Indian Journal of Pure & Applied Physics Vol. 43, December 2005, pp. 921-925

Electrochemical and optical studies of conjugated polymers for three primary colours Aparna Misra, Pankaj Kumar, Ritu Srivastava, S K Dhawan, M N Kamalasanan* & Subhas Chandra OLED Lab, Polymeric & Soft Materials Section, National Physical Laboratory, Dr K S Krishnan Road, New Delhi 110 012 * Email: [email protected] Received 29 April 2005; revised 20 September 2005; accepted 29 September 2005 The Cyclic Voltammetry investigation and optical properties of some of the commonly used conjugated polymers for the fabrication of organic LEDs, like poly [2-methoxy 5- (2′-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV), poly [2,3-bis (2-ethylhexyloxy)1,4-phenylenevinylene] (BEH-PPV) and poly[(9,9′-di(2′-ethylhexyl) fluoren-2,7-yleneethynylene] (PFE) for three principal emission colours red, green and blue, respectively have been reported. Both oxidation and reduction potentials of polymers were determined under the same experimental conditions to estimate the ionization potential (Ip: energy of Highest Occupied Molecular Orbital) and electron affinity (Ea: energy of Lowest Unoccupied Molecular Orbital). The optical band gaps of the polymers were obtained from their optical absorption spectra. A comparative study of the electrochemical and optical bandgap of the polymers has been made. The photoluminescence (PL) of the polymers has also been studied. Keywords: HOMO, LUMO, Optical band gap, Electrochemical band gap, Cyclic Voltammetry IPC Code: C08J, B23H3/00

1 Introduction Since the first report by J H Burroughes (1990) on poly (p-phenylenevinylene) (PPV) as an emitter for organic light emitting diodes1 (OLEDs), electroluminescent conjugated polymers have aroused a great deal of interest owing to their promising commercial application in polymeric light emitting diodes (PLEDs), field effect transistors, photovoltaic diodes, solar cells, electrochemical cells and lasers2-10. In the polymeric LEDs the matching of the HOMO and LUMO energy levels of the emissive materials with the work functions of the electrodes as well as their optical, chemical and electrical stabilities are of paramount importance. The electronic energy levels (HOMOs and LUMOs) of the polymers determine the PLED device structure and selection of electrodes and charge transporting materials. Generally, the ionization potential (Ip) of the organic molecules is measured by ultraviolet photoelectron spectroscopy (UPS), while the electron affinity (Ea) is determined from the difference of the ionization potential and the optical band gap, from optical absorption spectra. But this is not a direct measurement technique to determine the position of HOMO and LUMO energy levels. The electrochemical processes are similar to the charge injection and transport in the organic

LED devices. Therefore, Cyclic Voltammetry (CV) has been recognized as an easy and effective approach to evaluate the position of both the HOMO and LUMO energy levels and the band gap of the polymers11-13, 18,19. CV has also been used to determine the reversibility, reproducibility and stability of polymer films on the electrodes. In the present work, a comparative study of the electrochemical and the optical band gap in three conjugated polymers MEH-PPV, BEH-PPV and PFE, emitting principal colours red, green and blue, respectively has been reported. We synthesized MEHPPV in the laboratory while BEH-PPV and PFE were purchased from Sigma-Aldrich and used. The electrochemical band gaps were determined from the Cyclic Voltammetry of the polymers whereas the optical band gaps were determined from the wellknown optical absorption spectroscopy method. We also report the PL in solution as well as in films of the polymers. 2 Experimental Details Cyclic Voltammetry is a dynamic electrochemical method to investigate the electrochemical behaviour as well as to estimate the HOMO and LUMO energy levels of the polymers. Cyclic Voltammetry gives direct information of the oxidation and reduction

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potentials of materials. The oxidation process corresponds to removal of the electron from the HOMO energy level, while the reduction corresponds to electron addition to the LUMO energy level of the materials. The current arises from transfer of electrons between the energy level of the working electrode and the molecular energy levels of the materials under study. The onset potentials of oxidation and reduction of a material can be correlated to the ionization potential and electron affinity according to the empirical relationship proposed by Bredas et al.14,15 on the basis of a detailed comparison between valence effective Hamiltonian calculations and experimental electrochemical measurements. This correlation can be expressed as: Ip = - (Eox + 4.4) eV

