Modelling of solar cells with down-conversion of high energy photons, anti-reflection coatings and light trapping

In classical solar cells, each absorbed photon gives rise to one electron-hole pair, irrespective of the photon energy. By applying an appropriate photoluminescent layer in front of the solar cell semiconductor, one can convert one high energy photon into two low energy photons (so-called down-conversion). In the present study, we do not consider photoluminescent layers that merely shift down photon energies (without enhancing the number of photons). In principle, these two photons can then generate two electron-hole pairs in the solar cell, thus increasing the efficiency of the device. However, the two photons emitted by the converter, are not necessarily emitted in the direction of the semiconductor: they can also be emitted in the direction 'back to the sun'. As most semiconductors have a high refractive index, in case the luminescent material has a low refractive index, more than half of the photoluminescence emission is lost in the sun direction, resulting in a net loss of light current generated by the solar cell instead of an increase. On the other hand, a high refractive index of the conversion layer (e.g. equal to the solar cell refractive index) will lead to a bad optical coupling with the air and a good optical coupling with the semiconductor, and therefore, more than 50% of the emitted low energy photons will actually reach the solar cell. However, in the latter case, many solar photons do not reach the converter in the first place because of reflection at the air-converter interface. As a result, it turns out that, in the absence of any antireflection coating, a refractive index n(2) of the converting layer in the range between n(1)(1/2) and n(1) is optimal, where n(1) is the refractive index of the solar cell material. If, however, an anti-reflection coating is applied between air and the converter, the best choice for n(2) is n(1). Finally, if two anti-reflection coatings are applied (the former between air and the converter, the latter between the converter and the solar cell), then the efficiency of the overall device is independent of n(2). The influence of light trapping techniques on the conversion efficiency is also treated. in case the refractive index n is the same for both layers (n(1) = n(2) = n), the efficiency strongly increases with n. Only non-concentrated sunlight is considered in this paper. (C) 2008 Elsevier Ltd. All rights reserved.