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Henry J. Snaith, the founder of perovskite solar cells repos

Time:2021/03/05 丨 source:Fast Silver 丨 visit count:

Born in 1978, Ph.D. under the supervision of Richard Friend (a Fellow of the Royal Society, Fellow of the Royal College of Engineering, Sir, Professor of Cavendish Laboratory), and post-doctoral under the supervision of Michael Grätzel (a pioneer in the field of fuel-sensitized solar cells, perovskite The founder of solar cells), was elected a member of the Royal Society at the age of 37, is currently a professor of physics at Oxford University, and the founder of Oxford PV! The proper one in the perovskite world! In addition to scientific research, his articles have been cited more than 110,000 times, with an h index of 140, and won the Clarivate Analytics Citation Laureate. Perovskite, one of the "three founders" of perovskite solar cells; industrialization also Be a thief. On December 21, Oxford Photovoltaic Company set a new world record with 29.52% perovskite-silicon tandem cell efficiency. The National Renewable Energy Laboratory (NREL) confirmed this new record. Is it strong? !
On March 4, 2021, Henry J. Snaith reposted Nature, reporting the results on perovskite LEDs.

Author: Yasser Hassan, Jong Hyun Park
Corresponding author: Yasser Hassan, Cathy Y. Wong, Bo Ram Lee, Henry J. Snaith Communications Unit: Oxford University Department of Physics, Department of Chemistry and Biochemistry, University of Oregon, South Korea Pukyong National University Department of Physics
Halide perovskites are very promising light-emitting semiconductors because they show bright, adjustable band gap luminescence and high color purity. The photoluminescence quantum yields of perovskite nanocrystals in various emission color ranges are close to 100%, and light-emitting diodes with an external quantum efficiency of more than 20% have been proved in red and green materials (close to commercial The efficiency of organic light-emitting diodes). However, due to the formation of iodide-rich domains with a low band gap, high-efficiency and color-stable red electroluminescence formed by mixed halide perovskites has not yet been realized.
To this end, Yasser Hassan and Henry J. Snaith, Department of Physics, University of Oxford, United Kingdom, joined forces with Cathy Y. Wong, Department of Chemistry and Biochemistry, University of Oregon, and Bo Ram Lee, Department of Physics, Pukyung University, South Korea, to research and synthesize the treatment of mixed halides with multidentate ligands. Perovskite nanocrystals to suppress halide segregation under electroluminescence. The perovskite nanocrystal exhibits a stable red emission with a center wavelength of 620 nanometers and an external quantum efficiency of electroluminescence of 20.3%. Their research shows that the key function of ligand processing is to "clean" the surface of nanocrystals by removing lead atoms. Density functional theory calculations show that the binding between the ligand and the surface of the nanocrystal inhibits the formation of iodine Frenkel defects, thereby inhibiting the segregation of halides.
The author used an improved ligand-assisted reprecipitation method to synthesize MAPb(I1-xBrx)3 nanocrystals. The specific synthesis and ligand treatment steps are as follows: dissolve the perovskite precursor MAPbI2Br in acetonitrile and methylamine; use the modified ligand-assisted reprecipitation method to synthesize nanocrystals; after purification, use the ligand ethylenediaminetetraacetic acid (EDTA) and reduced 1-glutathione treated the nanocrystals (Figure 1).

Figure 1 Synthesis of perovskite nanocrystals
In order to evaluate the effect of ligand processing, the researchers analyzed time-resolved photoluminescence spectroscopy, PLQY, attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) ( figure 2). The MAPb(I1-xBrx)3 nanocrystals synthesized in toluene exhibited photoluminescence centered at 642 nm (Figure 2a). After treatment with glutathione, the intensity of the photoluminescence peak increased and blue shifted to 625 nm (Figure 2b). The high-resolution transmission electron microscope (HR-TEM) image shows that the E+G-treated nanocrystals are smaller than the untreated "pure" nanocrystals (the average cubic size is 12±1.7 nm and 16.3±2 nm, respectively; 2d and e). Before and after the ligand treatment, the cubic lattice constant of the nanocrystalline film was both 6.05Å (Figure 2f).

Figure 2 The effect of ligand treatment on solution photoluminescence and nanocrystalline structure characteristics
The device structure and energy level of the perovskite LED are shown in Figures 3a and b. Compared with other ligands, the current density of NC-LED treated with E+G is higher, and the external quantum efficiency (EQE) is significantly improved (Figure 3c, d). The maximum EQE measured when the current density is about 0.1 mA cm-2 is 20.3%, and the emission wavelength of the red perovskite LED is about 620 nm. The most pressing challenge in the production of red-emitting metal halide perovskites is to achieve band gap stability. Figures 3f and g show the emission spectra of the LED at a fixed current density and time, respectively. For the pure NC-LED at a constant current density of 1.5 mA cm-2, a broadening of the emission peak was observed within 20 minutes and a new peak appeared near 680 nm. These observations are consistent with halide segregation driven by electrical bias and/or current injection during LED operation, and result in lower energy release in iodide-rich regions. In contrast, the emission spectrum of the NC-LED treated with E + G is stable at 620 nm under the same working conditions and duration.

Figure 3 Characterization of MAPb(I1-xBrx)3 NC-LED device
In order to understand the reasons for the improved device performance when using ligand-treated nanocrystals, it is also necessary to determine the key ligand-perovskite interaction that stabilizes the surface of the nanocrystal. To this end, the authors conducted solid-state 13C nuclear magnetic resonance (NMR) studies on the nanocrystals before and after E+G treatment. Based on the NMR results, the researchers speculate that part of the role of these ligands is to remove insufficiently coordinated lead from the surface of the nanocrystals, thereby forming an electronic "clean" surface with fewer defects. Then, some excess ligand may bind to the remaining Pb on the "clean" perovskite surface, thereby further reducing the concentration of defects.

Figure 4 NMR spectroscopy to characterize the interaction between the ligand and the surface of the nanocrystal
To further understand ligand binding, the authors used density functional theory (DFT) to model the interaction of glutathione and EDTA with the surface of MAPbI3 (Figure 5). Studies have found that this specific ligand pairing has a strong affinity for the perovskite surface. A large number of calculated binding energies and a significant increase in the formation energy of iodide Frenkel defects indicate that glutathione and the combination of EDTA and glutathione significantly inhibited the insufficiently coordinated Pb atoms and stabilized these Frenkel defects.

Figure 5 The structural optimization of the calculated and simulated surface adsorption ligand
This research work illustrates how the functionality of metal halide perovskites is extremely sensitive to the properties of (nano) crystal surfaces, and proposes ways to control the formation and migration of surface defects. This is essential for achieving band gap stability of light emission, and may also have a greater impact on other optoelectronic applications (such as photovoltaics) that require band gap stability.
Hassan, Y., Park, J.H., Crawford, M.L. et al. Ligand-engineered bandgap stability in mixed-halide perovskite LEDs. Nature 591, 72–77 (2021). DOI:10.1038/s41586-021-03217-8
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