Cyclometalated ruthenium complexes for sensitizing nanocrystalline TiO2 solar cells

Cyclometalated ruthenium complexes of [Ru(C--arrow--N) (N--N--N)] configuration are a promising new class of molecular sensitizers for dye-sensitized solar cells, as a result of their broad and red-shifted visible absorption in comparison to the analogous [Ru(N--N--N)2] type coordinative complexes.     

A new family of heteroleptic ruthenium(II) polypyridyl complexes for sensitization of nanocrystalline TiO2 films

Two new heteroleptic ruthenium(II) photosensitizers that contains 2,2';6,2'-terpyridine with extended π-conjugation with donor groups, a 4,4'-dicarboxylic acid-2,2'-bipyridine anchoring ligand and a thiocyanate ligand have been designed, synthesized and fully characterized by CHN, mass spectrometry, UV-vis and fluorescence spectroscopies and cyclic voltammetry. The new sensitizers have either 3,5-di-tert-butyl phenyl (m-BL-5) or triphenylamine (m-BL-6) groups, where the molar extinction coefficient of both the sensitizers is higher than the analogous ruthenium dyes. Both the sensitizers were tested in dye-sensitized solar cells using two different redox electrolytes.     

Efficient charge separation in TiO2 films sensitized with ruthenium(II)-polypyridyl complexes: hole stabilization by ligand-localized charge-transfer states

We have studied the interfacial electron-transfer dynamics on TiO(2) film sensitized with synthesized ruthenium(II)-polypyridyl complexes--[Ru(II)(bpy)(2)(L(1))] (1) and [Ru(II)(bpy)(L(1))(L(2))] (2), in which bpy=2,2'-bipyridyl, L(1)=4-[2-(4'-methyl-2,2'-bipyridinyl-4-yl)vinyl]benzene-1,2-diol, and L(2)=4-(N,N-dimethylaminophenyl)-2,2'-bipyridine-by using femtosecond transient absorption spectroscopy. The presence of electron-donor L(2) and electron-acceptor L(1) ligands in complex 2 introduces lower energetic ligand-to-ligand charge-transfer (LLCT) excited states in addition to metal-to-ligand (ML) CT manifolds of complex 2. On photoexcitation, a pulse-width-limited (<100 fs) electron injection from populating LLCT and MLCT states are observed on account of strong catecholate binding on the TiO(2) surface. The hole is transferred directly or stepwise to the electron-donor ligand (L(2)) as a consequence of electron injection from LLCT and MLCT states, respectively. This results an increased spatial charge separation between the hole residing at the electron-donor (L(2)) ligand and the electron injected in TiO(2) nanoparticles (NPs). Thus, we observed a significant slow back-electron-transfer (BET) process in the 2/TiO(2) system relative to the 1/TiO(2) system. Our results suggest that Ru(II) -polypyridyl complexes comprising LLCT states can be a better photosensitizer for improved electron injection yield and slow BET processes in comparison with Ru(II)-polypyridyl complexes comprising MLCT states only.     

Optical modeling of nanocrystalline TiO2 films

The light transmittance, T, in nanocrystalline TiO2 films was studied as a function of the light wavelength, lambda, the nanocrystal radius, a, and the film thickness, d. Two types of TiO2 nanoparticles were employed: a commercial powder (P25) and synthesized particles from titanium isopropoxide (SP). The X-ray diffraction measurements revealed that both P25 and SP are mainly anatase and the average crystal sizes, 2a, of P25 and SP are 50.3 and 23.7 nm, respectively. Despite the visual difference between micron-order thin films of P25 and SP, the light hemispherical transmittance corrected with the surface specular reflectance has a clear dependence of ln(T) = -0.5beta lambda(-4)a(3)d, with beta = 1.5 x 10(3) from visible to near-infrared wavelengths. The dependence and beta value were successfully explained by the simplest model on the basis of the Rayleigh scattering theory. This indicates that the nanocrystalline TiO2 thin films are a typical medium where the simplest scattering model is a good approximation. However, the model was inapplicable to light scattering in relatively thick P25 films of 1.5-3.0 microm because of nonnegligible internal multiple scattering. For the moderate thickness films, ln(T) proportional to lambda(gamma), where gamma increases from -4 in proportion to the film thickness is an alternative approximation. With these light scattering models, the light absorption rate of the TiO2 crystal was successfully evaluated from experimental extinction rates.     

