Compositional analysis of (p) a-SiC_x:H emitter layers for enhanced short-circuit current density in SHJ solar cells

Abstract

One of the loss mechanisms in silicon heterojunction (SHJ) solar cells is the parasitic absorption in the p-type emitter layer. In order to increase the device performance, parasitic absorption losses should be decreased by widening the optical band gap of the emitter layer. The optical band gap of the boron-doped amorphous silicon can be increased from 1.6 to 2.3 eV by incorporating carbon in the amorphous matrix. In this work, the optical, electrical, and compositional properties of p-type amorphous silicon carbide (a-SiCx:H) deposited by Plasma Enhanced Chemical Vapor Deposition (PECVD) have been investigated. The dependence of the optical band gap and conductivity of the emitter layer under various deposition conditions, such as the RF plasma power and methane (CH4) gas flow rate, is studied in detail. Time of Flight Secondary Ion Mass Spectroscopy (ToF-SIMS) analysis clarifies the differentiation between active and passive forms of boron as a dopant compared to the total boron content within the thin films. Furthermore, the implementation of the wide band gap emitter in the SHJ solar cell and improvements in the solar cell's performance are discussed regarding the dependence on the applied emitter layer properties. Based on the optical simulation results, followed by endorsement from the EQE results, the (p) a-SiCx:H layer as the emitter improves the short current density by 0.5 mAcm−2 absolute in comparison to a conventional SHJ solar cell with a (p) a-Si:H emitter. The emitter layer investigated in this study consisted of a (p) a-SiCx:H with or without p-doped a-Si:H in the stack configuration.