Trimethylsilyl Iodine-Mediated Synthesis of Highly Bright Red-Emitting

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Trimethylsilyl Iodine-Mediated Synthesis of Highly Bright Red-emitting CsPbI3 Perovskite Quantum Dots with Significantly Improved Stability Yuting Cai, Haoran Wang, Ye Li, Le Wang, Ying Lv, Xuyong Yang, and Rong-Jun Xie Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b04049 • Publication Date (Web): 02 Jan 2019 Downloaded from http://pubs.acs.org on January 2, 2019

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Chemistry of Materials

Trimethylsilyl Iodine-Mediated Synthesis of Highly Bright Red-Emitting CsPbI3 Perovskite Quantum Dots with Significantly Improved Stability Yuting Cai,† Haoran Wang,‡ Ye Li,*,† Le Wang,§ Ying Lv,† Xuyong Yang,*,‡ and RongJun Xie*,† †College

‡Key

of Materials, Xiamen University, Simingnan-Road 422, Xiamen 361005, P. R. China

Laboratory of Advanced Display and System Applications of Ministry of Education, Shanghai

University, Yanchang road 149, Shanghai 200072, P. R. China

§China

College of Optical and Electronic Technology, China Jiliang University, Hangzhou, Zhejiang

310018, P. R. China

ACS Paragon Plus Environment

Chemistry of Materials 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

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ABSTRACT: Herein, we report highly bright and stable CsPbI3 (CPI) perovskite quantum dots (PQDs) synthesized with trimethylsilyl iodine (TMSI) under a reaction circumstance with I/Pb molar ratio of ~4.2. The obtained CPI (TMSI-CPI) PQDs show near-unity photoluminescence quantum yields (PLQYs) in solution and high stability (only 9% loss in PLQY after 105day storage) under ambient and dark conditions. The thermal stability of TMSI-CPI PQDs is also improved: the degradation temperature is higher than that of traditional hot injection synthesized CPI (Tra-CPI) PQDs. X-ray photoelectron spectroscopy (XPS) results show that the TMSI-CPI PQDs have a highly iodine-rich surface (the I/Pb atomic ratio is up to 4.4) which is believed to be responsible for such high stability and PLQYs. Further, the size and surface properties of CPI PQDs can be easily adjusted by changing the amount of TMSI. Finally, we fabricated QDs-based light-emitting diodes (QLEDs) utilizing TMSI-CPI PQDs as an emissive layer showing a maximum luminance of 365 cd m-2 and external quantum efficiency (EQE) of 1.8 %. During a working period of 2 hours, no shift and broadening of the electroluminance spectra happens for TMSI-CPI based QLEDs with an initial luminance of 100 cd/m2; the device lifetime for which the luminance drops to half of its initial value (100 cd m-2) reaches 3.11 h, which is nearly 7 times longer than that of Tra-CPI based QLEDs.

All-inorganic cesium lead halide (CsPbX3, X = Cl, Br, or I) perovskite quantum dots (PQDs), as a recently emerging member of the quantum dot (QD) family, have attracted increasing attention for their outstanding optical properties, facile synthesis and higher stability compared with the organic-inorganic hybrid counterparts. Since their first introduction in 2015,1 many applications based on this class of materials have been explored such as light-emitting diodes (LEDs),2-8 photodetectors,9-12 lasers,13, 14 15 photocatalysis, and so on. Especially for display applications, CsPbX3 PQDs have multiple advantages such as (i) near-unity photoluminescence quantum yields (PLQYs) in the green and red spectral regions, (ii) a broad wavelength coverage (400 - 700 nm) achieved through composition, size, morphology control, (iii) a narrow spectral line-width of 12 - 42 nm.16-19 Owing to these excellent optical properties, some high-performance QDsbased LEDs (QLEDs) utilizing CsPbX3 PQDs as an emissive layer have been reported. For example, the external quantum efficiency (EQE) of both green- and red-emitting CsPbX3 PQDs based QLEDs have exceeded 10 %; the luminance in green and red spectral regions can reach 24000 and 1444 cd m-2, respectively.20-23 However, their practical applications are severely hindered by the poor stability of CsPbX3 PQDs, either inherent or triggered.24 Especially for the red emitting CsPbI3 (CPI) PQDs, the phase transformation from the luminescent 3D perovskite phase (black phase) to the thermodynamically stable nonluminescent 1D orthorhombic phase (yellow phase) easily occurs within less than 3 weeks. Besides, this degradation process can be accelerated typically by moisture, heat, and/or UV light. Thus, tackling the stability issue of CPI PQDs is highly necessary and more urgent. So far, several methods have been proposed to enhance the stability of CPI PQDs, such as providing an inert protection layer,25-28 doping or alloying with hetero atoms,29-31 changing ligands23, 32, 33 and post-treatments34, 35. However, the CPI PQDs used in or synthesized by these methods are not state-of-the-art in stability and PLQYs. This has been demonstrated to be related to an iodineinsufficient synthetic environment where an iodine-rich surface structure beneficial for both high stability and PLQYs is not easy to form. 36, 37 In traditional methods, lead( Ⅱ ) iodine is chosen as lead and iodine sources and therefore the I/Pb molar ratio of only 2 in the reaction system is much smaller than the stoichiometric value of 3. In order to tackle this problem, recently, some syntheses of

