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Photophysical Study of Perylene/TiO2 and Perylene/ZnO Varying Interfacial Couplings and the Chemical Environment Antje Neubauer,*,†,‡ Jodi M. Szarko,†,§ Andreas F. Bartelt,† Rainer Eichberger,† and Thomas Hannappel† †
Helmholtz Centre Berlin for Materials and Energy, Department E-I5 - Materials for Photovoltaics, Hahn-Meitner-Platz 1, 14109 Berlin, Germany ‡ Institute for Physics, University of Rostock, Universit€atsplatz 3, 18055 Rostock, Germany § Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States ABSTRACT: The impact of the chemical environment on heterogeneous electron transfer processes of two perylene derivatives chemically bound to ZnO or TiO2 colloids in a film was investigated by means of absorption and fluorescence spectroscopy. The perylene molecules varied in their electronic coupling between the perylene moiety and a carboxylic acid group, which also bound the perylene dyes to the semiconductor surface. Significantly different excited state dipole moments for the two perylene derivatives were found for certain perylene-solvent interactions and were attributed to different intramolecular charge transfer characters of the excited state. It is concluded that lifetime broadening due to ultrafast electron injection plays only a minor role in explaining the observed strong broadening of the absorption spectra for the hybrid systems with strong electronic coupling. Rather, a strong environmental effect of the colloidal semiconductor has to be taken into account for the hybrid systems with strong interfacial couplings.
1. INTRODUCTION Heterogeneous electron transfer at organic/inorganic interfaces plays an essential role in so-called artificial systems, e.g., in molecular electronics1 or dye solar cells2 (DSCs). Despite the complex interplay of numerous processes, DSC devices with efficiencies of more than 11% (AM 1.5) were achieved, using ruthenium complexes chemically bound to colloidal TiO2 films, and surrounded by a wet-chemical redox system.3 During the past few years, increasing efforts were made to use pure organic dyes instead of transition metal organic complexes. A broad variety of pure organic dyes have been developed,4 e.g., triphenylamine, coumarin, perylene, or indoline dyes, achieving DSC efficiencies beyond 9%,5,6 when electrolytes were used with specific coadsorbents and additives. The electron injection from the excited dye state acting as an electron donor to a manifold of unoccupied electronic states in the semiconductor conduction band acting as an electron acceptor is the core process in DSC and depends strongly on the surrounding environment.7 In general, the electron injection rate for a heterogeneous electron transfer reaction from an excited dye state to a semiconductor depends on several parameters, namely, (a) the strength of the electronic coupling between the electron donor and acceptor, (b) the density of unoccupied states in the semiconductor conduction band, and (c) the reorganization energy, which is mainly determined in a liquid environment by the reorganization energy of the solvent. To achieve both a better understanding of the photoinduced heterogeneous electron r 2011 American Chemical Society
transfer mechanism and an increase of the electron injection to recombination ratio, many experimental and theoretical studies have been performed addressing at least one of these origins (a)-(c). For instance, to study the impact of electronic coupling between a dye and a semiconductor (a), theoretical and experimental studies have been performed, e.g., via the modification of the anchor group (Scheme 1), which chemically binds the dye molecule to the semiconductor surface. The electronic coupling strength was found to be stronger for dyes bound to a TiO2 surface via a carboxylic acid anchor group compared to a phosphonic acid anchor group, and was even stronger for two hydroxyl groups.8 For this case and for two carboxylic acid groups as anchor groups, ultrafast electron injection of some femtoseconds (fs) was observed experimentally.9-11 A different approach to control the electronic coupling strength is to modify the so-called bridge or spacer group, i.e., the molecular group between the electron donor moiety of the dye and the anchor group. (Within this paper, we will call this dye moiety “bridge”, regardless of its electronic distribution that correlates with its function as a spacer or a bridge group for electronic coupling.) By varying the molecular structure of the bridge (and anchor) group, and thus its electronic coupling strength, an electron injection deceleration by several orders of magnitude was observed for Received: October 7, 2010 Revised: February 3, 2011 Published: March 07, 2011 5683
dx.doi.org/10.1021/jp109615b | J. Phys. Chem. C 2011, 115, 5683–5691
