A zinc non-halide dopant strategy enables efficient perovskite CsPbI₃ quantum dot-based light-emitting diodes

Abstract

Metal ion doping has been considered as one of the most effective methods to achieve highly efficient perovskite light-emitting diodes (LEDs), which is significant for future high-definition displays and high-quality lighting. However, its exact function is still uncertain because the doping of metal ions is also accompanied by the introduction of halide ions and halide-rich circumstances, which can enhance the optoelectronic properties as well. In this work, a zinc non-halide dopant strategy was employed to specifically study the effect of Zn²⁺ on CsPbI₃ quantum dots (QDs). We confirmed that Zn²⁺ was doped into the perovskites interstitially via X-ray diffraction (XRD) and pair distribution function (PDF) analysis. Zn²⁺-doped CsPbI₃ QDs exhibited higher emission properties with 120% enhancement in the photoluminescence quantum yield (PLQY) compared with pristine CsPbI₃ QDs. The Zn²⁺-doped CsPbI₃ QD-based LEDs (QLEDs) showed nearly two-fold increase in the external quantum efficiency (EQE) versus the control device, and it improved from 7.5% to 14.6%. Besides, a maximum current efficiency of 0.83 cd A⁻¹ and the highest luminance of 378 cd m⁻² were achieved. These results certify that Zn²⁺ doping enables high-performance QLEDs without the introduction of halide ions, which is beneficial for the further research on doped perovskite fields and lays the foundation for the future practical applications of QLEDs.

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Introduction

Perovskite quantum dots (QDs) with superior properties, including high photoluminescence quantum yield (PLQY), narrow full width at half maximum (FWHM) and finely tunable emission spectra in the visible region, have attracted significant attention in next-generation high-definition displays and high-quality lighting.^1–3 On the basis of these excellent properties, a large number of studies have been carried out to improve the external quantum efficiency (EQE) of perovskite QD-based light-emitting diodes (QLEDs) to enable efficient light emission with less energy consumption.^4–6 During the past few years, there has been impressive progress in the EQE of perovskite LEDs. Up to now, the EQE values of perovskite QLEDs have been reported to be 16.48%,⁷ 21.3%⁸ and 2.12%⁹ for green, red, and blue QLEDs, respectively.

These high EQE values and other distinguished properties of QLEDs can be realized by different methods: doping,^10,11 surface ligand engineering,^12–14 defect passivation,^15,16 optimization of the device architecture,^17,18etc.^19–21 Among these methods, doping is commonly and frequently used to boost efficiencies. In traditional Cd-based QDs, transition metal-doped QDs have come up as a new class of light-emitting materials that can retain all the advantages of undoped QDs and also overcome some of the intrinsic disadvantages such as self-absorption and sensitivity to thermal, chemical, and photochemical disturbances compared to their undoped counterparts.^22,23 In 2009, Stouwdam et al.²⁴ reported efficient LEDs based on Cu-doped CdS QDs with an increased EQE of 5%. Later, Santra's group²⁵ employed Mn doping to boost the EQE of CdS-based QLEDs up to 5.4%.

In perovskite QDs, doping is also a frequently used method. With the structure of perovskites being ABX₃ (A is normally Cs⁺, formamidinium (FA) cation, methylamine (MA) cation and other organic molecules; B represents metal ions such as Pb²⁺; X represents halide ions like Cl⁻, Br⁻, and I⁻),^26,27 divalent metal ion doping has been considered as an effective method to enhance EQE because of the effect on reducing non-radiative recombination, lowering charge injection barriers, and enhancing carrier transport properties.^28–30 In 2018, Rogach's group reported that strontium doping and chlorine passivation enhanced the EQE of CsPbI₃ QDs to 13.5%.³¹ Recently, through the employment of Zn²⁺, their group further improved the EQE of CsPbI₃ QDs to 15.1%.³² The introduction of Zn²⁺ eliminates defects to some degree, which was validated by the improved radiative decay rates by 4.1-fold and increased PLQY values from 61.3% to 98.5%. Besides, many other papers have reported similar results that the doping of metal ions can improve the properties of perovskites.^33–35

