Enhanced photocurrent due to interband transitions from InAs quantum dots embedded in InGaAs quantum well solar cells
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Enhanced photocurrent due to interband transitions from InAs quantum dots embedded in InGaAs quantum well solar cells R. Vasan*, Y. F. M. Makableh, J. C. Sarker, and M. O. Manasreh Department of Electrical Engineering, 3217 Bell Engineering Center, University of Arkansas, Fayetteville, AR 72701, USA
ABSTRACT Solar cells based on InAs quantum dots embedded in InxGa1-xAs quantum wells grown on ntype GaAs substrate were fabricated and tested. Solar cells with In mole fraction (x) in the range of 0-40% were investigated. The performance of the solar cells was evaluated using currentvoltage characteristics, spectral response, and quantum efficiency measurements. The spectral response and quantum efficiency spectra possess several peaks along the lower energy side of the spectra, which are attributed to the interband transitions in the structure. These peaks are red shifted as x is increased above 0 %. The device power conversion efficiency was extracted from the current-voltage characteristics using an AM 1.5 solar simulator. The short circuit current density increased as the x is increased above 0 %. But the overall power conversion efficiency decreased due to decrease in the open circuit voltage. The decrease in open circuit voltage is due strain induced dislocations caused by lattice mismatch. INTRODUCTION Indium arsenide quantum dots solar cells are usually associated with what is called an intermediate band, which is a term assigned to a bound state within the quantum dots. Electronic transitions from the valance band to the conduction band via the intermediate band were the subject of several research activities in recent years [1-5]. Transition through the intermediate band was proposed to increase the power conversion efficiency of the solar cells [1]. The theoretical limit of this efficiency was predicted to be as high as 63 % for this class of solar cells [1], [2]. As mentioned above, the charge carriers undergo transitions to the conduction band through the intermediate band [3]. Furthermore, it was reported that that power conversion efficiency increases due to the additional charge carriers that are generated via the intermediate band [4], [5]. For the intermediate band to be effective in facilitating the generation of additional photocurrent, it has to be located below the band gap of the barrier, which is in this case GaAs. The location of the intermediate band can be engineered by embedding the InAs quantum dots into a quantum well, such as InGaAs with varying indium mole fraction. Recent studies of InAs quantum dot solar cells revealed extended spectral response in the near infrared spectral regions, while on the other hand, the current produced due to these transitions was reported to be very small [3]. High open circuit voltage of about 1.0 V was reported in InAs quantum dots solar cells due to introduction of strain compensations layers [6], [7]. Efforts have been made to further improve on the performance of the photovoltaic devices by introducing high density of InAs quantum dots [5], [8] and energ
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