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We show how semiconductor quantum dots may greatly increase photon conversion efficiencies by producing multiple excitons from a single photon. This is possible because quantization of energy levels in quantum dots enhances Auger processes, eliminates the requirement to conserve crystal momentum, and thus promotes multiple exciton generation, Quantum yields of 300% for exciton formation in PbSe, PbS,...
It is known that solar cells with high current, based on materials of low band gap, present low voltages and vice-versa. Intermediate band (IB) solar cells have been proposed as a means of increasing the current of solar cells without a substantial decrease of the voltage. The present status of the research on this topic is presented. IB materials and the related solar cells have been manufactured...
We propose a novel method to fabricate quantum dot (QD) arrays, utilizing nanometer-sized columnar structures of obliquely deposited thin films arising from the self-shadowing effect. We found that columns consisting of alternating arrays of QDs and barriers can be formed by means of alternate oblique depositions of the semiconductor and the dielectric onto rotating substrates. The deposition conditions...
we have grown InAs QDs on two types of vicinal GaAs substrates and under various growth conditions. QDs grown on 2deg offcut substrates show superior optical characteristics compared to QDs on 6deg offcut substrates, which showed no QD luminescence. The InAs growth temperature was shown to have an impact on QD nucleation, with higher growth temperature leading to both improved dot densities and coherence...
The suitability of using dual-mode PECVD, in conjunction with high temperature annealing, to fabricate arrays of silicon nanocrystals in a nitride matrix over large areas is investigated. The formation of nanocrystals is verified using TEM, XRD, micro-Raman and FTIR. Initial results show reliable growth of a superlattice of Si nanocrystals over an area of 14 by 10.5 cm
We have successfully prepared silicon quantum dots/amorphous silicon carbide multilayers by thermal annealing of stoichiometric hydrogenated amorphous silicon carbide (a-SiC:H)/ silicon rich hydrogenated amorphous silicon carbide (a-Si1-xCx) multilayers. Raman scattering spectroscopy and transmission electron microscope (TEM) measurement revealed that silicon quantum dots were formed in only a-Si1-x...
Luminescent concentrators may have significant promise for producing low-cost PV energy. Luminescent concentrators convert the solar spectrum and concentrate the sunlight, but the benefit that can be obtained by the spectral conversion is not obvious. This paper presents the results of a theoretical study of the impact of spectral concentration. A well-known set of relationships for PV cell efficiency...
We have investigated the characteristics of quantum dot (QD) solar cells with stacked multilayers of self-organized QDs. The GaAs-based p-i-n solar cells with 20 stacked InAs QD layers were fabricated by atomic hydrogen-assisted molecular beam epitaxy (H-MBE) on GaAs (001) substrates, and multiple stacking of InAs QDs was achieved by strain-compensation growth technique, in which the tensile strain...
We have theoretically analyzed the potential efficiency improvement to multi-junction solar cell efficiencies which are available through the incorporation of quantum dot using detailed balance calculations. We have also experimentally investigated the Stranski-Krastanov growth of self-organized InAs quantum dots and quantum dot arrays on lattice-matched GaAs by metallorganic vapor phase epitaxy (MOVPE)...
Photoactive nanomaterials are being investigated as multifunctional additives in polymeric photovoltaic devices. Polymer photovoltaic devices which rely solely on photon absorption by the conducting polymer are bandgap-limited (typically > 2 eV) with regard to the solar spectrum. Therefore, nanomaterials with optical absorption below the conducting polymer bandgap can allow composite devices to...
Quantum dot solar concentrators (QDCs) have been fabricated by the incorporation of quantum dots into highly transparent polymer host materials. UV polymerisation techniques were found to reduce the quantum dot quantum efficiency in comparison with thermally polymerised samples. The sample plates were characterised using photocurrent techniques in individual and stacked configurations. Due to the...
InAs quantum dot molecules (QDMs) are prepared by thin-capping-and-regrowth MBE process. The dot density can be varied between 1010 cm-2, for as-grown quantum dots (QDs), to 1012 cm-2, for multi-stack QDMs. Photocurrent measurements on 1-and 5-stack high-density QDM layers show that these InAs QDMs when embedded inside a GaAs bulk structure extend photon absorption beyond the 850-nm bandedge limited...
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