Effect of Electrolyte Composition on Electron ... - ACS Publications

Article pubs.acs.org/JPCC

Effect of Electrolyte Composition on Electron Injection and Dye Regeneration Dynamics in Complete Organic Dye Sensitized Solar Cells Probed by Time-Resolved Laser Spectroscopy Marcin Ziółek,*,†,‡ Cristina Martín,‡ Licheng Sun,§ and Abderrazzak Douhal‡ †

Quantum Electronics Laboratory, Faculty of Physics, Adam Mickiewicz University, Umultowska 85, 61-614 Poznan, Poland Departamento de Química Física, Facultad de Ciencias Ambientales y Bioquímica, and Inamol, Universidad de Castilla-La Mancha, Avda. Carlos III, S.N., 45071 Toledo, Spain § School of Chemical Science and Engineering, Department of Chemistry, Royal Institute of Technology (KTH), 10044 Stockholm, Sweden ‡

S Supporting Information *

ABSTRACT: Femtosecond time-gated fluorescence and nanosecond flash photolysis studies of seven complete, real titania nanoparticle solar cells sensitized with an efficient organic dye (TH305) were performed in order to investigate the role of the electrolyte composition on the charge transfer dynamics. The electron injection rate constants were found to range from 0.4 to 3.5 ps−1 in iodide-based electrolyte, and they well correlate with the shift of the conduction band edge potential of titania. The lithium cation additives resulted in 2 times faster electron injection rate constant (3.55 ps−1) with respect to that when larger sodium cations were used (1.86 ps−1). However, in the presence of a pyridine derivative component in the electrolyte solution, the electron injection rate constant decreased several times (0.38 ps−1 for Li+ and 0.54 ps−1 for Na+), while the electron injection efficiency was found to be still very high, 96−100%. The dye regeneration by the redox couple under relatively low fluence of excitation beam (0.4 mJ/cm2 giving about 4 electrons per titania nanoparticle) proceeds with an average rate constant of about 40 × 103 s−1 and efficiency close to 100%, independent of the electron composition. However, for a larger fluence (2 mJ/cm2) excitation, a titania−dye electron recombination process competes with the dye regeneration and lowers the solar cell efficiency. The effect of self-quenching, high vibrational levels of the dye excited state, and the neat solvent on the electron injection process are also discussed. This study clearly shows that for TH350-based DSSCs the best performance is obtained using Li+ and TBP as additives to the iodide electrolyte, giving the highest open circuit voltage and almost 100% efficiency of electron injection and dye regeneration. sensitizer on titania nanoparticle film exceeding 12% efficiency.9 Nevertheless, even though there has been a growing interest in the optimization of the different components of DSSCs, improving the energy conversion efficiency and stability of the cells is still an important challenge. In particular, there is a need for more time-resolved optical laser spectroscopy studies that directly probe the fast and ultrafast processes, especially in complete solar cells and for pure organic sensitizers. Such processes cannot be investigated by other electro-optical methods due to the temporal limitations. Moreover, as it has been recently revealed, the obtained time constants and quantum yields of the involved events in complete, working devices (with both electrodes, functioning electrolyte, etc.) may significantly differ from those assumed previously from the studies of separated, isolated components.10,11

1. INTRODUCTION The growth of the energy demand and, at the same time, the depletion of the energy resources in the World are encouraging toward the study of new alternatives for energy production. Among these, conversion of sunlight into electric power by solar cells is a good alternative for the energy production. More specifically, dye-sensitized solar cells (DSCCs) act as a promising solar converter being simpler and cheaper and having better efficiency at low light levels than the conventional photovoltaic devices.1−4 In DSSCs, after the light irradiation, the excited electrons are injected into the conduction band of the semiconductor oxide, and the formed dye-cations are regenerated by the redox couple.5,6 However, when the electron regeneration process is not fast enough, the oxidized dye tends to recombine with the electrons injected, in such a way to reduce the efficiency of the solar cell.5,6 So far, the DSSC energy conversion efficiency has reached a value close to 12% using ruthenium complexes7,8 and, very recently, a step forward was made using a porphyrin-based © 2012 American Chemical Society

Received: October 3, 2012 Revised: November 16, 2012 Published: November 29, 2012 26227

dx.doi.org/10.1021/jp3097988 | J. Phys. Chem. C 2012, 116, 26227−26238

The Journal of Physical Chemistry C

Article

2. EXPERIMENTAL SECTION TH305 dye (Scheme 1) was synthesized as previously described.14 Acetonitrile (ACN, spectroscopic grade >99.5%, Aldrich) and dichloromethane (DCM, spectroscopic grade >99.5%, Aldrich) were used as received. To make the TiO2 electrodes, Ti-Nanoxide HT paste (Solaronix SA) was employed. For the reference electrodes, the alumina paste was obtained by mixing 1 g of Al2O3 nanoparticles (d < 50 nm, Sigma Aldrich) dispersed in a solution containing 100 mL of ethanol, 1 mL of distilled water, and 1 mL of acetic acid. With the aim to obtain a homogeneous solution, the suspension was kept in an ultrasonic bath for 24 h. Then, 1.6 g of ethyl cellulose and 10 mL of α-terpineol were added into the solution (under stirring conditions and a temperature of 60 °C). In order to get the desired viscosity, the excess of ethanol was removed with a rotary evaporator. A conductive glass sheet (FTO) of 2 × 3 cm2 was first cleaned in a detergent with ion-exchanged water using an ultrasonic bath for 10 min, followed by thorough washing with water, and finally rinsing with ethanol. Then, the glass plates were immersed in isopropanol and sonicated for 30 min. After heating (400 °C) for 15 min, a layer of paste was coated on the FTO electrode using the doctor blade technique between two parallel adhesive Scotch tapes. Finally, the film on the glass was gradually heated under airflow up to 450 °C and kept at that temperature for 1 h to fabricate a TiO2 layer of about 5 μm thickness. Once the electrode was cooled to about 80 °C, the conductive glass with the titania or alumina film was immersed in a 5.50 × 10−5 M solution of TH305 in DCM, and kept at room temperature for 1−2 h to ensure optimum sensitizer uptake (absorbance of around 0.6 in the maximum of the TH305 band for the titania sample). No major photodegration was detected during the measurements. The platinized counter electrode was obtained by spreading a Pt-based solution (Platisol T, Solaronix) on FTO glass and heating at 450 °C for 10 min. To make a complete solar cell, the counter electrode was assembled with thermal adhesive film (25 μm Surlyn, Meltronix, Solaronix) that acts as a separator and sealing element. The different electrolytes (as well as neat ACN for the reference samples) were introduced in the complete cells with an analytical syringe through two holes drilled in the counter electrodes, which were later sealed by a piece of Surlyn and a microscope coverslip. The commercial iodide based redox electrolyte (SOL) was the one from Solaronix (Iodolyte AN-50), and it contains the I−/I3− redox couple ([I2] = 50 mM), ionic liquid, lithium salt, and pyridine derivative in ACN solution. The other six electrolytes (EL) were homemade ACN solutions, all containing the same amount of iodine ([I2] = 50 mM) and the following amounts of either lithium (LiI) or sodium (NaI) salts and tert-butyl pyridine (TBP): EL1: [NaI] = 0.5 M; EL2: [NaI] = 0.5 M and [TBP] = 0.58 M; EL3: [LiI] = 0.5 M; EL4: [LiI] = 0.5 M and [TBP] = 0.58 M; EL5: [LiI] = 0.5 M and [TBP] = 1.16 M; EL6: [LiI] = 0.5 M and [TBP] = 1.74 M. I2 and NaI were purchased from Panreac, and LiI and TBP were obtained from Sigma-Aldrich, all utilized without further purification. To desorb the TH305 from the electrodes, the conductive glass with the titania or alumina film was immersed in a DCM with a strong base solvent, 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU, 98%, Sigma-Aldrich).

