Structural and electrical characteristics of solution processed P3HT-carbon nanotube composite

Organic semiconductors have been identified as a fascinating class of low cost and flexible novel semiconductor materials that have the electrical and optical properties which can be easily processed. Due to their interesting physical properties, organic semiconductors have attracted tremendous research attention for next generation electronics and optoelectronics. Multiwalled carbon nanotubes (MWCNT) incorporated Poly[3-hexylthiophene-2,5-diyl] (P3HT) hybrid nano-composite film have been fabricated by solution processing technique followed by spin coating method using 1,2-dichlorobenzene as an intermediate solvent. Structural and morphological characteristics of the composite film have been studied by x-ray diffraction (XRD) and scanning electron microscope (SEM). The MWCNTs were observed to be well dispersed in the polymer matrix. Crystallites were found to be more ordered barely affecting the lamellar structure of P3HT in the nano-composite film. Structural and functional characteristics of P3HT and its hybrid nano-composite have been studied by UV-Visible (UV-Vis), Fourier transform infrared (FTIR) and Raman spectroscopic characterization. Excellent electrical properties have been observed from I-V and cyclic-voltammetric characterization of the well dispersed MWCNT in the P3HT composite. Improvement in electrical properties can be attributed to the higher carrier mobility of MWCNTs in the composites.

carrier mobility compared to their organic counterparts, have been widely researched for enhancing the physical properties of organic semiconductors. Incorporation of carbon based nanomaterials like multiwalled carbon nanotubes (MWCNT), graphene and fullerene [12] have been widely studied as filler material with improved physical properties [13][14][15].Due to excellent conductivity, high surface areas, low mass density, and large charge carrier mobility carbon nanotubes (CNTs) are being used in different optoelectronic devices as electrodes, semiconductors, transparent electrodes, gas sensors, etc. [16,17]. However, depositing an uniform thin film out of MWCNT is extremely difficult due to their chemical inertness and insolubility in most of the solvents. The improvement in the field effect mobility by incorporation of carbon based nanomaterials has already been reported [11]. P3HT based nanocomposites with better performance have been reported for solar cells, Light emitting diodes (LEDs), supercapacitor, thermoelectric devices, thin film transistors (TFTs) applications [18,19]. Only, solar cell device performance has been studied by of P3HT-single walled carbon nanotubes composite reported by Kyamkis et al. [18]. Photovoltaic device performance of P3HT-rGO composite has been investigated by Dingshan et al. [19]. In their work the used rGO was obtained during preparation of P3HT-rGO composites following many steps. Kuila et al. [20] have investigated photovoltaic properties of the composite using functionalized P3HT as well as MWCNT. They have focused mostly on the synthesis and device performance in which the structural properties have not been investigated in the composite. In this work, we report the structural and electrical properties of the solution processed P3HT-MWCNT composite without functionalizing either P3HT or MWCNT which is different from others [18][19][20].

Material synthesis:
Carbon nanotubes used for our studies have been synthesized by pyrolysis of benzene using ferrocene as catalyst for CNT growth using a cylindrical reactor tube at 900 0 C [21] and purified by oxidation and acid bath treatment [22].The purified CNTs were subjected for washing in distilled water many times and recovered followed by drying.

Composite preparation:
For composite preparation, around 40 mg of P3HT and 0.1 mg of CNT were dispersed in 10 ml of 1, 2-dichlorobenzene to prepare the P3HT-CNT solution mixture. The solution mixture has been stirred overnight to prepare the uniform dispersion.

Fabrication of composite film:
For the deposition of thin film, glass substrate was washed in detergent, rinsed in acetone and isopropanol for 10 minutes each and dried under a lamp. P3HT-CNT film have been prepared by spin coating the precursor dispersion over ITO substrate at 600 RPM for 1 min. 40 μl of the above dispersion was spun onto the cleaned glass substrate at 600 RPM for 1 min and dried over a hot plate at 70 0 C. All the chemical processing, deposition and measurements have been carried at room temperature.

Characterization:
X-ray diffraction data of MWCNT, P3HT and its MWCNT based composite recorded from a xray diffractometer (Ultima IV, RIGAKU) using a copper Kα-source (λ=1.54 Å) in the range of 4° to 60° are presented in Figure 1. SEM images obtained using a Nova NanoSEM 450/ FEI are presented in figure  2. Figure 2