CPI PQDs under a halide-rich reaction environment have been proposed and achieved a significant progress. Liu et al.36 replaced the frequently-used PbX2 with PbO and NH4X, and adjusted the X/Pb molar ratio in the reaction system to above 2. In that condition, the obtained CsPbX3 PQDs have improved durability against purification treatments. Woo et al.37 introduced metal iodine (e.g., ZnI2) into the reaction system to increase the amount of the iodine source and obtained CPI PQDs can stabilize for 3 days at ambient conditions with a high relative humidity up to 60 %. Dutta et al.38 synthesized stable CPI PQDs at a relatively high temperature (260 oC) in the presence of extra oleylammonium iodine. The product can be stored for a month under ambient conditions. Imran et al.39 creatively adopted a covalent compound (benzoyl iodine) as the iodine source that can be independently injected into the solution containing Cs and Pb precursors, and the CPI PQDs synthesized under an iodine-rich circumstance can be stored at room temperature for over 20 days. These interesting results validate the necessity and importance of an iodine-rich synthetic environment for synthesizing highly stable CPI PQDs. However, the aforementioned methods involve complicated procedures such as adding multiple raw materials or harsh preparation of the precursor. Besides, in some of these methods, because the iodine precursor and the lead source cannot be made independently, metallic lead particles would generate due to the reducibility of OLA, which can worsen the stability and optical properties of CPI PQDs.40, 41 Thus, it is urgent to develop a more effective iodine-rich synthetic route. Here, we report an iodine-rich synthesis of CPI PQDs based on trimethylsilyl iodide (TMSI) under a reaction circumstance with a TMSI/Pb-oleate molar ratio of ~4.2. The obtained CPI PQDs have excellent storage stability (over 85 % of PLQY after 105-day storage) and thermal stability. Making use of these CPI PQDs as an emissive layer, the QLEDs with a standard structure achieve a maximum EQE of 1.8 %, a luminance of 365 cd m-2 under the applied voltage of 7 V. The device lifetime (T50), defined as the time for the initial luminance (100 cd/m2) to drop by 50 %, is also improved to 3.11 h. Encouraged by these inspiring results and simplified procedures, we believe this TMSIbased method will become more universal in obtaining highly bright and stable CPI PQDs.


