1 Introduction

Due to their application in ultraviolet (UV) optoelectronic applications, wide-bandgap (WBG) oxide semiconductors have recently attracted a lot of attention. These compounds have enabled previously unthinkable uses and prospects for a multitude of smart technologies and applications, including phototransistors, solar cells, touch-sensitive display technologies, optoelectronic devices, and UV emitters [1]. Cu(In,Ga)Se2 (CIGSe)-based solar cells have gained a lot of attention as a result of their commercialization [2]. The direct band gap of copper indium diselenide (CuInSe2 or CIS) at room temperature is about 1.02 eV, and it has a high absorption coefficient in the range of 104–105 cm−1, making it one of the most promising materials being researched for solar cell applications. Thin-film solar cells made of CuInSe2 have remarkable conversion efficiency [3,4,5,6]. Due to its high absorption coefficient, an absorber layer only has to be between 0.5 and 1.5 μm thick. Several vacuum and non-vacuum approaches, including single-stage and two-stage growth processes, can be used to create these chalcopyrite thin films. The thermal evaporation of chalcopyrite compounds is an important step towards forming CuInSe2 (CIS) and Cu(In,Ga)Se2 (CIGS) thin films on heated substrates [7]. In our work, we used the thermal evaporation technique to create CuInGaSe2 thin films in a variety of thicknesses. The produced films’ structure and nonlinear optical characteristics have been carefully taken into account.

2 Experimental

The modified Bridgman technique has been used to prepare CuInGaSe2 crystals (Cu 23%, In 19%, Ga 10%, 50% Se). In this procedure, the silica tube was charged with the stoichiometric total of the pure components Cu, In, Ga, and Se (6 N). CuInGaSe2 thin films with distinct thicknesses (50–300 nm) were developed on a glass substrate using the evaporation process in a vacuum of 10–4 Pa. At room temperature, all of the films were deposited. A Dektak 150 surface profiler was used to measure the thickness of the films. X-ray diffraction (XRD) was used to examine the structural characteristics of CuInGaSe films using a Shimadzu 6000 (XRD-6000) diffractometer and a CuK line (λ = 1.5405 Å) with 2Ɵ ranging from 50° to 90°. The voltage and current of the X-ray tube were 40 kV and 30 mA, respectively. The detector moved at a pace of 1° per minute. To further understand the impact thickness proportion, structural and optical analyses of the films were conducted. The optical absorption spectra of these thin films were examined in the 250–2500 nm wavelength range using a Jasco V-670 spectrophotometer to figure out their spectral dependence. Using these samples, an X-ray diffractogram was produced in order to identify the film’s structure and provide structural data. The range of the scanning angle (2θ) (CuK-Line) was 5–90. A computer controls the diffractometer, and the software programme gives us a qualitative analysis of the X-ray pattern.

3 Results and discussion

Figure 1 illustrates the XRD patterns of CuInGaSe2 films produced at various thicknesses. With 50-nm films, no apparent diffraction peaks can be seen, pointing to the film’s amorphous nature. At 100, 150, 200, 250, and 300 nm, the films typically exhibit a maximum diffraction peak at 29°. There are several peaks in the 2Ɵ range of 28–32°, at 37.5°, and around 60°. Due to the CuSe compound’s reaction with the nearby In and Ga atoms, the peak at 29° becomes stronger for the 200 nm thickness [8, 9].

Fig. 1
figure 1

CuInGaSe2 films made at different thicknesses, as evidenced by their XRD patterns

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According to the measured XRD patterns, the film’s grain expansion has improved at 200 nm. The CuInGaSe2 films had better crystal quality at 200 nm, where the diffraction peak intensity increased to its highest level. The waveguide method is taken into consideration for determining thin films’ absorption spectra in the spectral region of their transparency. At room temperature, the wavelength range of 200–900 nm is used to record the UV–vis absorption spectra of CuInGaSe2 films.

