Investigating Morphology and Stability of Fac‐tris (2‐phenylpyridyl)iridium(III) Films for OLEDs

Abstract

Stable film morphology is critical for long‐term high performance organic light‐emitting diodes (OLEDs). Neutron reflectometry (NR) is used to study the out‐of‐plane structure of blended thin films and multilayer structures comprising evaporated small molecules. It is found that as‐prepared blended films of fac‐tris(2‐phenylpyridyl)iridium(III) [Ir(ppy)3] in 4,4′‐bis(N‐carbazolyl)biphenyl (CBP) are uniformly mixed, but the occurrence of phase separation upon thermal annealing is dependent on the blend ratio. Films comprised of the ratio of 6 wt% of Ir(ppy)3 in CBP typically used in OLEDs are found to phase separate with moderate heating while a higher weight percent mixture (12 wt%) is found to be stable. Furthermore, it is found that thermal annealing of a multilayer film comprised of typical layers found in efficient devices ([tris(4‐carbazoyl‐9‐ylphenyl)amine (TCTA)/Ir(ppy)3:CBP/bathocuproine (BCP)]) causes the BCP layer to become mixed with the emissive blend layer, whereas the TCTA interface remains unchanged. This significant structural change causes no appreciable difference in the photo­luminescence of the stack although such a change would have a dramatic effect on the charge transport through the device, leading to changes in performance. These results demonstrate the effect of thermal stress on the delicate interplay between the chemical composition and morphology of OLED films.

Diffusion – the Hidden Menace in Organic Optoelectronic Devices

“…However, the integrated PL decreased by 33% on thermal annealing showing that simple mixing of layers can have a dramatic effect on the photophysical properties. CBP [8] and BCP [33] are both used as efficient hosts for Ir(ppy) 3 and therefore the decrease in the PL signal is most likely due to phase separation and aggregation of Ir(ppy) 3 , [18] which leads to excited state quenching via triplet-triplet annihilation. [34] Given that the BCP diffused into the light-emitting layer we were interested to understand the process by which this was occurring.…”

“…The emissive guest:host layer comprised fac-tris(2-phenylpyridyl)iridium(III) [Ir(ppy) 3 ] and 4,4′-bis(N-carbazolyl)biphenyl (CBP) which were deposited simultaneously across the films to ensure that the thickness and 6 wt% Ir(ppy) 3 :CBP blend ratio (based on evaporation rates) were consistent. [18] Bathocuproine (BCP) and 4,4′,4′′-tris(N-carbazolyl)triphenylamine (TCTA) (both deuterated in order to enhance contrast) were used as the electron and hole transport layers as along with CBP both materials have been used in OLEDs and OSCs. [8,[19][20][21][22][23][24] The first film stack studied (Film 1, Figure 1) was composed of the three organic layers plus an aluminium electrode with the NR profiles shown in Figure 2.…”

Properties of materials for organic light emitting diodes

“…262 The reason for the different behaviour observed in this experiment is not certain, however it is possible that the very different heating rates to reach 100 °C (several hours for Film 1 in this experiment as opposed to four minutes in the earlier work) may be responsible. It is also possible that the blend ratio of the Ir(ppy) 3 :CBP layer in the film was different between the two experiments and that may have an impact on the emission intensity and morphological properties.…”

“…CBP 21 and BCP 132 are both used as efficient hosts for Ir(ppy) 3 therefore it is not likely that the interaction of d-BCP quenches Ir(ppy) 3 . Rather since Ir(ppy) 3 :CBP films formed by vapour deposition are not thermally stable and phase separate (as demonstrated in Chapter 5), 262 it is conceivable that the decrease in the PL signal observed here is due to phase separation and aggregation of Ir(ppy) 3 which leads to excited state quenching via triplet-triplet annihilation. 58 After cooling, the NR showed little change, indicating that the film morphology obtained at 100 °C was essentially 'frozen'.…”

“…The emissive guest:host layer Ir(ppy) 3 :CBP was deposited simultaneously across all four films, which ensured that the thickness, and more importantly, the blend ratio were identical. The blend ratio of guest:host layers can affect the morphological stability, 262 and so for a proper comparison of the diffusion processes in these films it is important that the composition of each layer is the same. The nominal blend ratio for all the films listed was 6 wt%.…”