The experimental results for the incident angle dependence are presented in Figure 8. For com
parison, the experiment was also simulated with the Raytrace Model. The short circuit current density measured by a reference PV cell at the LSC edge was used as the performance indicator. The values are plotted on an arbitrary scale since only the relative variation of the output with angle of incidence was under examination. The cosine factor in the photon flux arising from the angle of incidence has been excluded in this visualisation, so that purely the response of the LSC as a function of angle of incidence can be examined.
Surprisingly, the edge output was found to increase with increasing angle of incidence up to:70°, after which it dropped off sharply. This observation was confirmed by the Raytrace Model. The explanation for this behaviour is that the LSC used in this experiment was relatively lightly doped, which meant that the absorbance was low at the laser wavelength. In such a case, a large fraction of normally incident light simply passes through the LSC. Though a larger angle
of incidence increases the reflection off the top surface, it also increases the pathlength of the light in the material and hence the absorbance. In a lightly doped LSC, the gain from the additional absorbance can outweigh the loss from the reflectance up to large angles.
While the initial results show a very optimistic incident angle dependence of the LSC, they are not considered to be representative since the reference sample was not optimally doped. Therefore, a further raytrace study was carried out on an optically dense LSC. The results in Figure 9 show a slight increase of the output up to:60°. The transmittance of incident light through the top surface, calculated by subtracting the Fresnel reflectance from unity, is plotted for comparison. As expected, there is a strong correlation between the transmittance and the edge output, but the positive effect of increased pathlengths with increasing angle is noticeable. This effect exists not only in lightly doped LSCs, but also in optimally doped ones, since a longer pathlength of incident light improves the absorption without the drawbacks of a higher concentration of luminescent centres, which would escalate re-absorption losses. For
comparison the angular response of the LSC from Figure 9 is plotted next to the response of typical encapsulated silicon PV cells in Figure 10. Two curves are shown for the Si cell, one using our experimental measurements of the cell and one from the literature (Balenzategui & Chenlo,
2005) . . The latter was a multi-crystalline silicon substrate cell processed with Edge-Defined Film – F ed Growth (EF G) technology and encapsulated in a cerium doped low-iron front glass followed by an Ethylene Vinyl Acetate (EVA) layer and float glass. Interestingly, this comparison indicates that the reflectance of the encapsulated silicon cell, which contains several layers spanning a range of refractive indices (from that of glass to that of silicon), can be comparable with the reflectance of the LSC, i. e. the reflectance of glass or PMMA with a refractive index of 1.49.
In conclusion, it has been established that the LSC is reasonably insensitive to a change in angle of incidence up to approximately 70° from the normal. Yet the angular response of the LSC is not notably better than that of standard encapsulated silicon cells. However, compared to geo-
metric concentrators, the LSC has a clear advantage in terms of angular response.