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Highly Efficient Perovskite Light-Emitting Diodes Incorporating Full Film Coverage and Bipolar Charge Injection Ping Chen, Ziyang Xiong, Xiaoyan Wu, Ming Shao, Xingjuan Ma, Zu-hong Xiong, and Chunhong Gao J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b00368 • Publication Date (Web): 07 Apr 2017 Downloaded from http://pubs.acs.org on April 7, 2017

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The Journal of Physical Chemistry Letters

Highly Efficient Perovskite Light-Emitting Diodes Incorporating Full Film Coverage and Bipolar Charge Injection Ping Chen,1† Ziyang Xiong,1† Xiaoyan Wu,2 Ming Shao,3 Xingjuan Ma,1 Zu-hong Xiong1 and Chunhong Gao1*

1

School of Physical Science and Technology, MOE Key Laboratory on Luminescence and Real-Time Analysis, Southwest University, Chongqing 400715, China 2

Institute of Fluid Physics, Mianyang, China Academy of Engineering Physics, Mianyang 621900, China

3

Key Laboratory of Luminescence and Optical Information, Ministry of Education, School of Science, Beijing Jiaotong University, 100044, China

Corresponding Authors: 1* E-mail: [email protected]

ACS Paragon Plus Environment

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ABSTRACT: Solution-processable organometal halide perovskites have been emerging as very promising materials for light-emitting diodes (LEDs) due to their high color purity, low cost and high photoluminescence quantum yield. However, their electroluminescent performance is still limited by incomplete surface coverage and inefficient charge injection into perovskite. Here, we demonstrated a highly efficient perovskite LEDs (PeLEDs) incorporating full film coverage and bipolar charge injection within the active layer, by introducing perovskite precursor poly(9-vinylcarbazole):1,3,5Tris(1-phenyl-1H-benzimidazol-2-yl)benzene (PVK:TPBi) toluene solution into CH3NH3PbBr3 N,Ndimethylformamide solution. Both the film coverage and the charge injections were simultaneously improved by anti-solvent of toluene and PVK:TPBi matrix, respectively. After carefully adjusting the film morphology and weight ratio of PVK:TPBi, the optimal PeLEDs gave efficient emission with turn-on voltage of ~2.8 V, maximum luminance of ~7263 cd/m2, maximum current efficiency of ~9.45 cd/A and maximum external quantum efficiency of ~2.28%, which is among the best results based on MAPbBr3 reported so far.


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Earth-abundant metal trihalide perovskites (formulated as ABX3,

where A is the

methylammonium (CH3NH3+) (MA) or metal cesium cation (Cs+), B is the lead cation (Pb2+) and X is the halide anion (Br−, I−, Cl−)) have attracted enormous attention over past a few years due to their exceptional properties in developing low-cost and high power-conversion efficiency perovskite solar cells.1-4 These properties mainly include facile solution processability,5, 6 long charge carrier lifetime and diffusion lengths,7, 8 and low trap-state density9. More attractively, perovskite nanocrystals have also shown high color purity with full width at half maximum (FWHM) less than 30 nm,10 ultra high photoluminescence quantum efficiency (PLQY) of ~ 95% at room temperature,11 and tunable emission color from the ultraviolet to infrared wavelengths12-14. Thus, perovskite materials are promising in light-emitting diodes (LEDs),10-39 photodetectors,40 and lasing applications41, 42. During the past two years, impressive progresses have been made in perovskite LEDs (PeLEDs) with the maximum external quantum efficiency (EQE) jumping from 0.12% to 11.7%.5, 6, 10-39 So far, the best EQE are 1.38% for blue PeLEDs,15 8.53% for green PeLEDs,16 and 11.7% for near infrared PeLEDs,17 respectively. Such superior performance enables perovskite materials the great potential in replacing inorganic quantum dots (QDs) and conventional organic emitters for flat-panel display in commercial. To achieve highly efficient PeLEDs, two critical criteria must be satisfied: pinhole-free perovskite film to suppress leakage currents17-30 and optimized device structures with minimum injection barriers that guarantee balanced charge injection into perovskites10, 18-20, 28,

