ARTICLE pubs.acs.org/JPCC
Confined Photodynamics of an Organic Dye for Solar Cells Encapsulated in Titanium-Doped Mesoporous Molecular Materials Marcin Ziozek,† Cristina Martín,† Maria Teresa Navarro,‡ Hermenegildo Garcia,‡ and Abderrazzak Douhal*,† †
Departamento de Química Física, Facultad de Ciencias Ambientales y Bioquimica and Inamol, Universidad de Castilla-La Mancha, Avda. Carlos III, S.N., 45071 Toledo, Spain ‡ Instituto de Tecnologia Quimica, CSIC-UPV, Avda. de los Naranjos s/n, 46022, Valencia, Spain
bS Supporting Information ABSTRACT: A triphenylamine dye (TPC1) encapsulated in titanium-doped mesoporous silica structures as alternative materials for dye-sensitized solar cells has been studied by means of stationary absorption and emission as well as ultrafast emission spectroscopy. For the samples prepared by a grafting method, a TPC1 complex with titanium atoms within the mesoporous silica in dichloromethane (DCM) solution is formed, having a red shift of the visible absorption band by about 1300 cm1 with respect to that of the TPC1 in DCM (from 455 to 485 nm). For the complexes, multi- exponential emission quenching of the relaxed singlet excited state occurs with time constants from 300 fs to 30 ps and is assigned to the confined electron injection process into the TiO chromophore. The averaged electron injection rate from the higher energy levels gets smaller values for less energetic probing, from 2.7 1012 s1 at 600 nm to 1.5 1012 s1 at 700 nm. However, in the titanium-doped samples prepared by an impregnation method, we observed about 23 times slower injection. The difference is explained by different coupling between TPC1 and titania domains. As a reference to the confining effect on the dynamics, we also studied the behavior of TPC1 when interacting with amorphous silica and purely siliceous MCM-41 material in the same solvent. In amorphous silica, an equilibrium between neutral and anion structures of TPC1 is found to be shifted toward the anion form. For the MCM-41 material, the presence of a new absorption band at around 690 nm is revealed, assigned to the spontaneously created and remarkable stable TPC1 radical cation. The lifetimes of the normal and anion forms in both materials were found to be similar to those in solution. The femtosecond relaxation dynamics in these materials is also similar to that in solution (dominated by the solvation), but additional emission quenching in the TPC1/MCM-41 sample is observed, probably due to intermolecular energy transfer. The rate of energy transfer was estimated to decrease gradually when increasing the observation wavelength, from 1.11 1012 s1 at 500 nm to 0.13 1012 s1 at 700 nm. We believe that our results interrogating ultrafast dynamics of an efficient dye interacting with titania within the mesoporous materials will help in a better understanding and improvement of dye-sensitized solar cells.
1. INTRODUCTION Dye-sensitized solar cells (DSSCs) are one of the most promising photovoltaic systems to convert sunlight to electricity.1,2 The best DSSC energy conversion efficiency has reached 12% with the use of Ru complex dyes and titania nanoparticle film.3,4 To improve DSSC costs and performance, the synthesis of new, metal-free organic dyes for DSSC has been reported recently,59 with the aim to find alternatives to the Ru complexes. Moreover, the use of titania nanoparticle film in DSSC provides some drawback due to relatively slow charge collection (trap-limited electron diffusion) and the inaccessible interior for the sensitizing dyes (the individual titania particles are nonporous). To avoid the latter limitation, alternative materials based on titanium-doped zeolites10,11 and mesoporous hexagonal structures (MCM-41 type)12,13 have been recently proposed. Such types of materials were shown to successfully increase the photocatalytic activity of titania.13,14 The surface area of MCM-41 can reach 1000 m2/g, which is about 20 times more than that of standard nanoparticle film. It has been shown for the prototype solar cells made of Ru complexes and that kind of Ti-doped zeolites and MCM-41 r 2011 American Chemical Society
that, although the total photocurrent is smaller, the specific current density per titania atoms can be comparable to that of standard titania nanoparticles.11,12 Also, the open circuit voltage is similar to that of the standard cells, which makes these new materials very promising and worth further study. According to our knowledge, no time-resolved studies of the electron dynamics have been reported so far for such materials, in particular, the investigations of the electron injection rate, whose determination is one of the aims of this contribution. We used two kinds of Ti-doped mesoporous structures. In the first one, titanium atoms were incorporated into the previously prepared silica framework by the grafting method (at two doping concentrations), while in the second method the small titania nanoparticles were used as one of the components for building the hexagonal mesoporous structure. To probe the behavior of these materials, we have chosen a metal-free, TPC1 dye that Received: February 18, 2011 Revised: March 28, 2011 Published: April 12, 2011 8858
dx.doi.org/10.1021/jp201627t | J. Phys. Chem. C 2011, 115, 8858–8867
The Journal of Physical Chemistry C Scheme 1. Molecular Structure of TPC1 Dye and Schematic Representation of Its Attachment to Titania-Containing Mesoporous Structures of MCM-41 Type (Ti1MCM-41 and Ti2MCM-41) and SBA-15 Type (TiSBA-15)a
a
For clarity, the size of the TPC1 molecule is enlarged by about 2 times.
