Enhanced photoresponsivity in organic field effect transistors by silver nanoparticles

Organic semiconductors (OSC) such as thiophene-based oligomers exhibit useful electronic and optical properties making them applicable in photo-sensing devices. Generally, thiophene-based photodetectors exhibit a decent responsivity with a spectral sensitivity determined by the OSC's absorption properties. We present a simple, yet efficient method to enhance and spectrally tune the photoresponsivity of organic phototransistors (OPTs) utilizing the plasmonic properties of embedded metallic nanoparticles. The ability to fabricate and characterize nanostructures with high accuracy allows for tailoring of the optical properties of metal nanoparticles (NPs) and thereby tuning of the spectral range of enhanced photoresponsivity. In this work we have investigated OPTs based on the molecule 5,5″-bis(naphth-2-yl)-2,2':5′,2″-terthiophene (NaT3). We have fabricated OPTs with electron beam lithography-defined arrays of silver NPs under the organic thin-film. The photoconductivity of OPTs with and without NPs was characterized under spectrally resolved illumination. The OPTs exhibit significant responsivity at wavelengths below 500nm essentially coinciding with the absorption spectrum of NaT3. Finite element simulations of the optical properties of the NPs reveal large electric field enhancement at the plasmon resonances. At these wavelengths (350→500nm) the experimentally measured photoresponsivity of OPTs with NPs is significantly increased compared to the responsivity for OPTs without NPs. Due to a variation in particle size and shape of the fabricated silver NPs, a spectrally broad enhancement is predicted, which is in agreement with the experimental results. The results show that the photoresponsivity is dominantly enhanced in a spectral region of low OSC absorption coinciding with the localized surface plasmon resonances of the NPs. This work demonstrates that it is possible to improve and tailor the responsivity spectrum of photoactive organic devices by integration of metal nanoparticles.