Performance Enhancement of All-Inorganic Perovskite Quantum Dots

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C: Physical Processes in Nanomaterials and Nanostructures

Performance Enhancement of All-Inorganic Perovskite Quantum Dots (CsPbX) by UV-NIR Laser Irradiation 3

Ying Zhang, Haiou Zhu, Jilin Zheng, Guangyue Chai, Zongpeng Song, Yanping Chen, Yuanhai Liu, Siyu He, Yongqiang Shi, Yumin Tang, Meng Wang, Wenwen Liu, Lingfeng Jiang, and Shuangchen Ruan J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b11353 • Publication Date (Web): 04 Feb 2019 Downloaded from http://pubs.acs.org on February 4, 2019

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The Journal of Physical Chemistry

Performance enhancement of all-inorganic perovskite quantum dots (CsPbX3) by UV-NIR laser irradiation Ying Zhang†, Haiou Zhu‡, Jilin Zheng†,§, Guangyue Chai‡, Zongpeng Song†, Yanping Chen‡,Yuanhai Liu†，Siyu He†,Yongqiang Shi∥, Yumin Tang∥, Meng Wang†, Wenwen Liu⊥, Lingfeng Jiang† and Shuangchen Ruan*† †Guangdong Provincial Key Laboratory of Mico/Nano Optomechatronics Engineering, College of Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, China ‡College of New Materials and New Energies, Shenzhen University of Technology, Shenzhen, 518118, China §College of Communications Engineering, Army Engineering University of PLA, Nanjing 210007, China. ∥ Department of Materials Science and Engineering and the Shenzhen key laboratory for printed organic electronics, Southern University of Science and technology, Shenzhen, 518055, China ⊥ College of Mechanical and Electronical Engineering, Wenzhou University, Wenzhou, 325035, China

*Corresponding Author: Prof. Shuangchen Ruan College of Optoelectronic Engineering Shenzhen University, Shenzhen 518060, China. Email Id: [email protected] Telephone: +86-755-26536328; Fax: +86-755-26536338

Abstract: Perovskite quantum dots (QDs) have gained significant attention for both fundamental research and commercial applications, due to their outstanding optoelectronic properties and tremendous application potentials. Reducing defect density in all-inorganic perovskite QDs is of great importance for performance enhancement. In this work, it is found that the UV-NIR femtosecond laser pulse treatments could increase the photoluminescence quantum yield (PLQY) of the CsPbBr3 perovskites QDs from 71 % to 95 %. Such enhancement could be attributed to decrease of the defects after laser exposure, which is consistent with its red shift PL and other characterization results. Furthermore, the dynamics of energy transfer from exciton to trapping in irradiated CsPbX3 QDs are studied to gain an insight into the relative strength of exciton−trapping exchange coupling. The results confirm that the trapping defects are decreased

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by laser irradiation and the mechanism leading to enhancement of PLQY is revealed. In addition, the enhanced stability of laser-irradiated CsPbX3 QDs is also testified. Finally, the application of laser-treated perovskites QDs in liquid crystal display (LCD) backlight film is successfully demonstrated. It is further anticipated that this mechanism is helpful for design of perovskite-based optoelectronic applications such as LCD technology and light-emitting devices (LED). Keywords: trapping defects; femtosecond transient absorption spectroscopy; time-resolved photoluminescence decay

