Efficient Electron Injection from Acyloin-Anchored ... - ACS Publications

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Efficient Electron Injection from Acyloin-Anchored Semisquarylium Dyes into Colloidal TiO2 Films for Organic Dye-Sensitized Solar Cells Andreas F. Bartelt,*,† Robert Schütz,† Christian Strothkam ̈ per,† Joachim Schaff,† Stephan Janzen,† † † † Paja Reisch, Ivo Kastl, Manuel Ziwritsch, Rainer Eichberger,† Gerda Fuhrmann,*,‡ David Danner,‡ Lars-Peter Scheller,‡ and Gabriele Nelles‡ †

Helmholtz-Center Berlin for Materials and Energy, Hahn-Meitner-Platz 1, 14109 Berlin, Germany Materials Science Laboratory, Sony Deutschland GmbH, D-70327 Stuttgart, Germany

‡

S Supporting Information *

ABSTRACT: Semisquarylium dyes use a novel acyloin anchor group to strongly bind to TiO2 semiconductors. Efficient acyloin anchor mediated electron injection into nanocrystalline TiO2 is demonstrated, allowing highly efficient dye-sensitized solar cells with IPCEs > 80%. The acyloin anchor can thus be viewed as a true alternative to the standard carboxylic acid anchor group. The opto-electronic and electron injection properties of the most basic semisquarylium dye SY404 are compared to the modified semisquarylium dye DD1 and the carboxylic acid anchored indoline dye D131 using a combination of ultrafast and photoemission spectroscopy. For SY404, ultrafast injection times of ∼50 fs are found despite a small energetic driving force between dye excited states and TiO2 conduction band minimum. This is possible due to the strong electronic coupling of the semisquarylium dyes to the TiO2 surface mediated by the acyloin anchor. For a better overlap with the solar spectrum, the semisquarylium dyes are modified by substitution with a larger donor moiety (DD1). While for DD1 the overall absorption increases, the injection process slightly slows down; however, it still proves fast enough for very efficient injection. Compared to the carboxylic acid anchored indoline dye D131, the SY404 dye injects more than seven times faster despite a ∼150 meV smaller driving force.

1. INTRODUCTION Dye-sensitized solar cells (DSCs) are very promising candidates for a potentially low-cost alternative to standard silicon-based photovoltaic technologies.1−3 Usually, metal− organic complexes, mostly ruthenium-based, are employed in the best working cells, reaching power conversion efficiencies of 10−12%.4 Recently, pure organic dyes have become the subject of intensive research.5 These materials have several advantages as they combine potentially lower synthesis costs with the possibility of tuning physical properties such as molecular levels and their intrinsically high absorption coefficients. The high absorption allows reducing the film thickness of the semiconductor layer to which the dyes are attached, alleviating constraints connected to limited charge carrier diffusion lengths. The common form of these organic dyes follows a donor-π-bridge-acceptor (D-π-A) template which allows new structures to be designed for improved performance. The chemical binding of the dye onto the TiO2 surface is facilitated via the anchor group, which is usually attached to the main part of the chromophore and therefore acts as a link between the chromophore and the TiO2 surface. Various anchor groups have been shown to allow chemical coordination to the TiO2 surface like carboxylic acid, phosphonic acid, © 2014 American Chemical Society

sulfonic acid, hydroxyl, triethoxysilane, catechol, or boronic acid.6−9 By far the most employed and nowadays the standard is the carboxylic acid anchor −COOH, which was the only anchor group so far to yield acceptable solar cell efficiencies. Its derivatives, such as ester, acid chloride, acetic anhydride, carboxylate salt, or amide, have also been used. Phosphonic acid anchors provide a strong-binding link to the TiO2 surface, but the electronic coupling between the dye and TiO2 was inferior compared to the carboxylic acid group.10−14 In DSCs, the key photovoltaic task of photogenerated charge separation is facilitated by electron injection from the photoexcited dye into the TiO2 semiconductor. Hence, electron injection is at the heart of the DSC functionality. Charge separation at heterojunction interfaces is supported by potential energy drops; however, the higher the energy drop, the more potential energy is lost. For efficient electron injection, the rate of injection needs to be more than 2 orders of magnitude faster than the excited state relaxation rate.4 Ultrafast injection processes are often facilitated by large energy drops from the Received: December 21, 2013 Revised: February 24, 2014 Published: February 28, 2014 6612

