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

Abstract

The concept of the intermediate band (IB) solar cells (SC) offers the promiseof achieving 63% conversion efficiency devices. The effect of the type II bandalignment in the quantum dot (QD) IB SCs on the above percentage is analyzedand the potential of the Ge/Si system for fabrication of the type II QD IB SCis discussed. Also, it is shown that the increase of the sunlight concentrationleads to the rise of the potential barrier around QDs and the concentration ofSx≈700 caninduce the εL = 0.2 eV height barrier in the Ge/Si system, making this a significant result. Furthermore, theincrease of the sunlight concentration leads to the separation of the quasi-Fermi levels fromthe confined states and also leads to the decrease of the recombination activity in QDs. Thetwo-photon absorption in QDs increases rapidly and dominates over recombination at themoderate concentration. As the contributions of QDs to both the photo- and dark currentsin the type II QD IB SC are evaluated it is shown that, compared to the conventional SiSCs, the type II Ge QD IB Si SCs can generate about 25% higher photocurrent andconversion efficiency.