Quantum Dot Intermediate-band Solar Cell grown by Droplet Epitaxy

Quantum dot intermediate band (IB) solar cells have been proposed in order to increase the efficiency of traditional solar cells and immediately attracted a lot of attention and research since the time they were first proposed. The IB introduces an extension of the absorption coefficient of the semiconductor to lower energies, allowing a more thorough collection of the solar spectrum, via a two-step absorption of low-energy photons. Quantum dot (QD) structures are good candidates for IB solar cells, because their confined energy levels can overlap and form a miniband in dense arrays and the position of these levels and bands can be tuned varying the size and spacing of the QDs. The IB working mechanisms have been demonstrated for InAs QDs in GaAs, and many efforts are devoted to reduce the problems due to strain, and defect nucleation increasing carrier escape. Droplet epitaxy (DE) is a molecular beam epitaxy technique that allows for the growth of quantum dots of materials lattice matched to the barrier and the removal of the wetting layer. DE makes possible to indipendently control density, size and shape of the nanostructures. Densities as high as some 1011 cm-2 per layer have been reported and potentially a large number of layers can be stacked because the system is strain-free. Being a strain-free technique, there are virtually no defects in DE-grown materials. By tuning the size of the QDs it is obviously possible to change the position of the IB, and by tuning their aspect ratio the high energy states of the QDs can also be tuned in order to have a small electron-phonon coupling with the barrier. The lack of defect and wetting layer states can greatly reduce thermal escape of carriers from the IB, leaving photon-induced transitions the dominant ones, as requested by IB theory. For these reasons DE is a good candidate for the realization of QD-IB solar cells. We demonstrate the key working principle of IB materials, that is the production of sub-gap two-photon photocurrent, with Al0.3Ga0.7As solar cells containing GaAs QDs grown by DE. Measurements show that the shape of the two-photon photocurrent signal is different from the VB-CB photoresponse, indicating clearly that a two-photon process involving the QD levels is taking place. The two-photon signal response is in good agreement with the PL spectrum of the QD samples, that lets us easily understand the position of the energy levels of the system.