Advances in quantum dot intermediate band solar cells

Several groups have reported on intermediate band solar cells (IBSC) fabricated with InAs/GaAs quantum dots (QD) which exhibit quantum efficiencies (QE) for sub-bandgap photon energies. However, this QE is produced by the absorption of photons only through valence band (VB) to intermediate band (IB) transitions. The absorption of photons of that energy in IB to conduction band (CB) transitions is weak and is usually replaced by carrier escape. This mechanism is incompatible with the preservation of the output voltage, and therefore, it cannot lead to the high efficiencies predicted by the IBSC model. In this work, we discuss the contribution of thermal and tunneling mechanisms to IB-CB carrier escape in current QD-IBSCs. It is experimentally demonstrated that in QD-IBSC prototypes where tunnel escape has been eliminated, the sub-bandgap QE is suppressed at sufficiently low temperatures, and when this occurs, the only limit for the open-circuit voltage (V_OC) is the fundamental semiconductor bandgap, as stated by the IBSC theoretical model.