High-efficiency n-TOPCon bifacial solar cells with selective poly-Si based passivating contacts

Abstract

Improving the conversion efficiency of n-TOPCon solar cell is still a hot topic. The selective poly-Si based passivating contacts (Poly-SEs) are ideal candidates for reducing the parasitic absorption and contact resistivity of n-type silicon solar cells and for providing better current collection. In this work, we used LPCVD and the POCl3 tube furnace diffusion methods to fabricate the selective poly-Si based passivating contacts, and studied the influences of key process parameters of the SiOx layer formation process (the oxidation duration (toxidation) and the constant pressure duration (tpressure)), and POCl3 tube diffusion process parameters (the POCl3-N2 carrier gas flow rate at the deposition, deposition temperature, drive-in temperature) on the n+-poly-Si profiles, recombination current density (J0), contact resistivity (ρc) of n-TOPCon solar cells. The results showed that the toxidation and tpressure had a significant impact on J0 and ρc which were mainly related to the distribution number of O and Si4+ content on the growth of the SiOx layer. And the influence of the drive-in temperature of phosphorus (P) diffusion process on J0 value is stronger than that of the deposition temperature, which was mainly related to the chemical passivation of SiOx layer induced by P-indiffusion into Si at high temperature. The reduction in the thickness of poly-Si from 110 nm to 30 nm led to an increase in the short-circuit current density (Jsc) per nanometer of ∼0.0093 mA/cm2 per nm. The Poly-SEs were fabricated by 3D printing mask technology and secondary LPCVD/phosphorus diffusion with J0, n+ ≈ 5 fA/cm2 (n+-poly-Si layer ≈ 50 nm) and J0, metal,n++ ≈ 73.8 fA/cm2 (n++-poly-Si layer ≈ 110 nm), and the efficiency was improved by 0.12% owing to the increase in Jsc value of 0.28 mA/cm2. After optimizing the passivation process, the industrial-grade TOPCon bifacial cells reached an efficiency (Eff), Voc, Jsc, and FF values as high as 25.4%, 721 mV, 42.2 mA/cm2, and 83.5%, respectively.