Impedance spectroscopy study of solid-state dye-sensitized solar cells with varying Spiro-OMeTAD concentration

Transcription

1 Impedance spectroscopy study of solid-state dye-sensitized solar cells with varying Spiro-OMeTAD concentration Márcio S. Góes, 1 Francisco Fabregat-Santiago, 2 Paulo R. Bueno, 1 Juan Bisquert 2 1 Departamento de Físico-Química, Instituto de Química de Araraquara, Universidade Estadual Paulista, R. Prof. Francisco Degni s/n, Araraquara SP, Brazil. 2 Photovoltaic and Optoelectronic Devices Group, Departament de Física, Universitat Jaume I, Av. Sos Baynat, s/n, Castelló, Spain. ABSTRACT This work reports on the changes of performance of solid-state cells dye-sensitized solar cells with the variation of concentration of spiro-ometad between 5% and 25% in the fabrication of the cell. Variations of charge recombination and capacitance correlate with the improvement of current-potential characteristics at increasing spiro-ometad content, which is explained by reduction of transport resistance for hole transport, the increase of charge separation in the dye molecules, and importantly, with the increase of the -factor in the recombination resistance, that causes a reduction of the diode ideality factor. INTRODUCTION There has been a great interest recently in the study and improvement of solid dyesensitized solar cells using spiro-ometad hole conductor [1, 2]. It has been observed that filling the pores of nanoporous TiO 2 with the hole conductor poses some difficulties, and this limits the practical thickness of the active film, and consequently, the total light absorption and the photocurrent. On another hand, this kind of cells is rapidly increasing efficiency which has allowed a quantitative study of the electronic aspects of the device and the main photovoltaic parameters using impedance spectroscopy (IS) [3, 4]. While electronic aspects of the higher performance DSC using liquid electrolytes are well known [5, 6], the operation of the solid hole transport material (HTM) in the DSC requires further investigation [7]. Therefore we decided to study a set of solid DSCs where the OMETAD content in the TiO 2 pores was progressively increased from only 5% to 25% and here we report the main results. EXPERIMENT The solid-state DSCs were prepared according to a previous report [2]. The fluorine doped tin oxide (FTO) coated glass sheets (15 /, Hartford glass) were etched with zinc powder and HCl (1N) to give the required electrode pattern. The sheets were subsequently cleaned with soap (2% helmanex in water), distilled water, acetone, methanol and finally treated under oxygen plasma for 10 min to remove any organic residues. The FTO sheets were then coated with a compact layer of TiO 2 (100 nm) by spray pyrolysis deposition at 450º C using air as the carrier gas. A Dyesol TiO 2 nanoparticle paste was doctor bladed onto the compact TiO 2 to give dry film thickness between 1.6 and 1.8 μm, governed by the concentration of the paste. These sheets were then slowly heated to 500 ºC (ramped over 30 min) and baked at this temperature for 30 min under an oxygen flow. After cooling, the sheets were cut into slides of the required size and stored in the dark until further use. Prior to fabrication of each set of

