Chapter 3a
3.4. X-ray diffraction studies on the thin film of PVK:10 wt% LC1
involves charge carrier trapping in emissive layers. This is possible if the LUMO level of the guest molecule is lower than the host and HOMO level of the guest is higher than that of the host. In such cases the guest molecules constitute traps for both types of charge carriers (electrons and holes) injected into the matrix from the electrodes. Then the excitons are formed directly on the guest molecule without the need of Förster energy transfer from host to guest molecule.46 In Figure 3.11a, we can clearly distinguish between the energy levels of LC 1, 2 and 3 with respect to the energy levels of PVK. It is only in the case of LC 1, the LUMO and HOMO levels are situated between that of PVK, while in the case of LC 2 and 3, the HOMO levels are lower than that of PVK. This explains the higher electroluminescence efficiency of PVK:10 wt% LC1 in comparison to other two composites. It should be noted that all the above LCs have good absorption spectral overlap with the emission spectrum of PVK, but except compound 1, others do not show good electroluminescence. Thus the second mechanism plays a major role in determining the electroluminescence of this class of host-guest OLEDs.
Figure 3.13. The fluorescence decay of compound 1 (blue trace, λexc = 405 nm, monitored at λmax = 556 nm) and for the thin film of PVK:10 wt% 1 (red trace, λexc = 336 nm, monitored at λmax = 559 nm); (black trace shows IRF).
PVK:10% LC1 mixture (Figure 3.14a). Optically, the film formed by the above mixture was more birefringent in comparison to spin coated PVK film under same conditions.
Further, the film was annealed at 140 oC for 10 minutes to match the experimental condition. There was an increase in the birefringence of the thin film after annealing. The optical texture did not exhibit the characteristic pseudoisotropic pattern corresponding to homeotropic (face-on) orientation, instead a birefringent pattern was observed that corresponds to the planar orientation of Col LC phase. Usually, Col LCs when they are on single substrates exhibit a planar texture due to the forces acting at the LC–air interface, that favors planar alignment.47 Usually thermal annealing is done to provide good alignment and miscibility. The POM image of the film formed by the spin coating of LC1 on a single substrate showed a grainy texture as seen in Figure 3.14b. This texture remained same even after annealing at 140 oC for 10 min. This points to the superior homogeneous nature of the film formed by the PVK:10 wt% LC1 mixture.
Figure 3.14. (a) POM image of the annealed spin coated film of PVK:10 wt% LC1 at RT; (b) POM image of the spin coated annealed film of LC1 at RT.
In order to understand the microstructure of the thin film formed by the spin-coated film of PVK:10 wt% LC1, we have carried out the powder XRD studies. This revealed the presence of a single peak at low angle along with several peaks in the mid angle region, which points to the crystalline nature of the sample. The peak at low angle (2θ = 8.25o) corresponds to a d-spacing of 10.7 Å (Figure 3.15a). According to literature report, PVK (Molecular weight ≈ 40,000) has been reported to show a nematic LC phase. The XRD pattern for the reported sample showed a single peak at low angle, along with a diffused hallow at wide angle. Thermal annealing of this polymer resulted in stronger single peak at low angle (2θ = 8o) along with a diffused peak at wide angle. The authors interpreted the peak at low angle to a d-spacing of 11Å, that is corresponding to a long range order.48 In the present case, the PVK used was of molecular weight ranging from 25,000-50,000. Thus, it showed completely a different behavior from the reported sample even in the blended state.
Figure 3.15. (a) XRD profiles depicting the intensity against 2θ obtained for the spin coated film of PVK:10 wt% LC1 at RT (pink trace); For the same sample in capillary at 150 oC (blue trace); at RT (green trace) and for compound 1 at RT(red trace) (corresponding XRD image patterns are given on the right side);
(b) XRD profiles depicting the intensity vs 2θ obtained for the sample PVK:10 wt% LC1 on annealing from 175 oC to RT in capillary tube; (c) XRD profiles depicting the intensity vs 2θ obtained for the sample PVK alone on annealing from 175 oC to RT in capillary tube.
Further we were curious to know about the changes occurred during annealing.
