Ambipolar Host Research Articles

P‐188: Molecular Engineering of Host Materials for Blue Phosphorescent OLEDs: Past, Present and Future

AbstractWe report molecular design considerations for blue phosphorescent host materials, as well as propose design rules necessary to build ambipolar hosts and thus reach charge balance in a device. Our beginning developments are presented followed by the evolution of the original design to our state‐of‐the‐art, with the help of computational modeling

A spiro-configured ambipolar host material for impressively efficient single-layer green electrophosphorescent devices

A spiro-configured bipolar molecule (CSC) that possesses high triplet energy, suitable energy levels, and balanced ambipolar carrier mobilities was successfully applied as an efficient host material, compatible with various iridium-based green phosphors, to realize highly efficient single-layer PhOLEDs of a maximum external quantum efficiency up to 8.3% (31.4 cd A(-1)) at a practical brightness of 1000 cd m(-2) (8 V).

Molecular Host−Guest Energy-Transfer System with an Ultralow Amplified Spontaneous Emission Threshold Employing an Ambipolar Semiconducting Host Matrix

We report on the characteristics of a host-guest lasing system obtained by coevaporation of an oligo(9,9-diarylfluorene) derivative named T3 with the red-emitter 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran dye (DCM). We demonstrate that the ambipolar semiconductor T3 can be implemented as an active matrix in the realization of a host-guest system in which an efficient energy transfer takes place from the T3 matrix to the lasing DCM molecules. We performed a detailed spectroscopic study on the system by systematically varying the DCM concentration in the T3 matrix. Measurements of steady-state photoluminescence (PL), PL quantum yield (PLQY), time-resolved picosecond PL, and amplified spontaneous emission (ASE) threshold are used to optimize the acceptor concentration at which the ASE from DCM molecules takes place with the lowest threshold. The sample with a DCM relative deposition ratio of 2% shows an ASE threshold as low as 0.6 kW/cm(2) and a net optical gain measured by femtosecond time-resolved pump-and-probe spectroscopy as high as 77 cm(-1). The reference model system Alq(3):DCM sample measured in exactly the same experimental conditions presents an one-order-of-magnitude higher ASE threshold. The ASE threshold of T3:DCM is the lowest reported to date for a molecular host-guest energy-transfer system, which makes the investigated blend an appealing system for use as an active layer in lasing devices. In particular, the ambipolar charge transport properties of the T3 matrix and its field-effect characteristics make the host-guest system presented here an ideal candidate for the realization of electrically pumped organic lasers.

P‐152: Efficient Single‐Layer Phosphorescent Green, Blue and White OLEDs Employing Large‐Triplet‐Energy and Ambipolar Hosts

AbstractIn this paper, we report an ambipolar host material with large triplet energies for single‐layer phosphorescent devices. Using such host material, we demonstrate efficient single‐layer green, blue and even white phosphorescent OLEDs.

An ambipolar phosphine oxide-based host for high power efficiency blue phosphorescent organic light emitting devices

We report blue electrophosphorescent organic light emitting devices with an ambipolar host material, 4-(diphenylphosphoryl)-N,N-diphenylaniline (HM-A1), doped with FIrpic (iridium (III)bis[(4,6-difluorophenyl)-pyridinato-N,C2′]picolinate). The ambipolar nature of the host was verified using single carrier devices. The power efficiency of devices with PO15 (2,8-bis(diphenylphosphoryl)dibenzothiophene) electron transport layer (ETL) showed optimized performance when the ETL thickness was 500 Å, giving a peak power efficiency of 46 lm/W (corresponding external quantum efficiency (EQE) of 17.1%). The EQE and power efficiency at the brightness of 800 cd/m2 were measured with no light outcoupling enhancement and found to be 15.4% and 26 lm/W, respectively.

35.4: Stable and Efficient Organic Light Emitting Diodes Based on a Single Host of p‐Doped and n‐Doped Layers

AbstractA power‐efficient and stable organic light emitting diode with a simplified structure based on 2‐methyl‐9,10‐di(2‐naphthyl)anthracene (MADN) has been developed. The incorporation of 10% tungsten oxide (WO3) into MADN and 15% cesium carbonate (Cs2CO3) into MADN has been found to improve the hole and electron injection, respectively. Based on this work, it is anticipated that p‐i‐n OLEDs fabrication process can be considerably simplified and the bill of materials cost can reduced by using one ambipolar host for both the p‐ and n‐doped carrier transport layers.

