Effects of solution-processed polymer interlayers on hole injection and device performance of polymer light-emitting diodes

We present studies of current density and photometric efficiency using three well known, commercially available polyphenylenevinylene and polyfluorene based light-emitting polymers (LEPs) with different interlayers. The thin, spin-coated interlayers of poly(9,9-dioctyl-fluorene-co-N-(4-butylphenyl)-diphenylamine) (TFB) and poly[9,9-dioctyl-fluorene-co-(bis-N,N'-(3-carboxyphenyl)-bis-N,N'-phenylbenzidine)] (BFA) are placed between the poly(3,4-ethylenedioxythiophene)/polystyrenesulphonic acid (PEDOT:PSS) anode and the LEP. It is found that despite having very similar HOMO levels (+/- 0.1 eV) to the LEPs, the interlayers alter both the hole injection efficiency and the photometric efficiency of PLED devices. The increase or decrease of these depends on the particular interlayer-LEP combination involved, but there is a strong, general correlation between poorer hole injection resulting in a higher photometric efficiency. We attribute the variation in hole injection to the altered morphology and contact area at the anode interfaces, with the possible involvement of mobility-dependant space-charge effects or charge trapping. The dominant process in improving the photometric efficiency must be better electron-hole current balance, and/or the shift of the recombination zone to a more favourable position with less exciton quenching. The interlayers do not act as electron blocking layers, but hole injection enhancement by electron injection does seem to occur. These results show that interlayers can both increase and decrease device performance, depending on the interlayer-LEP combination involved. (c) 2007 Elsevier B.V. All rights reserved.