It has already been reported that the formed cations of the dye and constituents of the electrolyte can modify the electron injection yield, the open circuit voltage, and the electron diffusion due to their effect on the charge trap distribution and the conduction band position in the semiconductor material.2,10,12,13 Therefore, it is important to understand and control the influence of these additives on the mechanism of charge transfer, electron lifetime, and current−voltage characteristics with the aim to get highly efficient solar cells. On the basis of these considerations, for DSSC fabrication, we have chosen one of the recently introduced triphenylamine dyes, TPH305 (Scheme 1), which showed a 7.7% efficiency of Scheme 1. Molecular Structure of TH305 Dye

solar light conversion (η) in the optimum performance device with the iodide electrolyte.14 The dye consists of a phenoxazine unit with a long alkyl chain, a triphenylamine group (acting as energy antenna and electron donor), and the cyanoacrylic acid as an anchoring and acceptor group. TH305 has also been successfully used with iodine-free electrolytes, giving efficiencies up to 6%.15,16 The dye in ethanol solution has been studied in the ultrafast time scale by the femtosecond (fs) transient absorption technique, revealing an intramolecular charge transfer process from the triphenylamine antenna group to the cyanoacrylic acid part when it is pumped to the S2 state.17 The complete solar cells were studied by means of steadystate and time-resolved absorption and emission spectroscopy. We thus encompass the fast and ultrafast photobehaviors of complete solar cells to get more insights on the mechanism of charge separation in the real photovoltaic devices. In order to obtain the electron injection rate constant, kei, and the efficiency, φei, for the complete cells, we used fs up-conversion spectroscopy. Flash photolysis technique was employed to get the rate constant contributions of the recombination, krecom, and regeneration, kreg, processes, obtaining the regeneration efficiency, φreg. The values of kei and kreg were determined for the cells filled with the most often employed liquid electrolyte based on the I−/I3− redox pair and containing additives (tertbutyl pyridine and Li+ or Na+ cations) that are commonly used to increase the performance of the photovoltaic device. In this study, we discuss the influence of these additives on the dynamics of the charge separation and dye regeneration, and on the partial efficiencies of the complete cells, φei and φreg. The present study thus provides a detailed picture of the fast and ultrafast behavior of TH305 based complete solar cells, as well as valuable information on the factors which might limit the efficiency of the performance of DSSCs. This study shows that the best performance of TH305-based DSSCs has Li+, I−/ I3−, and TBP as additives to electrolyte, giving the high electron injection, small electron recombination rate constants, and the best open cicuit voltage. 26228



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The steady-state fluorescence and UV−visible absorption spectra were measured using FluoroMax-4 (Jobin-Yvone) and JASCO V-670 instruments, respectively. Pure films of nanomaterials (not covered with dyes) were used as references. The ns to s flash photolysis setup consists of an LKS.60 laser flash photolysis spectrometer (Applied Photophysics) and a Vibrant (HE) 355 II laser (Opotek) as a pump pulse source (5 ns time duration). The signal from OPO (pumped by Qswitched Nd:YAG laser, Brilliant, Quantel) at 490 nm was used for the sample excitation. The probing light source was a 150 W xenon arc lamp. The light transmitted through the sample was dispersed by a monochromator and detected by either visible or near-infrared photomultipliers (Applied Photophysics R928 and Hamamatsu H10330 A, respectively), both coupled to a digital oscilloscope (Agilent Infiniium DS08064A, 600 MHz, 4 GSa/s). The pump pulse energy was attenuated by a halfwaveplate and a polarizer pair. The experiments were performed at two pump pulse energy densities: 0.4 and 2 mJ/cm2. Because of the highly dispersive nature of the observed processes, the complete kinetics were obtained by combining the decays measured at several different time scales. The timeresolved spectra were constructed from fitted kinetic traces recorded in the spectral ranges 420−840 nm every 20 nm and 950−1600 nm every 100 nm. Femtosecond (fs) emission transients were collected using the fluorescence up-conversion technique. The system consists of a femtosecond optical parametric oscillator (Inspire Auto 100) pumped by 820 nm pulses (90 fs, 2.5 W, 80 MHz) from a Ti:sapphire oscillator Mai Tai HP (Spectra Physics) and coupled to an up-conversion setup. The pumping beam was adjusted at 530 nm, and its polarization was set to magic angle with respect to the fundamental beam. The typical energy of the pump pulse was 75 pJ (average beam power 6 mW), corresponding to energy density in pulse of 240 nJ/cm2. The sample was placed in a holder connected to a pair of translation stages (MTS series, Thorlabs). They move the sample in the x−y plane perpendicular to the pump beam preventing it from photodegradation during the measurements. The fluorescence was focused with reflective optics into a 1 mm BBO crystal and gated with the fundamental fs beam (820 nm). The IRF of the apparatus (measured as a Raman signal of neat solvent) was 300 fs (fwhm). To analyze the decays, a stretched exponential function (eq 1) convoluted with the IRF was used to fit the experimental transients. All the up-conversion measurements were performed in time windows of 30 ps, 200 ps, or 1 ns depending on the time scale of the decays. All experiments were done at 293 K. All the experimental kinetics were fitted with a stretched exponential function: A(t ) = A 0e−(t / τ)

