Mesoporous semiconducting films consisting of preferentially orientated monoclinic-phase nanocrystals of tungsten trioxide have been prepared using a novel version of the sol-gel method. Transformations undergone by a colloidal solution of tungstic acid, stabilized by an organic additive such as poly(ethylene glycol) (PEG) 300, as a function of the annealing temperature have been followed by means of a confocal Raman microscope. The shape and size of WO3 nanoparticles, the porosity, and the properties of the films depend critically on preparation parameters, such as the tungstic acid/PEG ratio, the PEG chain length, and the annealing conditions. Well-crystallized WO3 films combine excellent photoresponse to the blue region of the solar spectrum, up to 500 nm, with good transparency at wavelengths larger than 550 nm. Particular applications of these nanocrystalline WO3 films include photoelectrochemical and electrochromic devices.

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The photoelectrochemical characteristics of highly transparent nanoporous WO3 films are described. The photocurrent versus excitation wavelength spectra of these photoelectrodes exhibit a maximum close to 400 nm and a significant photoresponse to the blue part of the visible spectrum. The observed conversion efficiencies attain 75% for the photogeneration of oxygen from 1 M aq HClO4 and reach 190% in the presence of methanol in the solution, denoting in the latter case the occurrence of a perfect photocurrent doubling. Experiments conducted under simulated solar AM 1.5 illumination resulted in steady-state anodic photocurrents of the order of several mA/cm2.

The electrical properties of eumelanin, a ubiquitous natural pigment, have fascinated scientists since the late 1960s. For several decades, the hydrationdependent electrical properties of eumelanin have mainly been interpreted within the amorphous semiconductor model. Recent works undermined this paradigm. Here we study protonic and electronic charge carrier transport in hydrated eumelanin in thin film form. Thin films are ideal candidates for these studies since they are readily accessible to chemical and morphological characterization and potentially amenable to device applications. Current−voltage (I-V) measurements, transient current measurements with proton-transparent electrodes, and electrochemical impedance spectroscopy (EIS) measurements are reported and correlated with the results of the chemical characterization of the films, performed by X-ray photoelectron spectroscopy. We show that the electrical response of hydrated eumelanin films is dominated by ionic conduction (10 −4 −10 −3 S cm −1 ), largely attributable to protons, and electrochemical processes. To propose an explanation for the electrical response of hydrated eumelanin films as observed by EIS and I-V, we considered the interplay of proton migration, redox processes, and electronic transport. These new insights improve the current understanding of the charge carrier transport properties of eumelanin opening the possibility to assess the potential of eumelanin for organic bioelectronic applications, e.g. protonic devices and implantable electrodes, and to advance the knowledge on the functions of eumelanin in biological systems.

Light emitting field effect transistors based on molecular and polymeric organic semiconductors are multifunctional devices that integrate light emission with the current modulating function of a transistor. The planar geometry of organic light emitting field effect transistors (OLEFETs) offers direct access to the light emission region, providing a unique experimental configuration to investigate fundamental optical and electronic properties in organic semiconductors. OLEFETs show great potential for technological applications such as active matrix full color electroluminescent displays. In this Feature Article we review advances in OLEFETs since their first demonstration in 2003 and we highlight exciting challenges associated with their future development.