PHOTOVOLTAICS: Optical modeling determines luminescent solar-concentrator efficiency

A multi-institutional team of researchers, working through the FULLSPECTRUM project of the European Commission, has published encouraging research results from a five-year investigation of emerging luminescent solar-concentrator (LSC) materials and methodology for downconverting broad-spectrum solar radiation into the spectral-sensitive windows of silicon solar cells.^1,2

Luminescent particles in a clear plastic LSC film respond to both direct and diffuse sunlight by emitting narrow-spectrum light, which is then transmitted to the end of the sheet by total internal reflection, for photovoltaic conversion by much smaller and less expensive solar cells than otherwise might be required to gather and convert the same amount of solar radiation.

The European researchers evaluated currently available luminescent dyes and potentially tunable luminescent materials (quantum dots) using two different modeling approaches: a three-dimensional thermodynamic model based on radiative energy transfer between mesh points within the concentrator film, and a standard ray-tracing model extended to handle absorption and emission by luminescent particles.

Thermodynamic modeling revealed that even for an idealized LSC created from perfectly transparent host material with perfect mirrors on three edges and the bottom surface, and a luminescence quantum efficiency of one, more than three-fourths of the emitted luminescent energy would still be lost through the same top surface area that collects incident sunlight.

One strategy for reducing such losses involves wavelength-selective chiral nematic (cholesteric) liquid-crystal coatings applied to the top surface that would be transparent to incoming light but would reflect the emitted light within tunable bandwidths. Thermodynamic modeling is also likely to play a major role in developing parameters for effective coating materials of this type.

Edge mirrors with air gaps

Ray tracing was used to model the 2.45% efficiency of an idealized 5 x 5 cm² LSC with PMMA as the clear plastic material (with a refractive index of 1.49 and an absorption of 1.5 m^-1) doped with two luminescent dyes (CRS040 and Lumogen F Red 305) and to evaluate proposed parametric variations for potential efficiency improvements. Placing air gaps between the LSC polymer and its bottom and edge mirrors increased efficiency to 2.94% by enabling the internal reflection of the LSC substrate and reflections from the bottom and edge mirrors to work in a complementary fashion (see figure).

A further efficiency increase to 3.8% was achieved by decreasing optical absorption of the LSC polymer from 1.5 to 10^-3 m^-1 and increasing the refractive index from 1.49 to 1.7 in the model, even though a polymer with those characteristics is not yet available. Efficiency was further increased in the model from 3.8% to 6.5% and 9.1%, respectively, by replacing the multicrystalline silicon solar cell with a gallium arsenide cell or an indium gallium phosphide cell. The higher efficiencies come at a higher device cost, however.

As a possibly lower-cost alternative to changing solar-cell materials, the researchers also modeled an LSC in which a third luminescent material, an IR dye, was added to the two dyes in their basic model. Broadening the optical bandwidth of the luminescent materials in this manner could increase efficiency from 3.8% to 5.4%. In either case, a cost analysis would be required to determine which approach actually presents a more cost-efficient solution.

The researchers also tested the stability of luminescent materials in actual LSC polymers under real and simulated solar illumination for more than two years and found good material candidates for potential commercial use.

REFERENCES

1. W.G. Van Sark et al., Optics Express 16(26), p. 21773 (Dec. 22, 2008).
2. www.fullspectrum-eu.org/1_0.html