1 Introduction

Metal phthalocyanines (MPcs) have garnered significant interest in the field of organic electronics due to their exceptional performance, robust chemical and thermal stability, and cost-effectiveness [1, 2]. Derivatives of transition metals were synthesized. Moreover, there has been considerable interest in a number of MPcs compounds, which have been extensively researched and widely acknowledged for their applicability in electronic device applications, including gas sensors, dye-sensitized solar cells, organic field-effect transistors, and others [1,2,3,4,5,6].

Titanium Phthalocyanine Chloride (TiPcCl2) is a widely used derivative of metal phthalocyanines (MPcs), known for its exceptional stability and effective light absorption characteristics throughout the visible and near-infrared portions of the optical spectrum. Moreover, the compound has a conjugated system of 18π electrons. Additionally, TiPcCl2 exhibits a p-type transport characteristic, hence enabling it to function as a semiconductor. Titanium Phthalocyanine Chloride (TiPcCl2) is a widely used derivative of metal phthalocyanines (MPcs) that exhibits P-type mobilities, as reported in references [7,8,9]. Previous studies have mostly focused on investigating the influence of annealing temperature on the structure and features of TiPcCl2 thin films. Additionally, the effects of film thickness and the coordinated ligand on the optical and electrical properties have also been extensively explored in the literature [7,8,9]. Nevertheless, there is a lack of prior research describing the impact of growth factors such as substrate temperature, doping condition, preparation technique, and substrate nature on the structural properties, as well as the linear and nonlinear optical properties of TiPcCl2.

The type of substrate is regarded as one of the most critical elements influencing the structure, morphology, optical, and electrical characteristics of thin films. The significant impact of substrates on physical properties may be attributed to several variables, including substrate surface orientations, substrate surface conductivity, and lattice mismatch between thin films and substrates [10,11,12,13,14,15,16]. Therefore, it is important to consider the impact of substrates since they have a substantial influence on the physical characteristics of thin films. The linear optical characteristics of thin films may be influenced by the kind of substrate they are deposited on. An instance of this phenomenon is when a substrate with a high refractive index is used, resulting in an augmentation of the optical absorption of the thin film. This property may be advantageous in several applications that need light absorption, such as in the field of solar cell technology.

In recent studies, researchers have made efforts to create thin-film optical systems utilizing transparent conducting substrates (TCSs) like fluorine-doped tin oxide (FTO) coated glass and indium-doped tin oxide (ITO). This approach aims to capitalize on the advantageous characteristics of TCSs, such as their ability to abruptly terminate optical transmission in the near-infrared (NIR) range and their notable optical nonlinearities. The use of TCSs has significantly enhanced the process of light capture as well. The use of TCSs is possible with many optoelectronic devices owing to their distinctive properties. The use of TCSs has the potential to modify the nonlinear characteristics of organic materials such as CN-PPV/FTO [10], PTCDA/ITO [17] and NiTPP/FTO [18]. The investigation of the TCS substrate's impact on the linear and nonlinear optical characteristics of thin films is a necessary subsequent step, as shown by previous studies [10, 17, 18].

In general, the impact of the substrate on the structural properties and both linear and nonlinear optical behaviors of TiPcCl2 when it is applied onto several transparent glass substrates, including ITO and FTO, represents a multifaceted and underexplored domain of investigation. Nevertheless, the existing early investigations indicate that the qualities in question may be substantially influenced by the substrate. Further investigation is required in order to fully comprehend the impact of the substrate on thin films of TiPcCl2. The findings of this study have the potential to facilitate the advancement of novel and enhanced organic electronic gadgets using TiPcCl2 as a fundamental component.

The objective of this study is to examine the alterations in the crystal structure of thin films of TiPcCl2 resulting from the influence of transparent conducting substances. Additionally, this research aims to establish a connection between these structural modifications and the linear and nonlinear optical characteristics of the films. In order to achieve the objective, thin films of TiPcCl2 were fabricated through the thermal evaporation method on quartz, ITO, and FTO substrates under the same deposition conditions. The structural surface morphologies and molecular structure of these films were examined using XRD, FESEM, and FT-IR techniques, respectively. Additionally, the optical properties, including the optical band gap and optical constants, were determined by analyzing the transmittance T(λ) and reflectance R(λ) measurements at various wavelengths ranging from 200 to 2500 nm. Furthermore, we have calculated the non-linear optical parameters using a semi-empirical relation.

