Optical, morphological, structural, electrical, molecular orientation, and electroluminescence characteristics of organic semiconductor films prepared at various deposition rates

Extremely high deposition rates of ≈ 7200 nm s ^{- 1} for N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4, 4′-diamine (α-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 hand, 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 mobility 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.