A gold/organic semiconductor diode for ppm-level humidity sensing

The cooperative electric field of surface-adsorbed polar organic molecules has been shown to control charge transport in adjacent solid layers or interfaces. While the adsorption of water monomers on different metal surfaces has also been extensively investigated, the electronic implications of the mechanism have not yet been explored. Here, we show that H2O molecules, selectively adsorbed on a ∼10 nm-thick gold layer deposited on a hydrophobic semiconductor, substantially change the electron distribution in the gold layer, affecting the electron energy profile at the gold/semiconductor interface and altering the current–voltage characteristics of the structure. This concept is used for the fabrication of a resistive humidity sensor for ppm-level hygrometry. Made by depositing gold nanolayers on an air-stable hydrophobic organic semiconductor, oxidized poly[2-methoxy-5-(2-ethylhexyloxy)-p-phenylene vinylene] (MEH-PPV), the device demonstrates high sensitivity at H2O concentrations as low as ∼1 ppm in air, vacuum and inert backgrounds. The presence of gases such as CO2 and H2 in substantial concentrations and oxygen partial pressure variations in air do not interfere with the sensing process. Unlike common Kelvin condensation-based resistive humidity sensors, the electrical resistance of the presented device increases upon exposure to humid atmospheres.