InAs/AlGaAs quantum dot intermediate band solar cells with enlarged sub-bandgaps

Abstract

-In the last decade several prototypes of intermediate band solar cells (IBSCs) have been manufactured. So far, most of these prototypes have been based on InAs/GaAs quantum dots (QDs) in order to implement the IB material. The key operation principles of the IB theory are two photon subbandgap (SBG) photocurrent, and output voltage preservation, and both have been experimentally demonstrated at low temperature. At room temperature (RT), however, thermal escape/relaxation between the conduction band (CB) and the IB prevents voltage preservation. To improve this situation, we have produced and characterized the first reported InAs/AlGaAs QDbased IBSCs. For an Al content of 25% in the host material, we have measured an activation energy of 361 meV for the thermal carrier escape. This energy is about 250 meV higher than the energies found in the literature for InAs/GaAs QD, and almost 140 meV higher than the activation energy obtained in our previous InAs/GaAs QD-IBSC prototypes including a specifically designed QD capping layer. This high value is responsible for the suppression of the SBG quantum efficiency under monochromatic illumination at around 220 K. We suggest that, if the energy split between the CB and the IB is large enough, activation energies as high as to suppress thermal carrier escape at room temperature (RT) can be achieved. In this respect, the InAs/AlGaAs system offers new possibilities to overcome some of the problems encountered in InAs/GaAs and opens the path for QD-IBSC devices capable of achieving high efficiency at RT.

The feasibility of high-efficiency InAs/GaAs quantum dot intermediate band solar cells

“…This requires that thermal carrier escape from the QDs be suppressed [22] and that non-radiative processes in the cells be minimised. Recent results of reduced thermal escape due to a higher bandgap (AlGaAs) host material [23], and high QD-IBSC V oc s due to improved material quality [24] are promising in this regard. Second, the subbandgap photocurrent must be improved significantly by increasing subbandgap photon absorption in the QD array.…”

Two-photon photocurrent and voltage up-conversion in a quantum dot intermediate band solar cell

“…These improvements to the design have been proposed in [2], where it was demonstrated that the thermal activation energy of these InAs/AlGaAs QDs is about 250 meV higher than the energies found in the literature for InAs/GaAs QD, and almost 140 meV higher than the activation energy obtained in our previous InAs/GaAs QD-IBSC prototypes including a strain-relief-layer. It has been possible to resolve spectrally the contribution of the IB-CB transition to the sub-bandgap photocurrent [ Fig.…”

InAs quantum dot growth on AlxGa1−xAs by metalorganic vapor phase epitaxy for intermediate band solar cells

“…12 A better match to the solar spectrum can be achieved by replacing GaAs with Al x Ga 1Àx As spacers. 13 With a maximum lattice mismatch of only 0.1%, Al x Ga 1Àx As can be easily grown on a GaAs substrate, and a similar strain-induced Stranski-Krastanov growth mode can be expected for InAs QDs on this alloy. There are only a few studies of InAs QDs grown on Al 1Àx Ga x As, particularly by MOVPE, and a careful investigation is needed.…”