With the aim of producing simple and effective transparent conducting electrodes, the conducting polymer poly(3-hexylthiophene) (P3HT) incorporated with reduced graphene oxide film (rGO) (called rGO-P3HT) was prepared by spin-coating method. Structural, electrical and optical characterization showed that rGO-P3HT films 9.0 wt% P3HT exhibited good stability when exposed to the ambient atmosphere. These composite films of 200 nm thickness possess a sheet resistance and transparency of R<sub> □ </sub>~ 17 Ω and T ~ 72%, respectively. Owing to containing conducting polymer, rGO-P3HT-coated glass could be efficiently used in photovoltaic applications, in organic solar cells in particular, with the replacement of the indium tin oxide (ITO) and fluorine tin oxide (FTO) electrodes.
Transparent conducting electrodes (TCEs) have received increasing interest due to their various technological applications such as electrochromic display (ECD) [
In this work we present new results obtained in the investigation on the optical and electrical properties of rGO-P3HT composite films made by a simple spin-coating technique and we would like to suggest a potential application of rGO-P3HT coatings for transparent conducting electrodes in organic solar cells.
Preparation of rGO solutions was followed by the procedure described elsewhere [
Mixtures of 0.2 g GF, 0.3 g NaNO3 and 8.5 ml H2SO4 were put in a glass beaker, then 1.5 g KMnO4 and 30 ml of distilled water was poured into the glass beaker to get a liquid solution. Next, 12 ml H2O2 was added to this solution and ultrasonically stirred at room temperature for 10 hours to separate MnO4− and MnO2 into Mn+ ions, yielding a solution with a bright-yellow color. The obtained solution was unmoved for 24 hours waiting for a paste-like layer (with dark-yellow color) deposited at the glass beaker bottom, constituting rGO paste. By slowly sucking, the liquid solution above the rGO paste was completely taken off the beaker. Finally, 18 mg of rGO paste was diluted in 100 ml of N,N-dimethylformamide (DMF) solvent in 200 ml-volume glass beaker and ultrasonically stirred for 30 min for completely dispersing rGO in DMF (rGO-DMF). After 24 hours waiting for the solution stabilization, 60 ml of rGO-DMF solution from the glass beaker top was taken and kept in another beaker for further use.
P3HT with purity of ≥99.9% were purchased from Sigma-Aldrich Corporation. P3HT powder with a volume of 6 mg was mixed in 1.2 ml of rGO-DMF solution. This solution was ultrasonically stirred for 2 hours at room temperature. For all the volumes of chemicals of P3HT and rGO, a content ratio of P3HT:rGO chosen was ranging from 3.0 to 12.0 wt%. (Namely, the volume content of P3HT embedded in rGO matrix was 3, 5, 7, 9 and 12 wt%). Using spin-coating, rGO-P3HT solutions were deposited onto Corning-247 glass sheets of 10 mm ´ 10 mm in size. The following experimental parameters were used for coating: a delay time of 100 s, a rest time of 50 s, a spin speed of 2000 rpm, an acceleration of 650 rpm, and finally a wating time of 3 min. To dry the composite films, a flow of dried gaseous nitrogen was used for 8 hours. For a solidification avoiding the use of solvents, the film samples were annealed at 150˚C for 3 hours in a “SPT-200” vacuum drier.
The thickness of the films was measured on a “Veeco Dektak 6M” stylus profilometer, all samples chosen for further investigation have thickness of 200 nm. The electrical properties (sheet resistances) of composite samples were measured by four-point probe method. Atomic force microscope (AFM) images were obtained using a Belarusian “NT 206” atomic force microscope operating in a tunnel current mode.
Crystalline structure of the films was characterized by using D-5000 Siemens X-ray diffractometer (XRD). Transmittance spectra were measured on JASCO V-570 spectrophotometer using using “baseline elimination” from the substrate (Corning-247 glass) absorption and the sheet resistance of the samples were measured on a “KEITHLEY 2602” system source meter using four-point probe measurements.