2 devices, the nanoporous films were soaked in a 0.04M aqueous solution of TiCl 4 for 1 h at 70 ºC in an incubator. After rinsing with deionized water and drying in air, the films were baked once more at 500 ºC for 45 min subsequent cooling to 70 ºC and placed in a dye solution for 1 hour. The indoline based dye used for sensitization was D102. The dye solution comprised of 3 mg of D102 in 20 ml of acetonitrile and ter-butyl alcohol (volume ratio: 1:1). The hole transporting material used was spiro-ometad, which was dissolved in chlorobenzene at 5 different concentrations of 53, 111, 180, 250, 333 mg ml -1. After fully dissolving the spiro-ometad at 100 ºC for 30 min the solution was cooled and ter-butylpyridine (tbp) was added directly to the solution with a volume to mass ratio of 1:26 µl mg -1 tbp:spiro-meotad. Lithium bis(trifluoromethylsulfonyl)imide salt (Li-TFSI) ionic dopant was pre-dissolved in acetonitrile at 170 mg ml 1, then added to the hole-transporter solutionat 1:12 µl mg 1 of Li-TFSI solution: spiro-ometad. Upon changing the concentration of spiro-ometad in the solution, the ratio of spiro-ometad to tbp and Li-TFSI was kept constant. The dye-coated mesoporous films were briefly rinsed in acetonitrile and dried in air for one minute. A small quantity μl of the spiro-ometad solution was dispensed onto each dye-coated substrate and left for 20 s before spin-coating at 2000 rpm for 25 s in air. The films were then placed in a thermal evaporator where 50 nm thick gold electrodes were deposited through a shadow mask under high vacuum (10 6 mbar). The device area was defined as the overlap between the FTO anode and gold cathode and was approximately cm 2. The cells were illuminated with an AM1.5 solar light simulator (1000 W Class A Newport, A) and their current density-voltage (jv) characteristics registered with a PGSTAT-30 (Autolab). For the IS measurement potentials ranged between 0 an -1 V were applied in the illuminated cell. The amplitude of the ac signal used was 50 mv, and the frequency ranged between 1 MHz and 10 mhz. In this case, a potentiostat PGSTAT-30 with a frequency analysis module (FRA) from Autolab was used to perform impedance measurement RESULTS AND DISCUSSION The jv characteristic for a series of devices analyzed under AM 1.5 solar radiation conditions with increasing concentration of spiro-ometad are shown in Fig. 1. Characteristic parameters of the cells are given in Table I. A general improvement of all cell parameters is observed at increasing content of the HTM, and this will be explained with reference to the impedance parameters as a function of bias voltage that are shown in Fig. 2. The capacitance data are represented in Fig. 2(a). At low potentials, the capacitance is dominated by the blocking layer uncovered by the colloidal TiO 2 film in contact with the hole conductor. For cells with low spiro-ometad content the underlayer is only partially covered with the HTM, therefore, the capacitance has lower values with the lower HTM concentration. Samples with 20% and 25% concentration in the coating solution saturate to ~5 F/cm 2, suggesting full (or at least the maximum attainable) coverage of the underlayer and consequently of the pore holes. For the lower concentrations of HTM (5 and 10%), the chemical capacitance of TiO 2 is not observed. This indicates a failure of the rise of the Fermi level of electrons in TiO 2 with the applied bias. Coincidentally, those cells are the ones having the lower V oc. At the higher potentials these two cells present a drop in the capacitance related to the appearance of a negative capacitance.

3 Figure 1. Current density-voltage characteristics of solid state DSCs with 5 to 25% of the Spiro- OMeTAD. (a) 1 sun and (b) dark. Table I. Solid state DSCs performance parameters for the cells with different concentration of spiro-ometad. Concentration S (cm 2 ) V oc (mv) I sc (ma) (ma/cm 2 ) FF (%) 5% % % % % S, cell surface; j sc short circuit current density; V oc, open circuit potential; FF, fill factor and, overall efficiency. j sc

4 Figure 2. (a) Capacitance and (b) Recombination resistance of solid-state DSCs with increasing concentration of spiro-ometad under 100 mw/cm 2 illumination. The inset shows the values obtained from the slope for each graph. To complete the analysis of jv curves in Fig. 1 and performance parameters shown in Table I, we will focus the attention on recombination resistance shown in Fig. 2(b). As a general rule, the charge transfer losses from TiO 2 may be described with a recombination resistance, Fig. 2(b), that follows the expression [6]

