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Article

Lead-Free and Stable Potassium Titanium Halide Perovskites: Synthesis, Characterization and Solar Cell Simulation

Condensed Matter Research Laboratory, School of Advanced Sciences, Vellore Institute of Technology, Vellore 632014, India
*
Author to whom correspondence should be addressed.
Academic Editor: Abasifreke Ebong
Energies 2022, 15(19), 6963; https://doi.org/10.3390/en15196963
Received: 15 July 2022 / Revised: 13 August 2022 / Accepted: 29 August 2022 / Published: 23 September 2022

Abstract

Titanium based double perovskites have shown good optical properties along with remarkable stability, making them promising lead-free perovskites for optoelectronic applications. In this work, Potassium Titanium Halide (K2TiBr6, K2TiI6 and K2TiI2Br4) double perovskites are synthesized for the first time. Surface topology and chemical composition are studied. Photoluminescence characterization shows a peak in the UV region. The perovskites exhibit quasi bandgap with K2TiI6 having 1.62 eV direct bandgap, conducive for single junction solar cell fabrication. K2TiBr6 and K2TiI2Br4 have bandgaps 2.14 eV and 2.44 eV, respectively, which is favorable for tandem solar cell application. The decomposition temperature of K2TiI6 is notable at 678 °C, bestowing it with stability in ambient atmosphere. Inherent to its optical properties, Solar Cell Capacitance Simulator-1D (SCAPS-1D) is used to simulate perovskite solar cell (PSC) with K2TiI6 as the absorber. Utilizing the absorption data from UV-Vis spectroscopy, these PSCs are designed to give maximum efficiency. Upon numerical optimization of perovskite layer thickness, we propose an efficient, practically realizable PSC with a power conversion efficiency of 4.382%. Besides, various electron and hole transport layers are investigated and the effect of their thickness on the PSC performance are studied.
Keywords: lead-free perovskites; inorganic and stable perovskites; photovoltaic cells; optoelectronics lead-free perovskites; inorganic and stable perovskites; photovoltaic cells; optoelectronics

1. Introduction

The rapid development of the world’s economy and productive forces are depleting conventional fossil fuel [1,2]. In recent times, numerous challenges, such as global crises of energy and resources, the environment, and climate change, has propelled the world towards the green industrial revolution. Renewable and cost-effective alternative energy resources including wind, thermal, solar, and hydro energy are proposed by scientists as the solution for these problems [3]. Solar energy has a strong lead among other resources, based on its abundance, cost-effectiveness, and environment friendliness. Photovoltaic (PV) technology contributes major part in harnessing solar energy. PV technology has undergone noteworthy evolution over the past few decades, including silicon-based solar cells, Si–Ge thin-film solar cells, dye-sensitized solar cells, quantum dot sensitized solar cells, and perovskite solar cells [4,5].
The creation of low-cost, high-efficiency solar cells has been the prominent issue in the recent years. Perovskite solar cells (PSCs), a promising class of solar cells family, has drawn a lot of attention in the last decade, due to their high absorption coefficient (e.g., CsSnI3), long carrier diffusion length (e.g., CH3NH3PbI3, CH3NH3PbI3-xClx), low exciton binding energy (e.g., CsSnI3), low trap state density (e.g., CH3NH3PbI3, CH3NH3PbBr3), and tunable bandgap (e.g., CH3NH3Snx Pb(1−x)I3) [6,7,8,9,10,11]. From the initial value of 3.8% to the most current value of 26.19%, the power conversion efficiency (PCE) of PSCs based on lead (Pb) perovskites has been significantly increased [12,13].
However, there are inherent drawbacks in these Pb-based high-efficiency PSCs (e.g., MAPbI3, FAPbI3, Cs0.05FA0.85MA0.10Pb(I0.97Br0.03)3), the foremost drawback being the presence of lead, which is hazardous to organisms and the environment. Research shows that lead ion poisoning has a serious detrimental effect on the survival of people, animals, and plants [14,15]. Various degrees of lead poisoning have been studied, ranging from anemia, soft tissue damage to death in case of severe exposure [16,17,18]. Another notable problem is instability of Pb-based perovskites in the temperatures above 25 °C [19,20,21]. MAPbX3 (X = I or Br) perovskites decomposed rapidly on constant exposure to light (100 suns for 60 min) at elevated temperatures around ~45 °C − ~ 55 °C. Consequently, to ensure that people’s safety and pollution-free natural environment it is necessary to develop some non- or low-toxic metal ions to replace lead as perovskite absorbers in PSCs [22,23,24].
The universal chemical formula for metal halide perovskites is ABX3, where A is an organic cation, B is a metal cation, and X is a halogen anion [25,26]. The A cation in this case is usually methylammonium (MA), formamidinium (FA), Cs, or their mixture, and the halogen anion is normally Cl, Br, I, or their mixture. For B metal cation, previous research focused on less/non-toxic Sn2+, Bi3+, Ge2+, Sb3+, Mn2+, and Cu2+, as they have a similar electronic configuration to Pb [27,28]. The addition of these metal cations not only increases the diversity of perovskite species, but also improves PSCs’ environmental friendliness. Unfortunately, most of them have low optical efficiency and less stability at ambient temperatures.
Titanium (Ti) based perovskites have lately been recommended as a promising alternative to Pb-free perovskites for solar cell applications [29]. Recent research on cesium titanium bromide Cs2TiBr6 solar cell with conversion efficiency of 3.3%, paved the way for studies on this non-toxic alternative. Subsequent research on Cs2TiCl/Br6, proving them candidates for solar cells and other optoelectronic applications due to their high stability, non-toxic nature, visible band gap, and high conversion efficiency, piqued the interest of the scientific community [30,31]. The review by Ju et al. studied more Ti-based compounds, among which K2TiX6 (X = I, Cl, Br) demonstrated huge potential for optical and thermoelectric device fabrication [29]. K2TiCl6 has been fabricated and studied extensively while, K2TiI6, K2TiBr6 are getting attention gradually. The non-toxic nature of K2TiI6, K2TiBr6 and K2TiI2Br4 has been more advantageous for commercial applications than the present inorganic Cs2TiBr6.
Motivated by previous theoretical works on K2TiX6 (X = I, Br, Cl), this article uncovers the features of family of potassium titanium bromide iodide perovskites K2TiI6 K2TiBr6 and K2TiI2Br4 for photovoltaic and optoelectronic application [32,33]. The perovskites are synthesized by low-cost solution method and the structural stability, electronic and optical properties of K2TiI6, K2TiBr6 and K2TiI2Br4 are studied. These materials have quasi-direct band gaps. K2TiI6, K2TiBr6 and K2TiI2Br4 have bandgaps in the increasing order of 1.62 eV, 2.14 eV, and 2.44 eV. Photoluminescence peaks are recorded at K2TiI6 has bandgap closest to MAPbI3, the benchmark molecule that revolutionized PSCs. This perovskite also has high thermal stability, with decomposition temperature above ~670 °C. Owing to its bandgap and thermal stability, perovskite solar cell with K2TiI6 absorber is simulated using Solar Cell Capacitance Simulator-1D (SCAPS-1D) with different Electron Transport Layers (ETLs) and Hole Transport Layers (HTLs) recording the highest PCE of 4.382%. Therefore, the comprehensive study of their optical and thermoelectric properties provides deep insight to researchers for the use of these materials commercially for energy applications.

