The macroscopic electron transport in porous films of sintered 9 nm sized nanocrystals (NC-TiO2) is known to be 4–6 orders of magnitude worse than in polycrystalline (Pol-TiO2) dense films. To obtain fundamental knowledge regarding this large difference, we investigated the effects of spatial confinement and electron trapping processes on the charge transport in these samples. We determined the time-resolved real and imaginary microwave photoconductance (TRMC) on pulsed excitation. Large amounts of the photoexcited electrons are readily immobilized in deep traps in NC-TiO2 as concluded by comparing the TRMC decay kinetics with previously published transient absorption measurements. Our results show that trapped electrons do not give rise to a microwave photoconductance response, nor do they affect the motion of conduction band electrons. Additionally, by comparing a bare NC-TiO2 film with a dye-sensitized NC-TiO2 sample, the influence of the photogenerated holes on the photoconductance as a function of their locus is investigated. The positive charges either inside or outside the TiO2 nanocrystals contribute insignificantly to the photoconductance. On a nanosecond time scale only a minor fraction (maximum 2%) of the photoexcited electrons resides in the conduction band of NC-TiO2. Importantly, in both NC-TiO2 and Pol-TiO2 these electrons have the same intraparticle microwave mobility of 1.7 cm2/(V s) due to frequent backscattering events at a mean time interval of 85 fs. This mobility value represents the upper limit for the trap-free dc electron mobility in anatase TiO2 irrespective of the crystallite size. Hence, the photoconductance across a NC-TiO2 layer can be strongly enhanced by reducing the electron trap density and by eliminating the relatively inefficient electron hopping steps between adjacent nanocrystals.
C60 is used as an electron acceptor in small molecule photovoltaic devices in combination with various electron donors. The transport of excitons, i.e., bound electron/hole pairs, is an important factor determining the efficiency of such devices. Here we investigate the exciton diffusion length in C60 with the electrodeless time-resolved microwave conductance (TRMC) technique. Bilayers of 30 nm Zn-phthalocyanine with a C60 layer with variable thickness are prepared by physical vapor deposition. Analysis of the photoconductance with an exciton diffusion model yields a diffusion length of 7 nm, and the mobility of holes along Zn-Phthalocyanine stacks is close to 1 cm(2)/(V s). From analysis of the rise and decay of the TRMC transients, we attribute the photoconductance to diffusion and dissociation of singlet excitons rather than triplets. The energy transfer rate between C60 molecules exceeds 8 × 10(10) s(-1). Exciton diffusion cannot be described by the Förster model due to the close proximity of the molecules.
We propose an in-situ UV-vis monitoring technique called 'oxynitrogenography' as an approach towards the controlled and reproducible synthesis of thin films of different Ta-O-N phases, including the elusive β-TaON phase. The optical absorption changes are measured during annealing of the film at increasing NH 3 /H 2 O ratios, and can be directly correlated to the presence of different phases (Ta 2 O 5 , β-TaON, mixed TaON-Ta 3 N 5 , Ta 3 N 5 ) due to the abrupt change in the absorption edge. After additional XRD analysis, the thermodynamic equilibrium conditions to obtain these various phases are determined, and a phase diagram is constructed. We observe that there is a very narrow range of parameters for the thermodynamic stability of the β-TaON phase. We observe that the carrier mobility increases with the nitrogen content in the sample, from 10 -5 cm 2 /Vs in Ta 2 O 5 , to 10 -2 cm 2 /Vs in β-TaON and the mixed TaON-Ta 3 N 5 , until 10 -1 cm 2 /Vs in Ta 3 N 5 . While the carrier mobility of β-TaON and Ta 3 N 5 is comparable to that of BiVO 4 , the lifetime in the order of milliseconds is comparable to that of crystalline silicon. This is much higher than previously reported and compares favorably with the currently most promising metal oxide-based semiconductors (BiVO 4 , Fe 2 O 3 , WO 3 , Cu 2 O) for photo-electrochemical (PEC) water splitting. Although these long lifetimes may be partly caused by (de-)trapping from shallow trap states, these results clearly demonstrate that a high phase purity is an essential prerequisite for efficient (oxy)nitride-based absorber materials.
In this contribution we demonstrate a solid-state approach to triplet−triplet annihilation upconversion for application in a solar cell device in which absorption of near-infrared light is followed by direct electron injection into an inorganic substrate. We use time-resolved microwave photoconductivity experiments to study the injection of electrons into the electron-accepting substrate (TiO 2 ) in a trilayer device consisting of a triplet sensitizer (fluorinated zinc phthalocyanine), triplet acceptor (methyl subsituted perylenediimide), and smooth polycrystalline TiO 2 . Absorption of light at 700 nm leads to the almost quantitative generation of triplet excited states by intersystem crossing. This is followed by Dexter energy transfer to the triplet acceptor layer where triplet annihilation occurs and concludes by injection of an electron into TiO 2 from the upconverted singlet excited state.
We studied the temperature dependence of photophysical products on UV excitation of smooth dense anatase TiO2 with frequency- and time-resolved microwave conductance measurements. At 100 K, we observed the subnanosecond formation of microsecond-lived self-trapped excitons (STEs) with a 65% quantum yield, irrespective of the UV excitation wavelength. The remaining 35% photoexcitations results in mobile conduction band electrons (e CB –); the yield of e CB – gradually increases up to about 100% at 300 K. The complex mobility of e CB – is independent of temperature between 100 and 300 K. We explain the temperature-dependent quantum yield of e CB – by the thermally activated escape of the electron from its positive counter charge. At low temperature, the Coulomb attraction causes the electron to remain at short distance from the hole forming a neutral pair. This pair stabilizes by inducing a relaxation of the surrounding lattice resulting in a self-trapped exciton. The excess polarizability volume of each STE is found to be temperature independent and of the order of 104 Å3. Our results indicate that not only in dense smooth TiO2 but also in nanostructured TiO2 STEs are efficiently photogenerated at low temperature. For the first time we provide quantitative information about the quantum yield of excitons in bulk anatase TiO2, the existence of which has been previously demonstrated only qualitatively by means of photoluminescence measurements.
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