…(1)

Ea = - (Ered + 4.4) eV

…(2)

and Eg = Ip - Ea

…(3)

where Eox and Ered are the onset potentials of oxidation and reduction, respectively, while Eg is the band gap of the material. The onset potentials are determined from the intersection of the two tangents drawn at the rising current and baseline charging current of the CV traces. Cyclic Voltammograms of the polymers were recorded with a computer controlled Autolab PGSTAT 30 model potentiostat consisting of threeelectrode system electrochemical cell at a constant scan rate of 20 mV/s under nitrogen atmosphere. The polymers films (10 mg/ml in chloroform) dip coated on a Pt electrode (area = 9.4 mm2) were scanned positively and negatively together, while BEH-PPV film was dip coated and scanned negatively and positively separately, in 0.01 M tetra-butyl ammonium tetra-fluoroborate (TBABF4) dissolved in anhydrous acetonitrile against Ag/AgCl (3 M LiCl) reference electrode, separated from the electrolyte by a diaphragm. UV-vis optical absorption spectroscopy is a wellknown technique to evaluate the optical absorption band gap of the materials. The optical band gaps of the polymers were estimated from the optical absorption edge in thin films and using the Tauc relation16,17 Ahν = (hν - Eg) n

…(4)

where n is 1/2 for allowed direct, 3/2 for forbidden direct, 2 for allowed indirect and 3 for forbidden indirect transitions in the materials, A is the absorbance, Eg is the band gap corresponding to a particular absorption of photon energy hν. The direct optical band gap of the polymers were obtained from extrapolation of the straight line portion of their (Ahν)2 versus hν plots to A = 0. The absorption spectra of the polymers were recorded with a Shimadzu UV-2401 PC spectrophotometer at the room temperature. The optical absorption spectroscopic studies of the polymers were carried out in their chloroform solutions as well as in thin films. The thin films of the polymers were obtained by spin coating their solutions (10 mg/ml) on cleaned quartz substrates. The photoluminescence studies of the polymers were carried out with a HR 2000 Ocean Optics Spectrometer, having a CCD array sensor and fiber optic probe. 2.1 Synthesis of MEH-PPV

To 1 g (3mmol) of monomer 2-5-bis(chloromethyl) 1-methoxy-4-(2-ethylhexyloxy) benzene in 20 ml of anhydrous THF, 40 ml of 1 M solution of t-BuOK in THF available from Aldrich, was added dropwise and the reaction mixture was stirred at room temperature for 2 hrs. After the completion of the reaction, the reaction mixture was transferred into 500 ml of icecold methanol. The precipitate was washed with distilled water and reprecipitated from THF/methanol and dried under vacuum to yield 0.82 g of red powder as MEH-PPV. The molecular weight of MEH-PPV was determined by GPC VISCOTEK VE 8000 as ~ 100,000 daltons. The molecular structures of MEHPPV, BEH-PPV and that of PFE are shown in Fig. 1. 3 Results and Discussion In the voltammetric experiments, a potential is applied to a system in solution using two electrodes, a working electrode and a reference electrode and the current response is measured using the working electrode and a third electrode called counter or auxiliary electrode. The Cyclic Voltammograms of the polymers are shown in Fig 2. From the cyclic voltammetric studies, the onset potentials for oxidation of the polymers were observed to be 0.75, 1.0 and 1.49 V, while reduction potentials were -1.55, -1.7 and -1.75 V for MEH-PPV, BEH-PPV and PFE, respectively. According to the relationship proposed by Bredas, the onset potentials for oxidation and reduction, Ip, Ea and the electrochemical band gaps for

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Table 1 — Electrochemical band gaps for the polymers Ea ElectroPolymers Eox Ered Ip chemical (V) (V) (HOMO) (LUMO) band gap (eV) (eV) (Eg) (eV)

Fig. 1 — Molecular structures of (a) MEH-PPV (b) BEH-PPV (c) PFE

Fig. 2 — Cyclic Voltammograms of polymers films, dip coated on platinum electrodes in against Ag/AgCl reference electrode in acetonitrile at the scan rate of 20 mV/s

