Widespread application of solar water splitting for energy conversion is largely dependent on the progress in developing not only efficient but also cheap and scalable photoelectrodes. Metal oxides, ...which can be deposited with scalable techniques and are relatively cheap, are particularly interesting, but high efficiency is still hindered by the poor carrier transport properties (i.e., carrier mobility and lifetime). Here, a mild hydrogen treatment is introduced to bismuth vanadate (BiVO4), which is one of the most promising metal oxide photoelectrodes, as a method to overcome the carrier transport limitations. Time‐resolved microwave and terahertz conductivity measurements reveal more than twofold enhancement of the carrier lifetime for the hydrogen‐treated BiVO4, without significantly affecting the carrier mobility. This is in contrast to the case of tungsten‐doped BiVO4, although hydrogen is also a donor type dopant in BiVO4. The enhancement in carrier lifetime is found to be caused by significant reduction of trap‐assisted recombination, either via passivation or reduction of deep trap states related to vanadium antisite on bismuth or vanadium interstitials according to density functional theory calculations. Overall, these findings provide further insights on the interplay between defect modulation and carrier transport in metal oxides, which benefit the development of low‐cost, highly‐efficient solar energy conversion devices.
Overcoming poor charge carrier transport represents one of the biggest challenges in the development of metal oxide photoelectrodes. Time‐resolved conductivity measurements and density functional theory calculations reveal that a simple postsynthesis hydrogen treatment at 300 °C reduces the number of deep trap states in metal oxides. As a result, the charge carrier lifetime and overall photoelectrochemical performance are significantly enhanced.
BiVO4 is a promising photoanode candidate for water splitting applications, but its microscopic charge carrier transport properties are not yet fully understood. We investigated the photoinduced ...carrier mobility for undoped and 1% tungsten-doped BiVO4 thin films in an early time window from 1 ps to 1 ns using THz spectroscopy. The combined electron–hole effective mobility gradually decreases with time by 1 order of magnitude starting at an upper limit of ∼0.4 cm2 V–1 s–1. The loss is attributed to carrier localization. We provide for the first time direct time-resolved evidence of hole polaron formation accompanied by the temporal buildup of a polaron population in parallel to initial carrier trapping. A mobility of 0.02 cm2 V–1 s–1 is found for the self-trapped carriers, which leads to a thermal hopping activation energy of ∼90 meV.
The charge carrier dynamics of epitaxial hematite films is studied by time‐resolved microwave (TRMC) and time‐resolved terahertz conductivity (TRTC). After excitation with above bandgap illumination, ...the TRTC signal decays within 3 ps, consistent with previous reports of charge carrier localization times in hematite. The TRMC measurements probe charge carrier dynamics at longer timescales, exhibiting biexponential decay with characteristic time constants of ≈20–50 ns and 1–2 μs. From the change in photoconductance, the effective carrier mobility is extracted, defined as the product of the charge carrier mobility and photogeneration yield, of differently doped (undoped, Ti, Sn, Zn) hematite films for excitation wavelengths of 355 and 532 nm. It is shown that, unlike in conventional semiconductors, donor doping of hematite dramatically increases the effective mobility of the photogenerated carriers. Furthermore, it is shown that all hematite films possess higher effective mobility for 355 nm excitation than for 532 nm excitation, although the time dependence of the photoconductance decay, or charge carrier lifetime, remains the same. These results provide an explanation for the wavelength dependent photoelectrochemical behavior of hematite photoelectrodes and suggest that an increase in photogeneration yield or charge carrier mobility is responsible for the improved performance at higher excitation energies.
Time‐resolved terahertz microwave and terahertz spectroscopy are used to study the charge carrier dynamics of epitaxial hematite films. A physical basis for the wavelength dependent photoelectrochemical behavior of hematite is provided, demonstrating that an increase in photogeneration yield or charge carrier mobility is responsible for the improved performance at higher excitation energies.
The effect of compositional variation on charge carrier lifetimes of Cr1Fe0.84Al0.16O3, a promising material for solar water splitting recently identified using combinatorial materials science, is ...explored using ultrafast time-resolved optical reflectance. The transient signal can be described by a biexponential decay, where the shorter time constant varies over 1 order of magnitude with changing Cr content while the longer one stays constant. Intrinsic performance limitations such as a low charge carrier mobility on the order of 10–3 cm2/(Vs) are identified. Charge carrier lifetime and mobility are discussed as screening criteria for solar water splitting materials.
The temporal evolution of photogenerated carriers in CuWO
, CuO and WO
thin films deposited via a direct chemical vapor deposition approach was studied using time-resolved microwave conductivity and ...terahertz spectroscopy to obtain the photocarrier lifetime, mobility and diffusion length. The carrier transport properties of the films prepared by varying the copper-to-tungsten stoichiometry were compared and the results related to the performance of the compositions built into respective photoelectrochemical cells. Superior carrier mobility was observed for CuWO
under frontside illumination.
Time‐resolved conductivity measurements reveal that a mild hydrogen treatment at 300 °C for 10 minutes successfully extends the lifetime of carriers by more than two‐fold in bismuth vanadate (BiVO4) ...photoelectrodes. This enhancement is a result of passivation of deep trap states and/or reduction of their density. Consequently, the AM1.5 photocurrent and onset potential for the hydrogen‐treated BiVO4 are significantly improved. This is reported by Fatwa F. Abdi and co‐workers in article number 1701536.
The usefulness and acceptance of geo-information systems are mainly depends on the quality of the underlying geo-data. This paper describes a novel system for semi-automatic quality control of ...existing topographic geo-spatial data via automatic image analysis. The goal is to reduce
the manual effort for quality control of a GIS database to a minimum. The core of the system is a semantic network in which different image analysis operators can be included. The image analysis operators are created for specific applications, i.e., the quality control of specific object classes
which are most relevant. Images which can be used in the system are aerial images, high-resolution satellite imagery, and low-resolution satellite imagery. A prototype of the system has been in use for several years at public mapping organizations. From the experience gained during this time,
we give a detailed report on the system performance and an evaluation of the results.
Abstract
The temporal evolution of photogenerated carriers in CuWO
4
, CuO and WO
3
thin films deposited via a direct chemical vapor deposition approach was studied using time-resolved microwave ...conductivity and terahertz spectroscopy to obtain the photocarrier lifetime, mobility and diffusion length. The carrier transport properties of the films prepared by varying the copper-to-tungsten stoichiometry were compared and the results related to the performance of the compositions built into respective photoelectrochemical cells. Superior carrier mobility was observed for CuWO
4
under frontside illumination.
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