Chapter 3: Color Tunable Donor-Acceptor Electroluminescent Copolymers
3.3. Results and discussion
3.3.1. Synthesis and characterization of the polymers
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as a hole injecting layer and baked for 15 minute at 110 °C in argon environment. The emissive layer of ~75 nm was spin-coated above the PEDOT:PSS layer from the solutions of the copolymers in p-xylene (10 mg/mL) and thermally treated at 140 °C for 30 minute to remove the residual solvent. Finally, Bathocuproine (BCP) (10 nm), LiF (1 nm) and Al (100 nm) was thermally evaporated at a rate of 2 Å/s, 0.1 Å/s and 10 Å/s, respectively at a base pressure of 10-6 mbar as hole blocking layer and cathode, respectively. The emissive area of the PLEDs was 12 mm2.
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4000 3500 3000 2500 2000 1500 1000 500
Transmittance (a.u)
Wavenumber (cm-1)
50 35 25 10 05 01
Figure 3.2: FT-IR spectra of copolymers
in common solvents such as CHCl3, THF, chlorobenzene, p-xylene etc. The copolymers were found to show number-average molecular weights (Mn) in the range of 11519 to 18355 and a polydispersity index (Mw/Mn) of 1.12 to 1.42 as determined by GPC. Table 3.1 shows the results of polymerization of these newly synthesized carbazole based copolymers.
Table 3.1: Results of polymerization and thermal data of these newly synthesized carbazole based copolymers
Polymer Mna Mwa PDIa Yield (%) Tdb (˚C) Tgc (˚C)
PCzNPN01 12365 16386 1.32 72 422 143
PCzNPN05 13256 17102 1.29 67 396 145
PCzNPN10 18355 26202 1.42 61 347 142
PCzNPN25 15332 18736 1.22 78 327 140
PCzNPN35 13472 16336 1.21 73 316 142
PCzNPN50 11519 15713 1.36 64 302 151
aMn, Mw and polydispersity index of the copolymers as determined by GPC using polystyrene standards.
bOnset decomposition temperature calculated from TGA. cGlass transition temperature
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100 200 300 400 500 40
60 80 100
Weight %
Temperature (oC) 50%
35%
25%
10%
05%
01%
a)
50 100 150 200
-1.0 -0.8 -0.6 -0.4 -0.2
Heat Flow (mW)
Temperature (oC) 50%
35%
25%
10%
05%
01%
b)
Figure 3.3: TGA (a) and DSC (b) curves of copolymers
Thermal properties of the copolymers were studied by TGA and DSC at a heating rate of 10 °C/min under nitrogen flow. The TGA curves and onset decomposition temperature (Td) of the copolymers are presented in Figure 3.3a and Table 3.1. All the CPs exhibited satisfactory thermal stability with Td in the range of 302-422 °C andno significant weight loss observed at lower temperature. The DSC curves and glass transition temperature (Tg) of the copolymers are shown in Figure 3.3b and Table 3.1. The DSC results indicate that all copolymers were amorphous and therefore could be solution processed for PLED fabrication.
3.3.2. Photophysical Properties
The normalized absorption spectra of copolymers in THF solution (10-5 M) and spin coated thin films (thickness of approximately 80 nm) are shown in Figure 3.4a and 3.4b and summarized in Table 3.2. All the copolymers showed similar absorption spectra in the solution and in the thin films. Three distinct absorption bands were observed in both the solution and thin films. In the solution state, three absorption bands were observed at 306 nm, 345 nm and 405 nm. Similarly, in thin films also three absorption bands were observed at 305 nm, 350 nm and 420 nm. The first absorption band at ~306 nm is due to π-π* transition of the carbazole segments as observed in the absorption spectrum of pure PCz and the long absorption bands at ~350 and ~420 nm, which increased linearly with increasing NPN units (from 1 to 50 mol% in the polymer chain), can be assigned to the NPN unit. The absorption wavelength of copolymers are somewhat blue shifted, in both solution and thin films, which is probably due to the increasing feed of the NPN unit in the CP backbone, which induces a decrease in the effective conjugation length of carbazole moieties. Figure 3.4c and 3.4d present the normalized PL spectra of the copolymers in THF solution and thin films, respectively (350 nm excitation), (Table 3.2). In solution state, PCzNPN25, PCzNPN35 and PCzNPN50
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polymers showed single peak at 523 nm, due to efficient complete energy transfer from PCz to NPN moiety and the rest of the PCzN- PN10, PCzNPN05 and PCzNPN01 polymers exhibited three emission peaks at 402 nm, 421 nm and 523 nm due to incomplete energy transfer from PCz to NPN moiety. The peaks at blue region are due to the carbazole units, whereas, the peak at green region is due to the NPN unit. The intensity of the emission peak at green region (523 nm) was increased and the emission peaks at 402 nm and 421 nm decreases upon increasing the NPN unit in the CP chain.
300 350 400 450 500
0.0 0.2 0.4 0.6 0.8 1.0
Wavelength(nm)
50%
35%
25%
10%
05%
01%
PCz 0.0
0.2 0.4 0.6 0.8
50%
35%
25%
10%
05%
01%
PCz
Normalized absorbance (a.u.)Normalized Absorbance (a.u.)
Wavelength (nm)
0.0 0.2 0.4 0.6 0.8
1.0 50%
35%
25%
10%
05%
01%
PCz
400 450 500 550 600 650 700 0.0
0.2 0.4 0.6 0.8 1.0
50%
35%
25%
10%
05%
01%
Normalized Intensity (a.u.) PCz
Wavelength (nm)
c)
d) a)
b)
Figure 3.4: UV-Vis (a) & (b) and PL (c) & (d) spectra of copolymers in THF solution and thin film state
Table 3.2: Photophysical properties of the copolymers
THF solution Thin films
Polymer λ max, abs
[nm]
λ max, PL
[nm]
λ max, abs
[nm]
λ max, PL
[nm]
ФPLa
PCzNPN01 306 402, 421 300, 420 542 14.2
PCzNPN05 306 402, 421, 523 305, 420 544 14.5
PCzNPN10 306, 405 402, 421, 523 305, 420 552 11.6
PCzNPN25 300, 345, 405 523 304, 350, 420 560 12.5
PCzNPN35 292, 345, 405 523 304, 345, 420 571 8.3
PCzNPN50 292, 345, 405 523 304, 344, 420 583 2.4
aSolid state PL quantum yields are calculated with polydioctylfluorene standard
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In thin films, all copolymers exhibited the single emission peak due to the efficient intra and intermolecular energy transfer from PCz to NPN moiety. The PCz emission was completely quenched even when the NPN concentration was as low as 1% due to efficient charge transfer and strong energy transfer from PCz to NPN moiety. The PL emission peaks are gradually red-shifted with the increase in NPN content in the CP chain from 542 nm for the copolymer PCzNPN01 to 583 nm for the alternating copolymer PCzNPN50. This red shift could be attributed to the increase of the effective conjugation length of the copolymers due to electron-withdrawing nature of NPN units. The PL quantum yields (ФF) of copolymers were measured in thin film state by comparison their fluorescence intensity to polyfluorene (ФF = 0.55)33 relative intensity. The PL ФF values were tabulated in Table 3.2. The PL ФF values decreased on increasing the NPN mol% in the CP main chain due to the concentration quenching effect of the NPN moiety assigned to the aggregation of chromophores.34,35