AT-527

Oxalic Acid Enabled Emission Enhancement and Continuous Extraction of Chloride from Cesium Lead Chloride/Bromide Perovskite Nanocrystals

Shixun Wang, Xinyu Shen, Yu Zhang,* Xingwei Zhuang, Dingke Xue, Xiangtong Zhang, Jinlei Wu, Jinyang Zhu, Zhifeng Shi, Stephen V. Kershaw, William W. Yu, and Andrey L. Rogach*

Abstract

All-inorganic cesium lead halide perovskite nanocrystals (NCs) have demonstrated excellent optical properties and an encouraging potential for optoelectronic applications; however, mixed-halide perovskites, especially CsPb(Cl/Br)3 NCs, still show lower photoluminescence quantum yields (PL QY) than the corresponding single-halide materials. Herein, anhydrous oxalic acid is used to post-treat CsPb(Cl/Br)3 NCs in order to initially remove surface defects and halide vacancies, and thus, to improve their PL QY from 11% to 89% for the emission of 451 nm. Furthermore, due to the continuous chelating reaction with the oxalate ion, chloride anions from the mixed-halide CsPb(Cl/Br)3 perovskite NCs could be extracted, and green emitting CsPbBr3 NCs with PL QY of 85% at 511 nm emission are obtained. Besides being useful to improve the emission of CsPb(Cl/Br)3 NCs, the oxalic acid treatment strategy introduced here provides a further tool to adjust the distribution of halide anions in mixed-halide perovskites without using any halide additives.