Figure 2 shows the relationship between CuInGaSe2 film absorbance and wavelength for various thicknesses (50, 100, 150, 200, 250, and 300 nm). At a thickness of 50 nm, a high absorption value was seen. In the UV range, it appears that the absorbance tends to decrease as the wavelength increases, a characteristic that is related to the absorbance of CuInGaSe2 films with high energy. The potential applications for these films in UV protection zones are highlighted by the greater optical absorption of CuInGaSe2 films in the UV region [10].

Fig. 2
figure 2

Absorbance versus. wavelength plot for various film thicknesses

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Experimentally, this easy equation can be used to obtain the absorption coefficient (α) [11]:

$$ (\alpha ) = 2.303\;A/t $$
(1)

where thin film thickness (t) and absorbance (A) are specified.

The variation of the CuInGaSe2 thin films’ absorption coefficient for various thicknesses is shown in Fig. 3. Figure 3 depicts how the absorption coefficient varies with wavelength. According to published research, all films’ absorption coefficients dropped significantly around the fundamental absorption and then gradually at longer wavelengths [12].

Fig. 3
figure 3

The absorption coefficient (α) spectra of CuInGaSe2 films varied with thickness

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According to the graph, for the thin films under investigation, the absorption coefficient (α) rises as photon energy rises.

The CuInGaSe2 thin films exhibit a low absorption coefficient at low energy. With a reduction in thickness, the absorption coefficient rises. This is related to a rise in layer density and grain size, and its high surface roughness can be explained by the light scattering effect [13]. By projecting the straight line of the graph to the zero-absorption coefficient that passed over the energy axis, the band gap was calculated using the optical absorbance data [14]. In order to calculate the band gaps, the formula has been used [15, 16]:

$$ h\upsilon = A\left( {h\upsilon - E_{{\text{g}}} } \right)^{1/2} $$
(2)

where the letters α, hυ, Eg, and A stand for the absorption coefficient, photon energy, band-gap energy, and proportionality constant, respectively. Known as the band-tailing parameter, constant A [17]. Figure 4 displays the plot of (hυ)2 vs hυ to determine the range of band gaps for the films. By extending the linear portion of the best fit of (αhυ)2 vs hυ = 0, the direct band gap was calculated. There is no systematic relationship in the Tauc plot between the direct band gap energy and thickness in the CuInGaSe2 films. In this study, the Eg values ranged as follows: 3.69, 3.56, 3.82, 3.63, 3.4, and 3.66 eV. Table 1 discusses the band gap change with film thickness. Figure 5 displays the optical indirect energy gap (Eg indirect) values that were derived by extrapolating the linear section of the plots to (αhυ)1/2 = 0 after evaluating the straight line of the (αhυ)1/2 plot against photon energy in the high absorption range. The Eg values in this investigation ranged from 2.5, 2.7, 2.85, 3.1, 3.1, and 3.5 eV, according to Table 1. With thickness, the indirect band gap energy systematically varies. Where the values of the energy gap rely generally on the crystal structure of the composites as well as the distribution and arrangement of atoms inside the crystal lattice.

Fig. 4
figure 4

Relation between (αhυ)2 versus (hυ) for variation film thickness

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Table 1 The band gap change with film thickness
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Fig. 5
figure 5

Relation between (αhυ)1/2 versus (hυ) for variation film thickness

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The transmittance curve of the CuInGaSe2 films with respect to wavelength is shown in Fig. 6. Due to the logarithmic relationship between the absorbance and transmittance, as demonstrated by the following relationship, it should be noted that the transmittance is the opposite of the absorbance [18]:

$$A= {\mathrm{log}}_{10 }\left(\frac{1}{T}\right)$$
(3)

where T is the transmittance.

Fig. 6
figure 6

The transmission spectrum for different CuInGaSe2 film thicknesses as a function of wavelength

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4 Conclusions

CuInGaSe2 thin films with film thicknesses of 50, 100, 150, 200, 250, and 300 nm were deposited on glass substrates using thermal vacuum evaporation. Studies have been done on the optical characteristics. The measured band gap values for the direct and indirect transitions were calculated. The direct band gap energy does not change systematically with thickness, whereas the indirect band gap energy does. This finding shows that the 50 nm film is amorphous, but higher thicknesses yield crystalline films.