- Email: [email protected]
Efficient light-emitting diodes based on green perovskite nanocrystals with mixed-metal cations
Author’s Accepted Manuscript Efficient light-emitting diodes based on green perovskite nanocrystals with mixed-metal cations Xiaoli. Zhang, Wanyu Cao, Weigao Wang, Bing Xu, Sheng Liu, Haitao Dai, Shuming Chen, Kai. Wang, Xiao Wei Sun www.elsevier.com/locate/nanoenergy
PII: DOI: Reference:
S2211-2855(16)30457-8 http://dx.doi.org/10.1016/j.nanoen.2016.10.039 NANOEN1564
To appear in: Nano Energy Received date: 21 August 2016 Revised date: 14 October 2016 Accepted date: 18 October 2016 Cite this article as: Xiaoli. Zhang, Wanyu Cao, Weigao Wang, Bing Xu, Sheng Liu, Haitao Dai, Shuming Chen, Kai. Wang and Xiao Wei Sun, Efficient lightemitting diodes based on green perovskite nanocrystals with mixed-metal cations, Nano Energy, http://dx.doi.org/10.1016/j.nanoen.2016.10.039 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Efficient light-emitting diodes based on green perovskite nanocrystals with mixed-metal cations Xiaoli Zhang1, Wanyu Cao4, Weigao Wang2, Bing Xu2,3, Sheng Liu3, Haitao Dai4, Shuming Chen2, Kai Wang2*, Xiao Wei Sun1,2* 1
School of Electrical & Electronic Engineering, Nanyang Technological University,
50 Nanyang Avenue, 639798, Singapore 2
Department of Electrical & Electronic Engineering, Southern University of Science
and Technology, Shenzhen, 518055, China 3
School of Power and Mechanical Engineering, Wuhan University, Wuhan, 430072,
China 4
School of science, Tianjin University, Tinjin, 300072, China
[email protected] [email protected] *
Corresponding authors.
Abstract The recent success of solar cells has witnessed the emergence of metal halide perovskites as an important class of optoelectronic materials. Even with the record efficiency and inspiring performance, lead based perovskites are criticized for its toxicity, raising concerns about the environmental pollution. The reported divalent and heterovalent metal substitution for Pb indicates unfavorable performance. The lead-containing perovskite exhibits the outstanding activity compared to lead-free
analogues. It is speculated that the perovskite with mixed-metal cation is expected to enable the fabrication of efficient device and partially alleviate toxicity. For this reason, herein, we present the first synthesis of perovskite nanocrystals based on mixed-metal cation, and demonstrate the implementation of mixed-metal cation perovskite nanocrystals into LEDs for the first time.
Keywords: Perovskite; Nanocrystals; Light-emitting diode; Mixed-metal cation
Introduction The past 5 years have witnessed a revolution in optoelectronic research with the discovery of the metal halide perovskite family. These solution-processed perovskites are fast becoming the compelling materials, and have shown tremendous potential in the field of solar cells, LED, lasing, photodetectors, x-ray diffraction detection and plasmonics
[1-9]
. Lead-halide perovskite has been considered as the most promising
materials for the next generation of solar cells, achieving efficiencies above 20%
[1-5]
.
Moreover, the lead-containing hybrid perovskite CH3NH3PbBr3 thin film applied in LED has been recently reported to exhibit a current efficiency (CE) of 42.9 cd A-1 and an external quantum efficiency (EQE) as high as 8.53%
[10]
. Despite these
breakthroughs and inspiring records, the toxicity of heavy metal lead in current perovskite raises concerns about the environmental pollution. Given these imperfection, it is urgent to address this challenge by identifying a green and nontoxic
halide perovskite optoelectronic material. The lead-free perovskite can be realized by replacing Pb in the compound via substitution of other elements, such as Sn, Bi, Ge, Cu and Ag
[11-14]
. However, the
reported lead-free perovskite indicates some issues of concern. For example, the substitution of group-14 elements Sn for Pb tends to degrade the corresponding halide perovskite due to the oxidation from Sn2+ to Sn4+
[15-18]
, while other divalent cations
substitution of lead negatively impacts optoelectronic properties by increasing band gaps and effective masses
[19]
. The unfavorable replacing of homovalent cations
stimulates the considering of heterovalent substitution, such as the formation of a double perovskite structure with one monovalent and one trivalent cation [14], however, the broad photoluminescence from 480 to 650 nm limits its application in narrow emission, such as LED, photodetector and laser. Compared to the lead-containing perovskite, the unstable lead-free analogues under ambient conditions lead to inferior device performance due to higher defect densities
[20-23]
. In addition, lead-free perovskite applications have been rarely
demonstrated, which may be ascribed to low efficiency and unstability. In spite of great efforts in lead replacement, the lead halide perovskite family exhibits the best activity up to now. Inspired by the recent studies about mixed-cation perovskite based on MA-Cs, MA-FA and FA-Cs (MA: methylammonium; FA: formamidinium)
[24-26]
,
herein, we first report the mixed-metal cation based perovskite nanocrystals CsPb1-xSnxBr3 (x = 0-1), it is found that the emission property of the perovskite CsPb1-xSnxBr3 are significantly dependent on the substitution content of the Sn cations
in the perovskite composition. Based on the characterization of their optical and physicochemical properties, the mixed-metal cation perovskites CsPb1-xSnxBr3 were employed as light emitter in the LEDs. With an optimized composition of CsPb0.7Sn0.3Br3, the corresponding device exhibited encouraging performance. The partial lead substitution not only reduces the toxicity of material, but also improves the device performance, which may provide a potential and unlock a world of new optoelectronic materials for low-toxic and green perovskite LEDs.