Introduction

Benefitting from easily tunable band- gaps, excellent charge-transport proper- ties, high defect tolerance, and impressive photoluminescence quantum yields (PL QYs) with a narrow emission full width at half maximum (FWHM), cesium lead halide (CsPbX3, X  Cl, Br, and/or I) based perovskite light-emitting diodes (PeLEDs) have reached external quantum efficiencies (EQEs) of over 20% for emis- sion peaks in the green and the red.[1–4] Green and red PeLEDs based on cesium lead halide nanocrystals (NCs) with EQEs of up to 16% and 21% have been demon- strated as well,[5,6] while the blue-emitting PeLEDs still lag behind in terms of their performance, which impedes further developments of all-perovskite based red– green–blue displays.[7–10] Even though near-unity PL QYs have been demonstrated for CsPbCl3 NCs with a PL centered at 410 nm, this emission wavelength does not meet the requirements for displays. One way to adjust the emission wavelength toward a true blue color (450–460 nm) is to utilize mixed-halide CsPb(Cl/Br)3 perovskite NCs; but unfor- tunately, this system may suffer readily from lattice mismatch, phase segregation, and thereupon poor PL QY.[11–15]
Due to the dynamic and labile surface ligands of ionic lead halide perovskite NCs,[16] postpreparative treatment of perov- skite NCs by additives containing halides has emerged as a pow- erful tool to compensate their surface uncoordinated lead ions and thus to improve their PL QY.[17–19] We have previously dem- onstrated how the post-treatment with inorganic nitrate ions can mildly remove surface defects from CsPb(Cl/Br)3 NCs.[20] Herein, we present a strategy for achieving CsPb(Cl/Br)3 perovskite NCs with blue emission centered at 451 nm, with high PL QYs of 89% using anhydrous oxalic acid (OxA). Impor- tantly, the same kind of treatment executed for a continuous period of time (from a few seconds to 10 min) allowed us to continuously change the NCs’ bandgap, through the removal of chloride anions from the initial chloride/bromide perov- skites, so that ultimately a purely CsPbBr3 NC sample with strong green emission (PL QYs equal to 85%) at 511 nm could be obtained. Unlike previously used treatment methods with halide ions which compensate the halide vacancies, anhydrous OxA dispersed in toluene exercises a chelating action on the excess surface Pb and Cs cations to remove surface defects and eventually to decompose poorly crystalline perovskite NCs, thus resulting in improved PL QYs for the surviving NCs. Such a treatment is also useful to effectively control the Br/Cl ratio in the mixed-halide CsPb(Cl/Br)3 perovskite NCs and can be allowed to proceed up to the complete removal of Cl anions if desired.
100 L of CsPb(Cl/Br)3 perovskite NCs prepared by a hot- injection method (see the Experimental Section for details) were dispersed in 3 mL of toluene; this solution will be denoted as “pristine” in the following discussion. 50 mg of OxA powder was directly added into a solution of 100 L CsPb(Cl/Br)3 perovskite NCs dispersed in 1 mL toluene, and the mixture was vigorously stirred for some time, ranging from 2 s to 10 min. OxA would quickly react with the perovskite NCs once dissolved, leading to the changes discussed later on. After the treatment, non- dissolved OxA was removed by filtration, the filtered solution was centrifuged for 1 min at 6000 rpm, and the supernatant was collected and added into 2 mL toluene for further study. Figure 1a shows optical absorption spectra and photolumines- cence excitation (PLE) spectra of the pristine and the treated samples, labeled as “2 s OxA” and “10 min OxA” according to their treatment time. As compared to the pristine sample, the “2 s OxA” sample shows a slight blue shift originating from the quantum size effect during the surface defect elimina- tion process, while the “10 min OxA” sample has an obviously decreased optical density and a significantly red-shifted (from 443 to 501 nm) absorption spectrum, according to the variation of the PLE spectra. Figure 1b provides the respective PL spectra of the three samples; the “2 s OxA” sample shows a slight blue- shift of 3 nm and significantly enhanced PL QYs (from 11% for the pristine sample up to 89%), while the “10 min OxA” sample also has greatly improved PL QYs of 85% and a large red-shift from 454 to 511 nm. Figure 1c,d shows PL spectra and the PL FWHM of the set of samples obtained for different OxA treat- ment times; the PL peak continuously shifted to longer wave- lengths, and the Q-factor, which is defined here as the FWHM divided by the PL peak center wavelength, first decreased from 0.038 for the pristine sample to 0.036 for the “2 s OxA,” then increased to 0.041 for the “3 min OxA,” and finally reduced to 0.040 for the “10 min OxA” sample. There may be less anion composition dispersion as the result of the treatment, due to the removal of chloride in the mix-halide NCs by the oxalic acid induced chelating action, leading to a bromide-rich system with narrower proportion. Notably, the PL emission intensity reduced afterward (see details of the PL evolution shown in Figure S1, Supporting Information), though the treated sam- ples maintained high PL QYs (73%, 77%, 77%, 81%, and 84% for the 1, 2, 3, 5, and 7 min OxA treated NCs, respectively). It may also be attributed to possible lattice distortion occurring while removing the chloride, and the following lattice recon- struction occurring in response to the extraction of chloride ions during and/or after the OxA treatment process, in addi- tion to the obvious decrease of absorbance.[21] Further, to study the changes on PL performance from the pristine to “2 s OxA” samples, PL QYs were recorded by reducing the content of the OxA powders to 15 mg, though the PL enhancement would not be comparable with the treatment by adding 50 mg OxA powders due to the insufficient content of oxalic acid dissolved in toluene. The “less OxA” treated sample could also reach its highest PL QY after 6 s and showed continuously improved PL QYs of 11%, 41%, 64%, and 72% for the pristine, “2 s,” “4 s,” and “6 s” OxA treated NCs, respectively.
