Article pubs.acs.org/JPCC
Injection and Ultrafast Regeneration in Dye-Sensitized Solar Cells Liisa J. Antila,† Pasi Myllyperkiö,*,† Satu Mustalahti,† Heli Lehtivuori,‡ and Jouko Korppi-Tommola† †
Department of Chemistry, Nanoscience Center, University of Jyväskylä, Box 35, Jyväskylä 40014, Finland Department of Biological and Environmental Science, Nanoscience Center, University of Jyväskylä, Box 35, Jyväskylä 40014, Finland
‡
S Supporting Information *
ABSTRACT: Injection of an electron from the excited dye molecule to the semiconductor is the initial charge separation step in dye-sensitized solar cells (DSC’s). Though the dynamics of the forward injection process has been widely studied, the results reported so far are controversial, especially for complete DSC’s. In this work, the electron injection in titanium dioxide (TiO2) films sensitized with ruthenium bipyridyl dyes N3 and N719 was studied both in neat solvent and in a typical iodide/triiodide (I−/I3−) DSC electrolyte. Transient absorption (TA) spectroscopy was used to monitor both the formation of the oxidized dye and the arrival of injected electrons to the conduction band of TiO2. Emission lifetime of the dye-sensitized films was recorded with time-correlated single photon counting to reveal nanosecond time scales of injection. It was found that the injection dynamics of the N3 and N719 dyes are similar. In solvent the injection from both dyes occurs in the femto- to picosecond time scale while in the I−/I3− electrolyte, it slows down significantly, extending to the nanosecond time domain. The presence of the electrolyte was found to increase the excited state lifetime of the dyes, implying that injection efficiency remains high despite the slower kinetics of injection compared to neat solvent. A remarkable new finding was that the prominent absorption signal of the oxidized dye observed in neat solvent vanished almost completely in the presence of the electrolyte, while the arrival of electrons to the conduction band of TiO2 was practically unaltered, only slowed down. The observed disappearance of the oxidized dye population in the I−/I3− electrolyte is most likely related to the reduction of the oxidized dye by iodide I−, which is the first step of the dye regeneration process. To the best of our knowledge, this is the first time initial dye regeneration has been shown to occur in a few picoseconds after injection.
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INTRODUCTION The initial charge separation step in dye-sensitized solar cells1,2 is the electron injection, electron transfer from the excited dye molecule to the unoccupied states of the semiconductor. Injection is the first step in the charge separation process that is responsible for current production in the DSC. This process has received wide attention among researchers.3−27 Ruthenium bipyridyl dye N3 (Ru(dcbpy)2(NCS)2, dcbpy = 4,4′-dicarboxy2,2′-bipyridine)28 and its ditetrabutylammonium salt, N719,29 are among the most successful sensitizers of DSC’s. For a long time the best DSC conversion efficiencies were obtained with N3- or N719-sensitized titanium dioxide (TiO2) combined with the iodide/triiodide redox couple. However, recently a record efficiency for a cell using a liquid electrolyte has been reported for a cell utilizing a porphyrin dye and a cobalt complex redox couple.30 The electron injection process in N3- or N719-sensitized TiO2 films extends over several orders of magnitude in time. The following model has been presented for the observed kinetics in sensitized films in solvent (Scheme 1):8,9 Excitation at the lower energy metal-to-ligand charge transfer (MLCT) band of the dye leads to the instantaneous formation of the singlet state (1MLCT). The singlet state then undergoes ultrafast electron injection with time constant of 50 fs to the © 2014 American Chemical Society
Scheme 1. Simplified Energy Diagram of N3-Sensitized TiO2 and Related Kinetic Processes (Adapted from Refs 8 and 9)
unoccupied states of TiO2 (step 1, Scheme 1). Intersystem crossing (ISC, step 2) to the triplet state (3MLCT) (time constant 70 fs) competes with the injection resulting in ∼60% of singlet excited state population undergoing injection and ∼40% ending up in the triplet state. Only weak emission from Received: December 19, 2013 Revised: March 21, 2014 Published: March 21, 2014 7772
dx.doi.org/10.1021/jp4124277 | J. Phys. Chem. C 2014, 118, 7772−7780
The Journal of Physical Chemistry C
Article
the singlet state (3) can be detected.23 The initially excited state of the dye may be strongly coupled both energetically and spatially to the unoccupied states of TiO2 enabling even sub-10 fs injection to take place from nonthermalized singlet states.23,31 Mixing of the singlet and triplet states allows ISC to compete with injection. Mixing is in fact so extensive that the triplet state can be populated directly from the ground state by exciting in the red wing of the MLCT band.16 From the triplet state the injection occurs in the picosecond time scale with time constants ranging from 1 to 100 ps (4).5,7,11,16,17,21,25 This behavior has been suggested to originate from the dye intramolecular dynamics11 and heterogeneity of the interactions between the dye molecules and the semiconductor states.5,16,21 It has also been claimed that the slow components are due to injection from weakly bound dye molecules or from dye aggregates.15,19 The injection is believed to proceed with near unity quantum yield yet weak emission (5) from the triplet state can be observed.24,32 The injection process in DSC’s has been studied with transient absorption3−22,27 (TA) and transient emission23,24,32 spectroscopies as well as with terahertz spectroscopy.25,26 In TA spectroscopy, the electron injection kinetics is traced by monitoring the oxidized dye absorption in the near-infrared3−19 (near-IR) or the mid-infrared (mid-IR) absorption