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J. Phys. Chem. C 2010, 114, 17964–17974
New Type II Catechol-Thiophene Sensitizers for Dye-Sensitized Solar Cells Byeong-Kwan An,†,‡ Wei Hu,‡ Paul L. Burn,*,‡ and Paul Meredith*,‡ Department of Chemistry, The Catholic UniVersity of Korea, Wonmi-gu, Bucheon-si, Gyeonggi-do 420-743, Korea, and Centre for Organic Photonics & Electronics, School of Chemistry and Molecular Biosciences & School of Mathematics and Physics, The UniVersity of Queensland, Queensland 4072, Australia ReceiVed: June 21, 2010; ReVised Manuscript ReceiVed: August 16, 2010
We report a new class of thiophene-catechol light harvesting molecules for dye-sensitized solar cells. The catechol-titania combination has been described as a Type II injection system where it is thought charge injection occurs directly from the highest occupied molecular orbital of the dye to states near the titania conduction band edge. We show that the optical absorption of the catechol-titania complex moves to longer wavelengths with extension of the conjugation (1, 2, or 3 thiophene units). Binding of the dyes is via a bidentate catechol-titania bridge with the strength of the binding following the acidity of the catechol: the more acidic the stronger the binding. The binding strength was found to increase with increasing chromophore length. We have optimized device performance based upon dye loading and find open circuit voltage and short circuit current density are consistent with the observed improvements in light harvesting efficiency with extended conjugation. However, the maximum open circuit voltage is observed at low dye loadings, and dark current-voltage and open circuit voltage decay measurements indicate that recombination plays a significant role in suppressing device performance as dye loading increases. The interplay between open circuit voltage and short circuit current as a function of dye uptake means that efficiencies in these catecholthiophene sensitizers are limited. It is clear that these factors need to be independently controlled if Type II systems are to challenge Type I injection systems for high performance dye-sensitized solar cells. Introduction Dye sensitized solar cells (DSSCs) are widely viewed as a viable third generation photovoltaic technology with laboratoryscale verified conversion efficiencies exceeding 11% and onmodule efficiencies of ∼9.2%.1 DSSCs are photoelectrochemical devices where the light harvesting element is a nanoscopically distributed heterojunction composed of a UV-visible absorbing organic or organometallic dye bound to a wide band gap mesoporous metal oxide such as titanium dioxide (titania) which acts as the photoanode. Upon photoexcitation the organic dye is oxidized by the titania and is said to “sensitize” the photoanode to the solar spectrum. The oxidized dye is reduced via a liquid electrolyte redox couple (such as iodide/triiodide)2 or a solid-state hole-transport material/gel electrolyte.3 The circuit is completed by a high work function cathode (for example platinum) and the system is capable of many tens of thousands of cycles. Rapid (fs) and efficient (near unity quantum yield) charge generation by electron transfer across the photoanode-dye heterojunction interface are central to the overall external power conversion performance, as is the prevention of parasitic back-reactions that directly reduce photocurrent. The maximum achievable open circuit voltage generated by a DSSC is strongly dependent upon the energy difference between the photoanode quasi-Fermi level (under illumination) and the electrolyte redox potential.4 In the highest efficiency cells based upon heteroleptic ruthenium(II) complexes (for example, N3[cis-di(thiocyanato)-bis(4,4′-dicarboxylate-2,2′-bipyridyl)]ruthenium(II)), the maximum open circuit voltage of ∼0.7-0.8 V is * To whom correspondence should be addressed. E-mail: p.burn2@ uq.edu.au;
[email protected]. † The Catholic University of Korea. ‡ University of Queensland.
