We report preliminary studies of the nature of hole injection from poly(3,4-ethylenedioxythiophene)/polystyrenesulphonic acid (PEDOT:PSS) into three commercial conjugated light emitting polymers (LEPs). Sumation's LUMATION Green 1300, LUMATION Blue, and Merck's SuperYellow LEPs are studied in combination with interlayers of poly(9,9-dioctyl-fluorene-co-N-(4-butylphenyl)-diphenylamine) (TFB), and poly[9,9-dioctylfluorene-co-(bis-N,N'-(3-carboxyphenyl)-bis-N,N'-phenylbenzidine)] (BFA). Despite the highest occupied molecular orbitals (HOMOs) of the interlayers being close to that of PEDOT:PSS and the LEP, different interlayers have different effects on hole injection and OLED device performance. We use dark injection transient current method to show that interfacial morphology changes results in modulation of hole trap densities that in turn affect hole injection. Depending on the interlayer/LEP combination partial penetration of interlayer into the LEP layer may also occur resulting in additional changes in the bulk transport properties of the LEP. Our results show that it is not the interfacial energy level alignment but the physical morphology changes at the interface which are important for varying hole injection into the device. A combination of either improved or reduced hole injection due to variations in physical contact, intermixing and trapping at the interlayer/LEP boundary dominate device performance.
Here we report results of time-of-flight (ToF) measurements on blends of different ratios of poly(9,9-dioctylfluorene-cobis-
N,N'-(4-methoxylphenyl)-bis-N,N'-phenyl-1,4-phenylenediamine) (PFMO) and the structurally similar poly(9,9-
dioctylfluorene-co-N-(4-methoxyphenyl)diphenylamine) (TFMO). It is shown that the hole mobility can be tuned over
three orders of magnitude with a mobility minimum at 10% PFMO and 90% TFMO. We also use Raman microscopy to
demonstrate that the blends do not phase separate within the one micron resolution of our experiment.