Comparison of electron injection dynamics from re-bipyridyl complexes to TiO2 nanocrystalline thin films in different solvent environments

Factors that control photoinduced interfacial electron transfer (ET) between molecular adsorbates and semiconductor nanoparticles have been intensely investigated in recent years. In this work, the solvent dependence of interfacial ET was studied by comparing ET rates in dye sensitized TiO2 nanocrystalline films in different solvent environments. Photoinduced ET rates from Re(LA)(CO)3Cl [LA=dcbpy=4,4'-dicarboxy-2,2'-bipyridine] (ReC1A) to TiO2 nanocrystalline thin films in air, pH buffer, MeOH, EtOH, and DMF were measured by femtosecond transient IR spectroscopy. The ET rates in these solvent environments were noticeably different. However, differences between the rates in pH buffer and nonaqueous solvents (MeOH, EtOH, and DMF) were much smaller than the values expected from much more negative TiO2 conduction band-edge positions in the latter solvents under anhydrous conditions. It was suggested that the presence of adsorbed water, which was evident in FTIR spectra, lowered the band edge of TiO2 in these solvents and reduced the rate differences. The important effect of adsorbed water was verified by comparing two samples of Re(LP)(CO)3Cl [LP=2,2'-bipyridine-4,4'-bis-CH2PO(OH)2] sensitized TiO2 in DMF, in which the presence of a trace amount of water was found to significantly increase the injection rate.     

Transient absorption studies and numerical modeling of iodine photoreduction by nanocrystalline TiO2 films

We have used transient absorption spectroscopy to study the reaction between photogenerated electrons in a dye-free nanocrystalline titanium dioxide film and an iodine/iodide redox couple. Recombination kinetics was measured by recording the transient optical signal following band gap excitation by a UV laser pulse. In the presence of a methanol hole scavenger in the electrolyte, a long-lived (0.1-1 s) red/infrared absorbance is observed and assigned to photogenerated electrons forming Ti(3+) species. In the presence of iodine and excess iodide in the electrolyte, the signal decays on a millisecond-microsecond time scale, assigned to reduction of the redox couple by photogenerated electrons in the TiO(2). The electron lifetime decreases inversely with increasing iodine concentration, indicating that the back reaction is first order in [I(2)]. No evidence for I(2)(-) is observed, indicating that the reaction mechanism does not involve the formation of I(2)(-) as an intermediate. The shape of the kinetics evolves from monoexponential at low [I(2)] to stretched-exponential as [I(2)] increases. A Monte Carlo continuous-time random walk model is implemented to simulate the kinetics and its [I(2)] dependence and used to address the order of the recombination reaction with respect to electron density, n. The model incorporates the diffusion of oxidized species from the electrolyte toward the TiO(2) surface as well as electron trapping and transport in the TiO(2). In the limit of low [I(2)], the monoexponential kinetics is explained by the recombination reaction being rate limited by the diffusion of the oxidized species in the electrolyte. The stretched-exponential behavior at high [I(2)] can be explained by the reaction being rate limited by the transport of electrons through a distribution of trap states toward reactive sites at the TiO(2)-electrolyte interface, similar to the mechanism proposed previously for the kinetics of electron-dye cation recombination. Such trap-limited recombination can also explain the superlinear dependence of electron recombination rate on electron density, which has been reported elsewhere, without the need for a reaction mechanism that is second order in n. In contrast, a second-order reaction mechanism in a trap-free medium cannot explain the observed kinetics, although a second-order mechanism incorporating electron trapping cannot be conclusively ruled out by the data. We propose that the most likely reaction scheme, that is first order in both [I(2)] and n, is the dissociative reduction of I(2) onto the metal oxide surface, followed by a second electron reduction of the resulting adsorbed iodine radical, and that empirical second-order behavior of the electron lifetime is most likely explained by electron trapping rather than by a second-order recombination mechanism.     