where Γ is the gamma function. It has been shown that such calculated averaged rate constant is much more precise than the reciprocal of decay half-time,18 so far frequently used in nonexponential decay analysis. The photovoltaic response of the prepared solar cells was tested outside the laboratory building and under a blue sky in Toledo, Spain. The samples with a mask of 1 cm2 aperture were placed perpendicular to the Sun irradiation at the hours when the solar zenith angle corresponded to the conditions of AM 1.5 (1 Sun). The open circuit voltage (VOC) and short circuit currents (ISC) were measured with a standard current electric meter.

3. RESULTS AND DISCUSSION 3.1. Stationary Absorption, Emission, and Photovoltage Studies. Figure 1A presents a typical stationary UV−visible absorption spectrum of TH305 cells filled with ACN. The absorption maximum for both TiO2/ACN and Al2O3/ACN samples is around 480 nm. The different

β

(1)

This function contains two parameters: the characteristic time τ and the dispersion parameter β (0 < β < 1, lower β values mean more stretched decay, extending the decay time scale over many orders of magnitude). The justification for the use of this function for different processes will be given in the corresponding results and discussion sections. The averaged rate constant k of the process described by function (1) can be calculated as18,19 ⎛ τ ⎛ 1 ⎞⎞−1 k = ⎜ Γ⎜ ⎟⎟ ⎝ β ⎝ β ⎠⎠

Figure 1. (A) Stationary UV−visible absorption spectra of cells made of TiO2 or Al2O3 sensitized with TH305 and filled with neat ACN. (B) Stationary emission spectra of the same samples as in part A and under 530 nm excitation. The emission intensity of the TiO2 sample is multiplied 100 times. (C) Comparison of stationary emission spectra of cells made of Al2O3 sensitized with TH305 and filled with neat ACN and different indicated electrolytes, under 480 nm excitation.

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maximum absorbance intensities (A = 0.6−0.7 for TiO2/ACN and A = 0.3−0.4 for Al2O3/ACN) reflect the differences in the specific surface area for the nanoparticles of different diameter (about 10 nm for TiO2 and around 50 nm for Al2O3). We quantified the dye loading on TiO2 and Al2O3 by desorbing the dyes in DCM with DBU base (10% v/v). The results indicate that, in the case of TiO2, the TH305 concentration is 1.36 × 10−5 M, while for Al2O3 it is 5.6 × 10−6 M. Therefore, for the same layer thickness and immersion time, more TH305 dyes are adsorbed on a titania nanoparticle film than on an alumina one. Figure 1B shows stationary emission spectra of titania and alumina samples filled with ACN. For both of them, the maximum of the emission band is around 650 nm. Similar absorption and emission maxima on titania and alumina indicate that the ways at which the dye molecules are attached to semiconductor particles are similar for both materials. Therefore, as often used in many spectroscopic studies of DSSC systems, Al2O3 samples can be regarded as a reference for studying the electron injection process of TH305/TiO2 samples.20−23 The electron transfer from the excited dye at S1 (LUMO oxidation potential at −1.28 V vs NHE14) to the titania conduction band (potential of its edge at −0.55 V vs NHE for pH 62) is responsible for the excited state quenching of TH305. That is why the emission intensity of T305/titania is more than 100 times lower (Figure 1B) than that using alumina, for which the conduction band energy (conduction band edge potential at −4.5 V vs NHE24) is too high for the electron transfer to occur. The presence of different iodide-based electrolytes in the cell causes about a 25 nm red shift in the emission spectra of TH305 dye with respect to that in the neat ACN environment, as shown in Figure 1C. This fact is important for the comparison of time-resolved emission decays for different samples presented in the next section. As revealed by fluorescence excitation spectra, the absorption band shifts to the blue in the electrolytes studied with respect to that in ACN (Figure S1A in the Supporting Information). Therefore, the red shift in emission originates from a larger Stokes shift between the absorption and emission bands, which might be explained by the influence of a more polar environment (electrolytes vs neat ACN) on the stabilization of excited TH305. Its S1 state shows strongly charge transfer character, as it was recently shown for this dye,17 and as we have previously shown for other dyes for DSSC belonging to the triphenylamine family.25 Absorption and emission spectra were also measured for sensitized films in air for comparison, and both of them were found to be very similar to those of ACN cells (Figure S1B in the Supporting Information). The open circuit voltage (VOC) of the samples with different electrolytes measured under AM1.5 conditions is presented in the last column of Table 1. VOC is low for electrolytes without TBP (VOC = 430 mV and VOC = 460 mV for EL3 and EL1, respectively), while addition of TBP to the cell causes an increase in VOC by about 100−150 mV (VOC = 580 mV and VOC = 550 mV for EL4 and EL2, respectively). It is interesting to note that an increase in VOC is more prominent for Li+ than for Na+ cations. A further increase in TBP concentration did not increase VOC (for EL5 and EL6). The voltage is also high (VOC = 550 mV) for commercial SOL electrolyte, which contains, besides pyridine derivative, also other additives. The effect of additives will be discussed in the next section where the VOC changes are correlated with the electron injection rate

Table 1. Parameters of Solar Cells Made of TiO2 Sensitized with TH305 and Filled with Neat ACN or Different Electrolytes, Obtained from Time-Resolved Fluorescence Studiesa electrolyte

τ (ps)

β

kTi (ps−1)

kei (ps−1)

ϕei (%)

