Improving the efficiency of luminescent solar concentrators

Abstract

Luminescent solar concentrators (LSCs) are low cost photovoltaic devices, which reduce the amount of necessary semiconductor material per unit area of a photovoltaic solar energy converter by means of concentration. The device is comprised of a thin plastic plate in which luminescent species (luminophores) have been incorporated. The luminophores absorb the solar light and radiatively re-emit part of the absorbed amount of energy. Total internal reflection traps most of the emitted light inside the plate and wave-guides it to a narrow side facet with a solar cell attached, where conversion into electricity occurs. The efficiency of such devices is as yet rather low, due to several loss mechanisms, of which reabsorption is of high importance [1]. It is generally expected, that spectral separation between absorption and emission influences the losses due to reabsorption [2]. Semiconductor nano-crystals with a quasi-complete separation of absorption and emission spectra (large Stokes’ shift) may be good candidates to overcome reabsorption losses, however in practice their synthesis results in a drop in luminescence quantum efficiency [3]. The synthesis of highly luminescent nano-crystals with a small but not negligible spectral overlap promises to be less complicated [2], [4]. This work investigated the suitability of such quantum dots to circumvent the reabsorption problem in LSC devices by means of experimentally validated combined ray-tracing and Monte-Carlo simulations [2].

Combined ray-tracing and Monte-Carlo simulations is a widely used tool for efficiency estimations of LSC-devices prior to actual manufacturing. We have varied the LSC-size and luminophore concentration in the simulation for four different types luminophores: An hypothetical perfect luminophore, Lumogen Red 305 (the quasi-standard) and two different types of quantum dots, one with an almost complete absorption/emission-separation and one, where still a few percent of the light in the spectral region of emission is absorbed.

This thesis presents the simulation results of the LSC performance with four different luminophores under changing LSC-size and luminophore concentration. The work will present the impact of all the different loss processes. The simulations show that even the small absorption coefficients that overlap with the emission spectrum have a detrimental effect to the LSC by increasing the losses due to reabsorption.

It was found that semiconductor nano-crystals with a quasi-complete separation of absorption and emission spectra could be better luminophores as Lumogen Red 305. Only the absorption coefficients overlapping with the emission spectrum need to be negligible and the luminescent quantum efficiency needs to be at least 60%. However, it is demonstrated that a simple rectangle LSC geometry with PMMA as waveguide material is not economical viable.