Growth and Characterization of InAs Quantum Dot Enhanced Photovoltaic Devices
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Growth and Characterization of InAs Quantum Dot Enhanced Photovoltaic Devices Seth Martin Hubbard1, Ryne Raffaelle1, Ross Robinson1, Christopher Bailey1, David Wilt2, David Wolford2, William Maurer2, and Sheila Bailey2 1 Physics, Rochester Institute of Technology, 85 Lomb Memorial Drive, Rochester, NY, 14623 2 NASA Glenn Research Center, Cleveland, OH, 44135 ABSTRACT The growth of InAs quantum dots (QDs) by organometallic vapor phase epitaxy (OMVPE) for use in GaAs based photovoltaics devices was investigated. Growth of InAs quantum dots was optimized according to their morphology and photoluminescence using growth temperature and V/III ratio. The optimized InAs QDs had sizes near 7◊40 nm with a dot density of 5(±0.5)◊1010 cm-2. These optimized QDs were incorporated into GaAs based p-i-n solar cell structures. Cells with single and multiple (5x) layers of QDs were embedded in the iregion of the GaAs p-i-n cell structure. An array of 1 cm2 solar cells was fabricated on these wafers, IV curves collected under 1 sun AM0 conditions, and the spectral response measured from 300-1100 nm. The quantum efficiency for each QD cell clearly shows sub-bandgap conversion, indicating a contribution due to the QDs. Unfortunately, the overarching result of the addition of quantum dots to the baseline p-i-n GaAs cells was a decrease in efficiency. However, the addition of thin GaP strain compensating layers between the QD layers, was found to reduce this efficiency degradation and significantly enhance the subgap conversion in comparison to the un-compensated quantum dot cells. INTRODUCTION Recent proposals have pointed to alternate approaches to improving solar cell efficiency using low dimensionality nanostructured materials [1,2]. One approach involves insertion of low dimensional heterostructures (such as quantum wells, wires, and dots) into the intrinsic region of a standard single junction p-i-n solar cell, leading to formation of an intermediate band within the bandgap of the host. Given optimal host and intermediate bandgaps, initial theoretical predictions for conversion efficiency are 63% [1]. Unlike tandem cells, these low dimensional structures do not suffer from the added complexity of tunnel junctions or current matching requirements. A second approach takes advantage of the extended absorption spectrum of lower bandgap heterostructures [2]. In this approach, the sub-bandgap absorption of the nanostructures allows for enhancement of the short circuit current and overall efficiency improvements. Most commercial producers of III-V solar cells use organometallic vapor phase epitaxy (OMVPE) for growth of cell structures. However, progress in QD growth by OMVPE has just begun to accelerate over the last 5 years [3,4]; most QD lasers and detectors are still grown by MBE. The mechanisms of Stranski-Krastanow (SK) strain-induced QD growth in MBE are fairly straightforward. However, in the case of OMVPE, to initiate the SK growth mode, growth temperatures must be reduced to well below the values typically associated
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