VOC (mV)

ACN SOL EL1 EL2 EL3 EL4

0.450 0.800 0.320 0.850 0.250 1.250

0.618 0.516 0.554 0.480 0.805 0.485

1.533 0.662 1.856 0.544 3.546 0.377

1.506 0.648 1.842 0.530 3.532 0.363

98 98 99 97 100 96

550 460 550 430 580

τ and β are the parameters of stretched exponential function given by eq 1 convoluted with IRF fitted to the emission transient at 650 nm with the laser pulse energy density 240 nJ/cm2. kTi is the averaged decay rate constant on TiO2 calculated with eq 2. The electron injection rate constant, kei, was obtained from eq 5. The electron injection quantum yield, ϕei, was calculated from eq 6. The last column presents the open circuit voltage (VOC) of the cells measured under 1 Sun conditions. a

constants. Our maximum voltages are smaller than the one reported for the optimized DSSC of TH305 dye and iodide electrolyte (VOC = 730 mV).14 We think that the main reason for this is a lower Fermi level of electrons in titania (VOC is the difference between the Fermi level and the redox potential of the I−/I3− couple) in our devices due to the smaller number of photoinduced electrons. The photocurrents in our samples (ISC = 4−5 mA/cm2) are much smaller than those for the optimized cell made of TH305 (ISC = 15 mA/cm2).14 This is because in our cells the total absorbance of the dye is kept low (A < 0.7 in the maximum) to enable enough transparency of the sample for the spectroscopic studies. For DSSCs optimized for the best efficiency, the dye absorbance in the working electrode is usually about 4 times higher, which, according to our simulations for TH305, corresponds to at least 2 times more absorbed photons. Therefore, smaller dye loading in our samples might be an important factor for lower photocurrent. The rest of the differences in ISC (between our and the best devices) can be, in principle, due to worse electron injection or higher rate constants of electron recombination. Since, as we will show below, the efficiencies of electron injection and recombination from titania to oxidized dye are close to 100% in our samples, it is perhaps higher electron recombination from the titania network (or working electrode) to the electrolyte that accounts for the rest of the photocurrent losses. In total, our photocurrents are about 3.5 times smaller than the best for TH305 solar cells. For the same parameters of titania layer (thickness, porosity, particle size), the position of Fermi level is proportional to the logarithm of the number of photoinduced electrons in the conduction band.2,26 Thus, for the best ISC, the Fermi level is expected to be 1.25 times higher than that for our ISC, which is similar to the ratio of the best and our VOC (730 mV vs 580 mV). On the other hand, for similar photocurrents, the difference between conduction band edge and Fermi level should be constant and the same in different electrolytes. Thus, the observed differences in VOC directly reflect the changes in the conduction band edge position caused by different electrolyte additives. The increase in VOC value upon TBP addition in our samples is similar to the reported shift of the conduction band edge in ruthenium-based DSSC with 0.7 M LiI upon addition of 0.5 M TBP (160 mV).27 26230



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3.2. Time-Resolved Emission Studies. We have used ultrafast time-gated fluorescence to determine the electron injection rate constants for TH305 interacting with titania in complete solar cells filled with different electrolytes. In the past, the most common tool for studying this process was transient absorption, in which the observation of the radical cation band and/or the signals of electrons in the TiO2 conduction band was the direct proof of the occurrence of injection. However, time-resolved emission spectroscopy is very useful to investigate the evolution of excited population of molecules because it only involves the emission of the excited states, whereas transient absorption contains several overlapping emitting and non-emitting contributions due to excited state, radical cation and electron absorption, ground state bleach, and stimulated emission. Therefore, in the recent years, the timeresolved emission technique has been more and more frequently used to study DSSC systems.21,28−31 Since it gives information about the decay of the excited state (from which the electrons are transferred), it is sensitive to other ultrafast processes that might occur, like wavelength-dependent vibrational relaxation, solvation, and intermolecular quenching. Thus, a special procedure and careful treatment of the data have to be employed to properly extract the electron injection rate constants, which will be shown below. The fs up-conversion studies of solar and reference cells sensitized with TH305 were performed with the excitation at 530 nm, and the emission was gated at 600, 650, and 700 nm. The excitation wavelength lies on the long-wavelength side of the absorption band, which reduces the effect of vibrational relaxation on the observed transients (Figure 1A). The selected three emission wavelengths correspond to the blue, central, and red part of the emission band in ACN, while in electrolytes 25 nm red shift is observed (Figure 1B). Figures 2−4 show illustrative emission transients at different gating wavelengths and for different samples. Tables 1 and 2 give the results of stretched-exponential fits and calculated electron injection quantum yields. The formula for stretched exponential fits is given by eq 1, and the average rate constant of the decay is calculated using eq 2. The use of the stretched-exponential function for all samples requires some comment. As it was shown for DSSC systems, this function is very useful in describing the heterogenous nature of electron injection dynamics.28,32 Such heterogeneity is due to the certain energetic distribution of electron donor states (vibrational excited states of the dye) and energetically varying density of electron acceptor states in titania (decreasing below the conduction band edge with the exponential function of energy10,27 and increasing above the conduction band with the square root function of energy30,33). The stretched exponential time constant τ in eq 1 is proportional to the electronic coupling between the dye and TiO2 network, while the dispersion parameter β describes how much the density of acceptor states changes within the distribution of donor states. The observed decay of emission transient includes also, besides the electron injection rate constant kei, the contribution of vibrational relaxation rate constant (kvr), solvation (ksolv), and S1−S0 recombination process within the dye (kdye), which rather cannot be rigorously described by a stretched exponential function. However, the good fit results for all transients obtained by the use of one stretched-exponential function lead us to the assumption that the average rate constant on titania, kTi, obtained from eq 2 describes well the sum of the rate constants of all above-mentioned processes:

Figure 2. Femtosecond emission transients of Al2O3/SOL (A) and TiO2/SOL (B) samples observed at the indicated wavelengths, and upon excitation with 240 nJ/cm2 pulses. The solid lines are from the best fit using the convolution of Gaussian IRF of the setup with stretched exponential function given by eq 1 and the parameters given in Table 2.

k Ti = kei + ksolv + k vr + kdye

(3)

