Reflection at layer interfaces and absorption reduce the efficiency of all photovoltaic devices. Reflection and absorption are an inherent property of optical systems. Reflection is caused by the change in refractive indices as light travels from one medium to another, for instance from air (n=1) to glass (n=1.5), the reflection loss is proportional to the difference between the 2 refractive indices. Absorption is related to the absorption coefficient and is normally controlled through increasing or reducing the thickness of the materials the light is travelling through; packing density can also be decreased to produce, from the frame of reference of the light, material-air or material-vacuum hybrid solids with exceptionally low refractive indices.
The air-glass interface of superstrate solar cells and the cover glass (air-glass-air) of substrate solar cells is the largest source of reflection in solar cell technologies. To address this anti-reflective (AR) coatings are implemented. AR coatings work in a number of ways to reduce the reflection loss from air-glass interfaces. The simplest coating is single layer AR, single layer AR uses destructive interference and can reduce reflection at a certain wavelength to zero when the refractive index and thickness of the material is chosen correctly. Gradual index changing is an excellent but difficult method involving the change of refractive index throughout the coating to closely match that of air and glass at the respective sides of the coating; this gradual change means that mathematically the light hits no interface and thus no reflection occurs. Multilayer AR coatings rely on the same interference effects as the single layer coatings but implement more complicated interference patterns through multiple reflections from multiple interfaces, this results in a reduced reflection across a broader wavelength range than single layer AR.
Photon loss can also be reduced through manipulating the layers of the cell. Addition or subtraction of layers, layer thinning, or interference effects within the system can all be utilised to increase transmission to the photon absorbing layer. The transmission to the absorbing layer of any solar cell can be modelled using the transfer matrix method to predict the transmission accurately while varying different parameters in the system.
The overarching aim of the project is to understand the solar cell and AR markets and technologies. Applying this knowledge to maximize transmission through photonic manipulation; while also maintaining the electrical properties of the cell and market viability of the technologies.