Thermally Stimulated Luminescence of Naphthylsubstituted Benzidine Derivative and Triss-8s-(Hydroxyquinoline) Aluminum W
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EXPERIMENTAL SETUP The samples shown in Fig. l a were prepared by thermal evaporation at a base pressure of 5x10-7 Torr. A 1500 A organic layer was deposited onto a quartz substrate and capped with 1000 A of Ag. A second Alq 3 film was prepared where three 8 A metal layers where deposited between 150 A of Alq 3, starting at the quartz substrate followed by 1200 A of Alq 3 and the 1000 A Ag cap. These samples were intended to test if the metal layers induce trap states in the Alq 3 layer. Fig. lb shows the typical experimental setup for the TSL, similar to Ref 12. Here, the sample was cooled to 10 K with a CTI cryo-pump in a 1X 10"3 Torr vacuum. The samples were excited with 100 mW of 363 nm UV radiation from an Ar laser and a 4 cm 2 spot size. The radiation was turned off and the sample heated up at a linear rate of 20 °C/min. A Pritchard model 1980B collected the TSL emission. Then, the peaks were separated by thermal cleansing to more accurately determine the peak shape [15]. Here, the sample was cooled to 10 K and the excited with the UV radiation. The sample temperature was increased at a linear rate and the TSL spectrum measured up to a temperature above the first peak. The sample was then cooled back to 10 K followed by re-heating without the UV excitation. For Alq 3 , each thermally cleansed TSL peak was an average of three measurements due to the low signal to noise. All TSL spectra were taken at least three times to ensure the accuracy of the features and to test if the incident radiation result in damage to the molecules. In all cases, the TSL spectra were reproducible and did not show degradation after repeated exposure or thermal cycling. After the TSL measurements, the PL for Alq 3 was measured as a function of sample temperature from 10 K to 300 K in a SPEX Fluorolog, using 363 nm radiation from a xenon lamp.
A% =363 nm Agorai cap
Ag cap Quartz
qat EI~EZIquartz Cold head 8 K-450 K
Pritchard with PMT
Fig. I Schematic illustration of the device structure (a) and of the experimental setup used to measure the thermally stimulated luminescence (b). EXPERIMENTAL RESULTS Shown in Fig. 2a is the TSL for NPB and Alq3 . The TSL spectrum for the NPB film has four distinct peaks centered at 40, 65, 110, and 150 K. Conversely, the TSL spectrum for Alq3 had one broad peak centered at 156 K and smaller peaks at 67 and 60 K. The peaks were isolated by the thermal cleansing method and the results for NPB and Alq 3 are shown in Fig. 3a and 3b, respectively. Fig. 2b is the TSL spectrum for Alq 3 with the metal interlayers. The overall shape of the TSL peaks are similar to the undoped Alq 3 ,. However, the overall intensity is lowered, which indicates the metal layers induce non-radiative pathways for the TSL without creating distinct trap states, measurable by TSL. The amplitude of the TSL spectrum in Fig. 2b 754
has been scaled by the amplitude of the TSL for undoped Alq3, which demonstrates the reduced intensity due the presence of metal layers. 2.4
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2.0-
0.4
0
AIq3 Io C/min NPB lO°C/min
0.3
1.6(-.
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