Investigation of structural properties of organic thin films for solar cell and transistor applications

For the past several decades, organic materials including polymers, oligomers and small molecules have been of great interest for their various applications in the electronics and the semiconductor industry. The most appealing advantages of organic materials compared to their inorganic counterparts are their compatibility with flexible substrates and amenability to lowtemperature and low-cost fabrication processes such as evaporation, spin-coating and printing. Moreover, the ability to be utilized in fabrication of lightweight and large-area devices is among other reasons for popularity of organic materials. A large number of studies have reported on various aspects of the development and optimization of organic electronics such as organic light emitting diodes (OLEDs), solar cells (OSCs) and thin film transistors (OTFTs). Although significant progress has been made during this period, some of the intrinsic electrical properties of organic materials such as low carrier mobility have continued to hinder the full development and maturation of the organic electronics industry. In order to manufacture organic electronic devices with high performance, more detailed studies of the structure and the morphology of the organic materials as well as the underlying physical charge transport mechanisms should be performed. Additionally, growth, deposition and assembly processes need to be established and optimized for the new organic semiconductor technology. This work aims to advance the understanding of the effect of the structural properties of organic thin films on the charge carrier transport within the organic thin films as well as the charge carrier injection between the organic layers and the organic-inorganic materials such as metal or dielectric layers. Charge carrier transport mechanisms between different layers are crucial factors in determining the efficiency of organic electronic devices. These parameters rely largely on the molecular structure, morphology and ordering of the organic thin films. In order to investigate these intrinsic properties, several organic thin films were prepared using vacuum thermal evaporation method. Their morphology and structural properties were studied by the combination of various techniques including atomic force microscopy, X-ray reflectivity, spectroscopic ellipsometry and transmittance measurements. Based on the produced organic thin films, organic semiconductor devices such as OTFTs and OSCs were fabricated and their electrical and optical properties were characterized. Moreover, the effect of morphology and structure of the organic thin films on the organic device performance was studied. Ambipolar thin film transistors based on pentacene and PTCDI-C8 as the active layer and lithium fluoride (LiF) as the gate dielectric layer were fabricated and characterized. Conduction behaviors of these devices were modeled using Fowler-Nordheim (FN) tunneling theory. The results of this study suggest that the charge transport in OTFTs correlate not only with the organic semiconductor film structure, but also with the dielectric–semiconductor interfacial effects. Moreover, bilayer heterojunction OSCs based on CuPc/PTCDI-C8 as the donor/acceptor layers were fabricated and their electrical and optical properties were characterized. The effects of the active layers’ structures and morphologies as well as the buffer layers’ thickness variation on the device performance were studied. The results of this study emphasized the importance of the thin film structural properties on the device performance.