P R A S I C ' 02. Simpozionul na7ional cu participare interna7ional8 PRoiectarea ASIstat8 de Calculator

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1 UNIVERSITATEA TRANSILVANIA DIN BRAOV Catedra Design de Produs *i Robotic, Simpozionul na7ional cu participare interna7ional8 PRoiectarea ASIstat8 de Calculator P R A S I C ' 2 Vol. III Design de Produs 7-8 Noiembrie Braov, România ISBN TIO 2 FILMS FOR ORGANIC SOLAR CELLS Anca DU*, Mihaela ICA, Marius NANU* and ** * Transilvania University of Braov, Romania ** Delft University of Technology, the Netherlands Abstract: The development of the solar cell concept and design is reviewed. Solid state solar cells with a TiO 2 as a semiconductor electrode and solid sensitizer are presented. Obtaining TiO 2 thin films through Spray Pyrolysis Deposition is investigated and some parameters affecting the film morphology are pointed out. Keywords:solar cells, organic solar cells, spray pyrolysis deposition, TiO 2 anatase. 1. Introduction Solar cells operate by direct conversion of the sunlight energy into electricity using the properties of semiconductors. This process although not highly efficient (yet) is non-polluting and relates to an inexhaustible source, the sun. Sunlight incident on a solar cell creates electron-holes pairs within the semiconducting material in the cell. The cell has an asymmetrical electronic structure, which causes a separation of the pairs and a current flow in a load connected to the cell terminals, Fig. 1. Fig. 1. Schematic description of a p-n junction silicon solar cell The output parameters usually used to characterize a solar cell are: - Short-circuit current, I sc, equal ideally with the light generated current, I L (i.e. depending on the photon flux in sunlight). The maximum I sc value can be obtained by integrating the photon flux function over the entire wavelength domain where electron-holes pairs are generated. As it is expected, while the semiconductor band gap decreases the short-circuit current density increases. - Open circuit voltage, V OC, depends on the properties of the semiconductor, and is theoretically independent on the diode saturation current, I. Unlike to the I sc, the maximum value of the V OC decreases with decreasing the band gap. The power output of any operating point is the product I. V, i.e. the area of the dashed rectangle in Fig.2. The point describing the maximum area, M, is characterized by the parameters I mp and V mp. - The cell efficiency,, is the ratio of the maximum power output and the luminous input power, P L :

2 I L I Fig. 2 Properties of a p-n junction diode I mpvmp = 1 (1) P L The efficiency of the commercial cells does not exceeds 14% - The fill factor, FF is defined as the ratio of the maximum power output and the product of the cell short-circuit current and the open circuit voltage I mpvmp FF = (2) I V sc I mp I sc oc Dark V mp V oc The FF is high if the operating characteristic of the cell approaches a square, i.e. when operating with low overvoltages and without ohmic losses. The cells with good efficiencies, FF ranges from.7 to.85. But, it is the price that limits the use of the silicon cells not their performances because (micro)crystalline silicon used in solar cells is a by-product of the semiconductors/microchips industry. Alternatives have been investigated and the photoelectrochemical (PEC) cells represent a promising one. Fujishima and Honda described the first PEC with TiO2 as photoanode, in In a PEC cell one of the semiconductor/metal interface is replaced with a semiconductor/ electrolyte interface. The electrolyte is far less sensitive to the purity of the semiconductor so, the price of the cell decreases. The electrolyte contains a reversible Red/Ox couple, selected such as the reversible potential E o should be more positive than the flat band potential of the (n) semiconductor, E FB, promoting charge split. By illumination, electron-holes pairs are generated in the semiconductor. In the charge layer they move in opposite directions, holes are driven to the surface and electrons in the bulk. When closing the external circuit through consumer (R), M Illuminated V electrons are transferred to the metal counter electrode and enter the cathodic reaction, reducing the Ox to Red while holes cross the semiconductor/electrolyte interface and enter the photoanodic reaction, oxidizing Red to Ox. For high efficiency, the semiconductor and the electrolyte must be selected to maximize the E EFB difference. Such a cell, with an efficiency of 16% (Switzer, 1986) has a single crystalline photoelectrode of n-gaas and as electrolyte a mixture of K 2 Se- K 2 Se 2 -KOH.. Different other type of electrolytes are used for cells with photoelecrodes such as n-cds, n-cd(se, Te), p-inp, n-cuinse2, n-cds or n-ga(as,p), (Sequeira, 1994). An improvement in the field was induced by the photosenzitation of the semiconductor using dyes. The dye must have the energy of the HOMO in the band gap region and the LUMO at the bottom of the conduction band. By photoexcitation one electron from the dye molecule is inserted in the conduction band where it is used for the consumer (Maloney, 1996). Different types of organic dyes were tested, adsorbed as monolayers on TiO 2. Of course, the amount of dye depends on the semiconductor morphology. Gratzel, 1991, was the first to use nano-structured TiO 2 increasing the area by more than 1. A Gratzel cell using TiO2 and porphyrine dye has an efficiency of about 11%. The electrolyte (I 2 /I - 3 ) used in such cell is subject of temperature influences (vaporizing, decomposition) and is rather hazardous. This is why solid electrolytes are studied for Gratzel cells (Schoonman and co-workers). Thin films of FeS 2, CuS 2 and CuInS 2 are tested as sensitizer. Their efficiency is not very high yet, but the studies are still in the beginning. They are related to the electrolyte (type, deposition technique, stability, and electrical characteristics) and to the electrode. The TiO 2 thin layer must have the following features: - it is of anatase type, the TiO 2 polymorph with the band gap of 3.2 ev stabil under 5 o C, - it is nano-structured, - it has reproducibility in manufacturing. This paper presents the results of the experiments for obtaining nanostructured layers of TiO 2 by Spray Pyrolysis Deposition (SPD). 2. Experimental SPD is a gas-phase technique that allows obtaining homogenous, high quality films. The

