Article pubs.acs.org/JPCC
Combining a Small Hole-Conductor Molecule for Efficient Dye Regeneration and a Hole-Conducting Polymer in a Solid-State DyeSensitized Solar Cell Erik M. J. Johansson,*,† Lei Yang,† Erik Gabrielsson,‡ Peter W. Lohse,† Gerrit Boschloo,† Licheng Sun,‡ and Anders Hagfeldt† †
Physical Chemistry, Department of Chemistry - Ångström, Uppsala University, SE-751 20 Uppsala, Sweden Organic Chemistry, School of Chemical Science and Engineering, Royal Institute of Technology, Stockholm, Sweden
‡
S Supporting Information *
ABSTRACT: In dye-sensitized solar cells (DSC) an efficient transfer of holes from the oxidized dye to the contact is necessary, which in solid-state DSC is performed by hole-conductor molecules. In this report we use photoinduced absorption and transient absorption spectroscopy to show that a small hole-conducting molecule, tris(p-anisyl)amine, regenerates dye molecules in the pores of the dye-sensitized TiO2 nanoparticle electrode efficiently even for thick (>5 μm) electrodes. For similar thicknesses we observe incomplete regeneration using a larger polymer hole-conductor. However, the performance of the solar cells with the small hole-conductor molecules is poor due to that inefficient hole conduction in these small molecules may limit the collection of the charges at the contacts. Polymer hole-conductors, which may have a good hole conductivity, also have a high molecular weight, which makes these polymers difficult to infiltrate into the smallest pores in the electrode. We show that a conducting polymer, P3HT, may be added to the small molecule hole-conductor, to enable better transport of the charges to the contact and to reduce recombination and therefore increase the photocurrent. This new device construction with a small molecule efficiently regenerating the dye molecules, and a polymer conducting the holes to the contact is therefore a promising pathway for solid-state dye-sensitized solar cells.
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INTRODUCTION The nanostructured dye-sensitized solar cell (DSC) is a promising alternative to conventional inorganic solar cells. These solar cells have shown efficiencies over 12% using a dyesensitized nanostructured TiO2 film in combination with a redox couple in a liquid electrolyte.1−4 In the solar cell dye molecules absorb the light, and the excited electrons are injected into the TiO2. The electrons are thereafter transported through the mesoporous TiO2 electrode to one of the contacts. The redox couple in the liquid electrolyte regenerates the oxidized dye molecules and transfers the holes to the back contact. The liquid electrolyte may be replaced with a solid material, such as spiro-OMeTAD (2,2′,7,7′-tetrakis(N,N-dimethoxyphenylamine)-9,9′-spirobifluorene), 5−18 or conducting polymers18−33 to make a solid state DSC. This may have practical advantages compared to the liquid-electrolyte-based DSC. Although the highest energy conversion efficiency of solidstate DSC (sDSC) currently is lower than that in liquid electrolyte DSCs, it has recently been improved to over 7%.6 In the sDSC a thinner mesoporous TiO2 layer is needed in comparison with liquid electrolyte DSC due to the shorter electron diffusion length8−10 and possibly due to problems of infiltrating the hole-conductor molecules in the porous © 2012 American Chemical Society
nanoparticle electrode. The optimum thickness for the most efficient sDSCs is therefore about 2 μm. Many studies have been made on infiltration of molecular materials in mesoporous TiO2.10,18,19,27,33−40 For conducting polymers it has been shown that the infiltration is dependent on the molecular weight of the polymer, and high molecular weight limits the infiltration of the conducting polymer into the mesoporous system of nanoparticles.34 Regeneration of all the oxidized dye molecules in the porous system after light absorption and electron injection is therefore difficult using conducting polymers, whereas it is better by using smaller holeconductors.18 At the same time, the conductivity of the polymer may be advantageous for fast transport of the holes to the contact without recombining with the electrons in the TiO2 nanoparticles. New hole-conductor molecules are therefore developed to obtain higher energy conversion efficiency in the sDSC.41−43 It has also been shown that small liquid holeconductor molecules may work in the DSC.44 In this report, we show that a small solid molecular holeconductor, tris(p-anisyl)amine (TPAA), can efficiently regenReceived: May 30, 2012 Revised: July 26, 2012 Published: August 6, 2012 18070
dx.doi.org/10.1021/jp3052449 | J. Phys. Chem. C 2012, 116, 18070−18078
The Journal of Physical Chemistry C
Article
erate dye molecules and be infiltrated into a thick electrode of nanoparticles. We also investigate how an addition of a conducting polymer to the small hole-conductor affects the efficiency, recombination, and charge transport properties in the solar cell.
