劉 念， 羅家俊*， 杜培培， 劉征征， 杜 鵑， 唐 江*

（1.華中科技大學(xué) 武漢光電國家研究中心， 湖北 武漢 430074； 2.華中科技大學(xué) 集成電路學(xué)院， 湖北 武漢 430074；3.國科大杭州高等研究院 物理與光電工程學(xué)院， 浙江 杭州 310024；4.中國科學(xué)院 上海光學(xué)精密機械研究所， 強場激光物理國家重點實驗室， 上海 201800）

1 Introduction

Metal halide perovskites have attracted significant interest due to their excellent color purity, tunable emission wavelength covering the visible range,and low material cost[1-6].In recent years, there have been remarkable advancements in the device performance and operational stability of perovskite lightemitting diodes（PeLEDs）, driven by continuous innovation and breakthroughs in material properties and device structures[7-15].However, the widely used solution spin-coating method in laboratories for perovskite deposition has limitations in terms of fabrication area and process repeatability[16-20].Vacuum thermal evaporation, a technology extensively employed in OLED panel production lines, offers a promising solution for large-scale commercial production of PeLEDs, as it enables scale-up deposition, high repeatability, and precise deposition[17,21-24].

The research on vapor-deposited PeLEDs started relatively late, focusing mainly on efficiency enhancement strategies such as optimizing material composition, and process parameters, and introducing interfacial layers[23,25-30].These strategies aimed to improve the photoluminescence quantum yield（PLQY） of the perovskite film and enhance the electron-hole injection balance in the LED.In the optimization of vapor-deposited perovskite layers, commonly used techniques include introducing quasi-2D structures and encapsulating with Cs4PbBr6to create dielectric confinement.While these methods increased the exciton binding energy and PLQY by enhancing carrier recombination probability, the improvement is limited and may affect the carrier transport within the perovskite layer.Compared to enhancing radiative recombination in the perovskite,suppressing defect-assisted non-radiative recombination in the perovskite can better improve the PLQY while maintaining efficient carrier transport[31-32].The published studies have shown the presence of halide vacancies in the lattice and surface defects in pristine perovskite films[5-6,33].In solution-based methods,extensive research has been conducted to passivate these defects using Lewis-base additive molecules,such as organic ligands and conductive polymers, to interact with under-coordinated Pb2+ions[7,34-37].

In this study, bis（p-chlorophenyl） phenylphosphine oxide（Cl-TPPO） was first introducedin situduring the thermal evaporation deposition process of perovskite to achieve defect passivation and enhanced radiative recombination.The phosphine oxide with phosphine oxide group effectively complexes with undercoordinated Pb2+, suppressing defectassisted non-radiative recombination and controlling the high-energy and disordered crystallization process during thermal evaporation.By optimizing the ratio of raw materials during the evaporation process,the growth kinetics of the perovskite film were also optimized, leading to an improvement in radiative recombination efficiency.Based on the emission-enhanced perovskite layer, the LED device structure was carefully selected and optimized, resulting in thermal-evaporated green PeLEDs with a maximum EQE of 6.3% and 312 pixels per inch（ppi） highresolution perovskite LEDs with an EQE of 5.0%.

2 Experiment

2.1 Materials

Indium tin oxide（ITO） glasses were purchased from Liaoning Youxuan New Energy Technology Co., Ltd.Cesium bromide（CsBr, 99.99%） and lead bromide（PbB2, 99.99%） were purchased from Aladdin Reagent Ltd.Bis（p-chlorophenyl） phenylphosphine oxide（Cl-TPPO, 96%） was purchased from Alfa chemistry.4,4′-Bis[N-（1-naphthyl）-Nphenylamino]biphenyl （NPB, >99.5%） and 1,3,5-tris（1-phenyl-1H-benzimidazol-2-yl）benzene （TPBi,>99.5%） were purchased from Taiwan Luminescence Technology Co., Ltd.Molybdenum trioxide（MoO3,>99.9%） and lithium fluoride （LiF, >99.99%）were purchased from Sigma-Aldrich（Shanghai） Trading Co., Ltd.Aluminum pellets（Al, 99.999%）were purchased from Zhongnuo Advanced Material（Beijing） Technology Co., Ltd.

