Variations in Hole Injection due to Fast and Slow Interfacial Traps in Polymer Light-Emitting Diodes with Interlayers

• Published in: Advanced functional materials Vol. 20; no. 1; pp. 119 - 130
• Main Authors: Harding, M. James, Poplavskyy, Dmitry, Choong, Vi-En, So, Franky, Campbell, Alasdair J.
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag 08.01.2010; WILEY‐VCH Verlag
• Subjects: Light-emitting diodes; Polymeric materials
• ISSN: 1616-301X; 1616-3028
• DOI: 10.1002/adfm.200900352

Detailed studies on the effect of placing a thin (10 nm) solution‐processable interlayer between a light‐emitting polymer (LEP) layer and a poly(3,4‐ethylenedioxythiophene)/poly(styrenesulfonic)‐acid‐coated indium tin oxide anode is reported; particular attention is directed at the effects on the hole injection into three different LEPs. All three different interlayer polymers have low ionization potentials, which are similar to those of the LEPs, so the observed changes in hole injection are not due to variations in injection barrier height. It is instead shown that changes are due to variations in hole trapping at the injecting interface, which is responsible for varying the hole current by up to two orders of magnitude. Transient measurements show the presence of very fast interfacial traps, which fill the moment charge is injected from the anode. These can be considered as injection pathway dead‐ends, effectively reducing the active contact surface area. This is followed by slower interfacial traps, which fill on timescales longer than the carrier transit time across the device, further reducing the total current. The interlayers may increase or decrease the trap densities depending on the particular LEP involved, indicating the dominant role of interfacial chain morphology in injection. Penetration of the interlayer into the LEP layer can also occur, resulting in additional changes in the bulk LEP transport properties. In the negligible‐barrier regime, hole injection from PEDOT:PSS into conjugated polymers, both with and without an intervening 10 nm thick interlayer, can fall below its theoretical value by up to two orders of magnitude. This is due to fast and slow charge trapping in the region of the anode, which must be related to the morphology of the interfacial layers. In the figure, JTFSCLC and JDC represent the trap‐free space‐charge limited and the measured steady‐state DC currents.