Introduction In recent years, the perovskite semiconductor material has drawn great attention, due to its outstanding photoelectric properties.1-10 Perovskite crystals have ABX3 (X = Cl, Br, I) structure, usually belonging to the orthogonal, tetragonal or cubic system. Among the family of perovskite materials, organic-inorganic hybrid halide MAPbX3 (MA = CH3NH3; X = Cl, Br, I) can cover a wide spectrum of solar energy due to its wide absorption spectrum and has a high mobility of electrons and holes in various perovskites materials,11-13 so it can be used in solar cells with energy conversion efficiencies surpassing 20%.14,15 The organic-inorganic halides CH3NH3PbX3 (X = Cl, Br, I) perovskites can also be used as a luminescent material, but its application is limited due to the defect-rich structure and consequently, low quantum efficiency.16-18 Several efforts have been made to improve the photoluminescence quantum yield (PLQY) of organicinorganic halides CH3NH3PbX3,19-21 however, the inherent instability severely restricts their practical applications. Compared to organic/inorganic hybrid perovskites, full inorganic perovskite quantum dots (QDs), such as CsPbX3 (X = Cl, Br, I), exhibit higher stability and provide superior photoelectric performance.22-25 As is known, compared with bulk materials, QDs have quantum confinement effects and large exciton binding energies, so they usually have higher luminescence efficiencies.23 In addition, the surface of the QDs can be easily controlled so that it can be applied in various devices. L. Protesescu et al. found that all-inorganic CsPbX3 (X = Cl, Br, I) perovskite QDs can be obtained15. They can also be synthesized more conveniently and have the same excellent luminescence properties as CH3NH3PbX3 (X = Cl, Br, I) perovskite quantum dots. Recently, Yue Wang et al. have demonstrated the feasibility of single-photon and two-photon absorption in all-inorganic perovskite QDs and suggested the perovskite crystals as potential nonlinear absorbers and emitters.22,26 All-inorganic perovskite QDs materials exhibit many promising properties, such as photoluminescence, stimulated emission (SE), fluorescence and so on. However, as many literatures summarized, a certain density of trapping defects still exist in the all-inorganic perovskite QDs materials, which significantly influence their optical and optoelectronic properties and consequently, the device performance.27,28 Trap states defects of the perovskite films enhance nonradiative recombination, severely degrading the charge carrier lifetime and the PL, so to acquire high-performance perovskite devices, their trap defects should be removed as more as possible. To reduce defects and improve PLQY, a series of methods have been introduced recently, e.g. using poly (methyl methacrylate) (PMMA) as a template, a solution-based hot-casting technique, atomic substitution doping technique and so on.29-34 Those approaches have successfully reduced defects and improved PL to a certain degree, however, the methodologies are usually a bit sophisticated and other impurities are inevitably introduced. Therefore, challenges still remain on how to reduce defects and to improve the performance of all-inorganic perovskite QDs in order to fulfill some emerging applications, for example, further enhancement in photovoltaic performance and photoluminescence efficiency.29-34 In this study, a new method to eliminate the trapping defects and improve the PL is developed for the perovskites QDs. It is found that the performances of all-inorganic perovskite QDs (CsPbX3, X=Cl, Br and mixed halide system Cl/Br), including PLQY, crystallization quality and stability, have been apparently improved after being irradiated with UV-NIR femtosecond laser pulses at 400 nm and 800 nm under ambient conditions. The improvement of the PLQY and


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stability could be explained by the trapping defects elimination and crystallization quality perfecting, which is consistent with its red shift PL and other characterization results. An enhancement of 20% in the PLQY was achieved after UVNIR laser irradiation compared to that of the non-irradiated QDs. Furthermore, by studying the dynamics of energy transfer from exciton to trapping in irradiated CsPbX3 QDs, a mechanism that leads to enhancement of PLQY is proposed. It is evidenced that the decreasing of trapping defects arises from the exciton−trapping exchange coupling.35, 36

Finally, the superiority of laser-irradiated perovskite QDs applied as the green QD liquid crystal display backlight

films is experimentally verified. Since laser irradiation is helpful for the formation of the less surface defects and closepacked thin films of the CsPbX3 QDs, our findings provide a new effective option for boosting the practical application potentials of all-inorganic perovskite QDs in the field of optoelectronic devices.

Experimental Preparation of CsPbX3 QDs and the sample of CsPbX3 QDs films In this study, the perovskite QDs were synthesized by a chemical solution method and synthesized in an oxygen and water-free environment. Most chemicals used in the experiments including Cs2CO3(99.99%), octadecene (ODE, >90%), oleic acid (OA), oleylamine(OLA, 80-90%), PbCl2 (99.999%) and PbBr2 (99.999%) were purchased from Aladdin. Toluene (>99.5%) was purchased from Lingfeng Chemical Reagent Company (Shanghai, China). First, 0.0814 g cesium carbonate, 4 ml of octadecane and 0.25 ml of oleic acid were added into a 25 ml three-necked flask. Then they were heated under nitrogen to 120 °C for 1 hour until all the cesium carbonate reacted completely with oleic acid to form Csoleate precursor. Then, 5ml octadecene, PbX2 (0.188 mmol) such as 0.054 g PbCl2, 0.069g PbBr2, 0.5 ml of oleylamine and 0.5 ml of oleic acid were put into a 25 ml three-necked flask at the same time and heated under vacuum at 50°C for 30 minutes, and then heated under inert nitrogen gas to 120 °C for 1 hour. And then the temperature continued to rise to 180 °C under a nitrogen flow and 0.4 ml Cs-oleate precursor solution preheated at 160 °C was swiftly injected. After 5 seconds, the reaction mixture was cooled down to room temperature with ice-water bath. The crude solution was centrifuged for 5 min at 9500 rpm, the supernatant discarded and the precipitate redispersed in toluene to acquire a stable CsPbBr3 QDs solution. The precursor solution (10mg/ml) was spin coated onto fused silica substrates at 1000 rpm for 60 s to obtain the sample of CsPbX3 QDs films which is about 100 microns thickness, followed by drying at ambient conditions for 20 min.

Spectroscopy of the CsPbX3 QDs For two-photon spectroscopy, pump pulses at 800 nm (Legend Elite, Coherent, Repetition rate: 1 kHz, Pulse-width: 35fs) generate the two-photon excitations in the CsPbX3 QDs films. The optical wavelength of 400 nm was generated by frequency-doubling of output pulse (Wavelength: 800 nm, Repetition rate: 1 KHz, Pulse-width: 35 fs) from regenerative amplifier using β-barium borate (BBO) crystal. The laser beam was focused vertically onto the CsPbX3 thin film with quartz glasses (0.1 × 3 mm2) via a cylindrical lens with focal length of 75 mm. The florescence spectra from the sample were collected by a spectrometer (Avavtes AvaSpec-3648-USB2-UA). UV–vis absorption spectra were recorded on a Zolix Omni-DR600-SZU spectrophotometer. The prepared samples in 30mm quartz glasses were used to perform all UV–vis absorption and emission measurements. All the samples were measured at room temperature in air.