Supramolecular control of charge-transfer dynamics on dye-sensitized nanocrystalline TiO2 films

A [Ru(dcbpy)(2)(NCS)(2)] dye has been chemically modified by the addition of a secondary electron donor moiety, N,N-(di-p-anisylamino)phenoxymethyl. Optical excitation of the modified dye adsorbed to nanocrystalline TiO(2) films shows a remarkably long-lived charge-separated state, with a decay half time of 0.7 s. Semiempirical calculations confirm that the HOMO of the modified dye molecule is localised on the electron donor group. The retardation of the recombination dynamics relative to the unmodified control dye is caused by the increase in the spatial separation of the HOMO orbital from the TiO(2) surface. The magnitude of the retardation is shown to be in agreement with that predicted from the non-adiabatic electron-tunnelling theory.     

Multistep electron transfer processes on dye co-sensitized nanocrystalline TiO2 films

We report a method for achieving multilayer co-sensitization of nanocrystalline TiO2 films. The method is based upon an aluminum isopropoxide treatment of the monosensitized film prior to deposition of a second sensitizer. Appropriate selection of sensitizer dyes allows vectorial, multistep, electron transfer processes, resulting in a suppression of interfacial charge recombination and a significantly improved photovoltaic device performance relative to single-layer co-sensitization devices.     

Photochemical reduction of oxygen adsorbed to nanocrystalline TiO(2) films: a transient absorption and oxygen scavenging study of different TiO(2) preparations

Transient absorption spectroscopy (TAS) has been used to study the interfacial electron-transfer reaction between photogenerated electrons in nanocrystalline titanium dioxide (TiO(2)) films and molecular oxygen. TiO(2) films from three different starting materials (TiO(2) anatase colloidal paste and commercial anatase/rutile powders Degussa TiO(2) P25 and VP TiO(2) P90) have been investigated in the presence of ethanol as a hole scavenger. Separate investigations on the photocatalytic oxygen consumption by the films have also been performed with an oxygen membrane polarographic detector. Results show that a correlation exists between the electron dynamics of oxygen consumption observed by TAS and the rate of oxygen consumption through the photocatalytic process. The highest activity and the fastest oxygen reduction dynamics were observed with films fabricated from anatase TiO(2) colloidal paste. The use of TAS as a tool for the prediction of the photocatalytic activities of the materials is discussed. TAS studies indicate that the rate of reduction of molecular oxygen is limited by interfacial electron-transfer kinetics rather than by the electron trapping/detrapping dynamics within the TiO(2) particles.     

Influence of a TiCl4 post-treatment on nanocrystalline TiO2 films in dye-sensitized solar cells

In this study, the influence of the TiCl(4) post-treatment on nanocrystalline TiO(2) films as electrodes in dye-sensitized solar cells is investigated and compared to nontreated films. As a result of this post-treatment cell efficiencies are improved, due to higher photocurrents. On a microscopic scale TiO(2) particle growth on the order of 1 nm is observed. Despite a corresponding decrease of BET surface area, more dye is adsorbed onto the oxide surface. Although it seems trivial to match this finding with the improved photocurrent, this performance improvement cannot be attributed to higher dye adsorption only. This follows from comparison between incident photon to current conversion efficiency (IPCE) and light absorption characteristics. Since the charge transport properties of the TiO(2) films are already more than sufficient without treatment, the increase in short circuit current density J(SC) cannot be related to improvements in charge transport either. Transient photocurrent measurements indicate a shift in the conduction band edge of the TiO(2) upon TiCl(4) treatment. It is concluded that the main contribution to enhanced current originates from this shift in conduction band edge, resulting in improved charge injection into the TiO(2).     

