Abstract
The energy level alignment between a metal and an organic semiconductor (M/O) is regarded as the dominant factor affecting carrier injection efficiency. In particular, it plays a crucial role in the performances of organic light emitting diodes (OLEDs) including electroluminance efficiency and life-time. Through electrode surface modification, the size and direction of the interface dipole can be introduced to modulate the energy alignment at the interface. In this study, a series of thiol compounds, including fluorine-substituted benzyl mercaptans, n-alkanethiol, cyano-terminated (CN-), trifluoro-terminated (CF3-), perfluroinated substituents and binary mixtures with opposite dipoles, were used to modify the silver anode through the formation of well-ordered self-assembled monolayers (SAMs) and applied in the fabrication of top-emitting electroluminescent devices. The device performances were measured and analyzed to understand the effect of modification on the charge injection. It is demonstrated that the same substituent placed at different positions on a phenyl ring results in different dipole moment and modulate the work function/charge injection differently. Furthermore we demonstrated that besides the dipolar functional group, the chain length of alkanethiol can be used to tune the tunneling distance for charge injection. Through selection of the two parameters (dipolar functional group and tunneling distance), the energy alignment can be fine-tuned and influence the charge balance and luminescence efficiency. VII Furthermore, SAMs of binary mixtures of n-decanethiol and the perfluorinated analogue were formed on silver surface. Through change of mixing ratio, the work function of silver can be tuned continuously over a wide range: from 4.1 eV to 5.8 eV. The mixed SAM-modified Ag surfaces were used as the anode in the fabrication of hole-only devices and electroluminescent devices using different hole-transporting materials (HTL). Through the analysis of charge injection efficiency/luminescence efficiency, strategy of improving hole/electron carrier balance is proposed. With generally low mobility associated with organic semiconductors and limitations on the fabrication involving lateral transistors as driving component, we also extended our work on vertical type transistors, which uses greatly reduced channel length to lower driving voltage and increase current output. A large-area and periodically patterned nanoporous Al grids with controlled pore size were fabricated by poly(ethylene oxide)-assisted self-assembly of polystyrene nanospheres. This grid layer was used as the base electrode in a space-charge limited transistor (SCLT) with vertical architecture. A high performance device with poly(3-hexylthiophene) (P3HT) as conducting semiconductor was achieved with a high on-current