Ammonium Thiocyanate-Passivated CsPbI3 Perovskite Nanocrystals

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C: Energy Conversion and Storage; Energy and Charge Transport 3

Ammonium Thiocyanate-Passivated CsPbI Perovskite Nanocrystals for Efficient Red Light-Emitting Diodes Min Lu, Jie Guo, Po Lu, Liu Zhang, Ying Zhang, Qilin Dai, Yue Hu, Vicki L. Colvin, and William W. Yu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b06144 • Publication Date (Web): 21 Aug 2019 Downloaded from pubs.acs.org on August 21, 2019

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The Journal of Physical Chemistry

Ammonium Thiocyanate-Passivated CsPbI3 Perovskite Nanocrystals for Efficient Red Light-Emitting Diodes

Min Lu,†, ‡ Jie Guo,† Po Lu,† Liu Zhang,*, ‡ Ying Zhang,§ Qilin Dai,§ Yue Hu,|| Vicki L. Colvin,|| and William W. Yu,*, †, ±

‡College

of Instrumentation and Electrical Engineering, Jilin University, Changchun 130061,

China †State

Key Laboratory of Integrated Optoelectronics and College of Electronic Science and

Engineering, Jilin University, Changchun 130012, China §Department

of Physics, Jackson State University, Jackson, MS 39217

||Department

of Chemistry, Brown University, Providence, RI 02912

±Department

of Chemistry and Physics, Louisiana State University, Shreveport, LA 71115,

USA

ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT: Lead halide perovskite nanocrystals (NCs) as a promising material have been widely applied in optoelectronic devices, but the massive defects reduce their optical properties and consequently limits the device performance. In this work, we developed a method to reduce the surface defects caused by unbonded lead on the surface of NCs by introducing ammonium thiocyanate (NH4SCN) in the synthesis of CsPbI3 NCs. Thiocyanate ions (SCN-) can link to the unbonded lead on the surface of perovskite NCs, resulting in a prolonged lifetime and enhanced photoluminescence quantum yield. At the same time, the whole energy level of passivated CsPbI3 NC films goes down compared to the pristine CsPbI3 NC films, which contributes to better electron injection. As a result, the passivated NCs based light-emitting diodes (LEDs) realized 10.3% external quantum efficiency (EQE) and 823 cd/m2 luminance, higher than the LEDs made from the non-passivated NCs.


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1. INTRODUCTION In recent years, lead halide perovskite semiconductors have attracted significant attention in the fields of solar cells,1 photodetectors,2 and light-emitting diodes (LEDs)3, 4 due to their superior optical and electronic properties. In particular, perovskite nanocrystals (NCs) own their unique merits including higher photoluminescence quantum yield (PL QY), narrower full width at half maximum (FWHM), wider spectral coverage, easy synthesis and low fabrication cost, making them preferred materials for LED fabrication.5-7 But perovskite NCs have massive surface defects due to the large surface-to-volume (S/V) ratio. In addition, the dynamic nature and rather labile of ligand binding lead to the ligands to easily escape from the NC surface, resulting in increased surface defects.8-10 The existence of surface defects seriously degrades the luminescence properties and stability of perovskite NCs and further reduces the device performance of LEDs.11, 12 Numerous approaches and methods have been developed to passivate the surface defects of perovskite NCs. For instance, surface ligand regulation is an effective approach to passivate the surface defects,13, 14 but the introduction of the organic ligand with short chain still decreases the conductivity of materials and further influences the device performance. Other methods, such as reactant ratio regulation, surface cladding and post-treatment of pseudohalide are also adopted to reduce the surface defects and improve the optical properties of perovskite NCs.15-17 Recently, thiocyanate anion (SCN−) has been widely used as an additive in perovskite solar cells to stabilize the perovskite crystal structure, improve the crystal quality, decrease the trap density, and passivate grain boundaries, thereby, improving the efficiency and stability of perovskite solar cells.18-20 Here, we choose ammonium thiocyanate (NH4SCN) as an additive to passivate the surface defects of CsPbI3 NCs in the process of NC synthesis and further to improve the device performance of perovskite LEDs. By regulating the ratios of NH4SCN in the precursor



solutions, CsPbI3 NCs with longer average PL lifetimes and higher PL QY (84-89%) are obtained. Moreover, as the increase of NH4SCN contents, the whole energy level of NCs goes down, which reduces the electron injection barrier and increases the carrier injection efficiency, leading to more effective carrier recombination. Thus, better device performance was achieved. The maximum external quantum efficiency (EQE) and luminance were 10.3% and 823 cd m-2 for the perovskite LEDs based on CsPbI3 NCs with 20% NH4SCN.

