Materials Today Energy
In situ nanocrystal seeding perovskite crystallization toward high-performance solar cells
Graphical abstract
Introduction
Hybrid organic–inorganic perovskites (e.g. CH3NH3PbX3, X = Cl, Br, I) have recently attracted a worldwide research interest, owing to their advancing optoelectronic properties such as tunable band gap, high extinction coefficient, high charge carrier mobility, cost-effective manufacturing process [[1], [2], [3], [4]], and so on, and the highest efficiency of solar cells based on perovskites (PSCs) has increased linearly from the initial 3.8% to 25.5% in the last few years [5,6]. Even though the perovskites exhibited high tolerance on defect-related energy loss [7], the efficiency of PSCs was still lagged far behind the thermodynamic limit because some unintentional defects were inevitable to appear in simply solution-processed multicrystalline perovskites [[8], [9], [10], [11], [12]], which not only restricted the open-circuit voltage (VOC) of the device [13] but also directly correlated with hysteresis and stability issues [14,15]. Therefore, the trap state passivation or elimination is worthwhile and of great importance to further improve the device performance [[16], [17], [18], [19], [20], [21], [22]].
Crystallographic distortions in terms of element vacancies, interstitials, substitutional impurity, and grain boundaries are the main defects in the bulk or surface of perovskite thin films, which originally occur from the fabrication process. It was known from the crystallization dynamics that fast nucleation and the prolonged crystal growth process are prerequisites for the deposition of high-quality perovskite thin films, which should advantageously contain enlarged crystal size with improved surface coverage and minimized lattice distortion with preferred crystal orientation. Many strategies have been developed to accelerate the crystal nucleation, e.g. antisolvent dripping [23], vacuum flashing [24], high-temperature hot-casting [25], and so on, to speed up the solvent escape and reach a fast super-saturation concentration or to induce intermediate states by generating solvent-PbI2 via solvent engineering [26], creating additive-PbI2 complex via incorporating external functional additives [27] or adding some perovskite nanocrystals to seed the perovskite growth. In the meanwhile, some other methodologies were used to prolong the crystal growth process, e.g. using solvents with gradient boiling points [28], multistep thermal annealing [29], or annealing at solvent or moisture atmosphere, and so on, to retard the solvent exhaustion and prolong the crystal growth, or boosting the surface tension of the crystal grains by small ions doping [30] or light illumination [31] to enhance the possibility of coalescence at the expense of small grains. Therefore, perovskite crystals with grain size in the range from micrometer to millimeter can be designed.
In addition, some other efforts have been focused on defect passivation, and some functional agents have been intentionally incorporated in the perovskite preparation or after treatment to associate with antisite defects or under-coordinated ions, e.g. squaraine molecules with a zwitterionic structure have been used to associate with under-coordinated Pb2+ and passivate Pb–I antisite defects [32], iodofluorobenzene and Lewis bases (such as thiophene) can effectively passivate the defect states on the perovskite surface because of their interaction with insufficiently coordinated halide anions and metal cations [33], and neutral sulfonic acid–containing cationic quaternary ammonium salt and anionic sulfonate (–SO3−) was also used to effectively passivate defect states [34]. Hence, perovskite films with photoluminescence (PL) quantum efficiency up to 90% can be obtained [35]. In addition, some other additives were incorporated into the perovskite as a redox shuttle to oxidize Pb0 and reduce I0 [36] or turning the tolerant factor or dimensions to reduce the phase vibration.
