Micro- and macroscopic investigation of interfaces in organic light-emitting devices

This thesis presents investigations of the surface electronic structures and optical properties of organic luminescent materials such as poly(p-phenylene vinylene) (PPV), poly(2,3-diphenyl-phenylene vinylene) (DP-PPV) and tris-(8-hydroxyquinoline)aluminum (Alq{dollar}sb3{dollar}), during interface formation with metals. The characterization of light emitting diodes (LEDs) based on these materials is also presented. Surface electronic structure characterization was accomplished by x-ray and ultra-violet photoelectron spectroscopies (XPS and UPS). Characterization of the photophysical properties was obtained using UV/Vis, photoluminescence (PL) spectroscopy and time correlated single photon counting. Device characterization was done using electroluminescence (EL) spectroscopy. Chapter IV and V presents the XPS and UPS investigation and the PL studies of the metal/organic interface formation process, respectively. Combined, these investigations for the first time provide, from a fundamental and unique perspective, the physical processes that underlie the interfacial properties of these technologically interesting materials, and influence the performance of devices based on these materials. Ca was found to induce new states in the energy gap of 1,4-bis (4-(3,5-di-tert-butylstyryl)styryl) benzene (4PV) and react very strongly with Alq{dollar}sb3.{dollar} These new energy gap states were identified to be responsible for the dramatic reduction in 4PV PL. The slower rate of Alq{dollar}sb3{dollar} PL reduction is attributed to the strong reaction between Ca and Alq{dollar}sb3,{dollar} which reduces the amount of unreacted Ca that can induce quenching sites in Alq{dollar}sb3.{dollar} No interaction was found between the organic materials and ITO consistent with the observation that ITO does not affect the PL of these materials. In Chapter VI, the thermal conversion process and its effects on visible PL and EL are investigated in DP-PPV. For a 400 A thin film of DP-PPV, the conversion temperature was found to greatly influence both the PL and EL of DP-PPV. Although the conversion time was found not to be a factor for this thickness, it is expected to play a bigger role for thicker samples. Chapter VI also details the EL investigation of single and bi-layer LEDs based on PPV and poly(2,6-(4-phenylquinoline)) (PPQ) and its derivatives. Voltage tunability of color was demonstrated in both the single and bi-layer LEDs. The bi-layer devices were found to be more efficient and stable, and to exhibit a wider range of color tunability.