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Lead-Free Direct Bandgap Double Perovskite Nanocrystals with Bright Dual-Color Emission Bin Yang, Xin Mao, Feng Hong, Weiwei Meng, Yuxuan Tang, Xusheng Xia, Songqiu Yang, Weiqiao Deng, and Keli Han J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b07424 • Publication Date (Web): 19 Nov 2018 Downloaded from http://pubs.acs.org on November 19, 2018
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Journal of the American Chemical Society
Lead-Free Direct Bandgap Double Perovskite Nanocrystals with Bright Dual-Color Emission Bin Yanga,b, Xin Maoa,b, Feng Honga,b, Weiwei Menge, Yuxuan Tanga,b, Xusheng Xiab,d, Songqiu Yanga, Weiqiao Denga,c and Keli Hana,c* aState
Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Science, Dalian 116023, P. R. China. bUniversity cInstitute d
of the Chinese Academy of sciences, Beijing 100049, P. R. China.
of Molecular Sciences and Engineering, Shandong University, Qingdao, P. R. China.
Key Laboratory of Chemical Lasers, Dalian Institute of Chemical Physics, Chinese Academy of Science.
School of Physics and Technology, Center for Electron Microscopy, MOE Key Laboratory of Artificial Micro- and Nano-structures, and Institute for Advanced Studies, Wuhan University, Wuhan 430072, China. e
ABSTRACT: Lead-free double perovskite nanocrystals (NCs), i.e. Cs2AgInxBi1xCl6 (x = 0, 0.25, 0.5 0.75 and 0.9), that can be
tuned from the indirect bandgap (x = 0, 0.25 and 0.5) to the direct bandgap (x = 0.75 and 0.9) are designed. Direct-bandgap NCs exhibit 3 times greater absorption cross-section, lower sub-bandgap trap states, and >5 times photoluminescence quantum efficiency (PLQE) compared with those observed for indirect bandgap NCs (Cs2AgBiCl6). A PLQE of 36.6% for direct bandgap NCs is comparable to those observed for lead-perovskite NCs in the violet region. Besides the band edge violet emission, the direct bandgap NCs exhibit bright orange (570 nm) emission. Density functional theory calculations suggesting forbidden transition is responsible for the orange emission, which is supported by time resolved PL and PL excitation spectra. The successful design of lead-free direct bandgap perovskite NCs with superior optical properties opens the door for high performance lead-free perovskite optoelectronic devices.
INTRODUCTION Understanding the bandgap nature of semiconductor materials is crucial for high-performance optoelectronic applications. For direct-bandgap materials, absorption and recombination only involve photons (Figure S1),1 leading to high absorption coefficients, rapid carrier recombination rates, and high photoluminescence quantum efficiencies (PLQEs), which is benifical for light-emitting devices. In contrast, absorption and recombination involve not only photons but also phonons for indirect bandgap semiconductors,1 thereby leading to lower absorption coefficients and PLQE compared to direct semiconductors (Figure S1). Recently, all-inorganic lead-perovskite (CsPbX3, X: Cl, Br, I) nanocrystals (NCs) with a high absorption coefficient, low trap-state density, and near-unity PLQE have been widely examined for optoelectronic applications.2-8 Their superior performance is mainly related to the unique 3D perovskite structure and direct bandgap nature.2-9 Despite these superior characteristics of lead-perovskites NCs, the toxity of Pb is often considered to be a drawback and hinders commercial applications. Initially, lead-free perovskite has been synthesized by using Sn2+. However, Sn2+-based perovskite is extremely unstable under ambient conditions, with facile oxidation from Sn2+ to Sn4+.10-12 Subsquently, airstable lead-free perovskites NCs based on Bi3+, Sb3+, and double perovskite have been developed.13-23 However, these
reported lead-free perovskite NCs exhibit an indirect bandgap nature. For example, Bi3+, Sb3+ perovskite, and Ag+Bi3+ double perovskite NCs exhibit prominent sub-bandgap absorption, mainly corresponding to indirect transitions, thereby leading to a low PLQE.17-23 Thus far, to the best of our knowledge, lead-free direct-bandgap perovskite NCs have not been reported. In this study, we present the first report of lead-free direct bandgap double perovskite NCs. Direct bandgap Cs2AgInxBi1xCl6 (x=0.75, 0.9) NCs exhibit larger absorption cross-section, lower trap states and higher PLQE compared to the indirect bandgap double perovskite NCs (Cs2AgBiCl6). A PLQE of 36.6% for Cs2AgIn0.9Bi0.1Cl6 NCs is comparable to those observed for lead-perovskite NCs in the violet region. In addition, the direct bandgap NCs exhibit orange emission. By analyzing the results of density function theory (DFT) calculation, steady-state absorption, PL and transient absorption spectra, we conclude that dual-color emission originates from direct band to band transition (violet) and forbidden transition (orange), respectively.
RESULTS AND DISCUSSION 1． Synthesis and characterization Cs2AgInxBi1xCl6 (X = 0, 0.25, 0.5 0.75 and 0.9) NCs were synthesized by anti-solvent recrystallization18,21 as well as by the variation of the stoichiometry of In/Bi in the precursor. Details can be found in the Supporting Information (SI). X-
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ray diffraction (XRD) patterns revealed that these NCs are highly crystalline, and all these NCs occupy the Fm3m cubic space group similar to Cs2AgBiCl6 (Figure 1a).21 The careful examination of the XRD patterns revealed that the peaks are monotonically shifted to high angles with increasing In content, as can be observed in case of the (004) peak in Figure 1a. This result is consistent with the lattice contraction caused by the substitution of Bi3+ (ionic radius 108 pm) with smaller In3+ (81 pm). Figure 1b shows the transmission electron microscopy (TEM) images of Cs2AgIn0.9Bi0.1Cl6: A quasi-spherical shape with an average diameter of 4.3 nm is observed (Figure 1c). A high-resolution TEM (HRTEM) image of Cs2AgIn0.9Bi0.1Cl6 NCs (Figure 1d, e) revealed high crystallinity with lattice spacing values of 0.26 and 0.30 nm(Figure 1f), corresponding to the (024) and (004) diffraction peaks, respectively. The TEM images of other NCs can be found in the SI (Figure S4), which revealed a crystal size similar to those of Cs2AgIn0.9Bi0.1Cl6 NCs.
surfactants (Figure S6) such as oleic acid (OA), details are stated in our previous studies.18,21 On the other hand, an indirect bandgap transition corresponds to the intrinsic properties of materials that cannot be eliminated (Figure S6), leading to a low PLQE (