2. EXPERIMENTAL SECTION 2.1 Materials. Oleic acid (OA, 90%) and 1-octadecene (ODE, 90%) were purchased from Alfa Aesar. Oleylamine (OLA, 80-90%) and NH4SCN were acquired from Aladdin. Cs2CO3 was bought from J&K, and PbI2 was obtained from Sigma-Aldrich. 2.2 Synthesis of perovskite NCs. The synthesis of CsPbI3 NCs followed a previously published method.21,

22

Cesium oleate was prepared by adding Cs2CO3 (0.814 g), OA (2.5

mL), and ODE (30.0 mL) into a 100 mL three-neck flask; it was degassed and dried under vacuum for 1 h at 120 ˚С, and then heated to 150 ˚С under N2 until a clear solution was obtained. For the synthesis of CsPbI3 NCs with different NH4SCN ratios, 10.0 mL ODE, 0.173 g PbI2, 0.0057 g NH4SCN (20%) (or 0.0086 g NH4SCN (30%), 0.0114 g NH4SCN (40%)), 1.0 mL OA and 1.0 mL OLA were loaded into a 50 mL three-neck flask, degassed and dried by applying vacuum for 1 h at 120 ˚С. After the solution became clear, the temperature was raised to 170 ˚С, and 1 mL of cesium oleate solution was quickly injected. Five seconds later, the reaction mixture was cooled down to room temperature in an ice-water bath. The reaction product was separated by centrifugation for 10 min at 5000 rpm, the precipitate was redispersed in 2.0 mL of toluene. Adding 2.0 mL ethyl acetate and centrifuging once again for 10 min at 10000 rpm, the precipitate was redispersed in 1.0 mL of toluene to fabricate the LEDs.


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2.3 Synthesis of ZnO NCs. ZnO NCs were synthesized according to a previously published method.23, 24 A mixture of zinc acetate (0.4403 g) and ethyl alcohol (30.0 mL) were loaded into a 100 mL three-neck flask, degassed at room temperature for 10 min, and heated to boiling point temperature until zinc acetate powder was completely dissolved. After 30 min, the flask was cooled to room temperature naturally. Sodium hydroxide (0.2 g) in ethyl alcohol (10 mL) was injected into the flask quickly, which was maintained for 4 h; the product was purified, and the finally obtained precipitate was dissolved in 3 mL of ethyl alcohol. 2.4 Device fabrication. Patterned ITO-coated glasses were cleaned successively using soap, deionized water, ethanol, chloroform, acetone, and isopropanol, and subjected to UV-ozone treatment for 15 min. A solution of ZnO NCs was spin-coated onto the ITO substrate at 1000 rpm for 1 min and annealed in air at 150 ˚С for 10 min. The substrate was transferred into an N2 glove-box. A PEI 2-methoxyethanol solution (0.2% mass fraction) was spin-coated onto the ZnO film at a speed of 3000 rpm and annealed at 125 ˚С for 10 min. The perovskite NC emissive layer was spin-casted at 1000 rpm for 50 s. TCTA, MoO3, and Au layers were sequentially deposited by thermal evaporation in a vacuum deposition clamber (3 × 10−4 Pa). 2.5 Characterization. Absorption and photoluminescence (PL) spectra were measured on a PerkinElmer Lambda 950 spectrometer and a Cary Eclipse spectrofluorimeter, respectively. XRD data were collected on a Bruker SMART-CCD diffractometer. The absolute PL QYs of the samples were measured with a fluorescence spectrometer (FLS920P, Edinburgh Instruments) equipped with an integrating sphere with its inner face coated with BENFLEC. Time-resolved PL measurements were performed with a time correlated single-photon counting system of the FLS920P Edinburgh spectrometer. A 379 nm picosecond diode laser (EPL-375, repetition rate 5 MHz, 64.8 ps) was used to excite the



samples. Fourier transform infrared spectroscopy (FTIR) was conducted on an IFS-66V/S FITR spectrophotometer. Transmission electron microscopy (TEM) images were obtained on an FEI Tecnai F20 microscope. Ultraviolet photoelectron spectroscopy (UPS) spectra were collected using a PREVAC system. The J−V−L characteristics and the EL spectra of the LEDs were collected on a Keithley 2400 sourcemeter and a Photo Research spectrometer PR650 with an adhesive encapsulation in the dark room.

