Microampere-level piezoelectric energy generation in Pb-free inorganic halide thin-film multilayers with Cu interlayers
Graphical Abstract
Introduction
Perovskite halide materials possessing strong ferroelectricity and piezoelectricity have been actively investigated for potential applications such as sensors, generators, actuators, and transducers, with the aim of finding unique attributes that are not usually found in conventional oxide-based piezoelectric materials [1], [2], [3]. One of the applications that substantially exploit piezoelectricity is electromechanical energy generation, in which energy is typically harvested from free vibrational or bending source. The level of generated power is determined by the magnitude of polarization induced within the dielectric material, and thus, the subsequent potential difference between electrodes [4], [5]. Perovskite halide materials have advantages for such power-generation applications, including low relative permittivity and low-temperature processing compatible with flexible plastic substrates [6], [7]. Numerous examples of halide-based energy harvesters have been demonstrated, specifically using composite structures consisting of halide nanocrystals dispersed in a piezoelectric polymer matrix such as polyvinylidene fluoride (PVDF) or its derivatives [8], [9], [10], [11]. Owing to the benefits of this piezoelectric polymer, noteworthy harvesting performance has been reported, e.g., ~12 V and ~4000 nA for a methylammonium (MA) tin iodide (MASnI3)-PVDF composite [10] and ~22.8 V and ~6150 nA for a formamidinium lead iodide (FASnI3)-PVDF composite [11].
However, scarce research exists on energy harvesters made from thin films, particularly all-inorganic halide films, presumably owing to the difficulty of preparing high-quality but sufficiently thick thin films due to the limited availability of chemical precursors and processing adaptability. The form of thin films is generally preferred from the perspective of processing compatibility for the production of power-generating devices based typically on micro-machined cantilever structures [4], [12]. Beyond the broad adaptability of device designs with specific electrode structures, the thin-film harvesters generally have high power density due to the volume efficiency particularly from the thin thickness of submicron-scale, relative to the bulk or composite power-generators [13], [14]. Table S1 of the Supplementary Data summarizes the characteristics of previously reported piezoelectric energy harvesting devices based on halide thin films [5], [10], [15], [16], [17], [18], [19], most of which were composed of MA-based inorganic halides. For example, MAPbI3-based thin films demonstrated outputs of ~7.29 V and ~880 nA after poling at 30 kVcm−1 [15]. The only device based on an all-inorganic halide thin film of CsPbBr3 exhibited an energy-harvesting capability of ~16.4 V and ~640 nA after poling at 24.9 kVcm−1 [19].
There have been a lot of recent efforts incorporating Pb-free piezoelectric materials for power-generation purposes [20], [21], [22]. Herein, we introduce for the first time lead-free inorganic halide thin films for a highly efficient piezoelectric energy harvester, while aiming to optimize the quality of the flexible thin films on a plastic substrate primarily by controlling the solution deposition condition. The independent effects of poling and the multilayer structure (with metallic interlayers) are examined as critical contributors to the substantial enhancements in power generation. Depositing Cu interlayers enabled stacking the halide films to increase the total film thickness, accompanied by the positive effect of Cu on the polarization. Cu as an interlayer was selected owing to its high electrical conductivity and facile deposition as being used widely as conductors or fillers in rigid and flexible electronic devices [23], [24]. Total effective thickness of the halide films has been typically limited to ~200–500 nm because of the restricted availability of precursor chemicals and the dissolution of the first coated layer during the subsequent coating with the precursor solution [25], [26]. As a highlight, a CsSnI3 multilayer harvester incorporating three Cu interlayers delivered excellent harvesting performance of ~22.9 V and ~1233 nA, which are the highest values recorded thus far among halide-based thin films. The promising values are presumably due to the high effective piezoelectric coefficient attained by domain orientation caused by the electric poling and the extra space-charge polarization across the interfaces between the halide and Cu. The structural origin was further explored by estimating the poling-field-dependent structural changes along the c-axis, such as the distortion of SnI6 octahedra and the extensive off-centering of Sn atoms.
