Enhanced response in InAs quantum dots in an InGaAs quantum well solar cells by anti-reflection coatings
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Enhanced response in InAs quantum dots in an InGaAs quantum well solar cells by antireflection coatings Y. F. Makableh, R. Vasan, J. C. Sarker, S. Lee, M. A. Khan and M. O. Manasreh 3217 Bell Engineering Center, University of Arkansas, Fayetteville, AR 72701, USA
ABSTRACT A study on light absorption enhancement of an InAs quantum dots embedded into InxGa1quantum well with GaAs as a barrier solar cells was carried out. Solar cell devices were fabricated from different structures, which were grown by using molecular beam epitaxy, with the In mole fraction (x) varied between 0 – 25 %. Poly-L-Lysine ligands and ZnO sol-gel was used to modify the surface of the solar cells and act as anti-reflection coatings. The antireflection characteristic of the ligands and the sol-gel were investigated by measuring the solar cell characteristics before and after the solar cells surface modifications. The current-voltage characteristics were measured of the fabricated solar cells before and after Poly-L-Lysine and ZnO coatings. A significant enhancement on the order of 40 % of the solar cells performance was observed. This type of enhancement was observed in the power conversion efficiency, spectral response measurements, and external quantum efficiency. xAs
INTRODUCTION Through the last few years, InAs quantum dots solar cells gained an increased attention from the researchers worldwide [1 - 4]. The InAs quantum dots solar cells can reach theoretical power conversion efficiencies on the order of 63 % [2, 3]. This high power conversion efficiency is due to their capability to generate higher photocurrents than other solar cells without the quantum dots [4]. The quantum dots solar cells offers the ability of generating intermediate bands inside the quantum dots. These intermediate bands allow the generation of low energy charge carriers due to their quantized energy levels [5, 6]. Even with the promising high conversion efficiency of the InAs solar cells their performance did not reach high efficiencies in real devices [7]. On the other hand, several attempts are being under study to increase the InAs quantum dots solar cells efficiency by improving the growth techniques [8], plasmonic effect [9], hydrophobic surfaces [10], or anti-reflection coatings [11 - 13]. While several techniques are being studied to improve the device performance, anti-reflection coating has the ability to enhance the performance of the solar over wide spectral range [14 – 16]. Anti-reflection coating effect on device performance was reported for GaAs solar cells [17, 18], and for Si solar cells [19 – 21]. The anti-reflection coating can be generated using different techniques and materials. As an example, anti-reflection coating on GaAs was reported using AZO Sol-gel [22], and by using indium tin oxides nanocolumns [23]. Also, anti-reflection coating for Si solar cells was reported using plasma enhanced chemical vapor deposition of SiO2 and TiO2 [24, 25], reactive ion etching for reflection reduction [26], and nanolithography [27, 28].
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