An extended model for upconversion in solar cells

Here we analyze the system proposed by Trupke et al. (J. Appl. Phys. 92, 4117 (2002)) to increase solar cell efficiency. The system consists in adding to the cell a so-called upconverter, which is a device able to convert the low-energy (subband-gap) incident solar photons into photons of higher energy. The model takes account of (i) the nonradiative recombination in both solar cell and converter and (ii) the refractive index of both cell and converter. Two configurations are studied: cell and rear converter (C-RC) and front converter and cell. The main conclusions are as follows. (1) When nonradiative recombination is neglected for both cell and converter, the energy conversion efficiency of a C-RC system slightly exceeds the efficiency of a solar cell operating alone (under 1 sun illumination). (2) When similar realistic values for the radiative recombination efficiency are considered for both cell and converter, the energy conversion efficiency of a C-RC system is lower than the efficiency of a solar cell operating alone (under 1 sun illumination). (3) Adding a rear upconverter to the solar cell is beneficial in the case of present-day quality solar cells under concentrated solar radiation. (4) At small values of the cell refractive index (roughly less than 2), the conversion efficiency does not depend on the converter refractive index. (5) At higher values of the cell refractive index, the conversion efficiency decreases by increasing the converter refractive index. (6) The energy conversion efficiency does not increase by adding a front upconverter to the cell, whatever the values of the radiative recombination efficiency and solar radiation concentration ratio are. (C) 2008 American Institute of Physics. [DOI: 0.1063/1.3040692]