3. RESULTS AND DISCUSSION The CsPbI3 NCs in this work were synthesized following a previous report with minor modifications,21 and the passivated CsPbI3 NCs were prepared via adding different amounts of NH4SCN in the reaction solutions. Specific synthesis details are given in the experimental section, using four different feeding molar ratios of NH4SCN:PbI2 (0, 0.2, 0.3, and 0.4). Figure 1a shows the absorption and PL spectra of CsPbI3 NCs with different amount of NH4SCN. The first excitonic absorption peaks are barely changed at 671 nm, but it clearly shows that the band tail states disappeared after NH4SCN were introduced, demonstrating the surface passivation of CsPbI3 NCs.25 As the NH4SCN ratios increase from 0 to 40%, the PL peaks have a slight blue-shift from 686 to 691 nm. At the same time, the FWHM of PL peaks decreases from 36 to 32 nm. The absolute PL QY and FWHM as functions of the ratios of NH4SCN are summarized in Figure 1b. The PL QY of CsPbI3 NCs with different amounts of NH4SCN in toluene solution was determined under excitation by a 365 nm wavelength light, using a ﬂuorescence spectrometer equipped with an integrating sphere. They are 65%, 89%, 86%, 84% for pristine CsPbI3, adding 20% NH4SCN, 30% NH4SCN, and 40% NH4SCN, respectively. The increased PLQY presents the reduced nonradiative decay in the passivated CsPbI3 NCs.26, 27 To gain more insight into the exciton recombination dynamics, the time-resolved PL


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spectroscopy was performed, as shown in Figure 1c. The PL decay curves were fitted by a bi-exponential function to determine the decay times including two components. One is a fast component, which presents the nonradiative recombination; another is a slow component, which presents the radiative recombination. Corresponding data are summarized in Table 1. It is clear that the contribution of slow component and the average PL lifetimes become longer for the passivated CsPbI3 NCs, which demonstrates that the nonradiative channels are suppressed and the photon-generated carrier are more inclined to recombine with radiative channels when NH4SCN is available.27,

28

The nonradiative channels induced by surface

defects may originate from the under-coordinated Pb atoms on the surface of perovskite NCs to reduce the PL QY of perovskite materials.29,

30

When NH4SCN was introduced, the

under-coordinated Pb on the surface of CsPbI3 NCs can be effectively passivated by the SCNfrom NH4SCN due to the strong coordination between SCN−and Pb2+, leading to the enhanced PL QY and prolonged average PL lifetime. To explore the influence of NH4SCN on the crystal structure and morphology features of CsPbI3 NCs, X-ray diffraction (XRD) and transmission electron microscopy (TEM) were carried out. As shown in Figure 1d, the XRD patterns of CsPbI3 NC film with different amounts of NH4SCN have cubic phase with the same peak positions, and there is no obvious XRD peak shift as the content of NH4SCN increases, implying that the NH4SCN is not incorporated into the crystal structure of CsPbI3 NCs. Besides, the CsPbI3 NCs show enhanced crystallization that the diffraction peak of NCs gradually becomes stronger with the increase of the amount of NH4SCN, which may be attributed to the strong coordination of SCN- ions with CsPbI3 NCs. Thereby, the crystal growth was controlled effectively and a better crystal structure was formed. Figures 2a-d show the TEM images of all samples, they have a good size monodispersity and cubic shape. The average particle sizes are 11.0, 12.8, 13.1, and 13.9 nm for the pristine CsPbI3 NCs, adding 20% NH4SCN, 30% NH4SCN, and