Discussion:
The XRD pattern of MWCNT presented in figure 1 (a) reveals a broad peak at 26.6° corresponding to (200) planes of the hexagonal lattice. P3HT and its MWCNT composite reveal that scattering peaks corresponding to 2θ value 5.18° with d-spacing 17.13Å. The peak at 23.8° arising from the higher order reflection from large d-spacing represents the stacking distance of thiophene ring in P3HT [23][24][25][26][27]. With increase in MWCNT content in the composites, the intensity corresponding to (100) planes gradually increases and gradual decreases in (010) planes are observed. Hence the in-plane lamellar structure of P3HT is expected to be distorted while the out of plane inter-chains ordering is improved due to the incorporation of the fillers. FTIR spectra of MWCNT records small O-H bands that correspond to the unwanted residual intercalated water. The peak at 1634 cm -1 due to the C=C stretching vibration reveals hexagonal structure of MWCNT. The peak about 3400 cm -1 in FTIR is due to the adsorbed water or OH functional group [28]. No major peak shift in observed FTIR spectra of P3HT-CNT composites, suggests absence of strong interaction between CNT and P3HT except π-π stacking interaction [27].Employing FTIR spectra intensity ratio conjugation length of P3HT could be evaluated and has been reported by Furukawa et al. [29]. The absorption bands at 2855 cm -1 , 2927 cm -1 and 2950 cm -1 could be ascribed to the -CH 2 out-of-plane mode, -CH 2 in-plane mode and -CH 3 asymmetry mode of P3HT, respectively. The peaks at 1410 cm -1 and 821 cm -1 are the symmetric ring stretching and C-H out-ofplane mode of P3HT [30]. Blue shift in the absorption band at 821 cm -1 in the composite suggests the presence of weaker interaction between MWCNT with P3HT in the composite. The intensity ratio between the antisymmetric C=C stretching peak (1510 cm −1 ) and the symmetric stretching peak (1456 cm −1 ) directly affects the conjugation length of P3HT chain, as reported earlier [31,32]. The increase in absorption peak height at 1510 cm -1 signifies the increase in the conjugation length of the P3HT chain with increase in MWCNT concentration in the composite [31].
The Raman spectra of P3HT, MWCNT and P3HT-CNT [ Figure 4(a)] composite recorded at an excitation wavelength of 532 nm have been presented in figure 4(a). P3HT shows a strong peak at 1449 cm -1 and another at 1378 cm -1 representing the C-C skeletal deformation and C=C ring deformation of P3HT matrix [33]. The Raman active D-band and G-band of MWCNT has been recorded at 1347 cm -1 and 1580 cm -1 , respectively. Incorporation of MWCNT into the P3HT matrix has revealed widening of the D-band and G-band in the composite. From the UV-Vis spectra of P3HT and MWCNT-P3HT composites in 1,2-dichlorobenzene shown in figure 4(b), it can be observed that an absorption peak at 467 nm is observed. Ovsyannikova et al. [34] have reported no significant contribution of MWCNT addition to the absorption spectra of the composite. However, in the present investigation slight red shift of the band has been observed in the nano-composites upon addition of MWCNT to the P3HT suggesting the ground state interaction and appreciable charge transfer between MWCNT and the polymer. This phenomenon can be attributed to the increase in the conjugation length of the polymer chain [31,35].The observed increase in absorbance by incorporation of MWCNTs results in decrease in the torsional deformation in P3HT and thereby increase in the polymer ordering [36].
I-V characteristics of P3HT and its MWCNT composite [ Figure 5(a)] reveals the increase in conductivity by the incorporation of the highly conductive MWCNTs. Enhanced conductivity in the nanocomposite can be attributed to the conductive network created by carbon nanotubes that have high carrier mobility. The wrapping of MWCNT by P3HT in the composite eases the carrier transformation. To demonstrate the potential application of the prepared composite material, cyclic voltammetry measurement has been carried out for pristine P3HT and solution processed P3HT-CNT composite using glassy carbon electrode as working electrode, Platinum as counter electrode and Ag/AgCl as supporting electrode [ Figure 5(b)]. The HOMO level of P3HT was calculated to be 5.197 and that of P3HT-CNT was found to be 5.082. Incorporation of MWCNT in the P3HT matrix decreases the HOMO level of P3HT that

Conclusions:-
P3HT-CNT composite has been prepared successfully by solution processing method and improve in physical properties have been observed compared to pristine P3HT. Structural changes such as change in conjugation length of polymer chain and ordering of the stacked structure of P3HT has been observed due to incorporation of MWCNT into the P3HT matrix .Wrapping of P3HT around MWCNT decreases the agglomeration that accounts for better dispersion of fillers. Weaker π-π interaction between MWCNT and P3HT ease the flow of electrons from P3HT to MWCNT that leads to enhanced electrical properties of the composite. Smaller decrease in the HOMO level of P3HT-CNT composite compared to pristine P3HT might be useful for the improved performance of optoelectronic devices.

Acknowledgement:
The above research work has been supported by SERB, DST, Government of India through grant Number SB/S2/CMP-109/2013.