2 Experimental techniques

TiPcCl2 powder was obtained from Sigma-Aldrich (see Scheme 1) and deposited at room temperature on clean ITO/glass FTO/glass, and quartz substrates by the thermal evaporation method (Edwards type E306A-10–6 Torr). The film thickness (250 nm) was measured using the crystal thickness monitor (FTM5). The XRD patterns for TiPcCl2/TCSs were obtained using an XRD system (Rigaku RINT 2100 diffractometer). The surface properties of TiPcCl2/TCSs films were studied using FESEM (QUANTA FEG250) and a transmission electron microscope (FESEM, JEOL model JSM-7001F). The chemical structure of the powder and TiPcCl2/TCSs films was examined using the Fourier transform infrared (FT-IR) method (Jasco Model 6100) at room temperature in the spectral region 400–1600 cm−1. The optical measurements were performed using a spectrophotometer (JASCO V-570) in the wavelength range of 200–2500 nm.

Scheme 1
scheme 1

Molecular structure of TiPcCl2

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3 Result and discussion

3.1 Structural properties

The XRD patterns for TiPcCl2 thin films are deposited onto ITO, FTO, and quartz substrates are shown in Fig. 1. From this figure, TiPcCl2/ITO and TiPcCl2/FTO thin films have an amorphous structure. There is no significant TiPcCl2 peak in the case of ITO and FTO substrates, as they simply displayed the strong background peak of substrates. All of the peaks shown in these patterns are caused by ITO and FTO substrates. The amorphous phase in TiPcCl2/ITO and TiPcCl2/FTO thin films is caused by a lattice mismatch between TiPcCl2 and the ITO and FTO substrates. The disparity in the lattice constants of two materials is known as a lattice mismatch. When the lattice mismatch is too great, the two materials cannot form a crystalline structure. Instead, the two materials will combine to produce an amorphous structure, which is a disordered structure with no long-range organization. However, TiPcCl2/quartz has a predominantly single diffraction peak with preferred (-311) orientation and is centered at 2θ = 31.17°. This observation for TiPcCl2/quartz thin film is in agreement with results reported in the literature for the XRD pattern of TiPcCl2 in powder form [7]. Scherrer's equation [18, 19] is used to get the average crystallite size D. The nanostructure features of TiPcCl2/quartz thin film are confirmed by crystallite size values in the nanoscale (34 nm).

Fig. 1
figure 1

XRD patterns of as deposited TiPcCl2/quartz, TiPcCl2/ITO andTiPcCl2/FTO thin films

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FESEM is an extremely useful instrument for comparing the surface morphology of thin films produced on various substrates. This data may be utilized to better understand how substrate type and deposition factors impact thin film nucleation and growth, as well as to optimize the deposition process to generate thin films with the required surface shape. Figure 2 shows FESEM images of TiPcCl2/FTO, TiPcCl2/ITO and TiPcCl2/quartz thin films. It is obvious that the surfaces of TiPcCl2/FTO and TiPcCl2/ITO thin films exhibit non-smooth characteristics, displaying many voids and fractures in comparison to the TiPcCl2/quartz thin film. The morphology of TiPcCl2 thin films may be affected by lattice mismatch between TiPcCl2 and substrates. A larger lattice mismatch might result in the creation of islands and other film defects. These flaws have the potential to impair the efficiency of solar cells and other electrical equipment.

Fig. 2
figure 2

FESEM for TiPcCl2/FTO, TiPcCl2/ITO, and TiPcCl2/quartz thin films

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Figure 3 displays the grain size distributions of TiPcCl2/ITO, TiPcCl2/FTO, and TiPcCl2/quartz. The average grain sizes of TiPcCl2 thin films deposited onto ITO, FTO, and quartz substrates are computed using Image J software and found to be around 100 ± 2, 68 ± 2, and 52 ± 2 nm, respectively. The analysis reveals that the average grain size values suggest that the produced films possess a nanostructured characteristic. The different grain sizes observed in TiPcCl2 thin films on ITO, FTO, and quartz substrates might be related to the thermal expansion coefficients and lattice misfit between TiPcCl2 and the substrates. Notably, the lattice mismatch is more evident in FTO than in ITO. This process enhanced the chance of fractures and cavities, resulting in grain fusion, higher grain sizes, and increased TiPcCl2 surface roughness on the FTO substrate [10, 11].