Scheme 1. Sketch of the Solar Cell Structure with the Different Layers of Materials for the Device with P3HT (Symbolized by Purple Lines) Added to the TPAA HoleConductor (Symbolized by Green-Gray Color)a
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EXPERIMENTAL SECTION Sample Preparation. For the photoinduced absorption (PIA) and nanosecond transient absorption spectroscopy (TAS) measurements samples with a mesoporous TiO2 layer thickness of about 5 μm, as measured with a DekTak profilometer, were prepared by screen-printing the same colloidal TiO2 paste (Dyesol DSL 18NR-T) on FTO glass. After sintering the TiO2 film on a hot plate at 450 °C for 30 min, the film was cooled to room temperature and sensitized with the dye K77.45 The tris(p-anisyl)amine (TPAA) holeconductor was synthesized according to the literature procedure.46 A solution of TPAA (60 mg/mL) was spin-coated (2000 rpm, 30 s) on glass and also on the dye-sensitized TiO2 film. For samples with TPAA and P3HT additive, a solution of P3HT (Sigma-Aldrich, MW: 15 000−45 000), 20 mg/mL in chlorobenzene, was prepared. 50 μL of each the TPAA and the P3HT solutions was mixed to obtain the solution with P3HT added to TPAA. This solution was then spin-coated on a glass substrate and also on the dye-sensitized TiO2 film. Also, a sample of only the P3HT solution was prepared by spin-coating on glass. For the solar cell samples a compact TiO2 blocking layer was first deposited onto the surface of precleaned FTO substrate by spray pyrolysis on a hot plate at 450 °C using an air brush at a distance of 5 cm, and the thickness was controlled by the number of spray cycles. The solution used in the spray pyrolysis was 0.2 M Ti-isopropoxide and 2 M acetylaceton in isopropanol. Five spray cycles were used as standard parameter. Mesoporous TiO2 films were prepared on the compact TiO2 layer by spin-coating of the colloidal TiO2 paste diluted in terpineol in 42% weight ratio. A spin-coating rate of 2500 rpm for 30 s was used to obtain about 2 μm thick mesoporous film, as measured with a DekTak profilometer. After sintering the TiO2 film on a hot plate at 450 °C for 30 min, the film was cooled to room temperature and immersed in 0.02 M aqueous TiCl4 at 70 °C for 30 min. The film was then rinsed with deionized water and annealed on a hot plate at 500 °C for 30 min. After cooling to 90 °C, the film was inserted into the dye bath with the dye K77, and the sample was sensitized overnight. For the solar cell samples with TPAA as hole-conductor, a solution of TPAA (60 mg/mL), 60 mM 4-tert-butylpyridine and 15 mM LiN(CF3SO2)2 in chlorobenzene was applied to the films by leaving the solution to penetrate into the films for 1 min and then spin-coated for 30 s with 2000 rpm. For samples with TPAA and P3HT additive, a solution of P3HT, 20 mg/mL chlorobenzene, was prepared. 50 μL of each the TPAA (with additives) and the P3HT solutions was mixed to obtain the solution with P3HT added to TPAA. The solution afterward was applied to the films and after waiting 1 min spin-coated for 30 s with 2000 rpm. Finally, a 100 nm thick Ag contact (0.4 × 0.5 cm2) was deposited onto the organic semiconductor by thermal evaporation in a vacuum chamber (Leica EM MED020) with a base pressure of about 10−5 mbar to complete the cell. For a sketch of the solar cell structure see Scheme 1. PIA measurements were also performed on these solar cell samples (without silver contact).
a
In the solar cell devices the hole-conductor layer also contained the additives 4-tert-butylpyridine and LiN(CF3SO2)2.
