Improvement of performance of InAs quantum dot solar cell by inserting thin AlAs layers
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NANO EXPRESS
Open Access
Improvement of performance of InAs quantum dot solar cell by inserting thin AlAs layers Dongzhi Hu1*, Claiborne CO McPheeters2, Edward T Yu2, Daniel M Schaadt1
Abstract A new measure to enhance the performance of InAs quantum dot solar cell is proposed and measured. One monolayer AlAs is deposited on top of InAs quantum dots (QDs) in multistack solar cells. The devices were fabricated by molecular beam epitaxy. In situ annealing was intended to tune the QD density. A set of four samples were compared: InAs QDs without in situ annealing with and without AlAs cap layer and InAs QDs in situ annealed with and without AlAs cap layer. Atomic force microscopy measurements show that when in situ annealing of QDs without AlAs capping layers is investigated, holes and dashes are present on the device surface, while capping with one monolayer AlAs improves the device surface. On unannealed samples, capping the QDs with one monolayer of AlAs improves the spectral response, the open-circuit voltage and the fill factor. On annealed samples, capping has little effect on the spectral response but reduces the short-circuit current, while increasing the open-circuit voltage, the fill factor and power conversion efficiency. Introduction Group III-V compound semiconductor solar cells are the highest efficiency cells developed to date [1] due to the wide range of bandgaps that can be grown with high crystalline quality in this material system. Record efficiencies around 40% [2] are achieved in triple junction cells with an InGaP/InGaAs/Ge structure, in which lattice-matched InGaAs replaces a middle GaAs layer. Due to the three-dimensional confinement of quantum dots (QDs), their incorporation into the middle layer enhances the photo current of solar cells, which can be further improved by forward scattering techniques [3]. The electronic properties of QDs depend on their size, shape, and surrounding matrix [4] and can be tuned during molecular beam epitaxy (MBE) by growth rate, temperature, and in situ annealing procedures [5,6]. In particular, it is possible to tailor the absorption spectrum of the InAs QDs to the 1.0 to 1.2 eV range, which allows for enhanced absorption with respect to the solar spectrum. To tailor the absorption spectrum of the InAs QD, an in situ annealing procedure is often used. Annealing of InAs QDs at relatively low temperatures, i.e., lower than 470°C right after their deposition leads to classical Ostwald * Correspondence: [email protected] 1 Institut für Angewandte Physik/DFG-Center for Functional Nanostructures, Karlsruhe Institute of Technology (KIT), Karlsruhe, Germany Full list of author information is available at the end of the article
ripening. In this case, the dot density decreases with smaller dots disappearing while larger dots growing with annealing time. When the dot size becomes larger than a critical value, dislocations are formed, which is not preferred for solar cells. However, when annealing InAs QDs at relatively high temperatures, i.e., higher than 490°C, a c
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