Fig. 3
figure 3

Histogram of grain size distribution for TiPcCl2/TCSs thin films

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Furthermore, the observed grain size in the FESEM image of TiPcCl2/quartz is much greater than the crystallite size estimated using XRD data of the corresponding films, as determined by Scherrer's equation. The grain size seen in the FESEM image may exhibit a larger value compared to the crystallite size determined using XRD analysis. In this study, it is postulated that the observed increase in crystallite size on the surface morphology of the film may be attributed to the aggregation of smaller crystals. Additionally, XRD analysis is used to determine the average crystallite size, which is indicative of the coherent scattering of X-rays [20, 21].

Analyzing infrared spectra of organic compounds is a widely used and efficient method for determining their molecular structures. Infrared spectra in the range 400–1600 cm−1 (fingerprint region) for TiPcCl2 powder, TiPcCl2/FTO, TiPcCl2/ITO, and TiPcCl2/quartz thin films are shown in Fig. 4. Peak positions and assignments [22,23,24] are listed in Table 1. We noticed that the strength of the peaks reduced for compounds with varied substrates, indicating that the substrates had an influence on crystallization while keeping the chemical structure of the TiPcCl2.

Fig. 4
figure 4

FTIR spectra for TiPcCl2 powder, TiPcCl2/quartz and TiPcCl2/TCSs

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Table 1 The Peak Positions and assignments for TiPcCl2 powder, TiPcCl2/quartz and TiPcCl2/TCSs thin films
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3.2 Optical characterization

The spectral distributions of T(λ) and R(λ) for TiPcCl2/ITO, TiPcCl2/FTO, and TiPcCl2/quartz films over the wavelength range of 200–2000 nm are shown in (Figs. 5, 6). It is observed that at (λ > 1000 nm), transmittance values decline while reflectance values rise, owing to the free carriers in the ITO and FTO layers begin to absorb light, which reduces the transmittance and increases the reflectance. Also, the thickness of the ITO and FTO layers also affects the transmittance and reflectance of the films. A thicker ITO or FTO layer will absorb more light, which will reduce the transmittance and increase the reflectance. Furthermore, lower T (λ) and R (λ) for TiPcCl2/ITO, TiPcCl2/FTO, affirmed the increased scattering of photons with increasing crystal imperfections or structural disorder attributed to decreased crystallite size due to increased RMS roughness of the thin films [16]. Thus, structural alteration of the thin films on different glass substrates was responsible for the observed changes in transmittance and reflectance characteristics.

Fig. 5
figure 5

The spectral distribution of reflectance forTiPcCl2/quartz and TiPcC2/TCSs thin films

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Fig. 6
figure 6

The spectral distribution of transmittance forTiPcCl2/quartz and TiPcCl2/TCSs thin films

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Using the following relationship we can determine the absorption coefficient (α) of TiPcCl2/TCSs films from T (λ) and R (λ) measurements [25]:

$$\alpha = \frac{1}{d} ln \left( {\frac{{1 - R^{2} }}{2T} + \sqrt {R^{2} + \frac{{1 - R^{2} }}{{4T^{2} }}} } \right)$$
(1)

where d is the film thickness. The absorbance coefficient of the TiPcCl2/FTO, TiPcCl2/ITO, and TiPcCl2/quartz thin films is depicted in Fig. 7. The spectrum displays three distinct bands. The Q band corresponds to the transition between the ground state and the first excited state (S0 → S1). Q-band splitting can also be observed, while the Soret (B) band is associated with a strongly allowed transition from the ground state to the second excited state (S0 → S2). The Q and Soret (B) bands are frequently associated with π → π* singlet transitions [1, 2, 26,27,28,29,30,31]. In addition, a band called C occurs at 225 nm in the UV range due to the intrinsic transition of TiPcCl2 (n → π*) [1, 2, 26,27,28,29,30,31]. Using Tauc's relation, the optical band gap can be determined in the strong absorption area ((α ≥ 104 cm−1)) [25]:

