Metal Nanoparticles, Nanowires, and Contact Electrodes Self-Assembled on Patterned Monolayer Templates—A Bottom-up Chemical Approach
✍ Scribed by S. Hoeppener; R. Maoz; S.R. Cohen; L.F. Chi; H. Fuchs; J. Sagiv
- Publisher
- John Wiley and Sons
- Year
- 2002
- Tongue
- English
- Weight
- 248 KB
- Volume
- 14
- Category
- Article
- ISSN
- 0935-9648
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✦ Synopsis
reported elsewhere. This limits the peak brightness for this device to only 5500 cd m ±2 (12 V). The ratio of excimer to monomer emission gradually becomes richer in monomer emission as the brightness is increased, analogous to the FIrpic-FPt1 WOLEDs. The CIE coordinates of approximately (0.34, 0.35) are not severely affected by the shift in the mono-mer±excimer ratio, remaining nearly ideal white up to ~2000 cd m ±2 , with a CRI value at 75 or above for nearly all of the spectra considered. If we ignore the NPD emission, the CIE coordinates are slightly shifted to (0.33, 0.40). The CBP± FPt2 WOLEDs give g ext = 1.9 ± 0.2 % at a brightness of 100 cd m ±2 (J = 2 mA cm ±2 ). The power efficiency at this brightness is 2.5 lm W ±1 .
We have demonstrated that control of the molecular and electronic structures of an organic phosphor can lead to surprisingly efficient white light emission from an OLED containing only a single emissive dopant. We expect that these device structures will have several benefits over stacked or multiply doped single emissive layer devices. [6] The simple device structures make them well suited to low cost lighting applications. Indeed, a serious problem in multiple doped systems for white emission is differential aging. If one of the dopants degrades at a different rate from the others, the color of the device will change over time. This should not be the case for the single dopant monomer±excimer device. Degradation that results from charging or other reactions of the ground state molecules will equally affect the monomer and excimer emission. The photostability of these phosphorescent dopants is typically quite high, and device failure generally occurs through charged, rather than excitonic states. [19]
Experimental
The three phosphorescent dopants (FIrpic, FPt1, and FPt2) and NPD were prepared by literature procedures [20,21]. BCP and Alq 3 were purchased from Aldrich Chemical Company. PEDOT-PSS was purchased from Bayer Chemical. All of the molecular materials used to prepare WOLEDs were purified by thermal gradient sublimation before use.
Photoluminescent, photoluminescence excitation, and absorbance spectroscopy were performed on thin-films grown in vacuum (< 1 10 ±6 torr) by thermal evaporation on solvent-cleaned quartz substrates. The films were 1000 thick and were degassed with nitrogen during testing. Five films were grown: neat CBP, two films of CBP doped at < 1 wt.-% and ~7 wt.-% with FPt1, CBP doped with both FIrpic and FPt1 at 6 wt.-%, and CBP doped with 8 wt.-% of FPt2.
Electrophosphorescent double-doped monomer±excimer WOLEDs were grown with a 6 wt.-% FIrpic±6 wt.-% FPt1 doped CBP luminescent layer. The devices were fabricated on a glass substrate pre-coated with indium tin oxide (ITO, sheet resistance = 20 X/sq). The substrate cleaning and device fabrication were carried out by procedures described previously [6]. The device structure was ITO/PEDOT:PSS (400 )/NPD (300 )/CBP±FIrpic±FPt1 (300 )/BCP (500 )/LiF (5 )/Al (700 ) (PEDOT:PSS=poly(ethylene-dioxythiophene): poly(styrene sulfonic acid), NPD=N,N¢-diphenyl-N,N¢-bis(a-napthyl)-4,4¢-biphenyl, BCP=bathocuproine). A cross section of the device structure is shown in Figure 2. Devices were grown with areas as large as 2.7 cm 2 without loss in efficiency. The samples were exposed to air during tests. The device structure of the single dopant WOLEDs was ITO/NPD (400 )/CBP-8 wt.-% FPt2 (300 )/BCP (150 )/Alq 3 (200 )/LiF±Al (1000 ).