A new texturing technique for silicon solar cells using gas phase etching

Thesis

Solar energy is vital to combatting climate change. However, not all incident photons can transmit into a typical solar cell for electricity generation, a portion of the photons are lost to front surface reflectance. By changing the front surface texture of typical solar cells from microscale to nanoscale, the solar cell front surface reflectance (at wavelengths of interest), can be reduced to almost 0%. Such a change would increase the number of photons available in the solar cell for electricity generation. Despite the excellent optical properties nano-textures can offer, all current industrially relevant nanoscale texturing methods have drawbacks that prevent them from gaining a significant market share. This thesis explores texturing in the gas-phase to overcome the drawbacks of other nanoscale texturing techniques, whilst still providing exceptional optical properties to improve solar cell efficiency. Texturing Si in the gas-phase using ozone and hydrofluoric acid vapours at atmospheric pressure and low temperature was investigated. The premise of the texturing mechanism was to use the increased activity of ozone to oxidise the Si surface, at significantly lower temperatures than for O2 oxidation, then etch the oxide away using HF vapour, causing an ultra-low reflectance surface texture to form. A texturing tool was designed, built, and refined to study gas-phase texturing in this project. With this new tool, texturing reproducibility and uniformity were found to be dependent upon the surface chemistry of the Si wafers. By intentionally changing the surface chemistry via a precursor containing colloidal silica and IPA, reproducible and uniform texturing could be performed on sample sizes up to full-sized industrial Si wafers, with an average reflectance as low as 1.8% +/- 0.2%in 2 mins texturing. The mechanism behind the precursor was attributed to the Na+ counter-ions presentin the colloidal silica and a full texturing mechanism was proposed for the first time, accounting for the different morphologies gas-phase texturing could produce. Upon implementing gas-phase textured Si into a solar cell for the first time, a median cell efficiency of 17.24% was achieved, without any optimisation of the solar cell fabrication methods for the nano-texture.

Authors: Shaw, EC

• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: contamination; alkali metals; manufacturing; processing; silicon; gas phase; reflectance; sodium; chemistry; texturing; silicon solar cells; precursor; colloid; optical analysis; nanotexture
• Subjects: Manufacturing processes; Reflectance; Etching; Silicon solar cells; Nanoparticles