Results and discussion
Figure 1. (a) (b) TEM images of CsPb1-xSnxBr3 nanocrystals. (c) FFT and HRTEM of CsPb1-xSnxBr3 (x = 0.3) nanocrystals. (d) The variation of fringe distance with Sn content x in the compound. The perovskite nanocrystals with mixed-metal cation Pb and Sn were prepared
following literature methods
[27]
. In detail, the mixture of PbBr2 and SnBr2 was
dissolved in solvent tri-n-octylphosphine to get transparent solution, followed by the injection of cesium stearate and oleic acid and oleylamine to obtain stable CsPb1-xSnxBr3 nanocrystals. The as-prepared inorganic perovskite nanocrystals with mixed-metal cations were investigated by transmission electronic microscopy (TEM) as shown in Figure 1. The monodispersed CsPb1-xSnxBr3 nanocrystals have diameter of 8-10 nm with cubic shape determined by the perovskite crystal structure, which was clearly revealed by X-ray diffraction (XRD) patterns presented in Figure S1 (supporting information). For pure perovskite CsSnBr3 (x = 1), the resultant product is very unstable due to the Sn oxidation process, which is accompanied by a drop in PL efficiency to nondetectable values (<0.01%)
[13, 28]
. Thus, the Sn content x is varied
from 0 to 0.7 in this work for comparison. The lattice fringe is calculated from the fast fourier transformation (FFT) of high resolution TEM (HRTEM) (Figure 1c). As more Sn doped in the metal cation, the lattice fringe distance correspondingly reduced (Figure 1d). The observed spacing shrinkage in CsPb1-xSnxBr3 nanocrystals with an increasing Sn content x could be due to the smaller ionic radius of Sn cation (0.145 nm) than the Pb cation (0.180 nm). The elemental mapping result (Figure S2) detected by energy dispersive spectroscopy (EDS) exhibits that the Cs, Sn, Pb and Br atoms are uniformly incorporated into the nanocrystals.
Figure 2. (a) Absorption spectra and (b) PL plots of CsPb1-xSnxBr3 nanocrystals as a function of Sn content (x = 0-0.7). In order to identify the optical response of Sn doping in metal cation, we perform the UV-Vis absorption and photoluminescence (PL) measurement. According to Vegard’s law
[29]
, which is an approximate empirical rule, the band gap in
semiconductor is approximately a linear function of the lattice parameter, and the linear relationship also exists between the band gap and composition. In our work, the regular variation in lattice constant indicates the potential increased band gap of semiconductor CsPb1-xSnxBr3. As confirmed by UV-Vis absorption, as x increases, the absorption band is blue shifted from 520 nm (x = 0) to 496 nm (x = 0.7). The corresponding band gap of the perovskite is gradually shifted to the higher energy direction (from 2.38 to 2.50 eV). The normalized PL spectra are shown in Figure 2b, from which it can be clearly seen that the luminescence peaks can be effectively tuned towards shorter wavelength by varying ratio of the Sn content x. The quantum yield of the resultant nanocrystals CsPb1-xSnxPbBr3 is gradually decreased from 71% to 37% as more Sn incorporation, which is mainly ascribed to the unstable Sn oxidation in air. It is worth pointing out that the adding of Sn in Pb cation has not lead to external
emission from defects. Therefore, the use of mixed-metal cations in the perovskite materials provides versatility in the fine-tuning of the optical bandgap and the emission wavelength of provskite nanocrystals, which opens a new possible avenue to realize tunable wavelength of perovskite. The dependence of PL spectra, quantum yield and optical bandgap on Sn content x is summarized in Table S1.
Figure 3. (a) Schematic illustration of the perovskite LED device structure. (b) Energy band diagram of the different components in the perovskite LEDs. (c) Luminescnece photograph driven at 5 V for perovskite CsPb0.7Sn0.3PbBr3based LEDs. (d) EL spectra of perovskite CsPb1-xSnxPbBr3 based LEDs.
Figure 4. (a) Current density-voltage, (b) luminance-voltage, (c) current efficiency-current density of the perovskite LEDs based on CsPb1-xSnxPbBr3 nanocrystals and (d) EL spectra of device based on the best Sn substitution (x = 0.3) driven under different voltage. The excellent optoelectronics properties of the developed mixed-metal cation based perovskite nanocrystals encourage us to explore their application in LEDs. For comparison, devices based on CsPb1-xSnxPbBr3 nanocrystals with and without Sn doping were fabricated with the same device configuration. The typical device configuration consists of spin-coated layers of poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (poly-TPD)
and
the
(PEDOT:PSS), perovskite
poly(4-butylphenyl-diphenyl-amine)
nanocrystals,
and
evaporated
layers
of
1,3,5-Tris(1-phenyl-1H-benzimidazol-2-yl)benzene (TPBi) and LiF/Al, as shown in the schematic diagram in Figure 3a. The flat-band energy levels of the different components in the LEDs are illustrated in Figure 3b. In the developed devices,
electrons are injected from the LUMO (Lowest Unoccupied Molecular Orbital) level of TPBi into the conduction band of perovskite nanocrystals, while holes are transferred from the HOMO (Highest Occupied Molecular Orbital) level of poly-TPD into the valance band of perovskite NCs, followed by the radiative recombination inside the active perovskite layer. The matching of energy levels of the different components in the devices is one of the key factors that significantly affect the performance of LEDs. The fabricated perovskite LEDs show bright and uniform emission, as indicated by the photo of a CsPb0.7Sn0.3Br3 based LED which was turned on under a bias potential of 5 V. The normalized EL spectra can be readily tuned by substitution of Pb with Sn, as confirmed by the PL spectra. The detailed device performance is displayed in Figure 4. The current density-voltage
(J-V)
characteristics
of
the
perovskite
LEDs
based
on
CsPb1-xSnxPbBr3 nanocrystals are shown in Figure 4a. Without Sn doping (x = 0), the neat CsPbBr3 perovskite displays the luminance and current efficiency (CE) of 4814 cd m-2 and 0.93 cd A-1, respectively. The presence of Sn (x = 0.1) in the metal cation causes an improvement of luminance and CE about 1.05- and 1.77-fold, respectively. The best performance is achieved as x increases to 0.3, with maximum luminance and efficiency of 5495 cd m-2 and 3.60 cd A-1, respectively, indicating 1.14 and 3.87 times improvement compared to that of pure CsPbBr3. However, there is no expected performance improvement as excess Sn incorporation into the metal cation. The luminance of CsPb0.5Sn0.5Br3 CsPb0.3Sn0.7Br3 based LEDs dramatically drops down to 2203 and 1517 cd m-2, respectively. In addition, the device based on the best Sn
substitution (x = 0.3) is driven under different voltage from 5 to 7 V, no extra emission is detected in EL spectra (Figure 4d). The detailed device performance is summarized in Table S2. The improved performance can be ascribed to the gradually reduced bandgap as Sn content x increases, which results in the lowered valence band of mixed-metal cation perovskite. This facilitates the hole injection process because of the reduced hole injection barrier at the interface between the perovskite and the hole transport layer. However, the excess Sn doping (x = 0.5 and 0.7) causes the inferior device performance. To understand the kinetics of excitons and free carriers in mixed-metal cation
perovskite
CsPb1-xSnxBr3
nanocrystals,
we
conducted
time-resolved
single-photon counting measurements. Decay time of 4.5 ns is typically observed in CsPbBr3 nanocrystals (Figure S3), while time resolved PL decay of CsPb1-xSnxBr3 (x = 0.1 and 0.3) nanocrystals indicates slowly reduced radiative lifetime (4.3 ns and 3.8 ns), and the minimized value of 0.4 ns is obtained for x = 0.7. The decreased lifetime implies that the introduced Sn cation could increase free exciton emission decay that possibly derived from nonradiative energy transfer to the trap states. As reported in previous literature, the fast-decaying luminescence of Sn-based perovskite originates from band-edge states and shallow states induced by the intrinsic defects sites
[30-31]
.