The Journal of Physical Chemistry C hybrid systems with perylene dyes bound to TiO2 colloidal films.12 The relation between electron injection time and the density of unoccupied states in the semiconductor conduction band (b) has also been examined in several studies.13-15 For the different semiconductor substrates, e.g., colloidal TiO2, SnO2,13 or ZnO,13,15 an increase of the electron injection times from TiO2 to ZnO was observed due to the decrease in the density of unoccupied states in the conduction band. In addition, a decrease of the adsorbate excited state oxidation potential corresponded with a decrease of the semiconductor density of states, which led to a slower electron injection time.14 The impact of the reorganization energy (c) was investigated by theoretical simulations suggesting reduced electron injection as reorganization energy is increased.16,17 For a better understanding of these parameters, the effect of the individual parameters would be important to characterize. However, modifying one parameter will most likely affect at least one of the others. The chemical environment influences (a)-(c) as well. We use the term “(chemical) environment” within this paper for all parameters of the dye’s surrounding medium affecting the electronic states of the dyes, besides those directly associated with the electron transfer. These effects include both general interactions, where the environment acts as a continuum (e.g., refractive index, dielectric constant), as well as specific interactions, i.e., interactions between individual dye and solvent molecules (e.g., hydrogen bonding). More general, we applied this concept here to interactions of the dye with the semiconductor colloids acting as the surrounding medium and its electronic interactions with the dye molecules. Especially due to the crucial impact of the chemical environment on the DSC performance, environmental effects received recently increasing attention. The important effect of different dye solutions on the DSC efficiencies was explained by solvent-dependent absorption spectra and binding modes of the bound dyes.18 The impact of different solvents on the interfacial electron transfer dynamics was studied in dye-sensitized, colloidal TiO2 films.19 In different liquid environments the interfacial electron transfer dynamics for C3e/TiO2 and C3/TiO2 slowed by several orders of magnitude.7 All these studies showed the crucial impact of the chemical environment on the electron injection dynamics or the DSC performance. Many studies of electron injection dynamics actually relied on transient or steady-state absorption measurements and theoretical calculations.7,12,15,20 The spectral broadening of the steady-state absorption spectra was correlated with the lifetime broadening due to ultrafast electron injection dynamics and so far neglecting the environmental effect of the semiconductor.20 A detailed analysis of how the chemical environment affects the dye electronic states is still missing. Here we study in detail the impact of TiO2 and ZnO colloids as a chemical environment for the attached perylene derivatives C3e and C3 (Scheme 1). C3e and C3 are well suited for this study due to their different excited state behavior concerning electron injection into TiO2 and ZnO nanoparticles.15 Furthermore, due to their electronic properties perylene derivatives have been extensively investigated as model systems for the mechanistic study of the injection process in DSC,7,11,12,15,20-22 and hence many photophysical properties of these molecules are already well understood from an experimental and theoretical point of view. Thus, C3e, C3, TiO2, and ZnO make an ideal combination for studying the environmental effects of the semiconductor on the chromophore properties. In the course
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of this paper we show that the properties of the chemical environment are an essential factor for the absorption band broadening in strongly electronically coupled hybrid systems, which has been neglected so far. Recently, various solvent studies of similar perylene derivatives like nitroperylene or formylperylene have been performed.23-25 Here, the examination of the solvent effect on C3e and C3, which reveals the different dipole moments and (partial) intramolecular charge transfer characters (ICT) for C3e and C3 in the first excited singlet state, will serve as a basis for discussing the large broadening of the steady-state absorption bands for the organic/inorganic hybrid systems with strong electronic coupling.
2. EXPERIMENTAL DETAILS 2.1. Steady-State Absorption and Fluorescence Measurements. Steady-state absorption and fluorescence spectra were
recorded using a Perkin-Elmer Lambda 35 UV/vis, a Spex Fluoromax, or an SLM Aminco-Bowman AB2 spectrofluorimeter, respectively. For the spectroscopic measurements all solvents used were of Merck Uvasol quality, except toluene (Sigma Aldrich, spectrophotometric grade). To avoid self-absorption or aggregation effects, concentrations of less than 5 10-6 M were used. All emission spectra were recorded on a wavelength scale as quanta per nanometer. They were corrected for instrumental sensitivity. For conversion of the spectra to a wavenumber scale, the wavelength-dependent intensities were multiplied with λ2.26 The synthesis of the perylene derivatives 3-(8,11-di-tert-butyl-perylene-3-yl)propionic acid (C3e) and 3-(8,11-di-tert-butyl-perylene-3-yl)acrylic acid (C3) (Scheme 1) was described elsewhere.27 The tert-butyl groups at the perylene skeleton prevented the formation of dimers or excimers in sandwich configuration as is known for the unsubstituted perylene.28 The carboxylic acid group chemically bound the dye molecules to the surface. Quantum chemical calculations were performed using the Ampac program package29 and employing the semiempirical method AM130 to determine the ground state dipole moments for the fully optimized ground state geometries. 