consists of a triphenylamine unit as an electron donor and a cyanoacrylic acid group as an electron acceptor, separated by an oligophenylenevinylene unit (Scheme 1). Its promising use in DSSC has been recently reported.5,9 Moreover, we have previously measured the fast and ultrafast behavior of this dye in different solvents15 as well as interacting with all-titania nanoparticles and nanotubes in suspension.16 As a reference for the encapsulation effect, we have also studied in this contribution the behavior of TPC1 in purely siliceous mesoporous molecular sieves (MCM-41). To understand the interaction with the silica surface lacking the caging effects, we have also studied TPC1 interacting with amorphous silica. The samples have been studied by means of stationary absorption and emission spectroscopy and time-resolved emission spectroscopy in the temporal range from 50 fs to several nanoseconds. We believe that the present study is important to better understand the photobehavior of organic dyes interacting with new titania-doped materials for solar cells and catalysis. They should be also relevant for the fundamental knowledge on the real reaction times of the dyes encapsulated in host materials.1722
2. EXPERIMENTAL SECTION TPC1 dye was synthesized and purified as previously described.9 The purity of the dye was checked by measuring the absorption and emission spectra of TPC1 and comparing them with the previously published data on the same dye.9,15,16 Moreover, the same absorption and fluorescence excitation spectra indicate the presence of only one species in the ground state. Dichloromethane (DCM, anhydrous spectral, 99.9%, Aldrich) and acetonitrile (ACN spectroscopic grade >99.5%, Aldrich) were used as received. Purely siliceous mesoporous material MCM-41 (BET area: 1000 m2/g, 2.5 nm average pore diameter) was purchased from Sigma-Aldrich, and we reconfirmed its mesoporous structure using isothermal nitrogen adsorption and powder X-ray diffraction techniques. As a reference for a siliceous material having silanol groups but lacking
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the nanochannel structure of MCM-41, the silica particles were used (Merck, particle size: 0.0630.200 mm) after consecutive washing with n-hexane, tetrahydrofuran, and DCM. In some places, we refer to our previous studies16 of TPC1 interacting with titanium oxide particles (P25, Degussa, particle size: ∼21 nm, BET area: 50 m2/g). Titania-containing mesoporous structures (TiSBA-15, Ti1MCM-41, Ti2MCM-41) were prepared in the following way. Ti1MCM-41 and Ti2MCM-41 samples were prepared by grafting of titanium alkoxide using MCM-41 mesoporous silica as support. First, pure silica MCM-41 was synthesized from a gel having the following molar composition: SiO2:0.15CTABr:0. 26TMAOH:24.3H2O, where CTABr is cetyltrimethylammonium bromide and TMAOH is tetramethylammonium hydroxide. The crystallization was performed at 100 °C for 48 h in Teflon-lined stainless steel autoclaves in static conditions. The organic material was removed by calcination at 540 °C in N2 flow for 1 h and subsequently for 5 h in air flow. Ti-grafted MCM-41 samples with two different loadings of Ti were prepared using Ti(OEt)4 (Aldrich) as the source of titanium and dry ethanol as solvent. In a typical procedure, 3 g of calcined pure silica MCM41 was outgassed at high temperature (573 K, 2 h). Then, 30 g of an ethanolic solution containing 0.6 and 0.124 g of Ti(OEt)4 for Ti1MCM-41 and Ti2MCM-41, respectively, was added on the dehydrated MCM-41. The suspension was stirred for 12 h under N2 at room temperature, filtered, and washed with dry