Optimization of photonics for corrugated thin-film solar cells

The amount of solar energy reaching the active (photovoltaic) layer in a thin-film solar cell can be increased by reducing the Fresnel reflection losses at the interfaces. By using corrugated interfaces (at the wavelength scale), adiabatic propagation of the electromagnetic radiation is achieved over a broad wavelength range throughout the structure, which leads to an increase in the light that is absorbed in the active layer and, ultimately, to the improvement of the photovoltaic conversion efficiency. In this article, we have considered the case of corrugated thin-film solar cell structures and we have studied theoretically the optimization of such structures from the point of view of photonics. The focus was put on periodic pyramidal interface corrugations because they were similar to those existing at the surface of corrugated transparent electrodes on which active layers can be deposited. Because of their technological importance, we chose to work with fluorine-doped tin oxide as front electrode material and with amorphous silicon as active material. Using an original three dimensional transfer matrix method, we solved the electromagnetic wave propagation problem in the general case of laterally periodic stratified media and we compared this solution with effective medium approximated solution. On the basis of typical pyramid sizes, we demonstrated, through numerical simulations, the optimization of the global light energy intake by means of corrugations of increasing complexity. The best structures were found to be based on pyramid arrays having subwavelength periods and aspect ratio values close to one. Typically, a pyramidal structure with base and height both equal to 300 nm led to a global energy intake equal to I=0.98 (integrated over the spectral range 400-710 nm), which represented a 24% improvement in comparison with the global energy intake of a planar structure (I=0.79).