achievable with short circuit current densities of ∼15 mA/cm2.5 Higher open circuit voltages of 1 V have been achieved with a fully organic dye and magnesium oxide modified titania photoanodes.6 Dyes such as N3 based upon the ruthenium(II) complexes core have optical absorption bands in the visible dominated by transitions with metal-to-ligand and ligand-to-ligand character.7 They also possess excellent photochemical stability and have long-lived excited states because of intersystem crossing to the triplet state caused by the heavy atom effect of the ruthenium(II). In addition, when the ruthenium(II) complex is excited there is intramolecular separation of hole and electron density. In the case of N3, the hole-density is thought to be localized on the ruthenium atom and isothiocyanate ligands while the lowest unoccupied molecular orbital density (LUMO, to the first order the excited state electron density) is localized on the carboxylate functionalized bipyridyl ligand. The dicarboxylate groups form a bidentate attachment to the underlying photoanode (titanium dioxide), which facilitates the rapid and efficient electron transfer. The process can be considered as transfer of an electron from the dye HOMO (highest occupied molecular orbital) to states near the titania conduction band edge via the intermediate dye LUMO level. This type of photoinjection, often referred to as “Type I” (Figure 1), is energetically favorable as the LUMO has a lower electron affinity than the conduction band of the titania thus providing the necessary energy offset for exciton splitting. An additional benefit of a Type I process and spatial localization of excited state charge density is that parasitic back reactions (for example the transfer of the photoinjected electron back to the electrolyte) are minimized. As a result, N3-type dyes
10.1021/jp105687z 2010 American Chemical Society Published on Web 09/16/2010
New Type II Catechol-Thiophene Sensitizers
Figure 1. Schematic showing Type I and Type II photoinjection in a DSSC. Binding via a carboxylic acid such as in N3 allows Type I injection which occurs via the dye excited state. Binding via a bidentate catechol bridge yields Type II injection where excitation is a low energy charge transfer state of the dye-titania complex. The solid arrows in both cases indicate the actual pathway for excitation and photoinjection and the dashed arrow in the Type II case indicates “the apparent” pathway from the isolated dye ground state.
display sufficient dominance of the forward reaction versus all back reactions to generate almost unity internal photoelectron yield.8 Several other metal-free organic dye types have also been shown to be effective light harvesting systems in the dye sensitized architecture.9 Kim et al. have demonstrated a 9.1% device with a triphenylamine-based dye.10 Such “pure” organic dyes are perceived to have a number of notable advantages over their organometallic cousins including cost, ease of functionalization and modification to tune adsorbtivity and absorptivity, and compatibility with potential polymeric electrolyte replacements. However, to date, organometallic complexes remain the most efficient and widely used and are the first to be commercialized. It has also been shown that particular titania-dye binding configurations can produce so-called Type II photoinjection. In Type II injection it has been proposed that direct injection of an electron occurs from the dye ground state to the titania conduction band edge under photoexcitation (Figure 1). However, a two-step process via a ligand-to-metal transition from the catechol HOMO to a titanium d orbital followed by charge separation into a titania conduction band state is also a possibility. Type II injection has been less well studied than Type I injection, and in this work, we will just describe the charge injection process as Type II independent of the exact process responsible for charge separation. Reports on Type II injection have focused on bidentate catechol chelation to the titania which leads to very rapid processes ( C3 (0.26 V) measured in N,N′-dimethylformamide and referenced against the ferrocenium/ferrocene couple). Figure 4a shows standard titania photoanodes modified with C1, C2, and C3. The photoanode color changes from light orange (C1) to dark red/purple (C3), which indicates a decrease in the transition energy. This was confirmed by the catechol/ transparent titania absorption spectra in Figure 3 which also shows an increase in the optical absorption at longer wavelengths with increased conjugation length: A550nm - C1(0.23) < C2(0.67) < C3(1.26) (see also Table 1). Two possible reasons for the absorption being moved to longer wavelengths in these chelated systems has been advanced, namely: (i) destabilization of the catechol HOMO level and/or (ii) increased dipole moment of the surface-bound Ti-ligand complex via an induced charge transfer dipole under excitation.14a The electrochemical analysis clearly shows that increasing the conjugation length in going from C1 to C3 destabilizes the HOMO. In addition, as shown in Table 1 the Ti-ligand complex dipole moments (in Debye
Figure 3. Absorption spectra of catechol sensitizers in ethanol (5 × 10-5 mol/L) with corresponding molar extinction coefficients, and surface modified transparent TiO2 semiconductor substrates with different sensitizers with the optical density (OD) shown in arbitrary units. The bars on the bottom indicate the calculated absorption λmax of each catechol sensitizer determined by using a PM3 parameter and CI matrix (HyperChem).