dx.doi.org/10.1021/jp4125364 | J. Phys. Chem. C 2014, 118, 6612−6623

The Journal of Physical Chemistry C

Article

cellulose was screen printed on an optically transparent FTO substrate (fluorine doped SnO2, transmission >90% in the visible, sheet resistance 15Ω/square purchased by ATOC, Japan) by twice screen printing to give a 5 μm layer thickness. After drying, a second paste for the scattering layer containing ∼400 nm diameter nanoparticles was deposited. The sintering was carried out by gradually heating from room temperature to 510 °C in 90 min and then keeping the temperature steady for 120 min at 510 °C. The total resulting film thickness was 8 μm. To enhance the connectivity of the TiO2 particles within the layer, the photoanode was additionally treated with 40 mM TiCl4 solution for 30 min at 70 °C. After thoroughly washing with water, the TiO2 photoanode was sintered again at 510 °C for 30 min. After the film was cooled to 80 °C, dye sensitization of the photoanode was carried out as described above. The electrolyte was deposited by drop casting and the cell was assembled in a sandwich type geometry by using a 30 μm silicon spacer foil and a platinum counter electrode. The electrolyte consisted of 1.4 M 1,2-dimethyl-3-propylimidazolium iodide, 0.15 M sodium iodide, 0.075 M iodine, and 0.2 M tertbutylpyridine in methoxyacetonitrile. 2.4. Photovoltaic Measurements. The current−voltage characteristics of the DSCs cells were measured with a Keithley 2400 under AM 1.5 G simulated solar light. The incident light intensity was calibrated to 100 mW cm−2 with a standard silicon solar cell (Newport 91150 V). The conversion efficiency η was derived from the equation η = JscVocFF/light intensity. IPCEs were obtained with a Newport system at constant light intensity of 2 mW/cm2. The cell active area of 0.188 cm2 was controlled by use of a light shading mask. 2.5. Ground State Absorption and Photoluminescence. The ground state absorption spectra were measured with a UV−vis Perkin-Elmer Lambda 35 spectrometer. For the solutions, a dye concentration of 1.33 × 10−5 M was used in a quartz cuvette of 1 cm thickness. The TiO2/dye films were measured in air immediately following the dye treatment. The absorbance was determined from transmission T(λ) and reflection R(λ). Fluorescence spectra were recorded in air using an excitation spectroscopy setup (Fluoromax 1 - SPEX/ HORIBA Jobin Yvon GmbH). The time-resolved photoluminescence of the dyes in THF solution was recorded using a 405-nm 100-ps excitation pulse from a PicoQuant laser diode, a spectrally integrated detection involving a Si-APD and a PicoQuant time-correlated single photon counting setup. 2.6. Transient Absorption (TA) and Optical-Pump Terahertz-Probe Spectroscopy (OPTP). A 150 kHz repetition rate Coherent RegA fs laser system was used. For fs-TA, the 25-fs 430-nm pump pulses were obtained by frequencydoubling of a NIR noncollinear optical parametric amplifier (NOPA)19 output, yielding an excitation density of about 100 μJ/cm2 (focus diameter ∼150 μm). The probe pulses were derived from a white-light continuum, spectrally narrowed and chirp-compensated. The details of the TA setup can be found elsewhere.20,21 The time resolution measured as the crosscorrelation width between pump and probe pulses varied between ∼45 fs for all dyes at 1000 nm and ∼80 fs for D131 at 600 nm (∼70 fs for SY404 and DD1 at 600 and 800 nm probe wavelengths, ∼60 fs for D131 at 800 nm). The 490-nm pump pulse of the OPTP experiment generated with a NOPA yielded an excitation density of ∼5.5 μJ/cm2 (focus diameter ∼1.5 mm). The bandwidth of the few-cycle terahertz pulse is about 0.5−2.5 THz. The time resolution of ∼570 fs was estimated from the THz absorption rise of a thin