2. Materials and Methods

Potassium Titanium Bromide (K2TiBr6) powder was synthesized by the cost-effective ‘solution method’. Potassium Bromide (KBr, ≥99% trace metals basis, Sigma-Aldrich, St. Louis, MO, USA) powder was dissolved in water to form a saturated solution in which 1 mL of Hydro Bromic acid (HBr, 47% Avra, Telangana, India) was added. After the addition of acid, 2.3 g of Titanium (IV)Bromide (TiBr4, 98%, Alfa Aesar, Ward Hill, MA, USA) was added resulting in a brown solution. It was stirred at ~100 °C for 1 h to form residue. The pale-yellow residue (K2TiBr6) was collected after it was completely dried and ground to fine powder.
A similar process was employed to process K2TiI6 and mixed halide K2TiI2Br4 powder. The ratio changes in the case of mixed halide perovskite, where TiI4 and TiBr4 were taken in the ratio 1:2 and added to aqueous solution containing KI and KBr in equal parts. Before the addition titanium halides, acids HI and HBr were added in small amounts. The mixture was then subject to heat and stirred till the water evaporated, before the residue was scraped and ground to form fine powder.
Powder X-ray diffraction (XRD) measurements were performed using a diffractometer (Bruker D8 Advance, Karlsruh, Germany) with Cu Ka radiation. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) images were collected using Carl Zeiss EVO18 SEM microscope (Oberkochen, Germany). The absorption spectra were captured using a Jasco V-670 ultraviolet-visible-near infrared (UV-Vis-NIR, Tokyo, Japan) spectrophotometer with an integrating sphere. Thermogravimetric analysis and differential temperature analysis (TGA and DTA) were performed using SDT Q600, (TA instruments, New Castle, DE, USA) scanning from 30 °C to 1000 °C, with a ramping rate of 20 °C/min. The photoluminescence spectra were measured using a Hitachi F-7000 FL spectrophotometer (Tokyo, Japan) excited by 150 W xenon lamp.
The Tauc method is employed to calculate direct and indirect bandgap from optical absorption spectra by the following equation:
(α·hν)1/γ = B(hν − Eg)
where h is the Planck constant, ν is the photon’s frequency, Eg is the band gap energy, and B is a constant. The γ factor depends on the nature of the electron transition and is equal to 1/2 or 2 for the direct and indirect transition band gaps, respectively.

3. Results and Discussion

3.1. Structural Analysis

XRD studies of K2TiBr6, K2TiI6 and K2TiI2Br4 perovskites are given below in Figure 1a. All three perovskites have face centered cubic structure with Fm3¯m space group [32,33]. Based on the theoretical studies [32,33], the lattice constants of K2TiBr6 and K2TiI6 are 10.50 Å and 11.42 Å respectively. Values calculated from XRD pattern using Bragg’s law for K2TiBr6, K2TiI6 perovskites are a = 10.47 Å, 11.40 Å, agreeing closely to the theoretical values mentioned above. By Vegard’s law, lattice constant of K2TiI2Br4 is approximately a weighted mean of K2TiBr6 and K2TiI6, hence a = 10.965 Å. Lattice parameters calculated from the XRD pattern have descending values in the order K2TiI6 > K2TiI2Br4 > K2TiBr6. This is justified by the increasing atomic radii of the halides. Lattice parameters are calculated by Bragg’s law given below:
nλ = 2d sinΘ;
where the variable ‘d’ indicates the distance between the atomic layers, and the variable ‘λ’ specifies the wavelength of the incident X-ray beam and ‘n’ as an integer. ‘Θ’ represents the angle of incidence.
The perovskites show structural stability as predicted by Goldschmidt tolerance factor values, which are as follows: K2TiI6 = 0.9026 and K2TiBr6 = 0.9208. Despite the positive formation energy of K2TiI6 [32], it is thermodynamically stable and has similar synthesis procedure compared to K2TiBr6 and K2TiI2Br4.
The perovskites were stored for a period of 10 months at room temperature. XRD of the samples after 10 months is shown in Figure 1b, K2TiI6 shows change in peak intensity. Consequently, the FWHM of K2TiI6 changes from 523.42° to 146.33°, thus altering the crystallite’s size from 0.234 nm to 0.578 nm. FWHM stays the same for K2TiBr6 (0.14° to 0.14°), whereas K2TiI2Br4 has slight difference from 0.23° to 1.17°. Change in crystallites size might be the contributing factor to intensity changes in K2TiI6. However, there is no shift in peak positions in all the three perovskites. There appears no change in K2TiBr6 and K2TiI2Br4 pattern proving the stability of the perovskites. The physical appearance (i.e., the color, texture) of the powder sample remains the same even after this extended period. These perovskites are stable under light irradiation, providing a breakthrough in stable, lead-free alternative for lead based organic and inorganic perovskites.
Further analysis of structure was carried out using SEM and EDX. Figure 2a,d represents SEM images and chemical composition of K2TiI6. It showed clusters made up of cube like grains. Figure 2b,e is the SEM and EDX of K2TiBr6, exhibiting irregular shaped structure which shows cubic grains on closer magnification. Figure 2c,f denotes the SEM and EDX spectrum of mixed halide perovskite K2TiI2Br4, which exhibits irregular, angular shaped flaky grains. A closer look at the SEM images shows non-porous grains, contrary to their cesium-based counterpart formed by the same method [31]. In addition, these perovskites are soluble in water. This quality opens many avenues for easy deposition process, in turn greatly helping to realize efficient thin film devices.
The elemental composition of all three perovskites as found by EDAX is given in Table 1. No contamination or moisture is detected in the perovskites.

