**4. Conclusion**

In figure 14 the product of radiative recombination rate and the external fluorescence efficiency ( *f RR* ×*η fex*) is decreased from 1 to 10-3 to illustrate the effect radiative recombination and the external fluorescence on the cell efficiency. The maximum efficiency limit dropped from

**Figure 13.** The maximum efficiency against the energy band-gap of the solar cell, using the AM1.5G spectrum with the

Since the power output of the cell is determined by the product of the current and voltage, it is therefore imperative to understand what material properties (and solar cell geometries) boost these two parameters. Certainly, the short-circuit current in the solar cell is determined entirely by both the material absorption property and the effectiveness of photo-generated carriers collection at contacts. As previously mentioned (section 2.4.1), the manufactured solar cells with present technologies and materials have already achieved short-circuit currents close to predicted limits. Therefore the shortfall in efficiency could be attributed to the voltage. We show here that the key to reaching the highest possible voltages is first to have a recombination predominantly radiative with a maximal external emission of photons from the surface of the solar cell. Secondly we need a maximum solar concentration. The second condition could be achieved either by using sun concentrators, there are concentrators with concentration factor from ×2 to over ×1000 [23] or by non-concentrating techniques with emission and acceptance

At this level we can conclude that the efficiency limit of a single energy gap solar cell is bound by two intrinsic limitations; the first is the spectral mismatch with the solar spectrum which retains at least 50% of the available solar energy. The best known example of how

blackbody spectrum at *Ts*=6000°*K* for one sun and full concentration (*C=CMax*).

angle limited to a narrow range around the sun [24-26].

34.42% to 28.58%.

72 Solar Cells - New Approaches and Reviews

For single junction cell the record at present is 28.8% (GaAs) [14] compared to the SQ limit of 33.7% which is a significant accomplishment and little room has been left for improvement. Immense experimental research is now directed towards maximizing the external emission of photons from the solar cell. One way of getting beyond the SQ limit for a single junction is the use of concentrated radiation, the current record for concentrator cell is only 29.1% (GaAs) under 117 suns [14], this technology has a number of challenging problems (such as tracking and cooling systems) and there is still a long way to go. The same goal is accomplished by matching the angles under which light impinges from the sun and into which light is emitted from the solar cell. Recently it has been demonstrated that light trapping GaAs solar cell with limited emission angle efficiencies above 38% may be achievable with a single junction solar cell [25].

To overcome the restrictions of a single junction solar cell several directions were investigated during the last decades (i.e. hot carrier cells, carrier multiplication and down-conversion, impurity photovoltaic and multiband cells, thermophotovoltac and thermophotonic conver‐ sion…).

The most widely explored path has been tandem or stacked cells; they provide the best known example of how such high efficiency might be achieved. The present efficiency record for a triple junctions cell (InGaP/GaAs/InGaAs) is 37.9% compared to a predicted value of over 51% for an optimised set of three stacked cells [28, 29]. The major technological challenge with tandem solar cells is to find materials with the desired band gaps and right physical properties (i.e. lattice constant and thermal parameters). The ultimate efficiency target for this kind of configuration is 86.81% (for a set of an infinite number of stacked monochromatic cells under maximum solar concentration) which constitutes an arduous target, corresponding to an infinite number of stacked junctions radiated by a maximum solar concentration. The best performance that the present technology can offer is 44.4% using a triple junction GaInP/GaAs/ GaInAs cell under 302 suns [14].