excited dye state to the TiO2 conduction band minimum. However, another key factor governing the injection rates is the electronic coupling strength between excited dye states and TiO2 conduction band states. The overlap of the electronic wave functions of donor and acceptor controls the coupling and critically depends on the spatial charge distribution of the coupled dye−TiO2 system. Hence, in order to reduce the energy “consumed” in the injection process, a strong electronic coupling is desirable. Here, we describe the optical, electron injection and photovoltaic properties of two different semisquarylium dyes.15 Very recently, a similar dye was reported for the use in DSCs.16 Our basic semisquarylium dye SY404 consists of an indole donor and a squaric acid acceptor. It is shown to attach strongly to the TiO2 surface via the acyloin-type anchor group, which is part of the acceptor. The injection properties are compared to the well-known carboxylic acid anchored indoline dye D131 of similar optical characteristics.17,18 High injection rates are found for SY404 even though the energetic driving force for charge separations is small, reducing the potential energy loss consumed during electron injection. The favorable electron flow from the donor to the accepting squaric acid combined with the strong electronic coupling of the acyloin anchor to the TiO2 surface are found to be responsible for these efficient injection properties. Considering the compact size of SY404, the absorption coefficient is rather high. However, the absorption range is limited to the shorter wavelengths of the solar spectrum. In order to increase the absorption further into the red, additional donor moieties were attached to the indole group. The as modified semisquarylium dye DD1 is shown to inject slower than SY404 even though the driving force for injection is increased. The reason for this will be discussed as being rooted mostly in the reduced electronic coupling and the increased reorganization energy. Nevertheless, the injection efficiency of DD1 is unabatedly high, and outstandingly high incident-photon-to-current conversion efficiencies (IPCE > 80%) of both semisquarylium-based DSCs are obtained. This makes the acyloin-type anchor the first noncarboxylic acid anchor group to mediate efficient DSCs.

2. EXPERIMENTAL SECTION 2.1. Dye Preparation. The synthesis of the semisquarylium dyes and their analytical data are given in the Supporting Information. The reference indoline dye D131 was used as received from Mitsubishi Paper Mills Ltd., Japan. 2.2. Spectroscopic Material Preparation. The mesoporous TiO2 films were prepared by screen printing a paste composed of 20 nm TiO2 nanoparticles, terpineol, and ethyl cellulose. As substrates, alkali-free 50 μm thin glass, alkali-free 500 μm thick glass, 1.5 mm thick quartz, and ITO-coated alkalifree glass were used for transient absorption, UV−vis/ fluorescence, THz, and photoelectron spectroscopies, respectively. After sintering at 430 °C for 1 h in air, a film thickness of ∼2 μm was obtained, with a medium porosity of 60%. For dye sensitization, the TiO2 films were immersed in a 2 × 10−4 M THF dye solution for 3 h. Subsequently, a ∼5 min rinsing in pure THF solvent removed noncovalently attached dyes. After drying under a nitrogen stream, the dye-loaded TiO2 films were immediately transferred into UHV for transient absorption and photoelectron spectroscopy and into ∼10−4 mbar vacuum for OPTP. 2.3. Dye-Sensitized Solar Cell Preparation. A TiO2 paste composed of 20 nm nanoparticles, terpineol, and ethyl 6613



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shift in the absorption is expected. The hexyl chain was introduced to maintain good solubility of the material. For comparison, the indoline dye D131 is used as a reference. It contains the conventional carboxylic acid anchor including a cyano-group in its vicinity, which is the most popular unit used for organic sensitizers in DSCs.5,32,33 The semisquarylium dyes are presumed to adsorb onto the surface of TiO2 by a mononuclear dissociative bidentate geometry similar to anthocyanin dyes with α-hydroxy ketone moiety.30,31 3.1. Photovoltaic Performance. The photovoltaic performances of SY404 and DD1 as well as of D131 were characterized using DSCs made from thin 8 μm TiO2 films. The incident-photon-to-current conversion efficiencies (IPCEs) are plotted as a function of excitation wavelength in Figure 2.

