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Abstract
In this paper, the complex dielectric function of 2,5-bis(N,N-dibutyl-4-aminophenyl) thiazolo[5,4-d]thiazole is reported. Thin films of this material were obtained by spin coating on a silicon substrate. The samples were investigated using spectroscopic ellipsometry in the spectral range from 354 nm to 1907 nm at multiple angles of incidence. The ellipsometric data were analyzed using a stratified-layer model composed of a thiazolothiazole thin film, a native SiO2 oxide, and a Si substrate. The model dielectric function of the thiazolothiazole thin film was modeled using a series of Tauc-Lorentz and Gaussian oscillators. The best-model calculated data reproduces the experimental data very well. The bandgap of TTz is reported and found to be in good agreement with density functional theory calculations reported earlier.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Materials exhibiting multifunctional electro-chemical and optical properties like electrochromism, electrofluorochromism, photochromism and solvatofluorochromism are ubiquitous in contemporary technologies and can be found for instance in energy-conversion devices [1], molecular sensors [2], photochromic lenses [3], and biological imaging techniques [4]. Recently, thiazolo[5,4-d]thiazole (TTz), a class of highly fluorescent, solution-processable, heterocyclic aromatic compounds has gained considerable attention due to its reversible and high contrast multifunctional chromogenic properties [5–7]. The strong thermo-oxidative and photochemical stability of the TTz-based materials, are attractive for numerous organic optoelectronic devices [8]. For instance, TTz has been identified as a suitable material for the fabrication of organic solar cells [9], fluorescent sensors [10,11], and even redox flow batteries [12]. The design of TTz-based optoelectronic devices requires the accurate knowledge of the complex dielectric function. Several investigations have reported on the optical properties of fluorescent heterocyclic compounds with TTz backbone [13–15]. However, the complex dielectric function of TTz has not been studied yet.
In our manuscript, we report on ellipsometric measurements of the complex dielectric function of TTz thin films prepared using spin coating. A parameterized dielectric model function composed of a series of Tauc-Lorentz and Gaussian oscillators was developed. This model dielectric function accurately reproduces the experimental ellipsometric data within the measured spectral range from 0.65 eV to 3.5 eV (354 nm to 1907 nm). The bandgap of thiazolothiazole is identified using a Tauc plot and it’s found to be in good agreement with density functional theory calculations which have been previously reported [7].
2. Experiment
2.1 Sample preparation
Solution-processable 2,5-bis(N,N-dibutyl-4-aminophenyl)thiazolo[5,4-d]thiazole (TTz) dyes for thin film organic electronics applications were synthesized by refluxing 4-pyridinecarboxaldehyde, 4-(dibutylamino)benzaldehyde, and dithiooxamide in 40 mL of anhydrous dimethylformamide for 6 hours at 120$^{\circ }$C. The reaction solution was chilled overnight at 10 $^{\circ }$C. Crude precipitate was collected using vacuum filtration and was rinsed with dimethyl sulfoxide and water. The isolated product was purified using silica gel column chromatography (Silica Flash M60) with a 1:1 hexane/chloroform mixture. Si wafers were used as substrates. Prior to the spin coating of the TTz-films, the substrates were sonicated in acetone, deionized water, and isopropyl alcohol for 15 minutes. The substrates were dried with compressed nitrogen gas and treated with UV / ozone for 15 minutes. The Si substrates and a 16.1 g/L 1,2-dichlorobenzene solution of the thiazolothiazole dye were heated to 55 $^{\circ }$C in a nitrogen glove box. While in the glove box, 90 µL of the TTz solution was spin coated onto each of the Si substrates at 2000 RPM for 30 seconds to achieve a 20 nm nominal thickness. The samples were heated to 110 $^{\circ }$C for 20 minutes and stored covered in opaque (for visible through infrared) material in a glove box.
2.2 Data acquisition and analysis
A commercial spectroscopic ellipsometer (V-VASE, J.A. Woollam Co. Inc.) was used to investigate the TTz thin film samples. $\Psi$- and $\Delta$-spectra were obtained at room temperature in the spectral range from 0.65 eV to 3.5 eV (354 nm to 1907 nm) with a resolution of 0.025 eV at three angles of incidence $\Phi _a$=55$^{\circ }$, 65$^{\circ }$, and 75$^{\circ }$.