However, a divalent dopant element is introduced in the form of MX₂ (M = metal; X = Cl, Br, I and other halides).^36,37 Thus, halide ions are also introduced during divalent doping. Recently, researchers have presented that under halide-rich growth conditions, better emission properties and higher quality LEDs can be achieved.^38,39 For example, Liu et al.⁴⁰ found that under halide-rich circumstances, CsPbBr₃ QDs with a Pb/Br ratio of 1 : 4 show higher PLQYs and improved maximal luminance of 12 090 cd m⁻² compared with that in regular circumstances. Kye et al. also demonstrated that I⁻-rich growth conditions could control the formation of the cation vacancies of CsPbI₃ QDs and enhance the quality of QDs.³⁹ In addition, Dong's group reported that a synthetic approach involving a high Br/Pb ratio could achieve narrower photoluminescence by a high level of size control and ensemble uniformity.³⁸ Therefore, how M²⁺ really affects perovskite QDs and enhances the luminescence properties without the existence of X⁻ become meaningful problems that need to be addressed, and this is of great significance for the future development of highly efficient QLEDs.

Besides, CsPbI₃ perovskite QDs with deep-red emission (650–700 nm) have great potential in the next-generation high-definition displays and solid-state lighting, particularly for the specific demands of high visibility and applications like horticultural lighting, because deep-red light has better visibility than red light and the chlorophyll would have the most active synthesis under deep red luminescence.⁴¹ In addition, deep red light is promising in the construction of warm white light-emitting diodes in solid-state lighting applications, and numerous works about iodide-based perovskites have been extensively studied.^42–46

In this paper, we developed a zinc non-halide dopant strategy to clarify the internal mechanism of Zn²⁺ doping, figure out its impact on the structure and properties of CsPbI₃ perovskite QDs, and construct high-performance QLEDs. Zn²⁺ ions were doped in the interstitial sites and caused QD lattice expansion, as validated through X-ray diffraction (XRD) and pair distribution function (PDF). Besides, the Zn²⁺-doped CsPbI₃ perovskite QDs exhibited an improved PLQY of 76%, increased photoluminescence (PL) lifetime and lower trap density, which suggested less defect sites and more effective exciton recombination in the doped QDs. Hence, an EQE up to 14.6% for perovskite QLEDs was achieved through the proposed zinc non-halide dopant strategy. In addition to the use of zinc acetylacetonate (ZnAcAc) as the dopant that improved the EQE of QLEDs, it was proved that other zinc non-halide salts, including zinc acetate (ZnAc) and zinc stearate (ZnSt), were feasible for improving the performance of QLEDs. This result demonstrates the universality of the zinc non-halide dopant strategy and its potential in future practical applications.

Results and discussion

Zinc-doped CsPbI₃ QDs were synthesized by the common hot injection method,⁴⁷ in which different Zn²⁺ salts were added to the PbI₂ precursor as the zinc source (Fig. 1a); the detailed synthetic conditions have been listed in the Experimental section. To rule out the effect of halide ions and demonstrate the universality of the zinc non-halide dopant strategy, Zn²⁺ salts with different negative ions including Ac⁻, AcAc⁻ and St⁻, the structures of which are shown in the right part of Fig. 1a, were doped into CsPbI₃ perovskite QDs.

Fig. 1 (a) Left: The synthetic diagram of CsPbI₃ QDs by hot injection method; right: the corresponding structure of employed different negative ions of Zn²⁺ salts. (b and c) Schematics of radiative and non-radiative recombination of QDs and defect sites before and after Zn²⁺ doping. (d) The PL spectra of pristine and Zn²⁺-doped CsPbI₃ QDs and their photograph under UV lamps. (e) The PLQY spectra of pristine and Zn²⁺-doped CsPbI₃ QDs.

Our proposed zinc non-halide dopant strategy could effectively decrease the defect sites and non-radiative recombination, resulting in higher emission properties (Fig. 1b and c). After the doping of all the Zn²⁺ salts mentioned above, the perovskite QDs displayed higher emission properties. The Zn²⁺-doped solution emitted stronger red light versus the undoped QD solution, as shown in the inset of Fig. 1d. Accordingly, as shown in the steady-state PL spectra achieved under the same ultra-violet light excitation (Fig. 1d), the PL intensity of Zn²⁺-doped CsPbI₃ QDs is enhanced, which indicates the increased electron–hole radiative recombination. After doping, the peak slightly shifts from 684 nm to 682 nm, which might be attributed to the increased band energy with the introduction of Zn²⁺.³² Besides, the corresponding doped QDs exhibited more outstanding luminescence properties with the PLQY increasing to as high as 76%, as validated by the absolute PLQY measurements (Fig. 1e), compared with the undoped one.

