Multiple techniques have been employed to increase the amount of light that enters the cell and reduce the amount that escapes without absorption. The most obvious technique is to minimizing the top contact coverage of the cell surface, reducing the area that blocks light from reaching the cell.
The weakly absorbed long wavelength light can be obliquely coupled into silicon and traverses the film several times to enhance absorption.
Multiple methods have been developed to increase absorption by reducing the number of incident photons being reflected away from the cell surface. An additional anti-reflective coating can cause destructive interference within the cell by modulating the refractive index of the surface coating. Destructive interference eliminates the reflective wave, causing all incident light to enter the cell.
Surface texturing is another option for increasing absorption, but increases costs. By applying a texture to the active material’s surface, the reflected light can be refracted into striking the surface again, thus reducing reflectance.For example, black silicon texturing by reactive ion etching(RIE) is an effective and economic approach to increase the absorption of thin-film silicon solar cells. A textured backreflector can prevent light from escaping through the rear of the cell.
In addition to surface texturing, the plasmonic light-trapping scheme attracted a lot of attention to aid photocurrent enhancement in thin film solar cells. This method makes use of collective oscillation of excited free electrons in noble metal nanoparticles, which are influenced by particle shape, size and dielectric properties of the surrounding medium.
In addition to minimizing reflective loss, the solar cell material itself can be optimized to have higher chance of absorbing a photon that reaches it. Thermal processing techniques can significantly enhance the crystal quality of silicon cells and thereby increase efficiency.Layering thin-film cells to create a multi-junction solar cell can also be done. Each layer’s band gap can be designed to best absorb a different range of wavelengths, such that together they can absorb a greater spectrum of light.
Further advancement into geometric considerations can exploit nanomaterial dimensionality. Large, parallel nanowire arrays enable long absorption lengths along the length of the wire while maintaining short minority carrier diffusion lengths along the radial direction. Adding nanoparticles between the nanowires allows conduction. The natural geometry of these arrays forms a textured surface that traps more light.