Droplet Epitaxy and applications

The semiconductor quantum dot (QD) intermediate band solar cells (IBSC) design introduce an extension of the absorption coefficient of the host semiconductor to lower energies without voltage loss [1]. The IBSC working mechanisms have been demonstrated via Stranski-Krastavov self-assembly of InAs QDs in GaAs, but many issues such as strain defect nucleation, increased carrier escape, induced also by wetting layer states, are still unsolved [2]. A different approach is to self-assemble the QDs by droplet epitaxy (DE) [3], a molecular beam epitaxy technique. This technique allows for the growth of QDs without the introduction of strain, without a wetting layer, and with the independent control of number density, size and shape. DE also allows to tune the electronic states, controlling the size and aspect ratio of the QDs. Thus DE is a perfect candidate for the realization of IBSC. In this presentation the production of sub-gap two-photon photocurrent in solar cell containing lattice matched GaAs/Al0.3Ga0.7As QDs grown by DE will be shown [4]. By using DE for QD fabrication it is also possible to tune the size and the aspect ratio of the QD, to tailor proper electronic levels in order to reduce the temperature activated quenching of the sub-gap absorption spectrum of the IBSC (see Fig. 1). Moreover, the lack of defect and wetting layer states can greatly reduce thermal escape of carriers from the QDs [5], leaving photon-induced transitions the dominant ones, as requested by IBSC theory [1], and avoiding the quasi-Fermi energy pinning to QD states which causes the open circuit voltage reduction in QD based IBSCs. References [1] A. Luque, A. Martì, Physical Review Letters, 78, 5014 (1997). [2] A. Luque and A. Martì, Prog. Photovolt: Res. Appl. 9, 73-86 (2001) [3] N. Koguchi and K. Ishige, Japanese Journal of Applied Physics 32, 2052-2058 (1993). [4] S. Sanguinetti, K. Watanabe, T. Tateno, et. al., Applied Physics Letters 81, 613 (2002) [5] A. Scaccabarozzi, S. Adorno, S. Bietti, M. Acciarri and S. Sanguinetti, Phys. Satus Solidi RRL 7, 174-176 (2013)