2.2 Deposition of Perovskite Film

The deposition of perovskite was carried out by simultaneously evaporating multiple sources.The evaporation rates of the sources were monitored by quartz microbalance and calibrated with a stylus profiler.After optimization, the deposition ratios and rates of CsBr, PbBr2, and Cl-TPPO were determined to be 0.01, 0.01, 0.006 nm/s, respectively.During the deposition process of perovskite, the substrate rotated at a rate of 8 r/min to ensure the uniformity of the perovskite film.The vacuum chamber maintained a vacuum degree below 10-4Pa to ensure the reproducibility of the deposition process.

2.3 Device Fabrication

The etched ITO glasses were subjected to sequential immersion and ultrasonic cleaning with surfactant, acetone, isopropanol, and ethanol, they were dried by N2blowing and transferred to thermal evaporation equipment for the deposition of functional layers.The functional layers NPB and MoO3were deposited at evaporation rates of 0.2 nm/s and 0.04 nm/s, followed by the deposition of the perovskite layer, as mentioned above.TPBi, LiF, and Al were deposited at evaporation rates of 0.1 nm/s, 0.02 nm/s, and 0.3 nm/s, respectively, onto the rotating substrate to achieve the desired thickness.

2.4 Characterizations

The Shimadzu Solidspec-3700 spectrophotometer was used to measure the absorption spectra,while the Hitachi F-7000 fluorescence spectrometer was utilized for recording the PL spectra.The Zolix OmniFluo spectrofluorometer with a calibrated integrating sphere from Labsphere was employed to determine the absolute PLQY.The PL lifetime was measured using an Edinburgh Instruments Ltd EPL-370.X-ray photoelectron spectroscopy（XPS） was conducted using a Shimadzu-Kratos AXIS-ULTRA DLD-600W photoelectron spectrometer from Japan.Transient absorption（TA） spectroscopy experiments were performed using a commercial Helios-EOS Ultrafast system.The tests of PL, PLQY, TRPL, and TA were all performed under excitation light at 365 nm.The characterization of the PeLEDs was carried out in a N2glove box.A commercial measurement system （XPQY-EQE, Guangzhou Xi Pu Optoelectronics Technology） equipped with an integrating sphere was used to simultaneously record the current densityversusvoltage, luminanceversusvoltage, and EQEversuscurrent density curves.This measurement system was calibrated by the National Institute of Standards and Technology（NIST） using halogen lamps.

3 Results and Discussion

The optical properties of the perovskite films before and after the addition of Cl-TPPO are compared and shown in Fig.1.The test results indicate that the addition of Cl-TPPO does not affect the absorption edge of the perovskite, and the composition of perovskite remained unchanged.The photofluorescence（PL） intensity of the films is significantly enhanced, with the emission peak shifting from 520.7 nm to 518.2 nm, and the full width at half maximum（FWHM） decreasing from 20 nm to 18 nm.Overall, the optical properties of the perovskite thin film have been improved.

Fig.1 （a）Schematic diagram of co-evaporation deposition.（b）UV-Vis absorption spectra.（c）PL spectra of CsPbBr3 films without and with Cl-TPPO

The time-resolved PL（TRPL） and PLQY tests performed on perovskite films before and after Cl-TPPO addition are shown in Fig.2.The addition of Cl-TPPO would increase the PLQY of perovskite films from 21% to 57%, and the improved PLQY would contribute to the EQE improvement of PeLEDs devices.In the transient PL spectroscopy test, the film of CsPbBr3+Cl-TPPO showed a longer PL lifetime, and the PL lifetime was calculated by double exponential fitting and average lifetime according to the following formula[31]：

Fig.2 TRPL（a） and PLQY（b） test results of perovskite films before and after Cl-TPPO addition

the life-fitting results are shown in Tab.1.The addition of Cl-TPPO increased the average lifetime of perovskite films from 6.39 ns to 7.79 ns.

