**4. Conclusions**

A model for quantum well and superlattice solar cells was presented and applied to theoretically study qualitative trends in quantum well and superlattice solar cell performances. The findings from this study enhance our understanding of these devices and could provide a suitable guide for its designing. The model allows optimization the solar cell performance as a function of several of its parameters. The well and barrier band gaps, the width and depth of the wells, the amount of the wells in the intrinsic region are considered in the model to attain the best conversion efficiency.

We have shown theoretically that the insertion of multiple quantum wells into the intrinsic region of a p-i-n *AlxGa*1−*xAs* solar cell can enhance the conversion efficiency compared with its baseline cell. The quantum efficiency and the photocurrent for the AlGaAs/GaAs QWSC was calculated and compared with experimental results obtaining good agreement. These results, together with the agreement of the calculated open-circuit voltage with experimental values confirm the reliability of the model presented in this chapter.

GaAsP/InGaAs/GaAs strain-balanced solar cell was presented and studied. The effect of the electric field and the tensile and compressive stress, were carefully considered. The results of the modeling of SB-QWSC were also compared with experimental measurements successfully, validating again the suitability of the model. Following the model was used to determine the highest possible efficiencies for SB-QWSC containing quantum wells under varying degrees of strain. The strain-balanced multiple quantum well solar cells show a high conversion efficiency that makes it very attractive for their use in multijunction solar cells for space applications or concentrator photovoltaics based on a GaAsP/InGaAs/GaAs middle cell. Solar cells containing strain balanced QWs in a multijunction solar cell allow the absorption edge of each subcell to be independently adjusted.

We have shown a new type of photovoltaic device, the superlattice solar cell (SLSC), where coupled quantum wells or superlattices are inserted into the intrinsic region. The aim of this approach is the possibility of better tailor the photon absorption improving, at the same time, the transport of photogenerated carriers due to the tunneling along the nanostructure. The model adjusted to the superlattice solar cell case was then applied to GaAs/AlGaAs and GaAs/GaInNAs material systems. For the GaAs/AlGaAs case, was found photocurrent increase, and slight increments in the conversion efficiency over the QWSC. When applied the model to GaAs/GaInNAs, was obtained the dependence of conversion efficiency as function of nitrogen composition and quantum well width, which allowed determining the range of these parameters where higher conversion efficiencies than in QWSC of the same material system are reached.

Besides, the GaAs/GaInNAs QWSC or SLSC show high conversion efficiency. That make them very attractive for space applications or for a triple-junction concentrator cell based on a GaAs/GaInNAs bottom cell that could reach 50% conversion efficiency. Because of the results that our model predicts haven't been compared nor confirmed experimentally, it would be interesting to see if future experiments will corroborate our findings.

Solar cell efficiency potential remains far greater than actual solar cell efficiencies, including those achieved in research laboratories. New approaches are necessary in order to increase the conversion efficiency and the quantum based solar cells could be a road to reach this goal.
