Effect of TiO 2 -electrode properties on the efficiency of nanocrystalline dye-sensitized solar cells (nc-dsc)

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1 Effect of TiO 2 -electrode properties on the efficiency of nanocrystalline dye-sensitized solar cells (nc-dsc) J. Wienke, J.M. Kroon, P.M. Sommeling, R. Kinderman, M. Späth, J.A.M. van Roosmalen, W.C. Sinke Netherlands Energy Research Foundation ECN, P.O. Box 1, 1755 ZG Petten, The Netherlands Tel /Fax ; wienke@ecn.nl S. Baumgärtner Freiburger Materials Research Center, Stefan Meier Strasse 21, D-7914 Freiburg, Germany Tel / Fax ; baumgart@fmf.uni-freiburg.de ABSTRACT: The influence of the TiO 2-microstructure on the performance of the nc-dsc is investigated. Hence, four commercial TiO 2-powders with different particle diameters and sintering behaviour have been used: P25 (Degussa AG), H37 (Transcommerce International AG), AK1 and PK5585 (Bayer AG). With a view to large-scale production the TiO 2- electrode preparation process is changed by substitution of the well known squeegee (doctor-blade ) method by the screenprint process. The TiO 2-powders and electrodes are characterized by BET-analysis, SEM and TEM, pore size distribution analysis (Hg-porosimetry), X-ray diffraction, chemical (elemental) analysis and reflection/ transmissionmeasurements. The microstructure of the electrodes, prepared from different TiO 2-powders differs significantly, but the cell efficiencies at 1sun of AM1.5 solar light of the glass/sno 2:F/TiO 2/cis-(NCS) 2-bis(4,4-dicarboxy-2,2-bipyridine)- ruthenium(ii) /electrolyte/sno 2:F(Pt)/ glass-cells were found to be comparable, implying that additional factors limit the efficiency. In contrast, the change of the paste composition influences the efficiencies expanding from 2.5% for screenprint paste electrodes and 4.5% for squeegee paste electrodes. No microstrucutural changes could be observed by SEM for different paste composition. Hence, the different short circuit currents are probably caused by particle interconnection problems or incorporation of foreign particles during the preparation of the screenprint paste which definitely causes trapping of charge carriers. Keywords: TiO 2-1:Organic Solar Cells-2:Characterisation-3 1. INTRODUCTION The dye-sensitised solar cell of the Grätzel-type is a promising alternative to the inorganic solar cell [1]. Besides other parts of the cell, the microstructure of the nano-porous TiO 2-electrodes influences the solar cell efficiency significantly. The charge carrier transport can be controlled by an optimal nanoparticle interconnection and a pore size which guarantees an efficient electrolyte penetration [2]. The dye coverage can be optimized by use of small TiO 2 particles with a high internal surface area and by tuning the TiO 2 layer thickness. In this study we will focus on how the microstructure of the electrode affects the solar cell efficiency. Different commercial TiO 2 powders are used for this purpose, P25 (Degussa AG, Germany), H37 (Transcommerce International AG), and two powders from Bayer AG, Germany, AK1 and PK5585. All powders consist mainly of the anatase phase. A comprehensive characterization of the powders and produced electrodes is carried out. With a view to large-scale production it has been investigated if the squeegee method can be replaced by the screenprint process with the aim to get comparable photoelectric properties. The change of the deposition procedure also includes the transfer from the water-based squeegee TiO 2-paste to a paste, based on organic solvents. 2. EXPERIMENTAL 2.1. TiO 2-pastes and electrode manufacturing The preparation of the squeegee paste and deposition on the substrate took place according to the procedure described in the literature as doctor blading process [1].The paste quality is strongly dependent on the powder dispersion in the mortar. For the squeegee deposition one scotch tape (thickness around 5µm ) is used as spacer to adjust the thickness of the sintered electrode to around 1 µm (measured with Rodenstock profilometer). Table I: Paste composition squeegee paste (water-based) screenprint paste (based on organic solvents) paste 1 paste 2 12g TiO 2 4g TiO 2 with.7g 4- hydroxy benzoic acid 13 ml distilled water 73.3g terpinol (Fluka) 8 ml 1% acetylacetone in distilled. water 1.7g ethylcellulose (viscosity 3-5 mpas) (.12g 3.2%HCl/1g paste) The screenprintable TiO 2 paste is prepared by mixing 1g of 4-hydroxy benzoïc acid (Merck, p.a.) with 8 g ethanol (Merck, p.a. absolute); 6 g of TiO 2 powder is slowly added during mixing. This mixture is stirred until a homogeneous paste is formed. After attrition milling for 2 ½ hours, the ethanol is removed completely by a rotary evaporator delivering a dry compact powder. 