**4. Conclusions**

The aim of this chapter was to acquire a deep understanding of the working mechanism of excitonic solar cells and to improve the device performance. Therefore, we developed a new quantum formalism based on the wave function of excitonic solar cells. The basic idea of this new methodology was shown through the example of two-level excitonic solar cells. We demonstrated that this new methodology provides a quantitative picture of the fundamental processes underlying solar energy conversion, including photon absorption, exciton dissociation and charge separation as well as an understanding of their consequences on the cell performance. Interestingly, this theory could successfully analyze excitonic solar cell in the presence of strong Coulomb interaction between the electron and the hole. Here we highlight some of the important achievements of this study.