ZnTe crystal. Details of the THz spectrometer were described elsewhere.22 2.7. Photoinduced Absorption Spectroscopy (PIA). The 10 Hz PIA experiment involved a 532-nm 10-ns laser pulse with ∼2.3 mJ/cm2 excitation intensity. The white-light probe was generated with a 150 W Tungsten lamp, spectrally resolved at an Oriel Cornerstone 74000 (Newport) monochromator and detected with a Si avalanche photodiode. 2.8. Ultraviolet Photoelectron Spectroscopy (UPS). The UPS experiment used the 21.22 eV (He I) excitation at ∼10−8 mbar UHV, and a Specs Phoibos 100 hemispherical analyzer with an overall energy resolution of about 120 meV. No charging of the samples was observed. 2.9. Molecular Modeling. The calculations were performed using the Materials Studio Dmol3 program (Accelrys Inc., San Diego).23−25 The ground state structure was relaxed by means of density functional theory (DFT), and the electronic transitions were calculated by time dependent DFT (TDDFT) based on the optimized ground state structure using the BLYP 4.4 exchange correlation function with a double numerical basis set with double numerical plus polarization functions (DNP).25,26

3. RESULTS AND DISCUSSIONS The structures of the two semisquarylium dyes SY404 and DD1 and the indoline dye D131 used as reference in the present study are shown in Figure 1. Semisquarylium dyes are Figure 2. Incident-photon-to-current conversion efficiency (IPCE) spectra of dye solar cells made from TiO2 layers sensitized with SY404, DD1, and D131.

All three dyes show broad action spectra with maximal IPCE values around 80%. Taking into consideration the losses due to reflection and absorption in the transparent conductive oxide (TCO), the net conversion efficiency yielded by the semisquarylium dyes are, thus, higher than 90%. Thus, semisquarylium dyes perform well as sensitizers in DSCs. For DD1, the action spectrum is much broader than for SY404 and D131. As will be shown in the following section, this red-shift is in accordance with the absorption spectra of the dyes. The IPCE onset of DD1 almost reaches 700 nm. This extended absorption yields the highest photocurrents for DSCs based on DD1. We want to note that the distinct red-shift in the IPCEs compared to the corresponding absorption spectra is due to the scattering of the employed large TiO2 particles and reflectance of the Platinum counter electrode.34 The photovoltaic performances of the DSCs based on the semisquarylium dyes SY404 and DD1 are listed in Table 1, while the corresponding J-V curves are displayed in Figure S1 of the Supporting Information. The conversion efficiency η increases from SY404 to DD1, which is due to the short-circuit current density Jsc increase assigned to the enhanced spectral response. Even though the DD1 open-circuit voltage Voc decreases, the highest semisquarylium dye conversion efficiency of 5.35% is achieved with the DD1-based DSC. 3.2. Ground State Absorption and Fluorescence. In Figure 3a the absorbance spectra of the dyes SY404, DD1, and D131 dissolved in THF are shown, revealing the absorption red-shift for DD1 due to the increased dye donor strength at the 5-indole position. Also the extinction coefficients ε extracted from the absorbance maxima increased for DD1 (see Table 2). SY404 exhibits an unstructured absorption profile with a maximum at 404 nm. The DD1 absorption onset is red-shifted

Figure 1. Structures of the semisquarylium dyes and the reference indoline dye D131.

intermediates in the preparation of unsymmetrical squarylium dyes.27−29 The core structure of the semisquarylium dyes is based on an indole moiety as the donor and a squaric acid as the acceptor. The α-hydroxy ketone, also called acyloin moiety, acts as the anchor group and is part of the acceptor unit of the molecule. Efficient binding onto the TiO2 surface via α-hydroxy ketone functional groups has been shown previously in the case of anthocyanines that occur in various natural pigments.30,31 The synthesis and analytical data of the new semisquarylium dyes are given in the Supporting Information. At the 5-indole positions of the dyes different moieties were introduced with the aim to increase the donor strength of the indole moiety. SY404 is the simplest semisquarylium dye with a hydrogen atom at the 5-indole position. DD1 contains a hexylthiophene and an ethoxy functionalized triphenylamine thienyl moiety which extends the π-conjugation of the chromophore and is known for its strong donor abilities. As a result, a red 6614