Time-resolved PL decay curves of the three samples are shown in Figure 1e. The average (effective) PL lifetime (avg), the radiative rate Kr, and the apparent nonradiative rate Knr (ignoring the presence of dark nanoparticles relating to the blinking phenomenon) were calculated in ref. [19] and are listed in Table 1. While the true nonradiative lifetime and nonradia- tive transition rate cannot be calculated without the knowledge of any NC dark fraction, we have calculated the value assuming zero dark fraction in order to make comparisons between the different samples, as if the dark fraction did not change from sample to sample.[22] Both the “2 s OxA” and “10 min OxA” samples have longer average PL lifetimes of 5.9 ns as compared to 2.6 ns for the pristine sample. They also have increased radi- ative decay rates (over 3.4-fold of the pristine value) and greatly decreased (almost ten times) apparent nonradiative decay rates, which may indicate that the treated samples have fewer sur- face defects. Figure 1e compares the profiles of the PL decay curves for the pristine and treated samples, from which it can be appreciated that while the pristine sample decay is markedly heterogeneous in nature, the OxA treatment rapidly suppresses this resulting in decays that are far closer to single exponential in form. This correlates with the improvements in the PL QY. As for the strong redshift of the PL peak for the “10 min OxA” sample, as illustrated in Figure 1f, we infer that it is due to the removal of chloride ions when conducting the elimination of surface defects and the wholesale destruction of perovskites with poor-crystallinity, which reveals that the [PbCl6]4 octa- hedron in perovskites tends to have fewer well-coordinated lead ions and more defects, and thus, a comparatively poorer stability.
According to the transmission electron microscopy (TEM) images provided in Figure 2a and Figure S2 (Supporting Information), the pristine CsPb(Cl/Br)3 perovskite NCs are cubic particles with an average edge length of 9.9 nm, which have many dark dots (metallic Pb) due to continuous irradia- tion under the high-power electron beam.[23] The “2 s OxA” and “10 min OxA” samples have an average edge length of 9.4 and 9.6 nm, respectively (see size histograms in Figure 2b,c). They show better stability under electron beam due to the elimination of surface defects primarily originating from uncoordinated lead octahedra (halide vacancies) and lead vacancies formed during the growth of perovskite NCs.[18] More TEM images, including those of the “less OxA” treated samples presented in Figures S3 and S4a–c (Supporting Information) further dem- onstrate the size decrease from 9.9 to 9.6 nm, with fewer dark dots after the treatment. Notably, there is still some number of dark dots in the TEM image of the “2 s OxA” sample, which primarily locate on some connected and deformed NCs, while there are almost no such dots on the TEM image of the “10 min OxA” sample, indicating its significantly improved stability (Figure S2, Supporting Information). Energy dispersive spectrometer (EDS) data (Figures S4d and S5, Supporting Information) show that the (Cl  Br):Pb ratio increased to and “6 s” treated samples, respectively (Figure S4, Supporting Information). This is due to the removal of some lead ions and a decrease of the fraction of perovskite particles with poor crys- tallinity, in agreement with the previously reported findings.[20] The same peak of the “10 min OxA” sample shows a shift of 0.3 toward lower 2 angles compared with the pristine sample, due to the further removal of chloride ions from the mixed- halide perovskite lattice. Estimation of the crystal size from XRD data based on the Scherrer equation provides the values of 10.1, 9.6, and 9.6 nm for the pristine, “2 s OxA,” and “10 min OxA,” respectively. Similarly, the size evolution from 9.8, 9.7, to 9.6 nm was obtained for the “less OxA” treated samples. These support our assumption that the post-treatment leads to a decrease of NC size due to the removal of surface defects. The elemental mapping for the “10 min OxA” sample presented in Figure 2e also indicates that the pristine CsPb(Cl/Br)3 NCs become CsPbBr3 NCs, because only the distribution of Br maintains a full tight overlap with the TEM image, while Cs, Pb, and Cl elements become redistributed all over the place, due to the partial decomposition of some of the perovskite NCs, under the prolonged action of OxA.
XPS data provided in Figure 3a show that the Pb-4f spec- trum of the pristine CsPb(Cl/Br)3 NCs has two signature peaks of Pb-4f7/2 and Pb-4f5/2 at 138.0 and 142.9 eV, while these peaks for the “2 s OxA” sample slightly shifted toward higher binding energy (138.2 eV for Pb-4f7/2 and 143.1 eV for Pb-4f5/2), and for the “10 min OxA” sample they shifted even more (138.5 eV for Pb-4f7/2 and 143.4 eV for Pb-4f5/2). This indicates that some lead atoms at the surface, probably those coordinated to fewer halogen ions, were removed in the treated samples. The intensity of the N-1s core level in Figure 3b, corresponding to the amine group of oleylamine, was improved for the “2 s OxA” sample and became even stronger for the “10 min OxA” sample, which means that oleylamine ligands can better bind to the surface of those nanoparticles. Figure 3c shows negligible change on the intensity of the Br-3d core levels, but there is a shift toward lower binding energy of 0.1 eV for the “2 s OxA,” which is followed by a back-shift toward higher binding energies for the “10 min OxA” sample where the process of chloride removal and the formation of a more perfect perovskite lattice have been accomplished. From Figure 3d, the Cl-2p core level of the “2 s OxA” sample shows a shift of about 0.1 eV toward lower binding energy as compared to the pristine sample, and there are no such peaks detected for the “10 min OxA” sample, indicating the complete removal of chloride in the latter.
