Giulia Tagliabue1 Artur Davoyan1 Harry Atwater1

1, California Institute of Technology, Pasadena, California, United States

The design of ultra-thin, highly absorbing metallic nanostructures has become increasingly important for a variety of applications ranging from photo-detection to photo-catalysis. Indeed, it has been demonstrated that, despite their short mean-free paths (10-20nm), hot-carriers generated in a metal can be transferred to either an adjacent semiconductor or an adsorbed molecule leading to detectable photo-currents and measurable reaction products, respectively. Furthermore, localized heat generation can be beneficial for photothermal processes. Typically, plasmonic nanostructures are utilized for achieving broadband or narrowband, efficient light absorption at the small dimensions required for the efficient transfer of plasmonic hot-carriers or heat generation. However, relying on nano-patterned designs challenges the scalability of these structures for practical applications, in particular for solar-energy conversion devices.
Here, we study light absorption in ultrathin (10-15 nm thick) planar metal films. Our absorbers are comprised of a low-loss silver backreflector, a subwavelength transparent dielectric (such as SiO2) or wide-bandgap semiconductor (i.e NiO or TiO2), typically 50-250nm thick, and an ultrathin (5-30 nm) metallic active absorbing layer. As a thin absorbing metal layer we consider several different materials, including Au, Ti, Cu, Ni, Pt and Pd.
First, we demonstrate theoretically and experimentally that both broadband and tunable narrowband, near-unity light absorption is possible in these unpatterned, ultrathin metallic systems which could be easily upscaled. Next, we test our absorbers as photoelectrodes. We measure the photoresponse of our ultra-thin Pt broadband absorber in a 0.1M sulfuric acid solution under cathodic potentials and perform the hydrogen evolution reaction (HER). For the case of Pt-on-insulator, the electrode exhibit a response time of approx. 1s, consistent with fast-heating. Instead, for the case of Pt-on-semiconductor we observe a fast response (~1uA/cm2 of photocurrent at 4 Suns illumination intensity) which can be attributed to charge injection. We also study CO2 photoelectrochemical reduction (50mM K2CO3 solution saturated with CO2, V=-0.8 to -1.2V vs RHE) using a series of Cu narrowband absorbers with absorption peak tuned from 350nm to 750nm. Tuning the absorption peak, and hence the energy of the absorbed photons, enable us to control the energy distribution of the photo-excited hot-electrons. We report results of product analysis for our series of Cu absorbers and evaluate the impact of light energy on the selectivity of our electrodes.
Our work opens new pathway for efficient light collection and enables a simple platform for photon assisted hot carrier generation or localized heating.