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40% NH4SCN NCs, respectively; corresponding particle size distributions are given in insets of Figure 2 a-d. It is clear that the particle sizes of CsPbI3 NCs become larger with increasing NH4SCN content, which is consistent with the red-shift of PL peaks observed for CsPbI3 NCs with increasing NH4SCN concentration. In order to further demonstrate the existence of SCN- on the film, the X-ray photoelectron spectroscopy (XPS) of pristine and NH4SCN passivated CsPbI3 NC film was performed. As shown in Figure 3a, there is no S signal for the pristine CsPbI3 NCs, but S 2p peak appears for the NH4SCN passivated CsPbI3 NCs, which is attributed to the existence of NH4SCN. Next, Figure 3b shows the Fourier transform infrared spectroscopy (FTIR) of CsPbI3 NCs with different amounts of NH4SCN. It is noted that the peak at 1647 cm−1 corresponding to the N−H stretching and peaks at 2852, 2923, and 1463 cm−1 corresponding to the C−H stretching reveal the presence of OA and OLA ligands on the surface of perovskite NCs.31 Following the adding of NH4SCN, we observed the presence of a new peak at 2060 cm−1, consistent with the C≡N bond of a thiocyanate molecule to lead with a Pb−S bond,26, 27 which further demonstrated that SCN- ligands were bonded on nanocrystal surface. The CsPbI3 NCs with different NH4SCN ratios were further purified using ethyl acetate and they were used as emissive layers in electroluminescent LEDs. The ultraviolet photoemission spectroscopy (UPS) was characterized to confirm the energy level locations for the CsPbI3 NCs with different NH4SCN ratios, the corresponding data are shown in Figure 4b. The bandgap of all NCs that is 1.76 eV can be obtained from the Tauc plots (Figure 4a). From Figure 3b, the values of valence band maximum (VBM) of NCs were determined as -5.19, -5.38, -5.49, and -5.61 eV for pristine CsPbI3 NCs, 10% NH4SCN, 20% NH4SCN and 30% NH4SCN NCs. Consequently, the values of conduction band minimum (CBM) are calculated as -3.43, -3.62, -3.73 and -3.85 eV, respectively. In the perovskite NCs based

LEDs,

an

inverted

device

structure


with

indium

tin

oxide

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(ITO)/ZnO/polyethyleneimine (PEI)/perovskite NCs/4,4,4″-tris(carbazol-9-yl)triphenylamine (TCTA)/MoO3/Au was adopted, as shown in Figure 4c. Corresponding energy level diagram for all functional layers of these NCs based LEDs is given in Figure 4d. The ITO and MoO3/Au layers were used as cathode and anode, respectively, because of their ohmic carrier injection properties.32 An n-type ZnO NC film was chosen as the electron transport layer (ETL) due to its high electron mobility and deep-lying VBM (-7.2 eV) that is beneficial to blocking holes from the anode.33 So it also acted as a hole blocking layer (HBL). A thinner PEI film served as interlayer to modify the energy level of ZnO, thus reducing the work function of cathode contacts.34 A p-type TCTA film was used as the hole transport layer (HTL) because of its suitable highest occupied molecular orbital (HOMO) (-5.7 eV) and low electron affinity (-2.3 eV) that is beneficial to blocking electron from the cathode.35 Thereby, it is also called electron blocking layer (EBL). For the emissive layers, the CBM is gradually lower as the NH4SCN content increases, which reduces the electron injection barrier, thus improves electron injection efficiency. The appropriate energy level structure allowed for the confinement of injected charge carriers and enabled efficient radiative recombination of charge carriers in the emissive layer of LEDs. Figure 5a shows the current density–voltage–luminance (J–V–L) curves of LEDs based on CsPbI3 NCs with four different NH4SCN ratios. These four devices have a low turn on voltage (the applied voltage when the luminance is 1 cd m-2) of 2.2 V that is close to the bandgap energy of the perovskite NC emissive layers, illustrating a barrier-free and highly efficient charge injection into the emissive layers.36 The maximum luminance of pristine CsPbI3 NCs based LEDs was 200 cd m-2, while the NCs with 20% NH4SCN based device showed the maximum luminance of 823 cd m-2 with an enhancement of 3.1 times, which is attributed to the reduced electron injection barrier, thus results in higher charge injection efficiency. As shown in Figure 5b, the peak EQEs are 6.6%, 10.3%, 8.7%, 9.8% for the



