Lead halide perovskite light-emitting diodes (PeLEDs) have demonstrated remarkable optoelectronic performance1,2,3. However, there are potential toxicity issues with lead4,5 and removing lead from the best-performing PeLEDs—without compromising their high external quantum efficiencies—remains a challenge. Here we report a tautomeric-mixture-coordination-induced electron localization strategy to stabilize the lead-free tin perovskite TEA2SnI4 (TEAI is 2-thiopheneethylammonium iodide) by incorporating cyanuric acid. We demonstrate that a crucial function of the coordination is to amplify the electronic effects, even for those Sn atoms that aren’t strongly bonded with cyanuric acid owing to the formation of hydrogen-bonded tautomeric dimer and trimer superstructures on the perovskite surface. This electron localization weakens adverse effects from Anderson localization and improves ordering in the crystal structure of TEA2SnI4. These factors result in a two-orders-of-magnitude reduction in the non-radiative recombination capture coefficient and an approximately twofold enhancement in the exciton binding energy. Our lead-free PeLED has an external quantum efficiency of up to 20.29%, representing a performance comparable to that of state-of-the-art lead-containing PeLEDs6,7,8,9,10,11,12. We anticipate that these findings will provide insights into the stabilization of Sn(II) perovskites and further the development of lead-free perovskite applications.
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This work was partly supported by the National Natural Science Foundation of China (grant nos. 51972137, 12175298 and 62174104), the Science and Technology Planning Project of Jilin Province (grant no. 20190201306JC) and the start-up funding of Jilin University. Y.Y. acknowledges the financial support from the Shanghai Municipal Commission for Science and Technology (no. 20ZR1464100). U.R. acknowledges the Swiss National Science Foundation (grant no. 200020-185092) for funding as well as computational resources from the Swiss National Computing Centre CSCS. M.G. acknowledges financial support from the European Union’s Horizon 2020 research and innovation programme under grant agreement no. 881603. We thank the staff of beamlines BL17B1, BL19U1 and BL19U2 at SSRF for providing the beam time and the User Experiment Assist System of SSRF for their help. We thank M. Yao and J. Ning for discussions and G. Chen for TA measurements.
The authors declare no competing interests.
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Extended data figures and tables
a, Calculated electrostatic potential distribution for enol form of the CA molecule. b, Electron localization function image for the triketo CA-treated perovskite. c, Charge density difference between the tin perovskite and triketo CA. Obtained in systems with an iodide vacancy. d, Planar-averaged charge density difference of the pristine and the Sn perovskite with CA. e, Dielectric coefficients of the Sn perovskites with and without CA along the Z-axis. f, Density of states for the pristine (blue) and the CA-treated perovskite (red) with a surface iodide vacancy.
a, b, Built slab containing two inorganic layers (a) and a vacuum space of 17 Å. For the ab-initio molecular dynamics simulations, this model was expanded to a 4×3×1 supercell (b). Color code: Sn in grey, I in purple, S in yellow, C in black, H in pink, O in red, N in blue. c, AIMD snapshoot of a stable dimer at a perovskite grain boundary. The distances are in Å. Color code: Sn in grey, I in purple, S in yellow, C in black, H in pink, O in red, N in blue. d, Valeric acid dimer on top of a pristine TEA2SnI4 surface. The protons are shared between two oxygens with distances of 1.0 Å and 1.6 Å. Valeric acid only contains one functional group - thus if the carboxylic acid group is used to form dimers, the molecule can no longer bind to the perovskite surface, as observed with ab initio MD simulations. e, Dimer formation energies (blue) and adsorption energies (red) for CA and valeric acid (VA). f, Vibrational power spectra of the CA keto and enol monomers and dimers in gas phase. The mixed enol-keto form can also explain the observed CO stretch in FTIR. This figure shows the vibrational power spectra of the gas phase molecules of monomers and dimers of the pure keto and pure enol CA form. In the monomeric form, the CN and CO modes overlap at 1587 cm−1 and 1775 cm−1 for the enol and keto forms, respectively. Upon dimerization, these peaks split into three components with the CO contribution blue shifted compared to the CN one. From these results we can conclude that the experimental spectra contain contributions from mixed keto/enol dimers. Furthermore, Fig. 1 shows the computed vibrational power spectra of the most stable trimer configuration in vacuum and adsorbed on the perovskite surface. When the trimer is adsorbed on the surface, a blue shift of 25 cm−1 and 15 cm−1 is reported for CN and CO modes, respectively, in agreement with the experimental data. Another clear signature that CA adsorbs in the form of trimers and/or dimers is the peak around 3000~3300 cm−1 due to OH and NH groups involved in H-bond interactions (CO-HNC); indeed, this kind of peaks are not present for monomers.
a, b, SEM images of the perovskite films without (a) and with 5% CA (b). c, d, AFM images of the pristine (c) and the 5% CA-treated sample (d). e, KPFM measurements for the pristine and the CA-treated sample. f, Linear potential profiles for the pristine and the CA-treated sample.