ethanol. The solid was then dried at 373 K and calcined at 773 K for 6 h. The MCM-41 structure was preserved after grafting. Titanium loading and the textural properties of the Ti-grafted samples were evaluated (Ti1MCM-41: 3.5% TiO2, BET area: 1010 m2/g, pore volume: 0.70 cc/g, pore diameter: 3.5 nm; Ti2MCM-41: 6.3% TiO2, BET area: 991 m2/g, pore volume: 0.99 cc/g, pore diameter: 3.5 nm). The TiSBA-15 sample was prepared using colloidal TiO2 nanoparticles as the titanium source and following the method reported by García et al.13 TiO2 nanoparticles were prepared by controlled hydrolysis of Ti(OiPr)4 that renders a fairly homogeneous size of about 5 nm. These TiO2 nanoparticles were used in combination with Si(OEt)4 (1 wt %) that acts as a binder of the framework and Pluronic 123 as a template agent under conditions previously reported for the synthesis of SBA-15. The Ti content and the textural properties of the TiSBA-15 sample were determined (3% TiO2, BET area: 820 m2/g, pore volume: 0.89 cc/g, and pore diameter: 6 nm). Figure S1 (in the Supporting Information) presents the example TEM images of TiSBA-15 material with TiO2 particles, showing some of them isolated and some agglomerated. All mesoporous materials were dried overnight at 200 °C (to remove water) before adsorption of TPC1. Then, 100 mg of the powder was added to 20 mL of TPC1/DCM solution (c = 1.3 105 M), ultrasonicated for half an hour, and stirred for one day at room temperature. After that, the samples were washed 3 times by centrifugation with fresh DCM. The washing removes most of the dye loosely adsorbed on the external surface of MCM-41 grains. To check the concentration effect, a concentrated sample was prepared with 20 mL of TPC1/DCM (c = 3.9 105 M) per 50 mg of mesoporous powder. All the studied samples are summarized in Table 1. The experiments were done at 293 K in DCM suspension. The steady-state fluorescence and absorption spectra were measured using FluoroMax-4 (Jobin-Yvone) and JASCO V-670, respectively. The absorption spectrometer is equipped with a 8859
dx.doi.org/10.1021/jp201627t |J. Phys. Chem. C 2011, 115, 8858–8867
The Journal of Physical Chemistry C
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Table 1. Samples Used in the Present Studya sample name
nanomaterial
preparation ratio
molecules interacting (%)
TPC1 dye per Ti atom (103)
silica
SiO2 (Aldrich)
(1)
55
MCM-41
MCM-41 (Aldrich)
(1)
29
-
TiSBA-15
TiO2/SBA-15 (3% nanoparticles of TiO2)
(1)
63
4.3 (sat.)
Ti2MCM-41
TiO2-doped MCM-41 (6.3%)
(1)
100
Ti1MCM-41 Ti1MCM-41 conc.
TiO2-doped MCM-41 (3.5%) TiO2-doped MCM-41 (3.5%) concentrated sample
(1) (2)
100 99
5.9 35.5 (sat.)
P25
TiO2 nanoparticles (Degussa)
-
5.0 (sat.)
b
-
3.3
Preparation ratios: (1) 20 mL of TPC1/DCM, c = 1.3 10 M per 100 mg of the nanomaterial, (2) 20 mL of TPC1/DCM, c = 3.9 105 M per 50 mg of the nanomaterial. The calculation of the number of molecules interacting with the material is based on the total absorption in the liquids after three cleanings, and centrifugation of the solid sample by DCM. (sat.) indicates a saturated sample of Ti-doped material (dyes left in the supernatant liquid). b Prepared as previously reported.16 a
5
60 mm integrating sphere ISN-723 allowing the studies in scattering suspension (diffuse transmittance spectra). For the diffuse transmittance, the KubelkaMunk remittance function is used: F(R) = (1 R)2/2R, where R is the diffuse reflectance intensity from the sample. The picosecond (ps) emission decays were measured using a time-correlated single-photon counting (TCSPC) system.23 The sample was excited by 40 ps pulsed laser diodes centered at 433 nm (