5 R rec R0 exp EFn EFp (1) kbt being a constant (transfer factor) describing recombination of electrons from surface states in TiO 2 to the HTM, k B the Boltzmann constant, T the temperature, E the Fermi level of electrons in the TiO 2 and E the Fermi level of holes in the spiro-ometad. The difference of these last Fp two quantities is related to the forward bias potential as V EFn EFp / q. The first view of data concerns the cells made from 5, 10 and 15% HTM concentrated solutions. All these cells present successively increasing current collection in good accordance with the higher pore filling of the cells. The small difference in the current density obtained for all these cells suggest that although the pore filling is much poorer for the lower concentration, the surface of the TiO 2 is similarly coated with the spiro-ometad. Specifically, at the 5% concentration, the HTM is concentrated basically at the surface of the TiO 2, while the pores remain almost empty. The first observation in Fig. 2(b) is that, the factor related to the distribution of donor and acceptor states in both charge transport media [8], is lower for the cells with the two lowest HTM concentrations. Furthermore, at the lower potentials, these two cells present the lowest values of recombination resistance and thus the smaller R 0. These low values of R rec are attributed to the fact that most of the HTM is attached to the surface of TiO 2. Therefore the holes are always very close to the TiO 2 surface, increasing the recombination. This situation makes very difficult the process of screening of charge at the interface and could be the cause for the absence of rise in C and the inductance observed at high voltage [7]. In addition, the presence of insulated islands of HTM due to low pore filling may produce recombination centers at the surface of the TiO 2, reducing the value of R rec. In the case of 5% concentration cell, the low pore filling is also observed through the high series resistance found for the cell, in some cases more than twice the value of R rec. This high series resistance is attributed to the small quantity of HTM that contributes with a high transport resistance in this medium. As a consequence, the jv curve looks almost as a straight line yielding a very poor fill factor (FF). In the case of the 10% cell, as capacitance data has shown, the pore filling is better, so transport resistance in the HTM is reduced and the shape of the jv curve is much improved. Similarities between the recombination resistances and behavior of capacitance in samples with 5 and 10% HTM, suggest that the extra HTM has made the spiro layer on top of TiO 2 thicker. The cell with 10% concentration presents slightly lower R rec than 5% what explains the lower V oc obtained for this sample. On the contrary, cell with 15% presents much higher values of R rec, what yields a much larger V oc than in the previous cases. The increase in observed in this case, Fig. 2(b), is the origin, together with the decrease of the series resistance (transport resistance of HTM in the pores), of the increase of FF observed. In this case, coating of the semiconductor surface goes further than some layers distance, the holes can be transported far from the electrons and screening of charge may be better. A rise of the chemical capacitance can then be observed. However, these cells show lower current than those with higher pore filling, indicating that regeneration of oxidized dyes cannot be completely achieved. Samples with 20 and 25% present full pore filling, therefore all (or most) of the dye molecules attached to the surface of TiO 2 contribute to the photogeneration, yielding to the higher j sc shown in Table I. This higher coverage of the TiO 2 surface is also responsible for the lower Fn

6 recombination resistance with respect to the 15% cell, as the interfacial contact between the transporting media is higher. Besides the lower R rec, in the 20 and 25% cells, the higher j sc, allows obtaining a higher V oc for these cells. As R rec in 25% is higher than for 20% sample and the current is almost the same, V oc is higher in the first case. CONCLUSIONS A study of the progressive improvement of the performance of solid DSC by increasing the pore filling with the hole conducting material, shows two main separate factors that control the operation of the solar cell. The first is the series resistance associated with the hole transport. The second is the reduction of recombination at higher spiro-ometad content, not only by the absolute value of the recombination resistance, but also via the charge transfer number. Both factors, reduction of transport resistance and increase of, when combined produce a progressive improvement of the fill factor, and the reduced recombination also improves the open-circuit voltage of the solar cell. At high spiro-ometad content all the dyes attached to the surface become effective for charge separation and the photocurrent becomes larger. Even with low overall efficiencies (about 2.5%) the cells with higher spiro-ometad filling show good diode shape and a flat region of collection in the jv curves. ACKNOWLEDGMENTS We are very grateful to Pablo Docampo and Henry Snaith for supplying the cells used in this study. The Group of UJI has been supported by MCIN under project Consolider HOPE CSD M.S.G. thanks Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq Brasil) for the fellowship (201516/2007-1). REFERENCES [1] H.J. Snaith and L. Schmidt-Mende, Adv. Mat. 19 (2007) [2] H.J. Snaith, R. Humphry-Baker, P. Chen, I. Cesar, S.M. Zakeeruddin and M. Grätzel, Nanotechnol. 19 (2008) [3] F. Fabregat-Santiago, J. Bisquert, L. Cevey, P. Chen, M. Wang, S.M. Zakeeruddin and M. Grätzel, J. Am. Chem. Soc. 131 (2009) 558. [4] M. Wang, P. Chen, R. Humphry-Baker, S.M. Zakeeruddin and M. Grätzel, ChemPhysChem 10 (2009) 290. [5] F. Fabregat-Santiago, J. Bisquert, E. Palomares, L. Otero, D. Kuang, S.M. Zakeeruddin and M. Grätzel, J. Phys. Chem. C 111 (2007) [6] Q. Wang, S. Ito, M. Grätzel, F. Fabregat-Santiago, I. Mora-Seró, J. Bisquert, T. Bessho and H. Imai, J. Phys. Chem. B 110 (2006) [7] T.C. Li, M.S. Góes, F. Fabregat-Santiago, J. Bisquert, P.R. Bueno, C. Prasittichaia, J.T. Hupp and T.J. Marks, J. Phys. Chem. C 113 (2009) [8] J. Bisquert, F. Fabregat-Santiago, I. Mora-Seró, G. Garcia-Belmonte and S. Giménez, J. Phys. Chem. C 113 (2009)