3.2. Optoelectronic Properties

The perovskites were characterized to find their optical properties. Absorbance curves of K2TiI6, K2TiBr6 and K2TiI2Br4 are given in Figure 3. K2TiBr6 has high absorbance in the ultraviolet (UV) region with a peak at 327 nm, K2TiI6 and K2TiI2Br4 have high absorbance in the UV region with peak absorbance at 301 nm and 300 nm, respectively.
Absorbance in the visible region of the solar spectrum is desired for photovoltaic applications. While UV radiation makes around 4–8% of solar spectrum, they have longer wavelength, consequently having high energy per photon. UV photons have been reported to damage the perovskite solar cells, on exposure for a certain period. To minimize the damage caused by UV radiation, UV filtering, e.g., luminescent down shifting (LDS), can be used to preserve the solar cells [34].
Bandgaps for the perovskites are calculated by Tauc plots (Figure 4). K2TiI6 has direct bandgap of 1.62 eV and K2TiBr6 has direct bandgap of 2.14 eV (Figure 4a). Indirect bandgaps of K2TiI6, K2TiBr6 are 1.50 eV and 1.54 eV respectively as given in Figure 4b. The samples have quasi-direct bandgaps, since the difference between direct and indirect bandgap is very small (<30 meV). These values are in accordance with the theoretical studies published in various research articles [32,33]. The distinct difference in bandgaps of K2TiI6 and K2TiBr6 is due to Halide atom p- orbitals dominating VBM, while Ti-3d orbitals dominate CBM. The effect of halide atom ionic radius variation affects the CBM edges, and it increases the bandgap from Br to I. The 4 s orbitals of K contribute nothing to the VBM and CBM edges [32]. Direct bandgap of K2TiI6 is conducive to be an absorber in single junction solar cell. Large bandgap perovskite K2TiBr6 can be employed in multi-junction solar cell, in tandem with low bandgap absorbers like silicon [35,36]. K2TiI2Br4 has the direct bandgap of 2.44 eV (Figure 4a) and K2TiI4Br2 has bandgap 2.30 eV as represented in Figure 4c. Indirect bandgap of K2TiI2Br4 and K2TiI4Br2 are 1.58 eV and1.59 eV respectively (Figure 4d). Here, K2TiI2Br4 has a larger bandgap compared to K2TiI4Br2. Evidently, mixed halide perovskites have wide bandgap which can be tuned easily by changing the chemical composition of halides.
Photoluminescence spectroscopy results are shown in Figure 5. Emission peaks are in the visible region and red shifted in the order K2TiI2Br4 < K2TiBr6 < K2TiI6. PL intensity of mixed halide perovskite is less compared to bromide and iodide perovskites. Transition bandgap calculated from Figure 5 is similar to the bandgap intercepted from Tauc plot. This shows low defect density in all these perovskites. The FWHM of K2TiI6 is 7.15 nm, while K2TiBr6 and K2TiI2Br4 have values 1.06 nm and 16.76 nm respectively. PL spectra of CH3NH3PbI3 recorded broad high intensity peak at ~786 nm corresponding to its reported bandgap value (~1.60 eV) [37].
Since the bandgap of K2TiI6 is closer to the organic and most efficient CH3NH3PbI3 perovskite, it is extensively studied below [38,39]. For an efficient photovoltaic device application, PL studies on individual layers that constitute the cell can provide important information on carrier recombination and charge extraction from absorber layer. Specifically, observing the variation in PL intensity under continuous excitation, provides the knowledge about device stability and optimization [40]. To further explore this perovskite, its thermal stability is studied and SCAPS-1D is employed to give a suitable solar cell assembly to obtain maximum power conversion efficiency.

3.3. Thermal Stability

Thermal stability of perovskites is found using TGA. Figure 6a represents TG analysis of K2TiI6. The decomposition temperature is high at 678.36 °C, which makes it stable under high temperature conditions. Figure 6b compares the DTA curve of K2TiI6 and KI. K2TiI6 shows endothermic behavior till 157 °C, decomposition happens at 681 °C into KI and TiI4, and then proceeds to decompose completely at 880 °C.
Weight loss of the sample is very less till 678.36 °C, showing the absence of moisture. K2TiI6 displays high thermal stability compared to other Ti-based perovskites [31]. This property makes it promising for photovoltaic application in elevated temperature conditions.