Owing to such an approach, we obtain a single averaged decay rate constant for each sample at each wavelength and thus we can easily compare and calculate the electron injection rate constant, which would not be possible with multiexponential analysis. We will first discuss the results obtained for the reference Al2O3 samples. In this system, the electron injection from the S1 state of TH305 to the semiconductor is not possible, so the observed transient reveals the effect of vibrational relaxation, solvation, and the decay of the relaxed S1 state to the S0 state (due to internal conversion and intramolecular quenching) on the nanoparticle network. Similarly as mentioned above for titania samples, the averaged rate constant obtained from eq 2 with the stretched exponential fit parameters can be expressed as kAl = k vr + ksolv + kdye

(4)

As expected, at shorter gating wavelength, the hot vibrational levels of S1 state are more probed and the influence of the dynamic Stokes shift of the emission band due to solvation is also important.25,34 Therefore, the decay rate constant kAl is higher due to the contribution of kvr and ksolv. The measured value approaches the decay rate constant of the relaxed S1 state of TH305 dye as the emission is probed on the red part of the fluorescence band. For example, for Al2O3/SOL samples (Table 2 and Figure 2A), the rate constant changes from kAl = 0.024 ps−1 at 600 nm to kAl = 0.006 ps−1 at 700 ps. However, even at longer emission wavelength, the dispersion parameter β is still low (about 0.4, see Table 2) which indicates that, unlike the 26231



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Figure 4. (A) Femtosecond emission transients of solar cells made of TiO2 sensitized with TH305 and filled with the indicated electrolytes of different TBP concentration. The solid line is from the best fit for EL4 from Figure 3A. (B) Femtosecond emission transients of solar cells made of TH305 on TiO2 and filled with neat ACN and ACN with 0.5 M TBP, observed at 650 nm. The solid lines are from the best fit with the following parameters: τ = 0.45 ps, β = 0.62 for neat ACN and τ = 0.75 ps, β = 0.45 for ACN with TBP.

Figure 3. (A) Femtosecond emission transients of solar cells made of TiO2 sensitized with TH305 and filled with the different indicated electrolytes observed at 650 nm, and upon excitation with 240 nJ/cm2 pulses. The solid lines are from the best fit using the convolution of Gaussian IRF of the setup with stretched exponential function given by eq 1 and the parameters given in Table 1. (B) The dependence of electron injection rate constant kei obtained in the solar cells with different electrolytes on their open circuit voltage, VOC (from Table 1). The solid line represents the best fit of the exponential function f(VOC) = A exp(−VOC/ΔV), with fitted ΔV = 40 mV.

section) suggests that this process, even if present, has a small quantum yield. For other homemade electrolyte solutions the decay of alumina samples is the same as that for commercial SOL at the corresponding wavelength (checked for EL1 and EL2 electrolytes). However, for the Al2O3/ACN sample, the decay is faster. The kAl = 0.027 ps−1 value for Al2O3/ACN at 650 nm should be compared to the mean of the rate constants at 650 and 700 nm for Al2O3/SOL (due to the 25 nm shift of the fluorescence band and the necessity to compare the emission transients corresponding to the same vibrational energy excess), which is kAl = 0.010 ps−1 (Table 2). Therefore, the decay of TH305 on alumina is about 3 times longer when the electrode is immersed in the iodide electrolyte instead of the neat solvent. Having examined the reference emission transients on alumina, we can discuss now the results for complete solar cells with titania. Generally, the fluorescence decays of these samples are much faster than those for alumina ones (compare parts A and B of Figure 2, or see Figure S3 in the Supporting Information), indicating the dominant role of the electron injection in the excited state deactivation. As shown in the previous section, the TH305 emission bands are the same for titania and alumina, so the effect of vibrational relaxation and solvation dynamics should be the same at the same wavelengths for both materials. Therefore, on the basis of eqs 3 and 4, the electron injection rate constant kei can be simply calculated as the difference of the averaged constants obtained for titania and alumina:

free dyes in solution, the decay of the relaxed S1 state is highly nonexponential and some quenching mechanisms operate when the dyes are attached to the semiconductor nanoparticles. Indeed, our test studies of TH305 in DCM solution give the average decay rate constant of 0.005 ps−1 when measured near the maximum of the fluorescence band (650 nm), which can be compared to about 5 times higher rate constant kAl = 0.027 ps−1 obtained for the Al2O3/ACN sample measured at 650 nm. We have observed previously similar acceleration of the decay of the relaxed S1 state when studying time-resolved emission of different triphenylamine dye (TPC1) interacting with alumina particles in suspension (half-lifetime of 150 ps instead of about 1 ns in solution).35 One of the possible quenching mechanisms is singlet−singlet annihilation which can be significantly enhanced if the dye molecules with overlapping absorption and emission bands are closely packed on the nanoparticle surface.36,37 To verify this possibility, we checked the pump fluence effect on the Al2O3/ACN transients and observed doubling of the decay rate constant (from kAl = 0.015 ps−1 to kAl = 0.031 ps−1 for 40 to 480 nJ/cm2, respectively) when the pump pulse energy was increased 6 times (gating at 650 nm, see Figure S2 in the Supporting Information). Therefore, this kind of quenching can have a moderate effect on the lifetime of TH305 dye attached to alumina. It is also possible that electron injection to the deep trap states in the alumina bandgap can be responsible for some lifetime shortening. However, the lack of any radical cation signal of the alumina sample (see next

kei = k Ti − kAl 26232

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Table 2. Parameters of Cells Made of TiO2 or Al2O3 Sensitized with TH305 and Filled with SOL, Obtained from Time-Resolved Fluorescence Studies Observed at Different Indicated Wavelengthsa TiO2

Al2O3

wavelength (nm)

τ (ps)

β

kTi (ps−1)

τ (ps)

β

kAl (ps−1)

kei (ps−1)

ϕei (%)