Tab.1 Double exponential fitting results of PL attenuation curves of perovskite film without and with Cl-TPPO addition

By substituting the average lifetime and PLQY（ηPLQY） into the equations below, the proportion and rate of radiation and non-radiation recombination in perovskite films can be fitted[38]：

After adding Cl-TPPO, the radiation recombination rate constant of perovskites increased from 3.29×107s-1to 7.32×107s-1, while the non-radiative recombination rate constant decreased from 1.24×108s-1to 5.52×107s-1.It was found that non-radiative recombination was strongly correlated with defects in perovskite.These results also indicated that the introduction of Cl-TPPO inhibited the formation of defects in perovskite.

To investigate the interaction between Cl-TPPO and perovskite, XPS characterization of CsPbBr3films was carried out to explore whether Cl-TPPO affected CsPbBr3.The test results are shown in Fig.3.After the addition of Cl-TPPO, the binding energies of Pb 4f orbitals and Br 3d orbitals in CsPb-Br3films both shift towards lower binding energies,indicating that the addition of Cl-TPPO changed the electron cloud density around Pb2+and Br-.The decrease in binding energy corresponded to the gain of surrounding electrons.The electrons gained by Pb2+came from the P=O group in Cl-TPPO, where the lone pair of electrons of the oxygen atom occupied the Pb 6p orbital[35,39].This interaction reduced the defect charge density of undercoordinated Pb2+.The formation of the Pb—O—P bond between P=O and undercoordinated Pb2+affected the binding energy of the Br 3d orbital, and the electronegativity of O is stronger than that of Br, resulting in a decrease in the binding energy of the Br 3d orbital[8].

Fig.3 XPS spectroscopy of Pb 4f（a） and Br 3d（b） for different perovskite films

The transient absorption（TA） spectroscopy was used to analyze the carrier dynamics in the perovskite films before and after the addition of Cl-TPPO.The test results are shown in Fig.4.Both perovskite films, with and without Cl-TPPO, only exhibited one ground state bleaching（GSB） signal peak, consistent with the previous absorption spectroscopy results, indicating that the addition of Cl-TPPO did not change or introduce additional components.By analyzing the time evolution curve of the GSB peak in the perovskite film, it can be observed that the photogenerated carriers in the perovskite relaxed to the lowest energy state over time, resulting in a red shift of the GSB signal peak.The introduction of Cl-TPPO reduced this redshift from the initial 2.7 meV to 0.9 meV in the CsPbBr3film, indicating that the CsPbBr3+Cl-TPPO film possesses a narrower energy landscape and reduced density of trap states[23,40].This result is consistent with the previous finding that the addition of Cl-TPPO narrowed the FWHM of the perovskite film,confirming the working mechanism of Cl-TPPO in improving the performance of perovskite.The narrower energy landscape caused by the introduction of Cl-TPPO is beneficial for charge transport and radiative recombination in subsequent LED devices.

Fig.4 Pseudo color representations of TA spectra of CsPbBr3（a） and CsPbBr3+Cl-TPPO（b） films.TA spectra of selected pumpprobe delays of CsPbBr3（c） and CsPbBr3+Cl-TPPO（d） films