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Structural and Chemical Characterization of the CsPbX3 QDs Transmission electron microscopy (TEM) images of non-irradiated and irradiated CsPbX3 QDs were obtained on an FEI Tecnai G2 F30 transmission electron microscope operating at an acceleration voltage of 300 kV. X-ray diffraction (XRD) patterns of non-irradiated and irradiated CsPbX3 QDs were obtained using a Bruker AXS GADDS MWPC diffractometer equipped with Cu Kα GX-ray radiation with a multi-wire proportional counter. The operation voltage and current were 40 kV and 25 mA, respectively. The 2θ range was from 10° to 60° in steps of 0.02°. Scanning electron microscope (SEM) images and energy dispersive X-ray spectrometer (EDX) results were measured on Hitachi SU-70. The X-ray photoelectron spectroscopy (XPS) experiment was carried out on a PHI QUANTERA SXM instrument. The X-ray spot size is 200um and X-ray power is 40W. The survey spectra were taken with a pass energy 280eV and step size of 1eV. The narrow spectra of each element were taken with a pass-energy of 55eV and step size of 0.1eV.

Femtosecond transient-absorption spectroscopy (fs-TAS) and time-resolved photoluminescence spectroscopy (TR-PL) The femtosecond (0.1 ps-8 ns) transient absorption spectrometer (TAS) used in this study is based on a regeneratively amplified Ti: sapphire laser system (Coherent, Legend, 800 nm, 35 fs, 5 mJ/pulse, and 1 kHz repetition rate) and the Helios spectrometer (Ultrafast Systems LLC). Pump pulse at 400 nm were generated by OPA (Coherent, OperA Solo), the pump beam diameter at the sample is 100 μm, corresponding to an excitation density of 8μJ/cm2. A white light continuum (from 420 to 780 nm) was generated by attenuating and focusing ~10 μJ of the 800 nm pulse into a sapphire window, The probe beam was focused with an Al parabolic reflector onto the sample (with a beam diameter of 50 μm at the sample). The probe beams were focused into a fiber optics-coupled multichannel spectrometer with complementary metal-oxide-semiconductor (CMOS) sensors and detected at a frequency of 1 kHz. The delay between the pump and probe pulses was controlled by a direct-drive delay stage. The pump pulses were chopped by a synchronized chopper to 500 Hz, and the change in optical density at the sample resulted from the pump pulse was calculated. The instrument limited response time was 60 fs. Transient absorption data are calculated according to the following: Alog(I0/Iex)

(1)

Where Iex are intensity of probe light after the sample when the excitation light was incident on the sample, and I0 are intensity of probe light after the sample when the excitation light was blocked by optical chopper. The samples were pumped at wavelengths of 405 nm in the TR-PL experiments of fluorescence lifetime spectrometer (PicoQuant Fluo Time 300). A visible streak camera system (Olympus) was used as a detector during the TR-PL experiments. The PLQY of the samples were also measured by fluorescence lifetime spectrometer (PicoQuant Fluo Time 300). Current efficiency and power efficiency of the LCD devices were collected by using auto- temperature LED Opto-electronic analyzer ATA-500 (the measurements equipment was designed by EVERFINE PHOTO-E-INFO CO., LTD.)

Results and Discussion: Properties and Structures of CsPbX3 via Laser Irradiation: For simplicity and consistency, herein we chose CsPbBr3 QD for the analysis, and some analysis of the other components