600 650 700

0.850 0.800 1.200

0.609 0.516 0.474

0.797 0.662 0.376

6 22 40

0.322 0.405 0.379

0.024 0.014 0.006

0.773 0.648 0.370

97 98 98

τ and β are the parameters of stretched exponential function given by eq 1 convoluted with IRF and fitted to the emission transient. kTi and kAl are the averaged decay rate constants of TiO2 and Al2O3 samples, respectively, calculated with eq 2. The electron injection rate constant, kei, was obtained from eq 5, while the electron injection quantum yield, ϕei, was calculated from eq 6. a

and the electron injection quantum yield is the ratio ϕei = kei /k Ti

Scheme 2. Schematic Representation of Electron Injection (Rate Constant, kei), Electron Recombination (Rate Constant, krecom), and Dye Regeneration (Rate Constant, kreg) Processes Observed in the Complete DSSCs Studied in This Worka

(6)

Tables 1 and 2 present the obtained values of electron injection rate constants and efficiencies. The effect of different gating wavelengths was examined for the TiO2/SOL sample (Table 2 and Figure 2B). As can be observed, the electron injection rate constant is higher for shorter emission wavelengths when higher donor energy levels are more probed. For example, kei = 0.773 ps−1 at 600 nm is about 2 times higher than that at 700 nm (kei = 0.370 ps−1). There is more contribution of the injection from the relaxed S1 state in the emission transients at the latter wavelength with respect to that observed at 600 nm. This results are consistent with the electron injection model in which the density of the acceptor state in the conduction band (increasing with increasing energy) is responsible for different injection dynamics.10,29,30 We have observed previously a similar increase in the electron injection rate constant for high vibrational levels of the excited state for TPC1 dye.34,38 It is worth noticing that the ability of easy probing of the electron injection dynamics from different vibrational levels is one of the advantages of time-resolved emission techniques over transient absorption methods. Nevertheless, for all wavelengths, the electron injection rate constant in TH305 solar cells is so fast (in comparison to other relaxation processes) that the efficiency of electron injection is always almost 100%, despite large differences in kei (Table 2). One of the most interesting and important results is, however, the effect of additives in iodide-based electrolyte on the averaged electron injection rate constant. We investigated it for one gating wavelength (650 nm), and the results are plotted in Figure 3A. As can be seen in Table 1, electron injection rate constants correlate quite well with the open circuit voltages of the cells. This is because the voltage changes correspond to the variation in the potential of the conduction band edge of titania, as explained in the previous section. Decreasing VOC reflects the change of conduction band potential toward more positive values (energy changed to more negative values), which results in a higher density of electron acceptor states at the energy level of the electron donor and, thus, faster electron injection. This situation is illustrated qualitatively in Scheme 2. In line with this model, electron injection rate constants (proportional to the density of electron acceptor states) depend exponentially on open circuit voltage (proportional to the conduction band edge shift); see Figure 3B. To begin with, the longest kei values are found in electrolytes containing only iodide salts, without pyridine derivatives (EL1 and EL3), for which VOC is the smallest (Table 1 and Figure 3A). Among them, the effect is more pronounced for lithium cations (EL3 with kei = 3.53 ps−1) than for the sodium ones

a

The positions of potentials of different energy levels are not in scale. The injection rate constants are given for electrolyte EL3 (on the left) and EL4 (on the right) measured for emission at 650 nm. The regeneration rate constant is given for low pump energy (4 electrons per particle). DOS is the density of states of the semiconductor. CB is the conduction band, and TBP is tert-butyl pyridine.

(EL1 with kei = 1.84 ps−1). It is consistent with the reported effect of small cations on the conduction band shift, which is stronger for smaller cation sizes and higher cation charge.2,39,40 Moreover, due to the abnormal low shielding effect for Li+, its polarizability is higher than the expected one for its size. According to our knowledge, we confirmed for the first time this behavior for Li+ and Na+ cations by time-resolved spectroscopy in complete DSSSs. The small cation effect is explained in the similar way to the well documented Nernstian change in a flatband potential with pH of the aqueous solution.2 Protons (or small cations) when adsorbed on the semiconductor surface penetrate over some distance into its structure and neutralize partially the electron charge responsible for the band bending at the semiconductor−electrolyte interface. This phenomenon results in changing the flatband potential toward more positive values. If the involved cations are smaller (Li+ vs Na+) and penetrate easier, then the effect is more pronounced and the potential shift is larger. Next, the electrolytes with pyridine derivatives (SOL, EL2, and EL4−6) will be discussed. It is a common procedure to add TBP in order to increase VOC of DSSC when the position of the dye LUMO redox potential is high enough with respect to the conduction band edge.2 If the strongly proton accepting 26233



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nanoparticle structure contains some protic impurities.39 Such impurities shift the conduction band edge toward lower energies in TiO2/ACN samples, while the addition of TBP into ACN neutralizes this effect in the same way as for the electrolytes containing Li+ and Na+ cations. It can be noted that we have recently observed about 10 times faster electron injection in TiO2 sensitized films in air than in complete solar cells filled with SOL, both sensitized with TPC1 dye.42 Such films can easily absorb water from air, and the protons might modify the conduction band edge in the same direction as impurities in ACN. Finally, we have also checked the pump fluence effect on the observed emission transients. We found that, within the signalto-noise ratio, the normalized kinetics observed at 650 nm did not change when the averaged pump energy was changed between 40 and 480 nJ/cm2 for the TH305/TiO2/SOL cell (Figure S4 in the Supporting Information). This means that, first, the possible singlet−singlet annihilation do not influence the injection kinetics (which could be also deduced from the scale of changes observed under different pump energies for alumina samples, Figure S2 in the Supporting Information). Second, it also indicates that the number of injected electrons from previous excitations and present in one particle do not influence the injection rate constant. Although the energy density per pulse is small, the high repetition rate constant of the laser pulses (80 MHz) makes it so that much more than one electron per particle can be present in the titania network (for example, for 40 nJ/cm2 pulses, there are about 30 electrons injected, on average, into each nanoparticle during 1 ms). 3.3. Flash Photolysis Studies. Flash photolysis studies of the complete solar cells were performed to determine the slow dynamics of the dye regeneration by the redox couple (rate constant kreg) and the electron recombination from the titania conduction band to the dye (rate constant krecom). These processes can be followed by the observation of decay of the TH305 radical cation band (formed after the electron injection) in the presence (both regeneration and recombination) and absence (only recombination) of the redox couple. Knowing both constants (kreg and krecom) enables the determination of the quantum yield of the dye regeneration, which should be as high as possible for the best solar cell efficiency, and can be calculated as