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Scheme 1. Reaction mechanism for the TMSI-mediated synthesis of CPI PQDs. Trimethylsilyl halides have extensive use in organic synthesis, especially for protection of hydroxyl, amine and carboxyl groups.42, 43 When these groups react with trimethyl halides, hydrogen halides (HX) as one of the products are released. Taking TMSI for example, the specific reaction processes are illustrated in Scheme 1. Further, the acid HX can be captured by the basic alkyl amine and the generated alkyl ammonium halides can be made use of to synthesize metal halide PQDs (scheme 1). Recently, some groups have demonstrated the reliability and validity of trimethylsilyl halides in syntheses of some metal halide PQDs, such as Cs2AgBiX644, Yb-doped CsPbX3 (X = Cl or Cl/Br)45. Trimethyl halides are also an effective reagent for anion exchange in CsPbX3 PQDs with some outstanding advantages such as their easy elimination and inertness toward undesired side reactions.46, 47 Here, TMSI is used to synthesize CPI PQDs, given that the Cs : Pb : I molar ratio in the reaction system can be flexibly adjusted and the optimized ratio can be obtained to ensure high stability and performance of CPI PQDs. The experimental procedure is simple and easy to follow. When a certain amount of TMSI is injected into the mixed solution containing Pb-oleate, Cs-oleate, OA and OLA (for details see experimental section in the Supporting Information (SI)), a blood red solution appears immediately and emits bright red light under 395 nm excitation (Figure S1), indicating the successful synthesis of CPI PQDs. After several seconds, the reaction is stopped by being cooled down in an ice water bath. The stability of obtained crude CPI (TMSI-CPI) PQDs was firstly evaluated. After a 4-month storage, only a little yellow precipitate appears at the bottom of the bottle, while the supernatant still shows blood red in appearance. For unpurified traditional CPI (Tra-CPI) PQDs (detail preparation process can be seen in the SI), their PL completely disappears after only several days. Such a huge difference can be partly ascribed to the different content of free ligands in solution. Excess ligands can accelerate the degradation of PQDs.48 The volume of OA and OLA used in the TMSI based method is 0.52 and 0.90 mmol, whereas that is usually 1.58 and 1.29 mmol in the traditional method, respectively. Besides, the injected TMSI can consume a part of OA and OLA through the reaction illustrated in scheme 1. Thus, the crude TMSI-CPI PQDs solution has a lower probability of containing excess ligands. This feature makes it easier and convenient to store CPI PQDs as the immediate purification after synthesis is not necessary.

To analyze the inherent stability, excess ligands and unreacted starting materials in CPI PQDs should be excluded. Thus, the purification of CPI PQDs is still needed. For comparison, Tra-CPI PQDs are also treated in the same way. As seen in Figure S2, yellow precipitates appeared after the second washing of Tra-CPI PQDs, indicating the occurrence of the degradation. In contrast, the black precipitate of TMSI-CPI PQDs can still be collected after two cycles of washing. In addition to the enhanced stability during purification, an extra bonus is the high yield brought by the TMSI based method, which is important for promoting the utilization ratio of the toxic lead. In the TMSI based and traditional method, the amount of Cs-oleate and Pb-oleate used are ~0.048 and ~0.19 mmol, respectively, but the collected TMSI-CPI PQD precipitates after the first centrifugation have a darker black red color in appearance compared with the Tra-CPI PQD ones. The accurate mass of TMSI-CPI and Tra-CPI PQDs after one cycle of washing and drying is ~0.16 and ~0.07 g, respectively. The higher yield of TMSI-CPI PQDs is originated from the more complete conversion of Cs and Pb sources in the iodine-rich synthetic environment. Then, the purified CPI PQDs collected after one cycle of washing were dispersed in n-hexane for further characterization. As shown in Figures 1a-b, TMSI-CPI PQDs display an emission peak at ~ 680 nm with a full width at half maximum (FWHM) of 33 nm a little bit narrower than that of Tra-CPI PQDs (~37 nm). This small FWHM is indicative of a narrower size distribution of TMSI-CPI PQDs. Subsequently, the spectra were monitored at different intervals. During the measurement period of 58 days, no changes in the shape and peak position of the spectra are seen (Figure 1a). The bright red emission of the TMSI-CPI PQDs under the 395 nm excitation can still be observed on the 58th day (inset in Figure 1a). In contrast, the Tra-CPI PQDs totally lose their PL after 28-day storage (inset in Figure 1b). During this period, despite no obvious changes happen in the normalized PL spectra, the absorbance spectra of Tra-CPI PQDs change dramatically (Figure 2b) with (i) the gradual disappearance of the onset of the absorbance spectra; (ii) the increase of the intensity of the absorbance tail, which is caused by the scattering of the generated large particles; and (iii) the sharpening of the peak at ~ 424 and ~367 nm, indicating the occurrence of the phase transformation.49-51 The corresponding PLQYs were also recorded at different storage times. As shown in Figure 1h, the PLQY of TMSI-CPI PQDs always remains at a high level (no less than 86.0%) and the maximum can reach 96% during 105-day storage. However, Tra-CPI PQDs show a gradual decrease of the PLQY from the initial 70.2% to zero on the 28th day. Such a large difference can also be reflected by the change of the time-resolved PL spectra. The PL decay curves of TMSI-CPI PQDs in Figure 1c can be fitted by a monoexponential function (the fitting results are presented in Table S1). It is well known that the monoexponential PL decay curve implies less surface traps.18, 35, 52, 53 Thus, it is reasonable that the PLQY of TMSICPI PQDs is higher than that of Tra-CPI PQDs which show a biexponential decay curve (Figure 1d, h). Besides, the PL decay curve of TMSI-CPI PQDs remains almost unchanged within 58 days of storage. The average PL lifetime (τave) on the 1st, 28th and 58th days is 53.3, 52.8 and 54.2 ns,



respectively. But for Tra-CPI PQDs, the τave value continues to decrease from 85.5 ns on the 1st day to 77.6 ns on the 3rd day and to 65.6 ns on the 12th day. The gradual reduction of

