2D lead-bromide perovskite crystals were synthesized according to the previous reported anti-solvent diffusion method.15,16 A vial containing PbBr2, R-NH3Br and DMF/DMSO solution was put in a bigger sealed vial container with anti-solvent. Perovskites crystalized gradually as anti-solvent diffuses into perovskite precursor and the crystals were then taken out after several days (see Methods for details).
Phenylmethylammonium lead bromide ((PMA)2PbBr4) is a typical violet emissive perovskite without STEs phenomenon at room temperature.17 To investigate the influence of chemical bonds on the formation of STEs, halogen atoms were introduced as substituents in phenyl to tailor the chemical property of PMA cations. We performed first-principles density-functional-theory (DFT) to simulate (PMA)2PbBr4 and a series of its corresponding halogen-substituted (F, Cl and Br) perovskite samples. We take Cl-substituted perovskite (ClPMA)2PbBr4 as the model. Figure 1a shows the atomic structure of (PMA)2PbBr4, (2-ClPMA)2PbBr4 (ortho-substitution), (3-ClPMA)2PbBr4 (meta-substitution) and (4-ClPMA)2PbBr4 (para-substitution). As expected, (PMA)2PbBr4 shows a corrugated 2D perovskite structure with PbBr6 octahedrons connecting with corners, which crystalizes in the polar space group Cmc21.17 The interlayer spacing is 1.656 nm (Table S1). When Cl replaces ortho-hydrogen, the crystal lattice slightly expands in-plane-orientation due to the longer bond length of C-Cl (1.77 Å) compared to C-H (1.08 Å). Meanwhile, the interlayer spacing is compressed to 1.600 nm. However, in (3-ClPMA)2PbBr4 and (4-ClPMA)2PbBr4 perovskites, crystal lattices in-plane-orientation nearly remain unchanged, but the interlayers are pushed aside (Table S1). Powder XRD testing also verified these results. In Figure 1b, the well-defined diffraction peaks are corresponding to the (00h) reflections series of (PMA)2PbBr4 and (ClPMA)2PbBr4. All the diffraction peaks of (2-ClPMA)2PbBr4 shift to larger degrees in comparison to (PMA)2PbBr4, indicating a decreased interlayer distance. On the contrary, the diffraction peaks of (3-ClPMA)2PbBr4 and (4-ClPMA2)PbBr4 shift to smaller degrees. According to Bragg’s diffraction equation (2d×sinq = nl), interlayer distances of (PMA)2PbBr4, (2-ClPMA)2PbBr4, (3-ClPMA)2PbBr4 and (4-ClPMA)2PbBr4 were measured to be 1.669, 1.628, 1.744 and 1.750 nm, respectively, which are in fair agreement with above DFT simulation results (Table S1). As shown in Figure 1b, the crystal lattices of (2-FPMA)2PbBr4 and (3-FPMA)2PbBr4 remain almost the same as those of (PMA)2PbBr4, which is attributed to the smaller bond length of C-F (1.41 Å) compared to C-Cl (1.77 Å). The interlayer spacing of (4-FPMA)2PbBr4 increased to 1.710 nm, which is smaller than that of (4-ClPMA)2PbBr4. Therefore, it follows that perovskites (BrPMA)2PbBr4 with a larger bond length of C-Br (1.91 Å) induces larger expansion of crystal lattice (Figure 1b). However, the impact of halogen-substitution on distortion of the inorganic sheet is likely to be minimal. As shown in Figure S1, out-of-plane Pb-Br-Pb angles in the perovskites (PMA)2PbBr4 and (ClPMA)2PbBr4 are all 180 degrees, without any distortion as reported before in white perovskite.18 Meanwhile, the corresponding in-plane Pb-Br-Pb angles were measured to be around 140 degrees with small variations.