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CsPbX3 (X=Cl, and mixed halide system Cl/Br) can be found in the Supplementary Information (SI). The CsPbBr3 QD mentioned here was synthesized following a recipe reported by L. Protesescu et al. with slight modification.12 The CsPbX3 QDs solution was dropped on a quartz substrate of 20 mm diameter and spin-coated to obtain 100±10 microns thickness QDs films and got dried naturally in the air as shown in Figure 1(a). Detailed fabrication process can be found in the EXPERIMENTAL section. The molecular structural model of CsPbBr3 QD is illustrated in the inset of Figure 1 (a) (Left), it can be seen that the CsPbBr3 QD has a cubic structure. The PL spectrum is shown in the inset of Figure 1 (a) (Right), with the exciting wavelength of 400 nm and 800 nm, respectively, which correspond to one- and two- photon excitation, respectively. The PL peak was centred at 513 nm, with a bandwidth (full width at half maximum, FWHM) of 20 nm. The two-photon excited PL spectra have no remarkable differences with that of one-photon counterpart, which is a typical character similar with that of the traditional metal chalcogenide QDs.37 This suggests that the same electronic relaxation process existed in two different excitation types of the CsPbBr3 QDs.38 To study the effect of excitation light on the property of perovskite QDs, we conducted the laser irradiation experiments in the air circumstances. The CsPbX3 QDs film was irradiated by femtoseconds pulse laser of 800 nm and 400 nm with the power intensity of 10 mW, respectively, the repetition frequency was 1 KHz, and the laser focal spot diameter was about 4 mm, so the power density is 22 uJ/cm2 in the laser irradiation experiments. As shown in Figure 2 (a), the emission peak has a little blue shift in the first 5 minutes of irradiation and then a red shift of ~5 nm until 30 minutes. Thus the emission wavelength of laser irradiated CsPbBr3 QDs was finally shifted by 5 nm from 513 nm to 518 nm. The initial blue shift of PL luminescence peak can be explained by the heat effect of the perovskite QDs from the pump photon energy,39,40 because the continuous laser irradiation results in an increase of temperature at the excited site on the surface. This may also cause the decline of the PL intensity. While contrary to the expectation, PL intensity in irradiated CsPbBr3 QDs is stronger than that in non-irradiated QDs. This phenomenon suggests that there co-exists another process along with the oxidization. It is inferred that this property may arise from the relatively lower density of charge carrier-trapping defects in irradiated CsPbBr3 QDs compared to those in non-irradiated perovskite QDs. As schematically shown in Figure 1 (b), the defects may be partially removed by the laser irradiation which will be discussed later. Interestingly, we also found the blue shift first and then redshift trend of the emission peak with increasing excitation time, and the phenomenon appeared to be due to the decreased surface defects and improved crystal quality of irradiated all inorganic perovskites QDs. As the irradiation time accrues, the mixed exciton state gradually increases, and the body effect of excitons will slightly dominate, leading to the red-shift of luminescence peak over time. This is because the continuous illumination for CsPbBr3 QDs lead to decreasing the defects, and more and more excitons participate in the radiation recombination. Thus, this radiation recombination process dominates, and the overall performance shows a red shift trend. The similar behaviour can also be found in the two-photon counterpart scenario, which is shown in Figure 2 (b). In addition, another two important results are observed, which are shown in Figure 2 (c) and 2 (d), respectively. The blue dotted and solid curves are absorption and PL spectra of non-irradiated CsPbBr3 QDs, respectively. The red dotted and solid curves are absorption and PL spectra of irradiated CsPbBr3 QDs, respectively. The blue and red arrows indicate Stokes shift of non-irradiated and irradiated CsPbBr3 QDs, respectively. From those curves, the exciton absorption peak of irradiated CsPbBr3 QDs is more obvious and stronger, and irradiated CsPbBr3 QDs possess larger Stokes shift compare with that of non-irradiated CsPbBr3 QDs, which is considered to be more suitable for white lightemitting diodes when combined with blue LED ships.38 These results demonstrate that the laser irradiation treatment for CsPbBr3 QDs can obviously upgrade their optical superiority, as well as bring larger Stokes shift advantage.



Figure 1: (a) A schematic illustrating the fabrication procedure for the perovskite CsPbBr3 thin films in this work. The CsPbBr3 perovskite thin films was performed during spin-coating of perovskite precursor solution, followed by drying at ambient conditions for 20 min. The inset shows the cubic perovskite structure of CsPbX3 (left) and UV-vis absorption and PL emission spectra of CsPbBr3 QDs (right). (b) The schematic of perovskite CsPbX3 QDs after laser irradiation. The defects may be partially removed and photoluminescence (PL) is enhanced in the perovskites QDs.

Figure 2: The optical properties of CsPbBr3. PL spectra (a) 400 nm and (b) 800 nm laser irradiation with different irradiation time. Steady state absorption and absolute PL spectra of (c) 400 nm and (d) 800 nm laser irradiation, respectively. The optical properties of CsPbCl3 QDs. PL spectra (e) 400nm and (f) 800nm laser irradiation with different irradiation time. Steady state absorption and absolute PL spectra of (g) 400 nm and (h) 800 nm laser irradiation, respectively.

The redshift trend can also been found in the CsPbCl3 QDs, which are shown in Figure 2 (e) and 2 (f). Meanwhile,


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larger Stokes shift is also introduced after laser irradiation on the CsPbCl3 QDs, which are shown in Figure 2 (g) and 2 (h). In addition, the CsPb(Br/Cl)3 QDs show similar properties (see SI for details).