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τave is probably caused by the increased surface traps that act as nonradiative centers shortening the PL lifetime.34

Figure 1. Absorbance and PL spectra of (a) TMSI-CPI and (b) Tra-CPI PQDs. Time-resolved PL spectra of (c) TMSI-CPI and (d) Tra-CPI PQDs. XRD patterns of (e) TMSI-CPI and (f) Tra-CPI PQDs. (g) Photographs of the film of TMSI-CPI PQDs after 45 days of storage time. (h) PLQYs of TMSI-CPI and Tra-CPI PQDs as a function of storage time. X-ray diffraction (XRD) experiments were performed to examine the structure changes. TMSI-CPI PQDs have a cubic phase structure that can maintain unchanged within 43 days of storage under ambient and dark conditions (Figure 1e). Figure 1g shows that the film of TMSI-CPI PQDs still emits bright red color on the 45th day after preparation under the 395 nm excitation. In contrast, for Tra-CPI PQDs, the 1D orthorhombic phase of CsPbI3 appears after one day (Figure 1f). Besides, some extra weak and broad peaks located at 11.9o, 24.0o and 28.6o are identified and assigned to the Cs4PbI6 phase. According to the total stoichiometric ratio (Cs : Pb : I = 1 : 1 : 3), it is reasonable to infer that the PbI2 or (and) CsPb2I5 species should also exist. However, it is difficult to distinguish them due to the weak XRD signals caused by the low content and partial overlapping with the XRD patterns of 1D orthorhombic CsPbI3 and Cs4PbI6. All in all, the above results demonstrate that the TMSI-CPI PQDs have greatly enhanced phase stability. Transmission electron microscopy (TEM) was used to examine microstructural variations of CPI PQDs. Figures 2ab present the TEM images and Figure 2c shows the high

resolution TEM (HRTEM) image of TMSI-CPI PQDs obtained on the third day after preparation. Figure 2d shows the TEM image of the TMSI-CPI PQDs from the stored solution after 35 days of storage. During this storage period, the size of TMSI-CPI PQDs remains 10~11 nm and a relative narrow size distribution with a negligible change (the standard deviation increases from 1.1 to 1.3 nm). Besides, no particle agglomerations and changes in shape occur during aging. But for Tra-CPI PQDs, the size distribution becomes wider (the standard deviation is 1.9 nm) (Figure S3a), which is consistent with the broader FWHM of the PL spectra mentioned above. After 6 days of storage, some quasispherical nanoparticles appear (Figure S3b). The corresponding HRTEM image in Figure S3c shows a clear lattice spacing of 0.74 nm, which corresponds to the (012) plane of the rhombohedral Cs4PbI6. This observation is consistent with the XRD results above. In brief, these results confirm that TMSI-CPI PQDs have higher stabilities in the particle size and morphology during 35 days of storage under ambient and dark conditions.


