The Structural and Optical Properties of Perovskite Thin Films

Introduction

A perovskite structure has the form ABX₃, where A, and B are cations, and X is the anion (Wiley, et al. 2019; Korkmaz and Ozpozan-Kalaycioglu, 2012). Perovskites can have interesting a lot of properties because of the molecules in their structure (Kangning, et al. 1998). Therefore, studies on perovskite structures, in other words, perovskite thin films have increased in the last few years to improve the performance of devices such as optoelectronic devices. At end of the 19th century, perovskite structures were formed in an aqueous solution by researchers (Mitzi, 1999; Soto-Montero, et al. 2020). However, these structures contained lead which is so dangerous for the environment (Katsanoulas, 1999). Hence, the research groups have tried to find materials that can be not dangerous to environmental and human health (Daskeviciute-Geguziene, et al. 2022).

Different deposition techniques such as ink-jet printing (Xie, 2018), co-evaporation (Park, 2019), and spin coating (Fong, 2021) have been used to form perovskite thin films.

In this work, the chemical bath deposition technique was used to obtain the perovskite thin films. The technique involves controlled precipitation of a solution of the compound on the substrate. With this technique, the structural and optical properties of the film can be changed by variables, such as solution temperature and precursor concentration. In the CBD method, a film can be formed on any substrate (Güneri, 2019). The aim of this study is to obtain KCaCl₂I and KCaCl₃ perovskite thin films using the CBD technique. Additionally, it is to show how iodine affects the structural and optical properties of perovskite thin films.

Experimental Details

The chemicals calcium chloride (CaCl₂, Sigma Aldrich 99%), potassium iodide (KI, Sigma Aldrich 99%), dimethyl sulfoxide (DMSO), and dimethylformamide (DMF) were used in this study. The deposition of perovskite films was carried out using chemical bath deposition (CBD). Prior to the deposition of the thin films by CBD, the glass substrates were cleaned with detergent, deionized water, acetone, ethanol, isopropyl alcohol, and deionized water, respectively. To obtain KCaClI₂ perovskite thin films, CaCl₂, KI, DMSO, and DMF were used in the bath. First of all, the stoichiometric ratio with CaCl₂ and KI was separately mixed in DMSO: DMF (3:7). The solvent mixture DMSO: DMF was used to boost the morphology control. To obtain a clear and homogenous solution, these mixes were continuously blended at room temperature for 24 hours. After obtaining homogenous solutions, the solutions were poured into the beaker, where the substrate cleaned was put. The deposition temperature and time were fixed at room temperature and 48h, respectively, to fabricate the film. The chemical reaction of the composition is given as follows:

CaCl₂ +Kl KCaCl₂l

To obtain KCaCl₃ perovskite thin films, KCaCl₂, KCl, DMSO, and DMF, were used in the bath. The aforementioned process was repeated for KCaCl₂I perovskite thin film as well. The chemical reaction of the composition is given as follows:

KCl +KCaCl₂ KCaCl₃

The phase and orientation of the films were determined by XRD using a Panalytical Empyrean Model diffractometer. SHİMADZU & UV-2700 spectrophotometer were used to determine the optical properties of films. All measurements were made at room temperature.

Results and Discussion

Fig. 1. XRD diffractograms pattern of KCaCI₂I perovskite thin film and KCaCI₃ perovskite thin film.

Fig.1 shows the XRD diffractograms of KCaCI₂I/glass and KCaCI₃/glass, which have crystalline structures. The XRD diffractogram of KCaCI₂I perovskite thin film has four strong diffraction peaks, which are located at the angles 2θ: 28.39°, 40.56°, 66.4639°, and 73.7692°. These peaks didn’t index the other materials. In this context, the new perovskite structure was fabricated in this work. The XRD diffractogram of KCaCI₃ perovskite thin film has a background due to dirtiness on the glass. Furthermore, there are just two peaks in its spectrum at the angles 2θ: 28.29°, and 40.29°

