Abstract
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.
1. Introduction
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 [1]. Potential of Zinc(II) bis(8-hydroxyquinoline) (Znq2) has been amply recognized in the literature [2–5]. Znq2 devices have shown advantages over Alq3 in electron transport and have higher quantum yields in device performance which results in lower operating voltages [2]. Further, the devices have been shown to be comparatively more stable under influence of high operating voltages [3–5]. It is shown that electroluminescence (EL) of Znq2 devices does not shift with operating voltage [2]. As Znq2 is a symmetric molecule, it does not have variegated isomers as found in tris-quinalato structures [6]. Thermal analysis of Znq2 powders by differential scanning calorimetry (DSC) demonstrates a sole oligomeric species and no polymorphism [7]. It has also been shown that thermal stability of zinc complexes is higher than other transport layers [8]. In order to take advantage of various properties of this molecule, it is important to understand and correlate its properties in powders with thin films. While the lack of polymorphism is an important attribute in device application, the structure of the molecules in thin film forms is poorly understood. However, no clear study has yet been done on morphology and properties of Znq2 as thin film. In solution or powder form Znq2 is known to exist as monomer with H2O group attached to it. It has been shown that oligomers, in particular, tetramer of Znq2, are energetically favorable as well in solid state [7, 9]. Though many reports have suggested that tetramers may be present in thin films and may be responsible for lower operating voltages of Znq2 devices, it is not very clear with what geometry Znq2 molecules arrange themselves in as thin films [7, 10]. A lack of understanding of molecular arrangement in thin film forms further complicates the applicability of substituted bis-quinolates [11]. In this work, we aim to understand the structural transformation and changes in optical properties of Znq2 thin films deposited at varying deposition parameters and compare them with those of powdered Znq2.
2. Experiments
(Znq2)·2H2O and (Znq2)4 were synthesized in the laboratory [5]. The composition of the precipitate was Znq2·2H2O as ascertained by X-ray experiments. In order to obtain (Znq2)4 crystals, the precipitate was sublimed at ~200°C.
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 CuKα radiation with a wavelength of 1.541 Å. Photoluminescence data at room temperature was recorded using a spectrofluorometer (Fluorolog 3, Jobin Yvon) with front face detecting geometry, in which the emitted signal is collected at 22° with respect to the normal at the surface of the sample, coincident with the excitation light direction. Thermal analysis of the organic samples was carried out using thermal analyzer (Model TG/DTA 6300, SII NanoTechnology Inc., Japan).
3. Results and Discussion
3.1. Znq2: Powder Form
X-ray diffraction data on Znq2 samples before (Znq22H2O) and after sublimation are shown in Figure 1. The upper XRD pattern shows a signature peak at 6.96°, which is of Znq22H2O. The lower XRD pattern can be readily identified as (Znq2)4 [12].
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Figure 2 shows FTIR spectra of Znq22H2O and (Znq2)4. The high-frequency region lying between 3600 and 1700 cm−1 contains absorption bands mainly originating from localized hydrogen stretching vibrations. The middle region from 1700 to 1000 cm−1 relates to heavy atom in-plane stretching and bending vibrations. The last part of the spectrum comprising of the frequency region shows the out-of-plane and torsional modes [13].
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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)2H2O while it is missing in (Znq2)4. The intensity ratio of 3333 cm−1 band to 1110 cm−1 band, commonly used to study the water molecule number in metal-quinoline chelates, yields ratio of 0.7. This ratio is slightly higher than expected value (0.6) due to minor residual water content in the sample [14]. Peaks at 602 and 650 cm−1 are quite pronounced in (Znq2)4 indicating higher in-plane ring distortion. The vibrations at 1606, 1577, 1500, 1467, 1388, and 1327 cm−1 were allocated to the quinoline group of Znq2. The bands at 1500 and 1468 cm−1 should correspond to both the pyridyl and phenyl groups in Znq2.
