CSA Doped Poly ( aniline-coo-toluidine ) and Dispersed Zinc Oxide Nanoparticles : a Promising Material for Photovoltaics

Polyaniline (PANI) and its derivatives have been extensively studied in the last decade with improved processing and intrinsic electrical properties. They have received great attention owing to its ease of synthesis, good stability in the presence of oxygen and possessing electrochemical properties of unique conducting mechanism. This polymer can be obtained by electrochemical and chemical oxidative polymerization of aniline in aqueous solutions. Poly (o-toluidine) is a PANI derivative which contains the –CH3 group in the ortho position of the aromatic ring of the aniline monomer. Among the ring-substituted PANI derivatives, Poly (o-toluidine) has been most widely studied one. With regard to polyaniline based copolymers, a pioneering work has been reported about aniline copolymerization with o-toluidine and aminoacetophenone possessing conductivity property of broad range1-4. The copolymer was found to be soluble in common organic solvents like CHCl3, THF, DMF, NMP and DMSO5. The thermal stability of the copolymer was studied and found to vary irregularly with AN-OT molar ratio6-8. The copolymer shows new morphology with higher thermal stability and better solubility than its homopolymers. Potentiodynamic polarization and electrochemical impedance spectroscopy have been employed to study the corrosion inhibition effects of this copolymer on carbon steel9. Recently it has been reported that the metal composite of poly (aniline-co-o-toluidine)-BaFe12O19 composite exhibited a ferro magnetic behavior10. Juliet et al reported the characterization of copolymerization of aniline-co-o/m-toluidine and V2O5 nanocomposites 11. Optical band gap of PANI-co-POT thin film are found to be lower than that of POT films and shows a rectifying behavior12 and a spin coated thin film of polyaniline doped with camphorsulfonic acid (CSA) has been found to possess novel microstructure surface for NH3 detection at room temperature with better response13. Polyaniline has been used as an effective candidate as hole transporting layer in organic solar cells instead of PEDOT:PSS[14,15]. Polyaniline with amorphous silicon in heterojunction solar cells has been reported and found to have higher open circuit voltages in the solar cells16. Among metal oxide nanoparticles, zinc oxide nanoparticles possess their own importance due to their vast area of applications as gas sensor, chemical sensor, optical, electrical devices and solar cells17-21. ZnO nanoparticles are becoming promising semiconductor oxides when applied as an electron transport material22. Zinc oxide nanoparticles has been synthesized and used in dye sensitized solar cells23. Addition of ZnO nanoparticles into the copolymer chain will improve their compatibility and increases the thermal stability of the copolymer24. Polyaniline-ZnO nanocomposite thin film has been fabricated on glass substrates by vacuum deposition technique25. It has been reported that polymers doped with organic acids lead to possess high conductivity because organic solvents such as m-cresol and other substituted phenols act as secondary dopants26-33. CSA Doped Poly(aniline-co-o-toluidine) and Dispersed Zinc Oxide Nanoparticles: a Promising Material for Photovoltaics


Introduction
Polyaniline (PANI) and its derivatives have been extensively studied in the last decade with improved processing and intrinsic electrical properties.They have received great attention owing to its ease of synthesis, good stability in the presence of oxygen and possessing electrochemical properties of unique conducting mechanism.This polymer can be obtained by electrochemical and chemical oxidative polymerization of aniline in aqueous solutions.Poly (o-toluidine) is a PANI derivative which contains the -CH 3 group in the ortho position of the aromatic ring of the aniline monomer.Among the ring-substituted PANI derivatives, Poly (o-toluidine) has been most widely studied one.With regard to polyaniline based copolymers, a pioneering work has been reported about aniline copolymerization with o-toluidine and aminoacetophenone possessing conductivity property of broad range [1][2][3][4] .The copolymer was found to be soluble in common organic solvents like CHCl 3 , THF, DMF, NMP and DMSO 5 .The thermal stability of the copolymer was studied and found to vary irregularly with AN-OT molar ratio [6][7][8] .
The copolymer shows new morphology with higher thermal stability and better solubility than its homopolymers.Potentiodynamic polarization and electrochemical impedance spectroscopy have been employed to study the corrosion inhibition effects of this copolymer on carbon steel 9 .Recently it has been reported that the metal composite of poly (aniline-co-o-toluidine)-BaFe 12 O 19 composite exhibited a ferro magnetic behavior 10 .Juliet et al reported the characterization of copolymerization of aniline-co-o/m-toluidine and V 2 O 5 nanocomposites 11 .Optical band gap of PANI-co-POT thin film are found to be lower than that of POT films and shows a rectifying behavior 12 and a spin coated thin film of polyaniline doped with camphorsulfonic acid (CSA) has been found to possess novel microstructure surface for NH 3 detection at room temperature with better response 13 .Polyaniline has been used as an effective candidate as hole transporting layer in organic solar cells instead of PEDOT:PSS [14,15] .Polyaniline with amorphous silicon in heterojunction solar cells has been reported and found to have higher open circuit voltages in the solar cells 16 .Among metal oxide nanoparticles, zinc oxide nanoparticles possess their own importance due to their vast area of applications as gas sensor, chemical sensor, optical, electrical devices and solar cells [17][18][19][20][21] .ZnO nanoparticles are becoming promising semiconductor oxides when applied as an electron transport material 22 .Zinc oxide nanoparticles has been synthesized and used in dye sensitized solar cells 23 .Addition of ZnO nanoparticles into the copolymer chain will improve their compatibility and increases the thermal stability of the copolymer 24 .Polyaniline-ZnO nanocomposite thin film has been fabricated on glass substrates by vacuum deposition technique 25 .It has been reported that polymers doped with organic acids lead to possess high conductivity because organic solvents such as m-cresol and other substituted phenols act as secondary dopants [26][27][28][29][30][31][32][33] .
CSA Doped Poly(aniline-co-o-toluidine) and Dispersed Zinc Oxide Nanoparticles: a Promising Material for Photovoltaics The aim of the present research work is to synthesize, characterize and study the electronic properties of poly(aniline-co-o-toluidine) copolymer doped with CSA and dispersion of ZnO nanoparticles.The synthesized copolymer composites are spin coated to form a thin film using spin coating technique.The film is then subjected to various characterization techniques such as UV-Vis, FTIR, PL, XRD, SEM and EDAX.

