Optical and Electrical Properties of Organic Semiconductor Thin Films on Aperiodic Plasmonic Metasurfaces

Metal electrodes are playing an increasingly important role in controlling photon absorption and in promoting optimal light management in thin-film semiconductor devices. For organic optoelectronic devices, the conventional fabrication approach is to build the device on top of a transparent electrode, with metal electrode deposition as the last step. This makes it challenging to control the surface of the metal electrode to promote good light management properties. An inverted fabrication approach that builds the device on top of a metal electrode makes it possible to control the morphology of the metal surface independently of the organic semiconductor active layer to achieve a variety of photonic and plasmonic behaviors useful for devices. However, there are few reports of inverted fabrication of organic optoelectronic devices and its impacts on device properties. Silver (Ag) is the most suitable metal for fabrication of nanostructured electrodes with plasmonic behavior (i.e., plasmonic electrodes) because of its low parasitic absorption loss and high reflectivity. In this project, we describe the facile fabrication of silver nanoparticle (AgNP) aperiodic plasmonic metasurfaces and study their physical and optical characteristics. Then, we investigate the photonic and electrical behaviors of the aperiodic plasmonic metasurfaces when interfaced with poly(9,9-dioctylfluorene--benzothiadiazole) (F8BT) organic semiconducting polymer thin films. The luminescence quantum yield of F8BT thin films increases from 29% on planar Ag up to 66% on AgNP metasurfaces due to the Purcell effect and the improved extraction of emission coupled to surface plasmon polariton modes. In particular, we show that plasmonic enhancement can overcome ohmic losses associated with metals and metal-induced exciton quenching. According to the current-voltage characteristics of hole-only devices with and without aperiodic plasmonic metasurfaces, we conclude that AgNP aperiodic plasmonic metasurfaces have comparable electrical behavior to planar metal electrodes while having superior light management capability.