Colloidal-lithographed TiO2 photonic nanostructures for solar cell light trapping

Dielectric-based photonic structures, composed of a lossless but high refractive index material, are currently among the preferential solutions for light management in thin film photovoltaics, as they allow broadband manipulation of sunlight to strongly boost the absorptance in the thin solar cell layers. In this work, we present an innovative colloidal lithography nanofabrication method that allows the precise engineering of wavelength-sized features, with the materials and geometries appropriate for efficient light trapping when implemented on the front surface of the cells. The method is developed here with TiO2 nanostructures tested on amorphous-silicon absorber thin films coated on the rear side by a metallic reflector. It is a simple, low-cost and scalable approach consisting of 4 main steps: (1) deposition of periodic close-packed arrays of polystyrene colloids which act as the mask; (2) shaping the particles and increasing their spacing via dry etching; (3) infiltration of TiO2 into the inter-particle spaces and (4) removal of the polystyrene particles to leave only the structured TiO2 layer. The resultant array of wavelength-sized features acts as a nanostructured high-index anti-reflection coating, which not only suppresses the reflected light at short wavelengths but also increases the optical path length of the longer wavelengths, via light scattering, within the absorber. The optical results have been compared with numerical electromagnetic computations to provide a deeper understanding of the physical mechanisms responsible for absorptance enhancement in the cells. A notorious 27.3% enhancement in the cell photocurrent is anticipated with the fabricated structures, relative to conventional anti-reflection coatings.