Effect of Gas Flow Rate in PECVD of Amorphous Silicon Thin Films for Interface Passivation of Silicon Heterojunction Solar Cells

Abstract

Precursor gas flow rate variation (30–80 sccm) in the plasma‐enhanced chemical vapor deposition (PECVD) process of intrinsic a‐Si:H layer deposition using SiH4/H2 (equal ratio) plasma is explored and its effect on the i‐a‐Si:H/c‐Si interface passivation is investigated. A window of intermediate gas flow rates is determined for good quality surface passivation of n‐type c‐Si. Maximum effective minority carrier lifetime (τeff) above 1 ms, implied open‐circuit voltage (iVoc) ≈ 710 mV, and low interface defect density (Dit) ≈3.5 × 109 cm−2 eV−1 are obtained at an intermediate gas flow rate. The SiH4:H2 discharge emission characteristics, and the a‐Si:H film characteristics such as hydrogen concentration, film density, optical band gap, and refractive index, are also investigated. To examine the effect of the flow rate variation on the performance of the final device, front‐junction silicon heterojunction solar cells are fabricated on n‐type Si wafers, and ≈17% efficient cells are fabricated with an open‐circuit voltage (Voc) close to 690 mV at an optimized gas flow rate. This study provided information related to the transient plasma instability, SiH4 depletion, secondary reactions in the plasma, and flux of radicals toward the substrate for the film growth with a good level of surface passivation.