Development of a HWCVD epitaxial IBC solar cell as a test platform for novel antireflection and light trapping schemes

Abstract

Continuing to reduce the cost-per-watt is the key to providing affordable PV energy. In pursuit of reducing cost-per-watt of silicon PV, this work first focuses on the development of an all back contact silicon solar cell which incorporates a hot wire chemical vapour deposition (HWCVD) technique for p+ emitter deposition. This is as a simpler, lower cost alternative to the traditional high temperature diffusion processes. A proof-of-principle development batch is presented, which resulted in a device with low efficiency, however, through further characterisation, coupled with PC2D device modelling, various potential improvements to the design are identified. A detailed investigation into the as-deposited HWCVD emitter material reveals a polycystalline structure. Post-deposition annealing is shown to improve the crystallinity of the emitter but at the expense of a higher thermal budget. Good diode characteristics are demonstrated for the annealed p-type emitter on an n-type substrate. The focus then shifts to an experimental optimization of the front surface of the cell to achieve a low reflectance and low surface recombination, using conventional methods of thin film coating, pyramidal texturing and front surface field formation. The result is an improved device design and process listing which should result in a high efficiency baseline device on which further optimization can be carried out. A novel front surface antireflection scheme based on a two stage etch process that results in a hybrid nanowire-pyramid surface structure and a weighted average reflectance as low as 1.89% is then described. Conformal coating of the nanowires with alumina using atomic layer deposition is demonstrated, paving the way for effective passivation of these high surface area structures. Finally, a combination of finite difference time domain (FDTD) optical modelling and technology computer aided design (TCAD) electrical simulations predicts that a cell efficiency exceeding 20 % should be possible by combining an optimized HWCVD IBC process with a well-passivated hybrid nanowire-pyramid top surface antireflection scheme.

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ORCID for Stuart Boden: orcid.org/0000-0002-4232-1828

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