Structural Characterization of non-irradiated and irradiated CsPbX3 QDs: In order to ensure that the same nanocrystals that were laser irradiated were subsequently analyzed with each of all the analysis techniques, we remove all the non-irradiated region of the sample and just reserve the laser irradiated part. The transmission electron microscope (TEM) images of non-irradiated and irradiated CsPbBr3 QDs are shown in Figure 3 (a) and 3 (b), respectively, which shows that CsPbBr3 QDs are monodispersed with a nanocrystal with an average size of 131 nm.The HRTEM micrograph confirms a single-crystalline cubic phase with d-spacing of 5.7 Å. The selected area electron diffraction (SAED) result is shown as the inset in Figure 3 (c). The crystal structures of CsPbBr3 QDs both before and after the laser irradiation were analyzed by measuring X-ray diffraction (XRD) patterns. As shown in Figure 3 (d), five characteristic peaks are found. The XRD patterns of CsPbBr3 QDs films exhibit peaks at 15.18°, 21.58°, 30.68°, 34.43°, 37.78°, 43.83°, and 46.63° that can be assigned to (100), (110), (200), (210), (211), (220), and (300) planes, respectively. The lattice parameter is in accordance with a previous report,42 confirming that CsPbBr3 is of cubic perovskite phase.15 The laser irradiation had very little effect on the peak positions of CsPbBr3 QDs thin films. The XRD results of the irradiated CsPbX3 the peak positions for CsPbBr3 QDs (Figure 3 (d)). Therefore, no significant spectral or crystal structural changes are observed upon (X=Br/Cl, Cl) QDs are shown in SI, which is similar to that of CsPbBr3 QDs thin films. Elemental analysis was also performed by using an energy dispersive X-ray analyzer (EDX) to confirm the elemental composition ratios of non-irradiated and irradiated CsPbBr3 QDs thin film in SI. In order to better understand surface state situation, X-ray photoelectron spectroscopy (XPS) was conducted. Figure 3 shows XPS results of non-irradiated and laser irradiated CsPbBr3 QDs thin film. The high-resolution Cs 3d, Pb 4f, and Br 3d chemical states, which agree well with CsPbBr3 QDs thin film, are shown in Figure 3(e-g), respectively. After laser irradiation, no change is found for Cs 3d peaks, indicating the low bonding interactions of Cs with Br ions either on surface or in the crystal lattice (Figure 3 (e)). As shown in Figure 3 (f), there are two main peaks Pb 4f7/2 and Pb 4f5/2 at 138.4 and 142.8 eV, respectively. The main peak of Pb 4f have the two small shoulder peaks located at 136.5 and 141.0eV, which represents the presence of metallic Pb in non- irradiation.42 The presence of a huge number of metallic Pb indicate the existence of bromine vacancies (Figure 3 (g)).41,43 The metallic Pb species lead to non-radiative recombination for the CsPbBr3 QD film, so elimination of the metallic Pb content is critical to improve the electronic quality of perovskite films.42 Interestingly, the peak of metallic Pb disappears in the surface of thin film CsPbBr3 quantum dots after laser irradiated. It suggests that the metallic Pb is dramatically reduced in the irradiated CsPbBr3 QDs films, and the reduction of metallic Pb consequently results in a lower level of bromine vacancies in the irradiated CsPbBr3 QDs films. Elemental Pb could in fact easily act as an electron donor in the semiconducting perovskite. A lower level of metallic Pb can decrease the non-radiative recombination and increase the PL lifetime since the metallic Pb species in the film are likely to act as non-radiative recombination centres,

31, 44

which is consistent with the time-resolved

photoluminescence (TR-PL) decay data that will be given later.



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Figure 3: TEM images of a non-irradiated (a) and irradiated (b) CsPbBr3 QDs. (c) The HRTEM micrograph CsPbBr3 QDs. The inset in the bottom right corner is the corresponding diffraction images. (d) XRD patterns of perovskite CsPbBr3 QDs with different laser wavelength irradiation. The black line represents the non-irradiated, the red line represents the 400 nm irradiated, and the blue line represents the 800 nm irradiated. The corresponding Miller indexes are labeled at the top of the diffraction peaks. XPS spectra corresponding to Cs 3d (e), Pb 4f (f), and Br 3d (g) of non-irradiated CsPbBr3 QDs and 400 nm irradiated CsPbBr3 QDs.

Time-resolved Photoluminescence Decay & Femtosecond Transient Absorption Spectroscopy: To gain the dynamics of recombination, TR-PL decay and the absolute PLQYs were measured. The TR-PL decays for perovskite films with non-irradiation and irradiation are shown in Figure 4 (a) and 4 (b), and the TR-PL decay curves were well fitted to a bi-exponential function. A(t)=A1exp(-t/τ1 )+ A2exp(-t/τ2)

(2)

Where A1 and A2 are the relative amplitudes and τ1 and τ2 are the lifetimes for the fast and slow recombination, respectively.45 The TR-PL decays are attributed predominantly to exciton radiative recombination. The bi-exponential decay result powerfully demonstrates that two different particles take part in the emission process, shown in Figure 4


