Electrical characterization of optical resonance effects in laterally-nanostructured organic photodetectors.

Optoelectronic devices based on organic semiconductor materials are on the rise for sensing applications due to their integrability with a variety of substrates - including flexible substrates for wearables. For sensing applications often narrowband absorption is desired with suppression of light at other wavelengths. Here, we investigate narrowband absorption enhancement of organic photodetectors (OPD) with an integrated lateral nanostructure. We show with finite-element simulations, that resonant excitation of low absorbing wavelength regimes allow for up to 3 times the absolute absorption at wavelengths on resonance compared to wavelengths off resonance. We present experimental results for CuPc/C OPDs fabricated on grating nanostructures with periods of 350 nm and 400 nm and a grating depth of 140 nm as well as a grating period of 370 nm and grating depths of 30 nm. Angle-resolved transmission spectra clearly show the optical resonance effects. In order to evaluate the electrical resonance effects a measurement system is introduced based on angular laser excitation. An angular resolution of 0.1° is achieved in the analysis of the OPD photocurrent response. Using the measurement setup an increase of the photocurrent by up to 50% is observed for the TE-resonance. It is demonstrated that the resonance wavelength is tuned simply by adjusting the grating period without changes in the layer thicknesses. This opens up new opportunities in realizing pixels of different wavelength response next to each other employing a single active stack design.