Surface ligand modification of cesium lead bromide nanocrystals for improved light-emitting performance

Hua Wu a, Yu Zhang *a, Min Lu a, Xiaoyu Zhang a, Chun Sun a, Tieqiang Zhang b, Vicki L. Colvin c and William W. Yu *ad
aState Key Laboratory on Integrated Optoelectronics, and College of Electronic Science and Engineering, Jilin University, Changchun 130012, China. E-mail: yuzhang@jlu.edu.cn; wyu6000@gmail.com
bState Key Laboratory of Superhard Materials, and College of Physics, Jilin University, Changchun 130012, China
cDepartment of Chemistry, Brown University, Providence, RI 02912, US
dDepartment of Chemistry and Physics, Louisiana State University, Shreveport, LA 71115, USA

Received 8th December 2017 , Accepted 24th January 2018

First published on 25th January 2018


Cesium lead halide perovskite nanocrystals (NCs) possess excellent optical properties at visible wavelengths with great promise for applications in luminous display fields. We demonstrate a method to modify the surface ligand passivation of perovskite NCs for enhanced colloidal stability and emitting properties by incorporating didodecyl dimethyl ammonium bromide (DDAB). The photoluminescence quantum yield of the NC solution was improved to 96% from 70% and the perovskite film showed fewer trapped sites and enhanced carrier transport ability. The thus fabricated electroluminescent perovskite NC-LEDs exhibited a bright luminance of 11[thin space (1/6-em)]990 cd m−2, corresponding to 4-times improved external quantum efficiency (EQE), compared to the control device using regular NCs without DDAB.


1. Introduction

Recently, significant efforts have been applied to colloidal cesium lead halide perovskite (CsPbX3, X = Cl, Br, I) nanocrystals (NCs) since the first hot-injection synthesis method introduced by Kovalenko and coworkers.1 These NCs exhibit a high photoluminescence (PL) quantum yield (QY) of nearly 100%,2–4 a narrow emission line-width and a wide color gamut. They are widely applied in solar cells,5–9 LEDs10–16 and lasing.17,18 However, compared to the traditional cadmium chalcogenide quantum dots,19–21 CsPbX3 NCs showed poor colloidal stability during purification procedures and storage. Some surface chemistry research studies on CsPbX3 NCs suggest that oleylamine may be one of the main reasons for the instability.22,23 On one hand, oleylamine may detach lead atoms from the surface by coordination.24 On the other hand, ligands bonded to the surface atoms of perovskite NCs are highly dynamic. Therefore, ligands are easily lost during purification and storage in solution, making perovskite NCs unstable.25 Meanwhile, the loss of the oleylammonium halide surface species generates under-coordinated Pb atoms as electron-trapped surface sites.26 These issues limit the further application of these NCs.

Oleylamine is required in the synthesis for high-quality NCs.27 Therefore, oleylamine cannot be simply removed. Recently, Pan et al. utilized a ligand-exchange method to replace the long ligands with shorter ligands to enhance the performances of perovskite NC-LEDs greatly.28 Moreover, Yassitepe et al. proposed a strategy that utilized tetraoctylammonium halides to prepare CsPbX3 NCs, eliminating the use of oleylamine and enhancing the colloidal stability of the solution.23 Inspired by the solution-phase ligand exchange in chalcogenide quantum dots,29 we tried to use a mixture of oleylamine/DDAB to modify the surface passivation of NCs.

With the addition of DDAB, the PLQY of CsPbBr3 NCs was improved to near unity with high colloidal stability while maintaining the crystal structure and shape. Besides, this treatment also reduced the surface trap sites, enhancing the carrier transport ability of the NC film. When employing the modified NCs into LEDs, a maximum luminance and an EQE of 11[thin space (1/6-em)]990 cd m−2 and 2.1% were achieved, respectively. The performance is superior to that of the traditional (no DDAB) CsPbBr3 NC-LEDs in our previous work.30 Thus, we believe that this simple ligand change is an effective way to improve the colloidal stability and to reduce the trap sites of CsPbBr3 NCs, making the NCs more competitive for LEDs.

