Energy Level Alignment At Electrode Interfaces In Organic Electronic Devices And Related Studies

Organic electronics based on or ganic semiconductors have shown tremendous advantages compared to their inorganic counterparts,such as,low-temperature processibility,low manufacturing cost,high through-put,light weight and flexibility.There are two main types of semiconducting optoelectronic devices,i.e.,organic light-emitting diodes(OLEDs)and organic solar cells(OSCs).They are both thin-film devices constructed by different functional layers.The vertical dimension of each layer can almost be neglected compared to its dimension along the plane,indicating the importance of the thin-film interface.Various electronic processes including charge injection/extraction,charge recombination and exciton dissociation,take place at heterojunction interfaces in the device.Therefore,the quality of the interface directly influences device performance and stability.It is thus of paramount importance to study the interfacial energetics at organic/metal and organic/organic and understand their implications in device operation.Carriers have to overcome the energy barrier when moving across interfaces.A large barrier will cause voltage loss and hence reduce the output power in OSCs.In OLEDs,a large injection barrier could increase operation voltage and shorten device lifetime.Thus it is critical to achieve Ohmic contact at the electrode interfaces for both types of devices.In the recent years,a lot of dipolar interfacial materials,particularly organic electrolytes,have been developed to modify the electrode surface work-function to form an Ohmic contact.Various models have been proposed to explain their working mechanism.The consensus is that the interfacial material can form a dipole layer at the metal interface tuning the injection barrier.However,there are still much unknown about how the dipole takes effect in real a device.For example,there is prevalent controversy in the literature about the dipole direction in a regular device,and about the different functions of interfacial materials in carrier-injection devices(e.g.OLEDs)and carrier extraction devices(e.g.OSCs).The relationship between the molecular structure and its dipole formation capability is also yet to be clarified.In the thesis,we have studied the interfacial energetics of various organic semiconducting materials.The energy level alignment on different types of substrates is examined.Furthermore,interfaces of bulk-heterojunction thin films commonly seen in OSCs have been investigated for the first time.It was found that the Integer Charge Transfer(ICT)model can consistently describe the interfacial energetics of these organic materials.By charting the surface work-function of semiconductors on di fferent substrates,we have obtained the ICT levels of the organic active-layer materials.The knowledge provides essential basis for our interpretation of the energy-level diagram in a real device.Other newly developed organic materials can also be examined following the proposed experimental method.The result can help us optimize the device structure for specific materials.The second part of the thesis studies the dipole orientation of the interfacial layer in a regular device.A “peel-off” method has been specially employed to detach the fabricated device at different interfaces,thus circumventing the limitation in conventional methods where only one of the interfacial-layer surfaces can be characterized.In the experiment,a typical polyelectrolyte,PFN,is used as the cathode interfacial material.By measuring vacuum level shift across all interfaces,the complete energetic picture of the device is illustrated.The result indicates the existence of dipole layers at both surfaces of the PFN layer.In devices with wide-bandgap active-layer materials,like typical OLEDs,the net dipole of the interfacial layer lowers the injection barrier.In devices with materials of high electron affinity,like in most OSCs,Fermi-level pinning at the fullerene/electrode interface would occur to render the contact Ohmic and give a net dipole with a direction opposite to that in OLEDs.The transferred integer charges would also screen the PFN interfacial dipoles to be detected.Therefore,energy level alignment at the interfacial layer in different devices may not be the same depending on the specific materials.Since the interfacial dipole at post-deposited metal interfaces is critical in understanding device operation,in the third part of the thesis,we have systematically investigated the dipole formation on organic materials with various polarity.Traditional methods to characterize the interfacial dipole are complex and inefficient.To circumvent this problem,we have proposed a novel method by depositing Al through a shadow-mask with a slit opening.Due to metal vapor scattering,a small amount of Al can divert and land on the shadowed region on the organic substrate.Because of the ultrathin nature of this adsorbed metal layer,the formed dipole can be directly measured by scanning across the sample using a Kelvin probe.It is found that the polar species of the organic have a dominant role on the dipole layer formation.Different metal morphology and image charge effect could also lead to some slight difference.The dipoles at the organic/post-deposited Al were further compared to those formed at the pre-deposited Al/organic interfaces,distinguishing different dipole formation mechanisms.The proposed technique also provides a simple and fast characterization method for assessing the rapidly growing library of interfacial materials.The deposited thin film quality in solution processing is related to the wetting property of the solution.To form highly-uniform and pin-hole-free thin films,it is important to tune the contact angle of the solution on t he substrate.Particularly,superhydrophobicity is the phenomenon wherein the water contact angle exceeds 150 ° on a solid surface.Superhydrophobicity is commonly seen in nature,inspiring its many applications in medicine,architecture,and so forth.There is an elaborate three-phase(solid/liquid/air)distribution at the interface of water-solid contact on a superhydrophobic surface.However,buried under the liquid,this interface has seldom been directly studied due to the lack of in-situ and non-destructive observation technique.In the fourth part of the thesis,the synchrotron-radiation-based X-ray phase-contrast imaging(SR-XPCI)is exploited for the first time for direct observation of the superhydrophobic interfaces.The studied samples are the natural lotus leaf and man-made carbon-nanotube/polymer composite film.Unlike traditional absorption-based imaging,phase-contrast imaging enables better image resolution and contrast.The synchrotron radiation has further reduced the time consumption during sample scanning,so that destruction to the delicate interface is avoided.By assembling two-dimensional scanned slices,the three-dimensional distribution of the solid/liquid/air phases is reconstructed.It clearly reveals the abundant air trapped inside the micro-nano structures on the solid surface,which endows the samples strong superhydrophobicity.The proposed technique thus has achieved direct and non-destructive observation of the water-loaded solid surface.It could promote the study of the wetting physics in a whole new perspective,and help us design interesting wetting surfaces with various properties.