31-37

. On one hand, hybrid lead-

halide perovskite film coverage would be very poor if the film was directly prepared by a one step spincoating method from its pure precursor solution, resulting in seriously non-radiative leakage current


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losses in the PeLEDs.19, 23, 25-27, 29, 30 Different methods have been applied to improve the perovskite film coverage. J. Yu et al.23, 25 and B. Jiao et al.26 deposited the perovskite film from a precursor solution containing HBr and chlorobenzene as co-solvent respectively, to realize full film coverage. H. Cho et al. used a solvent treatment method of nanocrystal pinning (NCP) to realize fully covered perovskite film.16 Alternatively, incorporation of a dielectric polymer matrix of polyimide precursor dielectric (PIP)29 or poly-(ethylene oxide) (PEO)19, 30 into perovskite film have been demonstrated to effectively block the current leakage. On the other hand, many efforts have been also devoted into reducing the contact barrier between hole or electron injection layer (HIL or EIL) and perovskites, and enhancing the injection of holes and electrons into the perovskite layer. For example, Y. Kim et al.10 and D. Kim et al.31 replaced PEDOT:PSS with a self-organized Buf-HIL and PEDOT:MoO3 HIL, which both facilitates hole injection into perovskite layer and reduces exciton quenching at the HIL/perovskite interface. J. Wang et al. introduced a thin PEI modification layer on top of the zinc oxide (ZnO) layer, the work-function of cathode was reduced and the film morphology was improved, which led to a high EQE of ~3.5% in an infrared PeLEDs.28 And Y. Ling et al. spin-coated a poly(9vinylcarbazole):2-(4-biphenylyl)-5-phenyl-1,3,4-oxadiazole (PVK:PBD) mixing layer on top of the perovskite layer, effectively blocking the voids among perovskites polycrystallines and enhancing the overall charge transporting in PeLEDs.32 Clearly, to further improve the electrical performance of PeLEDs, it is desirable to simultaneously realize the high-quality perovskite film and bipolar charge injection. However, as far as we know, there is no report on simultaneous improvements of film


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morphology and charge injection by engineering the active layer, especially without changing the chemical structure of organometal halide perovskites. In this paper, we report a highly efficient PeLEDs incorporating full coverage of perovskites film and bipolar charge injection, through engineering the active layer MAPbBr3 (CH3NH3PbBr3) which exhibits high solution-processed capability and exciton binding energy (~84 meV)43. The precursor solution was prepared by mixing PVK:1,3,5-Tris(1-phenyl-1H-benzimidazol-2-yl)benzene (TPBi) toluene solution into MAPbBr3 N,N-dimethylformamide (DMF) precursor. The improvements of PeLEDs performance were attributed to two important factors. First, the anti-solvent of toluene accelerates the nucleation rate during the perovskites crystallization process,11, 28 resulting in much smaller MAPbBr3 crystallites and enhanced morphology of perovskites film. Second, the addition of PVK:TPBi matrix will facilitate the bipolar charge injection into MAPbBr3 perovskites layer, avoiding the exciton quenching at HIL/MAPbBr3 and/or MAPbBr3/EIL interface. By carefully adjusting the film morphology and the weight ratio of PVK:TPBi, the optimized PeLEDs exhibited turn-on voltage of ~2.8 V, maximum luminance of ~7263 cd/m2 and EQE of ~2.28% (corresponding to current efficiency of ~9.45 cd/A), which is among the best results based on MAPbBr3 reported by recent literatures16, 18, 20, 24, 38, 39, 44

. It is believed that our experimental results will provide valuable information for the facile

design and development of high-performance PeLEDs. Materials. Methylammonium Bromide (MABr, > 99.99%) and Lead Bromide (PbBr2, > 99.99%) were purchased from Xi’an Polymer Light Technology Corp. DMF, toluene, chlorobenzene and chloroform (anhydrous, > 99.9%) were purchased from Alfa-Aesar. PVK (99.99%, average Mw ≈


