Research Article |
© 2022 Barham K. Rahim, Fahmi F. Muhammadsharif, Salah R. Saeed, Kamal A. Ketuly.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Rahim BK, Muhammadsharif FF, Saeed SR, Ketuly KA (2022) A study on the optical properties and optoelectronic parameters of Sudan dye doped poly(5-hydroxy-L-tryptophane) and P(TER-CO-TRI) polymers. Modern Electronic Materials 8(3): 85-96. https://doi.org/10.3897/j.moem.8.3.91521
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In this paper, the optical properties and optoelectronic parameters of two newly synthesized polymers forming a donor : acceptor (D : A) binary system are investigated, followed by their subsequent doping with Sudan dye to obtain a ternary system. The donor polymer is P(TER-CO-TRI), while the acceptor is poly(5-hydroxy-L-Tryptophane). A cost-effective solution-processing was carried out to obtain different binary and ternary composites with concentration of 0.5 mg/ml. Optical absorption spectroscopy was used to measure the optical response and optoelectronic parameters, while FTIR and cyclic voltammetry were used to assess the structure and molecular energy levels of the polymers. The results revealed that the non-dispersive refractive index and energy gap of binary D : A was decreased from 1.56 to 1.52 eV and from 2.84 to 2.10 eV, respectively, when it was doped with Sudan dye. It was concluded that with the help of doping process, different values of energy band gap, refractive index, dielectric constant, and optical conductivity are achieved. This tuning achievement of the optoelectronic parameters is crucial in determining the possible applications of these materials in the organic electronics, photodiodes and photovoltaic devices.
poly(5-hydroxy-L-Tryptophane), P(TER-CO-TRI), sudan dye, energy band gap, extinction coefficient, refractive index, dielectric constant, optical conductivity
The study of optoelectronic parameters and physical properties of organic composites for possible applications in electronic and solution-processed optoelectronic devices has aroused tremendous research interest in recent years. This is primarily due to the low cost and flexibility of organic materials along with the low-temperature and large-scale production capabilities of these materials[
The efficiency of organic solar cells (OSCs) has been steadily improved over the past decade. Recently, a power conversion efficiency (PCE) of over 16% has been obtained in the single-junction OSCs [
The organic materials are currently attracting a considerable interest when it comes to the development of organic electronic devices such as transistors, sensors, memory, diodes and solar cells [
In the context of photovoltaic and photocatalysis activities, the interaction of light with materials, which allows photons to be absorbed at different energy levels, has become increasingly significant [
Thiophene-based materials have long been considered among the most promising organic semiconductors for application as active layers in the OSCs [
The contribution of organic materials has been well acknowledged in the application of electronic devices [
Because of the significant characteristics of soft organic semiconductors such as low weight, absorption strength, tuneability, and solution-processability with common chemical solvents, the development of organic-based photodiodes or photodetectors has gotten a lot of interest [
However, to explore the full potential of organic materials and their viability for different emerging applications, a comprehensive study on their optoelectronic parameters, and photo-physical response is necessary. Therefore, the current research paper was devoted to investigate the absorption response, optical energy gap, refractive index, dielectric constant, and optical conductivity of organic composite system made from synthesized electron donating and electron accepting polymers doped with organic dyes.
The host polymers of poly(5-hydroxy-L-Tryptophane) behaves like an electron acceptor and P(TER-CO-TRI) presents an electron donor whose molecular structure shown in Fig.
Fourier transformation infrared (FTIR) spectroscopy was applied to perform the vibrational analysis of the polymers. FTIR spectra can be used to reveal the molecular structure and molecular environment due to vibrational modes [
UV–VIS absorption spectroscopy was used to evaluate the photophysical properties of the two newly synthesized polymers. The polymer solution was prepared by dissolving 0.5 mg of each polymer in 1 ml of dimethyl sulfoxide (DMSO). As known from literary sources, the absorption bands in the UV region can be ascribed to the π–π* and n–π* transitions of delocalized excitons in the polymer chain, whereas the absorption bands in the visible range are assigned to intramolecular charge transfer (ICT) between electron-rich moieties and electron-deficient moieties in the main chain [
. (1)
where t is the thickness of the cuvette (1 cm) and A is the absorbance. All two polymers exhibited a sharp absorption band in the UV region which extended to the visible region. The absorption band for Acceptor poly(5-hydroxy-L-Tryptophane) was prolonged until 419 nm and the absorption band for P(TRI-co-TER) continued until 438 nm, whereas the absorption band for mixed continued till 430 nm. This is where the absorption band for the ternary system of D : A: sudan dye expanded to 580 nm. These indicate that the delocalized excitons’ transition from π–π* and n–π* take place in the polymer backbones for the polymers, whereas the differences of prolonged band in the visible region for the polymers is due to the degree of intramolecular charge transfer (ICT), which is related to the transition of excitons between benzenoid and quinoid rings [
In optoelectronic applications, it is imperative to have the measurement of the optical energy gap and the type of optical transitions in the conjugated polymers when considering the potential application of the polymers. From the absorption spectra, the optical energy gap and optical transition can be found using Tauc’s equation. Furthermore, the absorption edge from the absorption spectrum has been used to determine the optical energy gap, thereby measuring λonset as follows [
. (2)
However, Tauc’s equations can be applied directly to describe the nature of the transition, despite measuring the optical energy gap, i.e. by taking the natural logarithm and deriving Eq. 3,
αhν =α0(hν – Eg)n, (3)
, (4)
where Eg is the energy gap, α0 is the energy-independent constant, h is the Planck’s constant, ν is the frequency of the incident wave, and the value of n determines the type and nature of the transitions [
Absorbance spectra for all synthesized polymers (a–d), plot of (αhν)2 versus E for all synthesized polymers (e) and plot of d ln (αhν)/d (hν) versus hν for all synthesized polymers (f)
There are several parameters that should be considered in designing and optimizing organic photovoltaic devices which include charge transfer and charge collection at the active medium and electrodes. In this respect, electrochemical study provides information regarding the position of the HOMO and LUMO levels of organic materials prior to device fabrication. Cyclic voltammetry (CV) constitutes a reliable method for estimating energy levels from the oxidation and reduction potentials for the corresponding materials. The oxidation and reduction potentials are identified from the onset potential, which is defined as the potential where holes or electrons are initially injected into the HOMO and LUMO levels, respectively, and anodic or cathodic current growth becomes evident [
E HOMO = −(E (onset,ox.Fc+/Fc) + 5.39) (eV), (5)
E LUMO = −(E (onset,red.Fc+/Fc) + 5.39) (eV), (6)
E g Tauc = EHOMO – ELUMO. (7)
Illustrative CVs of the two polymers, versus Fc/Fc+, are presented in Fig.