For the pristine NCs, the value of the Cs:Pb ratio derived from the XPS measurements is 1.12, the (Cl  Br):Pb ratio is 2.85 and the Cl:Br ratio is 1.10, which is due to the pres- ence of excess cesium ions and halide vacancies resulting in a short-range disorder in the perovskite NCs. For the “2 s OxA” sample, the Cs:Pb ratio decreases to 1.01, the (Cl  Br):Pb ratio increases to 3.06, and the Cl:Br ratio slightly decreases to 1.06, indicating the removal of some uncoordinated lead and cesium ions from the surface, which eventually results in NCs with a more ideal perovskite lattice having better PL properties. This is in good agreement with the EDS data for the “less OxA” treated sample (Figure S4d, Supporting Infor- mation) and the inductively coupled plasma (ICP) analysis which revealed that the Cs:Pb ratio gradually changed from 1.14, 1.09, 1.05, to 1.02 for pristine, “2 s,” “4 s,” and “6 s” treated samples. For the “10 min OxA” sample, the Cs:Pb ratio further decreases to 1.02, the (Cl  Br):Pb ratio (2.95) constitutes an intermediate value between the previ- ously discussed cases, and the Cl:Br ratio becomes 0, due to the complete removal of chloride ions from the mixed-halide perovskite lattice. A plausible scenario is that the less-coor- dinated cesium and lead ions at the surface of CsPb(Cl/Br)3 NCs were first removed by a chelating reaction induced by some soluble form of OxA (such as C2O42 and/or HC2O4), during the short 2 s treatment. In the “2 s OxA” sample, the Cl:Br ratio decreased from 1.10 to 1.06, which implies that [PbCl6]4 octahedrons have more halide vacancies than [PbBr6]4 octahedrons. During the prolonged OxA treatment, which resulted in the “10 min OxA” sample, [PbCl6]4 octa- hedrons were completely chelated (Cl:Br ratio decreased to zero), while [PbBr6]4 octahedrons with better stability were retained. The enhanced XPS peak intensities of both C-1s and O-1s (Figure 3e,f) highlight the higher content of C and O (which can originate either from the oxalate ions or oleylamine ligands) as compared to the pristine sample. The peak at 288.5 eV in the XPS spectrum of C-1s core level of the “10 min OxA” sample can be attributed to lead oxalate and cesium oxalate formed from decomposed nonstoichiometric perovskite lattice fragments, as supported by Fourier trans- form infrared (FTIR) data presented below.
FTIR spectroscopy has been performed to further probe the presence of surface ligands on perovskite NCs in these three cases. As seen in Figure 4a, peaks at 3356 and 1641 cm1, corresponding to the NH stretching and NH bending vibra- tions, respectively, were remarkably enhanced for the treated samples. This evidences on better coordination of the treated NC surface by oleylamine ligands, which helps to passivate sur- face traps and thus to improve PL QYs. Besides, stronger CH stretching vibration peaks at 2925 and 2857 cm1 for the treated samples indicate that more organic ligands become attached to the perovskite NCs after the OxA treatment process. The relatively weak CO stretching signal at 1708 cm1, and CO stretching vibrations at 1194 and 1075 cm1 may originate both from oleic acid ligands and oxalate ions. Thus, we infer that the oxalate ions chelated with cesium and/or lead ions mostly reside in the decomposed fraction of the perovskite NC mate- rial instead of functioning as a ligand capping on the NCs with good crystallinity. However, with the characterization tools used herein, we are not able to conclusively resolve specific informa- tion on the spatial distribution of the halogen ions (Cl and Br) though it is possible that Br ions are mainly located in the inner part of pristine CsPb(Cl/Br)3 NCs similar with the findings for CsPb(Br/I)3 NCs.[24] Alternatively, there may be a lattice reconstruction occurring in response to the extraction of chloride ions during and/or after the OxA treatment process.[21] Raman measurements (Figure 4b; Figure S4f, Supporting Information) further revealed the advantages of the OxA treated samples even with a lower dosage treatment, as all of them pos- sess stronger vibrational mode 1 at 72 cm1 compared with the broader and weaker peak of the pristine sample, indicating less [PbX6]4 octahedra distortion in perovskite lattices based on the relevant literature.[25,26]
In conclusion, we have demonstrated that a short treatment with anhydrous oxalic acid is able to remove uncoordinated lead and cesium ions from the surface of CsPb(Cl/Br)3 NCs emitting at 451 nm, which results in an improvement of their PL QYs from 11% to 89%. For extended OxA treatment times, we were able to gradually and progressively extract chloride ions from the mixed halide CsPb(Cl/Br)3 perovskite NCs, thus ensuring a gradual shift of emission wavelength from 451 to 511 nm, while keeping the emission of the finally obtained CsPbBr3 NCs strong and narrow in width, with PL QYs of 85%. The facile treatment introduced here provides an effective strategy to prepare color tunable perovskite NCs with emission ranging from blue to green. Our study also sheds some light on the distribution of halogen ions in mixed halide CsPb(Cl/Br)3 perovskite lattices.