4. Solar Cell Simulation

One-dimensional solar cell capacitance simulator SCAPS (version 3.3.10) as a simulation platform is employed to study the application of the perovskites in solar cell devices [41,42]. This software is a general solar cell simulator program that can be used in the simulation, numerical analysis, and tracking the behavior of the photovoltaic structure. The software is mainly based on solving continuity, Poisson, and semi-conductor equations to describe the physics of the photovoltaic structure.
The three perovskites showed remarkable optoelectronic properties. However, K2TiI6 has bandgap value very close to that of CH3NH3PbI3 (~1.60 eV). And CH3NH3PbI3 has high power conversion efficiency with practical efficiency limit up to 29.53% [43]. Since they have similar optical properties, the bandgap of K2TiI6 is considered optimal and the solar cell devices based on it will have high efficiency.
SCAPS-1D is used to simulate a single-junction solar cell with the inorganic K2TiI6 as absorber, using the absorbance data from the sample processed earlier. Since the absorbance data for the material from experiment are applied, the chances of getting accurate results are high, which will help in fabricating the right device. The parameters used for simulation are given below in Table 2.
Parameters for K2TiI6 are from experiment carried out and from computational studies by various researchers. Two different electron transport layers (ETLs) and a Hole Transport Layer (HTL) are chosen [44,45,46,47,48,49] to optimize device configuration, for obtaining maximum efficiency. TiO2 and ZnO are the most widely used ETL, whose wide bandgap helps with higher extraction and transportation of electrons from perovskite layer to ETL. Commonly used HTL poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT: PSS) and Poly(3-hexylthiophene) (P3HT), possessing high hole mobility and stability are employed to study the performance of the perovskite K2TiI6 as absorber. Significant qualities, such as the low cost and easy deposition process, of these transport layers are highly desired over widely used HTL (Spiro-MeOTAD) and ETL (SnO2). The following are the four devices: Device 1- FTO/TiO2/K2TiI6/PEDOT: PSS/Metal contact, Device 2- FTO/ZnO/K2TiI6/PEDOT: PSS/Metal contact, Device 3- FTO/TiO2/K2TiI6/P3HT/Metal contact and Device 4-FTO/ZnO/K2TiI6/P3HT/Metal contact, to be studied in detail, and represented schematically in Figure 7. With the inclusion of defect density to all the layers, the results are obtained as shown below.

4.1. Effect of Absorber Layer Thickness

The PSCs are studied by changing their absorber layer thickness, while the electron and hole transport layers width were constant. To optimize the absorber width of the devices, the thickness of FTO conductive glass is fixed at 150 nm throughout the simulation. The thickness of ETLs TiO2 and ZnO are fixed at 400 nm and 500 nm respectively. HTLs PEDOT:PSS and P3HT are set as 400 nm and 50 nm respectively.
K2TiI6 layer was varied from 200 nm to 1000 nm, and their current density vs. Voltage behavior is observed. Figure 8 represents the current density vs. voltage graph of all the devices for 200 nm absorber thickness. Variation of K2TiI6 thickness from 200 nm to 1000 nm has no notable effect on the current density vs. voltage relation.
For all the devices, open circuit voltage (Voc), short circuit current (Jsc), and power conversion efficiency (PCE) decreases with increase in thickness. Fill factor keeps increasing with thickness (Figure 9). The charge carriers have a longer route with thicker absorber layer, which decreases the carrier collection at the electrodes, and the increase of carrier recombination results in decrease of Voc, Jsc and PCE [50]. The increase of series resistance with a thicker absorber layer shows an increasing trend in FF [51]. Hence, the 200-nm thick absorber layer is chosen for optimum performance of the PSC with various electron and hole transport layers. Table 3 presents the comparative analysis of cell parameters for all the devices studied at favorable thickness of absorber and transport layers.

4.2. Performance of PSC with Varying ETL Thickness

Electron transport layers are essential in providing various functions like, charge transfer and preventing charge recombination. The most common and efficient inorganic ETLs- Titanium dioxide (TiO2) and Zinc oxide (ZnO), are employed in these devices, whose width is varied from 100–500 nm, while keeping the HTLs (PEDOT: PSS) and P3HT thickness unchanged to optimize these PSC devices. With the thickness of FTO and K2TiI6 fixed at 150 nm and 200 nm respectively, ETL layers’ thicknesses are varied to optimize the devices. Figure 10 shows the Voc, Jsc, Fill factor and PCE curve for varying ETL thickness. Increase in the thickness of ETLs shows increase in Voc, Jsc and PCE, while FF decreases. Devices 1 and 3 registered the highest efficiency for TiO2 thickness of 400 nm. In the case of devices 2 and 4, ZnO thickness maintained up to 500 nm to get the highest possible output.

4.3. Performance of PSC with Different HTL Thickness

The variation of the simulated parameters as a function of the thickness of the p-layer in the range from 10 nm to 400 nm is displayed in Figure 11 where the thickness values for the ET layer and absorber layer are set at their optimal values of 400 nm and 200 nm, respectively for devices 1 and 3. Two other devices with ZnO ETL (devices 3 and 4), were fixed the values 500 nm and 200 nm for ETL and absorber layer as thickness to study the effect of HTLs. Polymers poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) and Poly(3-hexylthiophene) (P3HT) as HTL are simulated. Multiple tries were made to optimize the device. It is observed that the thickness of P3HT has direct effect on the device performance. For higher P3HT layer width (>40 nm), the device registered less PCE. Optimized device thickness for best efficiency is TiO2-400 nm, ZnO-500 nm; K2TiI6—200 nm and P3HT—40 nm. For the same transport layer thickness, the absorber layer width was varied which reduced the device efficiency. Device 3 had the configuration FTO/TiO2/K2TiI6/P3HT, which recorded the maximum power conversion efficiency of 1.0428%.
Device 1 and 2 demonstrated distinct behavior with different thicknesses of PEDOT:PSS. The optimal width of PEDOT:PSS is 400 nm for device 1 and 200 nm for device 2. These devices showed maximum efficiency for larger thickness compared to P3HT devices. Figure 11 compares the different parameters Voc, Jsc, FF, PCE for devices 1, 2, 3 and 4.