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Figure 2. (a, b) TEM images in different resolutions and (c) HRTEM image of TMSI-CPI PQDs obtained on the third day after preparation. (d) TEM image of TMSI-CPI PQDs obtained after storing the solution for 35 days. Temperature-dependent PL spectra of (e) TMSI-CPI PQDs and (f) Tra-CPI PQDs using 460 nm UV light as an excitation source. (g) Relative PL intensity of TMSI-CPI and Tra-CPI PQDs as a function of temperature. (h) Photographs of Tra-CPI PQDs film and TMSI-CPI PQDs film after experiencing the whole temperature-dependent PL measurement. Insets in (a) and (d) are the corresponding lateral size distribution. To evaluate the thermal stability of TMSI-CPI PQDs, temperature-dependent PL spectra were recorded. The heating rate was 5 oC/min, and upon reaching the measurement temperature, another 2 min was needed for the system to stabilize before measurements. As shown in Figures 2e-g, the PL intensity of both Tra-CPI PQDs and TMSI-CPI PQDs decreases gradually as the temperature increases due to the enhanced exciton-phonon interaction.54 But there are some obvious differences when the temperature exceeds 90 oC. For the film of the Tra-CPI PQDs, when the temperature reaches 110 oC, the electroluminance (EL) intensity of the excitation light increases rapidly, which is attributable to the decrease of the light absorption ability of the film as Tra-CPI PQDs partly degrade into high bandgap products (Figure 2f). Further increasing the temperature up to 130 oC, no emission can be detected, indicating that the Tra-CPI PQDs are totally degraded (Figure 2g). In contrast, the EL intensity of the excitation light of the TMSI-CPI PQD film remains unchanged during the whole measurement period (Figure 2e) and the red emission can still be observed at 130 oC (Figure 2g). After the finish of thermal quenching measurements, the film of Tra-CPI PQDs had transformed into yellow products and no red emissions under the 395 nm excitation can be observed; while the TMSI-CPI PQDs film is still black red in color and emits intense red light under the same excitation condition (Figure 2h). These results clearly show that TMSI-CPI PQDs have an enhanced thermal stability. The improvement is probably originated from a more iodine-rich surface structure (see below) which can induce surface strain and increase the energy barrier that a phase transformation needs to overcome.55-57 The photostability of TMSI-CPI and Tra-CPI PQD film was also evaluated (Figure S4). Upon exposure to UV illumination (100 mW/cm2), the PLQYs of TMSI-CPI and Tra-CPI PQD film display almost the same decay rates,

indicating that no obvious photostability enhancement of TMSI-CPI PQDs.

Figure 3. Deconvolution of the Pb 4f XPS signals detected from (a) the TMSI-CPI PQDs and (b) the Tra-CPI PQDs. (c) Atomic ratio (relative to Pb atoms for each sample) calculated based on the high-resolution XPS results. (d) Schematic diagram of the iodine-rich surface. Surface structure and composition were studied to check out critical factors for such stability enhancement of TMSI-CPI PQDs. FTIR spectra (Figure S5a) are similar for TMSI-CPI and Tra-CPI PQDs, indicating the same ligand composition. The surface ligand density was calculated based on the TEM and Thermogravimetric Analysis (TGA) results. Figure S5b shows the TGA curves of TMSI-CPI and Tra-CPI PQD. According to the previous works,58, 59 the thermal decomposition of capping ligands (OA and OLA) and CsPbI3 begins above ~300 and ~400 oC, respectively. Here, we assume that the mass loss occurring between



300~400 oC mainly arises from organic capping ligands. Thus, the weight percent of the capping ligands was estimated to be 7.0 % for TMSI-CPI PQDs and 5.9 % for TraCPI PQDs; the inorganic species of CsPbI3 was estimated to be 44.8 % for TMSI-CPI PQDs and 50.3 % for Tra-CPI PQDs. The surface ligand density is thus calculated to be 3.0 nm-2 for TMSI-CPI PQDs and 2.5 nm-2 for Tra-CPI PQDs. Such small difference in surface ligand density would not bring such obvious stability difference. X-ray photoelectron spectroscopy (XPS) measurements were also performed (Figures S6-7 and 3ab). The XPS spectra of TMSI-CPI PQDs and Tra-CPI PQDs are almost the same and the energy bands of Cs, Pb, I and N can be clearly identified, indicating the same elemental composition for these two kinds of CPI PQDs. An obvious difference in the high-resolution XPS spectra of Pb 4f is seen between TMSI-CPI and Tra-CPI PQDs. For Tra-CPI PQDs, besides two main peaks located at ~142.6 and ~137.6 eV corresponding to Pb atoms on the inner of PQDs, there are additional shoulders located at ~135.9 and ~141.2 eV (Figure 4b) which can be ascribed to Pb atoms in the outer surface of PQDs.60 It thus implies that the TMSI-CPI PQDs have a lead-insufficient or iodine-rich surface. The atomic ratio calculated based on the integration area and sensitivity factor is presented in Figure 3c. As expected, the TMSI-CPI PQDs have a higher I/Pb atomic ratio (4.4) than the Tra-CPI PQDs (3.4), which indicates more complete PbI64- octahedrons on the surface of the TMSI-CPI PQDs (Figure 3d). The inorganic surface is thus more negative, enabling to attract more oleylammonium cations. Indeed, the N/Pb atomic ratio of TMSI-CPI PQDs increases up to 4.3, which is higher than that of the Tra-CPI PQDs (2.5). Given that the Cs/Pb atomic ratio of the TMSI-CPI PQDs (0.48) is smaller than that (0.68) of the Tra-CPI PQDs, the high N content of TMSI-CPI PQDs is partly attributed to the replacement of more surface Cs ions by oleylammonium cations. These results verify that TMSI-CPI PQDs have an iodine-rich surface and are well passivated by the oleylammonium cations. The effects of different amounts of TMSI on the structure and optical properties were studied. When the amount of TMSI increases from 64 to 134 μL, the crystal structures keep cubic (Figure 4d); the FWHM of XRD peaks located at 14.1 and 28.6o broadens and the onset of absorbance and PL spectra blue shift gradually (Figure 4de), indicating the decrease of the size of PQDs. As expected, TEM results show that the size of TMSI-CPI PQDs decreases from 11.7 to 9.2 nm (Figure 4a-c). This is caused by the increased amount of oleylammonium iodine in the reaction system that suppresses the growth the CPI PQDs.61 Besides, the “black dots” that are adhered on the surface of CPI PQDs and have been confirmed to be PbI2 or metallic Pb particles reduce gradually (Figure 4a-c), which is beneficial for the stability and performance enhancement of TMSI-CPI PQDs.40, 62 As expected, the PLQYs increase gradually from 86.1 to 100 % (Figure 4f) and the PL decay curves change from bi- to monoexponential one (figure 4g and Table S1). The gradual decreasing τave is mainly caused by the increased exciton bonding energy induced by the small size. The Energy Dispersive X-ray spectroscopy (EDS) results (Table S2) show that the content of iodine element in CPI