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(a) and 4 (b). The 400 nm laser irradiated CsPbBr3 QDs with 95% PLQY exhibits fast and slow PL lifetimes of τ1 = 2.941 0.0971ns with a percentage of 73% and τ2 =20.870.278 ns with a percentage of 27%, respectively. Similarly, the 800nm laser irradiated QDs sample shows 94% PLQY, τ1= 2.80.1344 ns with a percentage of 25% and τ2= 19.180.921 ns with a percentage of 25%, which is nearly consistent with the 400 nm irradiated CsPbBr3 QDs. In contrast, the non-irradiated CsPbBr3 QDs with 71% PLQY reaches τ1 = 2.4480.098 ns with a percentage of 85% and τ2= 8.0620.266 ns with a percentage of 15%, respectively. The high PLQYs indicate the reduction of non-radiative decay in irradiated CsPbBr3 QDs. Our results are in line with those from the recent experiments on the surface-related emission of fluorescence decay time of CdSe QDs,46 the short-lived PL lifetime of several nanoseconds is ascribed to the initially generated excitons recombination upon light absorption, while the long-lived component can be explained by the excitons recombination being connected with surface states due to the stable excitons at ambitious temperature.44 Since the large lifetime of radiative recombination is corresponding to high PLQY, this result indicates a lower defects and defect-related non-radiative recombination for the irradiated CsPbBr3 QDs film.31 In addition, the enhancement of PLQY of CsPb(Br/Cl)3 QDs is also observed, which increases from 52% to 71%. The mechanism behind such an improvement is explained and discussed as follows. As mentioned before, the exciton luminescence quantum yield is about 71% for non-irradiated CsPbBr3 QDs, while for 400 nm and 800 nm irradiated CsPbBr3 QDs exciton luminescence increases substantially to 95% and 94%, respectively. As the PLQY is highly associated with the way how the excited electronics relax, the improvement of PLQY suggests that the competitive kinetics between radiative and non-radiative relaxation is altered by laser irradiation. As defects could provide extra non-irradiative relaxation pathway, it may be responsible for the lower quantum yield. Considering the higher quantum yield of exciton luminescence in laser irradiated CsPbBr3 QDs, some of the pre-existing structural defects may be removed by laser irradiation. This will reduce the concentration of defects and result in recombination of holes and electrons. For the number of non-radiative recombination pathways caused by defects is reduced, PLQY is increased. Thus, remove of defects can result in improved PLQYs for laser irradiated CsPbBr3 QDs. To obtain an intuitive understanding of the proposed mechanism, the dynamic processes in non-irradiated (black box) and irradiated (red box) CsPbX3 QDs are schematically visualized in Figure 4 (c). In non-irradiated CsPbX3 QDs, the radiative recombination of excitons co-exist with electron trapping and hole trapping defects, which induce nonradiative recombination pathway for excitons. While in laser-irradiated samples, a majority of defects including electron trapping and holes trapping are removed due to the exciton−trapping exchange coupling, thus excited exciton mainly relax in radiative recombination pathways, which result in higher PLQYs. In addition, this conclusion can be further confirmed by the following femtosecond transient absorption spectroscopy (fs-TAS) research. As a powerful tool for study of exciton dynamics, femtosecond transient absorption spectroscopy was measured from the comparative analysis of the exciton relaxation dynamics in laser non-irradiated and irradiated CsPbX3 QDs. Figure 4 (d) compares the transient absorption ∆A data obtained from non-irradiation (red), 400 nm irradiation (blue) and 800 nm irradiation (purple) CsPbBr3 QDs under the identical experimental conditions including the sample concentration and pump intensity. As is known, the two- and three-body mechanisms dominate short probe delay in higher exciton densities, while the density-independent single-particle process play a dominant role in the decays on long delay, including radiative recombination and defect-mediated relaxation,47, 48 In this study, this measurement was controlled to be in lower the pump intensity (no more than 8uJ/cm2). However, in lower pump intensity, some fast recovery components in ∆A data cannot be clearly detected, so to inquiry more elaborate physical processes, a relative lower pump intensity of 8uJ/cm2 was chosen. The time constants and relative amplitudes of non-irradiated CsPbBr3 QDs and irradiated ones obtained from multi-exponential fit of the transient absorption data, that is the decay curve can be well fitted as ∆A=A1e-t/τ1 + A2e-t/τ2 + A3e-t/τ3 + A4e-t/τ4 +B with four time constants, i.e., τ1, τ2, τ3 and τ4 for the different recovery components, respectively, are summarized in Table 1. The fast recovery component (τ1) is attributed to hot-electron