Despite the structural similarities between pristine and halogen-substituted (PMA)2PbBr4, their optical properties are distinct. At room temperature, (PMA)2PbBr4 displays only a narrow violet emission located at 410 nm (Figure 2a). However, the PL spectra of (2-F/Cl/BrPMA)2PbBr4 show broadband emissions from 400 to 800 nm, which are composed of a narrow free-excitons (FEs) peak and a much broad STEs peak (Figure 2a-c). The CIE chromaticity coordinates of the overall spectra of (2-FPMA)2PbBr4, (2-ClPMA)2PbBr4 and (2-BrPMA)2PbBr4 were determined to be (0.342, 0.361), (0.300, 0.330) and (0.312, 0.338), which give correlated color temperatures (CCT) of 5139 K, 7215 K, 6478K, respectively (Figure 2d). These locations are very close to the standard white light (0.33, 0.33). They display CRI values of 89, 86 and 88, respectively, which are higher than that of most commercial white light-emitting diodes (CRI of about 80). The PLQYs of (2-F/Cl/BrPMA)2PbBr4 crystals were measured to be 5%, 32% and 15%, respectively. The performance is much better than that of most low-dimensional white perovskites reported (Table S2). It should be noted that these FEs emissions of (2-F/Cl/BrPMA)2PbBr4 exhibit an asymmetric band spectra, including one main PL peak and two shoulder peaks. The location of the high-energy shoulder is corresponding to the absorption band edge. According to previous studies, this high energy shoulder peak may arise from the reabsorption of high energy region of the FEs emission spectrum in thick perovskite.16 The low-energy shoulder is similar to that reported in low-dimensional perovskites, which might derive from a vibrational replica of the intrinsic band.10,18 The STEs origin of the BWL emission was experimentally confirmed by implementing power dependent PL measurement, which shows a linear dependence from 0.024 to 23.5 W cm-2 (Figure 3a). Otherwise, PL from permanent defects usually shows a sublinear dependence on the excited power with a saturation of limited defect sites under high excitation intensity.2,7,8 We also observed a similar broad spectrum from (2-ClPMA)2PbBr4 powders synthesized by fast reprecipitation method (See Methods for details, Figure S2), which further confirms that the broadband emission does not originate from the defects in crystals. Compared to (2-F/Cl/BrPMA)2PbBr4 perovskites, the PL intensity ratios ISTEs/IFEs of (3-F/Cl/BrPMA)2PbBr4 decrease significantly. Para-substituted perovskites (4-F/Cl/BrPMA)2PbBr4 show negligible STEs emissions, almost the same as pristine (PMA)2PbBr4. This phenomenon could also be observed in the digital image of the samples (2-ClPMA)2PbBr4, (3-ClPMA)2PbBr4, (4-ClPMA)2PbBr4 under UV light (Figure 2e), as well as the image of (F/BrPMA)2PbBr4 perovskites crystals (Figure S3).
Figure 4b shows the temperature dependent PL spectra of (2-ClPMA)2PbBr4. The PL intensities of STEs and FEs both increase when temperature decreases from 298 K to 77 K. The ISTEs/IFEs increases from 2.9 at 298 K to 100 at 77 K (Figure 4c), which is determined by a strong electron-phonon interaction induced transformation between FEs and STEs and PL quenching. At high temperature, it is more probable that FEs relax into STEs, however, the PL quenching is dominated due to the strong electron-phonon interaction. At low temperatures, the reverse applies. During the cooling process, a reduced electron-phonon interaction would narrow the full width at half maximum (FWHM) of both FEs and STEs emission (Figure 4c). Figure 4d illustrates the PL lifetime of (2-ClPMA)2PbBr4 and (3-ClPMA)2PbBr4. The PL decay curves can be well fitted with a bi-exponential decay model. The PL lifetime is considered as a combination of a slow-decay component (radiative recombination) and a fast-decay component (nonradiative process) that give a long lifetime τ1 and a short lifetime τ2, respectively (Table S3). According to previous report, the emission ratio ISTEs/IFEs at a given temperature is related to △Gself-trap (self-trapping depth = ESTEs - EFEs) and the radiative emission rates from the STE and FEs states (kr,STEs and kr,FEs).18
Due to the similar radiative PL lifetime and much lower energy of STEs state than that of FEs state, the ISTEs/IFEs is dominated by the △Gself-trap (See Table S3 for detailed discussion).
