Controlling metal-polymer interfaces for optoelectronic devices
Creators: name: Sreekumar, Sneha
Publisher: name: No Publisher Supplied
Year: 2025

description: Organic semiconducting conjugated polymers are commonly used in optoelectronic devices, replacing inorganic materials due to their many advantages, such as, ease of processability, thin form factors, compatibility with solution deposition techniques, and mechanical flexibility. They are made of large number of aromatic monomer repeat units, creating macromolecular chains, whose electronic properties can be tuned by, for example, chemical doping, introducing chemical moieties or varying the side chain length. Organic semiconducting polymers are particularly advantageous because, they can be formed into thin films using atmospheric pressure deposition techniques, like spin coating or solution printingprocesses, compared to expensive vacuum deposition processes that are used for inorganic and organic small molecules. However, organic semiconducting polymers also suffer from several disadvantages, including low stability when exposed to air and moisture; and low device efficiencies, due to low charge carrier mobilities and the presence of triplet excited states. In optoelectronic devices, particularly in organic light-emitting diodes (OLEDs), only ~25 % of the light emitted is useful due to poor light extraction efficiency, and the device stability is low, especially for blue-emitting active layers, due to long lived triplet states. Previously, several methods have been used to overcome these issues by adding different additive and buffer layers or developing new device architectures. Commercially available OLEDs are fabricated with indium-tin oxide (ITO) transparent electrodes, which are extremely robust, although they involve high cost and high embodied energy. Alternatives forITO have been actively investigated, with research into new transparent conducting electrode materials (TCE) for bottom-emitting OLED devices. Another approach to this problem is to build the device on a metallic anode instead of ITO that can be patterned to give a plasmonic electrode, with the light coming out through a transparent top cathode. Plasmonic structures can enhance the light extraction efficiency due to resonant scattering or surface plasmon polariton extraction. Further, the overall device stability and performance can be improved due to the near-field optical effects of plasmonic structures, such as the Purcell effect, by strongly interacting with the organic semiconductor emitters. In the present thesis, we have investigated interfacial layers and plasmonic structures that can be effective for improving the performance of bottom-emitting and top-emitting polymer OLEDs. The first part of the thesis (Chapter 2) deals with one of the major problems in building a solution processed optoelectronic device on a metallic electrode, which is the creation of a Schottky barrier at the metal-polymer interface. This means that the injection of charges at the interface between the metal electrode and the polymer active layer depends on the difference between the work function of the metal electrode and frontier orbital energy levels of the polymer. Several methods may be used to overcome this barrier by using interfacial layers, oxides or buffer layers. One of the common methods used in organic field-effect transistors (OFETs) to overcome this barrier is to use a self-assembled monolayer (SAM) of thiol-containing compounds on metallic electrodes like Au or Ag. The other main challenge in using a metal like Ag is that it undergoes oxidation readily during processing of the device.In that regard, SAMs making S-Ag bonds can also help with passivation of the Ag surface. We employ (±) α-Lipoic acid derivatives to form SAMs on Ag for the study of electrical injection and electronic properties by building inverted-organic hole only conjugated polymer devices. (±) α-Lipoic acid with its two S atoms can form stronger bonds with Ag, and, by varying the functional groups at the end, we can vary the overall molecular dipole moment and interfacial interactions at the metal-polymer interface. We find significant improvements in the performance (about 1.5 times) and passivation with (±) α-Lipoic acid. Additionally, we studied the change in work-function, charge injection barrier and polymer-morphology in the presence of novel SAMs. In the second part of the thesis (Chapter 3), we develop a transparent conducting cathode material that can be used as a top contact in OLEDs built on Ag. For n-type electrode materials, a combination of ZnO and Ag layers are very popular, especially in solar cells. We have developed a multilayer of ZnO/Ag/ZnO suitable for deposition directly on the conjugatedpolymer active layer by optimizing and varying the ZnO and Ag thickness using mild sputtering conditions. We demonstrate a TCE that has a transparency of ~85% and a sheet resistance of 1.47 Ω/sq, which is lower than that of the industry standard TCE, ITO. We also demonstrate the usefulness of this TCE in a transparent OLED built on ITO. The third part of the thesis (Chapter 4) deals with fabricating solution processed OLEDs that incorporate plasmonic nanoparticles. The biggest roadblock in OLED display technology is the stability of blue OLED sub-pixels. Compared to the red and green sub-pixels, which have achieved an external quantum efficiency (EQE) of ~20%, the EQE of blue sub-pixels still remains at 5%. We first optimize a deep-blue phosphorescent OLED, and incorporate Ag nanoparticles onto the surface of the ITO electrode to study plasmonic and Purcell effects. We study transient photoluminescence lifetime, quantum yield and device stability to understand the effects of the plasmonic nanoparticles on the emitter, as well as on the device performance. We find that, even though there is a plasmonic and stability enhancement, especially in the case of colloidal Ag particles on ITO, the overall device performance is dominated by other optical and electronic effects. Further studies on the band alignment and charge transfer between the ITO, Ag nanoparticles and the hole transport layer (HTL), and electronic interactions between the HTL and emissive layer are necessary to isolate and improve the plasmonic and Purcell effects. In summary, in this thesis, we demonstrate different device architectures, electrodes and interfaces that can lead to efficiency and stability improvements in organic optoelectronic devices specifically, polymer-based OLEDs. We study the modification of metal-polymer interfaces for top-emitting OLEDs and develop a new transparent electrode material as thetop-contact electrode. We also study at the effects of plasmonic nanoparticles on the stability and efficiency in deep-blue phosphorescent OLEDs. These studies have the potential to impact OLED technology, specifically for display and solid-state lighting applications.