The solar-electric conversion efficiency of traditional semiconductor solar cells is limited by a fundamental trade-off between the current generated by photon absorption and the operating voltage. Photons with energies below the semiconductor band gap pass straight through the device and do not contribute to the photocurrent. High-energy photons can be absorbed, but the resulting electrons are collected and extracted at a lower voltage, limited by the intrinsic energy gap. Any difference in energy between the photon and the semiconductor band gap is lost as heat. The conversion efficiency of single-junction devices using optimal materials with mid-range energy gaps (~1 to 1.5eV) is typically limited to less than 25% of the incident solar power.
Quantum effects in nanostructured materials enable the development of new device concepts that can radically enhance the operation of traditional semiconductors. For example, a larger fraction of the optical spectrum can be harnessed while maximizing the solar-cell operating voltage by using quantum wells and quantum dots embedded in a higher-band-gap barrier material.
Nanostructured devices thus allow bypassing of the usual dependence of short-circuit current on open-circuit voltage, which limits conventional solar-cell design. Ultra-high conversion efficiencies are also predicted for photovoltaic devices that collect low-energy photons through a two-step process that pumps electrons from the valance to the conduction band via an intermediate stage. Theoretically, the maximum efficiency of a single-junction intermediate-band solar cell matches that of a three-junction tandem cell while avoiding the limitations of current matching and sub-cell interconnection.