5. Conclusions

In this article, inorganic vacancy ordered double perovskites K2TiI6, K2TiBr6 and K2TiI2Br4 are processed by a low-cost solution method [31] and subsequently characterized. Owing to the Goldschmidt tolerance factor, XRD studies confirm that potassium titanium halides perovskites exhibit single phase crystalline nature. On further structural characterization, SEM studies showed cube shaped grains, while EDAX gave an account of the chemical composition of the perovskites. In order to determine the optical properties, UV-Vis spectroscopy was employed. Absorption spectra shows absorbance in the UV region for all the three perovskites. To minimize the effect of UV photons degrading the perovskites on prolonged exposure, luminescent down shifting is recommended. The optical bandgap intercepted from Tauc plot shows all the three perovskites K2TiI6, K2TiBr6 and K2TiI2Br4 have quasi bandgaps. K2TiI6 has the desirable direct bandgap of 1.62 eV, K2TiBr6 has direct bandgap of 2.2 eV and K2TiI2Br4 has direct bandgap of 2.44 eV. The optical gap of Potassium Titanium Iodide is narrow like the most successful organic perovskite, Methylammonium Lead Iodide (MAPbI3), proving to be a promising candidate for lead-free, low-cost perovskite. K2TiBr6 and K2TiI2Br4 can help in realizing efficient tandem devices with CdTe, silicon, and CIGS. Narrow bandgap of CdTe, CIGS and silicon has proved to be more efficient in tandem with perovskites with optical gap larger than 2 eV [49,50]. As K2TiI6 perovskite appears more suitable for absorber in single junction solar cell, we check its thermal stability by TGA and DTA. With 678.36 °C decomposition temperature, it shows remarkable thermal stability. Based on its various merits, SCAPS-1D is used to simulate K2TiI6 based solar cells with the help of absorption data obtained from UV-Vis spectroscopy. By varying ETLs and HTLs, the device is optimized to attain maximum efficiency. The solar cell device with following configuration FTO/ZnO/K2TiI6/PEDOT: PSS showed maximum power conversion efficiency of 4.3821%. To conclude, Potassium Titanium Halides are synthesized and characterized, based on the results obtained using two sets of K2TiI6 perovskite solar cells simulated using SCAPS-1D, providing a promising solution for lead-free, cost effective, bio-friendly, thermally stable, and efficient perovskite.

Author Contributions

Conceptualization, S.K.; methodology, S.K.; validation, M.M.; formal analysis, S.K.; writing, S.K.; supervision, M.M. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Data Availability Statement

Not applicable.

Acknowledgments

The authors are grateful to Marc Burgelman, University of Gent, Belgium for providing the SCAPS-1D software for the studies. The authors extend their thanks to N. Krishnachandar, department of Physics, VIT University, Vellore, for his support and encouragement.

Conflicts of Interest

The authors declare no conflict of interest.

