Reduced material quality requirements for electronically coupled upconverters compared to intermediate band solar cells

Intermediate band (IB) materials allow the development of high efficiency solar cells by absorption of subgap photons. Standard IB devices have n-IB-p structures, which ensure that carriers are not directly extracted from the IB itself, but this geometry requires that holes from the front of the device must transit the IB region before being collected. This requirement forces IB materials to have relatively long carrier lifetimes before they can improve the efficiency of solar cells. The electronically coupled upconverter (ECUC) geometry puts the IB material at the back of the device. While more complicated 2D contact patterns are required to extract current directly from the standard semiconductor, carriers produced in the standard semiconductor do not need to traverse the IB region, allowing the ECUC to produce high efficiency with lower quality IB materials, as exist today. We study the potential efficiency of ECUC devices using the semianalytical modified Strandberg model, which describes the current-voltage performance of IB devices, including nonradiative recombination and the associated finite carrier collection. We find that even with moderate-to-low IB material quality, the ECUC geometry allows higher efficiency than the standard IB solar cell geometry. The ECUC is a promising architecture for near-term progress in IB devices.