Photovoltaic panels – improving methods of measuring electron transfer
Despite its importance in determining the potential of a photovoltaic device, current methods for monitoring the interfacial electron transfer remain ambiguous. Now, using deep-ultraviolet continuum pulses, Scientists at the École Polytechnique Fédérale de Lausanne (EPFL) have developed a substrate-specific method to detect electron transfer. Published in the Journal of the American Chemical Society, their paper entitled ‘Interfacial Electron Injection Probed by a Substrate-Specific Excitonic Signature’ describes how the team has developed a substrate-specific method to detect electron transfer. Sensitised solar cells, consisting of a molecular or solid-state sensitiser that serves to collect light and inject an electron into a substrate that favours their migration, are among the most studied photovoltaic systems. However, current methodologies, which all use light in the visible-to-terahertz frequencies (wavelengths around 400 – 30000 nm), can deliver ambiguous results. This approach is sensitive to carriers that remain free in the conduction band of the semiconductor substrate. They are therefore unspecific to the type of substrate and cannot be extended to the new generation of solid-state-sensitised solar cells. The EPFL team aimed to overcome the limitations of current methods of measuring electron transfer, employing two types of dye-sensitised solar conversion systems: one based on titanium dioxide, the other on zinc-oxide nanoparticles, both of which belong to the category of transition-metal oxide (TMO) substrates. Using deep-ultraviolet continuum pulses, EPFL scientists have developed a substrate-specific method to detect electron transfer. They explain in their paper, ‘(…) we demonstrate the use of deep-ultraviolet continuum pulses to probe the interfacial electron transfer, by detecting a specific excitonic transition in both N719-sensitised anatase TiO2 and wurtzite ZnO nanoparticles.’ The show that, ‘ (…) the signal upon electron injection from the N719 dye into TiO2 is dominated by long-range Coulomb screening of the final states of the excitonic transitions, whereas in sensitized ZnO it is dominated by phase-space filling.’ Transition metal (TM) oxides (TiO2, ZnO, NiO) are large gap insulators that have emerged as highly attractive materials over the past two decades for applications in photocatalysis, solar energy conversion. Despite the huge interest for such materials, the very nature of the elementary electronic excitations (Frenkel, Wannier or charge transfer exciton) is still not established. An Advanced Grant from the EU helped the research under the DYNAMOX (Charge carrier dynamics in metal oxides) project which is developing novel experimental tools that would provide us with hitherto inaccessible information about the charge carrier dynamics in TM oxides. Research conducted in Lausanne will help to identify excitonic transition with more clarity. For more information, please see: CORDIS project website
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