Fabrication and Photoelectric Properties of CdTe/TiO2 Nanocrystals Multilayer

In this paper, we report the fabrication and photoelectric properties of CdTe/TiO2 nanocrystal multilayer. Negatively charged CdTe nanocrystal hydrosols were synthesized in the aqueous phase with 3-mercaptopropionic acid as a surface modifier. The characterization of transmission electron microscopy (TEM) and UV-visible absorption spectrum indicates the resultant CdTe nanocrystal hydrosols are monodisperse and have a very narrow size distribution with a mean diameter of about 5 nm. Clear TiO2 aqueous colloidal nanocrystals with positive surface charges were prepared by using tetrabutyl titanate as precursor. Through the electrostatic interaction between the positive surface charges on TiO2 nanocrystals and those on CdTe nanocrystal hydrosols, multilayer of CdTe/TiO2 nanocrystals were fabricated on the pretreated quartz substrate by layer-by-layer electrostatic self-assembly method. UV-Vis absorption spectrum and atomic force microscopy (AFM) were used to characterize the as-prepared CdTe/TiO2 multilayer. The results show that the surface of the CdTe/TiO2 multilayer are flat and the adsorption intensity in UV-Vis spectrum increases with the layer numbers of the CdTe/TiO2 complex, indicating that CdTe/TiO2 multilayer could be fabricated successfully on the surface of quartz glass and the resultant multilayer have good quality. In the same way, CdTe/TiO2 multilayer were fabricated on the surface of Indium Tin Oxides (ITO) substrate disposed by the poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS). The CdTe/TiO2 complex is measured under AM1.5G simulated solar illumination with 100mWcm in air. The results show the maximum open circuit current density (Voc) and short circuit current density (Isc) of CdTe/TiO2 complex on ITO substrate is 0.45V and 0.050mA, which were better than TiO2 or CdTe nanocrystal hydrosols individual due to the interfacial hole-electron converter between the adjacent layers of CdTe and TiO2 nanocrystals. Introduction Energy shortage has become a serious problem in 21 century. Exploiting high efficient, low-cost, non-polluted and reproducible energy is important for human to settle this problem. In this process, inorganic semiconductor photoelectric materials used in solar cells have attracted more and more attention due to its low cost and simple preparation method. The electrostatic layer-by-layer self-assembly technology, based on alternate adsorption of opposite charged components, has been developed to coat nanoscaled films on any surface . It provides an effective way for controlling the molecular orientations and packing at the molecular level . In addition, the morphology and thickness of film can be controlled. Multilayer complex films like polymer/polymer, polymer/metal colloid particles, polymer/inorganic nano particles and inorganic nano particles/inorganic nano particles have been fabricated by using this self-assembly method . Due to the as-prepared multilayer possessing many unique properties different from those of the bulk materials such as optical, electric, magnetic, acoustic, and thermal properties, electrostatic self-assembly techniques offers substantial advantages over traditional manufacturing approaches in the fabrication of functional nanoscaled films or complex films . CdTe semiconductor nanocrystals have a narrow forbidden band (1.47eV) and high photoelectric transformation efficiency and TiO2 semiconductor nanocrystals are easily prepared, non-toxicity Advanced Materials Research Vols. 79-82 (2009) pp 481-484 Online available since 2009/Aug/31 at www.scientific.net © (2009) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/AMR.79-82.481 All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of the publisher: Trans Tech Publications Ltd, Switzerland, www.ttp.net. (ID: 141.211.175.139-06/08/10,06:57:54) ww w. sp m .co m .cn and stable. However, the chemical property of pure CdTe nanocrystal hydrosol is instable [9] and the forbidden band of TiO2 is wide (3.2eV). What’s more, TiO2 only absorbs ultraviolet light and its quantum efficiency is lower (about 10%). Therefore, CdTe and TiO2 nanocrystals both suffer disadvantage when they were used in photoelectric devices. It was reported that CdTe nanocrystals can enhance the charge separation ratio, extend spectrum response of TiO2 and improve quantum efficiency . In this paper, we report a novel nanoscaled CdTe/TiO2 complex film which could be applied in photoelectric system by layer-by-layer electrostatic self-assembly technology. The short circuit current density (Isc) and the open circuit current density (Voc) values of CdTe/TiO2 complex on ITO substrate are better than TiO2 or CdTe nanocrystals individual. The results demonstrate here are meaningful because they provide the promising prospect for self-assemble of CdTe and TiO2 nanocrystals toward nanoarchitecture with well photoelectric properties. Experimental Method Materials. NaBH4 (China Medicine (Group) Shanghai Chemical Reagent Corporation), Te powder (Aldrich), 3-mercaptopropionic acid (MPA, Aldrich), TiO2 sol (prepared according the method described in ref.11), poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS, 1.3wt%, Aldrich). The preparation of CdTe nanocrystal hydrosols. NaBH4 and Te powder with the mol ratio (NaBH4:Te=2:1) were added into a hermetic centrifuge tube. Then deionized water was added into ice-bath. NaHTe generated in tube after 4 hours. Next, fresh NaHTe was added into CdCl2 and MPA mixed solution with the ratio (Cd:Te:MPA=2:1:4.8), and the pH value was controlled between 8~8.2. Finally, the mixture was cohobated for 6 hours at 90 °C ~ 95 °C to generate CdTe nanocrystal hydrosols. Fabrication of CdTe/TiO2 multilayer. Quartz substrates were oxidized thoroughly in fresh piranha solution (98%H2SO4 and 30%H2O2 with a volume ratio of 7:3) for 20 minutes and washed with deionized water. ITO (2cm×2cm, 10Ω·cm per square) substrates were ultrasonicated in deionized water, acetone and isopropanol for 15 minutes, respectively. Then, PEDOT:PSS was spin-coated on the surface of ITO substrates and dried in N2 flow. The surfaces of quartz and ITO substrates treated took negative charges. The quartz and ITO substrates were put into fresh TiO2 sol and CdTe nanocrystal hydrosols respectively for 15 minutes and dried in N2 flow. Repeat above “dipping-drying” procedure from one to five times. Put these substrates into 450°C muffle for 2 hours after pretreatment. Measurement. Transmission electron microscopy (TEM, JEM-1230, JEOL) was used to characterize the morphology of CdTe nanocrystal hydrosols. UV-visible absorption spectra were measured with a Uv-vis absorption spectrophotometer (Agilent 8453). AFM (Benyuan Co. Ltd., CSPM-4000) was used to characterize the morphology of film. Electrochemical workstation (Chenghua Co. Ltd., CHI660) was used to measure photoelectric properties. The light source was Xe lamp (HAYASHI LA-410UV) under AM 1.5G simulated solar irradiation at 100mWcm. Results and discussions Fig.1 shows TEM image of CdTe nanocrystal hydrosols. The average diameter of CdTe nanocrystal is 5 nm with a spherical shape and CdTe nanocrystal individual disperses equably in the hydrosols. MPA which acts as a surface modifier plays an important role in the preparation of CdTe nanocrystals. It prevents CdTe particles to aggregate and improved their stability. Coated by MPA, CdTe nanocrystals take negative surface charges . As-prepared TiO2 nanocrystals take positive charges and the mean diameters which are about 8 nm in definitely spherical form . Therefore, TiO2 and CdTe nanocrystals can be layer by layer electrostatic self-assembled to shape multilayer complex films . Fig.2 shows typical AFM image of CdTe/TiO2 complex films on ITO substrate. The CdTe/TiO2 multilayer is flat and well shaped after heat treatment, which is good for electrons-holes transfer. 482 Multi-Functional Materials and Structures II