as calculated using a ZINDO-1 semiempirical methodology in HyperChem 7) increases across the series; C1(1.69) < C2(3.07) < C3(5.89), which is consistent with the proposed effect of the dipole moment. Clearly complexation of these catechols to the titania has destabilized the HOMOs of the catechols to form the titania-catecheol complex states. Luminescent quenching experiments are often used to probe charge separation in organic photovoltaic devices. However, it should be noted that luminescence quenching does not always lead to free charge carriers and that an excited state can decay nonradiatively. Nevertheless, we next investigated whether complexation of the catechols to the titania would lead to luminescence quenching. Figure 4b shows a series of images of C1, C2, and C3 adsorbed onto both silica and titania nanoporous substrates. When irradiated with 365 nm light (i.e., well above the predicted optical absorption edges shown in Figure 3), the catechol fluorescence is completely quenched on the titania substrate (Figure 4b) and not on the silica substrate. If the quenching of the luminescence is due to charge separation then photovoltaic activity will be seen in-device. ZINDO-1 calculations of the HOMO and LUMO electron densities show that the photostimulated exciton (LUMO density) is localized on the Ti-catechol chelate site in the case of the complex, and remains delocalized over the catechol-thiophene system in the unbound case [Figure 4c]. Effect of Conjugation Length on the Adsorption/Desorption of Catechol Sensitizers. Adopting an identical methodology to that described previously for ruthenium(II) dye uptake, we measured the kinetics of adsorption of the C1-C2-C3 family.22 Figure 5a shows dye adsorption (qt) as a function of time for each of the catechols onto standard titania photoanodes. The data can be fitted to a pseudo-second order model of the form (see Figure S1)
t 1 1 + t ) qt qe k2qe2
(2)
where qt and qe are dye uptake amounts at time t and equilibrium respectively, and k2 is the adsorption rate constant. This indicates that the rate determining step of the overall adsorption process
New Type II Catechol-Thiophene Sensitizers
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TABLE 1: Optical Properties of Catechol Dyes C1, C2, and C3 dye-adsorbed TiO2 substratea
in solution 2
dye
λmax (nm) ( (L mol-1 cm-1))b
EOX(V)c
HOMO (eV)d
onset absorption band (nm)
absorbance @ 550 nme
dipole moment of Ti-ligand complex (Debye)f
C1 C2 C3
372 (23 438) 411 (29 651) 429 (33 434)
0.31 0.29 0.26
5.11 5.09 5.06
571 596 636
0.23 0.67 1.26
1.69 3.07 5.89
a Catechol dyes were adsorbed from ethanol onto the transparent TiO2 (12 µm) substrates over a 24 h period. b 5 × 10-5 mol/L in ethanol. First E1/2 versus Fc+/Fc in N,N′-dimethylformamide. d EHOMO ) Eox - EFc/Fc+ + 4.8 eV. e Absorbance was measured by using a bare transparent TiO2 (12 µm thickness) substrate as a reference. f Dipole moment calculations were performed by using a ZINDO/1 semiempirical parameter (HyperChem 7.0). c
Figure 5. (a) Dye uptake versus time for the catechol sensitizers onto TiO2 anode substrates. Experimental data are plotted as points and the fit (curves) are based on a pseudo-second order model. (b) Desorption measurements of the catechol sensitizers using aqueous sodium hydroxide solution (1.0 M).
which is more consistent with intraparticle diffusion being the rate determining step.26 We have modeled this behavior (see Figure S2) according to the equation Figure 4. (a) Photo of the surface modified transparent TiO2 (12 µm) semiconductor anodes with the catechol sensitizers in ethanol (1 × 10-4 mol/L). (b) Photos of the fluorescence of the catechols adsorbed onto SiO2 (TLC substrates) and TiO2 substrates under illumination at 365 nm. (c) Electron density distribution of the HOMO and LUMO of C3 and Ti-bound C3 using a semiempirical calculation (ZINDO-1 parameter).
