Improving thin-film solar cells through optical modeling

Abstract

Thin-film solar cells convert solar energy into electrical energy, but require less material and lower temperatures than their conventional counterparts. Typically, thin-film solar cells comprise semi-transparent layers with thicknesses measured in microns and nanometers. Also, they have textured interfaces that scatter incoming light, prolonging the optical path. This improves light trapping and optical absorption by the layers, ultimately increasing the overall efficiency of solar-to-electric energy conversion. The texture of rough interfaces is random, making it difficult to to describe and model the light scattering. Neither scalar nor vector scattering theories, nor equations derived from them, can be directly used. These equations must be calibrated in order to yield realistic results when applied to thin-film solar cells. We developed a one-dimensional model for optical analysis of thin-film solar cells.1 Realistic complex refractive indexes were used to describe the optical properties of the layers, and both specular (non-scattered) and scattered light at the textured interfaces were taken into account. The specular light was analyzed in terms of electromagnetic waves, including interference in the simulated layers, while a ray-tracing method was used for scattered light. The scattered light at the textured interfaces was characterized by its the scattering level (’haze’), and angular distribution. Haze at the internal interfaces was determined from scalar scattering theory equations calibrated by measurements on solar cell textured substrates. The optical model was implemented in a user-friendly simulator named SunShine, and applied to different types of thin-film solar cells, such as silicon solar cells (amorphous,2 microcrystalline,3 micromorph,4 and HIT cells), Cu(In,GA)Se2based solar cells,5 and various amorphous silicon photodetectors. Figure 1. Absorptances are simulated in a top a-Si:H (thickness 200nm) and bottom μc-Si:H absorber layer (thickness 2.2μm) for a standard state-of-the-art micromorph solar cell and for an optically optimized cell.