The fundamental efficiency limit of a single bandgap solar cell is about 31% at 1 sun with a bandgap of about EG = 1.35 eV [1], determined by the trade-off of maximizing current with a smaller bandgap and voltage with a larger bandgap. Multiple bandgaps can be introduced to absorb the broad solar spectrum more efficiently. This can be realized in multi-junction cells, for example, where two or more cells are stacked on top of each other either mechanically or monolithically connected by a tunnel junction. An alternativeor complementary (see section 5.4)—approach is the quantum well cell (QWC).

5.2 Quantum well cells Quantum wells (QWs) are thin layers of lower bandgap material in a host material with a higher bandgap. Early device designs placed the QWs in the doped regions of a p-n device [2] but superior carrier collection is achieved when an electric field is present across the QWs. More recent QWC designs have employed a p-in structure [3] with the QWs located in the intrinsic region; a schematic bandgap diagram is shown in figure 5.1. The carriers escape from the QWs thermally and by tunnelling [4-6].