**5. Heterojunction cells**

Improved cell design has to include more than one band-gap for larger number of absorbed photons. P-i-n (from now on pin diode) diode designs offer wide intrinsic regions between the p- and n- regions of a p-n junction, where photo-carriers have a great chance to be generated and quickly swept away to the ends of the two-lead diode. This is possible due to the electrostatic field that develops at the depletion region. Illumination of the structure at the intrinsic region or a pin increases the chances of more photo-excited carriers. On the other hand, for a pin diode exposed to solar light and with a thin p-layer, minority electrons from the p-region may cross very fast (n ~ fraction of s) the junction at the p-i interface and be swept away to the load by the electric field in the mid-region of the cell. More than one band gaps in the mid region may lead to quantum wells where quantum size effects may take over as long as thickness values are in the order of 5 to 8 nm. Superlattice-like structures may be grown in the intrinsic region in order to accommodate both short and long solar wavelengths. It is commonly accepted that thin bulk window layers grown on top of a wide mid-region with quantum wells may offer a two-fold advantage (a) short wavelengths absorbed at the top and longer wavelengths absorbed in the mid region where a superlattice structure is essentially tuned at specific wavelengths. Thus, by growing a superlattice in the middle of a pin region (rather by changing the mid-region into a multiquantum well (mqw) sequence) one may reach the main objective: to capture more solar

The total current out of the cell is the sum of all currents minus recombination components from each region, especially recombination at the w region. Excess carriers in solar cells (as in any photonic device) are minority electrons and holes in the p and n regions respectively. When a cell is illuminated, solar photons excite electron hole pairs in all regions: the p-, nand depletion regions. The latter may be become of great significance for the following reason: excited electrons and holes do split away from each other due to the existing electrostatic field. This means that these excess carriers will reach the edges of the depletion region in a very short time. Note also that typically, mean diffusion lengths of these carriers are much longer than t he actual width of the depletion area (even in pin devices). This makes the depletion region especially attractive for illumination: electrons and holes will separate from each other quickly, and they will diffuse in the bulk parts of the cell very fast assisted by the electrostatic field. In addition, space availability in the mid-region provides a chance for excess layer s that can be tuned to desired solar photons for subsequent absorption, thus enhancing device performance. This is why multi-layers are used in the intrinsic region (long depletion region in p-n junctions). If tuned quantum wells are grown somewhere in the middle, incident solar illumination will push electrons in the quantum wells and to tunneling or thermionic escape. The notion of additional band gaps integrated in the intrinsic region has been adopted successfully recently. For instance, successful cells with more than one band gaps have been designed and realized, where two or three cells are connected in series forming tandem cells with the advantage of voltage increase. This is possible due to the series connection of the tandem cells. Tandems provide excess voltage but they lack in current, in other words, due to the differences of the layers involved, current matching will be enforced due to the series connection. If these structures can ensure relatively high current outputs, then, along with increased voltage one should expect efficiency improvements. In the next we outline the behavior of a cell in tandem: top cell of

AlAs/GaAs and bottom cell of a pin GaAs/Ge/Alloy for long wavelengths.

Improved cell design has to include more than one band-gap for larger number of absorbed photons. P-i-n (from now on pin diode) diode designs offer wide intrinsic regions between the p- and n- regions of a p-n junction, where photo-carriers have a great chance to be generated and quickly swept away to the ends of the two-lead diode. This is possible due to the electrostatic field that develops at the depletion region. Illumination of the structure at the intrinsic region or a pin increases the chances of more photo-excited carriers. On the other hand, for a pin diode exposed to solar light and with a thin p-layer, minority electrons from the p-region may cross very fast (n ~ fraction of s) the junction at the p-i interface and be swept away to the load by the electric field in the mid-region of the cell. More than one band gaps in the mid region may lead to quantum wells where quantum size effects may take over as long as thickness values are in the order of 5 to 8 nm. Superlattice-like structures may be grown in the intrinsic region in order to accommodate both short and long solar wavelengths. It is commonly accepted that thin bulk window layers grown on top of a wide mid-region with quantum wells may offer a two-fold advantage (a) short wavelengths absorbed at the top and longer wavelengths absorbed in the mid region where a superlattice structure is essentially tuned at specific wavelengths. Thus, by growing a superlattice in the middle of a pin region (rather by changing the mid-region into a multiquantum well (mqw) sequence) one may reach the main objective: to capture more solar

**5. Heterojunction cells** 

photons with energies higher than the band gap of the host material. The figure below depicts an intrinsic multi-quantum well area, where discrete energy levels cause a widening of the host material's gap (commonly GaAs with gap at 1.42 eV).

Fig. 4. Detail from a superlattice structure (typically GaAs/alloy and GaAs/Ge (as proposed in this study). The dashed line represents the Fermi level at thermal equilibrium. The optical gap can be tuned to desired energy values.

Figure 5 shows a superlattice covering the mid region of a pin cell:

Fig. 5. A p-i-n GaAs/alloy superlattice developed in the mid-region of a pin cell: the middle section depicts: the top layer (blue) is the wide gap alloy (e.g. AlAs) and the bottom layer is GaAs (host material); GaAs is also grown in the superlattice as the low gap medium.

Such a cell design (shown above) is an expanded p-n junction with a wide superlattice midregion occupying the intrinsic or low doped region between p and n. To reduce cost such a structure can be compromised by inserting a short period tuned superlattice as a small percentage of the device as a total.
