Efficiency Roll‐Off in Blue Emitting Phosphorescent Organic Light Emitting Diodes with Carbazole Host Materials

Abstract

1463wileyonlinelibrary.com dynamics. By nature, triplet excitons have a long lifetime. Under high current injection conditions, high density of triplet excitons leads to triplet-triplet annihilation (TTA) and triplet-polaron quenching (TPQ), which are the dominating loss mechanisms contributing to effi ciency roll-off. [ 3 ] Therefore, in order to determine the detail quenching mechanisms and the origin of effi ciency roll-off, it is important to characterize the interactions between triplet excitons and polarons. In this work, we use different carbazole hosts and study how these interactions affect the effi ciency roll-off. We studied the effi ciency roll-off in blue emitting phosphorescent OLEDs using iridium (III) bis(4,6-(di-fl uorophenyl) pyridinato-N,C 2 ′) picolinate (FIrpic) as the emitter and carbazole derivatives as the hosts. One carbazole compound commonly used as a host for FIrpic is 1,3-bis( N -carbazolyl)benzene (mCP). While high effi ciencies and long operating lifetimes have been realized in green and red emitting phosphorescent OLEDs, the performance of the blue emitting devices is still a bottleneck. [ 4 ] mCP is used as a host for FIrpic for the following reasons. First, carbazole compounds such as mCP have a high hole mobility value ( µ h ) of 5 × 10 −4 cm 2 V −1 s −1 . Second, mCP has a high triplet energy ( E T ) of 2.9 eV to prevent back energy transfer. [ 5 ] Because of the high E T , FIrpic doped mCP has a PL quantum yield of over 90% when used as the emitting layer. [ 6 ] Third, carbazole compounds have a small singlet and triplet energy splitting. This energy splitting can be as small as about 0.3-0.5 eV, [ 7 ] resulting in a lower turn-on voltage in blue phosphorescent OLEDs. [ 2 ] However, mCP has a low glass transition temperature ( T g ) of 60 °C, [ 8 ] due to its fl at molecular shape. It is therefore desirable to increase the T g of the carbazole materials to improve device stability.Here, we compare the device performance and effi ciency roll-off in blue phosphorescent devices using four carbazole derivatives as the host materials. In addition to the molecular weight of a compound, the T g of an organic material can be controlled by manipulating the steric hindrance of a molecule without lowering the E T . [ 9 ] For carbazole compounds, this can be done by adding additional carbazole units. Figure 1 shows the molecular structures of the derivatives in addition to mCP used in this study. While keeping the same peripheral carbazole groups, three additional derivatives were synthesized: a phenyl group is attached to the 9′-position of the carbazole core for 9′-phenyl-9,3′:6′9″-(9CI)-Ter-9H-carbazole (PTC), an ethyl group is attached to the 9′-position on the core carbazole for Effi ciency Roll-Off in Blue Emitting Phosphorescent Organic Light Emitting Diodes with Carbazole Host MaterialsChaoyu Xiang , Xiangyu Fu , Wei Wei , Rui Liu , Yong Zhang , Viktor Balema , Bryce Nelson , and Franky So * The effi ciency roll-off in blue phosphorescent organic light emitting diodes (OLEDs)...

Degradation Mechanism and Lifetime Improvement Strategy for Blue Phosphorescent Organic Light‐Emitting Diodes

“…PhOLEDs use triplet excitons for light emission. Triplet–triplet annihilation (TTA) and triplet–polaron annihilation (TPA) are the main degradation mechanisms and these occur due to the long excited state lifetime of the triplet excitons (≈ms range) . The TTA process is brought about by a triplet‐triplet quenching process and is accelerated when the excited state lifetime of the triplet excitons is long .…”

“…The TTA process is brought about by a triplet‐triplet quenching process and is accelerated when the excited state lifetime of the triplet excitons is long . The TPA process is attributed to triplet‐polaron interactions, which is activated by the co‐existence of triplet excitons and extra polarons in the emitting layer . Both the TTA and TPA processes are required in the operation of OLEDs, but they should be minimized to achieve PhOLEDs with long lifetimes.…”

Thermally Stable Donor–Acceptor Type (Alkynyl)Gold(III) TADF Emitters Achieved EQEs and Luminance of up to 23.4% and 70 300 cd m−2 in Vacuum‐Deposited OLEDs

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organic light-emitting diode (OLED) technology, examples of gold OLED with practical interests are scarce. [16][17][18][19][20] In this regard, thermally stable gold TADF emitters with high emission quantum yields are appealing. [3][4][5][6][7] Due to the electrophilic nature/relatively high reduction potential of Au(III) ion, the luminescent excited states of Au(III) complexes are usually ligand centered with little metal character, resulting in long triplet emission lifetimes, [8][9][10][11][12][13][14][15] thereby posing significant challenges to achieve high performance OLEDs with low efficiency roll-off at high luminance and driving voltage.

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10‐2: Invited Paper: Unified Analysis of Transient and Steady‐State Electroluminescence –Establishing an Analytical Formalism for OLED Charge Balance

“…[4]- [6] Many phosphorescent OLEDs suffer a pronounced efficiency reduction under high current (and often high brightness) excitation. [7]- [9] This steady-state efficiency roll-off has been treated by considering several factors, with most models invoking some form of bimolecular exciton quenching (either excitonexciton or exciton-polaron). [4], [5], [10]- [12] Often complementing these models is the potential for a reduction in the device charge balance factor, effectively an empirical factor that dictates the fraction of injected carriers that form excitons.…”