$$\left( {\alpha h\nu } \right)^{m} = H\left( {h\nu - E_{g}^{opt} } \right)\user2{ }$$
(2)

where H is a constant, \({E}_{g}^{opt}\) is the optical band gap, and m is an index dependent on the type of electronic transition occurring in the substances. Considering the optimum match is gotten with m = 2, it is concluded that the electronic transition is an indirect permitted transition for TiPcCl2/ITO, TiPcCl2/FTO and TiPcCl2/quartz thin films (Fig. 8). The first energy value \({{\text{E}}}_{{\text{g}}1}^{{\text{Opt}}}\) corresponds to the optical absorption Q band, whereas the second energy value \({{\text{E}}}_{{\text{g}}2}^{{\text{Opt}}}\) relates to the optical absorption B-band. The estimated values of the \({{\text{E}}}_{{\text{g}}}^{{\text{Opt}}}\) of the TiPcCl2/ITO, TiPcCl2/FTO and TiPcCl2/quartz thin films are listed in Table 2. Based on the findings presented in Fig. 8 and Table 2, it can be observed that the TiPcCl2/FTO film exhibits the lowest values for \({{\text{E}}}_{{\text{g}}1}^{{\text{Opt}}}\) and \({{\text{E}}}_{{\text{g}}2}^{{\text{Opt}}}\). Conversely, the TiPcCl2/quartz and TiPcCl2/ITO films demonstrate intermediate values, while the TiPcCl2/FTO film displays the maximum value. The decline in \({{\text{E}}}_{{\text{g}}1}^{{\text{Opt}}}\) values can be attributed to an increase in grain size and irregularity of films, resulting in the formation of defects in TiPcCl2 thin films. In the context of FTO substrates, it is worth noting that the surface of TiPcCl2/FTO has a rough texture when compared to microscope TiPcCl2/ITO and TiPcCl2/ITO, as evidenced by the FESEM photos. Also, Table 2 presents a comparison of the band gap energy values with those of other related metal MPcs compounds. Based on the data shown in the table, it can be inferred that the calculated \({{\text{E}}}_{{\text{g}}1}^{{\text{Opt}}}\) and \({{\text{E}}}_{{\text{g}}2}^{{\text{Opt}}}\) values for TiPcCl2 thin films are consistent with those seen in other MPcs thin films [1, 22, 26,27,28, 30,31,32,33,34,35]. Table 2 illustrates a marginal disparity in the \({{\text{E}}}_{{\text{g}}1}^{{\text{Opt}}}\) and \({{\text{E}}}_{{\text{g}}2}^{{\text{Opt}}}\).values observed between TiPcCl2 and other MPcs, which can be attributed to the distinctive characteristics of the central metal ion present in the phthalocyanine ring and substrate nature.

Fig. 7
figure 7

Absorption coefficient forTiPcCl2/quartz and TiPcCl2/TCSs thin films

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Fig. 8
figure 8

Energy gap for TiPcCl2/FTO, TiPcCl2/ITO and TiPcCl2/quartz thin films

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Table 2 Comparison of the optical parameters of TiPcCl2 with other phthalocyanine compounds
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The complex refractive index (N = n – jk) is a complex number that represents how the film interacts with electromagnetic waves. The real part of the refractive index, n, is connected to the phase velocity of light in the film as well as the material's electronic wave polarisation. The extinction coefficient, which is a measure of how much light is absorbed by the film, is related to the imaginary portion of the refractive index, k. The amplitude of electromagnetic waves is attenuated by a thin film when they pass through it, and this attenuation is proportional to the imaginary component of the refractive index, k. T(λ) and R(λ),are used to compute the optical constants (n and k) [20, 25].

$$k = \frac{\alpha \lambda }{{4\pi }}$$
(3)
$$n = \frac{1 + R}{{1 - R}} + \sqrt {\frac{4R}{{\left( {1 - R} \right)^{2} }} - k^{2} }$$
(4)