The decay behavior happened in Sn-based perovskite is from its high defect state densities (~1017 cm-3), while lead-containing perovskites indicates much lower defect densities
[13, 32-33]
. Therefore, the properly Sn doping can enhance the device
performance by reducing hole injection barrier and effective radiative recombination.
Conclusions We have fabricated perovskite CsPb1-xSnxBr3 nanocrystals based on mixed-metal cation, and systematically studied the role of Sn doping on optical properties and LED device performance. We find that Sn doping affects the absorption and PL spectra of perovskite nanocrystals. In detail, as Sn content is increased, the perovskite nanocrystals witness the blue-shift of bandgap and emission peak. This work, for the first time, used a mixed-metal cation, to achieve perovskite LEDs with improved luminance and efficiency. The excess Sn addition induced the fast decay of luminance, resulting in the inferior device performance. The use of mixed-metal cation provides additional versatility in fine-tuning optical bandgap and emission wavelength. This work opens the prospect for other cations, like Cu and Zn, to be explored as doping cations for perovskites. Therefore, mixed-metal cation is a novel compositional strategy on the road to the excellent and green perovskite LEDs. Aknowledgements This research is supported by the National Research Foundation, Prime Minister’s Office, Singapore under its Competitive Research Programme (NRF-CRP11-2012-01) and administered by Nanyang Technological University. This work is also supported by National Science and Technology Major Project of the Ministry of Science and Technology of China (2016YFB0401702) and Shenzhen Innovation Project (JCYJ20160301113356947, KC2014JSQN0011A and JCYJ20150630145302223).
References
[1] W. Yang, J. Noh, N. Jeon, Y. Kim, S. Ryu, J. Seo, S. Seok, Science 2015, 348, 1234-1237. [2] J. Lee, D. Kim, H. Kim, S. Seo, S. Cho, N. Park, Adv. Energy Mater. 2015, 5, 1501310. [3] A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, J. Am. Chem. Soc. 2009, 131, 6050–6051. [4] M. Saliba, S. Orlandi, T. Matsui, S. Aghazada, M. Cavazzini, J. Correa-Baena, P. Gao, R. Scopelliti, E. Mosconi, K. Dahmen, D. Angelis, A. Abate, A. Hagfeldt, G. Pozzi, M. Graetzel, M. Nazeeruddin, Nature Energy 2016, 1, 15017. [5] D. Bi, W. Tress, M. Dar, P. Gao, J. Luo, C. Renevier, K. Schenk, A. Abate, F. Giordano, J. Correa Baena, J. Decoppet, S. Zakeeruddin, M. Nazeeruddin, M. Gratzel, A. Hagfeldt, Sci. Adv. 2016, 2, e1501170. [6] J. Baena, L. Steier, W. Tress, M. Saliba, S. Neutzner, T. Matsui, F. Giordano, T. Jacobsson, A. Kandada, S. Zakeeruddin, A. Petrozza, A. Abate, M. Nazeeruddin, M. Gratzel, A. Hagfeldt, Energy Environ. Sci. 2015, 8, 2928–2934. [7] J. Pan, L. N. Quan, Y. Zhao, W. Peng, B. Murali, S. P. Sarmah, M. Yuan, L. Sinatra, N. M. Alyami, J. Liu, E. Yassitepe, Z. Yang, O. Voznyy, R. Comin, M. N. Hedhili, O. F. Mohammed, Z. H. Lu, D. H. Kim, E. H. Sargent, O. M. Bakr, Adv. Mater. 2016, DOI: 10.1002/adma.201600784. [8] M. Saliba, S. Wood, J. Patel, P. Nayak, J. Huang, J. Alexander-Webber, B. Wenger, S. Stranks, M. Horantner, J. Wang, R. Nicholas, L. Herz, M. Johnston, S. Morris, H. Snaith, M. Riede, Adv. Mater. 2016, 28, 923–929.
[9] R. Dong, Y. Fang, J. Chae, J. Dai, Z. Xiao, Q. Dong, Y. Yuan, A. Centrone, X. Zeng, J. Huang, Adv. Mater. 2015, 27, 1912–1918. [10] H. Cho, S. Jeong, M. Park, Y. Kim, C. Wolf, C. Lee, J. Heo, A. Sadhanala, N. Myoung, S. Yoo, S. Im, R. Friend, T. Lee, Science 2015, 350, 1222-1225. [11] M. Kumar, D. Sabba, W. Leong, P. Pablo, R. Rajiv, B. Tom, S. Chen, D. Hong, R. Ramamoorthy, A. Mark, G. Michael, G. Subodh, M. Nripan, Adv. Mater. 2014,26, 7122-7127. [12] B. Park, B. Philippe, X. Zhanng, H. Rensmo, G. Boschloo, J. Erik, Adv. Mater. 2015, 18, 6806-6813. [13] T. Jellicoe, J. Richter, H. Glass, M. Tabachnyk, R. Brady, S. Dubbon, A. Rao, R. Friend, D. Credgington, N. Greenham, M. Bohm, J. Am. Chem. Soc. 2016, 138, 2941-2944. [14] G. Volonakis, M. Filip, A. Haghighirad, N. Sakai, B. Wenger, H. Snaith, F. Giustino, J. Am. Chem. Soc. 2016, 7, 1254-1259. [15] C. Stoumpos, C. Malliakas, M. Kanatzidis, Inorg. Chem. 2013, 52, 9019. [16] T. Baikie, Y. Fang, J. Kadro, M. Schreyer, F. Wei, S. Mhaisalkar, M
ra tzel, T.
