Abstract
Controlling the crystallinity and surface morphology of perovskite layers by methods such as solvent engineering1,2 and methylammonium chloride addition3,4,5,6,7 is an effective strategy for achieving high-efficiency perovskite solar cells. In particular, it is essential to deposit α-formamidinium lead iodide (FAPbI3) perovskite thin films with few defects due to their excellent crystallinity and large grain size. Here we report the controlled crystallization of perovskite thin films with the combination of alkylammonium chlorides (RACl) added to FAPbI3. The δ-phase to α-phase transition of FAPbI3 and the crystallization process and surface morphology of the perovskite thin films coated with RACl under various conditions were investigated through in situ grazing-incidence wide-angle X-ray diffraction and scanning electron microscopy. RACl added to the precursor solution was believed to be easily volatilized during coating and annealing owing to dissociation into RA0 and HCl with deprotonation of RA+ induced by RA⋯H+-Cl− binding to PbI2 in FAPbI3. Thus, the type and amount of RACl determined the δ-phase to α-phase transition rate, crystallinity, preferred orientation and surface morphology of the final α-FAPbI3. The resulting perovskite thin layers facilitated the fabrication of perovskite solar cells with a power-conversion efficiency of 26.08% (certified 25.73%) under standard illumination.
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Data availability
The data that support the findings of this study are available from the corresponding authors on reasonable request.
Code availability
The code used for this study is available from the corresponding authors on reasonable request.
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Acknowledgements
This work was supported by the Basic Science Research Program (NRF-2018R1A3B1052820) through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (MSIP). H.J.M. and T.J.S. acknowledge the financial support from NRF contract number (NRF-2018R1A5A 1025224). Finally, we thank UCRF (UNIST Central Research Facilities) for support in using the equipment and the beamline staff at Pohang Accelerator Laboratory.
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J.P., J.K. and S.I.S. conceived this work and designed the experiment. H.-S.Y. performed a preliminary experiment. J.P. and J.K. fabricated the PSCs with various electrodes and characterized the perovskite films. M.J.P. and E.N. prepared the electrodes and samples for analysis. H.J.M. and T.J.S. conducted and interpreted the GI-WAXD measurements. M.G.K. measured and interpreted the EXAFS measurements. S.I.S. wrote the draft of the manuscript and all authors contributed feedback and comments for revising the manuscript. S.I.S. directed and supervised the study.
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Extended data figures and tables
Extended Data Fig. 1 Surface morphologies of thin films.
Thin films spin-coated with 10 mol% MACl (a) and BACl (b) added to FAPbI3 precursor containing 35 mol% MACl. Scale bars, 1 μm.
Extended Data Fig. 2 Changes in the width of the (100)α peak with time.
Shown during annealing from RT to 120 °C, using the thin films formed by dripping the antisolvent during spin-coating of the FAPbI3 precursor solutions containing 0, 5, 10 or 15 mol% PACl added to a 35 mol% MACl.
Extended Data Fig. 3 Representative 2D GI-WAXD images and enlarged 1D GI-WAXD profiles during in situ experiments.
a–e, Control. f–j, Target. k–p, Reference 1. q–u, Reference 2. GI-WAXD measured while increasing and maintaining the temperature from RT to 120 °C. v–y, Enlarged 1D GI-WAXD profiles of control (v), reference 1 (w), target (x) and reference 2 (y) samples around the (100)α peak to investigate the onset temperature of the δ-phase to α-phase transition. z, Plot of the onset temperature of the δ-phase to α-phase transition. 2D GI-WAXD shows that the transition temperature from δ to α phase gradually decreases as the length of the alkyl group in RACl increases from MACl to PACl and BACl. The boiling points and basicity of deprotonated MA0, PA0 and BA0 are different. Thus, strong acid–base interaction can occur between the Lewis acid PbI2 and the Lewis base R-NH2 on the surface of FAPbI3. Alkyl substituents such as methyl, propyl and butyl are electron-donating groups and tend to increase with increasing length. This makes BA0 more Lewis basic, increasing RH2N–PbI2 interaction. Eventually, BA0 deprotonated in BACl lowers more surface energy of δ-FAPbI3, leading to a faster and more effective δ-phase to α-phase transition.
Extended Data Fig. 4 In situ GI-WAXD.
Shown during heating from RT to 120 °C using the thin film deposited without dripping the antisolvent during spin-coating of the FAPbI3 precursor solutions with 10 mol% PACl added to a 35 mol% MACl.
Extended Data Fig. 5 Changes in colour observed at RT of thin films.
Thin films were deposited by dripping the antisolvent during spin-coating of the FAPbI3 precursor solutions containing 10 mol% of MACl, PACl and BACl, respectively, added to a 35 mol% of MACl.
Extended Data Fig. 6 Fourier-transformed RDF.
RDF obtained using theoretically calculated Pb LIII-edge EXAFS for the α-FAPbI3, δ-FAPbI3 and relaxed-FAPbI3, respectively.
Extended Data Fig. 7 Space-charge-limited current analysis.
Shown for reference 1 and reference 2. VTFL is trap-filled limit voltage.
Extended Data Fig. 8 Changes in the mutually normalized integrated area over time and representative 2D GI-WAXD images.
a, Changes in the mutually normalized integrated area obtained by α-phase peaks of in situ GI-WAXD measured while increasing and maintaining the temperature from RT to 120 °C. b, Representative 2D GI-WAXD images of target and reference 2 samples annealed at 120 °C for 30 min.
Supplementary information
Supplementary Information
Supplementary Figs. 1–8 and Supplementary Tables 1 and 2.
Supplementary Video 1
Preparing formamidinium lead triiodide (FAPbI3) black powder by heating at 120 °C in an oil bath with stirring.
Supplementary Video 2
Filtrating the precipitated FAPbI3 using filter paper.
Supplementary Video 3
Depositing perovskite thin films by spin-coating at a relative humidity of 20–30% in ambient air.
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Park, J., Kim, J., Yun, HS. et al. Controlled growth of perovskite layers with volatile alkylammonium chlorides. Nature 616, 724–730 (2023). https://doi.org/10.1038/s41586-023-05825-y
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DOI: https://doi.org/10.1038/s41586-023-05825-y
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