The Influence of Spin Coating Speed on the Optical Properties of P3HT Thin Film

Spin coating technique has been widely used in the fabrication of thin films due to its ease of use, inexpensive processing cost and providing control over the thickness of the film. Various polymers have been used in fabricating spin-coated thin film and poly(3-hexylthiophene) (P3HT) is one of the prevalent polymers utilized due to its semiconducting properties and flexibility in processing. In this study, P3HT is deposited on pre-cleaned quartz glass by the method of spin coating. The spin coating speed is varied from 500, 1000, 1500, 2000, 2500 rpm and the optical properties of the films are characterized by using a UV-Vis spectrophotometer. The UV-Vis absorption spectra of P3HT designated absorption peaks in the range of 520 nm to 550 nm. It is shown that the absorbance of P3HT is decreased with every increment of the spinning speed. Thus revealing the dependency of P3HT thin films’ optical properties on spin coating speed.


Introduction
Electronic industries frequently use basic methods in depositing solutions which are spin coating, dip coating, spray coating, and film deposition.In spite of there were several methods to use, the spin coating was chosen for this study.A technique of applying an even and uniform film onto a solid surface such as quartz glass, indium tin oxide coated glass, or silicon is known as the spin coating process.The process is generally involving the presence of centrifugal force and a liquid-vapour interface [1].The solution is being spread onto a substrate that is caused by the centrifugal force of the rotation.The coating of the substrate is completed after the rotation and hence, produces a film.A number of experimental parameters influenced the thickness of films, which are solvent evaporation rate, speed of the spinning [2], rotational time, the concentration of the solution or solid [3], and the viscosity of the coating solution.
These influential parameters are vital to be paid attention to in manipulating the film fabrication [4].to process, it is also economical, superior thermal, and environmental stability [5].P3HT is easy to process as it is soluble in many types of solvents and makes it inexpensive to process [6].P3HT is also a semiconductor polymer and polythiophenes comprise regioregular which is a combination of the different couplings and acts as a light-absorbing donor material [6].
In this study, P3HT is being used as the material to be deposited on the substrate and the effect of multiple spinning speeds on the optical properties of P3HT is studied.It is proven that different spinning speeds result in different absorbances of the material.Variations of spinning speed alter centrifugal force and therefore change the absorbance of material as well as the thickness of films.The absorbance of P3HT was increased as the spin coating speed was reduced and film that spin coated at only 500 rpm had the highest absorbance peak value (0.9 a.u.) while as the rotational speed escalated to a final value of 1000 rpm, the peak dropped to 0.5 a.u.[8].
Research has demonstrated that the absorbance of P3HT can be enhanced by reducing the rotation speed from 1000 rpm to 500 rpm [9].However, low rotational speed does not constantly increase the absorbance of material as it is evidenced that the highest absorbance value (2.38 a.u.) of zinc oxide was recorded at the speed of 4000 rpm and the absorbance value diminished to 0.95 a.u. at 2000 rpm [10].These show that rotational pace is important and able to determine the optical properties of P3HT and also affect the morphology of film as well as the band gap value.It has shown that the aggregates of P3HT increase when the speed upsurges and thus altering the structure of the material [11].The intensity of the centrifugal and shear force was changed by the spinning speed that varies film thickness and thus, impacting the film coarseness and crystalline structures [12,13].In addition, P3HT molecular orientation varies with the rotational speed which affects the morphology of regioregular P3HT [14].This study concentrates on the insight into the different spin coating speed that affects the optical properties of P3HT thin films.The optical properties of the films in analysed using different spinning speeds.
Film Preparation.Optical grade fused quartz glass (76.2 mm × 25.4 mm × 1 mm) is being used as the substrate and cleaned before the deposition process.It was soaked and slightly scrubbed in a decon bath and was cleaned by sonication for a few steps.The quartz was sonicated for 10 minutes in acetone directly afterwards and was then sonicated in DI water for 5 minutes.Then, it was sonicated in propanol for 10 minutes to cleanse it from any debris of acetone.The substrate was sonicated in DI water for another 5 minutes prior to drying with nitrogen gas.
After the cleaning process, the substrates were exposed to HMDS overnight before deposition.
P3HT solution (5 mg/mL) with chloroform as the solvent was then spin coated onto quartz at different rotational speeds involving 500, 1000, 1500, 2000, and 2500 rpm. Figure 2 shows the illustration of the spinning process as the solution is deposited at the centre of the substrate and is rapidly rotated according to the previously fixed rotational speed to fabricate uniform and even films that generally possess a thickness of 10-100 nm.The coating process was executed using a WS-400-6NPP-LITE spin processor purchased from Laurell Technologies Corporation.The system housing of the coater is able to resist strong acids and bases since it is manufactured from a solid co-polymer and hence, making the cleaning process of the coater easier and uncomplicated.The resulting films from the post-spin coating process were shown in Figure 3 and were set for optical properties characterization.Characterization.JASCO V570 UV-Vis (Ultraviolet-visible) spectrophotometers in the 350 -800 nm wavelength range were utilized to characterise the thin films' properties and operated at a resolution of 0.5 nm.The characterization will represent the absorption spectra of the elements and the spectrometer functions as light passes through the grating diffraction.