Figure 9 depicts the variation of k with wavelength for TiPcCl2/FTO, TiPcCl2/ITO and TiPcCl2/quartz thin films. It demonstrates that the k value for the extinction index of TiPcCl2/TCSs films decreases to a minimum of about 1000 nm after that, k begins to increase with increasing wavelength once more; this performance is due to the influence of their free carriers in FTO and ITO substrates. The absorption of light by free carriers is referred to as free carrier absorption. The quantity of free carrier absorption increases with wavelength, which explains why the k value for the extinction index of TiPcCl2/TCSs films likewise increases with wavelength. The effect of free carriers on FTO and ITO substrates on the optical characteristics of TiPcCl2/TCSs films is an important element to consider when creating and using these films for applications such as solar cells and optical sensors.

Fig. 9
figure 9

Extinction index forTiPcCl2/quartz and TiPcCl2/TCSs thin films

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The, n, is considered to be an important factor for optoelectronic devices, and the values of n for TiPcCl2 thin film are calculated using Eq. (4). Figure 10 illustrates the change of n with the wavelength for TiPcCl2/FTO, TiPcCl2/ITO, and TiPcCl2/quartz thin films. Similarly, the k case n for TiPcCl2/FTO and TiPcCl2/ITO begins to increase with increasing wavelength once more; this behavior is due to the contribution of their free carriers in FTO and ITO substrates [8]. The free carriers in also interact with electromagnetic radiation, such as light, and absorb photons. The absorption of photons raises the refractive index of the substance [36]. The influence of FTO and ITO on the refractive index of thin films is determined by a variety of parameters, including the concentration of free carriers, the thickness of the film, and the wavelength of light. In general, FTO and ITO will raise the refractive index of thin films, particularly at wavelengths over the plasma wavelength.

Fig. 10
figure 10

Refractive index forTiPcCl2/quartz and TiPcCl2/TCSs thin films

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The dielectric properties of the material affect how electromagnetic waves reflect and transmit, providing dynamic data on the electrical structure. The complicated dielectric constant components could be calculated using the optical constant [17]. The real component, denoted as ε1 = n2-k2, corresponds to the refractive index of the material. On the other hand, the imaginary part, represented as ε2 = 2nk, pertains to the energy absorption caused by interband transition and dipole dislocation.

The change in ε1 and ε2 for the TiPcCl2/ITO, TiPcCl2/FTO and TiPcCl2/quartz thin films with wavelength dependence on refractive index values is shown in Figs. 11, 12). As a result of the contribution of TCSs free carriers, TiPcCl2/TCSs thin films exhibit higher dielectric constants at high wavelengths than TiPcCl2/Quartz thin films. Free carriers can absorb photons by interacting with electromagnetic radiation such as light. Photon absorption raises the dielectric constant of the substance. Furthermore, TiPcCl2/ITO thin film has higher dielectric constant values than TiPcCl2/FTO thin film, particularly at long wavelengths. This is due to FTO having a lower concentration of free carriers than ITO. Quartz is a non-conductive substance having a low free carrier concentration. Because of the low concentration of free carriers, quartz has an extremely low dielectric constant regardless of the wavelength of light.

Fig. 11
figure 11

Real (ε1) part versus the wavelength forTiPcCl2/quartz and TiPcCl2/TCSs thin films

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Fig. 12
figure 12

The imaginary (ε2) part versus the wavelength forTiPcCl2/quartz and TiPcCl2/TCSs thin films

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The volume energy loss function (VELF) and the surface energy loss function (SELF) are optical features of materials that characterize electron energy loss owing to interactions with the material's bulk and surface, respectively. VELF is related to electronic transitions in the bulk material, while SELF is related to electronic transitions at the material's surface. The next empirical formula may be used to estimate these functions optically, which is linked to the ε1 and ε2 [10, 37]:

$$VELF = \frac{{\varepsilon_{2} }}{{\left(\varepsilon_{1}^{2} + \varepsilon_{2 }^{2} \right)}}$$
(5)
$$SELF = \frac{{\varepsilon_{2} }}{{\left( {\varepsilon_{1} + 1)^{2} + \varepsilon_{2}^{2} } \right)}}$$
(6)

