Enhancement of solar cell performance using surface morphology modification

Solar cell development has become more and more important due to the increase of worldwide energy demands, and conventional energy resources such as fossil fuels, will be exhausted soon. A key factor in applications and effectiveness of solar cell is to enhance its energy conversion efficiency. In this chapter, we reviewed and demonstrated innovative approaches to improve conversion efficiency of silicon solar cells by modifying surface morphology of the devices. Because more than 30% of incident light is reflected from the silicon surface back to the air, an anti-reflection (AR) layer is a typical type of coating which can be applied to the surface to reduce light reflection and to increase light absorption. Surface-relief gratings with sizes smaller than the wavelength of light, named sub-wavelength structures (SWS), can behave as antireflection surfaces. Using a mechanically continuous wavelike grating, the sub-wavelength structured grating acts as a surface possessing a gradually and continuously changing refractive index profile from the air to the substrate. We demonstrate polymer sheets with sub-wavelength AR structures using spin-coating replication and hot-embossing techniques with applications on silicon solar cells. The techniques provide simple and low-cost means for large-scale production of AR layers and to improve solar cell performance. On the other end of the spectrum, the results of the fabrication of ZnO nanorod and self-aggregated nanoparticles as the AR layers are also presented in this chapter. The vertically aligned and solution-grown ZnO nanorod arrays were deposited on the surface of the Si solar cells as the AR layer. We found that the nanorod morphology, controlled through synthetic chemistry, has a great effect on the AR layer performance. It's also known that nanoparticle monolayers composed of metal, silica or polystyrene have the characteristics of surface plasmon resonances and reducing light reflection on surface. Various solar cells substructures with the deposition of nanoparticles have been proved to be able to enhance the photocurrent, acceptance angles, and to reduce the surface reflection within specific wavelength (700-1100 nm) range. We demonstrate an appropriate and well-operated application of self-assembled nanoparticle layers on solar cell surface, which increases the light absorption and enhances the photocurrent so that the light conversion efficiency of the cells can be improved. The techniques not only provide enhancement of the light-harvesting capability of the device but also with a minimum cost.