Applied Science and Convergence Technology 2024; 33(1): 23-26
Published online January 30, 2024
Copyright © The Korean Vacuum Society.
kangwon national universityDepartment of Physics and Institute of Quantum Convergence Technology, Kangwon National University, Chuncheon 24341, Republic of Korea
†These authors contributed equally to this work.
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (https://creativecommons.org/licenses/by-nc-nd/4.0/) which permits non-commercial use, distribution and reproduction in any medium without alteration, provided that the original work is properly cited.
Formamidinium-based metal halide perovskites (FAPbI3) have gained prominence as materials for high-efficiency solar cells owing to their superior optoelectronic properties compared to those of traditional methylammonium-based perovskites. However, it is difficult to maintain the photo-active α-phase of FAPbI3 owing to the structural instability at room temperature. Although this issue can be addressed by partially substituting cesium (Cs), most reported CsFAPbI3 perovskites are produced in inert environments because of their low stability under ambient conditions. In this study, ethyl acetate, which protects the perovskite wet film from moisture during synthesis, was used as an antisolvent to optimize the Cs concentration in CsxFA1−xPbI3 under ambient conditions. Despite the formation of δ-CsPbI3 with increasing Cs concentration, the carrier lifetime was enhanced, resulting in improved power conversion efficiency. Highest reproducibility of solar cell was observed at a Cs concentration of 22 %.
Keywords: Cesium, Formamidinium, Mixed-cation, Perovskite, Photovoltaic
Formamidinium-based metal halide perovskite (FAPbI3) is a highly desirable material for high-power conversion efficiency (PCE) perovskite solar cells owing to its exceptional optoelectronic properties and high thermal stability . However, FAPbI3 typically maintains a photo-inactive δ-phase at room temperature (RT) while retaining the photo-active α-phase only at temperatures exceeding 300 K . The photo-inactive phase refers to a phase with a band gap greater than 2 eV; therefore, it cannot absorb light in the entire visible spectrum. The photo-active phase refers to a phase with a band gap that can absorb light in the visible spectrum. In solar cells, it is essential to absorb light in the visible spectrum, because a significant amount of light in the visible range reaches the Earth’s surface. Therefore, maintaining a photo-active phase that can absorb light over a broader range of the visible spectrum is crucial as opposed to a photo-inactive phase with a larger bandgap that cannot absorb light in the visible spectrum.
The phase instability of FAPbI3 can be improved by the addition of appropriate amounts of other A-site cations. For example, in the case of perovskite (MAFAPbI3) mixed with formamidinium [CH(NH2)2+, FA+] and methylammonium cations [CH3NH3+, MA+], FAPbI3 maintained the α-phase at RT because of the increased hydrogen bonding between hydrogen and iodine, which can be attributed to the higher dipole moment of the MA+ cation than the FA+ cation . However, MA cations are relatively volatile, leading to the formation of MA+ vacancies and thermal instability, which can negatively impact PCE [4, 5]. Substituting FA+ cations with inorganic cesium cations (Cs+) helps maintain the α-phase at RT and enhances the thermal stability compared to MAFAPbI3 [6–8]. Owing to these advantages, CsFAPbI3 has exhibited an improved PCE compared to that of FAPbI3 [8–10]. For example, in a study by Chen
While synthesizing CsFAPbI3 under ambient conditions, using ethyl acetate as an antisolvent could be an effective method to suppress the formation of δ-CsPbI3 because ethyl acetate absorbs moisture during the perovskite layer deposition process, allowing the synthesis of perovskites even in high-humidity environments. In a study by Troughton
In this study, we synthesized Cs
The 1.49 M FAPbI3 (Cs 0 %) precursor solution was prepared by mixing 172 mg (1 mmol) of formamide iodide (FAI) and 461 mg (1 mmol) of lead iodide (PbI2) in 71 µL of DMSO and 0.6 mL of
X-ray diffraction (XRD) measurements were performed using an X’ Pert Pro MPD X-ray diffractometer (PANalytical). The layer morphology was determined using scanning electron microscopy (SEM) (S-4800, Hitachi). Photoluminescence (PL) measurements were conducted using a Si photodiode (Electro Optical Systems Inc.) with the excitation source emanating from a 532-nm diode-pumped solid-state laser (CNI Optoelectronics Technology). The PL signals were accumulated using an SR510 lock-in amplifier (Stanford Research Systems). Time-resolved PL (TRPL) spectra were obtained using an FLS 920 spectrometer (Edinburgh Instruments) with a 656 nm picosecond pulsed diode laser (pulse width: 70 ps) as the excitation source. The photocurrent density–voltage (
As shown in Figs. 1(a)-(e), top-view SEM images were obtained to investigate the morphology of the Cs
Figure 2(a) shows the XRD spectra of the Cs
The occurrence of phase separation with δ-CsPbI3 was confirmed through PL and absorbance measurements. Figure 3(a) shows the normalized PL spectra of the Cs
Figures 4(a)-(d) show the photovoltaic parameters of the ITO/SnO2 /Cs
This research was supported by the Basic Science Research Program through the National Research Foundation of Korea, funded by the Ministry of Education, Science, and Technology (NRF-2019R1A2C-1086813). This study has been worked with the support of a research grant of Kangwon National University in 2023. Time-resolved photoluminescence measurements were performed at the central lab of Kangwon National University.
The authors declare no conflicts of interest.