The effect of Zn²⁺ on the crystal structure of CsPbI₃ QDs was first investigated by XRD to reveal the doping mechanism of Zn²⁺ without the introduction of halide ions. As shown in Fig. 2a, the main diffraction peaks are located at 14.1°, 20.0° and 28.6°, corresponding to the (100), (110) and (200) planes of the cubic perovskite structure.⁴⁸ After doping Zn²⁺, no new diffraction peaks appear and the existing peaks are slightly enhanced, indicating that the crystalline phase does not change and the crystallinity increases. After doping, the diffraction peak shifts to a smaller angle for all the Zn²⁺ salts with different negative ions (Fig. 2b). The peak shift towards smaller angles signified lattice expansion, which might result from the Zn²⁺ ions residing in the interstitial sites. Because the ionic radius of Zn²⁺ (74 pm) is smaller than that of Pb²⁺ (119 pm), the doping of Zn²⁺ into the crystal lattice of CsPbI₃ QDs through interstitial doping would be easy.

Fig. 2 (a and b) XRD patterns of pristine CsPbI₃ QDs and Zn²⁺-doped QDs, and the local enlarged (200) peak. (c and d) TEM images of the (left) pristine and (right) Zn²⁺-doped CsPbI₃ QDs with their corresponding enlarged TEM images as insets. (e–h) Element mapping of Zn²⁺-doped CsPbI₃ QDs, corresponding to Zn, Cs, Pb, and I.

Transmission electron microscopy (TEM) showed that the average diameter increased from 12.80 ± 0.3 nm up to 15.50 ± 0.2 nm (Fig. 2c and d). The relevant TEM images and lognormal distribution histograms of pristine and doped CsPbI₃ QDs by using different Zn salts are provided in Fig. S1 (ESI†). In agreement with the lattice expansion observed form the XRD results, the interplanar distance was enlarged from 6.1 Å to 6.3 Å (insets of Fig. 2c, d and Fig. S2, ESI†). Although doping Zn²⁺ resulted in crystal structure expansion, the film roughness showed no obvious degradation, as presented by atomic force microscopy (AFM) (Fig. S3 and S4, ESI†).

Taking the zinc source introduced in the form of ZnAcAc as an example, energy dispersive X-ray spectroscopy (EDS) chemical mapping was performed to further confirm the incorporation of Zn²⁺. It is shown from the EDS results (Fig. 2e–h) that the Zn element is incorporated in CsPbI₃ QDs, which is in accordance with previous reports.³² Meanwhile, as shown in the X-ray photoelectron spectroscopy (XPS) results (Fig. S5, ESI†), QDs synthesized with Zn²⁺ exhibited a new peak located at 1044.6 eV, which could be assigned to the 2p signals of Zn²⁺ and further proved that the Zn²⁺ ions were incorporated.

Although the whole lattice showed an expansion trend, as validated by the XRD and TEM results, it was still uncertain whether the Zn²⁺ ions were doped as interstitial ions. The perovskite structure is flexible, i.e., it can tolerate different types of cations on the A- and B-sites as long as the charge neutrality is maintained.^49,50 The XRD method, mainly used for long range structure characterizations, cannot provide detailed local structure information, which is important for the optoelectronic properties of perovskites.⁵¹ Therefore, we employed PDF to explore the local structure information and confirm the evolution of the CsPbI₃ QD structure after doping. Fig. 3a exhibits the characterized instrument schematic of PDF. With the high-energy X-ray beam focused on the CsPbI₃ QD sample, two-dimensional diffraction signals can be received, collected and converted to three-dimensional PDF information (G(r), where r is the atomic pair distance) by Fourier transforming S(Q):

Here, ρ(r) is the atomic pair density function, ρ₀ is the number density of atoms in the material, Q is the magnitude of the scattering vector (or momentum transfer), and r is the interatomic (atom–atom) distance.

Fig. 3 (a) Schematic of high-energy diffraction instrument set-up. (b) The pair distribution function G(r) of pristine CsPbI₃ QDs and Zn²⁺-doped QDs, respectively. (c and d) The Gaussian fitting of partial peak (Cs–Oct bond) to synchrotron X-ray PDF T(r) before and after Zn²⁺ doping.

