The Role of Random Texture Scattering on the Absorptance Enhancement in Halide Perovskite Layers

Hybrid perovskites are a class of thin-film semiconductors with remarkably steep absorption edges and high absorption coefficient. In the case of solar cells, a film thickness of less than a micrometer is usually sufficient to absorb most of the light when combined with a back reflector. Otherwise, an efficient light trapping strategy may be desired, e.g., in the case of tandem or semitransparent cells. Traditionally, light trapping is accomplished by employing randomly nanotextured substrates. In this contribution, absorption enhancements due to not only nanorough but also microrough substrates and with or without additional gold coating are evaluated from the point of gains in photocurrent and from the point of view of valid optical models. We find that light trapping from nanotextured substrates follows mainly the Yablonovitch model, leading to an apparent shift of absorption edge. This contrasts with microrough substrates and also the remarkable efficient light trapping capabilities of bare layers due to their native surface roughness, where the path enhancement in this case is almost uniform, making the layer optically thicker by factor two or more. Light trapping optical models as well as analytical techniques are reviewed, and new insights are presented.