a, b, GIWAXS images of TEA2SnI4 perovskite films without (a) and with 5% CA (b). c, XRD patterns of the pristine and the samples with 2%, 5% and 8% CA. d, Comparison of (002) peak for the samples with and without 5% CA.
a, XPS spectra of Sn 3d for the pristine and the CA-treated TEA2SnI4. b, XPS spectra of Sn 3d for the pristine and the CA-treated TEA2SnI4 stored in dry air for 12 h. c, XPS results of I 3d spectra for the tin perovskite films without and with 5% CA. d, Ratios of Sn versus I from the XPS spectra for the tin perovskite samples with and without CA. e, XPS spectra of Sn 3d for the pristine and the CA-treated TEA2SnI4 nanoplates. f, XPS spectra of Sn 3d for the pristine and the CA-treated TEA2SnI4 nanoplates stored in dry air for 12 h. g, XRD patterns of TEA2SnI4 nanoplates with and without CA. Time-dependent XRD patterns of TEA2SnI4 nanoplates without (h) and with CA (i) stored in dry air for 24 h. j-m, High-resolution TEM images of perovskite films. Insets: fast Fourier transform diffractograms. Scale bar, 5 nm. To evaluate the surface effect and analysis on how the air-stability relates with crystal stability, we prepared the TEA2SnI4 nanoplates with and without CA. From the XPS and XRD results (Extended Data Fig. 5e–i), the TEA2SnI4 nanoplates with CA showed less Sn4+ content, thus proving that CA can enhance the air stability of both perovskite films and nanoplates. From the TEM images (Extended Data Fig. 5j–m), It can be clearly seen that the perovskite with CA still show apparent crystal lattices even after 12h storage in dry air, indicating the better crystal stability with the introduction of CA.
a, PL spectra of the Sn perovskite films with various CA content. b, Logarithm curves of absorption coefficient (α) versus photon energy, the EU for the pristine and the treated sample are estimated. c, Visible absorption spectra of the Sn perovskite films with various CA content. d, TRPL spectra of the TEA2SnI4 films prepared with and without 5% CA. e, Non-radiative recombination coefficients B for the single-carrier devices with and without CA treatment, the dashed line indicates the condition of room temperature. f, Excitation-intensity-dependent PLQYs of the perovskite films with different CA content.
a, The fitted curves of the integrated PL intensity as a function of 1/T for the perovskites with and without CA. b, Kinetic traces at a probing wavelength of 615 nm for the Sn perovskite films with and without CA. c, d, Pseudo-color maps of femtosecond-transient absorption spectra of the pristine (c) and the sample with 5% CA (d) under an excitation wavelength of 400 nm.
a-d, Device performance for the Sn-PeLEDs with 2% and 8% CA: (a) J-V, (b) L-V, (c) CE-J, and (d) EQE-J curves.
a, XRD patterns of the perovskite films with and without 5% CA. b, Absorption spectra of the perovskite films with and without CA. c, Logarithm curves of absorption coefficient (α) versus photon energy, and the Urbach energy (EU) for the pristine and the treated sample. d, PL spectra of the films with and without CA. e, PLQYs of the perovskite films with and without CA. f, TRPL spectra of the films prepared with and without CA. g, FTIR spectra of the PEA2SnI4 with and without CA, and the pure CA. h, XPS spectra of Sn 3d for the pristine and the CA-treated PEA2SnI4. Time-dependent normalized PL intensity for the pristine (i) and the film with CA (j) exposed to dry air (20% humidity, RT) for 60 min. Time-dependent XRD patterns of the pristine (k) and the sample with CA (l) in dry air for 12 h. m, The most stable configurations for the tautomeric CA trimer bonded to the Sn perovskite surface. The distances are in Å. Color code: Sn in grey, I in purple, C in brown, H in pink, O in red, N in blue. n, Crystal orbital Hamilton population (COHP) plots of the local Sn-N bond after CA adsorption. o, Averaged integrated crystal orbital Hamilton populations (ICOHP) of the Sn-N bonds. p, EL spectra of one target device with constant peak emission wavelength of 625 nm under different voltages. J-V (q), L-V (r) and EQE-J curves (s) of the PEA2SnI4 based PeLEDs with (9.22%) and without (2.87%) CA.
This file contains Supplementary Figs. 1–19 and Supplementary Tables 1–5.
Visualization of CA configurations on the TEA2SnI4 surface. This video shows the visual coordination process of CA tautomeric mixtures on the TEA2Snl4 perovskite surface. CA monomers gradually form dimers and trimers over time to protect the perovskite surface.
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Han, D., Wang, J., Agosta, L. et al. Tautomeric mixture coordination enables efficient lead-free perovskite LEDs. Nature 622, 493–498 (2023). https://doi.org/10.1038/s41586-023-06514-6