White, Mater. Chem. A 2013, 1, 5628. [17] F. Hao, C. Stoumpos, D. Cao, R. Chang, M. Kanatzidis, Nat. Photonics 2014, 8, 489. [18] N. Noel, S. Stranks, A. Abate, C. Wehrenfennig, S. Guarnera, A. Haghighirad, A. Sadhanala, G. Eperon, S. Pathak, M. Johnston, A. Petrozza, L. Herz, H. Snaith, Energy Environ. Sci. 2014, 7, 3061.
[19] M. Filip, F. Giustino, Phys. Chem. C 2016, 120, 166−173 [20] N. Noel, S. Stranks, A. Abate, C. Wehrenfennig, S. Guarnera, A. Haghighirad, A. Sadhanala, G. Eperon, S. Pathak, M. Johnston, A. Petrozza, L. Herz, H. Snaith, Energy Environ Sci. 2014, 7, 3061. [21] Y. Takahashi, R. Obara, Z. Lin, Y. Takahashi, T. Naito, T. Inabe, S. Ishibashi, K. Terakura, Dalton Trans. 2011, 40, 5563. [22] Y. Takahashi, H. Hasegawa, Y. Takahashi, T. Inabe, Solid State Chem. 2013, 205, 39. [23] M. Kumar, S. Dharani, W. Leong, P. Boix, R. Prabhakar, T. Baikie, C. Shi, H. Ding, R. Ramesh, M. Asta, M. Graetzel, S. Mhaisalkar, N. Mathews, Adv. Mater. 2014, 26, 7122. [24] Y. Chen, J. Luo, S. Meloni, A. Boziki, N. Ashari-Astani, C. Grätzel, S. Zakeeruddin, U. Röthlisberger, M. Grätzel, Energy Environ. Sci. 2016, 9, 656– 662. [25] J. Lee, D. Kim, H. Kim, S. Seo, S. Cho, N. Park, Adv. Energy Mater. 2015, 5, 1501310. [26] D. McMeekin, G. Sadoughi, W. Rehman, G. Eperon, M. Saliba, M. Hörantner, A. Haghighirad, N. Sakai, L. Korte, B. Rech, M. Johnston, L. Herz, H. Snaith, Science 2016, 351, 151-155. [27] L. Protesescu, S. Yakunin, M. Bodnarchuk, F. Krieg, R. Caputo, C. Hendon, R. Yang, A. Walsh, M. Kovalenko, Nano Lett. 2015, 15, 3692
[28] M. Kwoka, L. Ottaviano, M. Passacantando, S. Santucci, G. Czempik, J. Szuber, Thin Solid Films 2005, 490, 36. [29] L. Vegard, Zeitschrift fur Physik, 1921, 5, 17-26. [30] N. Noel, S. Stranks, A. Abate, C. Wehrenfennig, S. Guarnera, A. Haghighirad, A. Sadhanala, G. Eperon, S. Pathak, M. Johnston, A. Petrozza, L. Herz, H. Snaith, Energy Environ. Sci 2014, 7, 3061. [31] Z. Xiao, Y.Zhou, H. Hosono, T. Kamiya, Phys. Chem. Chem. Phys. 2015, 17, 18900. [32] F. Deschler, M. Price, S. Pathak, L. Klintberg, D. Jarausch, R. igler,
u ttner,
T. Leijtens, S. Stranks, H. Snaith, M. tatu re, R. Phillips, R. Friend, Phys. Chem. Lett. 2014, 5, 1421. [33] AQ. kkerman, V. D’Innocenzo,
ccornero,
carpellini, A. Petrozza, M.
Prato, L. Manna, J. Am. Chem. Soc. 2015, 137, 10276.
Dr. Xiaoli Zhang is currently a research fellow at Nanyang Technological University (NTU) in Singapore and cooperates with group in South University of Science and Technology of China. Her research
focuses on the synthesis and characterization of nanostructures and their applications in novel energy storage and conversion devices. She studied Materials Physics & Chemistry at Tianjin University for master degree (2008-2010) and doctorate (2011-2014).
Wanyu Cao is currently a engineering master in School of Science at Tianjin University, China. Her research focuses on the nanostructured materials in photoelectronic device.
Weigao Wang obtained his master degree from Ningbo University. He is now works as Research Assistant in South University of Science and Technology of China. His research focuses on light-emitting diode fabrication and synthesis.
Dr. Bing Xu received his degree in Materials Physics and Chemistry from Tianjin University in 2015. After that he joined the Department of Electrical & Electronic Engineering, South University of Science and Technology of China (SUSTC), as a post-doctoral researcher. His current research interest is nano-photoelectrocatalysis and heterostructure devices.
Prof. Sheng Liu University in Mechanical interest is about material.
obtained his docterate degree at Stanford 1992. He is now the dean of the Power and college in Wuhan University. His research microelectron, optoelectronic and advanced
Haitao Dai received his BS and MS in Physical Education from Shaanxi Normal University; his PhD in Optics in 2005 from Fudan University. In 2010, he became an Associated Professor at Tianjin University. His current research interests include energy photonics (such as semiconductor illumination, semiconductor photocatalysis etc.) and tunable photonics devices based liquid crystal.
Dr. Shuming Chen University of Science professor in Southern (SUSTC), and his light-emitting diode thin-film transistor
obtained his docterate degree at Hong Kong and Technology in 2012. He is now the assist University of Science and Technology research interest is about organic (OLED), quantum-dot LED (QLED), and (TFT).
Dr. Kai WANG received his PhD degree in Optoelectronics from Wuhan National Laboratory for Optoelectronics (WNLO), Huazhong University of Science & Technology (HUST), in 2011. He is the assist professor in the Department of Electrical & Electronic Engineering,
South University of Science and Technology of China (SUSTC). His current research interests include core-shell nanocrystals and their potential applications in novel energy storage and conversion devices.
Prof. Xiao Wei Sun obtained a chemical engineering degree from the Tianjin, China (1990) and two doctorates from Tianjin University (1996) and Hong Kong University of Science & Technology, Hong Kong (1998). He is a tenured professor in Nanyang Technological University, and his research interest includes nanostructured materials, LSPR based catalysis, photovoltaic and 3D displays.