2. Results and discussion

We synthesized CsPbBr3 NC cubes following the method developed by Protesescu et al.,1 but DDAB was mixed with oleylamine when solubilizing PbBr2. To study the impact of the addition on the physical properties of the NCs, firstly, absorption and photoluminescence (PL) properties were measured (Fig. 1a and b). The no-DDAB CsPbBr3 NCs exhibited an absorption peak at 500 nm, while the PL peak was located at 513 nm with a full width at half-maximum (FWHM) of 18 nm. For the NCs with DDAB, a slight redshift was observed both in absorption and PL spectra. In perovskite NCs, the redshift was presumed to arise from changes in the crystal structure or some atomic doping effect. According to the X-ray diffraction (XRD) patterns for both samples, they were well consistent with the cubic CsPbBr3 phase, with no other observable phases and no peak shift compared to the untreated one, indicating the same lattice spacing for both samples (Fig. 1c). Therefore, these two factors can be ruled out based on the XRD patterns. Interestingly, the PLQY improved remarkably from 70% to 96% after the addition of DDAB. According to the previous research on halide treatment by Sargent and coworkers,31 we hypothesize that this change may be attributed to a better surface passivation. The enhanced PLQY is largely accounted for by the effective passivation of the NC surface. After being stored for one month, the no-DDAB NC solution precipitated some yellow solid; while the DDAB-treated solution still remained clear (Fig. S1). We will discuss this in a later section. Fourier transform infrared spectroscopy (FTIR) was applied to analyse the surface ligands of NCs. As shown in Fig. 1d, the N–H stretching mode at 3300 cm−1 almost disappeared after introducing DDAB. Therefore, we infer that DDAB may cause some changes in the surface properties of NCs, especially for the N-containing ligands; as a result, the quality and stability of NCs are improved.
image file: c7nr09126e-f1.tif
Fig. 1 Comparisons of absorption spectra (a), PL spectra (b), X-ray diffraction patterns (c) and FTIR spectra (d) of 0 (no-DDAB) and 0.1 M DDAB added CsPbBr3 NC samples. Inset in (a): Tauc plots data converted from the absorption spectra.

To further investigate the influence of DDAB on the properties of the NCs, four samples of CsPbBr3 NCs were prepared with different concentrations of DDAB. Sample 1, synthesized via a regular method (no DDAB), was set as a reference. The other three samples were prepared with DDAB at concentrations of 0.02 M, 0.05 M, and 0.1 M. The optical properties of NCs were all tested after the purification process and are illustrated in Fig. 2. With increased DDAB loading, the absorption edge red-shifted slightly. Transmission electron microscopy (TEM) measurements were performed for the 4 samples. According to the TEM images (Fig. 2c–f) and size distribution (Fig. S2), all the NCs show narrow size distribution and same average diameter. To better analyse the effect of the treatment, high-resolution TEM (HR-TEM) images for the samples were obtained (Fig. 2g).32 The distances between the lattice fringes of the four NC samples were all recorded as 0.41 nm for the (110) crystal face, indicating that here is no lattice change.


image file: c7nr09126e-f2.tif
Fig. 2 (a) Absorption spectra and (b) PL spectra of the 4 perovskite samples; (c)–(f) representative TEM images; all scale bars represent 15 nm. Inset in (b): average PL QYs of the NCs. (g) HR-TEM images of the corresponding four samples, and all scale bars represent 1 nm.

The PL peak of the NC solution exhibited the same trend with the increasing concentration of DDAB, but the PL QY concomitantly improved from 70% for the no-DDAB NC solution to 96% for the 0.1 M DDAB NC solution. The PL properties of the four samples are summarized in Table 1.

Table 1 Optical properties of 4 perovskite NC samples
Sample (DDAB concentration, M) PL peak (nm) FWHM (nm) PL QY (%)
1 (0) 513 18 70
2 (0.02) 515 19 87
3 (0.05) 515 20 91
4 (0.10) 515 19 96