of the electrons injected to the TiO2 conduction band.20−22,27 Most of the studies have been carried out on sensitized TiO2 films in solvent while the injection kinetics in the presence of the I−/I3− containing DSC electrolyte has received much less attention.14,19,22,24,25,27,32 The reports on the subject have been rather controversial. Teuscher et al. claim that the injection in a complete DSC with I−/I3− electrolyte takes place mostly in the femtosecond time scale.19 Haque et al. state that femtosecond injection is insignificant and picosecond injection dominates.14 Juozapavicius and co-workers found that injection extends from hundreds of femtoseconds to subnanosecond time domain.22,27 Another dye-related process that takes place in the presence of electrolyte is the reduction of the oxidized dye by the iodide ion, i.e., dye regeneration. The regeneration process has been suggested to proceed via the following steps:33−35 Iodide ion (I−) from the electrolyte attaches to the thiocyanate (NCS) ligand of the dye and reduces the oxidized dye molecule: D+ + I− → (D···I)
results supporting the formation of a (D···I) complex for Ru(dcbpy)2(CN)2.37 Formation of a stable (D···I2) complex in N3-sensitized TiO2 films in contact with iodine containing electrolyte has been observed both computationally and experimentally.33 N3···I2 complex is a stable compound, which has been crystallized from N3 and iodine-containing methanol solution, and its X-ray structure has been determined.38 To better understand the processes taking place in sensitized TiO2 films in the presence of electrolyte, we studied N3 and N719 on TiO2 films with femto- to nanosecond TA spectroscopy and probed both the formation of oxidized dye and arrival of injected electrons to the TiO2 conduction band. The measurements were first carried out for sensitized TiO2 films in neat solvent and then solvent was replaced the I−/I3− electrolyte. Time-correlated single photon counting (TCSPC) experiments in the subnano- to nanosecond time scale were performed on a separate but similar set of sensitized films. The near- and mid-IR TA signals of N3 and N719 dyes turned out to be similar. It was found that while electron injection in neat solvent takes place mostly in the femto- to picosecond time scale, in the presence of electrolyte it extends from the femtoto nanosecond time scale. However, in the presence of electrolyte the picosecond rise of the oxidized dye signal was negligible compared to the TiO2 electron signal. This indicates the presence of a process in addition to injection and excited state decay that affects the concentration of the oxidized dye. This process is most probably the initial step in the dye regeneration process, depicted in eq 1.
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EXPERIMENTAL METHODS Materials and Sample Preparation. TiO2 and Al2O3 films (∼3 μm) were prepared on CaF2 windows or microscope glass slides via doctor-blading (Solaronix Nanoxide-T or homemade Al2O3 colloid) and annealing at 450 °C for 30 min. The Al2O3 colloid was prepared by a previously reported method28 using Degussa Aluminum Oxide C as a starting material. Films were kept overnight in 0.3 mM ethanolic solution of N3 (Dyesol) or N719 (Solaronix). The optical density of the films after sensitization was ∼1 at the excitation wavelength. Upon removal from dye solution, the films were rinsed with absolute ethanol and dried under nitrogen flow. For TA experiments the CaF2 window with the sensitized film was assembled into a flow type cell39 by using a Surlyn frame as a spacer (25 μm) and another CaF2 window with two small holes for liquid injection. The cell was placed in a brass casing equipped with two Teflon tubes through which the liquid could be introduced or replaced in the cell without opening the casing. For TCSPC experiments the sensitized films on microscope glass slides were glued to a counter glass with a Surlyn frame, and the cell was removed from the brass casing after filling with the solvent or the electrolyte. The holes in the counter glass were sealed with Surlyn during the measurement. The measurements of the sensitized films were carried out first with the films in contact with the electrolyte solvent (3methoxypropionitrile, MPN, Aldrich) and then in contact with the full iodide/triiodide redox electrolyte. The electrolyte contained 0.6 M 3-propyl-1-methylimidazolium iodide (PMII, Solaronix), 0.1 M anhydrous LiI (Fluka), 0.05 M I2 (Aldrich), and 0.5 M tert-butylpyridine (TBP, Aldrich) in MPN. To minimize the water content of the electrolyte, MPN was dried by distillation and anhydrous LiI handled in argon glovebox. Steady-state absorption spectra of the films were recorded
(1)
Another iodide ion reacts with the dye−I complex and diiodine radical (I2−) is released to the electrolyte: (D···I) + I− → D + I 2−
(2)
The diiodine ion undergoes disproportionation36 2I 2− → I3− + I−
(3) 33
or it can reduce another oxidized dye molecule complex D+ + I 2− → (D···I 2)
and form a (4)
−
Upon encountering a further I from the electrolyte, complexed iodine can be released to the electrolyte as triiodide (I3−): (D···I 2) + I− → D + I3−
(5)
The feasibility of steps 1, 2, and 4 has been shown in ab initio molecular dynamics simulations.33 Clifford et al. have reported 7773
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Figure 1. Excited-state absorption and oxidized dye formation after exciting the MLCT band at 540 nm and probing in the near-IR (A) at 860 nm N3-sensitized and (B) at 820 nm N719-sensitized TiO2 films. Squares refer to films in MPN and circles to films in I−/I3− electrolyte. Also shown in (A) is N3 on Al2O3 in MPN (triangles) and in (B) N719 on Al2O3 in I−/I3− electrolyte (diamonds) that exhibit the time evolution of excited-state absorption of a noninjecting sample. Solid lines are fits to the experimental data that are normalized to unity at 200 fs.
