Self absorption in luminescent solar concentrators

Luminescent solar concentrators are photovoltaic devices made of thin transparent material, in which luminescent particles are dispersed. The incident light enters the device through its large facets and is subsequently absorbed by the luminescent particles, which re-emit it whilst changing its direction of propagation. Most of the re-emitted light hits the surfaces of the transparent plate in the regime of total internal reflection, which results in wave-guiding towards a facet with an attached solar cell, where the power conversion takes place. Loss mechanisms and their amplification by means of self-absorption prevent as yet the device prototypes from reaching their theoretical performance parameters (10% power conversion efficiency and concentration factor 4.5x ). In this thesis we investigate the impact of self-absorption in luminescent solar concentrators based on various luminescent species. This information is used to build a luminescent solar concentrator prototype which is capable of circumventing the loss amplification by self-absorption. The results are well understood in the light of validated ray-tracing/Monte-Carlo simulations, which predict that luminescent species whose absorption- and emission-spectra overlap are more prone to self-absorption effects. These simulations permit furthermore the separate investigation of the direct effects of losses, without the amplification effects. Based on this information the actual liquid-phase prototype of an LSC with the organic dye Lumogen Red 305 was built. A consequence of increased luminophore concentration is increased absorption. Increase of absorption in its turn increases self-absorption and thus increases the losses. On the other hand increased absorption contributes to more photons in the device, which means that more photons can be converted into electricity. This competition of losses and gains at increasing luminophore concentration leads to an expectation of the existence of an unknown optimal concentration. To find it, the luminophore concentration was increased stepwise and the device efficiency was determined at every step. It turned out, that upon addition of luminophores a saturation of the device efficiency is observed, which means that increase of luminophore concentration compensates the increased self-absorption losses. Furthermore the performance of CdTe/CdSe semiconductor nanocrystals – with highly reduces self-absorption but a weak luminescence quantum efficiency was compared with that of highly luminescent and highly self-absorbing CdSe-Multishell quantum dots. Both achieve approximately the same low efficiency of 1.2-1.3%, which indicates the pathway for further improvement: the increase of luminescence efficiency of CdTe/CdSe nanocrystals. Simulations predict that an otherwise identical CdTe/CdSe luminescent solar concentrator with an increased luminescence quantum efficiency (95%) could surpass the currently achieved performance with a device efficiency 3.7% with a concentration factor of 2.5. This is significant as so far high performance values could be reported for either high concentration factors (1.8) with low device efficiencies (2.7%) or high device efficiencies (7.1%) Finally, we describe the construction of a luminescent solar concentrator using Lumogen Red 305 with both performance quantifiers, concentration factor and optical efficiency, in the top of its category of concentrators.