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PQDs increases gradually as a whole. These results signify that an iodine-rich synthetic environment can effectively remove surface defect states and bring a desirable surface structure of CPI PQDs. The suitability of TMSI-CPI PQDs used in QLEDs was finally investigated. Figure 5a illustrates the structure of the QLEDs, where poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) on an indium tin oxide (ITO) glass substrate was used as a hole-injection layer (HIL); poly[bis(4phenyl)(4-butylphenyl)amine] (poly-TPD) and 2,2’,2’’,(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBI) were used as a hole-transporting layer (HTL) and an electron transporting layer, respectively. Figure 5b shows the corresponding flat-band energy-level diagram of the QLEDs (the energy values for TMSI-CPI PQDs were calculated by ultraviolet photoelectron spectroscopy (UPS) and optical measurements (Figure 1a & Figure S8)). The TMSI-CPI based QLEDs show bright red emissions with a peak wavelength of ~ 682 nm and a narrow FWHM of ~ 33 nm under various biases (Figure 5c). Compared to the PL emission of the TMSI-CPI PQDs solution, the EL emission is slightly red-shifted by ~2 nm, which can be attributed to the interdot and Stark effect.33 A photograph of the working TMSI-CPI based QLEDs is presented, giving a uniform red emission (inset in Figure 5c). The CIE coordinates of the TMSI-CPI based QLEDs at 6.5 V are (0.72, 0.28), showing a very high color purity (Figure 5d). From the J-V curves, it is found that the TMSI-CPI based QLEDs exhibit a higher current density than the Tra-CPI based QLEDs, indicating the remarkably improved carrier injection for TMSI-CPI based QLEDs (Figure 5e). The maximum luminance of TMSI-CPI based QLEDs reaches 365 cd·m-2 under an applied voltage of 7.0 V, which is more than 5 times higher than that of the Tra-CPI based QLEDs (69 cd·m-2). Compared to previously reported results (748, 827, 1106 and 1444 cd m2),5, 21, 33, 63 this value is not enough high, but it can be further promoted through optimization of device structure, ligand engineering, and so on. The turn-on voltage (where luminance is ＞1 cd·m-2) is 2.8 V for TMSI-CPI based QLEDs, lower than that (6.7 V) for Tra-CPI PQDs based QLEDs and that (4.1 V) reported by Bakr et al.33 who used the same device structure. The maximum EQE of the TMSI-CPI based QLEDs is 1.8%, higher than that of Tra-CPI based QLEDs (0.8%) (Figure 5f). In order to evaluate the device stability, normalized EL spectra of TMSI-CPI based QLEDs with an initial luminance of 100 cd/m2 for different operation times are collected. There is no shift and broadening of the EL spectra within a working period of 2 hours (Figure 5g). Meanwhile, the device lifetime (T50) for which the luminance (100 cd/m2) drops to half of its initial value (L0) is also calculated. As shown in Figure 5h, T50 for Tra-CPI based QLEDs at 50 cd m-2 is 1.31 h. By using the relation L0nT50 = const., where the acceleration factor is assumed to be 1.5,64 T50 for Tra-CPI based QLEDs when the initial luminance is 100 cd m-2 is calculated to be 0.46 h. For TMSICPI based QLEDs with an initial luminance of 100 cd m-2, the lifetime is 3.11 h, which is nearly 7 times longer than that of Tra-CPI based QLEDs. These results clearly show that TMSICPI PQDs enable to produce QLEDs with enhanced EL performance and reliability.