Considering the attenuation of N–H vibrations in the FTIR spectra (Fig. 1d), X-ray photoelectron spectroscopy (XPS) measurement was used to analyse the surface ligand composition in the 4 samples. Noticeable changes were observed in the high-resolution spectra of N 1s (Fig. S3). For the no-DDAB NCs, the N 1s spectrum can be fitted into two peaks at 399.9 eV and 401.8 eV, representing amine group (–NH2) and protonated amine group (–NH3+), respectively.28 For the remaining 3 samples, the N 1s spectra were fitted into three peaks due to the addition of DDAB. In addition to the above two components, the third peak at 402.2 eV was attributed to tert-ammonium cations from DDA+ cations. As shown in Fig. 3a, the contribution of –NH3+ and DDA+ ligands to total N 1s changed visibly with the increase of DDAB added. Clearly, with the increase of the DDAB concentration, the percentage of surface DDA+ cations increased linearly, from 0 to 64%, while the contribution of the surface –NH3+ group decreased almost linearly, too, from 62% to 4%. In other words, the main N-containing components on CsPbBr3 NC's surface changed from the –NH3+ group for no-DDAB NCs to DDA+ for modified NCs.


image file: c7nr09126e-f3.tif
Fig. 3 (a) The variation of the ratios of different ligands; (b) the relative contents of C and N to Pb and Cs of perovskite films from 4 samples by XPS; (c) XPS spectra of the Pb 4f (left panel) and Br 3d (right panel) peaks for no-DDAB and 0.1 M DDAB added NCs; (d) comparisons of TRPL spectra of 4 perovskite NC films. Current density–voltage characteristics of electron-only (e) and hole-only devices (f) with DDAB added NCs.

Furthermore, the surface ligand density was also investigated by comparing the atomic ratios of C/Cs, C/Pb, N/Cs, and N/Pb from XPS peak areas of the survey spectra. As shown in Fig. 3b, the relative atomic content of C and N is generally similar for the 4 samples, indicating a similar density of the N-containing ligand. Considering the fact that one DDA+ ion contains more C content than oleylamine, we hypothesize that the surface density of the oleate ligand decreased, due to the similar C content, especially for samples 3 and 4 binding with more DDA+. DDA+ has a stronger affinity with the negative sites (Br). In addition, DDA+ has large steric hindrance due to its branched structure. Thus, DDA+ ions are good to ensure high colloidal stability of the modified NC solution. Lastly, due to a stronger binding strength of Br to the positive sites, the addition of DDAB may bring more Br ions on the surface, replacing some oleate ligands. Fig. 3c shows the Pb/Br ratio changing from 1[thin space (1/6-em)]:[thin space (1/6-em)]2.5 for sample 1 to 1[thin space (1/6-em)]:[thin space (1/6-em)]2.7 for sample 4. Both the increased steric hindrance and more Br enhanced the colloidal stability.22,23

To further verify the above assumptions, time-resolved PL (TRPL) spectroscopy analysis was done and single carrier devices for these samples were prepared to analyse their optoelectronic properties. Fig. 3d presents PL decay curves of NC films prepared from the 4 samples. The curves were fitted to triexponential decay functions and the fitting results are listed in Table S1. The average PL lifetime increased from 13.6 ns to 19.6 ns after adding DDAB at a concentration of 0.02 M, indicating the efficient Br passivation effect and fewer traps for DDAB-modified CsPbBr3 NCs, which also corresponds to the enhanced PL QY. When changing the concentration from 0.02 M to 0.1 M, the average PL lifetime did not show a significant increase, which may be a result of fewer trap sites due to efficient passivation and faster charge transfer because of the reduced organic ligand density.

Fig. 3e and f present the current density–voltage characteristics of single carrier devices based on NC films from samples 1, 3 and 4. For the electron-only device, the current density enhanced with DDAB during the synthesis process, whereas similar improvement can be observed from the hole-only devices. By performing ultraviolet photoelectron spectroscopy (UPS), the energy level of CsPbBr3 NCs can be mapped as seen in Fig. S4. Considering that there is no observable shift of the energy levels for different samples, we attribute the improved carrier injection efficiency into the shortened organic ligands and fewer trap sites for the modified CsPbBr3 NCs. Therefore, the above characterization revealed that proper surface modification can lead to the coexistence of high colloidal stability, high PL QYs, and enhanced film carrier transport capabilities, which are beneficial for improved EL efficiency in LEDs.

