Patterned dye structures limit reabsorption in luminescent solar concentrators

Gain investigation of Perylene-Red-doped PMMA for stimulated luminescent solar concentrators

Luminescent solar concentrators (LSCs) utilizing stimulated emission by a seed laser are a promising approach to overcome the limitations of conventional LSCs, with a significant reduction of the photovoltaic material. In our previous work, we demonstrated the principle of a stimulated LSC (s-LSC) and correspondingly developed a model for quantifying the output power of such a system, taking into account different important physical parameters. The model suggested Perylene Red (PR) dye as a potential candidate for s-LSCs. Here, we experimentally investigate the gain of PR-doped polymethyl methacrylate (PMMA) required for s-LSCs using a single pump wavelength (instead of the solar spectrum) as a proof of principle. The results found from the experiment are well matched with the previously developed numerical model except for gain saturation, which occurs at a comparatively small seed laser signal power. To investigate the gain saturation, two approaches were taken: investigating (i) spectral hole burning and (ii) triplet state absorption. Experimental investigation of spectral hole burning with PR dyes showed a small effect on the gain saturation. We developed a general state model considering triplet state absorption of the PR dyes for the second approach. The state model suggests that the PR dyes suffer from significant triplet state absorption loss, which obstructs the normal operation of the PR-based s-LSC system.

Modeling of stimulated emission based luminescent solar concentrators

The efficiency improvement of luminescent solar concentrators (LSCs) necessary for practical realization is currently hindered by one major loss mechanism: reabsorption of emitted photons by the luminophores. Recently, we explored a promising technique for reducing reabsorption and also improving directional emission in LSCs utilizing stimulated emission, rather than only spontaneous emission, with an inexpensive seed laser. In this work, a model is developed to quantify the gain (i.e. the amount of amplification of a low power seed laser propagating through the solar-pumped concentrator) of stimulated-LSCs (s-LSCs) considering the effects of different important physical parameters. The net optical output power, available for a small PV cell, from the concentrator can also be determined from the model, which indicates the performance of s-LSCs. Finally, the performance of different existing material systems is investigated using literature values of the parameters required for the model, and a set of optimal parameters is suggested for practical realization of such a device.

Luminescent solar concentrators utilizing stimulated emission

Luminescent solar concentrators (LSCs) are an emerging technology that aims primarily to reduce the cost of solar energy, with great potential for building integrated photovoltaic (PV) structures. However, realizing LSCs with commercially viable efficiency is currently hindered by reabsorption losses. Here, we introduce an approach to reducing reabsorption as well as improving directional emission in LSCs by using stimulated emission. Light from a seed laser (potentially an inexpensive laser diode) passes through the entire area of the LSC panel, modifying the emission spectrum of excited dye molecules such that it is spectrally narrower, at wavelengths that minimize reabsorption to allow net gain in the system, and directed towards a small PV cell. A mathematical model, taking into account thermodynamic considerations, of such a system is presented which identifies key parameters and allows evaluation in terms of net effective output power.

Radiative transport theory for light propagation in luminescent media

We propose a generalization of radiative transport theory to account for light propagation in luminescent random media. This theory accounts accurately for the multiple absorption and reemission of light at different wavelengths and for anisotropic luminescence. To test this theory, we apply it to model light propagation in luminescent solar concentrators (LSCs). The source-iteration method is used in two spatial dimensions for LSCs based on semiconductor quantum dots and aligned nanorods. The LSC performance is studied in detail, including its dependence on particle concentration and the anisotropy of the luminescence. The computational results using this theory are compared with Monte Carlo simulations of photon transport and found to agree qualitatively. The proposed approach offers a deterministic methodology, which can be advantageous for analytic and computational modeling. This approach has potential for more efficient and cost-effective LSCs, as well as in other applications involving luminescent radiation.

Retention of power conversion efficiency--from small area to large area polymer solar cells

A light harvesting device geometry is implemented for polymer solar cells that retain efficiency when the device area is scaled up. Patterning devices and incorporating suitable fluorescent dye doped polymers in the spaces between the patterns leads to 12% efficiency enhancement. Vacuum free deposition of electrodes is carried out using meltable alloys giving rise to device efficiency of 6%. The fluorescent layer along with the electrode serves as an active encapsulant leading to improved device stability.

Increased efficiency of luminescent solar concentrators after application of organic wavelength selective mirrors

Organic wavelength-selective mirrors are used to reduce the loss of emitted photons through the surface of a luminescent solar concentrator (LSC). A theoretical calculation suggests that application of a 400 nm broad reflector on top of an LSC containing BASF Lumogen Red 305 as a luminophore can reflect 91% of all surface emitted photons back into the device. Used in this way, such broad reflectors could increase the edge-emission efficiency of the LSC by up to 66%. Similarly, 175 nm broad reflectors could increase efficiency up to 45%. Measurements demonstrate more limited effectiveness and dependency on the peak absorbance of the LSC. At higher absorbance, the increased number of internal re-absorption events reduces the effectiveness of the reflectors, leading to a maximum increase in LSC efficiency of ~5% for an LSC with a peak absorbance of 1. Reducing re-absorption by reducing dye concentration or the coverage of the luminophore coating results in an increase in LSC efficiency of up to 30% and 27%, respectively.

Cylindrical luminescent solar concentrators with near-infrared quantum dots

We investigate the performance of cylindrical luminescent solar concentrators (CLSCs) with near-infrared lead sulfide quantum dots (QDs) in the active region. We fabricate solid and hollow cylinders from a composite of QDs in polymethylmethacrylate, prepared by radical polymerization, and characterize sample homogeneity and optical properties using spectroscopic techniques. We additionally measure photo-stability and photocurrent outputs under both laboratory and external ambient conditions. The experimental results are in good agreement with theoretical calculations which demonstrate that the hollow CLSCs have higher absorption of incident radiation and lower self-absorption compared to solid cylindrical and planar geometries with similar geometric factors, resulting in a higher optical efficiency.