Efficient quantum well solar cells
Concentrator photovoltaics involves lenses and curved mirrors that focus sunlight onto small, but highly efficient, multi-junction solar cells. These highly efficient structures have been reported to convert more than 40 % of sunlight into electricity. However, a major consideration is the inability of some solar cells to extract all of the Sun's power because of the large bandgaps that some materials have. Quantum well solar cells, which increase solar cell efficiency by trapping solar energy in quantum wells, have recently gained acceptance as viable components for multi-junction solar cells. Such solar cells can overcome the limitations present in multi-junction solar cells by enabling flexible bandgap tuning while keeping good crystalline structure and optical and electrical properties. In PHOTOQWELL (Photonic optimisation of multiple quantum well structures for single and dual- junction solar cells), researchers aimed to further improve efficiency of multiple quantum well (MQW) solar cells. Researchers successfully developed computational tools for optimising MQW solar cell performance. By simulating the transport properties of MQW structures, they found that quantum well structures made by multiple layers offered potential for greater light absorption. In addition, the position of the quantum wells inside the solar cell played a major role in carrier collection efficiency. What was barely known was the strong dependence of MQW properties on carrier mobility across the quantum wells. An exciting finding that was never reported in the solar cell context was that quantum wells acted like quantum wires. Thin semiconductor layers suffered from severe thickness modulation and formed stripes of triangular section in a particular direction. Detailed analysis of their properties demonstrated that the photogenerated carriers survived without recombining for longer periods (10 times longer) than in quantum wells. Based on this finding, researchers considered using quantum wires to enhance carrier collection. By improving MQW solar cell performance, researchers open up the way for higher conversion efficiencies, making concentrator photovoltaics a viable option. In addition, the quantum wire exciting and challenging properties offer the opportunity to study new concepts in solar cells.