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dye-sensitized TiO2 films with the dyes dissolved in THF. As can be seen in Figure 3c−e, the semisquarylium dyes exhibit significant broadening and red-shift when adsorbed on TiO2, while D131 shows less broadening and a blue shift. While for SY404, the strong broadening is accompanied by an equally strong absorption center red shift of ∼29 nm, for DD1 only a small red shift of ∼4 nm is found. Rather, the absorption of DD1 attached to TiO2 is mostly broadened into the red when compared to the absorption in THF. In contrast, the absorption center of the indoline dyes D131 blue-shifts due to TiO2 adsorption by ∼20 nm. A blue shift had been observed for TiO2adsorbed D131 before, however significantly stronger, and was connected to a possible H-type aggregation of the adsorbed dyes.35 Also, on other cyano-carboxylic anchored organic dyes similar blue shifts had been observed, and connected either to aggregation36 or to the deprotonation of the dyes after adsorption.37,38 We did not find any signature of aggregation. However, the absorbance spectra of D131 recorded in an alkaline solution made by addition of tetrabutylammonium hydroxide shows a similar (almost 30 nm) blue shift comparable to the spectra shown in Figure 3e (see Figure S2 of the Supporting Information). This finding corroborates the assumption that the D131 blue shift in Figure 3e is caused by deprotonation.38 For the semisquarylium dye SY404, the strong red-shift and broadening of the absorbance spectra indicate strong electronic coupling of the dye chromophore to the TiO2 surface, additionally supplying the system with the possibility to harvest additional longer wavelength photons. Such red-shifts have been observed before for organic dyes strongly bound to TiO2 nanoparticles.39,40 Huber et al. associated the observed chargetransfer absorption band with a ligand to metal charge transfer interaction that might also involve a direct π(HOMO)3d(TiO2) charge-transfer excitation.40 The strong electronic coupling of the SY404 molecule to the TiO2 conduction band causes an extension of the electronic density in the excited states leading to a larger delocalization and hence a reduced optical band gap.41 In that sense, the observed red-shift and absorption broadening can be understood as a partial chargetransfer state formation,21 in which the intramolecular charge transfer is partially extended into the TiO2 conduction band. This is supported by recent DFT calculations of a very similar dye attached to TiO2, which report the LUMO electron density to be localized on both the acyloin anchor group and the TiO2.16 For DD1, the smaller red-shift indicates less electronic coupling compared to SY404; however, the strong broadening into the red indicates sufficiently strong interaction, opening optical transitions for longer wavelength photons inactive for the unattached dye. Large Stokes shifts are found for all dyes attached to TiO2 as can be seen from the comparison of absorbance and photoluminescence spectra shown in Figure 4. In all TiO2/dye photoluminescence spectra, the onset of the superimposed TiO2 photoluminescence was subtracted, and the independence of the photoluminescence spectra on the excitation energy was verified. The Stokes shift of SY404 was ∼90 nm and increases to ∼120 nm for DD1. This increase correlates with the increasing spatial separation of intramolecular donors and acceptors, and hence of occupied and unoccupied orbitals (see Figure 5). The optical band gaps of the semisquarylium dyes adsorbed on TiO2, which were obtained from the intersections of the absorbance and photoluminescence spectra shown in Figure 4, decrease from ΔE0−0 = 2.53 eV for SY404 to ΔE0−0 = 2.35 eV

Table 1. Photovoltaic Performance of DSCs Based on the Semisquarylium Dyes SY404 and DD1 As Well As the Indoline Dye D131a dyes

Jsc [mA/cm2]

Voc [mV]

FF

η [%]

SY404 DD1 D131

8.77 12.83 10.23

695 645 715

0.66 0.65 0.70

4.05 5.35 5.12

a

Conditions: irradiated light: AM1.5G (100 mW/cm2); photoelectrode: TiO2 (5 μm transparent +3 μm scattering layer), 0.188 cm2; electrolyte: 1.4 M 1,2-dimethyl-3-propylimidazolium iodide/0.15 M NaI/0.075 M I2/0.2 M tert-butylpyridine in methoxyacetonitrile. The estimated efficiency standard deviations of the measurements are ±0.5%.