4 g of this powder is dispersed with a sigma-kneader while adding slowly a mixture of 35 g of 5% ethylcellulose (Fluka, 3-5 mpa s) in terpinol (Fluka, anhydrous). After the mixture became homogeneous 4 gram of terpinol was added to the dispersion and the kneading was continued again until a homogeneous mixture was formed. Finally the paste was placed on a three-roller-mill for 3 min. The screenprinting process is carried out on an EKRA Microtronic II machine and a squeegee hardness of 6 shore is used. For the reproducible print of the TiO 2-paste a screen with 125 mesh/inch is installed and the parameters are adjusted to a snap-off of.8mm; a pressure of 3.8 bar; and a speed of 9 mm/sec. The layer thickness of the TiO 2-layer was varied by doing single and double prints. Before deposition of a second layer the electrode was first sintered at 45 C. To evaluate changes of the TiO 2- microstructure due to different paste composition electrodes with comparable

2 layer thickness are produced with the squeegee method for the deposition of the squeegee paste as well as the screenprint paste Electrode manufacturing After depositing the paste on SnO 2:F-glass supports (Glastron,7Ω/ ) the electrodes were dried for 1 minutes at 8 C and sintered for 3 minutes at 45 C in air. The dye-coverage was provided placing the electrodes for at least 1 hours in a.3 mm solution of cis-(ncs) 2 - bis(4,4-dicarboxy-2,2-bipyridine)-ruthenium(ii) in ethanol TiO 2-characterization methods The different TiO 2-powders are investigated by BETanalysis, SEM and TEM, pore size distribution analysis (Hg-porosimetry), X-ray diffraction and chemical (elemental) analysis. To evaluate the particle changes after some processing steps like preparation of the paste, deposition of the paste on the support and sintering the layers, the electrodes are characterized by SEM-microscopy and pore size distribution analysis as well. Reflection/transmission measurements are carried out using a spectroradiometer (Instrument systems) with an integrating sphere (Labsphere) IV-characteristics The final goal of this investigation is to test the TiO 2- material for the use in nc-dsc. The photoelectrical properties of a cell give beside others also information about the charge carrier transport through the electrode. The measurements are carried out in an open cell holder with the glass/sno 2:F/TiO 2/Ru-dye/electrolyte(methoxypropionitrile or acetonitrile) /SnO 2:F(Pt)/glass-composition. 3. RESULTS 3.1. Powder characterization The TiO 2-powders are produced by different production procedures. Therefore among others different particle sizes, particle agglomeration and TiO 2-phase distributions as well as deviating concentrations of trace elements in the powder are expected depending on the starting material and the apparature used for the TiO 2-production. Table II: X-ray analysis and BET-measurements on powders as received material X-ray (part.diam) BET-m 2 /g P nm 49 H nm 122(agglom) PK nm 34 AK1 17. nm 89 AK P25 is known as a powder with a low tendency to agglomeration. However, the primary particle diameter of 21nm is undesirable large as compared to the other powders of our selection. The most promising powder seems to be PK5585 with the highest BET-surface of 3 m 2 /g, due to the very small particle diameter of 8.5 nm. We have to take into account, however, that during sintering the agglomeration of the small particles is pronounced [3]. All TiO 2-powders mainly consist of the anatase phase Screenprint process compared to the squeegee method Screenprint pastes are made of the different powders P25, AK1, PK5585 and H37 using the same recipe regardless of variations in powder behaviour. Therefore, the viscosity of the resulting pastes differed and irregularities in the print behaviour of the pastes were expected. Comparing the