The influence of doping concentration andtemperature on magnetic field effect of delayed fluorescence in organiclight-emitting diode
2016
Up to date, both reverse intersystem crossing (RISC) and triplet–triplet
annihilation (TTA) are effective approaches to improve the internal
efficiency of fluorescent OLEDs, because they can make the best of
triplet excitons to generate delayed fluorescence. The energy difference
between singlet and triplet excited states ( D E S-T ) of special intramolecular
charge transfer (CT) species is quite small, which make it posible
for the participation of the CT species in RISC and TTA. When the
RISC and TTA coexist in fluorescent OLEDs, the utilization of triplet
excitons should be much better. The goal of this work was to demonstrate
the feasibility of the coexistence. However, determining whether the
coexistence happens in fluorescent OLEDs is not an easy task, because
the general methods can hardly distinct the delayed fluorescence is
originated from which approaches. Fortunately, both RISC and TTA are
highly spin-dependent processes that can generate sizable magneto-electroluminescence
(MEL) responses in OLEDs. Typically, the MEL response of RISC is a
low-field effect, i.e., the MEL decreases rapid within the low-field
regime (<5 mT) and then tends to saturation. Differently, the MEL
of TTA exhibits a slight increase within the range of 20 mT but a
remarkable decrease in the high-field regime (>20 mT). Hence, the
MEL can be used as a novel tool to discern RISC and TTA in fluorescent
OLEDs. Following such strategy, in this work, we firstly fabricated
the devices by doping different concentrations of intramolecular CT
states specie 4-(Dicyanomethylene)-2-tert-butyl-6-(1,1,7,7-tetramethyljulolidin-4-yl-vinyl)-4H-pyran
(DCJTB) into phosphor host material 1,3-bis(9-carbazolyl)benzene (mCP)
and then measured the MEL responses of these devices at room temperature.
It exhibited that the sign of low-field (<5 mT) component turned
from positive to negative as the doping concentration decreasing from
20% to 5%, this was same to the situation for the high-field ( B >20 mT) but was more prominent. These results are quite
similar to the theoretical MEL curves of RISC in low-field (<5
mT) and TTA in high-field ( B >20 mT), respectively.
Thus, we believe that the delayed fluorescence of DCJTB-doped devices
is caused by RISC and TTA simultaneously. Moreover, the RISC and TTA
rates increase as the doping concentration decreasing. Subsequently,
more remarkable RISC-dominanted low-field and TTA-dominanted high-field
decrease of MEL were observed with decreasing temperature from 300
to 20 K. It is attributed to the fact that low temperature can significantly
enhance RISC and TTA. In summary, low doping concentration and low
temperature are more favorable to the coexistence of RISC and TTA
in the DCJTB-doped devices. This work has a certain reference value
for the improvments of internal quantum efficiency of fluorescent
OLEDs via enhancing the utilization efficiency of triplet.
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