Thermal Dependence of Quantum Dot Solar Cells
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Thermal Dependence of Quantum Dot Solar Cells Cory D. Cress1, Seth M. Hubbard1,2, Christopher Bailey1, Ross Robinson1, Brian J. Landi1, and Ryne P. Raffaelle1,2 1 NanoPower Research Laboratories, Rochester Institute of Technology, 85 Lomb Memorial Drive, Rochester, NY, 14623 2 Physics, Rochester Institute of Technology, 85 Lomb Memorial Drive, Rochester, NY, 14623 ABSTRACT Various temperature dependent optoelectronic properties were measured for GaAs-based p-type / intrinsic / n-type (pin) solar cell devices containing 5-layers of InAs quantum dots (QDs) grown with strain-compensation layers. Curve fitting of the dark diode characteristics allowed for the temperature dependence of the saturation current and the ideality parameter to be determined. The resulting parameter values indicate high material quality. Air mass zero illuminated current density vs. voltage measurements were used to determine the temperature coefficients of the open circuit voltage, short circuit current, maximum power, and fill factor. A strong correlation between the temperature dependent quantum dot electroluminescence peak emission wavelength and the sub-GaAs bandgap spectral responsivity was observed. INTRODUCTION Many recent experimental investigations have demonstrated the ability to enhance the spectral responsivity of a GaAs solar cell by incorporating either InAs QDs [1-4] or GaSb QD [5]. In devices which utilized the stranski-krastanow QD growth mode, an improved longwavelength responsivity has been achieved, although a concomitant reduction in short circuit current is also observed. The reduction in short circuit current has been attributed to straininduced emitter degradation [4]. Electron trapping by the defects at the QD / GaAs interface, which increases the recombination current, may also contribute to the decreased performance [6]. As additional InAs QDs layers are grown using the Stranski-Kranstanow growth mode, residual compressive strain accumulates in the structure. To alleviate this strain in QD laser diodes [7, 8] and multiple-quantum well solar cells [9], a layer of material under tensile strain is grown to offset the compressive strain leading to a strain-neutral stack. Promising results have recently been demonstrated in GaAs-based InAs QD solar cells using this strain compensation technique, although the thickness of the strain compensation layers was still under investigation [10]. In this paper, the thermal behavior of a strain-compensated QD solar cell with a room temperature air mass zero efficiency in excess of 13% is reported. The thermal behavior of QD solar cells is an important characteristic that must be evaluated to determine the suitability of these devices for space and terrestrial concentrator applications where high temperatures may be encountered. In space, the operating temperature can vary drastically as the solar cells pass into and out of solar eclipse. Likewise, solar cells used in terrestrial concentrator applications are expected to be operated at 250 suns or higher and typically re