Figures 13 and 14 explains the fluctuation of SELF and VELF relative to λ for TiPcCl2/TCSs thin films. The obtained findings show unequivocally that the loss of energy of free charge carriers behaves about the same as that seen when it propagates over the surface. It is also clear that the volume and surface energy losses at lower wavelengths do not change considerably. This implies that energy is also attenuated by free carriers moving between the inner and outer surfaces. The high levels of VELF and SELF especially at high wavelengths are owing to a lattice mismatch between the TiPcCl2 and FTO substrates, which favors FTO over ITO. Because high-wavelength light is more readily dispersed than low-wavelength light, the impact of lattice mismatch on SELF and VELF is more severe at higher wavelengths. As a result, large levels of SELF and VELF are particularly visible at long wavelengths. By placing a buffer layer between the TiPcCl2 and FTO substrates, we can mitigate the impact of lattice mismatch on SELF and VELF. The buffer layer's crystal structure should be TiPcCl2 and FTO compatible. This will assist in decreasing strain and flaws at the two materials' contact. It is feasible to lessen the impacts of lattice mismatch and increase the performance of devices using TiPcCl2 and FTO substrates by following this step.

Fig. 13
figure 13

The variation of SELF versus the wavelength forTiPcCl2/quartz and TiPcCl2/TCSs thin films

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Fig. 14
figure 14

The variation of VELF versus the wavelength forTiPcCl2/quartz and TiPcCl2/TCSs thin films

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The optical response and electronic states within \({E}_{g}^{opt}\) are studied by measuring the optical conductivity (σop). Furthermore, σopt is dependent on optical dielectric constants and is composed of the real σ1 (ω) = ωε0ε2 and imaginary σ2 (ω) = ωεoε1 parts, where ω is the angular frequency ε0 is the free space electric permittivity [14]. The variation of σ1 and σ2 as a function of wavelength is seen in Figs. 15 and 16. From this figure, TiPcCl2/ITO is higher than TiPcCl2/FTO films of σ1 and σ2 in infrared regain. This is likely due to a combination of factors, including the different refractive indices, surface morphologies, and free carriers concentrations of the FTO and ITO as well as the lattice mismatch between the TiPcCl2 and TCSs.

Fig. 15
figure 15

The variation of real (σ1) part versus the wavelength forTiPcCl2/quartz and TiPcCl2/TCSs thin films

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Fig. 16
figure 16

The variation of imaginary (σ2) part versus the wavelength forTiPcCl2/quartz and TiPcCl2/TCSs thin films

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3.2.1 The nonlinear parameters

The study of nonlinear optical refractive indexes in the subject of nonlinear photonics provides information on the interaction of light with these things. It is required for basic research as well as applications in materials science and photonics technology [29,30,31]. Based on Miller's general rule the nonlinear third-linear optical susceptibility (χ(3)) and nonlinear refractive index (\({{\varvec{n}}}_{2}\)) can be calculated by following formulas[38,39,40,41]:

$$\chi^{(3)} = 1.7 \times 10^{ - 10} \left( {\frac{{n^{2} - 1}}{4\pi }} \right)^{4} esu$$
(7)
$$\varvec{n}_{2} = \frac{{12\varvec{\pi }}}{\varvec{n}}\varvec{\chi }^{{(3)}} \;\varvec{esu}$$
(8)

The variation of χ(3) and\({{\varvec{n}}}_{2}\)as a function of wavelength is seen in Figs. 17 and 18. As seen in these figures χ(3)and\({{\varvec{n}}}_{2}\) exhibit the same trend. The values of \({\upchi }^{\left(3\right)}\) and \({{\varvec{n}}}_{2}\) for TiPcCl2/FTO and TiPcCl2/ITO are significantly greater than those for TiPcCl2/Quartz. This increase in \({\upchi }^{\left(3\right)}\) and \({{\varvec{n}}}_{2}\) for the FTO and ITO substrates is due to the free carriers in the FTO and ITO substrates as well as the large nonlinear optical capabilities of the FTO and ITO layers, which contribute to nonlinear optical increases [11, 42, 43]. The free carriers can contribute to nonlinear optical increases through free carrier transport. When free carriers are transported through a material, they can interact with the lattice of the material and generate new photons. This process, known as free carrier bremsstrahlung, is also a nonlinear optical process. The calculation values of \({\chi }^{(3)}\) and, \({{\varvec{n}}}_{2}\) at the zero-frequency (hυ = 0) using Eqs. (8) and (9) are tabled in Table 2, and are compared with those of other related metal MPcs compounds. The obtained results of the nonlinear optical values for TiPcCl2/TCSs thin films suggest that TiPcCl2film is an ideal material for a variety of applications that require a strong nonlinear optical response, such as optical frequency comb generation, optical parametric amplification, and optical switching.

