Reports: DNI1048796-DNI10: Direct Growth of Type II Core/Shell Nanowire Array on Transparent Conducting Oxide (TCO) Substrate for Potential Solar Cell Application

Weilie Zhou, PhD , University of New Orleans

In our last year report, we have synthesized well-aligned ZnO/ZnS core/shell nanowire array and demonstrated a photovoltaic device, despite the relatively low conversion efficiency of ~0.1%. The inefficient conversion efficiency majorly originated from the absorption profiles and the interface recombination because of the large lattice mismatch. In light of this, we tried to explore new semiconducting combinations with better interface and appropriate absorption profiles. CdSe and ZnTe, two II-VI semiconductors, are recognized as promising candidates to achieve higher efficiency. First, they have a lattice mismatch of ~ 0.03% if they both crystallized in wurtzite structure, indicating the possibility of significantly depressing interfacial recombination; Moreover, they possess bang gaps of 1.7 and 2.4eV respectively, manifesting a good absorption profile in solar spectrum. In this narrative, we report the synthesis, structure analysis and the photovoltaic effect of CdSe/ZnTe core/shell nanowire arrays. Still, a two-step synthesis method combining thermal evaporation and pulsed laser deposition was applied to realize the core growth and shell coating. CdSe nanowires were synthesized via vapor-liquid-solid thermal evaporation at atmospheric pressure in our home-made horizontal tubular furnace. In order to achieve well-aligned CdSe nanowire array, we have investigated the influence of substrate, catalysts and seed layer on the morphologies/geometry of the nanowires. In details, silicon and muscovite mica were selected as the collect substrates; gold, silver and platinum was used as catalysts. Meanwhile, a layer of ~100nm CdSe thin film was deposited on substrates at 350oC to initialize the possible oriented growth, given the fact that the well-known epitaxial relationship between CdSe and muscovite mica. Our systematic investigation revealed that i) without any catalyst loading, no nanowire was observed on silicon substrate, while a few micro/submicrometer CdSe wire can be obtained on mica substrate; ii) only gold and platinum were able to initialize the nanowire growth; iii) CdSe thin films on mica were well oriented in c-axis; iv) no obvious alignment of CdSe nanowire can be realized without the seed layer. Taken together, a combination of these contributing factors, i.e., muscovite mica, platinum and CdSe seed layer was determined as the prerequisites of achieving well-aligned CdSe nanowire array. Apparently, two advantages of this catalyst-seed layer approach for CdSe nanowire array, distinguished from the reported mica-substrates approach, lie in the controllability in morphologies and the availability for further directly device fabrication. By manipulating the platinum catalysts and the duration, the density, length and the diameter of the nanowires can be tuned. The seed layer, evaporated CdSe thin film, enable to directly fabricate array based optronics, playing crucial roles as barrier layers for unfavorable carriers and the transport medium. More importantly, the seed layer is also indispensible for the following shell deposition, preventing the shell materials directly contacting the insulting substrate. Subsequently, we adapted two approached, pulsed laser deposition and e-beam evaporation, to form the shell on the as-grown CdSe nanowire array. It is worth noting that the vertically aligned, thicker in diameter and lower density of nanowire array, slow deposition rate and moderate temperature are the key issues need to be concerned, in order to increase the likelihood of successful coating. Generally, higher laser energy flux and higher temperature will induce the polycrystalline shell, unsmooth external surface and possible interfacial diffusion. After cooling to room temperature, the substrate with core-shell nanowire array were observed and identified by the scanning electron microscopy and X-ray diffraction. Final product then was deliberately removed from the substrate and dispersed in ethanol for the further structural analysis and device fabrication. The morphologies and the detailed structure analysis confirmed that the ZnTe shell was successful deposited over the CdSe nanowire with different thickness. The obvious contrast showed the interface of the core and shell, revealing that no interfacial diffusion occurred. Clear lattice fringes indicated both the shell and core are single crystalline and possess the wurtzite structure, which was further confirmed by the corresponding selected area electron diffraction. We also did the nanoprobe line scan to image the element distribution along the interface. The data obtained support our previous structure analysis. We also simulated the interface by CrystalKit, and the model of the interface consisting two wurtzite crystalline with close lattice constant is in good agreement with what we observed by transmission electron microscopy. So far, to demonstrate the efficient charge separation in such a lattice matched core-shell nanoheterostructure, we dedicated effort to fabricate nanowire device by a standard approach involving three-time e-beam lithography, chemical etching and two electrodes deposition. First the e-beam lithography was performed to expose half of the nanowire for chemical etching. Subsequently, another two lithography was employed to open widows to deposit the metals appreciate work function as electrodes. In our current device configuration, nickel and indium were chosen as the electrodes to ZnTe and CdSe, respectively, owning to the possible formation of metal-semiconductor ohm contact. We conducted measurement by exposing the devices to solar simulator. The photocurrent increase/drop instantly with on/off of the solar light exposure. In addition, we observed the obvious photovoltaic effect, which is similar to that observed in the solar cell based on single coaxial silicon nanowire. Further work including optimization of the materials electrical properties and the etching process are undergoing. Apart from the potential application in solar cells, this unique lattice-matched coaxial nanoheterostructure could be extended to other optronics in low dimensionality.

So far, the PI have been using ACS PRF fund to support one graduate student and one undergraduate student to work on type II core/shell solar cell study, which helped the PI established three dimensional nanowire cable arrays for photovoltaics application. Using the results achieved from this project, the PI have published 3 paper in highly ranked journals and contributed one book chapter to “Three-dimensional Nanostructures: Designing Next Generation Devices”, chief-edited by the Weilie Zhou and Zhong Lin Wang. Furthermore, the core-shell structure we developed in this project was applied in our three-dimensional nanosensor application, which greatly impacts the PI’s research career development. More recently, we are extending this research to supercapacitor research area, funded by LA Board of Regent grant.

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