Optical Properties of CdS and CdTe Sensitized ZnO Nanorods
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Optical Properties of CdS and CdTe Sensitized ZnO Nanorods C. J. Pereyra1, F. Ferrer1, R. E. Marotti1, C. Gómez1, L. Campo1, L. I. Amy1, F. Martín2, D. Leinen2, J. R. Ramos-Barrado2, E. A. Dalchiele1 1
Instituto de Física & CINQUIFIMA, Facultad de Ingeniería, Universidad de la República, Montevideo, Uruguay. 2
Lab. de Materiales y Superficies (Unidad Asociada al CSIC), Dptos. de Física Aplicada & Ingeniería Química, Universidad de Málaga, Campus de Teatinos s/n, E29071 Málaga, Spain. ABSTRACT Optical properties of ZnO-CdTe electrochemically prepared on a core-shell nanostructure (NS) were studied. Numerical simulations based on effective medium approximation give higher absorption than ZnO-CdS samples and a sensitive dependence on CdTe content. The absorption edges for deep black samples found by transmittance (T(λ)) and diffuse reflectance (Rdiff(λ)) measurements were at 1.33eV and 1.55eV, respectively. A split-off band edge was also found by Rdiff(λ) at ~2.5eV. The red shift observed in T(λ), previously observed in ZnO-CdS, and may confirm the enhancement of sub-bandgap absorption due to the NS nature of samples. INTRODUCTION ZnO is a transparent semiconductor with a bandgap energy (Eg) usually reported between 3.2-3.4eV [1]. Its nanostructures (NS), like nanorod (NR) arrays, have potential applications in photovoltaic solar cells due to their advantages in comparison with planar structures. NR based NS may increase cell efficiency by decoupling light propagation and carrier collection into orthogonal directions [2,3]. For this application, ZnO NR must be sensitized with another semiconductor [4-7]. The sensitizer (Sns) selection is critical for the performance of resulting NS, being CdS and CdTe great candidates [8,9]. ZnO-CdS core-shell NS were already prepared by Successive Ion Layer Adsorption and Reaction (SILAR) [4,5]. CdS increases light absorption in the visible region and the absorption edge depends on nanoparticle (NP) size and Sns content [5,6]. In effect, for CdS deposited by SILAR its Eg red-shifts with the number of coatings [5,10]. CdTe is an II-VI alloy which has an almost ideal Eg for photovoltaic applications [11-15]. The room temperature optical Eg of CdTe thin films lies between 1.47-1.53eV [11,12] and even as high as 1.57eV [13]. Also, the spin-orbit splitting (Δ0) produces another Eg (split-off) at ~2.5eV (Δ0~0.95eV) [14,16]. Usually this higher band is hard to observe in the dielectric constant [14,15]. Also, varying the stoichiometry between Cd and Te in preparation processes may result in n-type (Cd excess) or p-type (Te excess) conductivity [17]. For this reason, in the present work the optical properties of ZnO-CdTe on a core-shell NS are studied and compared with previous results on similar ZnO-CdS samples [4-6] by Effective Medium Approximation (EMA) numerical simulations, and transmittance T(λ) and diffuse reflectance Rdiff(λ) measurements. EXPERIMENT A two steps method was used for the preparation of CdS and CdTe sensitized ZnO NR arrays. In the first step ZnO NR were grown on a fl