The results of four-point probe measurements showed that sheet the resistance (R) of the rGO-P3HT was strongly dependent on the P3HT content (C) as shown in
Although the value of the sheet resistance of the composite film (i.e. 17 Ω) is still large in comparison with the one of the commercial ITO films (less than 10 Ω), it is quite comparative to the sheet resistance of AgNW-rGO films made by dip-coating technique as reported in [
A XRD pattern of a rGO-P3HT film is shown in
The full width at half maximum (FWHM) of these peaks is rather large. This shows that both the P3HT and rGO were formed in nanocrystalline particles. Using Scherrer formula, the calculated particles size (τ) can be found:
τ = 0.9 λ β × cos θ (1)
where λ is the wavelength of the X-ray used for incident radiation. In our experiment Cu-tube was used, thus λ = 0.15406 nm, β the FWHM (in radians) and θ is the diffraction angle of the considered peak (namely Bragg angle) [
From the XRD pattern in
The typical transmittance spectrum of the rGO-9.0 wt% P3HT film is shown in
The largest transparency (~80%) is observed at 400 nm, at 550 nm (the largest sensitivity of human eyes) it is about 72%; and the optical absorption edge―at around ultra-violet wavelengths. This is due to the strong absorption of rGO at a range from 240 to 275 nm that caused by π-π* transition of aromatic C-C rings and n-π* of C-O bond [
In this work, we used the UV-Vis spectra data to estimate the energy bandgap (Eg) of the composite film. For rGO combined with the conducting polymer (P3HT), Eg is the gap between the highest occupied molecular orbitals (HOMO) and the lowest unoccupied molecular orbitals (LOMO). It can be determined by using the expression [
α ( ν ) h ν = A ( h ν − E g ) n (2)
where h is Planck's constant, ν is the frequency of the incident UV-Vis radiation, A is a constant and n is 1/2 for direct band semiconductors and 2 for indirect band gap semiconductors. In case the reflectance of the films is ignored, the frequency dependence of α(ν) can be calculated from the experimental optical transmittance spectra (T), using following relation [
α ( ν ) = 1 d ln ( 1 T ) (3)
In our experiments, for the rGO-P3HT film the best fits were found for n = 1/2 (corresponding to the direct band). The hν-dependence of (αhν)2 is plotted in
Two importance parameters of a TCE are transmittance (in visible range of wavelengths―T) and sheet/square resistance (R). For our rGO-P3HT films, at room temperature and wavelength of 550 nm the sheet resistance and transparency of R ~ 17 Ω and T ~ 72% were obtained, respectively. Thus, for the 200 nm-thick composite film, the conductivity was found to be of σ = 3.0 ´ 105 S/m. With such a large conductivity and a high transmittance, rGO-P3HT-coated glass can be used as TCEs for optoelectronic devices like OLED and ECD. Especially for OSC, because rGO-P3HT films contain a conducting polymer (namely P3HT) which is commonly used as an efficiently photoactive layer in organic solar cells [
A simple and effective transparent conducting electrodes (TCEs) based on reduced graphene oxide film embedded with conducting polymer P3HT (rGO-P3HT) was developed. The rGO-P3HT composite electrode was fabricated by using a simple spin-coating method. This solution processed TCEs showed small sheet resistance (large electrical conductivity). rGO-P3HT TCEs also exhibited good stability when exposed to the ambient atmosphere. The optimum concentration of P3HT embedded in rGO was found to be of 9.0 wt%. The sheet resistance and transparency of the composite film attained a value of R ~ 17 Ω and T ~ 72%, respectively.
Our results provide a possible way to replace the indium tin oxide (ITO) or fluorine tin oxide (FTO) with rGO-P3HT films in the spin-coating technique, and make it promising to produce composite films for future flexible photovoltaic applications.
One of the authors (L. M. Long) expresses his sincere thanks to University of Engineering and Technology (VNU Hanoi) for support during his Ph.D. student research.
Long, L.M., Long, D.D., Nam, N.P.H., Tinh, N.T., Chizhik, S.A. and Dinh, N.N. (2018) Preparation and Characterization of Reduced Graphene- P3HT Composite Thin Films for Use as Transparent Conducting Electrodes. Materials Sciences and Applications, 9, 464-472. https://doi.org/10.4236/msa.2018.95032