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1,100,000) was purchased from Sigma–Aldrich. TPBi (> 99.0%) and cesium carbonate (Cs2CO3) (> 99.99%) was purchased from Jiangsu Fangsheng Light Technology Corp. All these materials were used as received. Precursor solution. The MAPbBr3 solution was prepared by dissolving MABr and PbBr2 with optimal 1.5:1 molar ratio in DMF (For the molar ratio optimization, please see figure S1 in Supporting Information) to give concentrations of ~40 wt%. PVK:TPBi (1:2 w/w, 8.3 mg/mL, 16.6 mg/mL) were dissolved in toluene solution. Both solutions were stirred at room temperature over three hours before use. Then, PVK:TPBi toluene solution was added dropwise into MAPbBr3 DMF solution until saturation. The final volume ratio of PVK: TPBi toluene solution to MAPbBr3 DMF solution is about 1:5. After 30 min stirring, the mixed precursor solution was obtained. PeLEDs fabrication. Patterned indium-tin oxide (ITO, 15 Ω/square) glass substrates were cleaned successively using ethanol, acetone, and detergent water in an ultrasonic bath. After 5 min treatment with UV-Ozone plasma (120 W), PEDOT:PSS (AI4083 CLEVIOS) was spin-coated onto the ITO substrate (4500 rpm/min, 40 s) and baked at 120 °C for 20 min under ambient conditions. The precursor solution was spin-coated onto PEDOT:PSS film in the glove-box (H2O and O2 ≤0.1 ppm) at a speed of 4000 rpm/min for 60 s, and annealed at 80 °C for 10 min (The perovskite films without thermal annealing was compared in figure S2 in Supporting Information). Subsequently, the substrates were transferred into a deposition chamber which was connected to the glove-box. Organic electrontransport layer (ETL) of TPBi with thickness of ~40 nm, and Cs2CO3/Al of ~120 nm were continuously


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deposited by thermal evaporation under high vacuum of ~5×10-5 Pa. The device active area was ~2 × 3 mm2. Characterizations. After fabrication, the PeLEDs were encapsulated immediately in the glove-box. The current density-luminance-voltage (J-L-V) characteristics were carried out by Keithley2400 source Meter and silicon photodiode calibrated by PR670 SpectraColorimeter (the calibrations were described in Supporting Information). The photoluminescence (PL) and electroluminescence (EL) spectra were measured by Hitachi F-2500 spectrometer and PR670 SpectraColorimeter, respectively. The PLQY was measured by fluoroSENS-9000. The surface morphology was investigated by scanning electron microscopy (SEM, JEOL JSM-7100F). The crystallinity characterizations were determined by X-ray diffraction (XRD) patterns characterized by Shimadzu XRD-7000. UV-vis absorption was measured on a Shimadzu UV-2600 spectrophotometer. And the time-resolved PL spectra were recorded by using a time-correlated single photon counting system (FL-TCSPC, Horiba Jobin Yvon) with ~375 nm picosecond pulsed laser. All the measurements were carried out in ambient air at room temperature.


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Figure 1. The scheme of preparations of (a) mixed precursor solution and (b) M-Pero film, (c) The PL spectra and absorption spectra of M-Pero film, the insets show M-Pero film deposited on ITO/PEDOT:PSS substrate without ① and with ① ~365 nm UV-light irradiation, (d) SEM images of the M-Pero film, (e) XRD pattern of the M-Pero film, (f) Time-resolved PL spectra of the M-Pero film (blue) and P-Pero film (red), the insets summarized the parameters used in triexponential decay model. All the PL decay signals have been deconvoluted by IRF signal.

The preparation of mixed MAPbBr3 perovskite (M-Pero) film was summarized in figure 1a and (b). The insets of figure 1c show the M-Pero film (deposited on ITO/PEDOT:PSS substrate) with and without ~365 nm UV-light irradiation. The PL spectra (with ~375 nm excitation) and absorption spectra were shown in figure 1c. It can be seen that the M-Pero film shows a typical green emission peaked around ~534 nm with narrow FWHM of ~24 nm. The PL spectra of M-Pero film and P-Pero film (directly prepared by MAPbBr3 DMF solution) were compared in figure S3 in Supporting


