Dissecting the electronic dynamics of a photovoltaic material

Transition metal oxides are among the most promising materials for the conversion of solar energy into electricity (photovoltaics) or into chemical energy such as the splitting of water (photocatalysis). Their structure makes them ideal for generation, transport and trapping of charge carriers, such as electrons and electron holes. Titanium dioxide is a promising transition metal oxide, but determining its electron dynamics at room temperature has proven very difficult. EPFL scientists have solved the problem by using X-ray absorption spectroscopy (XAS). Published in Scientific Reports, the study reveals new information about electron movement in the surface region of titanium dioxide, opening new potential for photovoltaic and photocatalytic systems.

The lab of Majed Chergui at EPFL has pioneered XAS for the study of femto- and pico-second phenomena in a wide range of molecular, chemical and materials science systems. In the present study, the team used their expertise to overcome one of main obstacles in solar converion technologies.

A solar material must be able to operate at room-level temperatures. However, electron dynamics take place too rapidly under such conditions, and until now, researchers usually had to first cool down the materials at very low temperatures, meaning that their models a limited as to how well they can represent reality.

The EPFL researchers focused on titanium dioxide, which is the most commonly transition metal oxide material in solar energy conversion applications. Given its low cost and exceptional ability to transport and trap electrical current in response to light, the properties of titanium dioxide have been studied extensively.

This study achieves an unprecedented view into the electronic dynamics of titanium dioxide. Chergui’s team used femtosecond X-ray absorption spectroscopy to probe the trapping dynamics of light-generated electrons in aqueous colloidal anatase titanium dioxide nanoparticles.

In this technique, electrons are generated by hitting the nanoparticles with an ultrashort laser pulse, while their evolution is probed by a second, femtosecond X-ray pulse. The study found that the electrons appear in less than 300 femtoseconds on titanium atoms, which act as traps with a pentacoordinated geometry and are mostly in the surface shell region of the titanium dioxide nanoparticles.

The study offers a new tool for exploring the electronic properties of materials under operating conditions of solar devices. “We have also shown that femtosecond, hard X-ray absorption can be used to gain detailed descriptions of charge-carrier dynamics in transition metal oxides, and our work is being extended to other such systems,” says Majed Chergui.

This work represents a collaboration of EPFL’s Laboratory of Ultrafast Spectroscopy, ETH Zürich, and the Paul Scherrer Institute. It was funded by the Swiss National Science Foundation (SNSF) via the NCCR:MUST and contracts, and by the COST actions PERSPECT H₂O and XLIC.

Reference

Santomauro FG, Lübcke A, Rittmann J, Baldini E, Ferrer A, Silatani M, Zimmermann P, Grübel S, Johnson JA, Mariager SO, Beaud P, Grolimund D, Borca C, Ingold G, Johnson SL, Chergui M. Femtosecond X-ray absorption study of electron localization in photoexcited anatase TiO2. Scientific Reports 5, Article number: 14834 (2015) DOI:10.1038/srep14834