Coherent, partially coherent and incoherent light absorption in thin-film multilayer structures

We present a model for determining the optical absorption spectrum and optical absorption profile for any layer in a one-dimensional thin-film multilayer structure, based on the coherent interference between light waves in the structure. We show how partially coherent and incoherent layers may be incorporated into the calculation by means of an appropriate integration. The work is an extension of previously published work regarding a particle (photon) model for absorption in such structures. We use a structure that is typical of an a-Si:H-based photovoltaic cell as an example, and find the optical absorption profile in the a-Si:H layer of this cell using the photon model and the coherence (wave) model presented in this paper. We find that the former is strictly monotonic with position in the layer, while the latter has a slight oscillatory character. We propose a simple model for determining the absorption profile in the a-Si:H layer when one or more layers in the structure are partially coherent or incoherent, including the a-Si:H layer itself. The absorption profile for such a case lies between the profiles for the cases of incoherence and coherence. The determination of such absorption profiles is very important in solar cell simulation, where the optical generation rate of electron-hole pairs must be known at each point in the a-Si:H layer. We stress that, although we have presented results specifically for an a-Si:H-based cell, the model is general and can be applied to any thin-film multilayer structure. It should be particularly useful for determining optical generation rate profiles of electron-hole pairs for use in photovoltaic device simulations.