To demonstrate the better light-emitting performance of DDAB modified CsPbBr3 NCs, LEDs were fabricated using NCs modified by 0.05 M DDAB (Fig. 4). The device structure and its energy levels for all layers are presented in Fig. 4a and b, with a cross-section scanning electron microscopy (SEM) image shown in Fig. S5. Following our previous work,16 the device consists of multiple layers: indium tin oxide (ITO, cathode), ZnO NC layer (electron transport layer, ETL, 40 nm thickness), CsPbBr3 NCs (20 nm thickness), 4,4′-bis(carbazole-9-yl)biphenyl and 4,4′,4′′-tris(carbazol-9-yl)triphenylamine organic molecules (hole transport layer, HTL, 50 nm thickness), and MoOx/Au (anode). With the exception of HTL and MoOx/Au layers being deposited by thermal vacuum deposition, ZnO and perovskite NC layers were sequentially spin-coated from their corresponding solutions. Here, the ZnO NC solution was synthesized by a solution method, and they were highly transparent in the visible region (Fig. S6). As shown in Fig. 4c (image), the device emitted bright green light. The EL spectrum was measured with an emission peak of 513 nm and a narrow bandwidth of 20 nm. Compared to the PL of the NC solution, the EL curve of the LED presented a slight red-shift, which can be explained by the change of the dielectric function of the solvent.33,34 Compared to most of the quantum dot-based devices, this shift is negligible, evidencing a slight energy transfer between perovskite NCs.


image file: c7nr09126e-f4.tif
Fig. 4 Device structure (a) and energy level diagram (b) of CsPbBr3 NC-LEDs. (c) PL spectrum of a CsPbBr3 NC in solution and EL spectrum from the corresponding LED. Inset is a photograph of a working device emitting bright green light. (d) Voltage dependent current density and luminance curves of LEDs; (f) LED's current efficiency and EQE as a function of luminance.

The performance comparison of the LEDs with modified NCs and control NCs is shown in Fig. 4d and e. A high luminance of 11[thin space (1/6-em)]990 cd m−2 was obtained, which is greatly improved compared to that of the control device employing no-DDAB CsPbBr3 NCs. The improved luminance can be explained by the boosted carrier injection and transport ability, as well as the enhanced rational recombination process due to the superior passivation. As a direct evidence of the more efficient light-emitting application, the luminance-dependent variations of current efficiency (CE) and external quantum efficiency (EQE) are shown in Fig. 4e. The peak CE and EQE for the LED based on modified NCs reached 6 cd A−1 and 2.1%, respectively, with an enhancement of 4.5 and 4 times, respectively. Although our devices are still lower than the best level of perovskite-based LEDs,35–37 this result is higher than most data of CsPbBr3 NC-LEDs in the literature (Table S2). Moreover, this method significantly improved the LED performance, indicating that this simple surface chemistry change is promising for further optimizing perovskite-based LEDs.

3. Conclusions

To summarize, we have demonstrated that the addition of DDAB ammonium halide in NC synthesis can tune the surface ligands of the obtained perovskite NCs and improve their PL QYs. The introduced organic cations replace protonated amine groups at the surface. Due to the large steric hindrance and more Br, enhanced colloidal stability is obtained. Our data suggest that CsPbBr3 films with this method exhibited enhanced carrier transport ability and reduced non-radiative recombination centers. The resultant LED's performance was correspondingly improved. Thus, it is worth noting that the surface chemistry of NCs plays an important role in optoelectronic device applications.

4. Experimental section

4.1 Chemicals

Oleic acid (OA, 90%) was obtained from Alfa. Lead (II) bromide powder (PbBr2, 99.999%), caesium carbonate (Cs2CO3, 99.9%), ethyl acetate, and oleylamine (OLA, 80–90%) were purchased from Aladdin. Zinc acetate (Zn(Ac)2, 99.999%), 1-octadecene (ODE, 90%), DDAB (98%), and sodium hydroxide (99.99%) were purchased from Aldrich. Hexane and toluene were obtained from Aldrich.

