 <p class="lead" id="first">Solar energy can directly drive electrochemical reactions at the surface of photoelectrodes. Photoelectrodes consist of semiconducting thin films on transparent conductive-glass substrates that convert light into electricity. Most photoelectrochemical studies have focused on water splitting, a thermodynamically uphill reaction that could offer an attractive pathway for the long-term capture and storage of solar energy by producing 'green' hydrogen.</p><p>Metal-oxide thin film photoelectrodes are particularly promising for these diverse functions. They comprise abundant elements, potentially offering infinite tunability to achieve the desired properties -- at potentially low costs.</p><p><strong>Made from plasma</strong></p><p>At the HZB Institute for Solar Fuels, several teams focus on developing such photoelectrodes. The usual method to produce them is pulsed laser deposition: an intense laser pulse hits a target containing the material and ablates it into a highly energetic plasma deposited on a substrate.</p><p><strong>Quality needs heat</strong></p><p>Further steps are needed to improve the quality of the deposited thin film. In particular thermal processing of the metal-oxide thin-film reduces defects and imperfections. However, this creates a dilemma: Reducing atomic defects concentration and improvements in crystalline order of the metal-oxide thin films would require thermal processing temperatures between 850 and 1000 degrees Celsius -- but the problem is that the glass substrate melts at 550 degrees Celsius.</p><p><strong>Flash-heating the thin film</strong></p><p>Dr. Ronen Gottesman from the HZB Institute for Solar Fuels has now solved this problem: After deposition, using high-powered lamps, he flash-heats the metal-oxide thin film. This heats it up to 850 degrees Celsius without melting the underlying glass substrate.</p><p>"The heat efficiently reduces structural defects, trap states, grain boundaries, and phase impurities, which would become more challenging to mitigate with an increasing number of elements in the metal-oxides. Therefore, new innovative synthesis approaches are essential. We have now demonstrated this on photoelectrodes made of Ta<sub>2</sub>O<sub>5</sub>, TiO<sub>2</sub>, and WO<sub>3</sub>, which we heated to 850 °C without damaging the substrates," says Gottesman.</p><p><strong>Record performance for α-SnWO<sub>4</sub></strong></p><p>The new method was also successful with a photoelectrode material that is considered a very good candidate for solar water splitting: α-SnWO<sub>4</sub>. Conventional furnace heating leaves behind phase impurities. Rapid thermal processing (RTP) heating improved crystallinity, electronic properties, and performance, leading to a new record performance of 1 mA/cm<sup>2</sup> for this material, higher by 25% than the previous record.</p><p>"This is also interesting for the production of quantum dots or halide perovskites, which are also temperature-sensitive," explains Gottesman.</p>