Highlights The perovskite nanocrystals with mixed-metal cations were prepared. The Sn doping in metal cation blue shifts the optical bandgap and emission wavelength. The proper Sn doping in Pb cation can enhance perovskite LED performance.
PII: DOI: Reference:
S2211-2855(16)30457-8 http://dx.doi.org/10.1016/j.nanoen.2016.10.039 NANOEN1564
To appear in: Nano Energy Received date: 21 August 2016 Revised date: 14 October 2016 Accepted date: 18 October 2016 Cite this article as: Xiaoli. Zhang, Wanyu Cao, Weigao Wang, Bing Xu, Sheng Liu, Haitao Dai, Shuming Chen, Kai. Wang and Xiao Wei Sun, Efficient lightemitting diodes based on green perovskite nanocrystals with mixed-metal cations, Nano Energy, http://dx.doi.org/10.1016/j.nanoen.2016.10.039 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Efficient light-emitting diodes based on green perovskite nanocrystals with mixed-metal cations Xiaoli Zhang1, Wanyu Cao4, Weigao Wang2, Bing Xu2,3, Sheng Liu3, Haitao Dai4, Shuming Chen2, Kai Wang2*, Xiao Wei Sun1,2* 1
School of Electrical & Electronic Engineering, Nanyang Technological University,
50 Nanyang Avenue, 639798, Singapore 2
Department of Electrical & Electronic Engineering, Southern University of Science
and Technology, Shenzhen, 518055, China 3
School of Power and Mechanical Engineering, Wuhan University, Wuhan, 430072,
China 4
School of science, Tianjin University, Tinjin, 300072, China
[email protected] [email protected] *
Corresponding authors.
Abstract The recent success of solar cells has witnessed the emergence of metal halide perovskites as an important class of optoelectronic materials. Even with the record efficiency and inspiring performance, lead based perovskites are criticized for its toxicity, raising concerns about the environmental pollution. The reported divalent and heterovalent metal substitution for Pb indicates unfavorable performance. The lead-containing perovskite exhibits the outstanding activity compared to lead-free
analogues. It is speculated that the perovskite with mixed-metal cation is expected to enable the fabrication of efficient device and partially alleviate toxicity. For this reason, herein, we present the first synthesis of perovskite nanocrystals based on mixed-metal cation, and demonstrate the implementation of mixed-metal cation perovskite nanocrystals into LEDs for the first time.
Keywords: Perovskite; Nanocrystals; Light-emitting diode; Mixed-metal cation
Introduction The past 5 years have witnessed a revolution in optoelectronic research with the discovery of the metal halide perovskite family. These solution-processed perovskites are fast becoming the compelling materials, and have shown tremendous potential in the field of solar cells, LED, lasing, photodetectors, x-ray diffraction detection and plasmonics
[1-9]
. Lead-halide perovskite has been considered as the most promising
materials for the next generation of solar cells, achieving efficiencies above 20%
[1-5]
.
Moreover, the lead-containing hybrid perovskite CH3NH3PbBr3 thin film applied in LED has been recently reported to exhibit a current efficiency (CE) of 42.9 cd A-1 and an external quantum efficiency (EQE) as high as 8.53%
[10]
. Despite these
breakthroughs and inspiring records, the toxicity of heavy metal lead in current perovskite raises concerns about the environmental pollution. Given these imperfection, it is urgent to address this challenge by identifying a green and nontoxic
halide perovskite optoelectronic material. The lead-free perovskite can be realized by replacing Pb in the compound via substitution of other elements, such as Sn, Bi, Ge, Cu and Ag
[11-14]
. However, the
reported lead-free perovskite indicates some issues of concern. For example, the substitution of group-14 elements Sn for Pb tends to degrade the corresponding halide perovskite due to the oxidation from Sn2+ to Sn4+
[15-18]
, while other divalent cations
substitution of lead negatively impacts optoelectronic properties by increasing band gaps and effective masses
[19]
. The unfavorable replacing of homovalent cations
stimulates the considering of heterovalent substitution, such as the formation of a double perovskite structure with one monovalent and one trivalent cation [14], however, the broad photoluminescence from 480 to 650 nm limits its application in narrow emission, such as LED, photodetector and laser. Compared to the lead-containing perovskite, the unstable lead-free analogues under ambient conditions lead to inferior device performance due to higher defect densities
[20-23]
. In addition, lead-free perovskite applications have been rarely
demonstrated, which may be ascribed to low efficiency and unstability. In spite of great efforts in lead replacement, the lead halide perovskite family exhibits the best activity up to now. Inspired by the recent studies about mixed-cation perovskite based on MA-Cs, MA-FA and FA-Cs (MA: methylammonium; FA: formamidinium)
[24-26]
,
herein, we first report the mixed-metal cation based perovskite nanocrystals CsPb1-xSnxBr3 (x = 0-1), it is found that the emission property of the perovskite CsPb1-xSnxBr3 are significantly dependent on the substitution content of the Sn cations
in the perovskite composition. Based on the characterization of their optical and physicochemical properties, the mixed-metal cation perovskites CsPb1-xSnxBr3 were employed as light emitter in the LEDs. With an optimized composition of CsPb0.7Sn0.3Br3, the corresponding device exhibited encouraging performance. The partial lead substitution not only reduces the toxicity of material, but also improves the device performance, which may provide a potential and unlock a world of new optoelectronic materials for low-toxic and green perovskite LEDs.