4.2 Synthesis of CsPbBr3 NCs

The CsPbBr3 NCs were synthesized following the reported procedures.18,38,39 Firstly, a Cs-oleate precursor was prepared. Cs2CO3 (0.8 g), oleic acid (OA, 2.5 mL) and ODE (30.0 mL) were mixed in a three-neck flask. After 10 min under flowing N2, the flask was heated to 120 °C under vacuum using a mechanical pump. After 1 h, the solution was heated to 150 °C under stirring for about 3 hours until the solid was completely dissolved. Another flask containing a mixture of ODE (10 mL) and PbBr2 (0.138 g) was dried at 120 °C under vacuum. After 1 hour, 1 mL OLA (or DDAB dissolved in OLA at different concentrations) and 1 mL OA were injected under N2 to dissolve PbBr2. When the solution turned clear, the heating temperature of the flask was increased to 180 °C, and the Cs-oleate (0.8 mL) precursor was quickly added to grow perovskite NCs for 5 s. Then, the flask was immersed in ice-water to quench the reaction. For purification, firstly, the crude solution was centrifuged at 10[thin space (1/6-em)]000 rpm for 5 min. The precipitate at the bottom of the centrifuge tube was dissolved in 2 mL toluene. Then, ethyl acetate was added into the solution with a volume ratio of 1[thin space (1/6-em)]:[thin space (1/6-em)]1. After the same centrifugation process, the precipitate was collected and dissolved in 2 mL toluene and stored in a glove-box.

4.3 Synthesis of ZnO NCs

The synthesis of ZnO NCs followed a published method.40 First, Zn(Ac)2 (0.4403 g) and ethyl alcohol (30 mL) were loaded into a 100 mL three-neck flask, and held under flowing nitrogen for 10 min. Then, the mixture was heated to reflux. After 20 min, the solid was dissolved completely and the solution became clear. Then the solution was cooled to room temperature naturally. The mixture turned turbid. A solution (0.2 g sodium hydroxide dissolved in 10 mL ethanol) was rapidly injected to the stirring mixture and the reaction was continued for 4 h. During the purification process, excess hexane was used to precipitate ZnO nanoparticles. Ethyl alcohol was then used to dissolve the products. This procedure was repeated twice and the final solution was filtered using a 0.45 μm filter.

4.4 Device fabrication

First, a cleaned ITO-coated glass substrate was treated with UV-ozone for 15 min. The prepared ZnO solution was spin-coated on the substrate at 1000 rpm for 40 s. The obtained layer was annealed in air for 15 min at 100 °C.41 The fabrication of a CsPbBr3 NC film was carried out in a glove-box, via spin-coating the NC solution. The HTL and MoOx/Au anode were sequentially deposited in a vacuum deposition clamber by thermal evaporation.

4.5 Characterization and device measurements

A PerkinElmer Lambda 950 spectrometer was used to measure absorption spectra. A Bruker SMART-CCD diffractometer was utilized to collect powder XRD patterns. PL characteristics were measured via a Cary Eclipse spectrometer, while EL spectra were recorded by using a Maya spectrometer (Ocean Optics). A JEOL JSM-7500F system was utilized to record SEM images. UPS data were taken on a PREVAC system. The current–voltage curves were measured through a Keithley 2612B sourcemeter. Luminance was recorded by using a calibrated Newport 1936-R power meter equipped with a 918D-SL-0D3R silicon photodetector. XPS measurement was performed on an ESCALAB250 spectrometer. The absolute PL QY measurement was carried out via a fluorescence spectrometer (FLS920P, Edinburgh Instruments), equipped with an integrating sphere with its inner face coated with BENFLEC. Time-resolved PL spectra were obtained using a time correlated single-photon counting (TCSPC) system, under right-angle sample geometry using a mini-τ miniature fluorescence lifetime spectrometer (Edinburgh Instruments). A 405 nm picosecond diode laser (EPL-375, repetition rate: 5 MHz, 64.8 ps) was used to excite the samples.

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

This work was financially supported by the National Key Research and Development Program of China (2017YFB0403601), National Natural Science Foundation of China (61675086, 61475062, 61722504, 51702115, and 51772123), China Postdoctoral Science Foundation (2017M611319), National Postdoctoral Program for Innovative Talents (BX201600060), BORSF RCS, Institutional Development Award (P20GM103424), and Open Research Fund of State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences.

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Footnote

Electronic supplementary information (ESI) available: Detailed size distributions of NCs; high resolution XPS spectra; calculation of energy levels and PL decay lifetime parameters for NC films; cross-section SEM image of a LED device; performance comparison of green perovskite NC-LEDs. See DOI: 10.1039/c7nr09126e

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