Influence of oxidized metal layers and plasmonic metasurfaces on the electrical and optical properties of organic semiconductor materials and devices
Creators: name: Cheng, Zhongkai
Publisher: name: No Publisher Supplied
Year: 2021

description: Organic semiconductors are composed of organic small molecules or polymers rather than traditional inorganic materials (e.g., silicon). Nowadays, organic semiconductors receive intensive attention in view of their great promise for large-area, light-weight and flexible electronics applications. Owing to their advantages compared to commonly used inorganic semiconductor materials in optoelectronic applications (such as, compatibility with plastic substances, low temperature processing, large area coverage, mechanical flexibility, and low cost), organic semiconductor materials are appealing for a broad range of devices, including transistors, sensors, solar cells, and light-emitting diodes. Since the active layer of emerging, next-generation optoelectronic devices are based on thin semiconductor films that have thicknesses less than 1 μm, organic polymer semiconductor materials (i.e., conjugated polymers) are more advantageous compared conjugated small molecules in terms of their better solution processability, lower process energies and better bio-compatibility. However, optoelectronic devices based on organic polymer semiconductor materials usually exhibit lower efficiency and poorer stability than more developed and well-known inorganic semiconductor optoelectronic devices. Therefore, in this thesis, the objective is to investigate electrode surface modification approaches to improve the optical and electrical properties of the organic polymer semiconductor thin films and devices to achieve high-performance and stable organic optoelectronic devices. Silver (Ag) is one of the most suitable metals for electrodes in optoelectronic devices because of its high reflectivity and its ability to support low-loss surface plasmon resonances that can enhance light-matter interactions. However, the electronic work function of Ag is not ideally matched to the frontier orbital energies of many organic semiconductors. In Chapter 2 of this thesis, we investigate the formation of an ultrathin surface oxide layer on a Ag electrode and its impact on hole injection into an organic conjugated polymer semiconductor. The surface oxide is intended to increase the work function of the Ag electrode to match the highest-occupied molecular orbital of the organic semiconductor. The surface oxide is formed by exposing the Ag electrode to a low-power O2/Ar plasma and it changes the electrical properties of the pure Ag electrode. We study the morphology and the chemical composition of the Ag surfaces after different plasma treatment times through X-ray photoelectron spectroscopy, scanning electron microscopy and dark-field optical microscopy. Inverted organic optoelectronic devices, especially solar cells, benefit from the use of plasmonic electrodes for increased light trapping and exciton generation. Yet, there are few studies of inverted fabrication of organic optoelectronic devices directly on top of plasmonic metasurface electrodes and its impacts on device properties. In Chapter 3, we describe the facile fabrication of aperiodic Ag nanoparticle plasmonic metasurfaces and study their physical and optical characteristics. The aim is to investigate how nanostructure parameters impact the optical and electrical properties of conjugated polymer thin films and devices. Then, we investigate the photonic and electronic behavior of the aperiodic plasmonic metasurfaces when interfaced with organic semiconducting polymer thin films. In particular, we show that plasmonic enhancement can overcome ohmic losses associated with metals and metal-induced exciton quenching.Photon recycling (PR), whereby photons generated by radiative recombination in semiconductors result in the regeneration of hole-electron pairs, plays an important role in the study of optoelectronic semiconductor materials and significantly affects the properties of their applications. PR has more obvious benefits for the performance of solar cells because solar cells have a greater demand for photon absorption than lasers and LEDs, and PR can significantly promote and meet this requirement. However, since PR has not been investigated and controlled in organic semiconductor devices, despite it’s potential to improve performance. In Chapter 4, we study PR in three different organic semiconducting polymer media on a variety of different surfaces. The aim is to demonstrate and explain the mechanism of PR in organic conjugated polymer films. We show that PR is strongly affected by the substrate surface type and the morphology of the semiconductor. This work demonstrates that PR can be achieved in organic semiconductors, and it is strongest for surfaces and structures that promote both high quantum yield and in-plane waveguiding at the semiconductor emission wavelength.