We show that independent size and morphology engineering of epitaxial quantum dots can be obtained using a kinetically controlled quantum dot fabrication procedure, namely droplet epitaxy. Due to the far-from-equilibrium droplet epitaxy procedure, which is based on the crystallization, under As flux, of a nanometric droplet of Ga, independent and precise tuning of quantum dot size, aspect ratio, and faceting can be achieved. The dependence of the dot morphology on the growth conditions is interpreted and described quantitatively through a model that takes into account the crystallization kinetics of the Ga stored in the droplet under As flux.

Intermediate band (IB) photovoltaic cells (IBSC) have been proposed as a novel concept in photovoltaics to increase the ultimate efficiency limit of semiconductor solar cells [1,2]. The IB introduces an extension of the absorption coefficient of the semiconductor to lower energies via a two-step absorption of low-energy photons without degrading the output voltage. This is permitted by a halffilled electronic band, lying within the bandgap of the host semiconductor, which mediates the absorption process. One possible approach to the construction of IBSC involves the insertion of quantum dots (QDs) into standard single junction solar cells in an attempt to form the isolated IB [3][4][5][6]. The key operating principle of the IBSC, the increase of the short circuit current due to the two-photon absorption process, has been demonstrated in a GaAs solar cell containing InAs quantum dots [7]. However, because the InAs QDs are grown using the Stranski-Kranstanov growth mode, wetting layer states and defects, the latter introduced by the residual compressive strain which accumulates in the IBSC due to the QD layers insertion [8], cause a sizeable deterioration of the cell and a lowering of the output voltage [9]. Recent results show that voltage preservation may be at reach using strain-compensation techniques which reduce the material defectivity [10][11][12][13]. A different strategy for IBSC defectivity reduction would be the use of lattice matched materials, like GaAs QDs in AlGaAs alloys.Here we report the fabrication and the test of an Al 0.3 Ga 0.7 As host IBSC cell obtained by the insertion of strain-free GaAs QDs in the active region. The GaAs/AlGaAs QDs were grown by droplet epitaxy [14,15]. Clear evidence of the key principle of IBSC, the twophoton absorption induced current, is reported.Droplet epitaxy (DE), a molecular beam epitaxy (MBE) variant, is a flexible growth technique which allows for the fabrication of quantum dots with designable size and density in strain-free materials [15,16]. In addition, DE allows to obtain a large variety of three-dimensional nanostructures with different geometries, ranging from rings to complex dot configurations [17,18], some of which have been recently proposed as active material for IBSC [19,20]. This intrinsic design flexibility is due to the We report the fabrication procedure and the characterization of an Al 0.3 Ga 0.7 As solar cell containing high-density GaAs strain-free quantum dots grown by droplet epitaxy. The production of photocurrent when two sub-bandgap energy photons are absorbed simultaneously is demonstrated. The high quality of the quantum dot/barrier pair, allowed by the high quality of nanostructured strain-free materials, opens new opportunities for quantum dot based solar cells.Left: Scheme of the fabricated quantum dot solar cell. Right: 1 × 1 µm 2 image of the dot layer.rrl solar

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To accommodate the need of energy‐related research for quality publications that keep the pace, pss (RRL) expands its coverage of photovoltaics and solar cells in a new regular section rrl solar, see the Rapid Research Letter by Andrea Scaccabarozzi et al. in this issue (). rrl solar calls for Rapid Research Letter and Review@RRL contributions in the following areas: • Photovoltaic materials preparation, optimization, characterization and physics • Significant solar cell advances, modeling or characterization results and techniques • Important and verified advances in PV efficiency • Novel and urgent results in photovoltaics concepts, physics and technology • All materials and device systems

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