Rapid vapor-phase direct doping for high-efficiency solar cells
An alternative boron emitter diffusion process called rapid vapor-phase direct doping (RVD) is studied and applied to n -type silicon solar cells with a tunnel oxide passivated electron contact (TOPCon). The RVD emitter diffusion process occurs under an atmosphere containing only the dopant gas and hydrogen. Thus, compared with standard tribromide diffusion processes, no oxygen is present. Hence, no boron glasses form during the RVD process. Consequently, a faster diffusion process with fewer chemical treatments after the diffusion process compared with standard tribromide processes is possible. In this paper, three different RVD emitter surface dopant concentrations and dopant depths were achieved by process parameter variations. These RVD emitters were applied to TOPCon cells, and their cell characteristics were compared with profiles of TOPCon reference cells with standard boron-diffused emitters. Up to 24.0% cell efficiency, 697.6 mV open-circuit voltage, 41.8 mA/cm2 short-circuit current density, and 82.1% fill factor were reached by the best TOPCon cell with an RVD emitter. Nevertheless, compared with the reference, all cells with RVD emitters exhibited efficiency losses. Hence, to further optimize cells with RVD emitters, in-depth characterizations were conducted. The cell efficiency of cells with an RVD emitter is mainly limited by two main reasons: First, effective carrier lifetime degradation was observed, resulting in voltage losses, and second, for RVD diffusion temperatures above 980 ∘C, a flattening of textured cell surfaces was detected leading to current losses. In order to overcome these issues, an adapted two-step RVD emitter diffusion process is suggested for future experiments.