Section snippets
Experimental section
Precursor solutions of CsSnI3 were prepared by completely dissolving cesium iodide (CsI, >99.0%, Tokyo Chemical Industry Co., Japan) and tin iodide (SnI2, 99.999%, Alfa Aesar, USA) in N,N-dimethylformamide (DMF, 99.9%, Sigma-Aldrich, USA) at 70 °C for 6 h. Tin iodide was added with an excess of 5 wt% beyond the stoichiometry to achieve a phase-pure structure. Solutions with different concentrations of 0.4, 0.6, 0.8, and 1.0 M were separately spin-coated at 3000 rpm for 60 s on an indium tin
Results and discussion
Flexible CsSnI3 thin films were deposited onto an ITO-coated PET substrate by one-step spin-coating of chemical solution. Fig. 1a shows the XRD patterns of the halide thin films prepared using precursor solutions ranging in concentration from 0.4 to 1.0 M. Irrespective of the precursor concentration, phase-pure orthorhombic perovskite structure with random orientation was observed, although more intense peak intensities were observed with a higher concentration, indicating stronger
Conclusion
CsSnI3 was investigated as a representative Pb-free all-inorganic halide material, while competitive electromechanical energy-harvesting performance was demonstrated with the unprecedented strategy of combining a poling process with a multilayered structure incorporating Cu interlayers. This is the first study demonstrating the harvesting performance of thin films composed of a Pb-free inorganic halide perovskite. As a highlight, excellent harvesting outcomes of ~22.9 V and ~1233 nA were
CRediT authorship contribution statement
Da Bin Kim: Investigation, Data curation, Writing – original draft. Kwan Sik Park: Investigation, Data curation. Sun Jae Park: Investigation, Data curation. Yong Soo Cho: Supervision, Writing – review & editing.
Author contributions
D.B.K., K.S.P., and S.J.P. performed the experiments with data analysis. Y.S.C. supervised the whole work and completed the preparation of the manuscript. D.B.K. and K.S.P. were contributed equally.
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.
Acknowledgement
This work was financially supported by grants from the National Research Foundation of Korea (NRF-2021R1A2C2013501 and NRF-2020M3D1A2102913), the Creative Materials Discovery Program of the Ministry of Science and ICT (2018M3D1A1058536), and the Graduate School of 2020 Yonsei University Research Scholarship Grants.
Da Bin Kim joined the Ph.D. program of Department of Materials Science and Engineering of Yonsei University, Seoul, Korea, in 2017. She has been actively involved in preparation and characterization of perovskite halides-based piezoelectric and optoelectronic devices. Direct correlations of strain-driven electrical and optical responses with structural aspects are being mainly pursued.
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Da Bin Kim joined the Ph.D. program of Department of Materials Science and Engineering of Yonsei University, Seoul, Korea, in 2017. She has been actively involved in preparation and characterization of perovskite halides-based piezoelectric and optoelectronic devices. Direct correlations of strain-driven electrical and optical responses with structural aspects are being mainly pursued.
Kwan Sik Park joined the Ph.D. program of Department of Materials Science and Engineering of Yonsei University, Seoul, Korea, in 2021. He started to do experiments on the chemical processing and characterization of perovskite halides-based electronic and optoelectronic devices.
Sun Jae Park completed the MS program of Department of Materials Science and Engineering of Yonsei University, Seoul, Korea in 2021. She studied the inorganic halide-based piezoelectric power-generators using thin-film structures processed by chemical solution deposition.
Yong Soo Cho is currently a professor of Department of Materials Science and Engineering of Yonsei University. He received his Ph.D. degree from New York State College of Ceramics at Alfred University in 1997. After employed at DuPont as a senior scientist for 6 years, he joined Yonsei University in 2004. His research focuses mainly on dielectric, ferroelectric and piezoelectric materials with various forms, including bulk, thin film, nano-scale structure and two-dimensional layer, for extensive electronic and optoelectronic applications.
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These authors contributed equally to this work.