As mentioned in a related paper, the main peaks of the CsPbI₃ pair distribution are located at 3.2 Å, 4.6 Å and 6.4 Å, corresponding to the Pb–I bond, Cs–Oct (octahedron) bond/I–I bond and Oct–Oct bond/Pb–Pb bond.⁵² As shown in Fig. 3b, no peaks corresponding to the Zn–I bond (2.94 Å) appear after Zn²⁺ doping, which means that the Zn²⁺ ions have not taken the place of Pb²⁺ ions and formed Zn–I octahedra. However, there were apparent movements and changes in the Cs–Oct bond. In order to seperate the peaks of near bonds from the main PDF peak, we converted G(r) into T(r):

T(r) = 4πrρ(r) = 4πρ₀r + G(r)

Here, the atomic pair distributions follow Gaussian profiles (Fig. S6, ESI†) and can fit different peaks. We conducted Gaussian fitting towards the peak of the Cs–Oct bond before (Fig. 3c) and after doping (Fig. 3d). The results showed that a peak located at 4.35 Å appeared. Because of the small radius of Zn²⁺, we attributed the appeared peak and the left shift in the Cs–Oct peak to the doping of Zn²⁺ as interstitial ions between the octahedral structure. Based on the XRD and PDF analyses, a schematic of Zn²⁺ doped in interstitial sites and lattice expansion is shown in Fig. S7 (ESI†).

Generally, the crystal structure of a material is related to its bandgap. To further evaluate the optical properties after the structural changes resulting from Zn²⁺ doping, the absorption spectra of pristine and Zn²⁺-doped CsPbI₃ QDs were measured. The band tails of CsPbI₃ QDs with or without doping in the absorption data show changes in the Urbach energy (E_u). The E_u values of these materials are calculated from the formula mentioned below:

Here, α₀ is a constant.^53,54 According to the formula, E_u can be obtained as the reciprocal of the slope near the band edge, as shown in Fig. 4a and Fig. S8 (ESI†). Because the Urbach energy represents the tail of the localized states in the bandgap,⁵⁵E_u is reduced from 34.3 eV to 23.9 eV, suggesting that the density of the localized states near the bandgap decreases after Zn²⁺ doping.

Fig. 4 (a) Relationship of ln(α) versus energy for the calculation of Urbach energy of pristine CsPbI₃ and Zn²⁺-doped CsPbI₃ QDs. (b) PL decay curves of pristine CsPbI₃ and Zn²⁺-doped CsPbI₃ QDs. (c) Device energy-level diagrams of hole-only devices. (d) Current–voltage characteristics of hole-only device.

Fig. 4b shows the PL decay curves obtained from pristine and doped QDs. The average lifetime of the pristine film was about 7.22 ns, which increased to 11.76 ns for Zn²⁺-doped QDs; the detailed information is shown in Table S1 (ESI†). Longer PL decay times manifest more effective radiative recombination pathways, which can suggest fewer defect centers, and this is favorable to reduce the trapping of excitons and increase the radiative recombination.

Attempting to further verify the difference in trap sites and densities, space-charge-limited current (SCLC) measurements were operated. A “hole-only” device with a structure of indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)/poly(bis(4-phenyl)(2,4,6-trimethylphenyl)amine) (PTAA)/QDs/poly[N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)-benzi(poly TPD)/Au] was fabricated (Fig. 4c). Under low applied voltages, the I–V relationship of the current density with the bias voltage was found to be linear and ohmic, indicating an ideal interface capable of injecting enough charges from the electrodes into the semiconducting materials to sustain the SCLC transport with negligible trap states. On increasing the bias voltage, the trap states were observed to be gradually filled by holes until the device reached the trap-filled limit (TFL) stage.^56,57

Here, n_t is the trap-state density, L is the layer thickness, ε is the relative dielectric constant and ε₀ the vacuum permittivity. The V_TFL values can be used to reflect the transition from the ohmic stage to the TFL stage, and the density of trap states (n_t) is proportional to V_TFL. From the SCLC data (Fig. 4d and Fig. S9, ESI†), the V_TFL values with and without Zn²⁺ doping were 0.62 V and 1.21 V, respectively. Hence, the obtained n_t calculated from the V_TFL value for the Zn²⁺-doped CsPbI₃ QD film was estimated to be 2.3 × 10¹² cm⁻³, which was lower than that for the pure CsPbI₃ QD film (4.6 × 10¹² cm⁻³). The SCLC measurements demonstrated lower trap density and less defect sites of Zn²⁺-doped CsPbI₃, which were in accordance with the results of PLQY and PL decay; this is extremely desired for constructing high-performance LEDs.

