Intermediate band mobility requirements in intermediate band solar cells

The term “intermediate band solar cells” invokes the idea of band transport in the intermediate band (IB), but since current is not extracted directly from the IB, it is not clear whether transport within the IB is required for high efficiency. We propose a simple rule to understand the requirements for IB transport in efficient devices. If, at each point in the IB, the generation rate of electrons optically excited from the valence band to the IB equals the generation rate of electrons moved to the conduction band from the IB, we call the device locally current matched (LCM). In the LCM case, IB transport is not required for high efficiency. When a device does not have local current matching, it must have IB transport or else recombination will decrease efficiency. Using the coupled Poisson/drift-diffusion model Simudo, we quantify the requirements for IB mobility in a range of cases. We verify that for LCM devices, IB mobility is not important. For mismatched devices, modest values of IB mobility near 1-30 cm
^2/
Vs are sufficient to attain high efficiency even in the absence of local current matching. We also show that device design can ameliorate the efficiency decrease from using locally mismatched materials with low mobility; choosing an appropriate polarity of the p-IB-n junctions can significantly restore efficiency even with low IB mobility. These results provide valuable guidance to developers of IB materials and devices, showing that IB mobilities are important when the subgap transition strengths differ significantly.