masses of the sintered electrodes the most liquid H37-paste indeed gives slightly lower coverage (see fig.1). mass mg/cm AK1 PK5585 H37 P25 Figure 1: Mass of electrodes prepared by single print (white), double print (gray) and squeegee of the screenprint paste (paste 2) (black) Going from single to double print, the resulting layer thickness is nearly doubled. However, the cross-section of a double-print shows a very sharp interface at the transition region between the first and the second print (fig.2). The mass (layer thickness) of the electrodes prepared by the squeegee-method is approximately the same as for a double screenprint. From SEM-photographs (fig.3-5) it is obvious that the microstructure of the electrodes prepared from a specific powder does not change even if the paste deposition method is varied (squeegee or screenprint) or if a different paste composition is applied (paste 1(water-based ) or paste 2 (based on organic solvents). Table III: X-ray diffraction analysis material w-% anatase w-% rutile P H37 >95 <5 PK AK1 >95 <5 Table IV: Chemical analysis Fe W Al Si P S d.l P H PK

3 Figure 2: SEM-photograph of a P25-electrode prepared by double screenprint with screenprint paste Figure 3: SEM-photograph of a P25-electrode prepared by squeegee deposition of squeegee paste 3.3. Electrode morphology As is obvious from table II and V, the BET-surface remains unchanged for P25 and AK1 powder, the powders with the highest particle diameter. For the other two powders, especially for PK5585, a dramatically decrease of the BET-surface is observed, as expected. The BETresults agree with the morphology of the electrodes. The P25-electrode consists of small homogeneously distributed secondary particles with a diameter of predominantly 1 nm (fig.2,3), which is also reflected in a sharp pore size distribution around a pore diameter of 5 nm. In contrast, the PK5585-electrode shows aggregates of different dimensions (fig. 4) and accordingly a broad pore size distribution with a maximum probability of pore diameters around 2 nm. The H37 differs by the existence of large agglomerates (>1µm ) which are observed even for the untreated powder. The crumbling morphology in fig.5 is reflected by the very broad pore size distribution ranging from 8 nm up to 1µm. Figure 4: SEM-photograph of a PK5585- electrode prepared by squeegee deposition of squeegee paste Table V: Electrode properties material BETm pore size distribution 2 /g P25 5 sharp at 5nm H37 75 very broad between 8 nm and 1µm PK sharp at 2 nm, broad 8-5nm AK1 83 sharp at 15 nm, broad 8-1nm As is obvious from Table V the BET-surface area of the sintered electrodes differs with respect to the powders. A drastically decrease of the originally high BET-surface values is observed for PK5585 and H37. This is a result of the rapid growth of the small particles during sintering of the electrodes. For powders with already larger particle diameter, like P25 and AK1, the BET-surface remains unchanged or slightly decreases during sintering of the electrode. However, the different package and agglomeration of the particles let assume that going from one to another electrode the light scattering properties as well as the transport properties including electrolyte diffusion into the pores. Figure 5: SEM photograph of a H37-electrode prepared by squeegee deposition of squeegee paste (magnification 2.) 3.4. Optical properties of the dye-coated electrodes The transmission and reflection of the untreated and coated electrodes have been measured. The calculated dye-absorption spectra indicate that the layer thickness of all electrodes is high enough to reach complete light