Material and Methods
The chemicals used for the synthesis are of AR grade.Aniline monomer and ammonium persulphate (APS) were purchased from Merck and o-toluidine from Lobachime.Camphorsulfonic acid and zinc oxide nanoparticles were purchased from Sigma Aldrich.0.1 M aniline (0.5 ml in 50 ml of 1 M HCl solution) and 0.1 M o-toluidine(0.5 ml in 50 ml of 1 M HCl solution) monomers were added in a 250 ml round bottom flask.0.2 M ammonium persulfate solution (4.56 gm of APS dissolved in 100 ml of 1 M HCl solution in separation funnel and it was added drop wise to the monomer mixture.As soon as the oxidant was added, the polymerization reaction began in the 1 M of hydrochloric acid medium.The reaction mixture was allowed to stir continuously for four hours.The temperature of the reaction mixture was maintained between 0°C to 5°C.The obtained copolymer was washed and filtered.A dark green precipitate is obtained and dried in vacuum at 60°C.The obtained green colored emeraldine salt was an HCl doped polymer.The copolymer was undoped by mixing with 50% of aqueous ammonia solution (50 ml of ammonia added in 50 ml of distilled water) and allowed to stir continuously for four hours.A dark bluish color was obtained and dried.The emeraldine base of the copolymer was doped CSA with a ratio of 1:5 in 1 ml of m-cresol in 5 ml vial which acts as a medium and secondary dopant.The solution was stirred for 5 days to get homogeneous solution.Finally the resultant solution was spin coated at 750 rpm for 20 secs.Zinc oxide nanopowder (2 mg) was dispersed into the CSA doped copolymer and then it was spin coated on a glass plate to form a thin film.The copolymer, CSA and ZnO in the composite are in the ratio of 1:5:1.

Characterization
The synthesized copolymer was characterized with FT-IR analysis which was recorded using Perkin Elmer FTIR Spectrometer as with KBr pellets.The samples were coated into thin film by using Programmable spin coater SCU 2008C.The thin film was characterized by UV-Vis spectroscopy using JASCO-V570 spectrophotometer.X-ray diffraction patterns were taken using Philips model TW 1710 diffractometer with Cu Kα radiation of 1.5417 angstrom.The SEM and EDAX were taken using Philips XL 30.Photoluminescence spectra were recorded using FT 600 spectroflurometer.

FTIR analysis
Figure 1 represents the FT-IR spectrum of the synthesized copolymer.The characteristic peak about 3350 cm -1 corresponds to the N-H stretching of the primary amine and confirms the amino group.The peak at 2920 cm -1 attributes to the C-H stretching vibration of methyl group.The absorption peak at 1590 cm -1 is assigned to the quinoid ring stretching.The peak observed at 1494.20 cm -1 corresponds to the presence of C=C stretching vibration in benzenoid ring.The sharp peak at 809.39 cm -1 attributes the para-coupled phenyl ring in the copolymer.The absorption band around 1145 cm -1 confirms the charge delocalization.The peak at 1297.78 cm -1 corresponds to the C-N stretching vibrations of the aromatic primary amine 5 .

UV-Visible analysis
The UV-Vis spectrum of the copolymer from NMP is shown in the Figure 2.There are two absorption peaks observed in the spectrum at 337 nm and 603 nm respectively.The peak at 337 nm corresponds to the π -π* transition.The peak at 603 nm corresponds to excitonic transition between highest occupied molecular orbital of the benzenoid ring and lowest unoccupied molecular orbital in quinoid ring or n -π* transition of the copolymer 34 .

