Manufacture and Photoelectrical Characterization of Poly(3-decylthiophene) Thin Films by Drop Casting Technique

Abstract

Poly(3-decylthiophene) is a polymer with conductive characteristics due to its conjugated polymeric chain. With the search for new applications and improvements of these materials, in sensors, and OPVs, there is a great demand for deeper knowledge about them. This work aims to characterize the P3DT, through analysis of optoelectrical measurements. Using the drop casting technique, thin films were made onto solid substrates. The films were subjected to optical characterization by UV-Vis and optical microscopy. The electrical characterization was obtained by IxV curves and then measuring the photoconductivity through Ixt curves, with a solar simulator. With UV-Vis measurements, it was observed that the absorption of light in the visible spectrum reached a peak of 520 nm in the film, blue-shifted in the solution attributed to the differences in the organization of the polymer chains. The optical microscopy measurements indicate the formation of aggregates with a higher concentration of aggregates observed for the film obtained with a more concentrated solution. Finally, the photoconductivity measurements carried out obtained a positive response to the photo-excitation of the material due to exposure to light, with an increase in current in the film as the photo-exposure cycles were repeated.

Keywords:
Drop casting; Chloroform; Optoelectronic properties

1. Introduction

Since the discovery of the high conductivity of doped polyacetylene made by Heeger, MacDiarmid, and Shirakawa in the 70s, the door was opened for the synthesis and study of the characteristics of a wide variety of materials identified by the π-conjugated polymeric chain that is responsible for their electrical and optical properties¹. Most conducting polymers are generally structured by alternating single and double bonds throughout their main chain, with a delocalized electron structure². Due to this structure, the π electrons can be delocalized along the polymer’s backbone chain, leading to one unpaired π electron per carbon atom³ creating the possibility of charge transport along the polymer chain, culminating in the polymer’s conductive behavior and semiconductor characteristics⁴,⁵.

The unique properties of these conjugated polymers draw attention based on their relative ease of processing, making it possible for such materials for a wide range of applications as technologically superior and more cost-effective alternatives⁶. The applications of these materials include areas such as the development of batteries⁷, transistors⁸, supercapacitors⁹, organic light emitting diodes (OLEDs)¹⁰, sensors¹¹-¹⁴, solar cells¹⁵, and others.

Representing the most classic type of conjugated polymers we can emphasize the polythiophene and its derivatives, which have been widely studied in recent decades¹⁶. This class of polymers is attractive for device applications¹⁷ due to their high stability in undoped states, can be structurally modified, and are easy to process in solution form¹⁸.

With advancements in polymer synthesis techniques, the first synthesis route for polythiophene was developed in 1980¹⁹. Due to their excellent stability, polythiophenes can be processed into various types of devices. As a conjugated polymer, the polythiophenes show strong π-stacking interactions between its thiophenic rings, which leads to low solubility. Attaching side chains to the main chain of the polymer promotes the increase in solubility²⁰. The first poly(3-alkyl thiophene) was synthesized in 1985, obtaining a material with good solubility, with regioregularities varying from 50 to 80%. Studies demonstrate that cyclic polythiophene oligomers present unique properties concerning their derivatives²¹ which highlight the interest in proposing new polythiophene derivatives. Furthermore, the asymmetry of the polythiophenes' backbone chain results in varying regioregularity, culminating in the possibility of several isomeric materials²².

Moreover, the optoelectronic properties of the polythiophenes and their derivatives create the opportunity for a plurality of studies revolving around their applicability as photodetectors, as demonstrated by an extensive number of works based on the optical and electrical characterization of such materials¹,²³. The high values of conductivity shown by these materials when suffering photo-excitation bring the possibility of fabrication of organic solar cells with increasing efficiency¹⁸,²⁴.

In this work, we study the optical and electrical characterization of drop casting films of poly(3-decylthiophene-2,5-diyl) (P3DT). The measurements were taken using glass laminae as substrates for the optical and morphological characterizations and using a gold interdigitated electrode for measurements of the current versus time with applied voltage at room temperature. The effect of different concentrations on the electrical conductivity of P3DT is investigated, indicating the potential for its application as a photosensor.

2. Methodology

2.1. Materials

In this work, a regioregular polythiophene poly(3-decylthiophene-2,5-diyl) (P3DT), acquired commercially from Sigma-Aldrich, CAS number 110851-65-5, product number 495344, was used in the study of its optoelectrical properties. Figure 1 presents the condensed chemical structure of P3DT.

Figure 1
Chemical structure of the P3DT.

