Light sensors based on organic phototransistors with absorption-enhancing nanoparticles

Organic semiconductors (OSCs) exhibit promising electronic and optical properties applicable in photo-sensing devices. Previous studies have found that thiophene-based semiconductors are suitable as the active layer in organic optoelectronic devices such as light-sensing transistors [1]. The ability to fabricate and characterize metal nanostructures allows for tuning of the optical properties of metal nanoparticles (NP). In this work we have investigated organic thin-film phototransistors (OPTs) based on the molecule 5,5’’-bis(naphth-2-yl)-2,2’:5’,2’’-terthiophene (NaT3). Using electron beam lithography (EBL) we have fabricated organic field effect transistors (OFETs) with arrays of silver NPs under the organic thin-film. Experimentally, the optical extinction of light incident on arrays of NPs has been measured by reflectance spectroscopy. The results have been compared to the predictions of Mie theory. The photoconductivity of OPTs with and without nanoparticles was characterized under white light and spectrally resolved illumination. The OPTs exhibit significant responsivity at wavelengths below 500 nm essentially coinciding with the absorption spectrum of NaT3. Recent work with reduced particle diameter and more particles shows indication of enhanced photoresponsivity by exploiting the localized surface plasmon resonance (LSPR) of the NPs.