In low-dimensional perovskites, excitons are confined in the inorganic sheets. Two kinds of bonds, namely Pb2+-Br- and NH3+-PbBr3-, might affect the formation of STEs. According to aforementioned crystal structural analysis, the structure of inorganic sheet containing PbBr6 octahedrons remains almost unchanged after the halogen-substitution, suggesting Pb2+-Br- bonds were not interfered and may not be the critical factor. Due to high electronegativity of halogen atoms, they would withdraw electrons from the phenyl group and the branched chain -CH2NH3+, induces more positive charge accumulating on -NH3+. As shown in Table 1, we obtained the charge quantities of eight -NH3+ in the unit cell using DFT calculations. The average charges of -NH3+ in (PMA)2PbBr4, (2-ClPMA)2PbBr4, (3-ClPMA)2PbBr4 and (4-ClPMA)2PbBr4 were calculated to be 0.272, 0.297, 0.279 and 0.285, respectively. We assume that the centers of the positive charges of -NH3+ cations and the negative charges of PbBr3- anions are located on the N atoms and Pb atoms, respectively. The average Coulomb force was calculated to be 0.83 nN for (2-ClPMA)2PbBr4 (Table S4), which is smaller than that of (PMA)2PbBr4 (0.73 nN). (3-ClPMA)2PbBr4 and (4-ClPMA)2PbBr4 have similar Coulomb force, 0.77 and 0.78 nN. Overall, the value of the Coulomb force is proportional to the ISTEs/IFEs ratio of perovskites. Therefore, it can be speculated that the strengthened ionic bonds would trigger the STEs formation in perovskites.
To further verify the effect of halogen substituents (electron-drawing group) on the formation of STEs, a typical electron-donating group (-CH3) ortho-substituted molecule 2-CH3PMA was used to synthesize the perovskite (2-CH3PMA)2PbBr4. The bond length of C-CH3 is close to that of C-Cl. As expected, the XRD diffraction peaks of (2-CH3PMA)2PbBr4 also shift to larger degrees in comparison to (PMA)2PbBr4 (Figure S4). However, we did not observe obvious broadband emission from STEs (Figure S4). Moreover, we fabricated the phenylethylammonium lead bromide ((PEA)2PbBr4) and corresponding halogen-substituted (FPEA)2PbBr4 and (ClPEA)2PbBr4 perovskites. The broad emissions are also observed in (2-FPEA)2PbBr4 and (2-ClPEA)2PbBr4 (Figure S5), but their PL intensity ratios ISTEs/IFEs are smaller than that of (2-FPMA)2PbBr4 and (2-ClPMA)2PbBr4. The PEA molecule has a longer branched chain (-CH2CH2NH3+) than PMA. Thus, the effect of electron-withdrawing from -NH3+ to halogen substituents is weaker than that in PMA. As a result, the Coulomb force increase in halogen-substituted (PEA)2PbBr4 is lower than that in halogen-substituted (PMA)2PbBr4.
In summary, we designed and synthesized novel halogen-substituted perovskite (F/Cl/BrPMA)2PbBr4 crystals by wet chemical methods. These perovskites display BWL emission with high PLQY, good CRI and excellent CIE coordinates close to standard white light (0.33, 0.33), indicating promising application for solid-state lighting. Combining DFT calculation and experimental analysis, we discovered that the halogen substituents withdraw electrons from -NH3+, leading to an increase in the Coulomb force in NH3+-PbBr3- bonds and enhancement of broadband emission from STEs. Thus, we believe that chemical bonds confining the excitons are of high importance for STEs formation, and new white perovskites can be designed and produced according to this approach.