Results and discussion
Figure 1. (a) (b) TEM images of CsPb1-xSnxBr3 nanocrystals. (c) FFT and HRTEM of CsPb1-xSnxBr3 (x = 0.3) nanocrystals. (d) The variation of fringe distance with Sn content x in the compound. The perovskite nanocrystals with mixed-metal cation Pb and Sn were prepared
following literature methods
[27]
. In detail, the mixture of PbBr2 and SnBr2 was
dissolved in solvent tri-n-octylphosphine to get transparent solution, followed by the injection of cesium stearate and oleic acid and oleylamine to obtain stable CsPb1-xSnxBr3 nanocrystals. The as-prepared inorganic perovskite nanocrystals with mixed-metal cations were investigated by transmission electronic microscopy (TEM) as shown in Figure 1. The monodispersed CsPb1-xSnxBr3 nanocrystals have diameter of 8-10 nm with cubic shape determined by the perovskite crystal structure, which was clearly revealed by X-ray diffraction (XRD) patterns presented in Figure S1 (supporting information). For pure perovskite CsSnBr3 (x = 1), the resultant product is very unstable due to the Sn oxidation process, which is accompanied by a drop in PL efficiency to nondetectable values (<0.01%)
[13, 28]
. Thus, the Sn content x is varied
from 0 to 0.7 in this work for comparison. The lattice fringe is calculated from the fast fourier transformation (FFT) of high resolution TEM (HRTEM) (Figure 1c). As more Sn doped in the metal cation, the lattice fringe distance correspondingly reduced (Figure 1d). The observed spacing shrinkage in CsPb1-xSnxBr3 nanocrystals with an increasing Sn content x could be due to the smaller ionic radius of Sn cation (0.145 nm) than the Pb cation (0.180 nm). The elemental mapping result (Figure S2) detected by energy dispersive spectroscopy (EDS) exhibits that the Cs, Sn, Pb and Br atoms are uniformly incorporated into the nanocrystals.
Figure 2. (a) Absorption spectra and (b) PL plots of CsPb1-xSnxBr3 nanocrystals as a function of Sn content (x = 0-0.7). In order to identify the optical response of Sn doping in metal cation, we perform the UV-Vis absorption and photoluminescence (PL) measurement. According to Vegard’s law
[29]
, which is an approximate empirical rule, the band gap in
semiconductor is approximately a linear function of the lattice parameter, and the linear relationship also exists between the band gap and composition. In our work, the regular variation in lattice constant indicates the potential increased band gap of semiconductor CsPb1-xSnxBr3. As confirmed by UV-Vis absorption, as x increases, the absorption band is blue shifted from 520 nm (x = 0) to 496 nm (x = 0.7). The corresponding band gap of the perovskite is gradually shifted to the higher energy direction (from 2.38 to 2.50 eV). The normalized PL spectra are shown in Figure 2b, from which it can be clearly seen that the luminescence peaks can be effectively tuned towards shorter wavelength by varying ratio of the Sn content x. The quantum yield of the resultant nanocrystals CsPb1-xSnxPbBr3 is gradually decreased from 71% to 37% as more Sn incorporation, which is mainly ascribed to the unstable Sn oxidation in air. It is worth pointing out that the adding of Sn in Pb cation has not lead to external
emission from defects. Therefore, the use of mixed-metal cations in the perovskite materials provides versatility in the fine-tuning of the optical bandgap and the emission wavelength of provskite nanocrystals, which opens a new possible avenue to realize tunable wavelength of perovskite. The dependence of PL spectra, quantum yield and optical bandgap on Sn content x is summarized in Table S1.
Figure 3. (a) Schematic illustration of the perovskite LED device structure. (b) Energy band diagram of the different components in the perovskite LEDs. (c) Luminescnece photograph driven at 5 V for perovskite CsPb0.7Sn0.3PbBr3based LEDs. (d) EL spectra of perovskite CsPb1-xSnxPbBr3 based LEDs.
Figure 4. (a) Current density-voltage, (b) luminance-voltage, (c) current efficiency-current density of the perovskite LEDs based on CsPb1-xSnxPbBr3 nanocrystals and (d) EL spectra of device based on the best Sn substitution (x = 0.3) driven under different voltage. The excellent optoelectronics properties of the developed mixed-metal cation based perovskite nanocrystals encourage us to explore their application in LEDs. For comparison, devices based on CsPb1-xSnxPbBr3 nanocrystals with and without Sn doping were fabricated with the same device configuration. The typical device configuration consists of spin-coated layers of poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (poly-TPD)
and
the
(PEDOT:PSS), perovskite
poly(4-butylphenyl-diphenyl-amine)
nanocrystals,
and
evaporated
layers
of
1,3,5-Tris(1-phenyl-1H-benzimidazol-2-yl)benzene (TPBi) and LiF/Al, as shown in the schematic diagram in Figure 3a. The flat-band energy levels of the different components in the LEDs are illustrated in Figure 3b. In the developed devices,
electrons are injected from the LUMO (Lowest Unoccupied Molecular Orbital) level of TPBi into the conduction band of perovskite nanocrystals, while holes are transferred from the HOMO (Highest Occupied Molecular Orbital) level of poly-TPD into the valance band of perovskite NCs, followed by the radiative recombination inside the active perovskite layer. The matching of energy levels of the different components in the devices is one of the key factors that significantly affect the performance of LEDs. The fabricated perovskite LEDs show bright and uniform emission, as indicated by the photo of a CsPb0.7Sn0.3Br3 based LED which was turned on under a bias potential of 5 V. The normalized EL spectra can be readily tuned by substitution of Pb with Sn, as confirmed by the PL spectra. The detailed device performance is displayed in Figure 4. The current density-voltage
(J-V)
characteristics
of
the
perovskite
LEDs
based
on
CsPb1-xSnxPbBr3 nanocrystals are shown in Figure 4a. Without Sn doping (x = 0), the neat CsPbBr3 perovskite displays the luminance and current efficiency (CE) of 4814 cd m-2 and 0.93 cd A-1, respectively. The presence of Sn (x = 0.1) in the metal cation causes an improvement of luminance and CE about 1.05- and 1.77-fold, respectively. The best performance is achieved as x increases to 0.3, with maximum luminance and efficiency of 5495 cd m-2 and 3.60 cd A-1, respectively, indicating 1.14 and 3.87 times improvement compared to that of pure CsPbBr3. However, there is no expected performance improvement as excess Sn incorporation into the metal cation. The luminance of CsPb0.5Sn0.5Br3 CsPb0.3Sn0.7Br3 based LEDs dramatically drops down to 2203 and 1517 cd m-2, respectively. In addition, the device based on the best Sn
substitution (x = 0.3) is driven under different voltage from 5 to 7 V, no extra emission is detected in EL spectra (Figure 4d). The detailed device performance is summarized in Table S2. The improved performance can be ascribed to the gradually reduced bandgap as Sn content x increases, which results in the lowered valence band of mixed-metal cation perovskite. This facilitates the hole injection process because of the reduced hole injection barrier at the interface between the perovskite and the hole transport layer. However, the excess Sn doping (x = 0.5 and 0.7) causes the inferior device performance. To understand the kinetics of excitons and free carriers in mixed-metal cation
perovskite
CsPb1-xSnxBr3
nanocrystals,
we
conducted
time-resolved
single-photon counting measurements. Decay time of 4.5 ns is typically observed in CsPbBr3 nanocrystals (Figure S3), while time resolved PL decay of CsPb1-xSnxBr3 (x = 0.1 and 0.3) nanocrystals indicates slowly reduced radiative lifetime (4.3 ns and 3.8 ns), and the minimized value of 0.4 ns is obtained for x = 0.7. The decreased lifetime implies that the introduced Sn cation could increase free exciton emission decay that possibly derived from nonradiative energy transfer to the trap states. As reported in previous literature, the fast-decaying luminescence of Sn-based perovskite originates from band-edge states and shallow states induced by the intrinsic defects sites
[30-31]
.