UV−vis Spectroscopy. UV−vis absorption spectra of the dye-sensitized TiO2 films (5 μm thick TiO2) and films with P3HT and TPAA (thickness of about 50 nm) on conducting glass (SnO2:F on glass) were recorded on an HR-2000 Ocean Optics fiber-optics spectrophotometer. For the absorption spectra shown in the figures, the conducting glass background was subtracted. Photoinduced Absorption Spectroscopy (PIA). PIA measurements were carried out on samples, prepared as described above, for the TiO2 layer thickness of about 5 and 2 μm, without the Ag contact. Also, dye-sensitized samples without hole-conductor were investigated. PIA spectra were recorded on the homemade setup as reported previously.47,48 White probe light generated by a 20 W tungsten−halogen lamp was superimposed with a square-wave modulated (on−off) green LED (Lasermate, GML 532-100FLE, 532 nm) or a red laser (Coherent, 632 nm) used for excitation. The transmitted probe light was focused onto a monochromator (Action Research Corp. SP-150) and detected by a UV-enhanced silicon photodiode connected to a current amplifier and lock-in amplifier (Stanford Research System models RS570 and RS830, respectively). The intensity of 6.1 mW cm−2 and a modulation frequency of 9.3 Hz were used for the excitation LED. The spectra in the figures were intensity normalized to simplify comparison of spectral shapes. Incident Photon to Current Conversion Efficiency (IPCE). IPCE spectra were recorded on a computer-controlled setup comprised of a xenon lamp (Spectral Products ASB-XE175), a monochromator (Spectral Products CM110), and a potentiostat (EG&G PAR 273). The setup was calibrated with a certified silicon solar cell (Fraunhofer ISE) prior to measurements. All sDSC samples were illuminated from glass side with an aperture area of 0.16 cm2 (0.4 × 0.4 cm2). Photocurrent Density−Voltage Measurement. The light source of solar simulator for measuring current−voltage characteristics was a 300 W collimated xenon lamp (Newport) calibrated with the light intensity to 1000 W m−2 at 1.5 AM global condition by a certified silicon solar cell (Fraunhofer ISE). Electrical data were recorded on a computer controlled by a digital sourcemeter (Keithley Model 2400) with the scan direction from open-circuit to short-circuit at 50 mV/s. The prepared sDSC samples were masked during the measurement with an aperture area of 0.20 cm2 (0.4 × 0.5 cm2) exposed under illumination. 18071
dx.doi.org/10.1021/jp3052449 | J. Phys. Chem. C 2012, 116, 18070−18078
The Journal of Physical Chemistry C
Article
Regeneration of the Dye Molecules. In the sDSC, the dye molecules are regenerated by the hole-conductor after electron injection. For an efficient regeneration the holeconductor should infiltrate the mesoporous TiO2 electrode to be able to regenerate the dye molecules. As described in the Introduction, this may be difficult using large hole-conductor molecules such as conducting polymers. Therefore, a small hole-conductor molecule might be better to regenerate the dye molecules, and in this report we therefore chose to use the small hole-conductor molecule TPAA (see Figure 1). In this section we use photoinduced absorption spectroscopy (PIA) to measure if the dye molecules are regenerated18,47,48 by the holeconductor molecule TPAA and compare with the regeneration by the P3HT polymer. In the measurements described below we use a relatively thick mesoporous TiO2 electrode (5.4 μm) to specifically probe if the hole-conductor infiltration is sufficient also for these thick electrodes. The results for thinner electrodes (2 μm) show a similar trend, and these results are presented in the Supporting Information. In Figure 2, the PIA spectra of the TiO2/K77, TiO2/K77/TPAA, TiO2/P3HT, and TiO2/K77/P3HT samples with the 5.4 μm TiO2 layer are shown.
Electron Lifetime and Light Intensity Dependence Measurement. Electron lifetime as a function of light intensity were measured by the custom-made “toolbox setup” using a white LED (Luxeon Star 1W) as light source to provide the base light intensity. The transient voltage and current response of the cells were recorded by using a 16-bit resolution digital acquisition board (National Instruments) in combination with a current amplifier (Stanford Research Systems RS570) and a homemade electromagnetic switching system. By superimposing the base light with a small square wave modulation (
