Effect of carbazole–oxadiazole excited-state complexes on the efficiency of dye-doped light-emitting diodes

Abstract

Interactions between hole-transporting carbazole groups and electron-transporting 1,3,4-oxadiazole groups were studied by photoluminescence and electroluminescence ͑EL͒ spectroscopy, in blends of poly͑N-vinylcarbazole͒ with 2-tert-butylphenyl-5-biphenyl-1,3,4-oxadiazole ͑PVK:PBD͒ and in random copolymers with carbazole and oxadiazole groups attached as side chains. Different excited-state complexes form in the blends, which exhibit exciplexes, and in the copolymers, which manifest electroplexes, due to topological constraints on the position of carbazole and oxadiazole units in the polymer. Both types of complex red-shift the EL spectra of the matrices compared with pure PVK homopolymer, although the shift is significantly greater for the electroplex. The presence of these complexes has a profound effect on the external quantum efficiency of dye-doped organic light-emitting diodes employing the blends or copolymers as matrices, as it strongly affects the efficiency of Förster energy transfer from the matrix to the dye. Single-layer devices doped with either coumarin 47 ͑C47͒, coumarin 6 ͑C6͒, or nile red ͑NR͒ were compared. Among the three dye-doped PVK:PBD devices, C6 doping yields the highest efficiency, while NR doping produced the most efficient copolymer devices, consistent with the degree of overlap between the EL spectrum of the matrix material and the absorption spectrum of the dye.

The nature of trapping sites and recombination centres in PVK and PVK–PBD electroluminescent matrices seen by spectrally resolved thermoluminescence

“…This indicates that PBD is not an electron acceptor which would quench the PL emission of PVK. Formation of exciplexes in PVK-PBD (40 wt %) films was observed by several groups using steady state and time-resolved photoluminescence spectroscopy [10,22,23]. Negres et al…”

“…The EL spectra for the PVK-PBD blends have dominant peak at 430 nm, which is red-shifted by ca 5 nm in comparison to the PL spectrum; this effect is explained by formation of the exciplex states. It was also shown that the EL spectra have some long-wavelength contribution dependent on composition of the blends and on applied electric field [4,10].…”

ZnS deposition on oxadiazole–terpyridine copolymer

“…1) were synthesized according to known procedures. 10,11 Azobis(isobutyronitrile) (AIBN) was recrystallized from methanol and stored in the freezer. All the other chemicals and solvents were used as received from Aldrich (Steinheim, Germany).…”

Effect of dye concentrations in blended-layer white organic light-emitting devices based on phosphorescent dyes

“…The emission for mixtures of PVK with the similar hole transport compound 2-͑4-biphenylyl͒-5-͑4-tert-butylphenyl͒-1,3,4-oxadiazole ͑PBD͒ has previously been assigned to an exciplex formed between a hole on PVK and an electron on PBD. 11 The addition of FIrpic to our PVK-OXD7 device results in blue emission, with a maximum in the EL spectrum at 480 nm together with a shoulder at about 505 nm. This emission is similar to that reported previously for FIrpic doped into PVK, 3,12 and is also characteristic of the emission observed when FIrpic is mixed in other host matrices, e.g., 4, 4Ј-bis͑9-carbozolyl͒-2, 2Ј-biphenyl ͑CBP͒, 13 4, 4Ј-bis͑9-carbazolyl͒-2, 2Ј-dimethylbiphenyl ͑CDBP͒, 8 or mCP.…”