Optical, morphological, structural, electrical, molecular orientation, and electroluminescence characteristics of organic semiconductor films prepared at various deposition rates
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Extremely high deposition rates of ≈7200 nm s^<-1> for N,N´-diphenyl-N,N´-bis(1-naphthyl)-1,1´-biphenyl-4,4´-diamine (a-NPD) and of ≈1700 nm s^<-1> for tris(8-hydroxyquinoline)aluminum (Alq_3) are found to be possible by controlling source-substrate distances and crucible temperatures. Shapes of ultraviolet-visible absorption spectra and photoluminescence (PL) spectra, atomic force microscope images, X-ray diffraction patterns, PL quantum yields, PL lifetimes, and PL radiative decay rates of the films remain independent of the deposition rates ranging from 0.01 to 1000 nm s^<-1>. On the other hands, hole currents of hole-only α-NPD devices increase ≈3 times while electron currents of electron-only Alq_3 devices decrease by ≈1/60 as the deposition rates are increased from 0.01 to 10 nm s^<-1>. The increase in hole current is confirmed to arise from an increase in hole mobility of α-NPD measured using a time-of-flight technique. The increase in hole moility is probably due to a parallel orientation of an electronic transition moment of α-NPD at the higher deposition rates. Moreover, the three orders of magnitude increase in deposition rate from 0.01 to 10 nm s^<-1> of α-NPD and Alq_3 results in a relatively small increase in voltage of ≈15% and a decrease in external quantum efficiency of ≈30% in organic light-emitting diodes (OLEDs). The reduction of the OLED performance is attributable to the marked decrease in electron current relative to the slight increase in hole current, indicating a decrease in charge balance factor at the higher deposition rates.
- 2011-10-01
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