Organic light-emitting diodes (OLEDs) based on conjugated small molecules and polymer semiconductors are being developed for applications in full-color displays and lighting [1]. Major challenges remain, including the need to significantly improve the emission color coordinates, performance, and durability of red, green, blue (RGB) and white OLEDs. In typical OLEDs consisting of multilayers or blends of hole-transport materials (HTMs, electron donors) and electron-transport materials (ETMs, electron acceptors), the electroluminescence (EL) arises from the radiative decay of excitons in the bulk of the emissive material. In some cases, EL originates from an intermolecular excited-state charge-transfer complex (exciplex) formed at the interface between the HTM and ETM and is often red-shifted compared to the bulk emission of the individual components [2,3]. The emission wavelength of the exciplex approximately depends on the difference between the ionization potential (IP) of the donor and the electron affinity (EA) of the acceptor, thereby providing a simple approach to tuning the EL colors by choice of the HTM and ETM with appropriate energy levels. In addition to the manipulation of OLED color, studies of exciplex formation at the donor/acceptor organic interface are important for a fundamental understanding of OLEDs. We herein report studies of the photophysics and exciplex EL of blends or bilayers consisting of blue-emitting oligoquinoline acceptor and different donor HTMs with varying IP values. The emissive n-type oligomers have a common 6,6'-bis(4-phenylquinoline) core as exemplified by 6,6'-bis(2,4-diphenylquinoline) (B1PPQ; EA = 2.7 eV) [4]. The HTMs studied include 4,4',4”-tris(3-methylphenylamino)triphenylamine (MTDATA; IP = 5.0 eV), 1,1-bis(di-4-tolylaminophenyl)cyclohexane (TAPC; IP = 5.3 eV) and poly(N-vinylcarbazole) (PVK; IP = 5.8 eV).

In the photoluminescence (PL) emission spectra of neat films of the donor molecules TAPC and MTDATA, weak violet-blue emission was detected; TAPC has two peaks at 370 and 435 nm, MTDATA has a peak at 424 nm and broad shoulder at ~540 nm. The thin films of acceptor B1PPQ showed strong blue fluorescence with a peak at 440 nm, making it an attractive emitter for blue OLEDs [4b]. The PL spectrum of an equimolar mixture of TAPC and B1PPQ has a peak at 495 nm (2.5 eV) and is thus red-shifted by 55 and 70 nm compared to neat films of B1PPQ and TAPC, respectively. Similarly, the PL spectrum of an equimolar mixture of MTDATA and B1PPQ has a peak at 587 nm (2.1 eV) and is dramatically red-shifted by 147 and 163 nm relative to neat films of B1PPQ and MTDATA, respectively. These new red-shifted emission bands in the binary blends are assigned to exciplexes between the donor TAPC (or MTDATA) and acceptor B1PPQ. No new bands were observed in the absorption spectra of the blends and also in their PL excitation spectra, clearly ruling out any ground state complexes. In addition, the emission energy of the red-shifted bands matches fairly well with the difference between the EA of acceptor and the IP of the donor. For example, in the case of the TAPC blend, the observed emission band is at 2.5 eV, and the difference in the IP of TAPC and EA of B1PPQ is 2.6 eV. Finally, we note that only blue emission from B1PPQ was observed in blends of B1PPQ with PVK; PVK does not form exciplexes with B1PPQ due to its high IP of 5.8 eV. Thus, with the same blue-emitting B1PPQ oligomer, we can systematically tune the emission color from blue to blue-green to orange by varying the IP of the hole-transport material in the OLED.

We fabricated various B1PPQ-based diodes, blend or bilayer, with different HTMs. The EL spectrum of bilayer diodes with PVK, ITO/PEDOT/PVK/B1PPQ/LiF/Al, had a maximum at 442 nm, which is identical to the PL emission maximum of B1PPQ, demonstrating that the blue emission arises from recombination in the bulk of the oligoquinoline film. The EL spectra of B1PPQ had very stable CIE coordinates (x=0.15, y=0.09) that did not vary with applied voltage. The maximum brightness of this device was 6025 cd/m² and the maximum external quantum efficiency was 3% at a brightness of 1215 cd/m² with a luminous efficiency of 2.4 cd/A.

Orange exciplex EL was achieved using blends of MTDATA and B1PPQ as the emissive layer in bilayer diodes (ITO/MTDATA:B1PPQ/TPBI/LiF/Al) with a thin film of 1,3,5-tris(N-phenylbenzimidizol-2-yl)benzene (TPBI; IP = 6.2-6.7 eV, EA = 2.7 eV) acting as the hole-blocking layer. Brightnesses varied from 200–1860 cd/m² and CIE coordinates were in the range of (0.45, 0.50) to (0.57, 0.42) as the molar fraction of B1PPQ in the blends varied from 10 to 80%.

White EL was observed from bilayer diodes based on neat layers of MTDATA and B1PPQ(ITO/PEDOT/PS:MTDATA/B1PPQ/LiF/Al) in which a 75 wt% dispersion of MTDATA in polystyrene acts as the hole-transport layer and a 7-nm thin B1PPQ film is the electron-transport layer. The EL spectrum has two bands, a blue band at 440 nm and an orange band at 575 nm, rendering a good white CIE coordinate of (0.35, 0.32). It is clear that the former emission arises from the bulk of the B1PPQ film, while the latter emission band originates from the exciplex at the interface of the bilayer. The orange exciplex emission only arises under electrical excitation; unlike the blends, the exciplex emission is not observed under photoexcitation of the bilayer MTDATA/B1PPQ films. The relative contributions to the EL from the bulk and exciplex emissions depend on the electric field, thereby resulting in a slight voltage dependence of the EL color. Optimization of the relative film thicknesses in the bilayer diodes is needed to minimize the voltage dependence. Brightnesses ranging from 600 to 3100 cd/m² were obtained in such bilayer diodes with the CIE coordinates in the (0.28, 0.26) to (0.36, 0.35) range.

Additionally, blue-green exciplex EL is observed from bilayer TAPC/B1PPQ devices (ITO/PEDOT/PS:TAPC/B1PPQ/LiF/Al). Thus, we can effectively manipulate the emission color of the OLED almost over the entire visible region by varying the IP of the HTM while keeping the same emissive ETM, exploiting the exciplex formation between the blue-emitting oligoquinoline and different HTMs. These results demonstrate that the oligoquinolines are promising emissive electron transport materials for full-color OLEDs and lighting.

Acknowledgements. This research was supported by the NSF STC-MDITR (DMR-0120967) at the University of Washington, the NSF (CTS-0437912), and in part by the Air Force Office of Scientific Research (Grant F49620-03-1-0162).

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