Luminescence down-shifting (LDS) of the incident solar spectrum was originally proposed by Hovel et al.58 in order to overcome the low spectral response in the blue region of the solar spectrum in some types of solar cells.
Figure 9.16 (a) A typical down-shifting structure for a CdTe solar cell. (b) A similar
structure as a fluorescent concentrator. Reproduced from ref. 27.
Proposed during the same period as luminescent solar collectors, both technologies share many similarities; a down-shifting structure for CdTe solar cells Figure 9.16(a) can be compared with a similar structure acting as a fluorescent collector in Figure 9.16(b). The solar cells in Figure 9.16(b), which cover only a part of the rear of the collecting structure, receive light that travels along the light-guiding collector by virtue of their higher refractive index.59 A recent review for luminescence down-shifting can be found in ref. 60.
Luminescent down-shifting, of course, is beneficial only if any losses are compensated by gains incurred through the frequency shift in the incoming light. The losses in the LDS layer are similar to those in the LSCs: (i) absorption, reflection and scattering losses in the matrix material; (ii) lower than unity fluorescence quantum yield of the luminescent species; (iii) escape cone losses from the top and the edge of the LDS layer; and (iv) reabsorption losses, although they play less significant role because of the shorter distances traversed by the emitted light.
There are two types of solar cells where luminescent down-shifting can be proven beneficial: solar cells with low quantum efficiencies at short wavelengths due to high minority carrier losses near the surface such as crystalline silicon (c-Si);61 and gallium arsenide (GaAs)60 and solar cells with a cutoff in the blue region due to absorption of the incoming light by the presence of window layers such as CdTe14 and CIGS.15 In principle, these shortcomings can usually be addressed by improvements in solar cell design, but it is frequently easier and cheaper to make use of LDS where light capture can be optimised independently of the solar cell fabrication process. Due to the availability of a wide range of dyes with high fluorescence efficiencies in the blue region of the spectrum, luminescence down-shifting offers the potential for improved light collection leading to increase of efficiencies in commercial solar cells.
Figure 9.17 Typical EQEs of thin film solar cells fabricated by the PV21 Supergen consortium.