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Information, they were identical to each other except the magnitude, indicating no signatures of confinement effects. The PLQY was estimated of ~44.1% for M-Pero film with excitation of ~375 nm. In addition, the absorption of the MAPbBr3 perovskite film gives three distinct peaks at ~520 nm, ~310 nm and ~236 nm. By comparing the UV absorption spectra of PVK film and TPBi film (See figure S4 in Supporting Information), it was found that the ~520 nm peak was assigned to the absorption of MAPbBr3, and ~ 310 nm and ~ 236 nm peaks can be attributed to absorption of PVK:TPBi mixture. The PL spectra and UV absorption spectra indicated that the addition of PVK:TPBi toluene solution will not lead to the formation of PVK:TPBi exciplex, and does not influence the color purity of PeLEDs. SEM images of the M-Pero film can be seen in figure 1d. It exhibited a quite uniform and dense surface morphology (full coverage) with average grain size of ~500 nm, which was critical to suppress the leaking current loss in the device. XRD pattern displayed in figure 1e gave diffraction peaks at 14.90°, 21.08°, 29.98°, 33.62°, 36.92°, 42.94° and 45.68° which were assigned to the (100), (110), (200), (210), (211), (220) and (300) planes of perovskite structure, respectively.26 The XRD results of M-Pero film and P-Pero film were compared in figure S5 in Supporting Information, they were identical to each other. Moreover, we found our results were consistent with the work carried by P. Kumar et al.,45 and H. Cho et al.,16 from which the lattice constant (a) was estimated to be ~5.945 Å. PL lifetime of M-Pero was determined by time-resolved PL spectra, as shown in figure 1f. For comparison, PL lifetime of P-Pero film was also displayed. Both PL decay curves of M-Pero and PPero can be well fitted by the following triexponential decay model which was summarized in figure 1f:


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−

I = A1e

t

τ1

−

+ A2 e

t

τ2

−

+ A3 e

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t

τ3

(1)

where I was the normalized PL intensity, τ1, τ2 and τ3 were the lifetimes of three decay components, and A1, A2 and A3 were the fractions of the three decay components. The three decay components can be denoted as τ1 (fast), τ2 (middle) and τ3 (slow). According to the study by K. Zheng et al.,46 the τ3 (slow) decay component was related to radiative recombination inside the grains, while the τ1 (fast) and τ2 (middle) decay component were attributed to “two kinds of trap-assisted recombination at grain boundaries.” As can be seen, PL lifetime of M-Pero film in both short and long time regime become longer. P. Kumar et al.45 using photothermal deflection spectroscopy (PDS) technique to estimate the energetic disorder of MAPbBr3 perovskite film. And they found a considerable decrease (more than an order of magnitude) in sub-band gap PDS absorption for antisolvent-treated films, as compared to that of untreated films. These results indicated that the antisolvent treatment can significantly reduced nonradiative recombination rate by decreasing the defect density, and therefore improved the PL yields and enhanced PL lifetimes. It can be expected that such good film morphology and enhanced optical properties of the M-Pero film will benefit the EL performance of PeLEDs.


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Figure 2. (a) The cross-section SEM image of the PeLEDs with M-Pero film, (b) The layered architecture of the PeLEDs, (c) energy level diagram of each layer in PeLEDs, (d) EL spectra of the PeLEDs under different driving voltages, the insets show the photographs of the device working at 4 V and 6 V, (e) J-L-V characteristics of the PeLEDs, (f) Current Efficiency and EQE versus Voltage of the PeLEDs.

Using the high-quality M-Pero film as light emitters, bright and efficient PeLEDs was created. The device

adopted

a

typical

layered

architecture

of

ITO/PEDOT:PSS/PVK:MAPbBr3:TPBi/

TPBi/Cs2CO3/Al. The cross-section SEM image of the PeLEDs was displayed in figure 2a, corresponding to the architecture in figure 2b. It was difficult to distinct the edges of PEDOT:PSS and TPBi layers because the PVK:MAPbBr3:TPBi layer was much thicker than PEDOT:PSS and TPBi layers. The energy level diagram of each layer was shown in figure 2c, and all energy level values were taken

from

literature.5

Under

electrical

excitation,

the

PeLEDs

emitted

bright

green

electroluminescence with excellent color purity (centered at ~534 nm with FWHM of ~24 nm), as


