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

Abstract

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.