compounds like TBP are added to the electrolyte, they probably neutralize the cation effect (by decreasing the amount of adsorbed cations at the surface27) and shift back the flatband potential toward more negative values. In our case, the electron injection rate constants are slowed down to kei = 0.53 ps−1 for EL2 and kei = 0.36 ps−1 for EL4 when the concentration of TBP is similar to that of Li+ and Na+ cations (about 0.5 M). For commercial electrolyte SOL (which also contains other additives), the injection rate constant is slightly higher, kei = 0.65 ps−1. Interestingly, when more TBP is added to the lithium salt ([TBP] = 1.16 M for EL5 and [TBP] = 1.74 for EL74), any further slowing down of the electron injection rate constant is not observed. Figure 4A combining the transients for EL4, EL5, and EL6 visualizes this effect. This means that all cations are already neutralized for the similar molar amount of TBP and iodide salt (0.5 M). Similarly, as discussed above for the emission wavelength dependence, there is a lot of redundancy in electron injection rate constant for all samples with respect to the other deactivation mechanism (measured in alumina samples), which causes the injection yield to be close to 100% in all cases. In particular, as shown in Table 1, it changes from 96% for the solar cell of the smallest rate constant (EL4) to 100% for the fastest injecting system (EL3). Therefore, for optimization of DSSC sensitized with TH305, the TBP concentration can be as high as 0.5 M to increase the VOC,14 because the electron injection yield is almost 100% for all conduction band positions. However, in the case of dyes that have lower LUMO energies and/or smaller coupling to the titania structure, the optimized TBP concentration has to be done to balance between higher VOC and lower ISC (a decrease in the latter can be due to lower electron injection efficiency). The electron injection rate constant in the cell filled with ACN is kei = 1.51 ps−1 at 650 nm gating wavelength (Table 1 and Figure 4B). Due to the shift of the fluorescence band of TH305 in electrolytes, this value should be compared with the mean of the rate constants measured at 650 and 700 nm in order to probe the same vibrational level. This mean value is, for example, kei = 0.51 ps−1 for the SOL sample (Table 2), which is 3 times smaller than that for ACN (1.51 ps−1). It agrees with what has been recently observed for several DSSCs with other dyes: the electron injection rate constant is significantly smaller in a complete system with electrolyte than that measured in neat solvent or in isolated electrodes.10,31,32,41 The usual interpretation of this effect is the different position of the conduction band edge of TiO2. However, if ACN can be treated as a neutral solvent (very small autoprotolysis constant39), then the conduction band edge of TiO2/ACN should be similar to that of EL2, EL4 (and perhaps also SOL), because in them, as concluded above, TBP neutralizes the effects of potential shift due to the presence of Li+ or Na+ cations. Therefore, the electrolyte additives might additionally modify the electron injection rate constant by decreasing the dye−titania electronic coupling when it is coadsorbed in the dye vicinity on the TiO2 surface. We performed a test measurement for the TiO2 cell filled with ACN and only TBP (0.5 M) and obtained about 3 times longer decay for the transient at 650 nm (Figure 4B), which gives the injection rate constant kei = 0.51 ps−1, similar to that for EL2, EL4, and SOL. Therefore, it might be the used pyridine derivative that is responsible for the weakening of the coupling between TH305 and the titania surface. An alternative explanation is that the neat ACN itself or the titania

φreg =

k reg k reg + k recom

(7)

Figure 5 presents the transient absorption spectrum for the TH305/TiO2/SOL cell gated at 1 μs after the excitation and constructed from the kinetic traces measured at different probing wavelengths in the visible (VIS, Figure 5A) and nearinfrared (NIR, Figure 5B) regions. The transient absorption signals in VIS decay to zero, within 300 μs (Figure 6C), and should be exclusively assigned to the interplay between the positive absorption of the TH305 radical cation (maximum around 660−680 nm) and the negative ground state depopulation band (between 440 and 540 nm). Our assignment is supported by the fact that, as revealed in the previous section, the electron injection yields were close to 100% for all cells. Moreover, the S1 state of the free TH305 dye lives no longer than 1 ns (see previous section). We also checked that there are no long transient absorption signals from the cells using TH305 interacting with alumina (without the electron injection process) in the flash photolysis experiment. Thus, the 26234



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Figure 5. Normalized transient absorption spectrum of the solar cell made of TiO2 sensitized with TH305 and filled with SOL, measured in a flash photolysis experiment at 1 μs after excitation with an intensity of 0.4 mJ/cm2 in visible (A) and near-infrared (B) ranges. In part B, the residual spectrum at 500 μs is also shown. The inset shows the kinetics measured at 1500 nm with the magnified residual part fitted with the stretched exponential function with offset.

radical cation and electrons in the conduction band are the only transient species that can be present in μs and sub-μs time scale in TH305/TiO2/SOL cells. However, the signal from the latter (electrons) should be present on much longer time scale (ms) under open circuit conditions. Similarly, the main contribution of the transient absorption signal in NIR (increasing gradually from 950 to 1600 nm, see Figure 5B) should be assigned to the radical cation, because it decays on the same time scale as the transients in VIS (see below). On the contrary, the small residual signal observed in NIR (Figure 5B) originates, most probably, from the injected electrons present in the trap states and/or the conduction band of titania nanoparticles.43 Importantly, the observation of a strong signal of TH305 radical cation in NIR (its amplitude at 1500 nm is comparable to that of the maximum in VIS, Figure 5) should be emphasized. It has been demonstrated that DSSCs made of ruthenium complexes do not give any signal in NIR except for the conduction band/trap state electrons, and therefore, this region is usually used for exclusive probing of electron population dynamics.18,43−45 Our result for TH305 (possessing a common triphenylamine electron donor unit) indicates that this assumption might not be valid for many solar cells sensitized with pure organic dyes. To get the dynamics of the radical cation decay, we probed at 660 and 1500 nm, near the VIS and NIR maxima of the transient absorption bands, respectively. Illustrative transients are shown in Figure 6, and Table 3 gives the results of the stretched exponential fit. The decays of the kinetics for the TH305/TiO2/ACN cell (Figure 6A) are stretched over many orders of magnitude in the time domain, with small dispersion