Figure 3. (a) Absorbance of SY404 (green circles), DD1 (pink triangles), and D131 (light blue diamond) dissolved in THF at a concentration of c = 1.33 × 10−5 M, showing an increase of semisquarylium absorbance and red-shift with increasing dye donor strength. (b) Normalized absorbance of 2-μm thin dye-sensitized TiO2 films, also revealing the red-shift and broadening for increased donor strength. The absorption spectra of TiO2/SY404 and the reference dye TiO2/D131 almost match. (c−e) Comparison of normalized absorbance spectra in THF solution vs adsorbed on TiO2. Due to adsorption, the SY404 absorption strongly red-shifts and broadens, which is less for DD1. The D131 absorption is blue-shifted and less broadened.

Table 2. Absorption Maxima in THF and Extinction Coefficients ε of the Semisquarylium Dyes and the Indoline Reference Dyea dye

λmax [nm]

ε [M−1 cm−1]

SY404 DD1 D131

405 456 451

3.40 × 104 ± 68 5.78 × 104 ± 116 4.69 × 104 ± 92

a

The averaged values were determined from three separate absorption measurements.

by about 30 nm and shows two peaks located at 434 and 451 nm. The shoulder on the blue side seems to consist of two bands at 357 and 378 nm. The reference dye D131 is spectrally close to DD1, however less broad and unstructured. The maximum is at 452 nm. The dye-sensitized 2-μm TiO2 films also show absorption redshift with increasing donor size (Figure 3b). Now, the absorption spectra of TiO2/D131 almost match the spectra of TiO2/SY404. It is instructive to compare the absorption spectra of the 6615



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the main optical transitions and the corresponding oscillator strengths. Inspection of the low energy transitions reveals for SY404 an intense absorption band with an oscillator strength of f = 0.68 at 3.09 eV (402 nm) which is in good agreement with the 404 nm peak in the experimental absorption spectrum shown in Figure 3a. The absorption band consists of π−π* transitions of predominant HOMO to LUMO character with a smaller contribution from the HOMO-2 to LUMO transition. The HOMO and the HOMO-2 orbitals of SY404 are delocalized over the entire molecule. In the LUMO the electron density is shifted toward the squaric acid moiety including the acyloin anchor group. The calculated spectrum of DD1 shows several bands. The main energy transition corresponding to the highest oscillator strength of f = 1.21 is calculated to be 2.31 eV (536 nm). It comprises almost equally of transitions from HOMO to LUMO+1 (44%) and HOMO-1 to LUMO (42%). Thus, while in SY404 photoexcitation predominantly populates the LUMO orbital, in DD1 the optical aborption leads to an evenly distributed population of LUMO and the LUMO+1. Significant excitation of higher-lying excited states are also observed experimentally as the peak at ∼380 nm in the absorption spectrum of TiO2/DD1 does not translate into the fluorescence spectrum. The DD1 HOMO electron density is centered on the triphenylamine thiophene moiety, while the anchoring group remains unpopulated. HOMO-1 exhibits a delocalization throughout the dye (Figure 5). In the DD1 LUMO the electron density is mostly localized on the squaric acid moiety bearing the anchoring group, while the LUMO+1 shows charge delocalization over the π backbone of the dye with little contribution of the squaric acid. For SY404 the favorable charge localization of the optically excited state around the anchoring group supports the

Figure 4. Normalized absorbance (left) and photoluminescence (right) spectra of the attached dyes TiO2/SY404, TiO2/DD1, and TiO2/D131 situated in air. The Stokes shift increases from SY404 to DD1. The intersection of absorbance and photoluminescence indicates the optical band gap ΔE0−0 of the dyes adsorbed on TiO2.

for DD1. For D131, ΔE0−0 = 2.48 eV was found. The optical band gaps are used below for the TiO2/dye band alignments. 3.2.1. Calculated Optical Transitions in Semisquarylium Dyes. We have performed detailed theoretical calculations of the unattached semisquarylium dyes. The ground state structure was relaxed by means of density functional theory (DFT) calculations and the electronic transitions were calculated by the Time-dependent density functional theory (TDDFT) method with an estimated accuracy of ±0.1 eV. A graphical representation of the theoretical results for the two semisquarylium dyes are shown in Figure 5, including the isodensity plots of all relevant orbitals, their energy positions,