Since the sample in the thin film could not be heated, we have carried out the XRD of this composite material in capillary. The sample filled in the capillary was heated up to 175 oC and then cooled slowly. Measurements were carried out at regular temperature intervals (Table 3.8, Figure 3.15b). The XRD pattern obtained after annealing at 175 oC showed an additional, relatively sharp peak at low angle and a diffused peak at wide angle. The two peaks obtained at the low angle correspond to the d-spacings of 23.4 Å and 12.8 Å, respectively. The diffused halo with a d-spacing of 4.74 Å is due to the packing of carbazole units. To understand this a control experiment was carried out by measuring the XRD of PVK sample alone as a function of temperature (Figure 3.15c, Table 3.8). These diffraction patterns were almost similar irrespective of the temperature with a single peak at low angle with a d-spacing of 12.4 Å and a diffused peak at wide angle with a d-spacing of 4.71 Å.
This also confirms that the PVK sample is also liquid crystalline in nature. We also should note the XRD pattern of LC1 at different temperature intervals showed several peaks at small angles (d10 = 20-21Å) with a d-spacing ratio fitting into a hexagonal lattice, along with diffused peaks at 5 Å and 3.4 Å. (Figure.3.3d, Table 3.2, Figure.3.15a). The first diffused peak corresponds to the packing of flexible tails and the second one corresponds to the packing of the cores. Thus the d-spacing values observed for the composite material were different than the one obtained for LC1 as well as PVK sample alone, though they showed the signatures of both the materials. This shows the well-dispersed nature of the composite material.
Table 3.8. Results of (hkl) indexation of XRD profiles of the PVK:10 wt% LC1 and PVK alone at different temperature intervals on coolinga
The first two peaks obtained at the low angle for PVK:10% LC1 mixture (at 175 oC) could be indexed into Miller indices (10) and (11) of a Col phase with hexagonal
lattice, with a lattice constant a = 27 Å (Table 3.8). This corresponds to the intercolumnar distance, i.e. the distance between two neighboring columns. Further cooling of the material to room temperature showed an increase in the intensity of the peaks at low angle, while maintaining the same d-spacing ratio. From 100 oC onwards we observed a second diffused peak at wide angle, which corresponds to the average intracolumnar distance, which is arising from packing of DLC cores or carbazole units within a column. Observation of this peak with a d-spacing of 3.74 Å is significant and supports the intimate packing of carbazole units in the polymer backbone and LC1 (Table 3.8). This distance is higher than the intracolumnar distance observed for LC1 alone, which is 3.45 Å. This may be due to
PVK:10 wt% LC1 PVK
(T/oC) dobs(Å) dcal(Å)
Miller indices (hkl)
Lattice
parameters (Å) dobs(Å)
175
23.42 12.75 4.74 (ha)
23.42 13.52
100 110
a = 27.04
c = 4.74 12.44, 4.72
150
23.12 12.66 4.72 (ha)
23.12 13.35
100 110
a = 26.7
c = 4.45 12.46, 4.73
125
23.12 12.64 4.72 (ha)
23.12 13.35
100 110
a = 26.7
c = 4.72 12.44, 4.71
100
23.11 12.65 4.72 (ha) 3.76 (hc)
23.11 13.34
100 110 001
a = 26.69 c = 3.76
12.44, 4.71
75
23.11 12.65 4.71 (ha) 3.76 (hc)
23.11 13.34
100 110 001
a = 26.69 c = 3.76
12.45, 4.71
50
23.11 12.59 4.71 (ha) 3.76 (hc)
23.11 13.34
100 110 001
a = 26.69 c = 3.76
12.44, 4.71
25
23.11 12.63 4.71 (ha) 3.74 (hc)
23.11 13.34
100 110 001
a = 26.69 c = 3.74
12.44, 4.71
adobs: spacing observed; dcal: spacing calculated (deduced from the lattice parameters; a for Colh phase).
The spacings marked ha and hc correspond to diffuse reflections in the wide-angle region arising from correlations between the alkyl chains and core regions, respectively.
the insertion of pendant carbazole moieties inside the columns. It should be noted that intercolumnar distance (a) observed in the case of LC1 alone is 23-24 Å (Table 3.2), while for the composite material, the observed intercolumnar distance is a = 26-27 Å. Since there is no much change in the intercolumnar distance, we can conclude that polymer chains are arranged along the columnar axis, with the pendant carbazole moieties arranging in a hexagonal columnar lattice. Thus the thermal annealing as a driving force, helps in the reorganization of the polymer chains and DLCs. In total, the thermal annealing enhances the alignment and intermixing of these two different components as shown in Figure.3.16.
Figure 3.16. Schematic showing the thin film processing of the mixture of PVK:10 wt% LC1. Expanded region shows the organization of PVK polymer chain and DLCs in Colh phase.