References

  1. Smith, C.J.; Forster, P.M.; Allen, M.; Fuglestvedt, J.; Millar, R.J.; Rogelj, J.; Zickfeld, K. Current fossil fuel infrastructure does not yet commit us to 1.5 °C warming. Nat. Commun. 2019, 10, 101. [Google Scholar] [CrossRef]
  2. Saeed, M.A.; Shahzad, A.; Rasool, K.; Mateen, F.; Oh, J.-M.; Shim, J.W. 2D MXene: A Potential Candidate for Photovoltaic Cells? A Critical Review. Adv. Sci. 2022, 9, 2104743. [Google Scholar] [CrossRef]
  3. Saeed, M.A.; Yoo, K.; Kang, H.C.; Shim, J.W.; Lee, J.J. Recent developments in dye-sensitized photovoltaic cells under ambient illumination. Dyes Pigm. 2021, 194, 109626. [Google Scholar] [CrossRef]
  4. Yao, Y.; Xu, J.H.; Sun, D.Q. Untangling global levelised cost of electricity based on multi-factor learning curve for renewable energy: Wind, solar, geothermal, hydropower and bioenergy. J. Clean. Prod. 2021, 285, 124827. [Google Scholar] [CrossRef]
  5. You, Y.J.; Saeed, M.A.; Shafian, S.; Kim, J.; Kim, S.H.; Kim, S.H.; Kim, K.; Shim, J.W. Energy recycling under ambient illumination for internet-of-things using metal/oxide/metal-based colorful organic photovoltaics. Nanotechnology 2021, 32, 465401. [Google Scholar] [CrossRef] [PubMed]
  6. Srivastava, S.; Singh, A.K.; Kumar, P.; Pradhan, B. Comparative performance analysis of lead-free perovskites solar cells by numerical simulation. J. Appl. Phys. 2022, 131, 175001. [Google Scholar] [CrossRef]
  7. Wolf, S.D.; Holovsky, J.; Moon, S.-J.; Löper, P.; Niesen, B.; Ledinsky, M.; Haug, F.-J.; Yum, J.-H.; Ballif, C. Organometallic Halide Perovskites: Sharp Optical Absorption Edge and Its Relation to Photovoltaic Performance. J. Phys. Chem. Lett. 2014, 5, 1035–1039. [Google Scholar] [CrossRef]
  8. Shi, D.; Adinolfi, V.; Comin, R.; Yuan, M.; Alarousu, E.; Buin, A.; Chen, Y.; Hoogland, S.; Rothenberger, A.; Katsiev, K.B.; et al. Low trap-state density and long carrier diffusion in organolead trihalide perovskite single crystals. Science 2015, 347, 519–522. [Google Scholar] [CrossRef]
  9. Adjogri, S.J.; Meyer, E.L. A Review on Lead-Free Hybrid Halide Perovskites as Light Absorbers for Photovoltaic Applications Based on Their Structural, Optical, and Morphological Properties. Molecules 2020, 25, 5039. [Google Scholar] [CrossRef]
  10. Kumar, A.; Kumawat, N.K.; Maheshwari, P.; Kabra, D. Role of halide anion on exciton binding energy and disorder in hybrid perovskite semiconductors PVSC. IEEE Xplore 2015, 42, 1–4. [Google Scholar]
  11. Frohna, K.; Stranks, S.D. 7—Hybrid Perovskites for Device Applications; Woodhead Publishing: Sawston, UK, 2019; pp. 211–256. [Google Scholar]
  12. Chen, Y.; Zhang, L.; Zhang, Y.; Gao, H.; Yan, H. Large-area perovskite solar cells—A review of recent progress and issues. RSC Adv. 2018, 8, 10489–10508. [Google Scholar] [CrossRef] [PubMed]
  13. NREL. Solar Cell Efficiency Chart—Photovoltaic Research—NREL, 2020. Available online: https://www.nrel.gov/pv/cell-efficiency.html (accessed on 1 July 2022).
  14. Hailegnaw, B.; Kirmayer, S.; Edri, E.; Hodes, G.; Cahen, D. Rain on methylammonium lead iodide-based perovskites: Possible environmental effects of perovskite solar cells. J. Phys. Chem. Lett. 2015, 6, 1543–1547. [Google Scholar] [CrossRef]
  15. Giustino, F.; Snaith, H.J. Toward lead-free perovskite solar cells. ACS Energy Lett. 2016, 1, 1233–1240. [Google Scholar] [CrossRef]
  16. Xiao, Z.; Zhou, Y.; Hosono, H.; Kamiya, T. Intrinsic Defects in a Photovoltaic Perovskite Variant Cs2SnI6. Phys. Chem. Chem. Phys. 2015, 17, 18900–18903. [Google Scholar] [CrossRef]
  17. Sani, F.; Shafie, S.; Lim, H.N.; Musa, A.O. Advancement on Lead-Free Organic-Inorganic Halide Perovskite Solar Cells: A Review. Materials 2018, 11, 1008. [Google Scholar] [CrossRef]
  18. Lee, B.; Stoumpos, C.C.; Zhou, N.; Hao, F.; Malliakas, C.; Yeh, C.-Y.; Marks, T.J.; Kanatzidis, M.G.; Chang, R.P. Air-stable molecular semiconducting iodo salts for solar cell applications: Cs2SnI6 as a hole conductor. J. Am. Chem. Soc. 2014, 136, 15379–15385. [Google Scholar] [CrossRef] [PubMed]
  19. De Oliveira, A.E.R.T.P.; Alves, A.K. Organic–Inorganic Hybrid Perovskites for Solar Cells Applications; Springer: Berlin/Heidelberg, Germany, 2019; pp. 89–101. [Google Scholar]
  20. Guo, R.; Khenkin, M.V.; Arnaoutakis, G.E.; Samoylova, N.A.; Barbé, J.; Lee, H.K.H.; Tsoi, W.C.; Katz, E.A. Initial Stages of Photodegradation of MAPbI3 Perovskite: Accelerated Aging with Concentrated Sunlight. Sol. RRL 2020, 4, 1900270. [Google Scholar] [CrossRef]
  21. Misra, R.K.; Aharon, S.; Li, B.; Mogilyansky, D.; Visoly_Fisher, I.; Etgar, L.; Katz, E.A. Temperature- and Component-Dependent Degradation of Perovskite Photovoltaic Materials under Concentrated Sunlight. Phys. Chem. Lett. 2015, 6, 326–330. [Google Scholar] [CrossRef]
  22. Saparov, B.; Sun, J.-P.; Meng, W.; Xiao, Z.; Duan, H.-S.; Gunawan, O.; Shin, D.; Hill, I.G.; Yan, Y.; Mitzi, D.B. Thin-film deposition and characterization of a Sn-deficient perovskite derivative Cs2SnI6. Chem. Mater. 2016, 28, 2315–2322. [Google Scholar] [CrossRef]
  23. Maughan, A.E.; Ganose, A.M.; Scanlon, D.O.; Neilson, J.R. Perspectives and Design Principles of Vacancy-Ordered Double Perovskite Halide Semiconductors. Chem. Mater. 2019, 31, 1184–1195. [Google Scholar] [CrossRef]
  24. Cai, Y.; Xie, W.; Ding, H.; Chen, Y.; Thirumal, K.; Wong, L.H.; Mathews, N.; Mhaisalkar, S.G.; Sherburne, M.; Asta, M. Computational study of halide perovskite- derived A2BX6 inorganic compounds: Chemical trends in electronic structure and structural stability. Chem. Mater. 2017, 29, 7740–7749. [Google Scholar] [CrossRef]
