Light Conversion by Electrochemiluminescence at Semiconductor Surfaces

ConspectusElectrochemiluminescence (ECL) is the electrochemical generation of light. It involves an interfacial charge transfer that produces the excited state of a luminophore at the electrode surface. ECL is a powerful readout method that is widely employed for immunoassays and clinical diagnostics and is progressively evolving into a microscopy technique. On the other hand, photoelectrochemistry at illuminated semiconductors is a field of research that deals with the transfer of photogenerated charge carriers at the solid-liquid interface. This concept offers several advantages such as a considerable lowering of the onset potential required for triggering an electrochemical reaction as well as light addressable chemistry, via the spatial confinement of redox reactions at locally illuminated semiconductor electrodes. The combination of ECL with photoelectrochemistry at illuminated semiconductors is termed photoinduced ECL (PECL). It deals with the triggering of an ECL reaction through the transfer of photogenerated minority charge carriers at the illuminated solid/liquid interface. PECL results in the conversion of incident photons (λ_exc), that are absorbed by the semiconductor photoelectrode to emitted photons (λ_PECL), produced by the ECL reaction. Although demonstrated in the 1970s by Bard et al. in ultradry organic solvents, PECL remained unexplored until the last five years. Nowadays, as a result of the considerable progress achieved in semiconductor photoelectrodes and ECL systems, a large variety of PECL systems can be designed by combining photoelectrode materials with ECL luminophores, making it a versatile tool for light conversion in aqueous media.In this Account, we introduce the fundamentals of ECL and photoelectrochemistry at illuminated semiconductors and review the recent developments in PECL. We discuss the two main PECL light conversion schemes: downconversion (where λ_exc < λ_PECL) and upconversion (where λ_exc > λ_PECL). Besides, PECL can be used to simplify considerably the common electrochemical setups employed for ECL. Indeed, by engineering the photoelectrode material and carefully considering the reactivity involved for ECL and its counter-reaction, PECL enables the ultimate concept of all-optical ECL (AO-ECL), i.e., ECL generation at an illuminated monolithic device immersed into the electrolyte solution. As discussed in this Account, AO-ECL is an important breakthrough that allows the simplest ECL experimental configuration ever reported, eliminating constraints such as an electrical power supply, wires, electrodes, connections, and specific electrochemical knowledge. As shown at the end of this Account, due to the robustness of recently manufactured PECL systems, several applications can already be envisioned for microscopy, elucidation of solar conversion mechanisms, near-infrared imaging, and bioanalysis.