Results and discussion
The P3HT thin film was spin coated at various rotational speeds which are at 500, 1000, 1500, 2000, and 2500 rpm. Figure 4 shows absorption spectra for films with different spinning speeds and each film exhibits dissimilar peaks of absorption.The absorption peak of film spin coated at 500 rpm is the highest (0.92 a.u.) and the maximum absorption was recorded at 520 nm.However, the peak absorption slightly shifted to 550 nm for 1000 rpm and the absorbance descents to 0.65 a.u.Films spun at 1500, 2000, and 2500 rpm retain absorption peaks at the same wavelength which is at 525 nm.The absorption peak at 525 nm is due to π-π conjugated chain transition and the vibronic structures from the P3HT polymer molecules aggregation [15].Additionally, it also has been stated that the P3HT possess an absorption peak of 525 nm with vibronic shoulders at 550 and 605 nm [16].
The presence of vibronic structures at 520-610 nm is an indicator that well-arranged structures are developed.Thus, it is confirmed that the structures were organized during the coating process.Meanwhile, the absorbance value dropped significantly to 0.45 a.u.for the film coated at 2500 rpm.Even though the peak absorption of P3HT is affirmed to be at 525 nm in a study, the maximum absorption can extend from 450 nm and the emission peak appears at 570 nm [17] and regioregular P3HT displays a peak maximum at 454 nm [6].In figure 4, the blue-shifted part of the spectrum is ascribed to the formation of intrachain disarrangement, assumed to be connected to chains in the amorphous part of the film.
While on the contrary, the red-shifted section is designating the insubstantial interaction of H-aggregates in crystalline regions [6].The absorption spectra acquired have shown peaks of 520-550 nm, indicating an intrachain transition of P3HT.However, two peripheral shoulders were not visible.The shoulders are usually detected at 558 and 603 nm and are an indicator for the formation of interchain π-π stacking interactions [18].
The Tauc relation method is being used to calculate the value of the energy bandgap.
The onset wavelength (λonset) is calculated as; h is a Planck constant and c represents the speed of light.Afterwards, the absorption coefficient of the thin film is calculated by using the expression of Beer-Lambert's law; The absorbance is denoted as A and l is the thickness of the film.The plot of αhv against hv is generated by using the expression of the Tauc relation [19]; Eg is the bandgap energy of the material, n is a constant value that stands for the nature of the electronic transition that were distinguished into four types,  = 2 (direct allowed transition),  = 1/2 (indirect allowed transition),  = 2/3 (direct forbidden transition), and  = 1/3 (indirect forbidden transition).The forbidden transitions are rare as allowed transitions usually dominate the absorption process which produces direct or indirect transitions.Plotting the (αhv)  versus hv is a final phase to determine whether  = 2 or  = 1/2 is giving a better fit in order to identify the correct transition type.The energy bandgap value is approximated from the extrapolation of a linear portion on the energy axis of the αhv against hv.The film coated at 500 rpm yields 1.98 eV while at 1000 it exhibits 1.93 eV which are indicating the common bandgap value for P3HT.P3HT is an organic semiconducting material with a bandgap value of approximately 1.90 eV [20,21].It is well-proven that the energy bandgap for semiconductors is lower than 3 eV [22] and for electronic insulating materials are ranging from 3 to 3.5 eV [23].The bandgap value decreased to 1.93 eV when the spinning speed was increased to 1000 rpm, and escalate to 1.99 eV at 1500 rpm.When the spin coating speed increased from 1000 rpm to 1500, 2000, and 2500 rpm, the bandgap value also gradually surges. 5th

Conclusion
P3HT thin films were fabricated on a quartz glass and the spinning speed during the P3HT solution was varied into five different rotational speeds.The effect of spin coating speed on the optical properties of P3HT and the results of the absorption spectrum obtained were aligned with the predicted outcomes.According to the data, the absorbance value of P3HT decreased as the spinning speed increased.A film spun coated with the speed of 2500 rpm has the lowest absorbance value (0.45 a.u.) and when coated at the reduced speed to 500 rpm resulted in the highest absorbance value (0.92 a.u.).It is concluded that the spin coating speed affects the optical properties, particularly the absorbance of P3HT thin film.

5th
photovoltaic (PV) cells and it is easily processed for thin film fabrication by spin coating and other techniques such as printing, Langmuir-Blodgett, and film casting.Apart from being uncomplicated

Figure 2 .
Figure 2. The schematic presentation of the spin coating process.

Figure 3 .
Figure 3.A schematic diagram of the P3HT thin film.

Figure 4 .
Figure 4.The absorption spectra of P3HT thin film with non-identical rotational speeds.

Table 1 .
The absorbance value and absorbance peak of P3HT thin film with spinning speed are varied.

Table 2 .
The energy bandgap values of the thin film spun coated at different spin coating speeds.

Table 2
is presenting the energy bandgap value that was generated by each thin film.