Figure 3 shows DTA and TGA results. First weight loss starts from 110°C in Znq22H2O, due to loss of water, and can be observed as endothermic peak at 122°C in DTA as well. Presence of minor peak at the same temperature in (Znq2)4 may be due to atmospheric moisture as such a peak is not observed in (Znq2)4 [2]. An additional exothermic peak for Znq22H2O is shown at 199°C. This peak may be due to release of energy by fusion of anhydrous Znq2 molecules into oligomers. Next strong endothermic peak occurs at 357°C followed by decomposition at 440°C. The sublimation temperature of Znq2 is 357°C.
Figure 4 shows photoluminescence (PL) emission spectrum of these two samples. PL spectrum of (Znq2)4 shows peak at 542 nm whereas PL spectrum of Znq22H2O shows PL peak at 506 nm. This indicates that HOMO-LUMO gap in (Znq2)2H2O is broader. Possibly, fusion of Znq2 into its oligomers leads to decrease in HOMO-LUMO gap due to improved π-π stacking.
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 [15].
Anhydrous Znq2 is amorphous in nature and tends to form tetramer at temperatures slightly higher than 135°C [12]. In order to form the tetramer, two anhydrous zinc quinolate molecules get connected by two bridging oxygen atoms to form an asymmetric unit. Two asymmetric units related by a center of symmetry, connected by two bridging oxygen atoms, make the whole molecule tetrameric.
Another pathway for obtaining tetrameric structure is combination of monomer and trimer [7]. The structure of (Znq2)4 contains four zinc atoms, two of which are in the center and two on the edges.
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 π-π interaction [2].
3.2. Znq2: Thin Films
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 1. Because of the fact that polymers [16], organic small molecules like Znq2 [17–22], and other organometallic complexes [23] are environment sensitive, a set of thin films deposited at variable deposition rates were exposed to environment in order to understand possible degradation and structural transformation mechanisms in Znq2.
Figure 5 shows X-ray spectrum of pristine and environmentally exposed thin films of Znq2. The fresh films show a broad peak at 8.35°. The presence of this peak points to the fact that structurally the molecules have a (Znq2)4 like composition in an amorphous structure. The exposed film shows the presence of an intense peak at 6.9 degrees, which is signature of Znq22H2O, and the absence of a broad peak at 8.35 degrees. This suggests that H2O molecule has now become attached to Znq2 leading to crystallization. This observation leads to conjecture that either there was a sizable fraction of anhydrous Znq2 or tetramer Znq2 has broken down in the thin film. The tetramer of Znq2 has been reported to be a stable compound which does not degrade under exposure to environment [7]. If the films were composed of fully tetrameric component, it is unlikely that H2O molecule could get attached to the compound. Thus, we can conclude that there may be a large fraction of disordered phase.
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Figure 6 shows FTIR spectra of thin films at different deposition rates (see Table 1). A comparison of thin films and powders (see Figure 2) shows that the molecular structure of all deposited films is close to tetrameric one; intensification and shifting of peak at 610 cm−1 indicating higher out of plane and torsional distortion. The broad band 3214 cm−1 which is missing in pristine thin films is quite apparent in the environmentally exposed films (see Figure 7) again indicating that the thin films are not fully tetrameric.
Figure 7 shows FTIR spectra of the thin films exposed to atmosphere. Broad peak due to hydration which was not present in pristine films can be seen in these films. The ratio of 1100 cm−1 and 3300 cm−1 was used to characterize Znq22H2O as material. This ratio is seen to be ~0.1 in thin films again indicating that pristine thin films must contain a fraction of anhydrous and amorphous Znq2.
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 8 shows that environmental exposure results in blue shift (506 nm from 540 nm) of the PL spectrum when exposed to air for extended period. Our results show that, whatever may be the starting compound for sublimation of Znq2, the thin films formed are of similar structure. As Znq2 sublimes at temperature around 350°C, it is natural that conversion of Znq2 powders to its stable tetramer form has already taken place.
4. Conclusions
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.
Acknowledgments
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.