Glass laminae substrates were used for the optical and morphological characterizations. The conductivity and optoelectrical measurements were possible using gold interdigitated electrodes (IDE), fabricated with photolithography onto glass substrates. Au-IDEs consist of electrodes containing 50 digits with dimensions of 110 nm in height, 8 mm in length, 100 μm wide for each digit, and spacing of 100 μm between each digit.

The Au-IDE electrodes are a vital part of this study since each pair of digits amplifies the total current measured²⁵, facilitating the electrical characterization of the active layers deposited as films over the IDEs, which may present too low conductivity otherwise. To prepare the solutions used in the confection of the drop casting films, the solvent CHCl₃ (99.9%), obtained from Synth, was selected due to the good solubilization of polythiophenes derivatives in such solvent.

2.2. Drop casting films

The drop casting technique is one of the simplest processing methods for obtaining polymeric films from solution²⁶. It consists of the dropping of the solution over the horizontally stable substrate, using an electronic pipette. Then, after all the solvent has evaporated, the thin film of the material is formed on the substrate, its molecules remaining linked by Van der Waals forces²⁷. Although this is a quick way of obtaining a thin film of a polymer such as polythiophenes, It does not allow for fine control over the film formation, concerning the film's organization. During the evaporation of the solvent, the formation of aggregates is facilitated, resulting in films with a heterogeneous surface.

Also limited by this technique is the control over the thickness of the films. Moreover, the evaporation of the solvent is a really important part of the film formation, for the film may be dried out naturally or by some kind of thermal process, which accelerates the evaporation. This, however, may cause too much disturbance in the system during evaporation culminating in a direct impact on the film morphology²⁸,²⁹.

For the confection of the films, a volume of 200 µL of the solution of P3DT (1.5 mg/mL and 2.0 mg/mL) was dropped onto the substrates (glass laminae and Au-IDE electrode), positioned over a horizontally stabilized support. The films then were left to dry out naturally at room temperature, with air humidity of approximately 30%, for 24 hours. This time was chosen to ensure the complete evaporation of the solvent.

2.3. UV-vis spectroscopy

In studying the electronic transitions that happen in the UV-Visible range, it is possible to assert a wide variety of characteristics presented by the materials, like the internal structural organization of a film³⁰, and the orientation of the molecules of the material on an organized thin film³¹,³². The UV-Vis spectroscopy measurements were realized using the Agilent Technologies model Cary 100 UV-Vis spectrophotometer, utilizing incident light comprehending a range of 900 to 350 nm. Wavelengths lower than 350 nm are avoided since the glass of the substrates is opaque in ultraviolet light. The intensity of light absorbed by wavelength is measured by a sensor located just behind the samples.

2.4. Electrical characterization in direct current

Curves of Current versus Voltage (I vs V) were measured utilizing a Keithley Instruments model 238 Source Measure Unit, gauged in a range from -10 V to 10 V, in a controlled temperature at 23 °C. The electrical conductivity was calculated by applying Ohm’s law equations in addition to the employment of the geometric factor of the Au-IDE electrode (cell constant), which was determined in previous works from the research group, following Olthuis’ method³³,³⁴. The cell constant has a value of 5.1 m^-1.

2.5. Optoelectronical characterization

In order to investigate the photoconductivity of the P3DT drop casting film, the Current versus time curve (I vs t) was obtained, utilizing a voltage font Keysight B2912A Precision Source Measure Unit (SMU), in addition to the Oriel VERASOL solar simulator. The measurements were made with illumination calibrated in 199 mW.cm^-2 (AM 1.5), applying intervals of 5 minutes of illumination and 5 minutes with the sample in the dark.

3. Results and Discussion

Figure 2 exhibits the absorbance spectra per wavelength obtained via UV-Vis absorption measurements for P3DT drop casting film (2.0 mg/mL), with 200 µL deposited over the glass substrate, as well as the P3DT solution (2.0 mg/mL).

Figure 2
UV-Vis spectra obtained from P3DT solutions and drop casting films.

It can be noted that the absorption for both is in the visible spectra. For the drop casting film, the absorption peak at 450 nm, is related to π - π* interactions³⁴. The spectra of the drop casting film are redshifted with a peak at 520 nm, this difference is attributed to the higher organization in the films in comparison to the solution. It is also possible to see two additional peaks in the film spectra, at 557 nm and 605 nm. The peaks at 520 nm and 557 nm are attributed to intrachain excitation, while the peak at 605 nm can be related to the interchain absorption³⁵,³⁶. Moreover, the difference in the intensity between drop casting film and solution is directly proportional to the amount of material.

Figure 3 shows the surface morphology of the P3DT drop casting films, produced by the deposition of 200 µL of poly(3-decylthiophene-2,5-diyl) in solution with concentrations of 1.5mg/mL and 2.0 mg/mL, and with different amplifications.