XRD analysis
Figure 3a shows the X-ray diffraction of the copolymer.It shows the broad amorphous diffraction peak due to the presence of benzoid and quinoid rings of copolymer.Figures 3b, c show X-ray diffraction of the pure CSA and copolymer doped with CSA thin film respectively.Since the  ratio of CSA is much higher than copolymer in the composites, the solvent vaporizes and excessive CSA crystallizes after spin coating and thus showing a high intensity peak at the 2θ value of 18.08°3 5 .This may be due to growth and formation of expanded chains of CSA doped copolymer and aligned orderly to one direction as clearly seen in the SEM image 36 .Macdiarmid et al has reported about XRD pattern of crystallized CSA doped polyaniline films and also emphasized the effect of secondary dopant m-cresol on the expanded coil like conformation 37 .Figure 3e shows the XRD pattern of copolymer with CSA doped and dispersed with ZnO nanoparticles.The three peaks at 15.39°, 17.56° and 20.59° indicate the crystalline camphorsulfonic acid and the values are confirmed with JCPDS data no.12-0907.Similar new peaks were observed with the same 2θ value of 15.44° and 17.61° in PPy-ZnO-CSA thin film 38 .After dispersing the zinc oxide nanoparticles, the ordered structure is disturbed and a hexagonal crystal structure is obtained giving rise to new peaks, which may reduce the electrostatic interaction between the copolymer and CSA.The molecular interaction between the m-cresol phenyl ring and CSA may also decrease and thus reducing the peak intensity.The XRD pattern of pure zinc oxide nanoparticles are shown in the Figure 3d.The high intensity peak for ZnO nanoparticles is at 36° and the characteristic peak at 36.32° in Figure 3e

SEM analysis
Figures 4a, b and c show the scanning electron microscope images of the copolymer doped with CSA at different magnification.Figure 4a shows highly grown hexagonal structure in the morphology of the film.All the crystals are aligned in the same direction and Figure 4b is the enlarged image shows growing crystals in the same direction.This structure may be due to the electrostatic interaction between the copolymer and CSA in presence of m-cresol and also due to the hydrogen bonding between the carbonyl group of CSA and hydroxyl groups in m-cresol 39 .There also exist phenyl-phenyl interactions between the phenyl ring of m-cresol and the neighboring copolymer ring.The van der Waals interaction between the phenyl rings of CSA and m-cresol is described by the Lennard potential.These molecular interactions are known to cause crystalline growth of CSA and these results in the formation of hexagonal structure 13 .
Figure 4c shows the porous morphology of copolymer matrix and appears to be composed of mostly fibrous structure.Figure 4d shows the SEM image of the copolymer doped with CSA and dispersion of zinc oxide nanoparticles.It shows that the rods like CSA are arranged in the same direction and the corresponding EDAX result is also shown in Figure 4e.The dispersed ZnO nanoparticles occupy in the porous copolymer film and therefore surface becomes smooth and hexagonal crystalline rods of CSA are uniformly distributed all over the polymer film.The EDAX confirms the presence of the carbon, sulphur, oxygen and zinc.

Photoluminescence studies
Figure 5 shows the photoluminescence spectra of the i) copolymer, ii) CSA doped copolymer and iii).ZnO nanoparticles dispersed CSA doped copolymer.The photoluminescence spectra of all the samples were taken with an excitonic wavelength of 380 nm.All the spectra show a common peak at 412 nm, 436 nm, 461 nm and a shoulder peak around 500 nm.The high intensity peak at 436 nm is due to the polaronic band of the copolymer.When the zinc oxide nanoparticles are dispersed, there is quenching in the PL spectrum showing the charge transfer between the copolymer and nanoparticles.The PL quenching is an evidence for exciton dissociation.As the photo excitons are dissociated, the probability for recombination will be less due to the ultra fast electron transfer reaction from donor to acceptor 40 .The quenching in emission spectrum is more in the case of ZnO nanoparticles dispersed polymer because of larger surface area leading to a higher possibility of charge transfer at the copolymer/ZnO interface.Here ZnO semiconducting nanoparticles act as an acceptor material.Similar quenching of PL spectrum has been reported when ZnO nanoparticles are added to MEH-PPV 41 , MDMO-PPV 42,43 and P3HT 44 films.This interesting quenching action in PL spectrum suggests that this copolymer composite material can be used for photovoltaic devices.

Conclusions
Poly (aniline-co-o-toluidine) copolymer has been successfully synthesized through chemical oxidation polymerization method.CSA doped Poly (aniline-co-o-toluidine) copolymer and dispersion of zinc oxide nanoparticles thin film has been successfully coated on glass substrates by spin coating technique.The synthesized copolymer is characterized by FTIR, UV, and XRD analysis.SEM study reveals the sponge like porous morphology of the copolymer composite.
π -π* and n -π* transition of benzonoid and quinoid groups of the copolymer are confirmed from the UV spectral studies.The FT-IR analysis confirms the chemical structure of copolymer.From the XRD pattern we can observe the crystalline nature of the CSA doped copolymer nanocomposties.Since the Poly (aniline-co-o-toluidine) copolymer is found to be highly soluble in NMP, it paves the way to have more application perspectives.The quenching in the photoluminescence spectrum indicates the charge transfer between the copolymer and the zinc oxide nanoparticles.These results strongly reveal that they are ideally suited for the manufacture of optoelectronic devices and could be used for photovoltaic application.Further extended research work in this direction will be highly useful for solar cell fabrication technology.