Experimental Section

Chemicals: Cs2CO3 (99.9%), PbCl2 (99.999%), and PbBr2 (99.999%) were purchased from Sigma-Aldrich. Oleic acid (90%) and 1-octadecene (ODE, 90%) were purchased from Alfa Aesar. Oleylamine (OAm, 80–90%) and anhydrous oxalic acid (OxA, 99.0%) were purchased from Aladdin. All the chemicals in this work were used without further treatment.
Synthesis of CsPb(Cl/Br)3 NCs: Cesium oleate was prepared by mixing Cs2CO3 (0.814 g), 2.5 mL oleic acid, and 30.0 mL ODE in a 100 mL three- necked flask and degassing the mixture under vacuum for 1 h at 120 C, followed by heating to 150 C under N2 until the solution became clear. For the synthesis of CsPb(Cl/Br)3 NCs, 20.0 mL ODE, 0.138 g PbBr2, and 0.104 g PbCl2 were added into a 50 mL three-necked flask, degassed, and dried by applying vacuum for 1 h at 120 C. Dried 2 mL oleic acid and 2 mL OAm were injected into the flask at this temperature, which was raised to 160 C once the solution became clear. 2 mL of cesium oleate solution was quickly injected, and 5 s later, the mixture was cooled down to room temperature in an ice-water bath. The solution was centrifuged for 10 min at 5000 rpm, and the obtained precipitate was dissolved in 3.0 mL of toluene, centrifuged again for 10 min at 10 000 rpm, and dissolved again in 2.0 mL toluene. 100 L of the resulting solution was dispersed in 1 mL toluene to conduct the OxA treatment process. After a centrifugation process, the supernatant was added into 2 mL of toluene for further characterizations.
Characterizations: Optical absorption and PL measurements were performed on a UV–visible spectrophotometer (PerkinElmer Lambda 950) and a Cary Eclipse spectrofluorimeter, respectively. TEM characterization was conducted on an FEI Tecnai F20 microscope. Powder XRD patterns were collected on a Bruker SMART-CCD diffractometer. FTIR spectroscopy was performed on an IFS-66 V/S FITR spectrophotometer. XPS was carried out on an ESCALAB250 spectrometer. Raman spectra were measured in backscattering geometry using a microspectroscopic Raman setup equipped with a 532 nm excitation laser. ICP measurement was conducted on a PerkinElmer NexION 350X ICP-MS Spectrometer. Absolute PL QYs were measured on a fluorescence spectrometer (FLS920P, Edinburgh Instruments) equipped with an integrating sphere with its inner face coated with BENFLEC. Quinine sulfate in 0.1 M H2SO4 with PL QY of 58% was used as a reference to ensure the test accuracy. Time-resolved PL lifetime measurements were carried out using a time-correlated single-photon AT-527 counting lifetime spectroscopy system with a picosecond pulsed diode laser (EPL-365 nm) as the single wavelength excitation light source.

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