Polymer | E onset,ox (V) | E onset,red (V) | E HOMO (eV) | E LUMO (eV) | E g opt (eV) |
P(TRIco-TER) | 0.31 | –1.24 | –5.70 | –2.78 | 2.92 |
poly(5-hydroxy-L-Tryptophane) | 0.14 | –2.20 | –5.53 | –3.19 | 2.34 |
Optical constants such as refractive index and extinction coefficient, and their derivative parameters like dielectric constant and optical conductivity, should be considered before applying the materials in photovoltaic devices. The way an electromagnetic wave propagates through materials and how the velocity within a material changes with respect to a vacuum is revealed by studying the refractive index. Furthermore, it constitutes a complex variable, the imaginary part of which indicates the amount of energy lost due to the medium, which is referred to as the extinction coefficient. The absorbance data were used to calculate both refractive index (n) and extinction coefficient (k) using Eqs. 8 and 9 [
, (8)
, (9)
where α is the absorption coefficient and R is the reflectance. They were calculated using Eq. 4 and the following formula R = 1 − T − A, where A is absorbance and T is transmittance and estimated from T = 10−A. Figs
Furthermore, the extinction coefficient (k) designates the loss of the incident photon due to scattering and absorption within the medium. Noticeably, the variation of (k) is almost comparable to the corresponding absorption coefficient (Eq. 9) [
The optical dielectric constant (ε) represents a frequency-dependent parameter and indicates the electronic response to the incident photon in the material. Meanwhile, the dielectric constant is a complex function and its real part is assigned to polarization upon the impact of an electromagnetic field whereas the imaginary part represents the optical loss and is described by the following equations [
ε = ε1 + iε2, (10)
ε1 = n2 – k2, (11)
ε2 = 2nk, (12)
(13)
where ε1 represents the real part and ε2 represents the imaginary part of the dielectric constant. Fig.
Fig.
Interestingly, with the help of poly(5-hydroxy-L-Tryptophane) (n = 1.416) dopant it is possible to increase the refractive index of P(TRI-co-TER) from (n = 1.39) to (n = 1.586) than (n = 1.52) in the Acceptor : Donor (1 : 2) system (binary) than this Acceptor 1 : 2 Donor : 2 sudan dye system. The refractive index peak shifts to the blue with increasing poly(5-hydroxy-L-tryptophan) content, reflecting the expectation of a corresponding decrease in the energy gap.
Furthermore, the real and imaginary components of optical conductivity (σ* = σr + iσi) can be investigated using the following formula:
σr = ωε0εi, (14)
σi = ωε0εr. (15)
Where σr is the real optical conductivity, σi is the imaginary optical conductivity, ω is the angular frequency and ε0 is free space permittivity (8.85 · 10–12 F/m). Fig.
Dielectric constant spectra for the synthesized polymers; (a) real part and (b) imaginary part
Refractive index spectra of the pristine of the donor, acceptor, binary and ternary systems
The studied optoelectronic parameters of the poly(5-hydroxy-L-Tryptophane), P(TRI-co-TER), binary and ternary systems
System | n | εr | σr · 10−4 (S/cm) |
P(TRI-co-TER) | 1.390 | 2.86 | 2.43 |
poly(5-hydroxy-L-Tryptophane) | 1.416 | 1.99 | 3.60 |
Donor : Acceptor (1 : 2) | 1.560 | 2.86 | 2.64 |
Donor : Acceptor : Dye (1 : 2 : 2) | 1.520 | 2.69 | 2.32 |
A broad spectrum of investigation on the optical properties and optoelectronic parameters of P(TER-CO-TRI), and poly(5-hydroxy-L-Tryptophane) along with their doping with sudan dye was successfully performed. Optical spectroscopy has been found to be extremely effective in measuring the optoelectronic parameters of binary and ternary composites of polymeric materials and dye. It was concluded that with the help of doping process, different values of energy band gap, refractive index, dielectric constant, and optical conductivity are achieved. This tuning achievement of the optoelectronic parameters is crucial in determining the possible applications of these materials in the organic electronics, photodiodes and photovoltaic devices.
Barham K. Rahim would like to Mr. Peshawa O. Amin for his ongoing support during the experimental setup and data collection process. Kamal A. Ketuly thanks the Erasmus+ scheme for facilitating collaboration between the University of Duhok and the University of Glasgow.