Highly transparent and highly passivating Silicon nitride for solar cells
Creators: name: Wan, Yimao
Publisher: name: The Australian National University
Year: 2014
Subjects: subject: Silicon solar cells
subject: Silicon Surfaces Optical properties
subject: Silicon nitride
subject: Thin films
subject: Plasma-enhanced chemical vapor deposition

description: This thesis concerns the optimisation and application of Silicon nitride (SiNx) films for silicon solar cells. Systematic and comprehensive studies of SiNx properties are undertaken to advance (i) the technology of SiNx synthesised by plasma enhanced chemical vapour deposition (PECVD), and (ii) the understanding of recombination at SiNx-passivated silicon surfaces. We examine the film properties of SiNx prepared by a microwave/radio-frequency dual-mode PECVD reactor. It is shown that there is no universal correlation between surface recombination and (i) bulk structural properties such as chemical bond densities, and (ii) bulk optical properties such as refractive index and extinction coefficient. Results of this study repudiate the common perception that surface recombination decreases as SiNx becomes Si-rich. The finding introduces the potential to independently control the optical and surface recombination properties of SiNx. This is of great importance for the industrial application of SiNx films to photovoltaic cells, as it allows the front surface transmission to be maximised while still attaining outstanding surface passivation. We attain a low and relatively constant surface recombination over a wide range of SiNx refractive indices. Notably, the behaviour is observed on several types of silicon surface surfaces-planar, textured, p-type, n-type, diffused and undiffused-with direct relevance to most silicon solar cell structures. The results confirm that the trade-off between the optical transmission and surface recombination is circumvented. In specific, we attain a highly transparent and highly passivating SiNx film. The value of this film is demonstrated on an n-type interdigitated back contact solar cell with no front surface diffusion, which makes the cell highly susceptible to the front surface passivation. On such a cell, the optimum SiNx developed in this thesis enables a conversion efficiency of 24.4 +/- 0.5% under standard testing conditions (25 Celsius degrees, AM1.5G spectrum). Besides the significant improvement in optical transmission and surface passivation, the results of this thesis also advance the current understanding of recombination at SiNx-passivated silicon surfaces. It is found that an increase in recombination of the textured surfaces is related to the presence of vertices and/or edges of the pyramids rather than to the presence of {111}-orientated facets. Furthermore, this thesis demonstrates that the increase in recombination introduced by (i) a lower pressure, leading to a higher refractive index, (ii) a higher NH3:SiH4 ratio, leading to a lower refractive index, and (iii) the vertices and/or edges of the pyramids, is primarily attributable to an increase in interface defect density rather than a decrease in SiNx charge density. In addition, we hypothesise that the increase in interface defect density is caused by an ion bombardment of the silicon surface at a lower pressure, and by an excessive incorporation of NHb radicals into the SiNx film network at a higher NH3:SiH4 ratio. The satisfactory resolution of the trade-off between optical transmission and surface passivation, and the improved understanding of recombination at SiNx-passivated silicon surfaces, represent significant contributions to the science and technology of silicon solar cells.