Analysis of Strain Compensation in Quantum Dot Embedded GaAs Solar Cells
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Analysis of Strain Compensation in Quantum Dot Embedded GaAs Solar Cells Christopher Bailey1, Cory Cress1, Ryne Raffaelle1, Seth Hubbard1, William Maurer2, David Wilt2, and Sheila Bailey2 1 NanoPower Research Laboratory, Department of Physics, Rochester Institute of Technology, 85 Lomb Memorial Drive, Rochester, NY, 14623 2 NASA Glenn Research Center, Cleveland, OH, 44135 ABSTRACT The effects of strain within stacked layers of InAs quantum dots (QDs) were investigated. InAs QD test structures with and without strain compensation (SC) were analyzed using atomic force microscopy, transmission electron microscopy, and X-ray diffraction. The affects of strain compensation on test structure morphology and on GaAs-based QD solar cell performance was studied as a function of the thickness of the SC layer. X-ray diffraction analysis of the QD embedded test structures reveals a relationship between the SC thickness and the observed crystalline quality. Air mass zero illuminated current vs. voltage data and spectral responsivity measurements were used for the solar cell comparison. When SC is employed, QD insertion shows a lower open circuit voltage, in reference to a baseline device without QDs, but leads to an enhancement in the short circuit current of the device. INTRODUCTION Ultra high efficiency InGaP/GaAs/Ge triple-junction solar cells (TJSC) are typically current limited by the middle GaAs junction [1]. Improving the production of photocurrent in this junction will improve the global conversion efficiency of the triple junction solar cell. There are a number of possible means to improve the current production of the middle GaAs junction including lattice-mismatched metamorphic InGaAs [2], multi-quantum wells [1] and quantum dot arrays [3]. All of these methods use lower bandgap materials inserted into the GaAs cell in order to enhance long-wavelength absorption and thus increase the short circuit current (ISC). It has been predicted that quantum dot enhanced TJSCs have an efficiency ceiling of 47% under one-sun 6000 K black body illumination spectrum [4]. Additionally, quantum dot array enhanced GaAs cells have the added benefit of possible intermediate band effects [5], anisotropic absorption [6] and enhanced radiation tolerance [7]. In the case of epitaxially grown QD structures, the growth mode is usually strain driven (Stranski-Krastanow technique) [8]. Despite the benefits of the additional absorption, the buildup of strain due to QD growth has been previously shown to induce dislocations, degrading the solar cell Isc and open circuit voltage (Voc), and thus leading to overall device degradation [9, 10]. However, strain compensation can be used effectively to balance the residual strain, impede dislocation formation, and lead to improvements in the solar cell characteristics [11, 12]. In this paper, we further investigate the effects of strain compensation by comparing uncompensated InAs QD solar cells and strain compensated InAs QD solar cells by both transmission electron microscopy (TEM) and high
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