Polyamorphism in Photodeposited Amorphous Metal Oxy(hydroxides) Electrocatalysts and Semiconductors

Metal oxide thin-film materials are ubiquitous in several devices and catalytic systems. However, methods amenable to large-scale processing, using low temperatures compatible with organic materials, are scarce. A perceived challenge with low-temperature method is that amorphous materials are obtained, which renders lesser-quality materials. In this talk I will present a photochemical method that affords amorphous metal-oxides with well-controlled composition. Our recent work has focused on characterizing these materials with synchrotron-based techniques, to better understand their short-range structure (at the nanoscale). A surprising behaviour has emerged, which appears to be more prevalent than hitherto appreciated: when annealed, these amorphous materials undergo amorphous-to-amorphous phase transitions which drastically alter their properties. Two families of materials will be discussed: <i>a</i>-(Fe,Ni)O_xwith applications in catalysing the oxygen-evolution reaction during water electrolysis, and <i>a</i>-CuOx, used as hole-transport layers in organic photovoltaics.<br/>Amorphous nickel–iron mixed metal oxides have been shown to be extremely efficient oxygen evolution reaction (OER) electrocatalysts with good stability in alkaline reaction conditions. Thus, they offer an economical alternative to expensive platinum-group metals based OER catalysts and could provide a crucial step towards a hydrogen-based energy economy. These favorable properties are presumably due to a synergistic effect between Fe and Ni, and the highly defective nature of the material. However, these synergistic effects strongly depend on the local structure of the catalyst and their origin are still poorly understood. In the first part of this talk I will present a combined electrochemical and X-ray absorption spectroscopy study of the thermal-annealing-induced structural evolution of amorphous (Ni,Fe)O_x thin films, and correlate this evolution to their OER catalytic capabilities. To reconcile our observations we put forward a dual-site OER reaction mechanism with potential- and rate-determining steps happening at Ni and Fe sites, respectively. We propose this synergistic effect is ultimately responsible for the superior OER performance of many (Ni,Fe)Ox catalysts.<br/>In the second part of this talk I will discuss our work to implement these photodesposited amorphous metal oxides in organic electronic devices, such as OLEDs and OPVs. Hole-selective charge transport layers are an important part of modern thin-film electronics, serving to direct electron flow and prevent leakage current. Crystalline metal-oxide hole-transport layers (HTLs) such as NiO and CuOx exhibit high performance and stability. However they are traditionally not amenable to scalable and sustainable solution-processing techniques. Conversely, amorphous metal oxides are much more readily prepared by low-temperature solution processing methods but often lack the charge transport properties of crystalline semiconductors. Herein we report the fabrication of amorphous a-CuOx thin films for use as HTLs in OPV devices. As was observed for the Fe,Ni oxide system, thermal annealing of the a-CuO_x induces an amorphous-to-amorphous phase transition, which results in p-type semiconducting behavior. The resulting thin films were used as HTLs in organic photovoltaic devices with power conversion efficiencies comparable to those fabricated with PEDOT:PSS. Ongoing work with electron-transport amorphous layers will be presented.