Image dipoles and polarons in organic semiconductors

Sammanfattning: The rapid development of organic electronics depends on the synthesis of new π-conjugated molecules/polymers and the exploration of fundamental physics. However, most of the efforts have been concentrated on the former, leading to a lack of thorough understanding of many important concepts, which will become the ultimate limiting factor for overall performance and applications. Thereinto, the interface energetics in multilayer stacked optoelectronic devices and the electronic structure of doped organic semiconductors are two of the most complicated and yet inevitable topics in this field. A better understanding of them can provide needed additional insights into the operation of devices and give valuable guidance for device and molecule design. Hence, the aim of this thesis is to investigate these two fundamental issues using various spectroscopic characterizations and supported by computational modeling.The cathode/organic interface plays a critical role in achieving balanced charge transport and improved stability in organic electronic devices. Employing stable cathode materials, however, typically results in large electron injection barriers and sub-optimal devices. Using small-molecule electron transport materials (ETMs) as interlayers is an effective approach to reduce the electron injection barriers, but the working mechanism is still under debate. By studying the energy level alignment behavior of ETMs on different types of substrates, we find the work function of the substrate is reduced by an extra “image” dipole formed at the interface and within the first layer of the ETM film. The use of non-reactive substrates and the results from X-ray photoelectron spectroscopy core level analysis exclude the orbital hybridization theory, which states that an ETM-metal complex may form at the interface. The characterization results on molecular orientation disqualify an explanation using intrinsic molecular dipole moments. Instead, experiments demonstrate that the interface dipole depends on the areal density and direction of lone electron pairs on the heteroatoms in ETMs, which is similar to previous observations in tertiary aliphatic amines. This behavior is well described by the so-called “double dipole step” model, where one dipole formed by the nitrogen nuclei and the lone pairs in the organic side points from the substrate surface to the organic film and the other one formed by their image charges in the electrode side shows the same direction.The polaron charge carrier is another important concept involved in multiple (opto)electronic processes during device operation, such as charge transport, exciton recombination/dissociation. Although numerous experimental and theoretical efforts have been made, there is still a lack of comprehensive studies on the electronic structure of negative polarons due to their high air sensitivity, including correlation between the valence band structure measured by ultraviolet photoelectron spectroscopy (UPS) and the optical band gap derived from the UV−vis−NIR absorption. In the present work, we are able to integrate the optical and electrical measurements with photoelectron spectroscopy to collect all information without breaking an ultra-high vacuum. Negative polarons formed in alkali metal-doped polymers are detected with new polaronic states below the Fermi level and lower energy absorption bands arising from the excitation from polaronic states to unoccupied states. In addition, the Fermi level shifts toward the conduction band with increasing the doping ratio, and the doubly-occupied polaronic state shows slightly lower energy than the topmost valance band peak of the neutral polymer. These observations are supported by the density functional theory (DFT) simulations, from which we also demonstrate that polaron pairs rather than bipolarons are preferentially formed at high doping ratios. By comparison of different polymer and dopant systems, we find the polymer-dopant interaction and the polaron delocalization are dependent on the distortion of the polymer backbone and the size of the dopant, properties that in turn affect the conductivity and air stability of the n-doped materials.I hope that the findings presented in the thesis can greatly promote the understanding of the energetics of the ETMs/cathode interface and the electronic structures of negative polarons in alkali metal-doped polymers, contributing to further providing new guidelines for the molecular design and improve the device performance.