Demonstration and Analysis of the Photocurrent by absorption of two sub-bandgap photons in a quantum dot intermediate band solar cell

Abstract

In order to surpass the efficiency limit of single gap solar cells, intermediate band solar cells (IBSC) have to fulfill two requirements: the production of extra photocurrent by absorption of sub-bandgap photons in electronic transitions involving the intermediate band (IB) and the preservation of a high output voltage, not limited by the existence of this band. This work presents experimental evidence of the production of electron-hole pairs by absorption of two sub-bandgap photons in IBSC prototypes fabricated with InAs/GaAs QD material. The experiments were carried out at low temperatures using a specifically designed modulated photocurrent measurement set-up with two light beams. The results are analysed with the help of a simple equivalent circuit model. This analysis is also used to highlight the relevance of the two-photon mechanism demonstrated in the experiment. It is discussed that, although the absorption of sub-bandgap photons in one of the IB transitions and subsequent thermal escape of carriers is a sufficient mechanism to obtain a photocurrent enhancement, the absorption of sub-bandgap photons in both transitions involving the IB is a requisite for the voltage preservation in IBSCs.