MEH-PPV

0.75 -1.55

-5.15

-2.85

2.30

BEH-PPV

1.0

-1.70

-5.40

-2.70

2.70

PFE

1.49 -1.75

-5.89

-2.65

3.24

the polymers are listed in Table 1. Comparing the Ip and Ea of the polymers, it can be seen that MEH-PPV and BEH-PPV have negatively higher values of Ea than PFE, which means that for MEH-PPV and BEHPPV the electron injection process is easier than that for PFE. However, Ip values of BEH-PPV and PFE are more negative than MEH-PPV suggesting that holes are difficult to be injected from ITO into the polymers when they are used as the active layer for single layer PLEDs. The optical absorption and photoluminescence spectra of the polymers in solution as well as in thin films are shown in Fig. 3(a-c). The polymers in the film state are bathochromically shifted compared to their solution samples. The broader absorption peak in films means that there is a distribution of energy levels corresponding to the π–π* transitions. The cutoff absorption wavelengths for MEH-PPV, BEH-PPV and PFE were estimated to be at about 610, 495 and 435 nm, respectively. The extrapolation of the straight-line portion of the (Ahν)2 versus hν plot to A = 0 for the polymers (Fig. 4), revealed the optical band gaps of the polymers to be 2.10, 2.62 and 2.89 for MEH-PPV, BEH-PPV and PFE, respectively. The UV-vis absorption and photoluminescence maxima in solution and thin film of the polymers samples along with their optical band gaps are summarized in Table 2. The electrochemical band gaps of the polymers, deduced from the onset potential of the oxidation and reduction, are higher than the values obtained from the optical absorption spectra. This difference can be attributed to the interface barrier between the polymer film and electrode surface. It can be suggested that the electrochemical band gaps may be the combination of the optical band gap and the interface barrier for charge injection. As the configuration of PLED devices is similar to the electrochemical system, the band gap obtained from the electrochemical method should be more meaningful.

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Fig. 3(a) — UV-vis absorption and photoluminescence spectra of MEH-PPV in solution and thin films

Fig. 3(b) — UV-vis absorption and photoluminescence spectra of BEH-PPV in solution and thin films

Fig. 3(c) — UV-vis absorption and photoluminescence spectra of PFE in solution and thin films

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Table 2 — Absorption and photoluminescence of the polymers in solution and thin film

Solution

Film

Solution

Film

Solution

Film

Optical band gap (Eg) (eV)

MEH-PPV

496

509

608

626

600

625

2.10

BEH-PPV

382

385

514

522

485

495

2.62

PFE

381

382

423,444,486

510,455

430

435

2.89

Absorption λmax (nm)

Polymers

Photoluminescence λmax (nm)

Cutoff Abs wavelength (nm)

Delhi, for his keen interest in this work. Thanks are also due to Dr Anil Kumar Gupta, Dr S S Bawa, for their help and cooperation. One of the authors (SC) is thankful to CSIR, India, for the award of Emeritus Scientist Scheme grant. References 1 2 3 Fig. 4 — Absorbance squared versus the photon energy (hν) extrapolated to zero absorption of the polymers

4 Conclusions We have made a comparative study of the electrochemical and optical band gaps of three principal colour emitting conjugated polymers for PLEDs application. For this purpose, MEH-PPV was chosen as red emitter, BEH-PPV as green emitter while PFE as blue emitter. We synthesized MEH-PPV in the laboratory by Gilch route while BEH-PPV and PFE were procured from commercial sources. The optical band gaps have lower values compared to the electrochemical band gaps. This difference in the band gap values from electrochemical and optical methods can be attributed to the interface barrier between the polymer film and electrode surface. However, as the configuration of PLED devices is similar to the electrochemical system, the electrochemical values are more meaningful than the optical for OLED applications. Acknowledgement Financial support by the Ministry of Information and Communication Technology, Govt of India and CSIR (India) Network project No. CMM-0010 is gratefully acknowledged. The authors would like to thank Prof Vikram Kumar, Director, NPL, New

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