The decay behavior happened in Sn-based perovskite is from its high defect state densities (~1017 cm-3), while lead-containing perovskites indicates much lower defect densities
[13, 32-33]
. Therefore, the properly Sn doping can enhance the device
performance by reducing hole injection barrier and effective radiative recombination.
Conclusions We have fabricated perovskite CsPb1-xSnxBr3 nanocrystals based on mixed-metal cation, and systematically studied the role of Sn doping on optical properties and LED device performance. We find that Sn doping affects the absorption and PL spectra of perovskite nanocrystals. In detail, as Sn content is increased, the perovskite nanocrystals witness the blue-shift of bandgap and emission peak. This work, for the first time, used a mixed-metal cation, to achieve perovskite LEDs with improved luminance and efficiency. The excess Sn addition induced the fast decay of luminance, resulting in the inferior device performance. The use of mixed-metal cation provides additional versatility in fine-tuning optical bandgap and emission wavelength. This work opens the prospect for other cations, like Cu and Zn, to be explored as doping cations for perovskites. Therefore, mixed-metal cation is a novel compositional strategy on the road to the excellent and green perovskite LEDs. Aknowledgements This research is supported by the National Research Foundation, Prime Minister’s Office, Singapore under its Competitive Research Programme (NRF-CRP11-2012-01) and administered by Nanyang Technological University. This work is also supported by National Science and Technology Major Project of the Ministry of Science and Technology of China (2016YFB0401702) and Shenzhen Innovation Project (JCYJ20160301113356947, KC2014JSQN0011A and JCYJ20150630145302223).
References
[1] W. Yang, J. Noh, N. Jeon, Y. Kim, S. Ryu, J. Seo, S. Seok, Science 2015, 348, 1234-1237. [2] J. Lee, D. Kim, H. Kim, S. Seo, S. Cho, N. Park, Adv. Energy Mater. 2015, 5, 1501310. [3] A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, J. Am. Chem. Soc. 2009, 131, 6050–6051. [4] M. Saliba, S. Orlandi, T. Matsui, S. Aghazada, M. Cavazzini, J. Correa-Baena, P. Gao, R. Scopelliti, E. Mosconi, K. Dahmen, D. Angelis, A. Abate, A. Hagfeldt, G. Pozzi, M. Graetzel, M. Nazeeruddin, Nature Energy 2016, 1, 15017. [5] D. Bi, W. Tress, M. Dar, P. Gao, J. Luo, C. Renevier, K. Schenk, A. Abate, F. Giordano, J. Correa Baena, J. Decoppet, S. Zakeeruddin, M. Nazeeruddin, M. Gratzel, A. Hagfeldt, Sci. Adv. 2016, 2, e1501170. [6] J. Baena, L. Steier, W. Tress, M. Saliba, S. Neutzner, T. Matsui, F. Giordano, T. Jacobsson, A. Kandada, S. Zakeeruddin, A. Petrozza, A. Abate, M. Nazeeruddin, M. Gratzel, A. Hagfeldt, Energy Environ. Sci. 2015, 8, 2928–2934. [7] J. Pan, L. N. Quan, Y. Zhao, W. Peng, B. Murali, S. P. Sarmah, M. Yuan, L. Sinatra, N. M. Alyami, J. Liu, E. Yassitepe, Z. Yang, O. Voznyy, R. Comin, M. N. Hedhili, O. F. Mohammed, Z. H. Lu, D. H. Kim, E. H. Sargent, O. M. Bakr, Adv. Mater. 2016, DOI: 10.1002/adma.201600784. [8] M. Saliba, S. Wood, J. Patel, P. Nayak, J. Huang, J. Alexander-Webber, B. Wenger, S. Stranks, M. Horantner, J. Wang, R. Nicholas, L. Herz, M. Johnston, S. Morris, H. Snaith, M. Riede, Adv. Mater. 2016, 28, 923–929.
[9] R. Dong, Y. Fang, J. Chae, J. Dai, Z. Xiao, Q. Dong, Y. Yuan, A. Centrone, X. Zeng, J. Huang, Adv. Mater. 2015, 27, 1912–1918. [10] H. Cho, S. Jeong, M. Park, Y. Kim, C. Wolf, C. Lee, J. Heo, A. Sadhanala, N. Myoung, S. Yoo, S. Im, R. Friend, T. Lee, Science 2015, 350, 1222-1225. [11] M. Kumar, D. Sabba, W. Leong, P. Pablo, R. Rajiv, B. Tom, S. Chen, D. Hong, R. Ramamoorthy, A. Mark, G. Michael, G. Subodh, M. Nripan, Adv. Mater. 2014,26, 7122-7127. [12] B. Park, B. Philippe, X. Zhanng, H. Rensmo, G. Boschloo, J. Erik, Adv. Mater. 2015, 18, 6806-6813. [13] T. Jellicoe, J. Richter, H. Glass, M. Tabachnyk, R. Brady, S. Dubbon, A. Rao, R. Friend, D. Credgington, N. Greenham, M. Bohm, J. Am. Chem. Soc. 2016, 138, 2941-2944. [14] G. Volonakis, M. Filip, A. Haghighirad, N. Sakai, B. Wenger, H. Snaith, F. Giustino, J. Am. Chem. Soc. 2016, 7, 1254-1259. [15] C. Stoumpos, C. Malliakas, M. Kanatzidis, Inorg. Chem. 2013, 52, 9019. [16] T. Baikie, Y. Fang, J. Kadro, M. Schreyer, F. Wei, S. Mhaisalkar, M
ra tzel, T.