The robustness of the higher emission properties and lower trap density of the QD films is expected to provide high exciton recombination efficiency in devices. The device performances of QLEDs are presented in Fig. 5. The device (Fig. 5a) is made up of multiple layers in the following order: ITO, PEDOT:PSS, PTAA, perovskite QDs, 1,3,5-tris(1-phenyl-1H-benzimidazol-2-yl)benzene (TPBi, 40 nm), and LiF/Al (1/100 nm). The corresponding device energy-level diagrams are shown in Fig. S10 (ESI†).

Fig. 5 (a) Schematic illustration of multilayer perovskite QLED device. (b) Normalized EL spectra at an applied voltage of 5 V before and after Zn²⁺ doping. (c) The luminance–current density characteristics of QLEDs; inset shows photographs of the two devices. (d) Current efficiency of the devices as a function of luminance. (e) EQE of the devices as a function of current density. (f) EQE values for doping with different Zn²⁺ salts.

The typical electroluminescence (EL) spectrum of the device containing CsPbI₃ and Zn²⁺-doped CsPbI₃ QDs consists of peaks located at 689 nm, and 687 nm, showing a similar shift to that of the PL peak (Fig. 5b). The Commission Internationale de l’Eclairage (CIE) chromaticity coordinates of CsPbI₃ and Zn²⁺-doped CsPbI₃ QD films are (0.72, 0.27) and (0.72, 0.28), showing nearly no difference (Fig. S11, ESI†).

The luminance–current density curve of the devices (Fig. 5c) was measured and it was found that the devices with Zn²⁺ ions showed higher luminance under the same current density. The luminance after Zn²⁺ doping improved from 221 cd m⁻² to 378 cd m⁻², which validated the enhanced exciton recombination in Zn²⁺-doped CsPbI₃ QDs. Also, luminance and current density are the key factors of current efficiency, which is directly proportional to luminance and inversely proportional to current density. Therefore, the higher luminance under the same current density of Zn²⁺-doped CsPbI₃ QDs resulted in higher current efficiency than that for pristine CsPbI₃ QDs (Fig. 5d), which increased from 0.75 cd A⁻¹ to 0.83 cd A⁻¹.

Based on the enhanced exciton recombination, the devices containing Zn²⁺-doped CsPbI₃ QDs demonstrated higher EQE; the peak EQE reached as high as 14.6%, as shown in Fig. 5e, which was higher than that of the pristine one (7.5%) and achieved nearly 100% increase. Similar results were also obtained for other Zn salts (Fig. 5f and Fig. S12, S13, ESI†). The increase in EQE indicates that Zn²⁺ doping can effectively enhance the radiative recombination, which is in accordance with the characterization results mentioned above. The achieved 14.6% EQE is in the forefront for reported CsPbI₃ QLEDs. The EQE progress of CsPbI₃ QLEDs and detailed information are provided in Fig. S14 and Table S2 (ESI†).

Conclusions

In conclusion, we investigated the effects of Zn²⁺ on CsPbI₃ perovskite QDs through different zinc non-halide dopants. It was observed that Zn²⁺ was doped into CsPbI₃ QDs in an interstitial position, as elucidated by the PDF and XRD methods. Resulting from higher PLQY, lower trap density, and more effective exciton recombination after doping, high-performance perovskite QLEDs were achieved. The EQE of the device increased from 7.5% to 14.6%, which was nearly 2-fold increase versus that for the device based on undoped CsPbI₃ QDs. These findings would help us achieve a better understanding of the effect of doping metal ions, especially divalent ions, on the structure and properties of perovskite materials and devices, which is of great value for the further development of perovskite optoelectronic devices.

Experimental section

Materials

PbI₂ (from Aladdin, 99.99%), cesium stearate (CsSt, 98% pure, from J&K), ZnAc (from Macklin, 99.99%), ZnAcAc (from Macklin, 98%), ZnSt (from Macklin, 98%), oleic acid (OA, from Aladdin), oleylamine (OAm, 80–90% pure, from Aladdin), 1-octadecene (ODE, >80% pure, from Aladdin), and n-octane (>98% pure, from TCI) were used. Toluene, isopropanol and ethyl acetate were of analytical grade and were used as received without further purification. PTAA and TPBi were purchased from Xi’an Polymer Light Technology Corp.

