Investigating the Morphology and Stability of Thin-Film Organic Light Emitting Diodes

Authors

Arthur Smith, Jeremy Ruggles, Hamish Cavaye, Paul Shaw, Ian Gentle and Paul Burn (University of Queensland), Tamim Darwish and Michael James (ANSTO)
 

Stable film morphology is critical for long-term high performance in organic light-emitting diodes (OLEDs).  Such devices are typically formed by a light emitting layer (in this instance formed by an organic iridium complex [Ir(ppy)₃] dispersed in a layer of CBP)* sandwiched between an electron transport layer (BCP)* and a hole transport later (TCTA)*  (Figure 1).

Fig 1. Schematic of a thin-film trilayer OLED device.  The thickness of each layer was ~ 30nm, and the BCP and TCTA layers were deposited using deuterated molecules to enhance scattering contrast.

Neutron reflectivity data collected using the Platypus time-of-flight reflectometer has used to study the out-of-plane structure of blended thin films and multilayer structures comprised of the above molecules; with this work recently featured in Advanced Functional Materials [1] (Figure 2). 

Key to the success of this study has been the deuteration of the heterocyclic aromatic molecules BCP and TCTA by members of the National Deuteration Facility at ANSTO.  By replacing the H atoms of these molecules by D atoms (see Figure 1), we have enhanced the scattering contrast between these molecular layers (from having a Scattering Length Density of ~2.4×10-6 Å-2in the protonated forms to ~4.9×10-6 Å-2 in the deuterated forms). 

In doing so we have enabled the capacity to see the structure of these multilayer devices, as well as follow inter-diffusion between the different molecular layers.

Figure 2

Figure 2.  Photoluminescence microscopy image of phase separation of Ir(ppy)₃ complex and CBP in the emissive layer upon thermal annealing.  The figure is a composite of two images taken at different excitation and emission wavelengths (blue–340 nm excitation, 370–400 nm capture; green–390 nm excitation, 495–550 nm capture) to discriminate between CBP (blue) and Ir(ppy)₃ (green) emission.

(Cover image from Advanced Functional Materials, Volume 21, Issue 12 (2011).

Thermal annealing of a multilayer film comprised of typical layers found in efficient devices: TCTA / Ir(ppy)₃:CBP / BCP caused the BCP layer to become mixed with the emissive blend layer, whereas the TCTA interface remained unchanged (Figure 3).

This significant structural change caused little appreciable difference in the photo luminescence of the stack although such a change does have a dramatic effect on the charge transport through the device. These results demonstrate the effect of thermal stress on the delicate interplay between the chemical composition and morphology of OLED films.

Figure 3. (a) Neutron reflectivity and (b) scattering length density profiles for a TCTA / Ir(ppy)3:CBP / BCP multilayer film on Si; with data taken before, during and after annealing.

 
In addition our study found that as-prepared blended films of Ir(ppy)₃ in CBP were uniformly mixed, but phase separation occurred upon thermal annealing and was dependent on the blend ratio. Films comprised of 6 wt% Ir(ppy)₃ in CBP (typically used in OLEDs) were found to phase separate with moderate heating.  An example of this behaviour is shown by the cover image - Figure 2.

*Ir(ppy)₃  =  fac-tris(2-phenylpyridyl)iridium(III)
CBP  =  4,4′-bis( N -carbazolyl)biphenyl
BCP  =  bathocuproine
TCTA  =  tris(4-carbazoyl-9-ylphenyl)amine

Reference

1.   A. R. G. Smith, J. L. Ruggles, H. Cavaye, P. Shaw, T. A. Darwish, M. James, I. R. Gentle and P. L. Burn, “Morphology and Stability of Fac-tris(2-phenylpyridyl)iridium(III) Blended Films: a Neutron Reflectometry Study”, Advanced Functional Materials, 21, 2225-2231 (2011). (Cover of issue)  DOI: 10.1002/adfm.201002365