International Conference on Ceramics, Bikaner, India International Journal of Modern Physics: Conference Series Vol. 22 (2013) 18 23 World Scientific Publishing Company DOI: 10.1142/S2010194513009872 SYNTHESIS AND PHOTOSENSITIVE PERFORMANCE OF NANOSTRUCTURED ZnO/DYE HYBRID FILM FOR ENERGY CONVERSION MAMTA RANI Physics Department, Panjab University, Chandigarh,160014, India. mamtagarg2211@gmail.com S. K. TRIPATHI * Physics Department, Panjab University, Chandigarh,160014, India. * Corresponding author email: surya@pu.ac.in; surya_tr @yahoo.com The ZnO film prepared by doctor blade method are highly porous in presence of structure directing agent PEG and then can be easily sensitized by various molecules. In the present work, the nanostructured ZnO/dye hybrid film prepared by doctor blade method has been investigated in conductivity and photoconductivity measurements in view of applications in dye-sensitized solar cells (DSSC) and in optoelectronics, since electron transport in these devices plays a decisive role for the electron collection efficiency and therefore for the overall efficiency. The absorption of ZnO film sensitized by Rose Bengal dye (RB) has been studied by UV spectroscopy which indicates that after the dye is adsorbed on the ZnO electrode, its absorption spectra showed redshift in the peak position compared to the absorbance spectra of dye in ethanol. Highly porous ZnO films are found to have high dark conductivity, probably because of a higher n-doping, which is due to higher concentration of Zn atoms in the film. On the other hand, ZnO/RB hybrid film is found to show a much higher sensitivity to illumination with visible light in photoconductivity measurements due to a higher absolute photoconductivity and lower conductivity in the dark. Keywords: ZnO; Absorption; Photosensitivity; Dye; Solar cell. 1. Introduction Solar electricity is a steadily growing energy technology. The photovoltaic has become more and more important because sunlight is a clean and limitless energy source compared to the traditional fossil energy sources. Up to now, silicon solar cell (so called the first generation solar cell with market share more than 80%) must be refined to high purity, required in a form of a thin wafer (200 micron thick or lower); and hence a large amount of energy is required for purification and crystal growth. The processing of crystalline silicon wafers is high-level semiconductor technology and capital intensitive. Dye sensitized Solar Cell (DSSC) based on mesoporous TiO 2 /organometallic dye molecule junction appears to be very promising due to its low cost fabrication together with other attractive features like flexibility. 1-5 Undoped and doped oxide thin films have a wide range of applications as an important semiconductor material such as solar cells, 18
Synthesis and Photosensitive Performance of ZnO/Dye Hybrid Film 19 electrical, piezoelectric or luminescent devices and also as gas sensors and chemical sensors 6-9. Replacing the conventional TiO 2 nanocrystalline (nc) electrode with organic/inorganic hybrid nanostructures will undoubtedly improve overall performance of dye-sensitized solar cells (DSSC). Inorganic nc-zno is a promising candidate for such hybrid structures, due to its unique properties, such as wide bandgap (3.2eV), high conductivity and high excitonic binding energy (60Mev). 10 In addition, the conduction band edge position of ZnO is similar to that of TiO 2. Dye-sensitization has provided a successful solution to extending the absorption range of the cell to low-energy light with effective result. This approach presents advantages over the direct band to band excitation in conventional solar cells. Here attached dye rather than the semiconductor itself are the absorbing species and the processes of the light absorption and charge separation are separated by the semiconductor/sensitizer interface preventing electron hole recombination. 11, 12 In the present work, ZnO films are coated on FTO substrate by doctor blade method. Conductivity (σ d ) and photoconductivity (σ ph ) are important factors in view of a use of the porous materials in DSSCs, since electron transport in these devices plays a decisive role for the electron collection efficiency and therefore for the overall efficiency. 13 Photoconductivity measurements at ZnO/RB film are also interesting in view of applications in optoelectronics. The structural formulae of RB dye is shown in Fig. 1. Pure ZnO films are prepared and photo response of sensitized film is studied and investigated for comparison. Fig. 1. Structural formula of Rose Bengal dye. 2. Experimental 2.1. Synthesis of ZnO dye hybrid films For preparation of ZnO film, porous nanosize (~20 nm) ZnO powder prepared by sol gel method (reported elsewhere) was used. For the present work porous nanosize ( 20 nm) ZnO powder prepared by sol gel method was used. In order to break the aggregates into separate particles, the ZnO powder was ground in a porcelain mortar with a small amount
