ZnO/ZnSe Type II Core-Shell Nanowire Array Solar Cell
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ZnO/ZnSe Type II Core-Shell Nanowire Array Solar Cell Y. Zhang,1* D. Li,1 Z. Wu,2* J. Zheng,2 X. Lin,2 H. Zhan,2 S. Li, J. Kang,2* J. Bleuse,3 L. Grenet,3 D. Rapisarda,3 and H. Mariette3 1 Department of Electrical and Computer Engineering, and Energy Production and Infrastructure Center, The University of North Carolina at Charlotte, Charlotte, NC 2 Fujian Key Laboratory of Semiconductor Materials and Applications, and Department of Physics, Xiamen University, Xiamen, Fujian, China 3 CEA and CEA-CNRS Group “Nanophysics of Semiconductors”, Grenoble, France *[email protected]; [email protected]; [email protected] ABSTRACT Shockley-Queisser detailed balance theory predicts that under one sun a semiconductor with its bandgap in the range of 1.0 – 1.6 eV can potentially achieve an energy conversion efficiency > 30%. Therefore, the conversional wisdom would suggest looking for a semiconductor with a bandgap in this range for a single junction solar cell. Here we explore an alternative way of selecting the absorber material for PV, which allows using semiconductors with much larger bandgaps, in conjunction with new device architecture. Specifically, our device is based on an array of core-shell semiconductor nanowires, such as ZnO-ZnSe, where the two components exhibit type II band alignment. Our approach relies on the most basic property of a type II heterojunction, i.e., the staggered band alignment, that provides the function of charge separation, as in the case of dye-sensitized solar cell or (organic) bulk heterojunction solar cell. However, they differ in two important aspects: (1) the current structure is all inorganic, thus, expected to offer better chemical and photo- stability; and (2) In this approach, the interfacial transition provides an effective absorption or photo-response threshold that can be much lower than that of either component. In this work, using a ZnO-ZnSe core-shell nanowire array, we report the observation of the key signatures associated with the type II optical transition, and the demonstration of a solar cell based on the core-shell nanowire array. INTRODUCTION Conversional wisdom would suggest to find a semiconductor with a bandgap in the range of 1.0 – 1.6 eV for a single junction solar cell, because the Shockley-Queisser detailed balance theory predicts that under one sun a semiconductor with its bandgap in this range can potentially achieve an energy conversion efficiency > 30%. However, it has been recently predicted that when two large bandgap semiconductors (e.g., ZnO and ZnSe or GaN and GaP), which on their own neither of them can be an efficient light absorber, form a type II heterojunction (T2HJ), it can absorb a much broad spectrum of light due to the interfacial transition, as though the heterojunction has a much lower bandgap than any of the components.[1] Therefore, an array of type II heterojunctions of two large bandgap materials, core-shell nanowires in particular, can potentially form an efficient solar cell. In principle, a solar cell using a T2HJ does not require any inte
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