Organic Photovoltaics where Water is the Semiconducting Medium and Protons and Hydroxides are the Charge Carriers

Electrochemical technologies require ion-conducting electrolytes that are passive in that electric bias drives ion migration in the thermodynamically favored direction. Recently, my group engineered two important features into passive ion-conducting polymers to introduce the active function of photovoltaic action due to ion transport. These features were covalent bonding of photoacid dyes to the polymers such that absorption of visible light resulted in liberation of protons, and synthesis of polymer membranes with charge-selective contacts to facilitate separation and collection of H⁺ and OH^–. Joining a cation-selective polymer to an anion-selective polymer forms an ionically conductive bipolar membrane, which mimics a rectifying electronic semiconductor pn-junction diode in both form and function. Using a photoacid-dye-modified bipolar membrane, solar-simulated illumination generated a ~120 mV photovoltage. Insights into materials function were obtained using impedance spectroscopy, Mott-Schottky analysis, finite-element numerical modeling of photoacid kinetics, and analysis of drift–diffusion generation–recombination membrane physics. These photo-responsive ion-conducting polymers represent a new class of materials that we envision using for solar desalination of salt water.