Application of plasma-enhanced chemical vapor deposition in order to improve the efficiency of crystalline silicon solar cells

This work deals with the development and in-depth characterization of various thin dielectrics for their application as anti-reflective coatings and passivation layers in state-of-the-art passivated emitter and rear cell (PERC)-type solar cell architectures. The layers are deposited using plasma-enhanced chemical vapor deposition (PECVD) processes.

PECVD-deposited SiNx is the typical choice as the anti-reflection coating (ARC) for silicon-based solar cells. However, there still exists room for improvement in passivation quality of SiNx, while maintaining its good optical properties for the front side of the solar cell. A good compromise between optics and surface passivation is struck to prepare an optimized SiNx layer enabling low average emitter saturation current density Joe of 82 ± 2 fA/cm2 on an industrial–type emitter.

In the later section of this thesis, silicon-rich oxynitride (SiriON) layers are deposited using silane (SiH4) and nitrous oxide (N2O) for their application as ARC and passivation layers stacked with SiNx. An optimized thickness of ~3-5 nm and ~5-10 nm at N2O/SiH4 =0.21 and 0.30, respectively, was found.

Plasma-based oxidation of c-Si substrate utilizing N2O as the only gas-phase precursor is also developed. Application of the ultra thin SiOx layer below SiNx is found to significantly improve the passivation of the c-Si surface and n+ emitter, in comparison to single layer SiNx. Low J0e values of 15±2 fA/cm2 for planar and 24±2 fA/cm2 for textured surfaces, respectively, are achieved for an n+ emitter with a sheet resistivity of 161 Ω/sq after the fast firing process. Application of the plasma based SiOx/SiNx stack in PERC type solar cells also slightly outperformed the benchmark passivation stack of outgassing SiOx/PECVD-SiNx at Fraunhofer ISE, with the former leading to a peak conversion efficiency η = 20.9% in comparison to η =20.8%.