20 M. Rani & S. K. Tripathi of polyethylene glycol to prevent re-aggregation of the particles. The nanoporous semiconductor electrode was prepared by doctor blade method on FTO substrate. After air drying, the electrode was sintered for 30 min. at 450 C in air. The RB dye was dissolved in ethanol at concentration of 6.4 10 4 M. Colouring of the ZnO surface with three dyes were carried out by soaking the film for 48 hr in a solution of the dye in ethanol. The electrode was dipped into the dye solution while it was still hot at 80 C. After dye adsorption, dye-coated films were rinsed in ethanol, dried by blowing argon stream and were kept in dark. 2.2. Characterization UV Visible absorption spectrum of ZnO/dye samples have been recorded on a Perkin Elmer LS 35 spectrometer. To carry out the electrical characterizations on these films, a specially designed metallic (stainless steel) sample holder, fabricated in the laboratory is used. The sample holder is connected to a rotary vacuum pump (British Thomson Houston Ltd., Type BC2410) to maintain a vacuum 10-3 mbar throughout the measurements. Temperature dependent conductivity studies is performed using Keithley 6517A. Heaters are connected to a variac through outside the sample holder to vary the rate of heating. The heating rate is monitored through the display of the digital panel which is connected to the copper-constantan thermocouple to find the activation energy of the all samples. The light is shone through the quartz window by using a tungsten bulb (200W) as a photo excitation source to carry out the photoconductivity measurements. To cut off the IR part of light, water in transparent petridis is kept above the quartz window, while taking readings in the presence of light. Before measuring photoconductivity of the sample, it is first kept in dark till it attains equilibrium. Fig. 2. Normalized absorbance vs wavelength spectra of ZnO and Dye/hybrid film.
Synthesis and Photosensitive Performance of ZnO/Dye Hybrid Film 21 3. Results The UV-Vis spectra of as-synthesized ZnO, all dye solutions in ethanol, ZnO/dye hybrid film is shown in Fig. 3. The absence of any absorption peak in the spectra of as synthesized ZnO film is in good agreement with the wide band gap nature of the material and its inability to absorb in the visible range. Organic dye RB has absorption maxima at 558 nm, respectively. After the dye is adsorbed on the ZnO electrode its absorption spectra broadened slightly with a redshift in the peak position. The visible absorption is attributed to the π-π * transition of the respective π-conjugating molecule of FGF dye. Small shift in peak position of ZnO/dye hybrid film as compared to absorption spectra of dye in ethanol shows good anchoring of RB dye with ZnO Figure 3 shows that the representative graph of lnσ dc vs. 1000/T. The semiconductor nature of films follow the following relation σ = σ o (-E a /KT) (1) where σ is the resultant conductivity, σ o is the conductivity at initial thermal equilibrium, k the Boltzmann constant, E a the activation Energy and C is the constant due to leakage current 14. Highly porous ZnO film is found to have a very high conductivity already in the dark, probably because of a higher n-doping, which is due to a higher concentration of Zn atoms in the film and defects such as presence of oxygen vacancies. Table.1 shows the value of dark activation energy for all samples using equation 1. Dark conductivity increases with increase in temperature which is an indication of semiconductor behaviour for all films. ZnO film shows linear behaviour in full region (2.2 3.4)/K, while dye hybrid sample show linear behaviour in two regions. The first activation energy at higher temperature gives information about postion of donor level below conduction band of ZnO, while second activation energy may be due to trap levels which can be introduced by addition of dye. Fig. 3. Variation of dark conductivity (σ dc ) with temperature for ZnO and ZnO/dye hybrid film.
22 M. Rani & S. K. Tripathi For photoconductivity measurements, sample was exposed to light for 30 sec through transparent window of sample holder. Fig. 4. shows σ ph increases exponentially with temperature. The well-defined activation energy involved in the temperature dependence of σ ph suggests that the recombination centres are located at relatively discrete levels of localized states. As different dyes can lead to different film structures and morphologies, therefore ZnO/dye hybrid film have different photoactivation energies as given in table 1. Fig. 4. Variation of photo conductivity (σ ph ) with temperature for ZnO and dye/hybrid film. As photosensitivity (S) is independent of film thickness, therefore it provides a good measure for comparision of ZnO/dye hybrid films. It is defined as S=σ ph /σ dc (2) ZnO/RB film has high photosensitivity. The high value of S indicates that a high proportion of surface dye molecules are being excited by π- electrons acting as sensitizers whose transition energy is lower than that of trapped state. The absorption in visible region and photosensitivity factor suggests that the ZnO/ FGF dye hybrid film can be used in photovoltaic and optoelectronic devices. Further work on photoconductivity measurements will be required to elucidate the exact mechanism of charge transfer between dye and ZnO film. Acknowledgments This work is sponsored by Department of Science and Technology (Major Research Project), New Delhi. Author (MR) thankful to CSIR, New Delhi, for providing fellowship.
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