LEDs based on pristine CsPbI3 NCs, adding 20% NH4SCN, 30% NH4SCN, and 40% NH4SCN NCs, respectively. Moreover, these three devices based on passivated CsPbI3 NCs displayed lower efficiency roll-off compared with the pristine CsPbI3 NCs based one, suggesting that NH4SCN in emissive layers reduces carrier loss due to nonradiative recombination and keeps low charging degree even at high current densities, thereby improves the device performance.34 Figure 5c shows normalized EL spectra of these four devices. All devices exhibited single EL peak from perovskite NCs without any other emissive peaks from charge transport materials, indicating that perovskite NC emissive layers served as the primary exciton recombination centers to realize balanced charge carrier transport when the device was operating.37 But the EL spectra have broadenings and red-shifts compared with the PL spectra and exhibit similar to each other, which is attributed to the dielectric function of surrounding medium and the energy transfer from smaller to larger NCs in the ensemble.38-40 As shown in Figure 5d, the symmetric EL emission corresponds to Commission Internationale de l’Eclairage (CIE) color coordinates of (0.72, 0.28) that are close to the spectral locus. The inset of Figure 4a is a photograph of the working device with bright red emission, which fully meets the requirements of display applications.

4. CONCLUSIONS We demonstrated that the introduction of NH4SCN in perovskite NC synthesis can passivate the surface defects of perovskite NCs and improve their optical properties. Surface defect passivation enables narrowed PL spectrum and improved PL QYs. In addition, the electronic properties are also changed with reduced electron injection barrier and enhanced charge injection efficiency. The device performance of resultant LEDs was correspondingly improved. The best device efficiency of 10.3% was obtained when the NH4SCN ratio is 20%.


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Thus, it is worth noting that surface passivation is an effective strategy to improve the optoelectronic properties of materials and corresponding device performance.

AUTHOR INFORMATION Corresponding Authors *E-mails: (L. Zhang) [email protected]; (W. W. Yu) [email protected]. Notes The authors declare no conflict of interest. ACKNOWLEDGEMENTS We acknowledge financial support from the Natural Science Foundation of China (51772123), and the Special Project of the Province-University Co-constructing Program of Jilin University (SXGJXX2017-3).

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FIGURES

Figure 1. (a) UV−vis absorption and PL spectra of CsPbI3 NCs with different NH4SCN ratios in toluene. (b) PL QY and FWHM values of perovskite NCs as a function of NH4SCN ratios. (c) PL decay curves of CsPbI3 NCs with different NH4SCN ratios. (d) XRD patterns of CsPbI3 NC films with different NH4SCN ratios on quartz substrates.


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Table 1. Time-resolved PL decay profiles for CsPbI3 NCs with different NH4SCN ratiosfitted with a bi-exponential function.

Sample

τ1 (ns)

τ2 (ns)

f1 (%)

f2 (%)

τavg (ns)

CsPbI3

20.71

87.02

57.03

42.97

49.20

20% NH4SCN

19.86

126.60

52.98

47.02

70.02

30% NH4SCN

18.11

108.90

54.13

45.87

59.76

40% NH4SCN

16.24

122.10

53.19

46.81

65.79



Figure 2. (a-d) TEM images of CsPbI3 NCs with different NH4SCN ratios (0, 20%, 30%, and 40%, respectively). Insets show corresponding particle size distributions calculated from TEM images.


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Figure 3. (a) XPS spectra of the S 2p for pristine CsPbI3 and 30% NH4SCN passivated CsPbI3 NC films. (b) FTIR transmission spectra of CsPbI3 NCs with different amounts of NH4SCN.



Figure 4. (a) Tauc plots and (b) UPS spectra of CsPbI3 NC films with different NH4SCN ratios. (c) Device structure of the perovskite NC based LEDs. (d) Device energy level diagram for all functional layers.


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Figure 5. (a) Current density and luminance versus driving voltage for CsPbI3 NC LEDs with different NH4SCN ratios. (b) External quantum efficiency versus current density of the four devices. (c) EL spectra of the four devices. (d) CIE coordinates for the EL spectrum; inset shows the photograph of a working device.



TOC Graphic


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Ammonium Thiocyanate-Passivated CsPbI3 Perovskite Nanocrystals

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