cooling process, which is originated from excited electron relaxation via a non-radiative recombination pathway.49, 50 This process would result in a lower PLQY for non-irradiated CsPbBr3 QDs, as the relative amplitude of A1 is large compared to that of irradiated CsPbBr3 QDs. The ultrafast component is accounted for a vast majority in the nonirradiated CsPbBr3 QDs, which is responsible for the lower PLQY. The second recovery component (τ2) could be caused by non-radiative recombination via Auger recombination and charge transfer from excited state to trapping states. The decrease of the relative amplitude of A2 after laser irradiation on CsPbBr3 QDs is attributed to a decline of non-radiative recombination associated with traps.48 The hundreds of picoseconds component (τ3) can be attributed to electron–hole radiative recombination, which is corresponding to the slower bleach recovery long lifetime component. As mentioned in Table 1, the relative amplitudes of A3 are dramatically increased in the 400 nm or 800 nm irradiated CsPbBr3 QDs, which is considered to be attributed to the passivation of the trap states. As a consequence, the band edge photoluminescence process improved in the irradiated CsPbBr3 QDs leads to higher PLQY. For a persistent and slowly recovery component τ4=1.90.088 ns for the non-irradiated CsPbBr3 QDs and corresponding component no less than 4.50.255 ns for the irradiated ones in ∆A data, one possible explanation is the absorption from the trapped exciton.51,52 Because there are many defects in the nonirradiated CsPbBr3 QDs, and exciton participating in non-radiative recombination is relative large and lead to the relatively short lifetime of radiative recombination, while for irradiated CsPbBr3 QDs, the defect exciton absorbing energy from the laser irradiation become normal excitons, then more radiative recombination excitons result in a longer lifetime of radiative recombination. This is the reason that leads to decreased defects and the higher PLQYs in irradiated CsPbBr3 QDs. This conclusion is consistent with the result of TR-PL decays. Therefore, these results suggest that laser irradiation have removed some of the electron or hole trapping defects abundant in CsPbBr3 QDs. The similar conclusion can also be found in the other CsPbX3 (X=Cl, Br/Cl) (see SI for details) and the time constants of corresponding QDs obtained from multi-exponential fit of the transient absorption data absorption data are listed in SI. Therefore, the irradiated CsPbX3 QDs seems to be a better candidate for optoelectronic applications due to its low traps density and long radiative lifetime.


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Figure 4: TR-PL decays for perovskite films with 400nm -irradiated (a) and 800nm-irradiated (b) CsPbBr3 QDs. (c) Competing dynamic processes in non-irradiated and irradiated perovskite QDs. Dashed lines within the gap represent trap states. (d) Transient absorption data of non-irradiated and irradiatedCsPbBr3 QDs. Experimental data (symbol) and fit (solid lines) are shown for non-irradiated (red), 400 nm irradiated (blue) and 800 nm irradiated (purple) QDs. The time constants were obtained from multi-exponential fitting of the data. Table 1: Fitting Parameters of Normalized Transient Absorption Data to −ΣAi exp(t/τi)

Sample non-irradiation 400nm irradiation 800nm irradiation

A1

A2

A3

0.34

0.19

0.19

0.019

0.009

0.12

0.02

0.006

0.0057

0.13

0.04

0.008

0.0019

0.009 0.55 0.036 0.44 0.025

A4

τ1(ps)

τ2(ps)

τ3(ps)

0.28

0.476

9.8

773.0

1.9

0.014

0.03

41.65

0.088

0.31

1.00

327.0

8.3

0.021

0.06

6.63

17.33

0.55

0.39

1.04

24.99

405.3

4.5

0.023

0.069

1.45

22.25

0.255

0.56 80.0

τ4(ns)

Thermal-stability and photo-stability tests: In association with the improvement of trapping defects, the stability of inorganic perovskite QDs can also be enhanced by laser irradiation. This enhancement is of great importance to the application in an on-chip system. In this work, a thermal controller system was used to test the thermal stability. During the test, the experimental temperature range was from -25℃ to 100℃. The relative intensity of CsPbBr3 QDs declined with increasing the temperature, which is shown in Figure 5 (a). The relative intensity of laser-irradiated CsPbBr3 QDs was decreased to 60% after the temperature



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reached 100 ℃, while that was decreased to 25% for the non-irradiated CsPbBr3. The same phenomenon can also be observed in 800 nm laser irradiated CsPbBr3 QDs (see SI for details). In addition, the thermos-cycling results were illustrated in SI. When the temperature was decreased to the initial temperature, the intensity of irradiated CsPbBr3 was almost the same as that before heat treatment, while the relative intensity of non-irradiated CsPbBr3 was decreased to 65% after the temperature treatment was implemented. Results indicate that laser-irradiated (400 nm laser) CsPbBr3 demonstrated higher thermal stability than non-irradiated CsPbBr3. The thermal stability of 800 nm laser irradiated CsPbBr3 are shown in SI, which is similar to that of 400 nm laser. A photo-stability test of CsPbBr3 QDs was conducted under continuous UV-light (365 nm, 6W) illumination. During the test, the CsPbBr3 QDs thin film was illuminated by UV light ranging from 30 min to 72 h. Figure 5 (b) shows that the relative intensity of non- irradiated CsPbBr3 QDs was reduced to 30% after 72 h illumination. As can be seen from previous analysis on structures of irradiated QDs, a surface passivation structure was able to prevent photooxidation in the process of UV light illumination, whereas non-irradiated perovskite QDs was easily exposed to oxygen and thus results in surface defects. The relative intensity decreased fast due to surface defects, and perovskite QDs decomposed. However, the surface passivation can act as a protective structure for the interior of CsPbBr3 QDs, so the PL intensity of irradiated (400 nm laser) CsPbBr3 intensity roughly decreased to 70% after UV light illumination about 72 h. The same conclusion can be had in 800 nm laser irradiated CsPbBr3 QDs (See SI). Therefore, laser-irradiated CsPbBr3 QDs exhibits not only better thermal stability but also photo stability. In addition, the similar enhancement of stability of CsPb(Br/Cl)3 QDs after laser-irradiation can also be observed, which is shown in SI.

