Green emission devices

Another example of spiro-derivatized PF was demonstrated by Bo and coworkers [319], who synthesized soluble spiro-bifluorene-based polymer 219. This polymer showed stable bright-blue PL (d>PL = 91% in toluene), and showed no green emission in the annealed film (although no device preparation has been reported as yet) (Chart 2.52). 

Introduction of electron-accepting hi thieno[3,2-6 2, 3 -e]pyri dine units resulted in copolymer 308 with ca. 0.5 V lower reduction potential compared to the parent homopolymer PFO 195 [398]. Upon excitation at 420 nm (A ax =415 nm), copolymer 308 exhibited blue-green emission with two peaks at 481 and 536 nm. Preliminary EL studies of an ITO/PEDOT/308/A1 device showed two peaks positioned as in the PL spectra. The PLED exhibited low turn-on voltage ( 4 V) but at higher voltages of 6-9 V, a slight increase in the green component was observed (Chart 2.83). 

The emission spectrum of some PT and PBD polymer bilayer devices cannot be explained by a linear combination of emissions of the components. Thus, white emission of the PLEDs ITO/422/PBD/A1 showed Hof 0.3% at 7 V, and consisted of blue (410 nm), green (530 nm), and red-orange (620 nm) bands. Whereas the first and the last EL peaks are due to the EL from the PBD and the PT layers, respectively, the green emission probably originates from a transition between electronic states in the PBD layer and hole states in the polymer... 

Several groups have studied naphthalene substituted anthracene derivatives as hosts or emitter materials in blue OLEDs (121, 202-205) (Scheme 3.63). The Kodak group used ADN as a host and TBP as a dopant in ITO/CuPc/NPD/ADN TBP/Alq3/Mg Ag [241]. They achieved a narrow vibronic emission centered at 465 nm with CIE (0.154, 0.232) and a luminescent efficiency as high as 3.5 cd/A. In comparison, the undoped device shows a broad and featureless bluish-green emission centered at 460 nm with CIE (0.197, 0.257) and an EL efficiency below 2.0 cd/A. The operational lifetimes of the doped device and the undoped device were 4000 and 2000 h at an initial luminance of 636 cd/m2 and 384 cd/m2, respectively. 

Ma et al. at PPG recently applied for patents on a series of iridium star-like bidentate complexes [300], Examples of two such green dopants are shown in Scheme 3.82 (254, 255). OLEDs fabricated using the dopants showed green emission with higher EQE and enhanced stability compared with a similar Ir(ppy)3-based device. 

The concept of using HBMs in OLEDs started with the pioneering work of Kijima et al. when they were trying to get pure blue emission from an EL device with Alq3 as ETM and NPD as an EML [346], An undesired green emission color from Alq3 was suppressed when a thin layer of BCP was added between the NPD and Alq3 layers. 

CIE coordinates of the TOLED (—0.06, —0.01) more than that of the bottom-emitting device (—0.03, +0.01) when one considers a viewing angle of 60°. However, this is of minor concern for green emission but care must be taken when applying this design to blue and red pixels. 

Copolymers 11 derived from 2,7-fluorene and 2,3,4,5-tetraphenylsilole are green electroluminescent SCPs.28 The EL spectra of the copolymers show exclusive green emissions (AEL 528 nm) from the silole units, which are almost not changed with the copolymer compositions. A maximum 7yEL of 1.51% can be achieved with the copolymers as the emissive layer in EL devices. 

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