Synthetic procedures

First, 0.15 M cesium precursor was prepared by loading 2.5 g CsSt and 2 mL OA into 40 mL ODE, and the solution was completely dissolved after heating to 140 °C for 0.5 h. Ten mL of ODE, 1.5 mL of OAm, 2 mL of OA, PbI₂ (0.25 g) and ZnAc (0.045 g)/ZnAcAc (0.07 g)/ZnSt (0.17 g) were loaded into a 100 mL four-neck flask, degassed at 110 °C for 10 min, mixed at 110 °C for 20 min and heated to 180 °C in 10 min under Ar flow. Then, 0.75 mL of CsSt solution (0.15 M in ODE) was quickly injected. After 15 s, the reaction mixture was cooled by the ice-water bath. Then, isopropanol was added into the crude solution with a volume ratio of 2 : 1; the precipitate was collected separately after centrifugation and dispersed in toluene. Ethyl acetate was added into the toluene dispersion; the precipitate was collected and redispersed in n-octane.

Device fabrication

PEDOT:PSS solutions (Baytron P VPAl 4083, filtered through a 0.22 μm filter) were spin-coated onto the ITO-coated glass substrates at 4000 rpm for 60 s and baked at 140 °C for 15 min. The PTAA (in chlorobenzene 5 mg mL⁻¹) and CsPbI₃ QDs (in n-octane 20 mg mL⁻¹) were deposited layer-by-layer by spin coating at 2000 rpm for 60 s. The PTAA and QD layers were baked at 120 °C for 15 min and 60 °C for 10 min, respectively, before the deposition of the next layer. TPBi (40 nm) and LiF/Al electrodes (1 nm/100 nm) were deposited using a thermal evaporation system through a shadow mask under a high vacuum of ∼2 × 10⁻⁴ Pa. The device active area was 4 mm², as defined by the overlapping area of the ITO and Al electrodes.

Characterization and device measurements

One drop of the diluted as-synthesized CsPbI₃ QD dispersion was placed onto a carbon-coated Cu grid, and octane was allowed to evaporate at room temperature. TEM images were taken on a Tecnai G2 F30 S-TWIN TEM instrument operated at an acceleration voltage of 200 kV and equipped with an EDX/STEM HAADF detector. XRD samples were prepared by dropping the purified QD inks onto glass substrates. XRD patterns were acquired using a Bruker D8 Advance X-ray diffractometer operating with Cu Kα radiation (λ = 1.5406 Å). PDF data were achieved by high energy XRD experiments conducted at the beamline 11-ID-C, Advanced Photon Source, Argonne National Laboratory. The wavelength of the X-ray was 0.1173 Å. The diffraction spectra were acquired in transmission geometry by a 2D detector. To evaluate film roughness, QD inks were spin-coated onto Si substrates and then analysed by a Veeco D3100 AFM tool. The absorbance of the QD octane solution was measured by a Shimadzu UV-3600 UV/VIS/NIR spectrophotometer. The PL spectra of the QDs were obtained by using a Varian Cary Eclipse spectrometer. XPS spectra were recorded on a Thermo ESCALAB-250 spectrometer using a monochromatic Al KR radiation source (1486.6 eV). XPS samples were prepared by dropping the purified QD inks onto glasses. The absolute PLQY was measured using a Horiba Fluorolog system equipped with a single grating and a Quanta-Phil integration sphere coupled to the Fluorolog system. The PL decay was measured on a time-correlated single-photo counting (TCSPC) spectrofluorometer (FLS920, Edinburg Instrument, U.K.) at room temperature (EPL-455 nm). The EL spectra, L–J–V characteristics and EQE were collected by using a Keithley 2400 source, a fibre integration sphere, and a PMA-12 spectrometer for light output measurements in a glovebox filled with N₂ at room temperature (the measurement equipment was designed by Hamamatsu Photonics Co., Ltd).

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by NSFC (51922049, 61604074), the National Key Research and Development Program of China (2016YFB0401701), the Natural Science Foundation of Jiangsu Province (BK20180020), the Fundamental Research Funds for the Central Universities (30917011202), and PAPD of Jiangsu Higher Education Institutions. This research used the resources of the Advanced Photon Source, a US Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c9qm00734b

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