Fabrication of a 100 μm thick crystalline silicon solar cell

Abstract

Direct energy conversion from sun light to electricity using crystalline silicon based solar cell modules has been commercially available for several years. Above 20% conversion efficiencies have been realised in laboratory scale since 1990 [1]. The more important issue in the present solar cell fabrication development is to adopt such procedures and equipment that allow practical module efficiencies with less material and processing costs. One way to cut the material costs is to use thinner silicon wafers, which gives more wafers per length of a silicon log.In a high conversion efficiency cell the thickness has to be well below electron diffusion length for collection of the energy from long wavelength part of the solar spectrum. The diffusion length is an indicator of the material quality. Thin crystalline silicon solar cell technology gives the opportunity to choose cheaper, lower quality silicon as a starting material for the cell process without severe reduction in the conversion efficiency.Optimisation of a thin crystalline silicon solar cell design includes efficient light trapping and passivation of the cell surfaces. The proportion of a cell surface area to the cell volume increases as thickness is reduced. This magnifies the relative importance of surface recombination of the charge carriers as compared to the volume recombination in a solar cell. Although the quality demand on the cell material is relaxed, high conversion efficiency thin solar cell technology puts more weight on the quality of the surface passivation.In this paper, the fabrication procedure of thin crystalline silicon solar cell samples used in H.U.T. Electron Physics Laboratory is presented. The sample material is thinned using a 20% hot KOH bath to a nominal 1007 μm thickness. The standard cell sample is a 24 mm × 24 mm square with a p-type substrate, aluminium doped back surface field and phosphorous front diffusion. Aluminium evaporation through a stainless steel mask is adopted as a quick and simple metallisation scheme for both front and back contacts [2].The current-voltage-characteristics together with spectral response data of a typical thin crystalline silicon solar cell is also illustrated for the evaluation of the process. The measurement data is evaluated using the procedures and software developed by the author [2, 3]. The software includes diode modelling of the solar cell current-voltage data and breaking the spectral response to contributions from the emitter, depletion region and the base with numerical estimates on diffusion length, surface recombination velocities and charge carrier lifetimes.