A Faster Path to Next-Generation Materials for Solar Energy Conversion

To achieve a sustainable society with an energy mix primarily based on solar energy, we need means of storing energy from sunlight as chemical fuels (“solar fuels”) that have up to 100 times higher energy and power densities than the best batteries. Viable, global scale photoelectrochemical (PEC) energy conversion of cheap, abundant resources (such as water and CO₂) into solar fuels depends on the progress of semiconducting light absorbers with enhanced carrier transport properties, suitable band edge positions, and stability in direct-semiconductor/electrolyte junctions.

The search has concentrated mainly on metal oxides that offer good chemical stability and are wide-ranging, highly tunable multi-functionalities, unparalleled among other materials classes. However, oxide light absorbers tend to suffer from poor charge transport compared to non-oxide semiconductors (e.g., Si, GaAs) due to the formation of polarons. The good news is that only a fraction of the possible ternary and quaternary oxides (together ~ 10⁵ – 10⁶ combinations) have been studied so far, making it likely that the best materials are still waiting to be discovered. The bad news is two-fold: 1) with an increasing number of elements, designing highly controlled synthesis routes of "semiconductor-grade" (i.e., high phase purity, low concentration of bulk and surface defects) oxides will become more thermodynamically and kinetically challenging, and 2) there are currently no robust and proven strategies to identify promising multi-elemental systems.

To overcome these challenges, before exploring different compositions, the first step is to place as the central science novel non-equilibrium synthesis methods by high-throughput combinatorial investigations of synthesis parameter spaces. This would open new avenues for stabilizing metastable materials, discovering new chemical spaces, and obtaining light absorbers with enhanced properties to study their physical working mechanisms in PEC energy conversion. The knowledge gained from studying various “tuning knobs” of non-equilibrium synthesis methods would be used to create optimized boundary conditions platforms to maximize the potential of exploring polyelemental compositions of light absorbers via data-driven combinatorial studies of materials and devices.

I will introduce an original approach for exploring non-equilibrium synthesis-parameter spaces (e.g., temperature, thickness, chemical reactivity) without changing concentration and stoichiometry by employing two non-equilibrium synthesis key components: pulsed laser deposition (PLD) and rapid radiative heating. A significant advantage of combining these two components is the ability to conduct a highly reproducible, high-throughput combinatorial synthesis that enables high-resolution observation and analysis. Even minor changes in synthesis can have a significant impact on the material properties, physical working mechanisms, and performances, as demonstrated by studies of the relationship between the synthesis conditions, crystal structures of α-SnWO₄, and properties over a range of thicknesses of CuBi₂O₄, both emerging photoabsorbers for PEC water splitting that were used as model multinary materials.

Near the end of my talk, I will briefly present my plans and vision for researching heteroanionic metal oxynitride perovskite materials for PEC and solid-state devices for sustainable energy applications.  Oxynitrides have shown in ideal cases greatly enhanced transport properties, high performances, and increased stability in aqueous solutions compared to oxides. However, difficulties in synthesizing "semiconductor-grade" oxynitrides model systems leave many questions unanswered about their physical working mechanisms owing to differences in polarizability, electronegativity, nitrogen, and oxygen anion charge. Innovative breakthroughs in the non-equilibrium synthesis of oxynitrides will transform them into disruptive and innovative multi-functional materials with the desired physicochemical and optoelectronic properties to address next-generation major challenges.