Figure 3
Optical microscopy images of P3DT drop casting film with different amplifications.

It is possible to note the presence of aggregates in the first two magnifications for the optical microscopy images of the drop casting film produced using the 1.5 mg/mL solution, represented by dark dots along the surface of the film, in the third amplification, however, it is virtually impossible to distinguish any aggregates on the film. In the drop casting film produced using the 2.0 mg/mL solution, however, several aggregates can be noted in the three magnifications. The formation of various aggregates is a direct consequence of the usage of the drop casting technique since this technique doesn´t offer the possibility of morphology control during the film formation²⁶,³⁷. Additionally, the higher concentration of the P3DT solution employed resulted in a film with high thickness, and difficulting the obtention of a good focus during the OM measurements.

Figure 4 shows the electrical characteristic curve (Current versus Voltage) carried out for the P3DT drop casting films deposited on the Au-IDE electrode, with the solutions of 1.5 mg/mL (black) and 2.0 mg/mL (red).

Figure 4
Electrical characteristic curves (I vs. V) of the P3DT drop casting film.

The linearity of the results shows the ohmic behavior of the P3DT drop casting film, which is a typical characteristic of such a configuration of its contacts (Au/active layer/Au)³⁸,³⁹. The ohmic pattern shown in the I vs V curve allows the applicability of Ohm’s laws to calculate the electrical conductivity of the films. The value of conductivity calculated was 3.50 x 10^-6 S/m for the drop casting film obtained using the 1.5 mg/mL solution, while the one manufactured using the 2.0 mg/mL solution reached a value of 2.57 x 10^-6 S/m, indicating a slight lower conductivity for the higher concentration solution, which might be a consequence of the difference in organizations achieved in both films.

The curve of Current versus time for the photoconductivity response of the P3DT drop casting films with 1.5 mg/mL (red) and 2.0 mg/mL (black) is exhibited in Figure 5.

Figure 5
Photoconductivity response of the P3DT drop casting film.

It is possible to observe in the photoconductive curve of the P3DT drop casting films, that the films respond to light in such a manner that the current measured always rises when the material is exposed to light. This indicates the formation of charge carriers, originating from the photoexcitation of the polymer, promoting the decrease of the drop casting film’s resistance, as it is already expected for polythiophene derivatives when exposed to light in similar conditions³⁴,⁴⁰. It is also possible to note that for each repetition of the photoexcitation cycle, the current assumes a higher value, for both concentrations, which may indicate the possibility of the trapping of charge carriers in structural defects along the morphology of the film. Furthermore, the concentration of the solution used in the drop casting processing technique, seems to have influenced the overall increase in current when comparing both films, since the drop casting film made using a solution of 2.0 mg/mL reached higher values in current compared to the 1.5 mg/mL film.

4. Conclusions

Through this study, it was possible to carry out characterizations of P3DT thin films made using the drop casting technique and deposited on glass laminae and gold interdigitated substrate (Au-IDE electrode) for two different concentrations, 1.5 mg/mL and 2.0 mg/mL. By carrying out UV-Vis absorption measurements, an absorption band with peaks of 520 nm and 450 nm was obtained, for the casting film and the solution, in that due order, exposing a red-shift that indicated in the thin film the presence of a better molecular organization compared to the solution. Concerning optical microscopy, an indication of the material's tendency to form polymeric agglomerates in the film during the evaporation of the solvent was observed, with an apparent higher density of agglomerates obtained for the higher concentration of the P3DT solution. The ohmic character of the P3DT drop casting film was shown in the Current vs Voltage measurements presented, making possible the calculation of the conductivity of the thin films at 3.50 x 10^-6 S/m for the film produced using the 1.5 mg/mL solution and 2.57 x 10^-6 S/m for the film made with the 2.0 mg/mL solution. Finally, the photoconductivity measurements that were carried out obtained a positive response of the material to light exposure, showing higher values of photocurrent obtained for the film made with the higher concentration solution and with the photocurrent presented by the drop casting films rising at each exposition cycle.

5. Acknowledgments

This research was funded by São Paulo Research Foundation (FAPESP), and National Council for Scientific and Technological Development (CNPq, INEO). The authors are grateful to the Brazilian Nanotechnology National Laboratory (LNNano/CNPEM, LMF project 2023090) for supplying the gold interdigitated electrodes. This study was financed by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - Brazil (CAPES - Finance cod. 001).

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Publication Dates

• Publication in this collection
  30 Aug 2024
• Date of issue
  2024

History

• Received
  30 Apr 2024
• Reviewed
  11 July 2024
• Accepted
  23 July 2024