absorbed photons. All measurements were carried out at room temperature and ambient conditions. Analysis of the timeresolved data was performed using multiple exponential decay schemes in a least-squares fitting program.
before and after the measurements to ensure good quality of the sample. Spectroscopic Measurements. TA was studied in two different laser setups with different probe wavelengths. An integrated one-box Ti:sapphire laser (Quantronix Integra C) was used to pump both setups. In setup I home-built nonlinear optical parametric amplifier (NOPA) was used to produce the 540 nm pump pulses. White light continuum generated in a sapphire crystal was used for probing. After the sample, the probe and the reference beams entered a monochromator (Acton, SpectraPro SP-150i, 300 grooves/mm), and the outcoming 8 nm bands were detected with a pair of photodiodes. In setup II, a traveling-wave optical parametric amplifier of superfluorescence (TOPAS, LightConversion, Inc.) was used to produce 540 nm pump pulses. The mid-IR probe pulses centered at 1990 cm−1 were generated by using a homebuilt double pass optical parametric crystal in combination with difference frequency generation in an AgGaS2 crystal. Setup II is described in more detail in ref 40. The time resolutions of the TA experiments were determined to be ∼200−250 fs. In both experiments an excitation intensity of 3 × 1014 photons/(cm2 pulse) was used. Samples were stationary during the collection of a single time trace and translated between scans to obtain each trace from a fresh spot of the sample. This procedure yielded similar traces as continuous sample translation indicating that no sample degradation took place during a single scan. Better signal-to-noise ratio of the time traces was achieved from measurements on stationary samples. Emission decays of the samples in the sub-nanosecond and nanosecond time scales were measured using a time-correlated single photon counting (TCSPC) system consisting of a HydraHarp 400 controller and a PDL 800-B driver (PicoQuant GmbH). The samples were excited at 483 nm (spectral fwhm 4 nm) with a diode laser head LDH-P-C-485 at a repetition frequency of 5 MHz driven by the PDL 800-B control unit. The excitation intensity was 4 × 107 photons/(cm2 pulse). Long pass filters were used to detect the emission above 670 nm with a single photon avalanche photodiode (SPAD, MPD-1CTC). The time resolution of the experiment was determined to be approximately 140 ps (fwhm of the instrument response function (IRF)). Several similar samples were measured, and the fluorescence intensity was normalized to the number of
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RESULTS Femtosecond to nanosecond transient absorption (TA) measurements were carried out by pumping the MLCT bands of the dyes and probing the oxidized dye formation in the near-IR and arrival of the electrons to the TiO2 CB in the mid-IR. The same N3- or N719-sensitized films, first in contact with the solvent and then with the electrolyte, were measured at both probe wavelength regions. Steady-state absorption spectra of the samples showed no degradation taking place during the measurements. Solvent covered N3- and N719-sensitized films were first measured by probing the excited and oxidized state of the dye in the near-IR. After excitation at 540 nm the time evolution of the TA signals (squares in Figure 1) at 860 nm and at 820 nm for N3- and N719-sensitized TiO2 films, respectively, turned out to be almost identical. The data were fitted with a sum of two exponentials convoluted with a Gaussian instrument response (Table 1). Sensitized Al2O3 films were used as reference to obtain the dye excited state absorption response Table 1. Time Constants (τ) and Relative Amplitudes (in Percentages of the Total Amplitude) Obtained from Fitting of the TA Signals in Figure 1 τ1 (A1) in neat solvent N3 at 860 nm N719 at 820 nm N3 at 1960 cm−1 N719 at 1960 cm−1 in I−/I3− electrolyte N3 at 860 nm N719 at 820 nm N3 at 1960 cm−1 N719 at 1960 cm−1 7774
τ2 (A2)
τ3 (A3)
