J. Phys. Chem. C 2010, 114, 11903–11910
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Surface Molecular Quantification and Photoelectrochemical Characterization of Mixed Organic Dye and Coadsorbent Layers on TiO2 for Dye-Sensitized Solar Cells Tannia Marinado,† Maria Hahlin,‡ Xiao Jiang,† Maria Quintana,† Erik M. J. Johansson,‡,§ Erik Gabrielsson,| Stefan Plogmaker,‡ Daniel P. Hagberg,| Gerrit Boschloo,§ Shaik M. Zakeeruddin,⊥ Michael Grätzel,⊥ Hans Siegbahn,‡ Licheng Sun,| Anders Hagfeldt,†,§ and Håkan Rensmo*,‡ Inorganic Chemistry, Center of Molecular DeVices, Chemical Science and Engineering, Royal Institute of Technology, 100 44 Stockholm, Sweden, Department of Physics and Astronomy, Uppsala UniVersity, Box 516, SE-75120, Uppsala, Sweden, Organic Chemistry, Center of Molecular DeVices, Chemical Science and Engineering, Royal Institute of Technology, 100 44 Stockholm, Sweden, Laboratoire de Photonique et Interfaces, Institut des Sciences et Inge´nieurie Chimiques, Ecole Polytechnique Fe´de´rale de Lausanne, Station 6, CH-1015, Lausanne, Switzerland, and Department of Physical and Analytical Chemistry, Uppsala UniVersity, Box 259, 75105, Uppsala, Sweden ReceiVed: March 16, 2010
Different molecular layers on TiO2 were prepared by using the p-dimethylaniline triphenylamine based organic dye, D29, together with the coadsorbents decylphosphonic acid (DPA), dineohexyl bis(3,3-dimethylbutyl)phosphinic acid (DINHOP), and chenodeoxycholic acid (CDCA). The surface molecular structure of dye and coadsorbent layers on TiO2 was investigated by photoelectron spectroscopy (PES). A focus was to determine the surface molecular concentrations using characteristic photoelectron core levels. Dye-sensitized solar cells (DSCs) were prepared from the same substrate and were further characterized by photoelectrochemical methods. Together the investigation gives information on the arrangement of the mixed molecular layer and a first insight to the extent to which the coadsorbents exchange with dye molecules on the TiO2 surface for the examined conditions. Introduction The dye-sensitized solar cell (DSC) is a promising, efficient, low-cost molecular photovoltaic device. The function of the DSCs originates from the charge transfer reactions that follow from light absorption at a molecular interface between an oxide and an electrolyte. In the photocurrent generation process photons are absorbed in a dye molecule, which is followed by a fast injection of electrons from the dye to the conduction band in the oxide.1,2 The dye is regenerated through an electron transfer from the redox couple in the electrolyte to the oxidized dye. Much research effort has been invested into the understanding of the complex interplay of the dye/electrolyte/semiconductor interface of the DSCs.2,3 The sensitizing dye is central in DSCs, and the use of conventional ruthenium based dyes has reached promising photovoltaic efficiencies of 11-12%.1,4-8 However, the use of metal-free organic dyes has lately become attractive.9-17 This is partly due to facile dye structural modification, high absorption coefficients and reduced material cost. In many studies it has also been shown that the photovoltaic efficiencies of DSCs based on novel organic dyes can be improved by the employment of coadsorbents in the dye bath solution.18-22 * Corresponding author. Address: Ångstro¨mlaboratoriet, La¨gerhyddsv. 1, Box 259, 751 20 Uppsala, Sweden. Tel: 018-471 35 47. E-mail:
[email protected]. † Inorganic Chemistry, Center of Molecular Devices, Chemical Science and Engineering, Royal Institute of Technology. ‡ Department of Physics and Materials Science, Uppsala University. § Department of Physical and Analytical Chemistry, Uppsala University. | Organic Chemistry, Center of Molecular Devices, Chemical Science and Engineering, Royal Institute of Technology. ⊥ Ecole Polytechnique Fe´de´rale de Lausanne.