4 absorption at the maximum wavelength of 54 nm (fig. 6). Going from single to double screenprint the increased layer thickness results in a slightly larger absorption in the higher wavelength region. The shape of the absorption curves is not significantly influenced by variation of the paste composition and deposition method. Additional information about the microstructure of the TiO 2-electrodes can be received by evaluation of the electrode scattering. The reflection spectra of the dyecoated electrodes prepared from the selected TiO 2- powders differ significantly although the layer thickness is kept constant by depositing them with the squeegee method (fig.7). At wavelengths above 1Tm the TiO 2- scattering can be observed exclusively. The different reflection values and slopes of the curves indicate that the electrodes differ significantly in agglomeration and pore size distribution. P25 electrodes gave the lowest reflection (scattering) values with a pronounced increase of the scattering intensity at shorter wavelengths. This is another indication for the uniform structure and the absence of large agglomerates. For the other electrodes strong scattering intensities even at higher wavelengths indicate the presence of large agglomerates. This phenomena are consistent to the results from SEM and BET-analysis. If the paste composition is varied the scattering properties for one specific powder remain unchanged. absorbance wavelength /µm Figure 6: Absorption spectra of Ru-dye coated PK5585 electrodes deposited by squeegee with paste 1 (black, solid) or paste 2 (black,dotted) and single (gray, solid) or double screenprint (gray, dotted) with paste 2 reflection /% PK5585 H37 AK1 P wavelength /µm Figure 7: Reflection spectra of Ru-dye coated electrodes prepared by squeegee method using the screenprint paste IV characteristics The photovoltaic parameters are measured at 1sun of AM1.5 solar light in an open cell holder. The open circuit voltage of the electrodes range from.6v (H37 double print) to.69v (P25 single print); most electrodes give a mean value of.63v (excl. P25 with a mean voltage of.67 V). Going from single to double screenprint a small voltage drop is registered. The differences in open circuit voltage are small regardless of the TiO2-powder used, the paste composition or the paste deposition method. The fill factor is around.6 for all electrodes. The parameter which influence the cell efficiency most is the short circuit current. There are only small differences between the cell efficiencies of electrodes prepared by different powders, but the change of the paste composition causes a doubling of the short circuit current going from electrodes prepared with the screenprint paste 2 (Isc 6.5 ma/cm2, η 2.5%) to the ones deposited with the squeegee paste 1 (Isc up to 12mA/cm2, η up to 4.5%) (fig.8). energy efficiency /% AK1 PK5585 H37 P25 Figure 8: Cell efficiency at 1sun AM1.5 solar light for different TiO 2-powders used; white-single screenprint (paste 2), light gray-double screenprint (paste 2), graysqueegee of paste 2, black -squeegee of paste 1 (electrolyte paste 1: acetonitrile, paste 2: methoxypropionitrile) 4. CONCLUSIONS Each of the selected TiO 2-powders yields a specific electrode microstructure. P25 differs by formation of homogenous layers with a small pore size distribution, indicating low agglomeration of the primary particles. PK5585 represents an example for strong BET-surface decrease during sintering of small particles. For one specific powder the morphology of the electrode does not change, even if the paste composition is changed or the deposition method is varied. The screenprint method is suitable to produce electrodes with comparable layer thickness as obtained with the squeegee method. Therefore a double print is applied to the substrate. The double screenprint increase the dye absorption at higher wavelength, but introduces an interface with deviating morphology. The microstructural changes of electrodes prepared from different TiO 2-powders do not affect the cell efficiency significantly, implying that beside paste composition and layer morphology there are other factors (e.g. the presence of surface-states which lead to trapping), which limit the rate of the overall charge carrier transport process. The dramatically decrease of the cell efficiency using screenprint paste with respect to squeegee paste can probably be explained by weak particle interconnection, due to organic residuals, which act as quenchers, or incorporation of foreign particles leading to electron trapping. 5. ACKNOWLEDGEMENTS

5 We are indepted to NOVEM for financing this project in the framework of their NOZ-PV programme. REFERENCES [1] M.K. Nazeeruddin et al., J. Am. Chem. Soc. 15(1993), Baker, E. Muller, P. Liska, N. V [2] C.J. Barbé, M. Grätzel, manuscript, submitted for publication publication [3] L. Kavan et al., J.Electrochem. Soc. 143(2) (1996), 394-4