3 reactants (precursors) are mixed in a glass then are sprayed over the heated substrate. The technique is very simple; the layer can be grown on surfaces of very different dimensions or shapes. For obtaining regular nanostructures a careful adjustment of some parameters must be considered: the composition of the liquid, the temperature of the substrate, the time of spraying and most important- the dimensions of the drops of liquid reaching the substrate i.e. the nozzle used for spraying, Fig. 3 Fig. 3. The nozzle used for SPD (manufactured in TU Delft, the Netherlands) The experimental setup belongs to the TU Delft University and a similar one exists in the Transilvania University of Brasov. The precursor used in our experiments is titanium tetrakis-isoprpoxyde (TTIP 97%, Aldrich) and the reaction expected is Ti (OCH(CH 3 ) 2 ) 4 (l) + 18 O 2 (air) K TiO 2 (s) + 12 CO 2 (g) + 14H 2 O (v). Secondary reactions mentioned in literature are forming ethanol or methanol but are of no consequence in our experiment considering the temperature and the open atmosphere used in the experiments. Ethanol solutions of TTIP and acetylacetonate, (AcAc, 99+%, Aldrich) were prepared with different component ratios, as presented in Table 1. with SnO.. 2 In 2 O 3 (ITO). The plates were rinsed in an ultrasonic bath with ethanol, acetone and again ethanol and dried in a nitrogen stream. The plates were put on a thermostatic heater (CERAN 1), fixed at 45 o C. After heating the spray deposition was done in pulses, and samples were removed from the heater after 1, 15, 2 and 25 minutes. After cooling, the structure of the samples was analyzed by XRD (Bruker D8, Cu K ) and laser 1 Raman spectroscopy (on a homemade device, TU Delft). For the very thin layers, grown at 1 and 15 minutes, the Energy Dispersive X Ray spectroscopy (Oxford Link-Isis) is supplementary used. The morphology of the layers was investigated by Scanning Electron Microscopy (Joel JM 58 LV). For a better resolution before the scanning microscopy the samples were sputtered with platinum (Edward Sputter Coater S15B). 3. Results and Discussions The EDX analysis shows very low concentrations of TiO 2 in the samples after 1 and 15 minutes of spraying for each sample. Therefore the further tests were done on samples sprayed 2 and 25 minutes grown on TCO glass if other not specified. The Raman spectra for the three samples at their maximum deposition time are presented in Fig. 4a c. All the spectra exhibit the significant peaks, confirming that the final layer consists almost of anatase. The significant peaks (398, 513 and cm -1 ) are developed even from the first tem minutes, as Fig. 5 shows and no phase transition occurs during heating. 8,k 6,k TCO Sample 1, 25 min Table 1 Precursors mixtures used for SPD Sample V TTIP / ml V AcAc / ml V EtOH / ml Special care must be taken to avoid the water contact of the precursor mixtures, when TiO 2 powder is formed with a high reaction rate. The deposition substrates were of glass coated with a thin layer of SnO 2 (TCO) and glass coated Intensity (a.u.) 4,k 2,k, Raman shift (cm -1 ) Fig. 4, a