Fig. 2 shows the transmission, reflection, and absorption spectra of the KCaCI₂I and KCaCI₃ perovskite thin films. The transmission ratio of the KCaCI₂I perovskite thin film was lower than that of the KCaCI₃ perovskite thin film despite depositing time the same. There can be a lot of reasons. One of them is deposition velocity. The deposition velocity of the KCaCl₂I perovskite thin film can be faster than that of the KCaCI₃ perovskite thin film. When the refractive spectrum of both films was investigated, the refractive spectrum of KCaCl₂I perovskite thin films is higher than that of the KCaCI₃ perovskite thin film (Fig. 2). The origin of this difference can be iodine. The KCaCl₂I perovskite thin films have two maximum absorption peaks at 307 nm and 362 nm. These values show that there are two transmissions in the films. One of them can be connected from the lower valence band (VB2) to the minimum conduction band (CB), and the other can be fatigued to the transition from the higher valence band (VB1) to the CB (Al-Asbahi, et al. 2020). The KCaCI₃ perovskite thin film absorbed most light below 350 nm.

Fig. 2 The transmission, reflection, and absorption spectra of the KCaCI₂I and KCaCI₃ perovskite thin films.

The values of the direct energy band gaps of perovskite thin films were estimated from the Tauc plot of the absorption spectrum (Tauc, 1974). As seen in Fig. 3, the band gap values of the KCaCI₂I perovskite thin film are 2.45 eV (VB1 →CB) and 2.65 eV (VB2 →CB) while that of the KCaCI₃ perovskite thin film is 3.85 eV. These band gap values show that these films can be used as a window layer of a solar cell (Sharmin, et al. 2019).

Fig. 3 (αhν)² versus photon energy for the KCaCI₂I and KCaCI₃ perovskite thin films.

The refractive index and extinction coefficient of the films were determined using equations (Benramdane, et al. 1997). The non-linear dependence of the refractive index (n) and extinction coefficient (k) on the wavelength at room temperature can be seen in Fig. 4. Fig. 4 shows that KCaCI₂I and KCaCI₃ have different behavior for both parameters. The extinction coefficient of KCaCI₂I perovskite thin film increased from 425 nm. The refractive index of this film has also a reverse situation after 362 nm. The extinction coefficient of KCaCl₃ perovskite thin films increased from 325 nm. Additionally, the refractive index of KCaCl₃ perovskite thin films is almost constant and approximately 2.24–2.18. These values show that this film can be used for antireflection coating (Al-Asbahi, 2020).

Fig. 4 The refractive index and extinction coefficient on wavelength in KCaCI₂I and KCaCI₃ perovskite films.

The real and imaginary parts of the dielectric constant of the films were calculated using the equations (Hodgson, 1970). According to Fig. 5, while the real and imaginary parts of the dielectric constant for KCaCI₂I perovskite films were found to be 16.23 and 0.78 at 600 nm, respectively, the real and imaginary parts of the dielectric constant for KCaCI₃ perovskite films were determined to be 4.99 and 20.21 at 600 nm, respectively.

Fig. 5 The dependence of dielectric constants on wavelength in KCaCI₂I and KCaCI₃ perovskite films.

Conclusions

In this study, we successfully obtained KCaCI₂I and KCaCI₃ perovskite films via the CBD technique and compared the effect of iodine on the structural and optical properties of films by using characterization methods. According to XRD patterns, KCaCI₂I perovskite films have better crystalline structures than KCaCI₃ perovskite films. The band gap values of the KCaCI₂I perovskite thin film are 2.45 eV and 2.65 eV. The band gap value of the KCaCI₃ perovskite thin film is 3.85 eV. These band gap values show that both films can be used as a window layer of a solar cell. However, the KCaCI₃ perovskite thin film can be a better performance than the KCaCI₂I perovskite thin film as a window layer. To understand the effect of iodine on the properties of thin films, we plan to fabricate new films which will include different perovskite. Additionally, we will try using water instead of DMF: DMFO for green chemistry. In addition to all these, obtaining new perovskites using the solid state method and their differences can be investigated.

Acknowledgments

This work was fully supported by Erciyes University Scientific Research Projects Coordination Unit under the project numbers FBA-2022-11988. The authors wish to thank to Erciyes University.

References