Direct Time Resolved Observation of Carrier Trapping and Polaron Conductivity in BiVO4

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

Efficient Electron Injection from Acyloin-Anchored Semisquarylium Dyes into Colloidal TiO₂ Films for Organic Dye-Sensitized Solar Cells

Semisquarylium dyes use a novel acyloin anchor group to strongly bind to TiO₂ semiconductors. Efficient acyloin anchor mediated electron injection into nanocrystalline TiO₂ is demonstrated, allowing highly efficient dye-sensitized solar cells with IPCEs > 80%. The acyloin anchor can thus be viewed as a true alternative to the standard carboxylic acid anchor group. The opto-electronic and electron injection properties of the most basic semisquarylium dye <b>SY404</b> are compared to the modified semisquarylium dye <b>DD1</b> and the carboxylic acid anchored indoline dye <b>D131</b> using a combination of ultrafast and photoemission spectroscopy. For <b>SY404</b>, ultrafast injection times of ∼50 fs are found despite a small energetic driving force between dye excited states and TiO₂ conduction band minimum. This is possible due to the strong electronic coupling of the semisquarylium dyes to the TiO₂ surface mediated by the acyloin anchor. For a better overlap with the solar spectrum, the semisquarylium dyes are modified by substitution with a larger donor moiety (<b>DD1</b>). While for <b>DD1</b> the overall absorption increases, the injection process slightly slows down; however, it still proves fast enough for very efficient injection. Compared to the carboxylic acid anchored indoline dye <b>D131</b>, the <b>SY404</b> dye injects more than seven times faster despite a ∼150 meV smaller driving force

Enhancing Charge Carrier Lifetime in Metal Oxide Photoelectrodes through Mild Hydrogen Treatment

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