Multiple exciton generation regions in phosphorescent white organic light emitting devices

We demonstrate a white electrophosphorescent organic light emitting device (WOLED) with a three-section emission layer (EML) where excitons are formed in the multiple emission regions. The EML consists of a stepped progression of highest occupied and lowest unoccupied molecular orbital energies of the ambipolar hosts. Analysis shows that (36 ± 6)% of the excitons form in the blue emitting region, while (64 ± 6)% form in the green emitting region at 100 mA/cm 2. The doping of the red, green and blue phosphors, each in its own host, allows for efficient utilization of excitons formed in these multiple regions. Based on this architecture, the WOLED has an internal quantum efficiency close to unity. The WOLED has total external quantum and power efficiencies of η ext,t = (26 ± 1)% and η p,t = (63 ± 3) lm/W at 12 cd/m 2, decreasing to η ext,t = (23 ± 1)% and η p,t = (37 ± 2) lm/W at 500 cd/m 2. When an undoped electron transport layer is used, the peak efficiency is η ext,t = (28 ± 1)%. Due to the distributed exciton formation in the EML, the WOLED exhibits higher total efficiency than monochromatic devices employing the same red, green and blue dopant–host combinations.

Electron and hole transport in a wide bandgap organic phosphine oxide for blue electrophosphorescence

We report blue phosphorescent organic light-emitting devices (OLEDs) using an ambipolar host, N-(4-diphenylphosphoryl phenyl) carbazole (MPO12), doped with iridium (III) bis[(4,6-difluorophenyl)-pyridinato-N,C2′]picolinate (FIrpic). The external quantum efficiency and operating voltage is 9.1(±0.1)% and 4.8V, respectively, measured at a brightness of 800cd∕m2 with no outcoupling enhancement. By varying the layer structure of the OLEDs, we show that MPO12 is capable of transporting both electrons and holes, in contrast to previous demonstrations using diphosphine oxides, which only transported electrons. The improved hole transport results in improved device efficiency.

An ambipolar host material provides highly efficient saturated red PhOLEDs possessing simple device structures

A highly efficient red electrophosphorescent device exhibited saturated red emission and an impressive external quantum efficiency [eta(ext) = 10.8% (ph/el)] with simple device configuration of doping an iridium complex (Mpq(2)Iracac) into a novel ambipolar spiro-configured donor-acceptor host material (D2ACN) has been developed.

Phosphorescent organic light-emitting device with an ambipolar oxadiazole host

In this latter, the authors demonstrated a phosphorescent organic light-emitting device (OLED) with a lower driving voltage and a longer operation lifetime based on 2,2′-bis[5-phenyl-2-(1,3,4)oxadazolyl]biphenyl, an oxadiazole (OXD) derivative, as the host of the emitting layer doped with a common green-emitting phosphorescent dopant, fac-tris(2-phenylpyridine) iridium [Ir(ppy)3]. Rather than an electron transporting materials, the OXD exhibits an ambipolar transport characteristic with suitable Ir(ppy)3 concentrations due to its low highest occupied molecular orbital value (5.9eV) and the hole-transporting characteristics of Ir(ppy)3. It results in a 2.4V voltage reduction and a 2.6 times lifetime elongation of the OXD-based device, as compared to the conventional phosphorescent OLED.

Architectures for efficient electrophosphorescent organic light-emitting devices

We discuss several device architectures leading to high-efficiency organic electrophosphorescent (EP) light emission. An external electroluminescence efficiency (/spl eta//sub ext/) of (10.0 /spl plusmn/ 0.5) % was realized by doping fac-tris(2-phenylpyridine)iridium (Ir(ppy)/sub 3/) into a 2,9-dimethyl-4,7-diphenyl-1,10-phenenthroline (BCP) electron transport layer. Direct exciton formation on the phosphor dopant avoids exciplex formation at the interface of unipolar hole and electron transport layers. Further, triplet exciton and carrier dynamics in a double heterostructure were investigated to determine the location and width of the exciton formation zone. High-efficiency EP is also demonstrated in a simplified two layer architecture using a 4,4'-N, N'-dicarbazole-biphenyl (CBP) ambipolar carrier transport host.