2.2. Preparation of Dye-Sensitized Colloidal Substrates. The colloidal TiO2 (anatase) and ZnO (wurtzite) films have been synthesized by adapting previously described methods.31,32 The TiO2 colloids had an average diameter of roughly 10 nm, while the ZnO colloids were roughly 30 nm in diameter (measured by TEM). The thickness of the TiO2 and ZnO films was in the range of 3-4 or 1-2 μm, respectively. The glass substrate was of 50 μm thickness (Schott display glass AF45). More details were described elsewhere.15 Prior to adsorption of the perylene dyes, the TiO2 or ZnO films were heated in a muffle furnace at 430 °C for 45 min or at 400 °C for 60 min, respectively, and cooled to about 50 °C under a light stream of argon. The dye molecules were adsorbed by immersion of the TiO2 or ZnO films for 50 or 30 min in 10-4-10-5 M solution of the perylene derivatives in dried and freshly distilled toluene in an argon environment. During immersion, the films were gently shaken with an IKA MS3 shaker. After adsorption of the dyes, the films were rinsed with dried toluene and immersed in toluene for 10 min to remove the excess dye molecules that did not bind to the surface. Prior to steady-state absorption measurements, the dye/ semiconductor samples were evacuated in a desiccator (p ∼ 10-4 mbar) for two hours. 5684
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The Journal of Physical Chemistry C Scheme 1. Investigated Perylene Dyes: 3-(8,11-Di-tert-Butylperylene-3-yl)propionic Acid (C3e) and 3-(8,11-Di-tert-butyl-perylene-3-yl)acrylic Acid (C3)
2.3. Transient Absorption Measurements. The transient absorption measurements were performed using a Coherent RegA 9050 regenerative amplifier operating at a repetition rate of 150 kHz and providing 50 fs fwhm (full width at halfmaximum) pulses with a central wavelength of λ0 = 800 nm. Roughly 85% of the output was used to generate sub-15 fs pulses with a central wavelength of about 440-450 nm using an infrared noncollinear optical parametric amplifier (NOPA), which has been described previously.33 Briefly, 880 nm pulses were compressed using standard fused silica prisms and then frequency doubled using a 100 μm type I BBO crystal. The average pump pulse energy was typically less than 10 nJ. The remaining output from the amplifier was focused into a 2 mm sapphire crystal to create a white light continuum probe. For the transient absorption spectra, the uncompressed white light spectrum was optimized for the 530-750 nm region. The fundamental 800 nm light was cut off using a high pass filter. The cross correlation at the different wavelengths ranged from 40 to 100 fs and was broader for the shorter wavelengths.
3. RESULTS AND DISCUSSION 3.1. Steady-State Absorption Spectra of the Dye/Semiconductor Systems. Figure 1 shows the normalized absorption
spectra for the hybrid systems C3e/ZnO, C3e/TiO2 (a), C3/ ZnO, and C3/TiO2 (b). Absorption spectra of the bare nanostructured ZnO and TiO2 films were also plotted (dashed lines in Figure 1a and b). In the spectral range shown in Figure 1, two types of absorption transitions could be assigned. The transition at lower energies originated from the S1 r S0 absorption band of the perylene molecules C3e (a) and C3 (b) bound to the semiconductor. The absorbance at higher energies originated from optical transitions within the nanostructured semiconductor films and reflected the absorption band edges of the bare ZnO or TiO2 films at about 385 nm (3.22 eV) and 365 nm (3.40 eV), respectively. The observed absorption band edge depended strongly on the properties of the semiconductor samples, such as colloid size, layer thickness, etc. To correct the absorption spectra for the background originating from the optical transitions within the semiconductor films, the absorption spectra of bare TiO2 or ZnO films from the same batch and of neighboring positions were measured and subtracted from the ones for the dye-sensitized films. These background corrected spectra are shown in Figure 1c and d. The assignment of the absorption
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bands for the optical transitions of the dyes is discussed in section 3.2 on the basis of the unbound dyes. The steady-state absorption spectra of these four hybrid systems showed the following features: (i) the highest transition probability was observed for the 0-1 Franck-Condon transition of the S1 r S0 absorption band, (ii) an energetic red shift of the S1 r S0 absorption band was detected for the C3 compared to the C3e hybrid systems, (iii) the energetic position of the S1 r S0 absorption band for C3e hybrid systems did not depend on the choice of the semiconductor, in contrast to the absorption band for the C3 hybrid systems, and (iv) a strong broadening of the S1 r S0 absorption band occurred for the C3 hybrid systems. The origins of these trends are diverse and will be discussed below: (i) For each of the four hybrid systems the 0-1 vibrational transition of the S1 r S0 absorption band was the Franck-Condon transition with the highest intensity (Figure 1a-d). In contrast, the S1 r S0 absorption band of the unbound molecules C3e and C3 in toluene and methanol solution (see also Figure 1c and d) showed the highest intensities at the 0-0 vibrational transition. A slight distortion of the relative intensities for the hybrid systems has to be considered: Especially in the ZnO hybrid systems the optical transition of the semiconductor in the spectral range of dye absorption >390 nm (
