**1. Introduction**

General p-i-n solar cell structures are suitable hosts for regions where quantum effects might occur; excess photo-carriers can be developed in quantum traps (quantum wells) grown along the direction of the cell (from p to n region). The cell absorbs at two different wavelengths (a) the host **(Ehost=hc/λhost band-gap)** and (b) the optical gap wavelength (**Esolar-photon=hc/λoptical gap)**. The optical gap of the superlattice can be tuned to desired solar photon energy values (e.g. 1eV photons, Figure-1), with optical gap matching the desired solar photon flux (Figure 1) [1]. General p-i-n solar cell structures are suitable hosts for regions where quantum effects might occur; excess photo-carriers can be developed in quantum traps (quantum wells) grown along the direction of the cell (from p to n region). The cell absorbs at two different wavelengths (a) the host (**Ehost = hc/host band-gap)** and (b) the optical gap wavelength (**Esolar-photon = hc/optical gap)**. The optical gap of the superlattice can be tuned to desired solar photon energy values (e.g. 1eV photons, Figure-1), with optical gap matching the desired solar photon flux (Figure 1) [1].

**Figure 1: Solar spectrum (AM1.5 Direct) and the region of interest at primary wavelength 1240nm**. **Figure 1.** Solar spectrum (AM1.5 Direct) and the region of interest at primary wavelength 1240nm.

to effective mass separation of photo-carriers and recombination loss reduction; the advantage of using quantum wells in the intrinsic region of the host material (here GaAs) is the widening of © 2015 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited.

the gap of low-gap material (here Ge); the optical gap (Ee1 – Ehh1) [4] caused by the narrow gap layer in the cell, can in essence be tuned to equal-energy incident solar wavelengths. This means that excess carriers may be trapped in quantum wells and thus thermionic escape will lead to *excess* currents in the cell. A cell design that could lead to excess current may occur in an all-

Superlattices (large number of periods with thin quantum wells) in the intrinsic region of p-i-n cells provide photo-carrier separation (electron-hole pairs or EHP's) via carrier drifting, leading Superlattices (large number of periods with thin quantum wells) in the intrinsic region of p-in cells provide photo-carrier separation (electron-hole pairs or EHP's) via carrier drifting, leading to effective mass separation of photo-carriers and recombination loss reduction; the advantage of using quantum wells in the intrinsic region of the host material (here GaAs) is the widening of the gap of low-gap material (here Ge); the optical gap (Ee1 – Ehh1) [4] caused by the narrow gap layer in the cell, can in essence be tuned to equal-energy incident solar wavelengths. This means that excess carriers may be trapped in quantum wells and thus thermionic escape will lead to *excess* currents in the cell. A cell design that could lead to excess current may occur in an all-GaAs p-i-n cell with a lattice-matched GaAs-Ge superlattice, grown in the middle of the intrinsic region (Figure 2b), and where excess photo-electrons are *thermionically* reaching the conduction band and are swept away by the built-in electrostatic field. In this chapter we provide a first-principles derivation of excess thermionic current, after radiative recombination losses are taken into account. We propose a short superlattice implanted in the middle of a bulk p-i-n solar cell (Figure 2a), with the expectation of excess current generation via concentrated optical illumination. Such a superlattice will be expected to trap photo-carriers with the chance of overcoming the potential barrier. Once such carriers are out of the quantum traps, they will be likely to join the flux of bulk currents in the p-i-n device. These carriers will essentially be swept away by the built-in electrostatic field, espe‐ cially in the mid-region of the structure. GaAs p-i-n cell with a lattice-matched GaAs-Ge superlattice, grown in the middle of the intrinsic region (Figure 2b), and where excess photo-electrons are *thermionically* reaching the conduction band and are swept away by the built-in electrostatic field. In this chapter we provide a firstprinciples derivation of excess thermionic current, after radiative recombination losses are taken into account. We propose a short superlattice implanted in the middle of a bulk p-i-n solar cell (Figure 2a), with the expectation of excess current generation via concentrated optical illumination. Such a superlattice will be expected to trap photo-carriers with the chance of overcoming the potential barrier. Once such carriers are out of the quantum traps, they will be likely to join the flux of bulk currents in the p-i-n device. These carriers will essentially be swept away by the built-in electrostatic field, especially in the mid-region of the structure.

**Figure 2: (a) A portion of a superlattice embedded in the intrinsic region and illuminated at X suns. Carriers are excited in the quantum wells and thermionically escape to the conduction band (b) Bird's eye view of the intrinsic region with the SL (0.5 m) portion illuminated at concentrated light. Figure 2.** a) A portion of a superlattice embedded in the intrinsic region and illuminated at X suns. Carriers are excited in the quantum wells and thermionically escape to the conduction band (b) Bird's eye view of the intrinsic region with the SL (0.5 μm) portion illuminated at concentrated light.