After verifying the optimization effect of Cl-TPPO, the doping ratio of Cl-TPPO was further optimized.According to the EDS test results, we determined the doping ratio of Cl-TPPO and found that with the increase of the Cl-TPPO doping ratio, the grain of the perovskite film gradually became smaller and the flatness of the film improved.As the Cl-TPPO doping ratio increases, the PL emission peak of the perovskite films remains relatively stable, in contrast to the peak shift observed with the mixing of Br and Cl halogens[41].Additionally, the angle of the diffraction peaks in XRD showed minimal variation,suggesting that Cl did not significantly impact the lattice structure[42].The main effect of Cl-TPPO is to passivate perovskite grain boundaries and serve as a ligand to decrease the defect density of the perovskites.By using Cl-TPPO passivated CsPbBr3as the light emitting layer, we constructed relevant LED devices to explore the improvement in device performance after defect passivation.As shown in Fig.5（a）, the choice of the hole transport layer（HTL） NPB was based on the conduction band energy level of the perovskite layer, with the HOMO level of NPB positioned at 5.5 eV.Additionally, a hole injection layer of MoO3was introduced to reduce the device's turn-on barrier and enhance hole injection.The structure of the LED device was ITO/MoO3（2 nm）/NPB（30 nm）/CsPbBr3+Cl-TPPO（40 nm）/TPBi（40 nm）/LiF（1 nm）/Al（80 nm）.The corresponding device performance curves are shown in Fig.5（b）and 5（c）.Benefiting from the optimization of surface defects in the perovskite layer by Cl-TPPO, the leakage current of the device before turn-on decreased by an order of magnitude.As the current density increased after turn-on, the device brightness significantly improved, consistent with the previously verified suppression of non-radiative recombination.Ultimately, the external quantum efficiency of the device increased from an initial value of 0.23% to 6.3%, with a maximum brightness of 35 642 cd/m2, making it one of the brightest devices among reported organo-inorganic hybrid perovskite LED devices prepared by thermal evaporation[24,28,30,43].

Fig.5 （a）Energy level diagram of each layer in PeLEDs.（b）Current density and luminance versus voltage characteristics of the PeLEDs.（c）EQE-current density curves of these two devices

To expand the application scenarios of vacuum evaporation PeLEDs, we constructed pixelated PeLEDs based on the previously optimized 6.3% device structure.The specific preparation process is shown in Fig.6.First, the photoresist pixel pits wereprepared on the ITO substrate, and then the subsequent functional layer and perovskite layer were all deposited by vacuum evaporation.At the bottom of the pixel pit, there is ITO and the transport layer contact, and the rest of the place is blocked by the insulating photoresist, so under the voltage drive, only the perovskite layer in the pixel pit is electroluminescent（EL） to achieve pixelated PeLEDs lighting.

Fig.6 Flow chart of pixelated PeLEDs prepared by vacuum evaporation

In the performance curve of the obtained pixelated PeLEDs shown in Fig.7, the maximum brightness of the device is about 2 100 cd/m2, and the maximum EQE of the device is about 5.0%.The overall current density of the device is lower than that of the PeLEDs prepared on the whole surface, which is presumably related to the possible photoresist residue at the bottom of the pixel pit, resulting in carrier injection and device brightness being affected.Fig.7（d）shows the EL spectra of pixelated PeLEDs and a device lighting photo at 4 V, in which a single pixel pit of 60 μm × 20 μm horseshoe-shaped pixels yielded a pixelated PeLEDs resolution of 312 ppi.In addition, we collected EL intensities of 400 pixels, and their spatial uniformity is shown in Fig.S3, and the pixelated PeLED exhibits negligible intensity fluctuations, indicating the uniformity of perovskite preparation by vapor deposition of perovskites and its superiority in large-area production.

Fig.7 （a）The diagram of pixelated PeLEDs structure.（b）Current density and luminance versus voltage characteristics.（c）EQEcurrent density curves of pixelated PeLEDs.（d）EL spectra at different driving voltages of pixelated PeLEDs， the inset is the photo of pixelated PeLEDs driven at 4 V

4 Conclusion

In this work, we have reported phosphine oxide Cl-TPPO as a passivating ligandin situinto thermally evaporated perovskite films for efficient thermal evaporated PeLEDs.The P=O functional groups in Cl-TPPO interact with the undercoordinated Pb2+ions, reducing trap density and enhancing radiative recombination in the perovskite film, resulting in PLQY of 57%.By incorporating Cl-TPPO, the EQE of thermally evaporated PeLEDs improved from 0.23% to 6.3%, with a maximum brightness of 35 642 cd/m2.Moreover, using the fully thermal evaporated device process, we achieved pixilated PeLEDs with an EQE of 5.0%at 312 ppi.This work explores the pixelated emission of thermally evaporated PeLEDs, highlighting the high applicability of thermal evaporation technology in the commercialization of PeLEDs for displays.

Supplementary Information and Response Letter are available for this paper at: http://cjl.lightpublishing.cn/thesisDetails#10.37188/CJL.20230231.