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 degrees C for the NAOS-SiOx, 880 degrees C for the PANO-SiOx , and 930 degrees 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 similar to 1 x 10(18) 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 tau(eff) 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 (rho(c)) measurements show that the Thermal-SiOx leads a higher p c than the others, but its value is still low enough for high-efficiency solar cells.