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Figure 4. TMSI-CPI PQDs synthesized using different amounts of TMSI. (a-c) TEM images. (d) XRD patterns. (e) Absorbance and PL spectra. (f) PLQYs. (g) Time-resolved PL spectra. Insets in (a-c) are the corresponding lateral size distribution.

Figure 5. (a) Illustration of the device structure. (b) Flat-band energy-level diagram. The energy levels for PEDOT:PSS65, polyTPD36 and TPBI65 are taken from references. (c) EL spectra of the TMSI-CPI based QLED under various biases. (d) CIE coordinate for the EL spectrum of TMSI-CPI based QLED at 6.5 V. (e) Current density and luminance and (f) external quantum efficiency of TMSI-CPI based QLEDs and Tra-CPI based QLEDs as a function of applied bias. (g) Normalized EL spectra of TMSICPI based QLEDs with an initial luminance of 100 cd m-2 at different time intervals. (h) Relative luminance (L/L0) of TMSI-CPI based and Tra-CPI based QLEDs as a function of operating time, the initial luminance (L0) of TMSI-CPI based and Tra-CPI based QLEDs are 100 and 50 cd m-2, respectively. In summary, we presented a facile iodine-rich synthesis of highly stable and bright CPI PQDs based on trimethylsilyl iodine (TMSI). The PLQY of TMSI-CPI PQDs synthesized under a reaction circumstance with I/Pb molar ratio of ~4.2 is near-unity and after 105 days of storage, it still exceeds 85%. The thermal stability of the TMSI-CPI PQDs is also improved compared to the traditional hot injection synthesized ones. Such obvious improvement is attributed to a highly iodine-rich surface structure of the TMSI-CPI PQDs (the I/Pb atomic ratio is up to 4.4). The QLEDs

utilizing these TMSI-CPI PQDs as an emissive layer also have the promoted performance and stability: the maximum luminance and EQE reach 365 cd·m-2 and 1.8%, respectively; the device lifetime (T50) is increased 3.11 h when the initial luminance is 100 cd m-2. These results further confirm the importance of a halide-rich reaction environment for obtaining highly stable PQDs.

ASSOCIATED CONTENT Supporting Information



The Supporting Information is available free of charge on the ACS Publications website. Additional data about experimental section, characterizations, the photographs of the obtained solution after injection of TMSI, photographs recorded for every step of ethyl acetate washing process for Tra-CPI and TMSI-CPI PQDs, TEM images of Tra-CPI PQDs on the 1st day and on the 6th day, the lateral size distribution of Tra-CPI PQDs on the 1st day, PLQYs of TMSICPI and Tra-CPI PQD film upon UV exposure for different minutes, FTIR spectra and TGA curves of TMSI-CPI and Tra-CPI PQDs, XPS spectra for TMSI-CPI and Tra-CPI PQDs, XPS spectra corresponding to Cs 3d, I 3d5/2, and N 1s of TMSI-CPI PQDs and Tra-CPI PQDs, secondary electron cutoff region and valence band region of UPS spectra of TMSI-CPI PQDs, the illustration of energy levels of the TMSI-CPI PQDs, Quantitative EDS analysis of the element proportion (atomic %) in the TMSI-CPI PQDs synthesized using different amounts of TMSI.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. Funding Sources Any funds used to support the research of the manuscript should be placed here (per journal style).

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 51561135015, 51572232, 51602771), National Key Research and Development program (2017YFB0404301) and the “DoubleFirst Class” Foundation of Materials and Intelligent Manufacturing Discipline of Xiamen University. X. Yang also thanks the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning (No. TP2015037).

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Trimethylsilyl Iodine-Mediated Synthesis of Highly Bright Red-Emitting

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