White, Mater. Chem. A 2013, 1, 5628. [17] F. Hao, C. Stoumpos, D. Cao, R. Chang, M. Kanatzidis, Nat. Photonics 2014, 8, 489. [18] N. Noel, S. Stranks, A. Abate, C. Wehrenfennig, S. Guarnera, A. Haghighirad, A. Sadhanala, G. Eperon, S. Pathak, M. Johnston, A. Petrozza, L. Herz, H. Snaith, Energy Environ. Sci. 2014, 7, 3061.
[19] M. Filip, F. Giustino, Phys. Chem. C 2016, 120, 166−173 [20] N. Noel, S. Stranks, A. Abate, C. Wehrenfennig, S. Guarnera, A. Haghighirad, A. Sadhanala, G. Eperon, S. Pathak, M. Johnston, A. Petrozza, L. Herz, H. Snaith, Energy Environ Sci. 2014, 7, 3061. [21] Y. Takahashi, R. Obara, Z. Lin, Y. Takahashi, T. Naito, T. Inabe, S. Ishibashi, K. Terakura, Dalton Trans. 2011, 40, 5563. [22] Y. Takahashi, H. Hasegawa, Y. Takahashi, T. Inabe, Solid State Chem. 2013, 205, 39. [23] M. Kumar, S. Dharani, W. Leong, P. Boix, R. Prabhakar, T. Baikie, C. Shi, H. Ding, R. Ramesh, M. Asta, M. Graetzel, S. Mhaisalkar, N. Mathews, Adv. Mater. 2014, 26, 7122. [24] Y. Chen, J. Luo, S. Meloni, A. Boziki, N. Ashari-Astani, C. Grätzel, S. Zakeeruddin, U. Röthlisberger, M. Grätzel, Energy Environ. Sci. 2016, 9, 656– 662. [25] J. Lee, D. Kim, H. Kim, S. Seo, S. Cho, N. Park, Adv. Energy Mater. 2015, 5, 1501310. [26] D. McMeekin, G. Sadoughi, W. Rehman, G. Eperon, M. Saliba, M. Hörantner, A. Haghighirad, N. Sakai, L. Korte, B. Rech, M. Johnston, L. Herz, H. Snaith, Science 2016, 351, 151-155. [27] L. Protesescu, S. Yakunin, M. Bodnarchuk, F. Krieg, R. Caputo, C. Hendon, R. Yang, A. Walsh, M. Kovalenko, Nano Lett. 2015, 15, 3692
[28] M. Kwoka, L. Ottaviano, M. Passacantando, S. Santucci, G. Czempik, J. Szuber, Thin Solid Films 2005, 490, 36. [29] L. Vegard, Zeitschrift fur Physik, 1921, 5, 17-26. [30] N. Noel, S. Stranks, A. Abate, C. Wehrenfennig, S. Guarnera, A. Haghighirad, A. Sadhanala, G. Eperon, S. Pathak, M. Johnston, A. Petrozza, L. Herz, H. Snaith, Energy Environ. Sci 2014, 7, 3061. [31] Z. Xiao, Y.Zhou, H. Hosono, T. Kamiya, Phys. Chem. Chem. Phys. 2015, 17, 18900. [32] F. Deschler, M. Price, S. Pathak, L. Klintberg, D. Jarausch, R. igler,
u ttner,
T. Leijtens, S. Stranks, H. Snaith, M. tatu re, R. Phillips, R. Friend, Phys. Chem. Lett. 2014, 5, 1421. [33] AQ. kkerman, V. D’Innocenzo,
ccornero,
carpellini, A. Petrozza, M.
Prato, L. Manna, J. Am. Chem. Soc. 2015, 137, 10276.
Dr. Xiaoli Zhang is currently a research fellow at Nanyang Technological University (NTU) in Singapore and cooperates with group in South University of Science and Technology of China. Her research
focuses on the synthesis and characterization of nanostructures and their applications in novel energy storage and conversion devices. She studied Materials Physics & Chemistry at Tianjin University for master degree (2008-2010) and doctorate (2011-2014).
Wanyu Cao is currently a engineering master in School of Science at Tianjin University, China. Her research focuses on the nanostructured materials in photoelectronic device.
Weigao Wang obtained his master degree from Ningbo University. He is now works as Research Assistant in South University of Science and Technology of China. His research focuses on light-emitting diode fabrication and synthesis.
Dr. Bing Xu received his degree in Materials Physics and Chemistry from Tianjin University in 2015. After that he joined the Department of Electrical & Electronic Engineering, South University of Science and Technology of China (SUSTC), as a post-doctoral researcher. His current research interest is nano-photoelectrocatalysis and heterostructure devices.
Prof. Sheng Liu University in Mechanical interest is about material.
obtained his docterate degree at Stanford 1992. He is now the dean of the Power and college in Wuhan University. His research microelectron, optoelectronic and advanced
Haitao Dai received his BS and MS in Physical Education from Shaanxi Normal University; his PhD in Optics in 2005 from Fudan University. In 2010, he became an Associated Professor at Tianjin University. His current research interests include energy photonics (such as semiconductor illumination, semiconductor photocatalysis etc.) and tunable photonics devices based liquid crystal.
Dr. Shuming Chen University of Science professor in Southern (SUSTC), and his light-emitting diode thin-film transistor
obtained his docterate degree at Hong Kong and Technology in 2012. He is now the assist University of Science and Technology research interest is about organic (OLED), quantum-dot LED (QLED), and (TFT).
Dr. Kai WANG received his PhD degree in Optoelectronics from Wuhan National Laboratory for Optoelectronics (WNLO), Huazhong University of Science & Technology (HUST), in 2011. He is the assist professor in the Department of Electrical & Electronic Engineering,
South University of Science and Technology of China (SUSTC). His current research interests include core-shell nanocrystals and their potential applications in novel energy storage and conversion devices.
Prof. Xiao Wei Sun obtained a chemical engineering degree from the Tianjin, China (1990) and two doctorates from Tianjin University (1996) and Hong Kong University of Science & Technology, Hong Kong (1998). He is a tenured professor in Nanyang Technological University, and his research interest includes nanostructured materials, LSPR based catalysis, photovoltaic and 3D displays.
Highlights The perovskite nanocrystals with mixed-metal cations were prepared. The Sn doping in metal cation blue shifts the optical bandgap and emission wavelength. The proper Sn doping in Pb cation can enhance perovskite LED performance.