The analysis of the spectroscopic ellipsometry data obtained from the TTz thin film samples was carried out using stratified-layer optical model calculations with a commercial software package (WVASE32, J.A. Woollam Co. Inc.). The optical model is composed of four layers and consists of an undoped Si substrate, a native SiO$_2$ oxide layer, the TTz thin film, and the air ambient. The thickness of the substrate and the native oxide were determined to be $(360\pm 35)$ µm and $(2.8\pm 0.1)$ nm, respectively, by using ellipsometric measurements of a bare Si substrate. To determine the optical properties of the Si substrate and the native SiO$_2$ layer, standard model dielectric functions were used [16]. The parameterized model dielectric function for Si uses a combination of oscillators with Gaussian broadening and critical point features composed of continuous polynomial sections, which ensure Kramers-Kronig consistency. The complex dielectric function of SiO$_2$ employs a Sellemeier function. In order to describe the optical response of the TTz layer a parameterized dielectric function was developed.
The analysis of the spectroscopic ellipsometry data requires Kramers-Kronig consistent dispersion functions that provide sufficient flexibility to render the experimental lineshape accurately (see Fig. 2, for the experimental and model calculated ellipsometric angles $\Psi$ and $\Delta$, respectively). The Tauc-Lorentz model, originally proposed by Jellison and Modine, is frequently used to parameterize the complex dielectric function of amorphous semiconductors and insulators [17,18]. It has been shown by Synowicki and Tiwald that complex dielectric function models consisting of multiple oscillator types can provide a very flexible approach for analyzing amorphous and single crystalline semiconductors [19]. This approach ensures that the extracted complex model dielectric function is Kramers-Kronig consistent. The parameterization is advantageous over point-by-point analysis approaches where $\varepsilon _1$ and $\varepsilon _2$ are determined independently for each wavelength and the extracted complex dielectric function may not fulfill the Kramers-Kronig relationship. The analysis using parameterized dielectric functions furthermore prevents measurement noise to become part of the complex dielectric function. To accurately render the ellipsometric measurements of the investigated sample, the parameterized model dielectric function of the TTz thin film is composed of a combination of an oscillator with Gaussian broadening $\text {Gau}_i(A,E_0,\Gamma )$ and a set of four oscillators with Tauc-Lorentz broadening $\text {TL}_i(A,E_0,\Gamma,E^{\text {TL}}_{\text {g}})$:
The imaginary part of the dielectric function with Tauc-Lorentz and Gaussian broadening $\varepsilon _2^{\scriptscriptstyle \text {TL} \scriptstyle }(E)$ and $\varepsilon _2^{\scriptscriptstyle \text {Gau} \scriptstyle }(E)$, respectively, can be expressed analytically [19,20]:
During the analysis relevant parameters, such as layer thickness of the TTz thin film and the Si substrate as well as the oscillator parameters are varied using a Levenberg-Marquardt algorithm until the best match between the experimental and the model calculated data is achieved by minimizing the weighted error function $\chi ^2$ [21,22].
3. Results and discussion
Figure 1 shows the experimental (green dashed line) and best-model calculated (red solid line) $\Psi$- (a) and $\Delta$-spectra (b) for the substrate measured at three angles of incidence $\Phi _a$=55$^{\circ }$, 65$^{\circ }$, and 75$^{\circ }$. The most prominent feature in the spectra is observed at approximately 1.12 eV. For energies below 1.12 eV, the Si substrate is transparent and some fraction of the detected light is reflected at the backside of the silicon substrate. This effect was considered appropriately during the stratified layer optical calculations. The experimental (green dashed lines) and best-model calculated (solid red lines) $\Psi$- and $\Delta$-spectra for the TTz thin film sample measured at three angles of incidence $\Phi _a$=55$^{\circ }$, 65$^{\circ }$, and 75$^{\circ }$, are shown in Fig. 2. The $\chi ^2$ for this best-model fit is 1.6, indicating a good agreement between the experimental and best-model calculated data over the measured spectral range [22]. A comparison of Figs. 1 and 2 shows that, the $\Psi$-spectra for the bare substrate and the TTz thin film sample are similar for energies below 2.4 eV. The features in this energy range are primarily due to Si with TTz being mostly transparent. The contribution of the TTz thin film on the experimental and best-model calculated is evident for energies 2.5 eV to 3.5 eV. The bare substrate does not exhibit any notable features this spectral range. On the other hand, the $\Psi$- and $\Delta$-spectra in Fig. 2 are dominated by the absorption features due to the TTz thin film located at approximately 2.7, 2.9, 3.1 and 3.25 eV.