The aforementioned methods were favorable to design high-quality perovskites, but complicated the fabrication process with limited reproducibility. In addition, the incorporation of external organic or metallic additives might need aftertreatment to remove them or parasitically reduce the conductivity or induce stability issues. Therefore, it is highly desirable to eliminate defect states in a feasible one-step deposition method. Recently, we reported a new route to fabricate high-quality perovskite thin films by using methylammonium acetate (MAAc) ionic liquid as the solvent instead of traditional toxic and highly coordinating solvents [37,38]. MAAc can strongly associate with the perovskite precursor via H-bonding, and the low vapor pressure of MAAc enabled a prolonged crystal growth, and the high viscosity of the precursor solution with enhanced surface tension significantly improved the moisture stability of the solution, which facilitated the perovskite fabrication in ambient air condition with the absence of antisolvent and low precursor solution concentration. However, the highly bonded MAAc with the perovskite precursor parasitically retarded solvent evaporation and limited mass transport during the crystallization process, which slowed down the crystal nucleation process and led to the aggressive perovskite growth on the limited nucleus on post-thermal annealing, resulting in numerous unintentional defects within the as-deposited thin films.
Herein, a series of lead halides (PbCl2, PbBr2, PbI2) were used as functional additives for the preparation of high-quality perovskite thin films when using ionic liquid MAAc as the solvent, and the influence of the additives on the perovskite crystallization and device performance was scrutinized. It was found that the addition of excess lead halides can associate with MAAc and form MAPbX3-xAcx nanocrystals, which can be used as the nucleus to seed the perovskite growth, thus accelerating the nucleation. Moreover, the PbCl2 was found to be superior to PbBr2 and PbI2, by simultaneously reducing the trap-state density and shifting the trap depth to a shallower one, resulting in a significantly enhanced VOC in solar cells based on PbCl2-doped perovskites. As a result, a champion efficiency of 21.26% with high storage stability for more than 1300 h was achieved by adding PbCl2 into the perovskite precursor, which provided a feasible route to obtain high-performance perovskite solar cells in a highly reproducible way.
Section snippets
Results and discussions
Lead halides, such as PbCl2, PbBr2, and PbI2, have different association abilities with MAAc owing to their different electronegativity of the halide elements and different ionization potential in the solvent. Liquid-state 1H nuclear magnetic resonance (NMR) spectroscopy was performed to investigate how the lead halides interact with MAAc in solution. As shown in Fig. 1a, a characteristic peak at 7.97 ppm arising from the amino protons of MAAc [39] was observed in 1H NMR spectra of the pure
Conclusion
In summary, we demonstrate a novel route to fabricate high-quality perovskite thin films by using lead halides PbX2 (X = Cl, Br, I) as additives when using MAAc as the solvent. It is evidenced that the lead halides can associate with MAAc and generated MAPbX3-xAcx perovskite. The as-formed perovskites can be used as the nucleus to seed the perovskite growth, resulting in enlarged perovskite crystal size with eliminated defect states. Among them, the incorporation of PbCl2 exhibits the best
Materials and reagents
Methylamine solution (33% in H2O), ethylic acid (99%), and N, N-dimethylformamide (anhydrous) were purchased from Sigma-Aldrich. Lead iodide (PbI2, 99.99%) was purchased from Nano-C. Methylammonium iodide (CH3NH3I, 99.5%), Spiro-OMeTAD, 99.5% bis (trifluoromethane) sulfonamide lithium (Li-TFSI, 99%), and 4tert-butylpyridine (TBP, 98%) were obtained from Shanghai MaterWin New Materials Co., Ltd. Tin (IV) oxide SnO2 solution (15% in H2O colloidal dispersion) was purchased from Alfa Aesar (China)
CRediT author statement
Wen Wu: Investigation, Writing - original draft, Data curation. Min Fang: Investigation, Validation. Lingfeng Chao: Investigation. Lei Tao: Investigation. Hui Lu: Data curation. Bixin Li: Data curation. Xueqin Ran: Formal analysis. Ping Li: Formal analysis. Yingdong Xia: Formal analysis. Hui Zhang: Supervision, Writing - review & editing, Funding acquisition, Visualization. Yonghua Chen: Conceptualization, Supervision, Writing - review & editing, Funding acquisition.
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 article.
Acknowledgments
This work was financially supported by the Natural Science Foundation of China (grants 51972172, 61705102, and 91833304), the Young 1000 Talents Global Recruitment Program of China, the Jiangsu Specially Appointed Professor Program, and a fellowship from the China Postdoctoral Science Foundation (2020M672181).
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