Composition-tuned MAPbBr 3 nanoparticles with addition of Cs + cations for improved photoluminescence† Sai S. H. Dintakurti, * abc Parth Vashishtha, b David Giovanni, d Yanan Fang, b Norton Foo, b Zexiang Shen, ade Claude Guet, ab Tze Chien Sum d and Tim White * b Hybrid organic–inorganic lead halide perovskite nanoparticles are promising candidates for optoelectronic applications. This investigation describes the structural and optical properties of MA x Cs 1x PbBr 3 mixed cation colloidal nanoparticles spanning the complete compositional range of Cs substitution. A monotonic progression in the cubic lattice parameter (a) with changes in the Cs + content confirmed the formation of mixed cation materials. More importantly, time-resolved photoluminescence (TRPL) revealed the optimized 13 mol% Cs nanoparticle composition exhibits the longest charge carrier lifetime and enhancement in radiative pathways. This sample also showed the highest photoluminescence quantum yield (PLQY) of 88% and displays 100% improvement in the PLQY of pure MAPbBr 3 and CsPbBr 3 . Prototype LEDs fabricated from MA 0.87 Cs 0.13 PbBr 3 were demonstrated. Introduction Hybrid organic–inorganic lead halide perovskites (LHPs) have been widely explored for optoelectronic applications resulting in solar cells with a record power conversion efficiency (PCE) of 25.5% and light emitting diodes with an external quantum efficiency (EQE) exceeding 20%. 1–7 The key advantages of this class of materials are solution processability, inexpensive starting materials, composition tunable band gap, high hole and electron mobility, and high defect tolerance. 8,9 Despite showing high efficiency, prototype perovskite materials such as MAPbX 3 and FAPbX 3 (where MA ¼ methylammonium; FA ¼ formamidinium; X ¼ Br, I) suffer from phase instability, poor reproducibility, and thermal instability. 10–12 Saidaminov et al. 13 found lattice strain in perovskite induces point defects leading to structural degradation and PLQY quenching. Lattice perfec- tion can be improved by the addition of inorganic Cs + that improves phase stability by strain relaxation leading to more reproducible optoelectronic devices with improved thermal stability. 14,15 A similar strategy of strain relaxation in FAPbI 3 produced solar cells with PCE of 24.4%. 16 In contrast, compressive strain in CsPbBr 3 has been also found to be detrimental towards stability. 17 On the other hand, Zhang et al. 18 reported signicant improvement in thin-lm LEDs using mixed cation Cs 0.87 MA 0.13 PbBr 3 material when compared to pure CsPbBr 3 . In general, it is likely that strain relaxation can be achieved by incorporating additional cations in the ABX 3 LHPs, to improve structural stability and optoelectronic properties. Following the synthesis of mixed-cation perovskite powders and appreciable crystals, research focused on analogous nano- particles to exploit their high PLQY, quantum connement, narrow emission linewidth, and size-tuneable optical proper- ties. 19–21 These studies were motivated, in part, because perov- skite nanoparticle LEDs show comparable performance to state- of-the-art CdSe based QDLEDs. 5,22,23 Song et al. 24 synthesised 15 mol% FA doped CsPbBr 3 (Cs 0.85 FA 0.15 PbBr 3 ) demonstrating 61% PLQY. Later, Vashishtha et al. 13,19 demonstrated the rst triple cation synthesis of Cs x (MA 0.17 FA 0.83 ) 1x PbBr 3 (x ¼ 0–0.15) for LED applications. These nanoparticles exhibit high phase stability and reproducibility conrming the expected advan- tages of mixed-cation perovskites. Premkumar et al. were the rst to synthesize cesium–methylammonium mixed-cation lead bromide nanoparticles exhibiting twin PL emission peaks for Cs 20% and 40% compositions (Cs 1x MA x PbBr 3 , x ¼ 0.2, 0.4). 25 The twin emission was attributed to the bi-phase nature of the samples; although Cs-MA should show a continuous solid solution and be completely miscible according to perovskite tolerance factor considerations. 25,26 It was also observed that these Cs-MA based LHP nanoparticles were larger than 12 nm a ERI@N, Interdisciplinary Graduate School, Nanyang Technological University, Singapore 639798. E-mail: sriharsh001@e.ntu.edu.sg b School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798. E-mail: tjwhite@ntu.edu.sg c Department of Physics, University of Warwick, Coventry, West Midlands, CV4 7AL, UK d School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 639798 e Centre for Disruptive Photonic Technologies, CNRS International NTU Thales Research Alliance, Nanyang Technological University, Singapore 639798 † Electronic supplementary information (ESI) available. See DOI: 10.1039/d1ra03965b Cite this: RSC Adv. , 2021, 11, 24137 Received 21st May 2021 Accepted 25th June 2021 DOI: 10.1039/d1ra03965b rsc.li/rsc-advances © 2021 The Author(s). Published by the Royal Society of Chemistry RSC Adv. , 2021, 11, 24137–24143 | 24137 RSC Advances PAPER Open Access Article. Published on 08 July 2021. Downloaded on 9/26/2023 8:36:29 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online View Journal | View Issue