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shown in figure 2d. It should be noted that PL and EL spectra overlapped with each other very well (See figure S6 in Supporting Information), indicating the emission of the PeLEDs merely came from MAPbBr3 perovskite. From the J-L-V characteristics in figure 2e and 2f, we achieved the turn-on voltage of ~2.8 V (determined as the driving voltage at ~1 cd/m2), the maximum luminance of ~7263 cd/m2, the maximum current efficiency of ~9.45 cd/A (at current density of ~24.7 mA/cm2) and the maximum EQE of ~2.28% (the EQE was calculated referred to Ref. [47]). To evaluate the reproducibility of PeLEDs performance, four devices were fabricated in a single batch at one time with the same emissive layer (the inset of figure 2d). The device performances were summarized in Table S1, which show small fluctuations among these devices. And we selected the champion value for exhibition. To the best of our knowledge, our results are one of the most efficient PeLEDs based on MAPbBr3 which have been demonstrated so far (Table S2)16, 18, 20, 24, 38, 39, 44. The excellent EL performance of PeLEDs can be attributed to the good morphology of M-Pero film and efficient bipolar charge injection, which were related to the addition of toluene solution and PVK:TPBi matrix, respectively, and would be discussed in detail below:


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Figure 3. SEM images of M-Pero film with (a) PVK:TPBi DMF solution, (b) PVK:TPBi toluene solution, (c) PVK:TPBi chlorobenzene solution, (d) PVK:TPBi chloroform solution. (e) J-V characteristics and (f) L-V characteristics of the PeLEDs with M-Pero films mentioned above, (g) Current Efficiency and (h) EQE versus Voltage of the PeLEDs.

The influence of toluene on MAPbBr3 perovskite film morphology can be seen by comparing figure 3a and 3b. As shown in figure 3a, M-Pero film with PVK:TPBi DMF solution composed of isolated crystallites with grain size larger than ~1 µm. The film coverage was quite low (only ~50% estimated from SEM images by using ImageJ software26), resulting in tremendous pinholes for leaking current. When PVK:TPBi toluene solution was added into the MAPbBr3 solution, the grain size decreased dramatically from ~1 µm to ~500 nm, and M-Pero film became dense and uniform. From figure 3b, nearly full coverage of MAPbBr3 film was obtained. The improvement of morphology can be understood by accelerated crystallization process when decreasing the solution solubility.48 DMF and toluene are good and anti-solvents for MAPbBr3 perovskite, with MAPbBr3 solubility higher than ~0.2


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g/mL and smaller than ~10−7 g/mL, respectively.11 However, DMF and toluene were mutual soluble. After mixing DMF with toluene, the solubility of MAPbBr3 decreased dramatically, producing a highly supersaturated state in the mixed solvent and then inducing faster crystallization during the film formation. Fast crystallization deposition (FCD) method has been demonstrated as an effective method to achieve dense and uniform perovskite film.16, 27, 28, 48 The SEM results indicated that our M-Pero film should be “polycrystalline film” (the perovskite size was usually in the range of hundreds of nanometers) instead of nanoparticles (the perovskite size was usually in the range of several nanometers) embedded in PVK: TPBi matrix. Moreover, we have conducted a comparative experiment that only mixing toluene in perovskite precursor solution (DMF/toluene mixture without PVK and TPBi). The results are shown in figure S7 in Supporting Information. The morphology (figure S7a) was most identical to that of DMF/toluene mixture with PVK: TPBi, but the PeLEDs show lower maximum luminance of ~196.8 cd/m2, and lower maximum current efficiency of ~1.4 cd/A than the PeLEDs with PVK: TPBi which exhibited maximum luminance of ~7263 cd/m2 and maximum current efficiency of ~9.45 cd/A. To verify our conjecture of improved morphology by adding anti-solvent, another two anti-solvents of chlorobenzene and chloroform were also mixed into MAPbBr3 solution, respectively. As shown in figure 3c and 3d, similar to the results of toluene solution, morphologies of MAPbBr3 perovskite film with high quality were obtained by chlorobenzene (grain size ~300 nm, ~83% coverage) and chloroform (grain size ~550 nm, ~90% coverage). Chlorobenzene and chloroform were also mutual soluble to DMF. The solvent polarity order is: toluene

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