Limiting efficiency for a multi-band solar cell containing three and four bands

Multi-band solar cells provide a possible approach to obtain photovoltaic efficiencies that are greater than that of a single junction solar cell. In the three-band case, an intermediate band harnesses photons of energy less than that between the two main bands, allowing these photons to contribute to the power output of the device. Previous work has shown that introducing a third band offers an efficiency of 63.2%. This paper extends the theory to four bands and calculates a limiting efficiency of 71.7%. Finite bandwidths of all bands can be used to ensure photon absorption selectivity, assumed in deducing the previous limits, but at the cost of reduced limiting efficiency. The maximum efficiency using this feature for the three and four bands is 58.9% and 59.0%, respectively. Superlattices and quantum dots offer flexibility in artificially designing the energy and widths of the bands.