Typical external quantum efficiency (EQE) curves for thin film solar cells fabricated in the PV21 consortium are shown in Figure 9.17 for three different platforms—amorphous silicon (a-Si), CdTe and CIGS—together with the AM1.5G solar spectrum photon flux. The short wavelength region (300-500 nm), which is of interest in the down-shifting process, shows a reduced spectral response.
From an inspection of the EQE curves at wavelengths below 500 nm it can be seen that the most promising improvements in relative efficiency increase could be achieved with CdTe solar cells. Recent down-shifting studies have also been carried out on c-Si cells but with only small increments observed in efficiencies.61 The two CdTe EQE curves shown in Figure 9.17 are fabricated from different methods. One contains a thick (240 nm) CdS layer fabricated by metal organic chemical vapour deposition (MOCVD),62 and the other one has a normal thickness CdS and the cell was fabricated via close space sublimation.63 The thicker CdS layer was grown to demonstrate the feasibility of the LDS layer and also due to the similarities of the spectral response with commercial produced PV modules. The higher efficiency CdTe spectral response can be used to investigate potential efficiency improvements due to luminescence down-shifting in state-of-the-art laboratory CdTe solar cells.
Examples of absorption and emission spectra of LDS layers spin coated on glass containing one, two and three BASF Lumogen F dyes are shown in Figure 9.18. The mixing dye ratios were optimised in the layer for efficient energy transfer. The combination of two (V560 and Y083 or Y083 and O240) and three (V570, Y083 and O240) dyes together can increase the absorption range of the LDS layer and utilising efficient energy transfer between the
Figure 9.18 Normalised absorption and emission spectra of LDS dye mixtures containing: (a) one (Y083); (b) two (V570 and Y083); (c) (Y083 and O240); and (d) three (V570, Y083 and O240) dyes. A typical EQE curve of a CdTe solar is also shown.
dyes emit the absorbed light in a wavelength region where the EQE response of the solar cell is high. The two dye (V570 and Y083) and one dye (Y083) layers have the same emission profile, but the two dye layer increases the absorption range of the LDS layer into the UV region (350400 nm). Using the O240 dye as the acceptor dye in a two dye (Y083 and O240) LDS layer, the emission can be shifted further into the red end of the spectrum shown in Figure 9.18(c). If we add the R305 dye, the effect of absorption of the dye becomes detrimental to the efficiency of the cell. Lastly, the LDS layer containing three dyes shown in Figure 9.18(d) absorb almost all the incident light below 550 nm and emission occurs in a region of the solar spectrum with improved spectral response; in this case the maximum of the emission peak is at 584 nm.
These LDS layers have been tested with CdTe solar cells to estimate the increase in EQE from the action of the LDS layer.64 A blank PMMA layer of the same thickness as the dye-doped LDS layer was used to measure the reference EQE. An example of an improved spectral response of a CdTe solar cell with LDS dye layers on top when compared with a blank PMMA layer is
Figure 9.19 (a) EQE comparison of CdTe solar cells (Glyndwr) with the application
of fluorescent down-shifting structures. A single, two and three dye LDS layer was applied and the blank PMMA EQE curve is also shown for comparison. (b) Comparison of short circuit current density (Jsc) of a CdTe solar cell with and without concentration of light using fluorescent structures (CE: concentration enhancement).
shown in Figure 9.19(a). The external quantum efficiency (EQE) of the cell is improved in the blue region of the spectrum due to the action of the LDS layer. The EQE values are improved for wavelengths 400-500 nm mirroring the absorption spectrum of the dye in the LDS layer. By taking advantage of the wave guiding structure of Figure 9.16(b), a further increase in current output can be observed when the down-shifting structure is used as a fluorescent collector. The short-circuit current density output is shown in Figure 9.19(b) with and without the effect of concentration. Although further work is needed to optimise the LDS structures, the examples given here give an indication of the efficiency increase that can be achieved.