Although changes in the DSC functional properties due to the presence of coadsorbents can be measured, difficulties in identifying and quantifying the coadsorbents on a sensitized surface partly limit further understanding of the interfacial properties. The function of the molecules at the interface will depend on the inherent properties of the individual component. However, the dye-coadsorbent molecular structure (including surface coverage) as well as the electronic structure of the molecular layer formed during the sensitization process may vary as a consequence of dye, dye solution, electrolyte and the effect of different coadsorbents. PES (photoelectron spectroscopy) is a technique that can be used to gain information on surface molecular and electronic structure.23-33 Specifically, the technique can give element specific information that can be used to measure relative surface concentrations of a mixed molecular layer of for example dyes and coadsorbents adsorbed on a TiO2 surface. The triphenylamine (TPA) based organic dye chosen in the present study, D29, has good spectral properties due to the substituted TPA in the p-position with additional phenyl groups, functionalized with p-N,N-dimethylanilinyl units, see Figure 1. DSCs based on the D29 dye have previously shown respectable efficiencies when used together with the coadsorbent (CDCA).34 However, it was also observed that a possible drawback was mediocre electron lifetimes, in particular due to recombination to the redox electrolyte, limiting the photovoltaic performance to some extent. The present investigation will focus on the effects from different coadsorbents by investigating films sensitized with solutions containing D29 as well as similar solutions also containing the coadsorbents decylphosphonic acid
10.1021/jp102381x 2010 American Chemical Society Published on Web 06/17/2010
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J. Phys. Chem. C, Vol. 114, No. 27, 2010
Figure 1. Molecular structure of organic dye D29.
(DPA), dineohexyl bis(3,3-dimethylbutyl)phosphinic acid (DINHOP) and chenodeoxycholic acid (CDCA), see Figure 2.18,22 The sensitized electrodes are investigated by PES, and an important part of the present study is to show how this technique can be used to obtain quantitative information about the mixed molecular surface concentrations of a dye-sensitized TiO2 surface. In combination with photoelectrochemical characterization of analogous DSC samples the study allows new molecular level understanding of the functional properties of the dye/ coadsobent/TiO2 interface. Experimental Section Sample Preparation. The Preparation of Working Electrodes and Dye-Sensitized Solar Cells. Fluorine-doped tin oxide (FTO) glass plates (Pilkington-TEC8) were cleaned in detergent solution, water and ethanol using an ultrasonic bath. The FTO substrates were immersed into 40 mM aqueous TiCl4 solution at 70 °C for 30 min and washed with water and ethanol. The screen printing procedure was repeated with TiO2 paste to obtain 4 µm thick nanocrystalline films and an area of 0.32 cm2. The preparation of TiO2 paste (∼25 nm colloidal particles) is described elsewhere.35 The TiO2 electrodes were sintered at 500 °C, for 60 min. Nine solutions were prepared, four containing dye, [0.2 mM D29], [0.2 mM D29 and 0.05 mM DPA], [0.2 mM D29 and 0.05 mM DINHOP], [0.2 mM D29 and 6 mM CDCA] and five samples containing only coadsobents (no dye), [0.05 mM DPA], [5 µM DPA], [0.05 mM DINHOP], [5 µM DINHOP] and [6 mM CDCA] dissolved in ethanol solution. The electrodes were immersed in respective solution for 16-17 h in the dark at room temperature. All solvents and chemicals were used without further purification. The electrode were thereafter rinsed in ethanol and subsequently dried in air flow. The electrodes were used for the PES measurements, and identical electrodes were used for DSCs assembly. The DSCs used for photovoltaic measurements consisted of a dye-sensitized working electrode, 50 µm Surlyn frame used as a hot melt sealant, a liquid electrolyte and a counter electrode of platinized FTO. The area of the TiO2 film electrodes was 0.32 cm2. The electrolyte
