Modeling upconversion of erbium doped microcrystals based on   experimentally determined Einstein coefficients
  Upconversion of infrared photons is a promising possibility to enhance solar cell efficiency by producing electricity from otherwise unused sub-band-gap photons. We present a rate equation model, and the relevant processes, in order to describe upconversion of near-infrared photons. The model considers stimulated and spontaneous processes, multi-phonon relaxation and energy transfer between neighboring ions. The input parameters for the model are experimentally determined for the material system \beta-NaEr0.2Y0.8F4. The determination of the transition probabilities, also known as the Einstein coefficients, is in the focus of the parameterization. The influence of multi-phonon relaxation and energy transfer on the upconversion are evaluated and discussed in detail. Since upconversion is a non-linear process, the irradiance dependence of the simulations is investigated and compared to experimental data of quantum efficiency measurements. The results are very promising and indicate that upconversion is physically reasonably described by the rate equations. Therefore, the presented model will be the basis for further simulations concerning various applications of upconversion, such as in combination with plasmon resonances in metal nanoparticles. 
S. Fischer, H. Steinkemper, P. L\"oper, M. Hermle, J. C. Goldschmidt
physics.optics