Fig. 17
figure 17

The variation of χ(3) versus the wavelength for forTiPcCl2/quartz and TiPcCl2/TCSs thin films

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Fig. 18
figure 18

The variation of n(2) versus the wavelength for TiPcCl2/quartz and TiPcCl2/TCSs thin films

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Due to light's interaction with the semiconductor, organic thin films display a two-photon absorption (TPA) process, resulting in significant optical nonlinearity. In this TPA, two or more photons are absorbed, and the electrons are excited from the ground state to higher energy levels simultaneously, making this process largely a third-order nonlinear phenomenon. \({{\text{E}}}_{{\text{g}}}^{{\text{Opt}}}\) and hv. As a result, the empirical relationship presented by Sheik-Bahae et al. provides a connection between the βc coefficient and \({{\text{E}}}_{{\text{g}}}^{{\text{Opt}}}\) [44,45,46]:

$$\beta _{c} \left( \omega \right) = \frac{{3100\sqrt {21} \left[ {\left( {2h\nu /E_{g}^{{Opt}} } \right) - 1} \right]^{{3/2}} }}{{n^{2} E_{g}^{{Opt^{3} }} \left( {2h\upsilon /E_{g}^{{Opt}} } \right)^{5} }},\;{\text{cm/GW}}$$
(9)

The (βc) value upsurges with hν until it reaches a maximum value, then is reduced again (see Fig. 19 and Table 2). This is because the nonlinear absorption process is resonant, meaning that it is most efficient when the photon energy (hν) is equal to (\({{\text{E}}}_{{\text{g}}}^{{\text{Opt}}}\)⁄2 < hυ < \({{\text{E}}}_{{\text{g}}}^{{\text{Opt}}}\)). The resonant nature of nonlinear absorption is due to the fact that the nonlinear absorption process involves two or more photons. In order for two photons to be absorbed simultaneously, they must have the same energy. This is why the nonlinear absorption coefficient is highest when the photon energy is equal to half the optical band gap of the material [44, 45]. Also, according to Fig. 19 TiPcCl2/FTO thin film has the highest \({{{\varvec{\beta}}}_{{\varvec{c}}}}_{{\varvec{m}}{\varvec{a}}{\varvec{x}}}\) this is due to the lowest \({{\text{E}}}_{{\text{g}}}^{{\text{Opt}}}\) for TiPcCl2/FTO thin film.

Fig. 19
figure 19

The variation of βc, versus hν for TiPcCl2/quartz and TiPcCl2/TCSs thin films

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4 Conclusions

The observed changes in the structural, morphological, linear and nonlinear optical characteristics of TiPcCl2 with various transparent conducting substrates are shown. XRD, FESEM and FTIR spectra verified the influence of transparent conducting substrates on the structure morphology and molecular structure of TiPcCl2. The study of grain sizes from FESEM images indicates that the generated films have a nanostructured property. The drop in \({E}_{g}^{Opt}\) values is due to increased particle size and film irregularity, which causes flaws in TiPcCl2 thin films, notably on FTOsubstrate. Free carriers in FTOand ITO substrates affected dielectric constants, energy loss functions, and optical conductivity constants, notably at higher wavelengths. Another important finding of the present work is the higher values of the nonlinear optical χ(3) (esu), \({n}^{\left(2\right)}\) (esu) and β(c) parameters for TiPcCl2/TCSs than TiPcCl2/Quartz. Based on those findings, it can be concluded that TiPcCl2 films exhibit very desirable characteristics for a diverse range of applications that need a robust nonlinear optical response. These applications include optical frequency comb creation, optical parametric amplification, optical switching, optical computers, and ultra-pulsed lasers.