Band alignment and conversion efficiency in Si/Ge type-II quantum dot intermediate band solar cells

The concept of the intermediate band (IB) solar cells (SC) offers the promise of achieving 63% conversion efficiency devices. The effect of the type II band alignment in the quantum dot (QD) IB SCs on the above percentage is analyzed and the potential of the Ge/Si system for fabrication of the type II QD IB SC is discussed. Also, it is shown that the increase of the sunlight concentration leads to the rise of the potential barrier around QDs and the concentration of S-x approximate to 700 can induce the epsilon(L) = 0.2 eV height barrier in the Ge/Si system, making this a significant result. Furthermore, the increase of the sunlight concentration leads to the separation of the quasi-Fermi levels from the confined states and also leads to the decrease of the recombination activity in QDs. The two-photon absorption in QDs increases rapidly and dominates over recombination at the moderate concentration. As the contributions of QDs to both the photo- and dark currents in the type II QD IB SC are evaluated it is shown that, compared to the conventional Si SCs, the type II Ge QD IB Si SCs can generate about 25% higher photocurrent and conversion efficiency.