Studying the Conductive Polymers

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Table of Contents

Introduction
Synthesis
Characterization
Structure-property-performance
Application of SY PPV
Conclusion
Works Cited

Introduction

Naturally, conventional polymers e.g. rubbers have long been used for their poor conductive properties as insulator. However, post-1970s, its usage has transformed from being an insulator to a conductor after the discovery of polyacetylene. Today, since polyacetylene, conductive polymers e.g. poly (aniline), polydialkylfluorines, Polythiopene and poly (phenylenevinylene) or PPV have continued to dominate the market. Specifically, PPV continues to enjoy wide usage as an important component in the manufacture of OLEDs (organic light emitting diodes) which in turn form the basic components in electronic display screens. In this article we feature the synthesis, characterization, the structure-property-performance and the application of Super Yellow Poly (phenylenevinylene) or SY PPV.

Synthesis

PPV is a common conjugated polymer with excellent electroluminescence properties that can be manufactured and commercialized “as important components in full-color flat panel displays” (Mikroyannidis 90). As such, in this article we explore the synthesis of SY PPV, a derivative of PPV, using 2-bromo-1, 4-xylelyne diacetate as a precursor element.

In order to understand the process involved in the synthesis of SY PPV, a schematic presentation of the steps involved and their intermediate products are shown below (Scheme 1). Importantly, this process explores an archetypal Suzuki coupling reaction in its initial process to synthesize two analogous compounds including 2-(3-(3′, 7′-dimethyloctyloxy) phenyl)-1, 4-xylylene diacetate (compound 2) and 2-(4- (3′, 7′-dimethyloctyloxy) phenyl)-1, 4-xylylene diacetate (compound 5). The reagent used to spark this reaction is 2-bromo-1, 4-xylelyne diacetate (compound 1) in presence of phenylboric acid. The two compounds (2 and 5) then undergo a reduction process in presence of LiA1H4 to form two other matching products of 2-phenyl-1, 4-xylylenediols denoted as compounds 3 and 6 in the schematic diagram below. A subsequent reaction with SOCl₂ yields other twin monomer compounds (4 and 7). These two monomers plus another monomer- 2-methoxy-5-(2′-ethylhexyloxy)-1, 4-bismethychlorobenzene (compound 8) are then subjected to a copolymerization process to yield SY PPV. Of note, the molar feed factor for monomer 8 is always kept at less than 2% to avoid undesirable effect that includes a decrease in electro-luminous (EL) efficiency.

(Mo et al. 1192).

Characterization

For this conductive polymer we investigate its electroluminescent properties. As such, polymer light emitting diodes (PLEDs) combined with “structure of ITO (indium tin oxide)/PEDOT: PSS (poly (3, 4-ethylenedioxythiopene): poly (styrenesulfonate))/emission layer/ Ba/Al was fabricated to inspect the electroluminescent properties of SY PPV” (Mo et al. 1193). As it has been portrayed in the figure below (figure 1), the electroluminescence band of this conductive polymer exhibits a small red light shift relative to photoluminescence (PL) band. Moreover, the trend of EL assumes a bi-peak shoulder with the highest peak at 552 nm while the lowest peak coincides with the 528 nm mark. This behavior is a consequence of increased intensity of vibrations emanating from long wavelength bandwidth. Importantly, the CIE coordinates computed for the product obtained is found to be (0.388, 0.578) (Spiliopoulos 34). Figure 2 exhibits current density-luminance-voltage behavior of the polymer’s PLEDs. The turn-on voltage as exhibited by the graph shows that the luminance is triggered when the voltage is at 2.4V. The minimum luminance- 1 cd.m^-2 is what defines the turn-on voltage. From the trend, it can be seen that the optimum luminance of this SY PPV PLEDs exceeds 49000 cd.m^-2 and that it is achieved at a voltage of 8 V. This component has a peak LE of 21 cd.A^-1, and is equivalent to an 8% EQE. For this device, it is important to note that the presence of impurities greatly affect its performance. In this one though the effects of impurities were significantly minimized courtesy of the efficient synthesis method adopted (Hsieh 19). The influence of impurities on the EL efficiency is, however, a topic for another day.

Fig.1.

Fig.2. (Mo et al. 1193).

Structure-property-performance

With respect to structure-property-performance of SY PPV, we investigate the electrochemical nature of the polymer. As such, we employ cyclic voltametry (CV) to investigate the doping process (p-type) (see figure 3 below). The graph shows that the doping process starts at approximately 1.1V. Consequently, the polymer’s HOMO level is -5.5 eV (E_HOMO=-(E_ox+4.4). Its corresponding LUMO level is -3.06 eV. Importantly, electronically, holes are the dominant charge carriers; hence, “exciton formation is usually limited by bottleneck of inefficient electron contribution” (Hsieh 42). For this case, however, with the high-lying LUMO, the performance of SY PPV is anticipated to be efficient.

(Mo et al. 1193).

Application of SY PPV

SY PPV is applied in solar cells to enhance its efficiency. In an experiment, it can be shown that a cell mounted with a photoluminescent SY PPV layer exhibits a “lower optical transmission between 380 and 480 nm, where the polymer has a high absorption” (Hsieh 45). Consequently, the overall absorption is optimized courtesy of photoluminescence of the layer which does not affect its transparency over a wide range of spectrum. In an experiment, it can be shown that the efficiency is not owed to an overlaying polymer layer that comes after a cathode. To this end, a 100nm-thick layer of SY PPV that is mixed with a 3 % PCBM is superimposed on the cathode. This layer results in a quenching effect, with the PCBM coming in hand as an effective exciton quencher (see figure below).

(Mo et al. 1193).

Conclusion

Conductive polymers have truly transformed the usage of conventional polymers. For this article, featuring SY PPV, a novel approach was used in the synthesis process, yielding a product with excellent properties.

Works Cited

Hsieh, Rayn, and Yu Young. Electroluminescent polymer compositions and processes thereof. Boston: Allyn, 2000. Print.

Kavarnos, Georsek. Fundamentals of Photoinduced Electron Transfer. New York: New York University Publications, 1997. Print.

Mikroyannidis, John. Synthesis of Poly(p-phenylene vinylene)- and Poly(phenylene ethynylene)-Based Polymers Containing p-Terphenyl in the Main Chain with Alkoxyphenyl Side Groups. American journal on micromolecules 27.5 (2010): 90- 94. Print.

MO, Yue-Qi, Chang Xue-Yi, Hu Su-Jun, Han Shao-Hu, Wu Hong-Bin and Peng Jun- Biao. Synthesis and Electroluminescent Properties of Phenyl-Substituted Poly(phenylenevinylene). Acta Phys. Chim. Sin. 27.5 (2011): 1188-1194. Print.

Spiliopoulos, Ioakim. Macromolecules 2001, 34, 5711. South Melbourne, Victoria: Thomson Learning, 2007. Print.

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