The environment sensitivity of organic semiconductors may change their molecular structure and hence optical properties. Exploiting this concept, experiments were performed on a green light emitting material bis(8-hydroxy quinoline)Zinc, (Znq2) used in organic light emitting diodes (OLEDs). Thin films were deposited at varying deposition parameters, and their properties were compared. We investigated that as deposited films have a significant component of Znq2 tetramer out of two known forms, that is, dihydrate and anhydrous tetramer (Znq2)4, the films deposited at lower deposition rates have higher anhydrous content. The degradation of thin film is shown, that changes the optical properties of film from green emission to blue which may be due to water adsorption and crystallization.
Small molecules such as metal based quinoline derivatives like Tris-(8-hydroxyquinoline) aluminum (Alq3) have been shown to have high efficiency as well as stability in organic light emitting devices, OLEDs [
(Znq2)·2H2O and (Znq2)4 were synthesized in the laboratory [
Thin films of Znq2 were deposited on glass and polished Si substrates in ultra-high vacuum system (~10−8 mbar) with multiple thermal evaporation sources. The crucible temperature was varied from 290°C to 350°C for a series of samples which were deposited from 0.1 Å/s to 10 Å/s. Film thickness was kept at ~100 nm.
X-ray and FTIR studies were conducted in order to probe structural order of the films. Grazing angle X-ray measurements were taken at incident angle of 0.5° and 4°, and 2Θ were scanned from 2° to 40° using CuK
X-ray diffraction data on Znq2 samples before (Znq2
X-ray diffraction data on Znq2 samples before (Znq2
Figure
FTIR spectra of (Znq2)
Comparison of FTIR spectra of two phases shows a variation in characteristic spectral parameters. Broad band at 3100–3500 cm−1 attributed to stretch in OH bond is present in (Znq2)
Figure
Thermal Analysis of (Znq2)
Figure
Photoluminescence spectrum of Znq2 powders.
It is interesting to note that molecular structure of Znq2 changes appreciably in its various forms. The water molecules are loosely attached to zinc atom and are easily removed by heating the compound to 135°C [
Anhydrous Znq2 is amorphous in nature and tends to form tetramer at temperatures slightly higher than 135°C [
Another pathway for obtaining tetrameric structure is combination of monomer and trimer [
The structure around zinc atoms at the edge is that of distorted trigonal bipyramid, and the zinc atoms lie in pentacoordinate geometry whereas the central atoms are hexacoordinate. (Znq2)4 is a highly symmetric almost planar molecule with several inversion symmetry points. The presence of this symmetry forces the (Znq2)4 molecules to arrange bridging terminal ligands in parallel fashion adjacent to tetrameric units, resulting in a close intermolecular
A series of thin films of Znq2 were deposited on different glass and Si substrates with deposition rate as a variable. Details of the films are given in Table
Details of Znq2 powders and thin films. Note that the powder may be at lower temperature than temperature of crucible made of ceramic.
Sample name | Sample form | Deposition rate (Å/s) | Crucible |
---|---|---|---|
Znq2 |
Powder (as prepared) | — | — |
(Znq2)4 | Powder (sublimed) | — | — |
Zn0p2 | Thin film | 0.2 | 290 |
Zn0p5 | Thin film | 1 | 310 |
Zn1p0 | Thin film | 1.5 | 312 |
Zn2p0 | Thin film | 2 | 316 |
Zn2p5 | Thin film | 2.5 | 320 |
Zn5p0 | Thin film | 5 | 330 |
Zn10p0 | Thin film | 10 | 350 |
Figure
X-ray spectrum of a thin film of Znq2 deposited at 0.2 Å/s (Zn0p2) and the same film exposed to laboratory ambient for a week.
Figure
Infrared transmission spectra (in arbitrary unit) of two fresh Znq2 thin films on Si substrates deposited at the rate of 1 Å/s and 10 Å/s (from top to bottom).
Infrared transmission spectra (in arbitrary unit) of environmentally exposed Znq2 thin films deposited at different rates of 1, 1.5, 2.0, and 2.5 Å/s.
Figure
We have also observed that films deposited at lower rates show antistokes shift in PL spectrum faster (2–4 weeks, laboratory ambient) than films deposited at higher rates with time. This suggests that tetrameric component has increased in the composition of thin films and the films are denser in nature. The absorption spectrum also shows blue shift in the absorption band.
All the pristine films deposited at different rates show PL excitation at 375 nm and emission about 540 nm. PL spectrum again suggests that pristine films contain tetramer as well as disordered component. PL spectrum can be fitted with two Gaussians suggesting two major levels in the band gap.
Figure
Photoluminescence peak shift from 540 nm in pristine thin film to 500 nm in environmentally exposed film deposited at 1 Å/s.
Znq2 comprising different isomeric phases was synthesized. Thin films of Znq2 were deposited as a function of deposition rate. X-ray diffraction data on Znq2 powders comprising different phases have been presented. The photoluminescence data corresponding to each phase have been clearly identified. As deposited thin films are shown to be amorphous in nature with a strong presence of (Znq2)4. These films show a PL peak at ~540 nm. Exposure to oxygen environment leads to water adsorption and crystallization of thin films leading to a blue shift in the PL peak position. Films deposited at lower temperatures (lower rates) convert more quickly (approximately two weeks, laboratory ambient) to dihydrate Znq2. This suggests that (Znq2)4 is present in higher density in films deposited at higher deposition rates.
Technical support from Samtel Center for Display Technologies, Indian Institute of Technology Kanpur, is gratefully acknowledged. Authors also thank Dr. Satyendra Kumar for useful discussions.