Environmentally stable lead-free cesium bismuth iodide (Cs3Bi2I9) perovskite: Synthesis to solar cell application
Introduction
The organic metal halide perovskites are promising materials for opto-electronic devices due to their unique properties such as tunable bandgap, diffusion length of charge carrier, high absorption coefficient and excellent carrier transport. Due to these excellent properties, perovskite materials have attracted numerous researchers to seek their applications as light-absorbing material in thin film solar cells. The solution-processed methylammonium lead (Pb) halides (CH3NH3PbX3 or MAPbX3) have shown remarkable light-absorbing properties in solar cells and enhanced power conversion efficiency (PCE) [[1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12]]. In recent years, the conventional Pb-based perovskite solar cells have passed 22% PCE [[13], [14], [15], [16], [17], [18]]; however, the lifetimes of such solar cells are low due to chemical instability. Another detriment of CH3NH3PbX3 perovskite solar cells is the presence of toxic Pb. As a result, development of Pb-based perovskite materials for solar cell application is restricted and it is necessary to replace the Pb with a non-toxic element.
Several attempts have been made to replace Pb in perovskite material by group 14 elements such as tin (Sn) and germanium (Ge) and moderate PCE of up to 6.4% has been reported [19,20]. However, the insertion of Sn and Ge in perovskites results in poor stability because Sn2+ and Ge2+are oxidized to Sn4+ and Ge4+. Above all, when Pb is replaced by Sn, the perovskites show high mobilities in thin film transistors [21] but the material becomes metallic due to the self-doping effect [22,23]. Ogimi et al. [24] attempted a mixed metal (Sn–Pb) perovskite with tailorable band gap by changing the Sn-to-Pb ratio. They reported that Sn was a better alternative metal ion for solar cells with a lower band gap. Nonetheless, the CH3NH3SnI3 perovskite has not shown improvement in PCE because a small amount of Pb is essential to stabilize Sn in its Sn2+ oxidation state. Bismuth (Bi) is the element adjacent to Pb in the periodic table and has similar properties; Bi-based halide perovskites have structural formula A3Bi2X9, where A indicates monovalent cation (Na+, K+, Rb+, Cs+or CH3NH3+) and X indicates halogen atom (F, Cl, Br or I). The A3Bi2X9 perovskites have received considerable attention due to better stability under humid environments compared to Pb-based halide perovskite, and they also exhibit low-temperature solution process ability and non-toxicity. Few reports are available on Bi-based halide perovskite materials, and most earlier studies focused on the crystal structure and phase transition. There are few available reports concerning applications of A3Bi2I9 as light-absorbing material in solar cells [[25], [26], [27], [28], [29], [30], [31], [32], [33], [34]]. Currently, the highest PCE for Bi-based perovskite solar cells is 3.20% [35].
Making single-step Pb-free perovskite thin films for opto-electronic applications is critical. In the present study, we developed a solution process for a single-step synthetic route that allows the preparation of pure-phase Pb-free Cs3Bi2I9 perovskite thin films. We prepared an inorganic Pb-free Cs3Bi2I9 perovskite layer with prolonged environmental and thermal stability at low temperature (~80 °C) using a simple and cost-effective spin coating method. The structural, optical and morphological properties of the Cs3Bi2I9 perovskite layer were meticulously examined. The synthesized Cs3Bi2I9 perovskite layer was successfully employed in AZO/compact-TiO2/Cs3Bi2I9/CuSCN/graphite solar cells. The solar cell based on Cs3Bi2I9 perovskite showed a maximum PCE of 0.17% under 100 mW/cm2, which was much lower than the previously reported value of 3.20% [35]. With further optimization of synthesis conditions and the device fabrication process, there is an enormous opportunity to increase the device efficiency toward the development of highly efficient, environment friendly Cs3Bi2I9 perovskites for diverse opto-electronic applications. Absence of Pb in the perovskite material with its improved environmental and thermal stability makes it advantageous over its counterparts. Furthermore, fabrication of the entire device at a lower temperature (>100 °C) opens the window for producing flexible perovskite solar cells.
Section snippets
Preparation Cs3Bi2I9 thin films
In order to synthesize Cs3Bi2I9 films, a solution of Cs3Bi2I9 was prepared by dissolving high-purity powder of cesium iodide (CsI, 99.99% pure, Alfa Aesar, 2.1 M) and bismuth iodide (BiI3, 99% pure, Sigma Aldrich, 0.7 M) in 0.5 ml of di-methyl-formamide [(CH3)2NCH, DMF] and stirring it for 1 h at 70 °C. A one-step spin-coating solution process method was used for the deposition of Cs3Bi2I9 perovskite films. The precursor solution of Cs3Bi2I9 was dropped onto the substrates and initially
XRD analysis
Formation of Cs3Bi2I9 perovskite material was first confirmed from XRD graphs. The XRD patterns of Cs3Bi2I9 perovskite films deposited on AZO and glass substrates (using one-step spin coating method) are shown in Fig. 2a and b, respectively. The XRD patterns of the same Cs3Bi2I9 perovskite film taken after 1 and 3 months of exposure to ambient environment conditions (room temperature ~27–28 °C and humidity~55–60%) are shown in Fig. 2c and d, respectively. All the Cs3Bi2I9 perovskite films were
Conclusion
We successfully synthesized Pb-free Cs3Bi2I9 perovskite thin films using a one-step spin coating method. Formation of Cs3Bi2I9 perovskite was confirmed by XRD, Raman spectroscopy, XPS and TEM. The XRD clearly showed that all Cs3Bi2I9 perovskite films were polycrystalline and with hexagonal structure. Furthermore, the XRD pattern of Cs3Bi2I9 perovskite layer taken after 3 months of exposure to ambient environment conditions showed that the material had high stability. The UV–visible spectroscopy
CRediT authorship contribution statement
Ravindra Waykar: Data curation, Investigation, Formal analysis, Writing - original draft. Ajinkya Bhorde: Investigation, Data curation, Methodology. Shruthi Nair: Data curation, Methodology. Subhash Pandharkar: Data curation, Formal analysis. Bharat Gabhale: Data curation, Methodology. Rahul Aher: Validation, Methodology. Sachin Rondiya: Data curation, Methodology. Ashish Waghmare: Investigation, Methodology. Vidya Doiphode: Data curation, Methodology. Ashvini Punde: Investigation, Methodology.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
Ravindra Waykar, Subhash Pandharkar, Shruthi Nair and Ashish Waghmare are thankful to the Ministry of New and Renewable Energy (MNRE), Government of India, for the financial support under the National Renewable Energy Fellowship (NREF) program. Ajinkya Bhorde is thankful to the Department of Science and Technology (DST), Government of India, for an INSPIRE Ph.D. Fellowship. Rahul Aher is thankful to Savitribai Phule Pune University, Pune, for the award of Bharatratna J.R.D. Tata Gunwant
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