Figure 5: (a) Thermal stability and (b) photo-stability tests of non-irradiated (black) and 400 nm irradiated (red) CsPbBr3 QDs in ambient atmosphere, respectively.

Demonstration of application in liquid crystal display backlight film: The enhanced PL properties of the as-fabricated QDs films makes them to be promising luminescent materials in many optical systems. Recently, perovskite QDs emerged as low-cost alternative materials for efficient and wide color gamut (WCG) display technology .11,12,53 Usually, the RGB light sources stem from a blue light emitting diodes (LED) chip, a red phosphor (e.g., K2SiF6:Mn4+, KSF) and a green perovskite quantum dots film (PQDF). Consequently, the key of WCG backlight unit for LCD is the high-performance and high-efficient green light generated by the PQDF. In the present work, due to the limitation of experimental conditions available in our lab, an elementary demonstration that employs the laser-treated PQDF as the green QD LCD backlight films is illustrated, which is sufficient in evidencing the superiority of laser-irradiated perovskite QDs. Thus, we focused on green QD backlight film which is supposed to be applied in the display in combination with magenta LEDs (blue LEDs with a red KSF phosphor). The green QD backlight films are encapsulated in a green perovskite QD layer which is sandwiched between fast curing epoxy adhesive


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films. The thickness of QD layer itself is about 100 micron. The configuration of the device is shown in Figure 6 (a), and the blue light emitted from LED chip is partly converted into green light by green perovskite QDs. The corresponding non-irradiated and laser-irradiated CsPbBr3 QDs PL spectrums under the same condition are shown in Figure 6 (b) and 6 (c), respectively. As can be seen from the green peaks, the PL intensity of the irradiated CsPbBr3 QDs is significantly higher than that of non-irradiated ones. Considering the high luminance with small currents, the current efficiencies of irradiated CsPbBr3 LCDs exhibit a larger peak value of 45.4 cd A-1, much higher than that of non-irradiated CsPbBr3 (Figure6(d)). These devices also demonstrate higher power efficiency of 79.6 lm W-1 than that of the non-irradiated CsPbBr3 QDs device (72.34 lm W-1) (Figure 6(e)). It suggests that irradiated perovskite QDs have excellent performance with respect to energy saving aspects which is considered resulting from the very high quantum efficiency of laser irradiated perovskite QDs.

Figure 6: (a) The left is schematic diagram of the configuration of the prototype LED device, and the right is photographs of blue LED under the operation of 5 mA. Normalized emission PL spectra at an applied current of 20mA using green emissive non-irradiated (b) and irradiated (c) CsPbBr3 QDs films. (d) Current efficiency of the devices as a function of current. (e) Power efficiency of the devices as a function of luminance.




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Conclusion: In summary, in this work, the surface electronic trapping defects that lead to a lower PLQY of all-inorganic perovskite CsPbX3 (X=Cl, Br/Cl, Br) QDs are effectively decreased by laser irradiation. Consequently, the PLQY is finally improved by up to 20%. TR-PL and fs-TAS that were employed to study the excited state dynamics processes confirmed the decreasing of trapping defects after laser irradiation and revealed the underlying mechanism. Moreover, the thermalstability and photo-stability of irradiated CsPbX3 QDs were also improved. Finally, an elementary demonstration of their application in liquid crystal display backlight film is given. Though more investigations are needed to reveal the correlations between structural (especially the surface states) and physical properties, this novel passivation technique is likely to prove useful for many high-quality all-inorganic perovskite-based optoelectronic applications including lasing, lighting, and display, pushing it to a new application prospect in the field of optoelectronic devices. AUTHOR INFORMATION Corresponding Author To whom correspondence should be addressed. Email: [email protected] Author Contributions All authors contributed in this manuscript and approved the final version. Supporting Information The details of femtosecond TA, and optical properties of CsPb(Br/Cl)3 and CsPbCl3 QDs, and structural information: XRD and EDX, and the fitting analysis of four decay components in TA decay curves. Conflicts of interest: None

Acknowledgments: The research is partially supported by the National Key R&D Program of China (Grant No. 2016YFA041100), China Postdoctoral Science Foundation (Grant 2016M592528 and 2016M602517), Guangdong Provincial Natural Science Foundation (2017A030310130) and National Natural Science Foundation of China (Grant No. 61575129，51502175, 11704285 and 61805156), Shenzhen Science and Technology Project (Grant No. JCYJ20160328144942069)

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Laser Irradiation

Reduced Defects & Enhanced PL

Trapping Defect 30 min


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Laser irradiation can reduce surface electronic trappings defects of all-inorganic perovskite CsPbX3 QDs, and improve PLQY by up to 20%.

Laser Irradiation

Reduced Defects & Enhanced PL

Trapping Defect 30 min


Performance Enhancement of All-Inorganic Perovskite Quantum Dots

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