Figure 6. Illustrative normalized kinetics for solar cells made of TiO2 sensitized with TH305 measured in a flash photolysis experiment after pump excitation at 490 nm. The black lines show the best fits using a stretched exponential function from eq 1 and parameters given in Table 3. The time axis is in logarithmic scale. (A) Effect of two different pump pulse energy densities for the TiO2 samples filled with neat ACN and probed at 660 nm. The normalization factor for kinetics at 2.0 mJ/cm2 is 2 times larger than that for kinetics at 0.4 mJ/cm2. (B) Effect of different electrolytes probed at 1500 nm with pump energy density 0.4 mJ/cm2. (C) Effect of two different pump pulse energy densities for the TiO2 samples filled EL3 and probed at 660 nm. The normalization factor for kinetics at 2.0 mJ/cm2 is 4 times larger than that for kinetics at 0.4 mJ/cm2.

parameters (β = 0.20−0.26) and the averaged decay rate constants kACN = 9 s−1 and kACN = 59 s−1 for excitation with 0.4 and 2.0 mJ/cm2, respectively. According to our estimation, these excitation photon densities per pulse correspond to the average excitation of about 4 and 20 electrons per one titania nanoparticle, respectively, taking into account some parameters of the used samples (absorbance, nanoparticle diameter, and layer thickness). There is no redox couple in TiO2/ACN cells, so the long lifetimes are due to the exclusive presence of the electron recombination process. It is a common assumption that the rate constants obtained in neat solvents can be taken as the value of recombination rate constant in the devices filled with electrolyte (krecom = kACN).5,6,46 The use of a stretched exponential function to fit the kinetics of TH305/TiO2/ACN cells is based on the nearest-neighbor 26235



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Table 3. Results of the Stretched Exponential Function Fit of the Kinetics Measured for the Indicated Samples in Flash Photolysis Experiment under Excitation with the Indicated Pump Pulse Energy Densitya sample

probing wavelength (nm)

pump pulse energy density (mJ/cm2)

τ (μs)

β

k (s−1)

τ1/2 (μs)

TiO2/ACN TiO2/ACN TiO2/SOL TiO2/EL4 TiO2/EL3 TiO2/EL3 TiO2/EL3

660 660 1500 1500 1500 660 660

0.4 2.0 0.4 0.4 0.4 0.4 2.0

5600 170 14.0 13.0 13.3 11.6 1.4

0.256 0.204 0.473 0.455 0.544 0.556 0.362

8.65 58.7 32 × 103 32 × 103 43 × 103 52 × 103 160 × 103

1500 81 9.0 7.5 7.5 5.7 1.1

τ and β are the parameters of stretched exponential function given by eq 1, while the averaged decay rate constant k was calculated from eq 2. For the comparison, the decay half-times (τ1/2) are also given. a

TH305 cation, the driving force is about 0.5 V, which is on the edge of the efficient regeneration by iodide electrolytes and enables high VOC of the cell. This means that, at least from the point of view of dye regeneration, the system is well optimized. Third, the regeneration dynamics are nonexponential, like the electron injection and electron recombination, with a dispersion parameter of about β = 0.5. Unlike the electron injection and recombination processes (discussed before), there is no well documented explanation why dye regeneration dynamics should be described by a stretched exponential function instead of a single-exponential one. One of the proposed possibilities are a range of activation energies for the regeneration reaction or a gradient in local iodide concentration across the nanoparticle film.18 The relatively, so far, clear picture of the regeneration dynamics in TH305 solar cells is, however, disturbed by the results obtained at higher excitation energy density, 2.0 mJ/ cm2. As Figure 6C and Table 3 show, the radical cation signal decays significantly faster than that at 0.4 mJ/cm2 excitation, and the average decay rate constant is about 4 times higher, kEL = 160 × 103 s−1. We have also observed an increase in the radical cation decay rate constant when the probing light from a xenon lamp additionally excites TH305 molecules. For example, for the measurement of kinetics at 660 and 1500 nm (presented in Figure 6), we used color filters that blocked the whole spectral range of the light that could overlap with the absorption band of the samples (Figure 1A). However, we could not maintain this condition for probing in the bleach region, and the repopulation dynamics measured at 520 nm was much faster than that measured in the red part with proper filters (Figure S6 in the Supporting Information). If the averaged rate constant obtained from the decays in ACN can be regarded as the true recombination rate constant, it cannot explain the observed shortening of radical cation lifetime in electrolytes under higher excitation energy. It is too small (kACN = 59 s−1 at 2.0 mJ/cm2) when compared to those observed in the presence of the redox couple (kEL = 160 × 103 s−1 at 2.0 mJ/cm2), so ϕreg = 100% (as for excitation at 0.4 mJ/ cm2). There is also no reason why the increased number of radical cation molecules (under higher excitation fluence) could result in higher regeneration rate constant (in fact, if the concentration of iodide was insufficient, one would expect slowing down of this rate constant).18 Another possibility is that the recombination rate constant measured in ACN could not be correct for electrolyte samples because the position of the conduction band might be different. However, as shown in Figure 6B for EL3 and EL4, the shift of the conduction band due to TBP addition does not effect the kinetics. It should be emphasized that the iodide concentration can influence another

random walk model that describes the electron recombination process.47 In this widely accepted picture, the electron transport within the particle proceeds through the jumps between adjacent trap sites until the electron reaches a recombination center. Thus, the time scale is determined by the trap-limited diffusion. In this model, the recombination is faster for a larger number of electrons in the particle, because the probability of the recombination is higher, which has often been confirmed.5,6,12,47,48 This agrees also with our observation: the recombination rate constant increases about 7 times when the density of electrons increases 5 times (Figure 6A and Table 3). It can be noted that the amplitude of the initial signal at 5 times higher excitation energy is only 2 times higher than that at lower energy (before normalization, see caption to Figure 6A). Therefore, most probably a part of the recombination at 2.0 mJ/cm2 excitation occurs already in the time scale shorter than the time window of the experiment (

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