Figure 5. Schematic representation of the excitation energies of SY404 (left) and DD1 (right) with the corresponding oscillator strengths (f) including the main transition with their probabilities (italic), the isodensity plots of involved orbitals and the corresponding absorption spectrum obtained from TDDFT calculations. 6616



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excited state relaxation competes with the injection process, the semisquarylium dye injection times need to be faster than ∼1 ps in order to allow efficient injection. For D131 a singleexponential decay with τ ≈ 1 ns is found, which is about three to four times longer than for the semisquarylium dyes. 3.4. Electron Injection Dynamics. The photoexcitation of the dyes attached to TiO2 will lead to electron injection into the TiO2 conduction band. This injection can be followed either by the evolution of the cationic states of the dyes or the arrival kinetics of the electrons in the TiO2 conduction band. First, we will describe the formation dynamics of excited and cationic dye states following photoexcitation observed using fs-transient absorption spectroscopy. 3.4.1. Excited and Cationic State Absorption. The four dyes dissolved in THF exhibit broad excited state absorption spectra covering the entire optical spectrum, with absorption maxima at λ0 ≈ 630 nm for SY404 and at λ0 ≈ 680 nm for DD1 (see Figure 7). The peak width is broadest for SY404, while for

assumption of a strong electronic coupling to the semiconductor surface. In contrast, for DD1 the optically excited states of LUMO and LUMO+1 show both a strong and a weak charge localization on the anchoring group. Hence, a weaker electronic coupling to the TiO2 surface is theoretically expected, and confirms the experimental observation of a slightly reduced DD1 electronic coupling compared to SY404 (see Figure 3c,d). In Table 3 the Kohn−Sham HOMO and LUMO energy values are listed. The general trend of the band gap values Table 3. Calculated Energies of Kohn−Sham Frontier Molecular Orbitals of SY404 and DD1 HOMO-2 HOMO-1 [eV] [eV] SY404 DD1

−5.93 −5.22

−5.19 −4.67

HOMO [eV]

LUMO [eV]

−4.69 −4.09

−2.50 −2.67

LUMO+1 LUMO+2 [eV] [eV] −1.79 −1.99

−1.06 −1.93

estimated from the Kohn−Sham HOMO and LUMO orbital energy differences is in full agreement with the band gap decrease observed in the spectroscopic data. However, while for SY404, also the absolute values of experimental and calculated absorption are in good agreement, for DD1 a pronounced deviation is observed. Such considerable differences are, unfortunately, quite common for optoelectronic materials.42,43 Generally, the calculated optical band gaps are overestimated. This is because traditional exchange-correlation functions underestimate the energies for charge-transfer transitions. As confirmed also in our studies, the error increases usually with the distance between the negative and positive charges of the charge transfer states.44 3.3. Excited State Relaxation. In order to efficiently inject electrons into TiO2, the injection time needs to be at least 2 orders of magnitude faster than the dye relaxation into the ground state,4 which is the competing depopulation channel of the excited states. Using time-resolved photoluminescence, the lifetimes of the excited states were measured for the noninjecting case of the dyes dissolved in THF (concentration 10−5 M). For SY404 and DD1, lifetimes of 360 and 240 ps were found (see Figure 6). While the SY404 decay is monoFigure 7. Comparison of normalized excited state (in THF solution at Δtexc ≈ 50 ps) and cationic state absorption (dyes adsorbed on TiO2 in vacuum at Δtcat ≈ 1 μs) of the semisquarylium dyes and the reference dye. The excited state signals were about 3−4 times stronger than for the cation.

DD1 a second shoulder is found. The negative signal of DD1 at 500 nm is caused by the bleaching of the ground state absorption. The excited state spectra were recorded with a spectrally broad white-light probe pulse 50 ps time-delayed to the 430-nm pump pulse. The photogenerated cation absorption spectra were determined from the photoinduced absorption experiment (PIA) with the TiO2/dye samples situated in vacuum (

Efficient Electron Injection from Acyloin-Anchored ... - ACS Publications

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