Marinado et al. consisted of 0.6 M tetrabutylammonium iodide (TBAI), 0.1 M lithium iodide (LiI), 0.05 M iodine (I2), 0.05 M guanidinium thiocyanate (GuSCN), 0.5 M 4-tert-butylpyridine (4-TBP) in acetonitrile. Synthesis of the Dye. The synthesis of the D29 dye was presented in our previous work.34 The coadsorbents DPA and CDCA were purchased from Lancaster and Sigma Aldrich, respectively, and were used without any further purification. DINHOP was synthesized as described previously. Experimental Techniques. The photoelectron spectroscopy (PES) measurements were performed with an in-house ESCA 300 spectrometer, using monochromated Al KR radiation (1486.7 eV). The electron takeoff angle was 90°. The PES spectra are energy calibrated by setting the Ti2p substrate signal to 458.56 eV.27 Charging and radiation effects were checked for by measuring the specific core level repetitively and were found negligible for all spectra reported in the present investigation. Small amounts of surface adsorbed species are always present on ex situ prepared samples. However, according to the molecular structure the relative number of sulfur (S) versus carbon (C) is 1:42, see Figure 1, and this value is close to the experimentally observed value of 1:41, see Table 2, showing that the amount of contaminants on the TiO2 surface is small relative to the amount of dye molecules. For photovoltaic measurements, a xenon lamp with a Schott Tempax 113 filter was used to simulate sunlight (AM 1.5) and was calibrated to 1000 W/m2 with a reference silicon diode. The DSCs were masked to 0.45 cm2, a mask somewhat larger than the active surface area, to capture both the direct and the diffuse light.36 Monochromatic incident photon to current conversion efficiency (IPCE) was recorded using a computerized setup consisting of Keithley 2400 source/meter as a current meter and a xenon arc lamp (300 W Cermax, ILC Technology), followed by a 1/8 m monochromator (CVI Digikrom CM 110) as a light source. Electron lifetimes and charge extraction measurements of the complete dye-sensitized solar cell devices were performed using a green-light emitting diode (Luxeon K2 Star 5W, λmax ) 530 nm) as light source. Voltage and current traces were recorded with a 16-bit resolution data acquisition board (DAQ National Instruments) in combination with a current amplifier (Autolab PGSTAT12). Electron lifetimes were determined by monitoring the transient voltage responses after a small light intensity modulation (square wave modulation), recording the step response with a DAQ board. The voltage response was well fitted to a first-order decay, and time constants were thus obtained. Charge extraction measurements were performed in the following way: the solar cell was biased in the dark (5 s) and set to open-circuit conditions, the cell was then shortcircuited and the current was measured under 10 s and then integrated to obtain the accumulated charge, Qoc (V). For the transient absorption measurements a laser flash photolysis setup was used. A Surelite Nd:YAG laser at 10 Hz,
Figure 2. The molecular structure of the coadsorbents used in the present study: (left) DPA, (center) DINHOP, and (right) CDCA.
Mixed Organic Dye and Coadsorbent Layers on TiO2
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TABLE 1: The Intensities of the Measured S2p, P2p and Ti2p Signalsa S2p Int P2p Int Ti2p Int mean thickness (Å)
D29
D29/DPA
D29/DINHOP
D29/CDCA
169
79 197 11463 6.03
127 31 9368 9.67
178
8967 11.5
6604 16.0
DPA
DINHOP
CDCA
DPA(low)
TiO2
301 12057 5.12
34 14630 1.64
12028 5.17
228 13825 2.66
16027 0
a The intensities of the S2p signal can directly be correlated to the relative amount of D29 on the different surfaces, and the intensity of the P2p signal can be correlated to the amount of DPA/DINHOP on the different surfaces. Also in the table is the calculated mean thickness (in Å) of the adsorbed layer.
with a pulse length of about 1 ns, was used together with a Continuum Surelite OPO plus to obtain the pump pulse at 456 nm and a pulse length
