Motivated to minimize the effects of solid-state solvation
and
conformation disorder on emission properties of donor–acceptor-type
emitters, we developed five new asymmetric multiple donor–acceptor
type derivatives of tert-butyl carbazole and trifluoromethyl
benzene exploiting different electron-accepting anchoring groups.
Using this design strategy, for a compound containing four di-tert-butyl carbazole units as donors as well as 5-methyl
pyrimidine and trifluoromethyl acceptor moieties, small singlet-triplet
splitting of ca. 0.03 eV, reverse intersystem crossing
rate of 1 × 106 s–1, and high photoluminescence
quantum yield of neat film of ca. 75% were achieved.
This compound was also characterized by the high value of hole and
electron mobilities of 8.9 × 10–4 and 5.8 ×
10–4 cm2 V–1 s–1 at an electric field of 4.7 × 105 V/cm, showing relatively good hole/electron balance, respectively.
Due to the lowest conformational disorder and solid-state solvation
effects, this compound demonstrated very similar emission properties
(emission colors) in non-doped and differently doped organic light-emitting
diodes (OLEDs). The lowest conformational disorder was observed for
the compound with the additional accepting moiety inducing steric
hindrance, limiting donor–acceptor dihedral rotational freedom.
It can be exploited in the multi-donor–acceptor approach, increasing
the efficiency. Using an emitter exhibiting the minimized solid-state
solvation and conformation disorder effects, the sky blue OLED with
the emitting layer of this compound dispersed in host 1,3-bis(N-carbazolyl)benzene displayed an emission peak at 477 nm,
high brightness over 39 000 cd/m2, and external
quantum efficiency up to 15.9% along with a maximum current efficiency
of 42.6 cd/A and a maximum power efficiency of 24.1 lm/W.
The multifunctional materials for application in organic
light-emitting
devices (OLEDs) based on a single structural motif are very desired
but quite rare species. Such structures allow simplifying the chemical
variety within OLED heterostructures and thus reducing their cost,
manufacturing time, and logistic efforts. In this paper, we report
the 9-(2,3,5,6-tetrafluoro-4-vinylphenyl)carbazole molecule (Cz4FS)
utilized as a fluorescent emitter, host material for quantum dot based
OLEDs (QLEDs), acceptor part of the exciplex active layer, and monomer
that can be used for the preparation of emissive polymers and copolymers.
The external quantum efficiency (EQE) of the corresponding fluorescent
OLED based on a Cz4FS single emitter doped into a 1,3-bis(carbazol-9-yl)benzene
matrix is 4.2%, which is close to the theoretical limit and maximum
brightness at the level of 3600 cd/m2. An OLED based on
exciplex emission obtained utilizing Cz4FS as an acceptor demonstrates
higher efficiency (5.3%) and much higher brightness near 25 000
cd/m2. A QLED based on Cz4FS as a host for CdSeS/ZnS core–shell
quantum dots demonstrates excellent energy transfer from the Cz4FS
matrix that results in a clear spectrum of quantum dots with an EQE
of 2.3%, maximum of 19 000 cd/m2, and narrow spectral
distribution. An OLED based on a Cz4FS-based polymer and copolymer
demonstrates not extraordinary efficiency but low-efficiency roll-off
in a wide range of current densities.
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