  25. Roy, P.; Kumar Sinha, N.; Tiwari, S.; Khare, A. A review on perovskite solar cells: Evolution of architecture, fabrication techniques, commercialization issues and status. Sol. Energy 2020, 198, 665–688. [Google Scholar] [CrossRef]
  26. Chakraborty, S.; Xie, W.; Mathews, N.; Sherburne, M.; Ahuja, R.; Asta, M.; Mhaisalkar, S.G. Rational Design: A High-Throughput Computational Screening and Experimental Validation Methodology for Lead-Free and Emergent Hybrid Perovskites. ACS Energy Lett. 2017, 2, 837–845. [Google Scholar] [CrossRef]
  27. Sun, S.; Noor, T.P.H.; Zekun, D.R.; Felipe, O.; Antonio, M.B.; Mariya, L.; De, X.; Tofunmi, O.; Janak, T.; Savitha, R.; et al. Accelerated Development of Perovskite-Inspired Materials via High-Throughput Synthesis and Machine-Learning Diagnosis. Joule 2019, 3, 1437–1451. [Google Scholar] [CrossRef]
  28. Park, B.W.; Philippe, B.; Zhang, X.; Rensmo, H.; Boschloo, G.; Johansson, E.M. Bismuth based hybrid perovskites A3Bi2I9 (a: Methylammonium or cesium) for solar cell application. Adv. Mater. 2015, 27, 6806–6813. [Google Scholar] [CrossRef] [PubMed]
  29. Ju, M.G.; Chen, M.; Zhou, Y.; Garces, H.F.; Dai, J.; Ma, L.; Padture, N.P.; Zeng, X.C. Earth-Abundant Nontoxic Titanium (IV)-based Vacancy-Ordered Double Perovskite Halides with Tunable 1.0 to 1.8 eV Bandgaps for Photovoltaic Applications. ACS Energy Lett. 2018, 3, 297–304. [Google Scholar] [CrossRef]
  30. Chen, M.; Ju, M.G.; Carl, A.D.; Zong, Y.; Grimm, R.L.; Gu, J.; Zeng, X.C.; Zhou, Y.; Padture, N.P. Cesium titanium (IV) bromide thin films based stable lead-free perovskite solar cells. Joule 2018, 2, 558. [Google Scholar] [CrossRef]
  31. Kong, D.; Cheng, D.; Wang, X.; Zhang, K.; Wang, H.; Liu, K.; Li, H.; Sheng, X.; Yin, L. Solution processed lead-free cesium titanium halide perovskites and their structural, thermal and optical characteristics. J. Mater. Chem. C 2020, 8, 1591–1597. [Google Scholar] [CrossRef]
  32. Radhakrishnan, A.; Manickam, S.; Jeyaperumal, K.S.; Perumalsamy, R. First principle study of structural, mechanical, electronic and optical properties of K2TiX6 (X = Cl, Br, I) for photovoltaic applications. Solid State Sci. 2021, 118, 106654. [Google Scholar] [CrossRef]
  33. Ghrib, T.; Rached, A.; Algrafy, E.; Al-nauim, I.A.; Albalawi, H.; Ashiq, M.G.B.; Ul Haq, B.; Mahmood, Q. A new lead free double perovskites K2Ti(Cl/Br)6; a promising materials for optoelectronic and transport properties; probed by DFT. Mater. Chem. Phys. 2021, 264, 124435. [Google Scholar] [CrossRef]
  34. Anizelli, H.S.; Stoichkov, V.; Fernandes, R.V.; Duarte, J.L.; Laureto, E.; Kettle, J.; Visoly-Fisher, I.; Katz, E.A. Application of luminescence downshifting materials for enhanced stability of CH3NH3PbI3(1-x)Cl3x perovskite photovoltaic devices. Org. Electron. 2017, 49, 129–134. [Google Scholar] [CrossRef]
  35. Siegler, T.D.; Shimpi, T.M.; Sampath, W.S.; Korgel, B.A. Development of wide bandgap perovskites for next-generation low-cost CdTe tandem solar cells. Chem. Eng. Sci. 2019, 199, 388–397. [Google Scholar] [CrossRef]
  36. Tong, J.; Jiang, Q.; Zhang, F.; Kang, S.B.; Kim, D.H.; Zhu, K. Wide-Bandgap Metal Halide Perovskites for Tandem Solar Cells. ACS Energy Lett. 2021, 6, 232–248. [Google Scholar] [CrossRef]
  37. Basumatary, P.; Kumari, J.; Agarwal, P. Probing the defects states in MAPbI3 perovskite thin films through photoluminescence and photoluminescence excitation spectroscopy studies. Optik 2022, 266, 169586. [Google Scholar] [CrossRef]
  38. Oskar, S.; Peter, W.; Shreetu, S.; Ievgen, L.; Mykhailo, S.; Gebhard, J.M.; Andres, O.; Miroslaw, B.; Wolfgang, H.; Christoph, J.B.; et al. Looking beyond the Surface: The Band Gap of Bulk Methylammonium Lead Iodide. Nano Lett. 2020, 20, 3090–3097. [Google Scholar]
  39. Hutter, E.M.; Gélvez-Rueda, M.C.; Osherov, A.; Bulović, V.; Grozema, F.C.; Stranks, S.D.; Savenije, T.J. Direct–indirect character of the bandgap in methylammonium lead iodide perovskite. Nat. Mater. 2017, 16, 115–120. [Google Scholar] [CrossRef]
  40. Mahon, N.S.; Korolik, O.V.; Khenkin, M.V.; Arnaoutakis, G.E.; Galagan, Y.; Soriūtė, V.; Litvinas, D.; Ščajev, P.; Katz, E.A.; Mazanik, A.V. Photoluminescence kinetics for monitoring photoinduced processes in perovskite solar cells. Sol. Energy 2020, 195, 114–120. [Google Scholar] [CrossRef]
  41. Bansal, S.; Aryal, P. Evaluation of New Materials for Electron and Hole Transport Layers in Perovskite-Based Solar Cells through SCAPS-1D Simulations. In Proceedings of the IEEE 43rd Photovoltaic Specialists Conference, Portland, OR, USA, 5–10 June 2016; pp. 747–750. [Google Scholar]
  42. Kunal, C.; Mahua, G.C.; Samrat, P. Numerical study of Cs2TiX6 (X = Br, I, F and Cl) based perovskite solar cell using SCAPS-1D device simulation. J. Sol. Energy 2019, 194, 886–892. [Google Scholar] [CrossRef]
  43. Baloch, A.B.A.; Hossain, M.I.; Tabet, N.; Alharbi, F.H. Practical Efficiency Limit of Methylammonium Lead Iodide Perovskite (CH3NH3PbI3) Solar Cells. J. Phys. Chem. Lett. 2018, 9, 426–434. [Google Scholar] [CrossRef]
  44. Helander, M.G.; Greiner, M.T.; Wang, Z.B.; Tang, W.M.; Lu, Z.H. Work function of fluorine doped tin oxide. J. Vac. Sci. Technol. Vac. Surf. Film. 2011, 29, 011019. [Google Scholar] [CrossRef]
  45. Lakhdar, N.; Hima, A. Electron transport material effect on performance of perovskite solar cells based on CH3NH3GeI3. Opt. Mater. 2020, 99, 109517. [Google Scholar] [CrossRef]
  46. Lin, L.; Jones, T.W.; Yang, T.C.-J.; Duffy, N.W.; Li, J.; Zhao, L.; Chi, B.; Wang, X.; Wilson, G.J. Inorganic Electron Transport Materials in Perovskite Solar Cells. Adv. Funct. Mater. 2021, 31, 2008300. [Google Scholar] [CrossRef]
