Advances in solution processing of organic semiconductors for organic semiconducting devices

Abstract

Organic semiconductors have emerged as an intriguing and well-explored class of materials in recent years, with the potential to replace traditional inorganic semiconductors. One advantage of organic semiconductors is their reliance on hydrocarbons, eliminating the need for expensive and typically toxic inorganic elements, which pose risks to both human health and the environment. Another benefit of organic semiconducting materials lies in the ability to precisely tailor their properties through molecular design and synthesis, yielding organic semiconductors specifically optimized for desired functionalities. Common to all organic semiconductors is the presence of a conjugated system comprising alternating single and double bonds. Such systems can be realized in both small molecules and polymers. Charge transport within these systems occurs through hopping of charge carriers within this conjugated framework. By judiciously selecting side chains for polymers or functional groups for molecules, the properties of the organic semiconductor can be tailored. For instance, the emission wavelength of emitters for organic light-emitting diodes (OLEDs) can be tuned across the entire spectrum, or side chains can be added to enhance solubility. This work focuses on the fabrication of films of organic semiconductors from solution and the preparation of blends of organic semiconductors with other materials to enhance the properties of the films. In Chapter 4 of this study, the possibility of increasing the permittivity of organic semiconductor films is investigated. A high permittivity is necessary to achieve high efficiency in organic solar cells (OSCs). Organic semiconductors typically have a low permittivity. Therefore, they were mixed with materials possessing a high permittivity to achieve a film with an increased permittivity. It was found that this approach is feasible in principle; however, it negatively affects the charge transport through the organic semiconductor. In Chapter 5, the improvement of electron injection into organic semiconductors is investigated. To achieve this, a method is employed whereby a thin layer of approximately 4 nm thickness is placed between the semiconductor and the electrode. Previous studies have demonstrated that this approach is effective for efficiently injecting holes. It was found that this principle can also be applied to negative charge carriers. Additionally, a method was developed to apply this thin layer from solution onto an organic semiconductor film. In Chapter 6, the feasibility of manufacturing OLEDs from solution is investigated, aiming for comparable efficiency to OLEDs produced through other manufacturing methods. Initially, the processability of various emitters from solution was examined, along with the blending of emitters with polymers to enhance the physical stability of films against subsequently applied layers. It was observed that blending films with polymers for use in OLEDs is feasible in principle; however, a reduction in efficiency occurs when an additional layer from solution is to be processed onto it.