Analysis and characterization of silicon textured by metal assisted etching technique

Abstract

The efficient reflection suppression of a solar cell over a broad spectral range can be achieved by nano-scale surface textures. The fundamental reason is that the textured surface behaves like an effective medium with graded refractive index, resulting - in the optimal case - in vanishing reflectivity and appearing black under sunlight. The surface morphology is commonly known as "black Silicon" or bSi. Various methods for fabricating bSi have been developed such as reactive ion etching (RIE), metal-assisted etching (MAE) and laser texturing. MAE appears to be an interesting method because it is a flexible and potentially simple process for producing bSi. In fact, highly efficient bSi solar cells have been reported recently. Nevertheless, the energy conversion efficiency of these cells is still well below the highest achievable efficiencies. The high aspect ratio and surface area of the bSi can result in high rates of surface recombination, and therefore high values of Seff on the resulting structure. Nonetheless, no results of a systematic investigation of the correlation between optical performance, surface area increase and Seff in silicon have been reported to the best of author's knowledge. Another reason for the lower-than-optimal performance of the cells is the poor metal contact that contributes to the series resistance of the cell. Potential origins of this detrimental effect have only briefly been investigated through visual inspection, and measurements of the contact resistivity have not been made. In this thesis, an alternative 3-step MAE texturing technique which produces nanoporous silicon (MAE nSi) will be investigated using Silver (Ag) and Palladium (Pd) as catalyst. The thesis consists of two parts, namely experimental investigations and modelling. Both Ag and Pd are used to produce nano-scale structures. The optical properties of the nSi surfaces produced using these two metals are shown to be very similar. The majority of the work focuses on Ag due to the demonstrated superior electronic properties of the resulting samples. The investigation of Ag textured MAE nSi extends from basic optical performance to the effect on optical performance of high temperature process steps, initial surface roughness, the addition of a dielectric coating and encapsulation. In addition, the surface passivation and contact quality of Aluminium evaporated contacts on the nS surfaces are investigated. Optical modelling of MAE nSi provides greater insight into the light propagation behaviour of the structure. In this case, focus ion beam (FIB) has been used to acquire SEM images of the surface morphology. The reconstructed surface was then imported into GDCalc software for calculating the reflectance and transmittance matrices which can then be used to further analysis. Light propagation inside the bulk can then be calculated using an incoherent model. The modelling results were then compared to experimental results and also an ideal Lambertian model. Finally the generation profile was obtained. The generation profile is useful for investigating the potential efficiency of a solar cell. Show more