Optimization of In-situ and Ex-situ doped p+ Passivating Contact for High Efficiency p-TOPCon Solar Cell Application

This paper presents fabrication and optimization of p+ passivating contact (p+ poly-Si/SiOx/c-Si) for high efficiency p-TOPCon solar cells. The p+ passivating contacts were formed by (i) in-situ doped p+ poly-Si via LPCVD and (ii) ex-situ doping of intrinsic poly-Si via APCVD boron diffusion. We studied the relationship between the passivation quality of this contact as a function of boron diffusion profile varied by altering (i) the crystallization annealing temperature, and (ii) the APCVD precursor gas flow ratio $\left( {{\emptyset _{{B_2}{H_6}}}/{\emptyset _{Si{H_4}}}} \right).$ In-situ doping resulted in higher B concentration in poly-Si, which improves field induced passivation, as well as in Si absorber near the interface, which enhances Auger recombination to degrade passivation. This trade-off resulted in lower optimum annealing temperature for in-situ process (875°C vs 950°C). However, process optimization resulted in excellent passivation with comparable iVoc and J0 for both in-situ (719mV/6.3fAcm-2) and ex-situ doped (716mV/6.6fAcm-2) un-metallized p-TOPCon. Greater than 22.0% screen-printed large area (244.32cm2) bifacial p-TOPCon solar cells were fabricated by replacing LBSF of p-PERC cells with 270 nm p+ TOPCon junction with ~10% metal coverage on the rear side. TOPCon cells gave 0.4% higher efficiency than their PERC counterpart.