Effect of Lead:Halide Precursor Ratio on the Photoluminescence and Carrier Dynamics of Violet- and Blue-Emitting Lead Halide Perovskite Nanocrystals

Abstract

Highly luminescent cesium lead halide (CsPbX 3 , X = Cl, Br, I) perovskite nanocrystals (NCs) are promising materials for a number of optoelectronic applications like LEDs and other display technologies. However, low photoluminescence (PL) quantum yield (QY) of the large-bandgap violet-and blue-emitting CsPbCl 3 and CsPb(Cl/Br) 3 NCs is an obstacle to the development of blue-and white-emitting LEDs. In this work, we show that these NCs with high PLQY can be obtained directly by employing an appropriate halide precursor and by optimizing the Pb:X precursor ratio. Specifically, employing N-chloro-and Nbromophthalimides as halide precursors and varying the Pb:X precursor ratio, we have obtained stable and highly luminescent (PLQY 80−99%) perovskite NCs emitting in the blue-violet region extending to green by direct synthesis. Time-resolved PL and ultrafast pump−probe studies of these systems reveal the effect of Pb:X precursor ratio on the carrier recombination processes. Rapid carrier trapping is found to be the dominant process that impairs the PLQY of the NCs obtained by using a stoichiometric (1:3) Pb:X precursor ratio. This trapping of carriers is effectively alleviated by using an excess amount of the halide precursor during the preparation of the NCs. The results brighten the potential utility of these high-quality perovskite NCs emitting in the blue-violet region in optical applications.

On direct synthesis of high quality APbX3 (A = Cs+, MA+ and FA+; X = Cl−, Br− and I−) nanocrystals following a generic approach

“…S6G†) is quite remarkable as it represents one of the highest reported values for this system obtained by any direct method of synthesis (Table S2†). 6,24,25,32 An important point to note here is that a stoichiometric amount of DCDMH produces these brightly luminescent CsPbCl 3 NCs (Table S2†), unlike earlier cases, where a much higher amount of chloride precursor was necessary. 6,24,25,32 We found that an excess amount of DCDMH did not have much influence on the PLQY of the system initially, but the PLQY dropped for Pb : Cl precursor ratios exceeding 1 : 7 (Fig.…”

“…The shortest and major component (0.47 ns, 76%) is due to the rapid non-radiative relaxation of the carriers through the trap states. 25,37 The absence of this trapping component for the CsPbBr 3 NCs explains its higher PLQY compared to the CsPbCl 3 NCs. The calculated radiative ( k r ) and non-radiative ( k nr ) rate constants of the systems give further insight into the charge carrier recombination processes.…”

“…[9][10][11][12][13][14] Increasing demand for various lighting and display technologies has generated huge interest in developing methodologies for the preparation of high quality perovskite NCs. 8,12,15,16 While these studies helped targeted synthesis of violet, green or redemitting all-inorganic and/or hybrid NCs, 6,[17][18][19][20][21][22][23][24][25][26][27][28][29][30][31] no generic method of obtaining all these systems with good optical properties and stability is reported to date despite intense efforts. For example, Pradhan and co-workers obtained CsPbX 3 (X = Cl, Br and I) NCs with near-unity PLQY, but not the hybrid systems.…”

Room‐Temperature Treatment with Thioacetamide Yielding Blue‐ and Green‐Emitting CsPbX3 Perovskite Nanocrystals with Enhanced Photoluminescence Efficiency and Stability

“…In contrast, the trap states are deep in nature in violet-emitting CsPbCl 3 and blue-emitting CsPb(Cl/ Br) 3 NCs and the PLQY of these systems are typically < 5% and < 40%, respectively. [15][16][17] It is quite challenging to produce brightly luminescent blue-emitting NCs with near-unity PLQY. The literature suggests that post-synthetic surface modification is an effective way of eliminating excess lead from the surface [5,[18][19][20] and suppressing halide deficiency related defects.…”

“…An important point to note here is that as these trap states are shallow in nature in green‐ emitting CsPbBr 3 NCs, these systems possess fairly high PLQY (typically, ∼50–80%). In contrast, the trap states are deep in nature in violet‐emitting CsPbCl 3 and blue‐emitting CsPb(Cl/Br) 3 NCs and the PLQY of these systems are typically <5% and <40%, respectively [15–17] . It is quite challenging to produce brightly luminescent blue‐emitting NCs with near‐unity PLQY.…”