is chemisorption of the dye onto the titania surface.25 Looking at the initial rates of dye uptake in the first 6 h, one observes quite marked differences between C1 and C2/C3 and behavior
qt ) Kpt1/2
(3)
which gives intraparticle diffusion rate constants (Kp × 1015 cm-2 h-1/2) of C1 (7.0) > C2 (5.2) > C3 (5.0). Irrespective of this early time behavior, the total dye uptake amount (or qe) was found to be C1 (117.6) < C2 (122.8) < C3 (138.7) (×1015 cm-2 h-1/2). We calculated the molecular volumes of each catechol by the semiempirical PM3 method (Figure S3) giving C1 (694.92 Å3) < C2 (833.33 Å2) < C3 (1073.34 Å2). In
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previous work with N3-like ruthenium(II)-complexes we observed the inverse relationship between uptake and molecular volume (i.e., smaller dyes have higher uptake),22 and hence, this total dye uptake result was somewhat surprising since the larger dyes have lower diffusion coefficients. We believe that for these catechol dyes the relative binding constants is an important factor in the dye uptake. In the study on the effect of molecular volume on the performance of the ruthenium(II) dyes the binding of the carboxylate anions was the same for all the dyes whereas for the catechol dyes the chromophore is different and hence the relative binding energy can be different. In the catechol-titania surface reaction the -OH catechol group substitutes for the OH group in tTiOH1/3- to form a surface complex,27 and therefore, one may expect that the acidity of the catechol group plays an important role in the reaction. Calculations of the relative energies of the protonated and conjugate base forms show that the order of acidity for the catechol dyes should be C1 < C2 < C3 (see Figure S4). To determine whether the strength of binding followed the relative acidities we undertook a Benesi-Hildebrand analysis to calculate the catechol binding constant K (Figures S5 and S6).14 K
Tisurface + catechol y\z CT complex
We found K (M-1) to be C1 (6,296) < C2 (12,837) < C3 (30,261), confirming the more acidic catechol binds more strongly. We also estimated the occupant areas of each dye on the titania surface and found that there was only a small difference from C1 to C3 (Figure S3), and this is due to the cylindrical shape of the catechol dyes rather than the more globular shape of the ruthenium(II) complexes. In summary, it is clear that although molecular volume through pore diffusion affects initial dye uptake, the equilibrium value is determined by the chemical reactivity of the catechol group with the titania surface. It should also be noted that the bidentate binuclear bridging geometry required for the formation of the Ti-catechol HOMO state was confirmed using attenuated total internal reflection Fourier transform infrared spectroscopy (ATR-FTIR). We observed the typical ν(C-O) and ν(aromatic CC) modes at 1300 and 1490 cm-1, respectively which have been reported to be due to bidentate catechol binding to titania (Figure S7).14b,17 With respect to dye desorption stability (an important characteristic for long-term device performance) we performed an “accelerated” desorption study using aqueous 1.0 M NaOH solution as a means to enhance detachment by nucleophilic attack. Note, in a similar study on dyes bound via carboxylate ligands we found that 0.1 M NaOH was sufficient to desorb simple dyes in short time scales.22 The catechol binding is stronger than carboxylate and we found the stability against desorption to be in the order C1 < C2 < C3 [Figure 5b] consistent with the binding constant measurements. It is also possible that the enhanced stability with C2 and C3 is related to, at least in part, the dye hydrophobicity which increases with conjugation length (log P: C1 (2.71) < C2 (3.45) < C3 (4.19)).28 Summarizing the findings so far we have observed (i) strong tunable optical absorption in the visible region, (ii) evidence for exciton quenching possibly leading to charge generation, and (iii) stable binding. Effect of Conjugation Length and Dye Uptake on DSSC Photovoltaic Performance. Multiple devices for each dye were fabricated at a range of dye uptake amounts. The cells were fabricated as detailed in the experimental section and tested
An et al.
Figure 6. Representative J-V (current density versus voltage) curves of catechol dye (C1, C2, and C3) based DSSCs.
under closely controlled and calibrated AM1.5G conditions. It is also important to note that spectral mismatches were maintained