In situ FTIR studies of primary intermediates of photocatalytic reactions on nanocrystalline TiO2 films in contact with aqueous solutions

Multiple internal reflection infrared spectroscopy was applied to in situ investigations of surface intermediates of photocatalytic reactions on nanocrystalline TiO(2) films in contact with aqueous solutions. UV irradiation in the presence of dissolved O(2) caused the appearance of new bands peaked at 943, 838, and 1250-1120 cm(-)(1) together with intensity changes in other bands. Investigations of influences of the solution pH, the presence or absence of hole and electron scavengers, and isotopic H(2)O --> D(2)O exchange on the spectral changes have revealed that the primary step of photocatalytic O(2) reduction is the formation of the surface peroxo species, Ti(O(2)), giving the 943 cm(-)(1) band, probably with the surface superoxo species, TiOO., as a precursor, in neutral and acidic solutions. The surface peroxo species is then transformed to the surface hydroperoxo, TiOOH, giving the 838 and 1250-1120 cm(-)(1) bands, by protonation in the dark. This is, to our knowledge, the first direct in situ spectroscopic detection of primary intermediates for the photocatalytic O(2) reduction in aqueous solutions. On the basis of the assignment, a possible reaction scheme for various processes of the photocatalytic O(2) reduction is proposed, which is in harmony with other spectral changes induced by the UV irradiation.     

Efficient co-sensitization of nanocrystalline TiO(2) films by organic sensitizers

Dye-sensitized solar cells based on co-sensitization of organic dyes having complementary spectral absorption in the visible region resulted in a panchromatic response, which exhibited 86% incident monochromatic photon-to-current conversion efficiency in the visible region; the optimized cell gave a short circuit current density of 15.5 mA cm(-2), an open circuit voltage of 685 mV and a fill factor of 0.70 corresponding to an overall conversion efficiency of 7.43% under solar simulated light irradiation of 100 mW cm(-2).     

Molecular control of recombination dynamics in dye sensitised nanocrystalline TiO2 films

Modification of the structure of a porphyrin dye shows a significant change in the rate of charge recombination between injected electrons in the TiO2 and the oxidized dye anchored to it following optical excitation, offering an insight into fundamental understanding of processes occurring at the dye/semiconductor interface.     

Dye-sensitized solar cells based on nanocrystalline TiO2 films surface treated with Al3+ ions: photovoltage and electron transport studies

Nanocrystalline TiO2 films, surface modified with Al3+, were manufactured by depositing a TiO2 suspension containing small amounts of aluminum nitrate or aluminum chloride onto conducting glass substrates, followed by drying, compression, and finally heating to 530 degrees C. Electrodes prepared with TiO2 nanoparticles coated with less than 0.3 wt % aluminum oxide with respect to TiO2 improved the efficiency of the dye sensitized solar cell. This amount corresponds to less than a monolayer of aluminum oxide. Thus, the Al ions terminate the TiO2 surface rather than form a distinct aluminum oxide layer. The aluminum ion surface treatment affects the solar cell in different ways: the potential of the conduction band is shifted, the electron lifetime is increased, and the electron transport is slower when aluminum ions are present between interconnected TiO2 particles.     

