Characterization of electrodeposited zinc oxide/tetrasulphonatedcopper phthalocyanines (ZnO/Ts-CuPc) hybrid films and their photoelectrochemical properties

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Abstract

Hybrid films of zinc oxide (ZnO) and tetrasulphonatedcopper phthalocyanine (Ts-CuPc) have been prepared by cathodic electro-deposition from aqueous O2-saturated solutions of ZnCl2 and Ts-CuPc. The Ts-CuPc content of the films can be varied in a wide range by variation of dye concentration in the electro-deposition bath – from single Ts-CuPc molecules embedded in compact crystalline ZnO to films based on an amorphous Ts-CuPc framework. With increasing dye content the colour of the films changes from light blue to deep blue. All films were characterized by XRD, SEM and AFM. Photoelectro-chemical characteristics of the electrodes were studied by photocurrent spectra and by time-resolved photocurrent measurements in 0.1 M KCl electrolyte. At the same time, changes in the electrical and photoelectrical properties of the films are observed, enabling the tuning of these properties in view of optoelectronic applications. Theoretical calculations based on density functional theory (DFT) models were made to understand the optoelectronic properties of the hybrid films. Different parallel and perpendicular orientations were tested. For the most stable hybrid structure, total and projected densities of states of the system were obtained and the alignment of the levels observed.

Introduction

Recently metallo-phthalocyanines complexes have attracted much attention due to their application indifferent areas such as display devices, data storage, solar cell, photo-reduction of water to hydrogen and other UV–vis light driven process [1], [2], [3], as well as their role in the photodynamic therapy of cancer [4], [5]. Metallo-phthalocyanines such as copper or zinc–phthalocyanines complexes are promising materials for dye-sensitized photoelectrochemical solar cell. The conversion of solar energy into electricity is based on nano-porous–nano-crystalline photo-electrode [6], [7], [8]. In such cell, light absorption of the dye is followed by electron injection from the excited adsorbed dye into the conduction band of the semiconductor; the electrons are brought back to the oxidized dye through an external way using a counter electrode (Pt) and a redox system. It is desirable that a dye to be used as a sensitizer [9], [17]; should absorb light in the whole solar spectrum, with energy of its lowest unoccupied molecular orbital (LUMO) higher than the conduction band edge of the semiconductor. Also the dye should be attached strongly to the semiconductor surface to allow the fast electron injection into the conduction band with a quantum yield of unity. It is known that through dye-sensitization of a wide gap semiconductors, like TiO2 or ZnO, electron can be injected from the excited state of the dye into the semiconductors [10], [11], [12], [13], [14], [15], [16], [17], some of the systems investigated are merocyanines-ZnO [18], CuPc/TiO2 and TiO2/ZnPc [19]. For a recent review see Ref. [20]. These devices were investigated in the form of bilayer devices. In bilayer heterojunction devices, the photoelectrons generated in organic dye diffuse to the dye/oxide interface in order to achieve charge separation. But the photovoltaic efficiency is limited due to the short diffusion length of excitons (less than 10 nm) in the organic dye. The efficiency of these devices can further be increased by the proper choice of optical band gap and right alignment of donor acceptor molecular orbital. The former will lead to more overlapping with solar spectrum and the latter will result in higher open circuit voltage. In recent years one-step electrodeposition of nanostructured ZnO/dye hybrid films on transparent conducting substrates such as glass covered with ITO or FTO have been deposited and characterized [21], [22], [23]. Especially dye molecules with carboxylic or sulphonic acid groups were found to adsorb strongly on the growing ZnO surface, leading to their co-deposition and a significant influence on the morphology and crystallographic orientation of the ZnO [21], [23], [24], [25], [26]. The most prominent example is the dye eosin Y, which leads to highly porous and sponge-like ZnO structures that can be described as porous single crystals and proved to be very efficient in dye sensitized solar cells [21], [23]. Beside organic dye molecules, tetrasulphonated metal phthalocyanines (Ts-MePc) (Fig. 1) with Zn, Alor Si as metals were also co-deposited with ZnO [24], [25], [26], [27]. These films also exhibited some porosity of the ZnO framework, so that the Ts-MePc could act as photo-sensitizers in photo-electrochemical measurements.

In this paper, nanoporous and nanocrystalline hybrid films of ZnO/Ts-CuPc of various thicknesses are electrodeposited on ITO substrate and are characterized by X-ray diffraction (XRD), Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM). The optical and electrical properties of hetero-junction photovoltaic device consisting of hybrid organic (Ts-CuPc)–inorganic (ZnO) structure thin film are presented. Aside from wavelength dependent photocurrent measurements, time-resolved photocurrent measurements are also discussed. Theoretical calculations based on DFT models have been made to study how the dye interacts with the surface and how the bands of the hybrid system are aligned. This is a crucial issue for knowing the possibilities of charge transfer between the donor and the acceptor. Therefore two main goals are addressed in the present paper; the preparation and characterization of hybrid TS-CuPc/ZnO thin films and the understanding of their photoelectronic properties by means of a theoretical level alignment calculation using DFT methods.

Section snippets

Experimental

High purity ZnCl2, KCl and 1α3β (NaSO3)CuPc (here by referred as Ts-CuPc) dye from Aldrich chemicals were taken as starting materials. The experimental setup used to prepare ZnO/Ts-CuPc thin films of various thicknesses by cathodic electrodeposition technique was consisted of a computer-controlled potentiostat/galvanostat and a classical three-electrode electrochemical cell filled with an aqueous solution containing 5.0 mM ZnCl2 and 0.1 M KCl as supporting electrolyte. Oxygen was bubbled through

Theoretical

Calculations based on density functional theory (DFT) were made using a PBE-GGA [28] functional. The used code was SIESTA [29], which projects electron wave functions and density onto a real space grid and uses as basis set a linear combination of numerical, finite-range localized atomic orbitals. We also used the DFT+U method [30] included in SIESTA that allows a semi-empirical inclusion of electronic correlations in the description of electronic spectra for systems such as zinc oxide. The

Crystallinity

XRD patterns of ZnO films prepared with dye of different concentrations in electro-deposition bath varying from 10 μM to 200 μM at −0.9 V of deposition potential at 70 °C for all samples are shown in Fig. 2a and b. A total charge of 1.5 C was passed in each case. It is well known that pure ZnO films show reflexions corresponding to the wurtzite structure, however, with strong (0 0 2) but weak (1 0 0) and (1 0 1) reflexions (Fig. 2a). The intensity of (0 0 2) peak increases with the rise of deposition

Conclusions

Electrodeposited ZnO/Ts-CuPc films represent a hybrid material system that can be varied in its contents in a wide range simply by variation of the Ts-CuPc concentration in the electro-deposition bath. At the same time, the optical as well as the electrical and photoelectrical properties can be tuned. Electrodeposited hybrid films with high phthalocyanine content behave as a thin film of Ts-CuPc. Photocurrent can only be generated in the films having a low concentration of dye. The probable

Acknowledgments

Spanish Government supported this work through MEC Grant MAT2009-14625-C03. M. Moya acknowledges the GeneralitatValenciana (Santiago Grisolia Program) for fellowship funding. P. Palacios acknowledges the hospitality of the Earth Sciences Department in Cambridge. The authors acknowledge the computer resources provided by the Madrid Supercomputing Center (CeSViMa).

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