4 Intensity (.u.) Intensity (.u.) Fig. 4, b Raman shift, cm -1 Fig. 4, c Sample 2; 3 min Raman shift, cm -1 Sample 3; 3 min Fig. 4 Raman spectra of TiO 2 and TCO a- Sample 1, b- sample 2, c- sample 3 Anatase is the meta-stable polymorph of TiO 2 (rutile is the only thermodynamically stable form, at each temperature) and literature mentions it as majority in thin films at temperature lower then 5 o C. In each sample prepared, the XRD analysis exhibits, beside the anatase structure, also a small quantity of the non-stoechiometric compound, Ti 8 O 15, Fig. 6. These type of compounds are usually obtained in an atmosphere with low oxygen content. Bessergenev and coworkers, 22, obtained these compounds as by-products in the CVD deposition of thin layers of anatase, at low pressures. Intensity (cps) (11) (**) (2) theta Fig. 6. XRD of titania, Sample 2, after 3 min of spraying; (**) is the peak of Ti 8 O 15 The uv-vis spectra can be used for the calculation of the film thickness, Fig. 7. For deposition time higher than 2 minutes the thickness was above 1 nm. 3x x min 8 Intensity (a.u.) 2x1 4 1x1 4 5x1 3 2 min 15 min 1 min Transmision Raman shift, cm Wavelength (nm) Fig. 5 Raman spectra of the TiO2 layer grown at 1, 15, 2 and 25 minutes (sample 1). Fig. 7 Titania uv-vis spectrum (sample 2, 25 min)

5 Reference Fig. 9 TiO 2 thin layer grown on ITO (sample 2) The SEM pictures shows that the morphology of the samples depends on the TTIP solutions: concentrated solutions (sample 2) forms nonhomogenous films, with pin holes, while diluting the mixture and increasing the acetilacetonate proportion leads to smooth films, Fig. 8. It is also important the substrate used for deposition. When using ITO glass, with more nucleation centers, the layer is less regulate and exhibits the growth of TiO 2 powder grains, Fig. 9. Increasing the spraying time has as consequence not only increasing the thickness but also the bound of TiO 2 particles in the film. 1. Aperathitis, E. et. al. Sol. Energy Maater. 2, 15, Bessergenev, V.G., Khmelinski, I.V. et.al. Vacuum. 6 (22), p Fujishima, A., Honda, K., Nature. 1972, 238, p Gratzel M., et. al. J. Am. Chem. Soc. 115, 6382, Green, M. Solar Cells. Prentice Hall, K. Nishida, Moriswa, K., Hiraki, Murishi, S., Katoda, T. Applied Surface Science , 2, p Maloney, E. Synthesis and Characterization of TiO2 films made by CVD for applications in organic solar cells. Ph.D. Thesis, TU Delft, O Regan, M. Gratzel, Natur. 353, 737, Pleskov, Y.V. Semiconductor photoelectrochemistry for cleaner utilization of solar energy. In Environmental Orientated Electroche-mistry, Elsevier, Switzer, J.A. Electrochem. Soc. 1986, 133, Conclusions The paper presents experimental aspects for obtaining thin films of TiO 2 used in organic solar cells. The nanostructured films are obtained by spray pyrolysis deposition. The films characterized by Raman laser spectroscopy, XRD and uv-vis, consist of anatase with a very low impurity content of Ti 8 O 15. The use of dilute precursor solutions is a condition for obtaining homogenous films.

6 Fig. 8a Sample 1, 2 min sputtering Fig. 8d 8b Sample 3; 1; 3 25 min sputtering Fig. 8c Sample 2; 2 min sputtering Fig. 8d Sample 2; 25 min spraying Fig. 8d Sample 3; 25 min sputtering Fig. 8b Fig. Sample 8b Sample 3; 33; min 3 spraying min sputtering