  47. Karimi, E.; Ghorashi, S.M.B. Investigation of the influence of different hole-transporting materials on the performance of perovskite solar cells. Optik 2017, 130, 650–658. [Google Scholar] [CrossRef]
  48. Gan, Y.; Bi, X.; Liu, Y.; Qin, B.; Li, Q.; Jiang, Q.; Mo, P. Numerical Investigation Energy Conversion Performance of Tin-Based Perovskite Solar Cells Using Cell Capacitance Simulator. Energies 2020, 13, 5907. [Google Scholar] [CrossRef]
  49. Abdul, K.K.; Sadia, S.U.; Tasnim, T.F.; Sakibul, A.; Mohammad, A.A. Highly efficient Cesium Titanium (IV) Bromide perovskite solar cell and its point defect investigation: A computational study. Superlattice. Microst. 2021, 156, 106946. [Google Scholar] [CrossRef]
  50. Kanoun, A.A.; Kanoun, M.B.; Merad, A.E.; Goumri-Said, S. Toward development of high-performance perovskite solar cells based on CH3NH3GeI3 using computational approach. Sol. Energy 2019, 182, 237–244. [Google Scholar] [CrossRef]
  51. Raghvendra; Kumar, R.R.; Pandey, S.K. Performance evaluation and material parameter perspective of eco-friendly highly efficient CsSnGeI3 perovskite solar cell. Superlattice. Microst. 2019, 135, 106273. [Google Scholar] [CrossRef]
Figure 1. XRD pattern of K2TiI6, K2TiI2Br4 and K2TiBr6 (a) when prepared (b) after 10 months.
Figure 1. XRD pattern of K2TiI6, K2TiI2Br4 and K2TiBr6 (a) when prepared (b) after 10 months.
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Figure 2. SEM images of (a) K2TiI6; (b) SEM of K2TiBr6; (c) SEM image of K2TiI2Br4. (d) EDX spectrum of K2TiI6 (e) EDX spectrum of K2TiBr6 (f) EDX spectrum of K2TiI2Br4.
Figure 2. SEM images of (a) K2TiI6; (b) SEM of K2TiBr6; (c) SEM image of K2TiI2Br4. (d) EDX spectrum of K2TiI6 (e) EDX spectrum of K2TiBr6 (f) EDX spectrum of K2TiI2Br4.
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Figure 3. Absorption curve for K2TiI6, K2TiBr6 and K2TiI2Br4.
Figure 3. Absorption curve for K2TiI6, K2TiBr6 and K2TiI2Br4.
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Figure 4. Tauc plot for (a) direct bandgap of K2TiBr6, K2TiI6 and K2TiI2Br4 and (b) indirect bandgap of K2TiBr6, K2TiI6 and K2TiI2Br4. (c) Direct bandgap of K2TiI4Br2 and (d) indirect bandgap of K2TiI4Br2.
Figure 4. Tauc plot for (a) direct bandgap of K2TiBr6, K2TiI6 and K2TiI2Br4 and (b) indirect bandgap of K2TiBr6, K2TiI6 and K2TiI2Br4. (c) Direct bandgap of K2TiI4Br2 and (d) indirect bandgap of K2TiI4Br2.
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Figure 5. PL spectra of K2TiI2Br4, K2TiBr6, and K2TiI6.
Figure 5. PL spectra of K2TiI2Br4, K2TiBr6, and K2TiI6.
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Figure 6. (a) Thermogravimetric analysis of K2TiI6; (b) Differential thermal analysis of K2TiI6 and KI.
Figure 6. (a) Thermogravimetric analysis of K2TiI6; (b) Differential thermal analysis of K2TiI6 and KI.
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Figure 7. Schematic representation of solar cell devices.
Figure 7. Schematic representation of solar cell devices.
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Figure 8. Current density versus voltage plot for 200 nm thickness of absorber layer in the devices 1, 2, 3 and 4.
Figure 8. Current density versus voltage plot for 200 nm thickness of absorber layer in the devices 1, 2, 3 and 4.
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Figure 9. Voc, Jsc, fill factor and PCE of devices 1, 2, 3 and 4 for different absorber layer thickness.
Figure 9. Voc, Jsc, fill factor and PCE of devices 1, 2, 3 and 4 for different absorber layer thickness.
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Figure 10. Comparing the values of Voc, Jsc, Fill factor and PCE for varying ETL thickness.
Figure 10. Comparing the values of Voc, Jsc, Fill factor and PCE for varying ETL thickness.
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Figure 11. Values of Voc, Jsc, FF and PCE for varying thickness of HTL with fixed thickness of ETL and K2TiI6.
Figure 11. Values of Voc, Jsc, FF and PCE for varying thickness of HTL with fixed thickness of ETL and K2TiI6.
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Table 1. Energy dispersive X-ray analysis of K2TiI6, K2TiBr6 and K2TiI2Br4.
Table 1. Energy dispersive X-ray analysis of K2TiI6, K2TiBr6 and K2TiI2Br4.
ElementWeight%Atomic%ElementWeight %Atomic%ElementWeight%Atomic%
(K2TiBr6)(K2TiI6)(K2TiI2Br4)
K K18.6526.37K K24.5043.83K K21.1134.96
Ti K37.4943.28Ti K16.0123.38Ti K12.8617.39
Br L43.8630.35I L59.4932.79Br L46.4937.68
I L19.549.97
Total100.00 Total100.00 Total100.00
Table 2. Parameters used in simulation of K2TiI6 based PSC.
Table 2. Parameters used in simulation of K2TiI6 based PSC.
ParametersFTOZnOTiO2K2TiI6PEDOT: PSSP3HT
Eg (eV)3.53.33.21.612.22
χ (eV)4.44.14.142.93.2
εr9991033
Nc (cm−3)2.2 × 10184 × 10181 × 10211 × 10192.2 × 10151 × 1020
Nv (cm−3)1.8 × 10191 × 10192 × 10201 × 10191.8 × 10181 × 1020
μn (cm2/Vs)20200204.4100.0001
μp (cm2/Vs)1025102.5100.0001
ND (cm−3)1 × 10191 × 10181 × 10193 × 101900
NA (cm−3)0003 × 10183.17 × 10141 × 1016
Ref.[44][45,46][45,46][32,33][47,48][49]
Table 3. Comparative analysis of cell parameters for K2TiI6 based PSCs.
Table 3. Comparative analysis of cell parameters for K2TiI6 based PSCs.
DeviceConfigurationVoc (V)Jsc (mA/cm2)FF %PCE %
1FTO/TiO2/K2TiI6/PEDOT: PSS1.15577.864747.95854.3593
2FTO/ZnO/K2TiI6/PEDOT: PSS0.94505.358045.90632.3245
3FTO/TiO2/K2TiI6/P3HT1.00032.290545.50321.0428
4FTO/ZnO/K2TiI6/P3HT1.07012.655451.29581.4576
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