Sensitization and stabilization of TiO(2) photoanodes with electropolymerized overlayer films of ruthenium and zinc polypyridyl complexes: a stable aqueous photoelectrochemical cell

Overlayer thin films of vinylbipyridine (vbpy)-containing Ru and Zn complexes have been formed on top of ruthenium dye complexes adsorbed to TiO(2) by reductive electropolymerization. The goal was to create an efficient, water-stable photoelectrode or electrodes. An adsorbed-[Ru(vbpy)(2)(dcb)](PF(6))(2)/poly-[Ru(vbpy)(3)](PF(6))(2) surface composite displays excellent stability toward dissolution in water, but the added overlayer film greatly decreases incident photon-to-current conversion efficiencies (IPCE) in propylene carbonate with I(3)(-)/I(-) as the carrier couple. An ads-[Ru(vbpy)(2)(dcb)](PF(6))(2)/poly-[Zn(vbpy)(3)](PF(6))(2) composite displays no loss in IPCE compared to ads-[Ru(vbpy)(2)(dcb)](PF(6))(2) but is susceptible to film breakdown in the presence of water by solvolysis and loss of the cross-linking Zn(2+) ions. Success was attained with an ads-[Ru(vbpy)(2)(dcb)](PF(6))(2)/poly-[Ru(vbpy)(2)(dppe)](PF(6))(2) composite. In this case the electropolymerized layer is transparent in the visible. The composite electrode is stable in water, the IPCE in propylene carbonate with I(3)(-)/I(-) is comparable to the adsorbed complex, and a significant IPCE is observed in water with the quinone/hydroquinone carrier couple. The assembly [(bpy)(2)(CN)Ru(CN)Ru(vbpy)(2)(NC)Ru(CN)(bpy)(2)](PF(6))(2) ([Ru(CN)Ru(NC)Ru](PF(6))(2)) adsorbs spontaneously on TiO(2), and electropolymerization of thin layers of the assembly to give ads-[Ru(CN)Ru(NC)Ru](PF(6))(2)/poly-[Ru(CN)Ru(NC)Ru](PF(6))(2) enhances IPCE and has no deleterious effect on the IPCE/Ru.     

Electron injection dynamics of Ru polypyridyl complexes on SnO2 nanocrystalline thin films

Ultrafast infrared spectroscopy was utilized to investigate the electron-transfer dynamics from Ru(dcbpy)(2)(X)(2) complexes (dcbpy = 4,4'-dicarboxy-2,2'-bipyridine; X(2) = SCN(-), 2CN(-), and dcbpy; referenced as RuN3, Ru505, and Ru470, respectively) to nanocrystalline SnO(2) films. For both films exposed to air (dry) and submerged in a pH 2 buffer solution, all traces show biphasic dynamics with a small ultrafast component (less than 10%) and nonexponential slow component, indicating that most injection occurs from thermalized excited state of the dye. In the dry film, the injection rate becomes slower, comparing RuN3, Ru505, and Ru470, correlating with decreasing excited-state oxidation potentials in these dyes. However, the variation of injection rate with dye potential is less noticeable at pH 2. The possible reason for the different injection dynamics in these dyes and under different environments are discussed. These injection dynamics are also compared with those on TiO(2) and ZnO.     

Self-assembly and photoelectric properties of cerium complexes with 3, 4, 9, 10-perylenetetracarboxylic acid on nanocrystalline TiO2 films

Self-assembled (SA) films (PMP, M = Ce³⁺ or Ce⁴⁺) of 3,4,9,10-perylenetetracarboxylic acid (PTA) on nanocrystalline TiO₂ films with Ce³⁺ or Ce⁴⁺ as a bridge were fabricated and characterized with UV-Vis, IR, and XPS synchrotron radiation photoelectron spectroscopy (SRPES) which gave the HOMO energy levels for the SA films. It was shown that thin-layer sandwich-type solar cells based on these SA films possess good properties for photoelectric conversion. While PTA-loaded TiO₂ electrode (P) generated 26.9% of incident monochromatic photon-to-electron conversion efficiency (IPCE), PMP-sensitized TiO₂ electrodes yielded 55.8% and 39.1% for Ce⁴⁺ and Ce³⁺ respectively. PMP films can be considered as a kind of complexes whose HOMO energy levels were proved to be higher than that of film P, which is one of the major reasons for the increase in IPCE from film P to film PMP.