The presence of a carrier selective contact is fundamental for photoelectrochemical (PEC) optoelectronic devices which convert solar energy to chemical/electrical energy. For example, during the overall water splitting (OWS) reaction in an acidic electrolyte, the generated electrons and holes must selectively transport to their respective contacts where they reduce protons and oxidize water, forming H2(g) and O2(g) respectively. Often, bare semiconductor (SEM) absorbers are poor catalysts and metals (or metal oxides) (CAT) are alternatively used, although forming a new junction with the underlying SEM. In some cases, the CAT/SEM junction does not theoretically form the required carrier selective junction. A real photocathode consisting of a p-InP and Pt junction should theoretically form an ohmic or non-selective junction, however, this photocathode efficiently reduces protons in operation. In extension, many single OWS particles decorated with cocatalysts theoretically would not form selective contacts producing sufficient open-circuit voltages required to split water (by simple thermionic-emission theory alone). This is further debated in the mechanism for carrier separation during photoelectrodeposition of catalyst in such OWS particles. Here, we exploited conductive probe microscopies and surface-sensitive spectroscopies to reveal the underlying mechanism driving carrier separation towards selective contacts such as alloying transformation and heterojunction formation in the Pt/p-InP system. Additionally, I will demonstrate the unique behavior of nanoscopic electrical contacts by individual characterization (operando and in-situ), demonstrating selectivity by a manifestation of the pinch-off effect. On the Pt/p-InP model system, individual contact selectivity, by the pinch-off effect, on a chemically controlled surface seen in Figure 1.a later evolves due to the formation of an InOx/p-InP heterojunction after operation (after CV). Furthermore, we investigate SrTiO3 as a planar model system proxy to OWS particles and show only small variation in the oxygen evolution reaction (OER) current in macroscopic cyclic voltammograms of <100> and <110> terminated surfaces (Figure 1.b). By revealing the underlying mechanisms and the degree that they play, PEC devices can be engineered utilizing an optimal absorber and catalyst whilst their pristine junction would theoretically be non-selective or producing an insufficient photovoltage. Figure 1
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