Fig. 1. Experimental (green dashed line) and best-model calculated (red solid line) $\Psi$-spectra (a) and $\Delta$-spectra (b) for the substrate measured at three angles of incidence (55$^{\circ }$, 65$^{\circ }$, and 75$^{\circ }$) in the spectral range from 0.65 eV to 3.5 eV.
Fig. 2. Experimental (green dashed line) and best-model calculated (red solid line) $\Psi$-spectra (a) and $\Delta$-spectra (b) for the investigated TTz thin film sample measured at three angles of incidence (55$^{\circ }$, 65$^{\circ }$, and 75$^{\circ }$) in the spectral range from 0.65 eV to 3.5 eV.
Figure 3 shows the real (a) and imaginary part (b) of the best-model dielectric function for TTz in the spectral range from 0.65 to 3.5 eV. TTz is mostly transparent below the Tauc-Lorentz bandgap at $E^{TL}_g$ = (2.41 $\pm$ 0.01) eV. Figure 3(b) also shows the contributions of the individual oscillators constituting the imaginary part of the best-model dielectric function. The strongest absorption occurs at 3.25 eV. Three additional absorption peaks can be recognized at the low energy flank of the main absorption peak. In order to describe the experimental $\Psi$- and $\Delta$-data accurately, a broad absorption with Gaussian broadening was included below the onset of the main absorption features.
Fig. 3. Real $\varepsilon _1$ (a) and imaginary $\varepsilon _2$ parts (b) of the best-fit parameterized model dielectric function. In (b), the contributions of the individual oscillators used to model the complex dielectric function are shown. The best-model parameter values are summarized in Table 1.
The best-model transition energies, amplitudes, and broadening parameters of the TTz dielectric function are given in Table 1. The thickness of the TTz thin film was determined to be t = 21.5 $\pm$ 0.1 nm. For the series of samples fabricated and characterized for this investigation the observed thickness values, determined using spectroscopic ellipsometry, were found identical within the experimental uncertainty.
Table 1. Summary of the best-model parameters obtained for the dielectric function of TTz during the optical model analysis. The error limits correspond to a 90% confidence level.
Figure 4 shows the Tauc plot for TTz obtained from $\varepsilon _2$ depicted in Fig. 3. Using a linear extrapolation a bandgap value of $E^{\scriptscriptstyle \text {TP} \scriptstyle }_{\scriptscriptstyle \text {g} \scriptstyle }=3.04$ eV is found. TTz has been investigated numerically using density functional theory with the B3LYP density functional and the 6-31G$^{\ast }$ basis set [7]. The observed energy difference between its highest occupied molecular orbital (HOMO) and its lowest unoccupied molecular orbital (LUMO) is 3.1 eV. The $E^{\text {TP}}_{\text {g}}$ found here corresponds very well to these density functional theory results.
Fig. 4. Tauc plot for thiazolothiazole and linear extrapolation (green dashed line) to determine the physical band gap $E^{\text {TP}}_{\text {g}}=3.04$ eV.
The absorption bands found below the HOMO-LUMO gap (see Figs. 3(b) and 4) are tentatively attributed to polaron effects which have been observed in ellipsometric measurements of semiconducting polymers [23]. Similar observations were made in Ref. [24], where the absorption characteristics of TTz-based polymer thin films were studied by gradually oxidizing the thin film. The oxidization resulted in the generation of polaronic and bipolaronic bands at longer wavelengths, which caused the TTz-based polymers to exhibit broad absorption bands. For the TTz thin film studied here the observed absorption bands below $E^{\text {TP}}_{\text {g}}=3.04$ eV are therefore tentatively attributed to polaronic effects caused by oxidation in ambient air.
4. Conclusion
In this paper, the complex dielectric function of a solution-processable TTz thin film has been reported for the first time using spectroscopic ellipsometry. A model dielectric function composed of Tauc-Lorentz and Gaussian oscillators has been developed. This mixed oscillator model dielectric function accurately reproduces the optical features of the ellipsometric spectra obtained from the TTz thin film sample in the measured spectral range from 0.65 to 3.5 eV. The bandgap of TTz was determined using a Tauc plot and is in good agreement with the HOMO-LUMO gap obtained in a recent density-functional theory calculation reported in the literature. A broad absorption band is observed below the bandgap and tentatively attributed to the formation of polarons due to oxidization in ambient air.
Funding
National Institutes of Health (1R15GM140392-01); National Science Foundation (1828430, 2052745).
Acknowledgments
The authors would like to acknowledge the support from the Department of Physics and Optical Science and the Department of Chemistry of the University of North Carolina at Charlotte. We further acknowledge support from the Center for Optoelectronics and Optical Communications.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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