best result is achieved using the Thermal-SiOx, while the NAOS-SiOx is slightly inferior, but better than the PANO-SiOx. The p+-poly-Si/SiOx structures with the three SiOx layers achieve the optimized passivation quality at different annealing temperatures of 820 °C for the NAOS-SiOx, 880 °C for the PANO-SiOx, and 930 °C for the Thermal-SiOx. The other potential factors affecting the passivation quality are also studied. The most important observation is that the optimized p-TOPCon structures with the three SiOx layers have a similar B diffusion profile, which penetrates into the c-Si wafer about 50 nm with B concentration decreasing to ~1 × 1018 cm−3. However, the overall p+-poly-Si/SiOx is still much poorer than n+-poly-Si/SiOx in terms the passivation quality. The comparison of the τeff versus carrier injection intensity spectra suggests that the B–O complex is the passivation killer possibly, and the approaches to improve the p-TOPCon are searching the other elements to reduce the B–O defects. In addition, contact resistivity (ρc) measurements show that the Thermal-SiOx leads a higher ρc than the others, but its value is still low enough for high-efficiency solar cells.