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Abstract
Solution processed organic-inorganic hybrid perovskites are emerging as a new generation materials for optoelectronics. However, the electroluminescence is highly limited in light emitting diodes (LED) due to the low exciton binding energy and the great challenge in stability. Here, we demonstrate a super air stable quasi-two dimensional perovskite film employing hydrophobic fluorine-containing organics as barrier layers, which can store in ambient for more than 4 months with no change. The dramatically reduced grain size of the perovskite crystal in contrast to three dimensional (3D) perovskites was achieved. Together with the natural quantum well of quasi-two dimensional perovskite confining the excitons to recombination, the LED exhibited the maximum luminance of 1.2 × 103 cd/m2 and current efficiency up to 0.3 cd/A, which is twenty fold enhancement than that of LED based on 3D analogues under the same condition.
© 2017 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The solution-processed organic-inorganic hybrid three dimensional (3D) perovskites have emerged as a new generation of optoelectronic materials since 2009. The power conversion efficiencies (PCEs) of the perovskite solar cell (PSC) have boosted from 3.8% to the certified 22.1% within the past 8 years. The fast progress is due to the merits of long-range balanced charge carrier diffusion lengths, low trap density, high charge carrier mobility, and low exciton binding energy [1]. Unprecedentedly, these metal-halide perovskite have also demonstrated great potential for using in light emitting diodes (LED), due to their direct band gaps and high photoluminescence quantum efficiencies (PLQEs). Furthermore, the emission color of these perovskites can be easily tuned across the whole visible range (from 390 nm to 800 nm) by simply tailoring the halide composition in the deposition precursors. Since the first demonstration of 3D perovskite LED (PeLED) [2], the optimization methods of interface engineering [3] and perovskite morphology control [4, 5] have been tried. The performance of PeLEDs was significantly improved on a certain degree. However, the small exciton binding energy resulted slow free electron-hole bimolecular radiative recombination and poor stability fundamentally limit the progress of 3D PeLEDs [6, 7].
In order to further overcome these problems, a class of quasi-two dimensional (2D) layered perovskites has recently emerged, generally known as L2MAn-1PbnX3n-1, in which the inorganic layers [PbX6]4- octahedral is separated by long organic chain (L) barrier layers with periodic array. In the quasi-2D layered perovskites, electrons and holes can be confined in inorganic layers to recombination due to the nature of quantum well structure [8, 9], leading to increased exciton binding energy and reduced exciton diffusion distance. In comparison to 3D analogues, the key of quasi-2D layered perovskites is the introduction of long chain organic molecules. Therefore, various long chain organic molecules have been explored to construct quasi-2D perovskites for high performance LEDs. For instance, phenylethyl ammonium (PEA) was first used [10] in both near-infrared (PEA2MAn−1PbnI3n + 1) and green emission (PEA2MAm−1PbmBr3m + 1) PeLEDs with external quantum efficiency (EQE) of 8.8% and [11] maximum luminance of 2935 cd/m2 [12], respectively. Similarly, 2-phenoxyethylamine (PEOA) was employed in green emission (PEOA2MAn–1PbnBr3n + 1) PeLEDs with EQE of 2.82% [13]. Recently, Wang et. al. have achieved red emission PeLEDs with highest EQE of 11.7% based on 1-naphthylmethylamine (NMA) with layer number of n = 2, (NMA)2FAPb2I7 [14]. Although high performance has been achieved, the film stability of quasi-2D layered perovskites, which is necessary in achieving long-term stability of devices, have not been demonstrated yet.
In this work, we demonstrate an super air stable quasi-2D perovskites (TFA)2MAn-1PbnBr3n + 1, by adding 3,4,5-Trifluoroaniline (TFA) hydrobromide (C6H2F3NH2HBr, TFABr) into CH3NH3PbBr3 (MAPbBr3) precursor solution. We found that the stability of (TFA)2MAn-1PbnBr3n + 1 film was dramatically improved with no change in XRD pattern in 2688 hours in air, because of the hydrophobicity of F atom and the formation of intermolecular hydrogen bond to further stabilize the quasi-2D perovskite structure. The addition of organic macromolecule TFA promoted the formation of uniform and dense (TFA)2MAn-1PbnBr3n + 1 quasi-2D perovskite films with reduced grain size, which thereby increased the radiative recombination. Based on this strategy, maximum luminance of 1.2 × 103 cd/m2 and current efficiency of 0.3 cd/A at layer number of n = 3 were achieved in our PeLEDs, which is much higher than device based on 3D CH3NH3PbBr3 perovskite.
2. Method
2.1 Synthesis and materials preparation
TFABr was synthesized by mixing 3,4,5-Trifluoroaniline (Alfa Aesar) and aqueous HBr (45 wt% in water, Alfa Aesar) with the 1:1.1 (mol:mol) ratio at 0 °C with constant stirring. The reaction took over 2 hours. The resulting white precipitate was collected by evaporating the solvent with rotary evaporator, then purified by dissolving in ethanol and recrystallized. The TFABr was washed with diethyl ether several times to remove the unreacted HBr stabilizer and then dried at 60 °C in a vacuum oven for 12 h. The other precursor materials (MABr: methylamine bromide; PbBr2: lead bromide) were purchased from Xi’an polymer light technology corp.
Perovskite precursor solutions were prepared by dissolving TFABr, MABr, and PbBr2 with a molar ratio of 2:1:2, 2:2:3, 2:3:4, 2:4:5 in DMF (30 wt%) corresponding to n = 2, n = 3, n = 4, and n = 5, respectively, followed by stirring at 60 °C for 2 h in a nitrogen-filled glove-box.
2.2 Device fabrication
Glass substrates coated with indium tin oxide (ITO) were sonicated sequentially in detergent, deionized water, acetone and ethanol for 15 min in sequence and bake at 120 °C for 12 h after dried with a N2 gun. Substrates were exposed to UV light for 15 min for ozone treatment, then the PEDOT:PSS (4083, Heraeus) aqueous solution was spin-coated on the substrate at 5,000 rpm for 50 s and annealed at 120 °C for 20 min in ambient. Then the substrates were transferred into the glove-box. Next, a solution of PVK in chlorobenzene at a concentration of 2 mg ml−1 was spin-coated above on PEDOT:PSS film at 5,000 rpm for 50 s, followed by thermal annealing at 150 °C for 15 min. Then the substrates were rinsed twice by DMF. The perovskite films were prepared by spin-coating at 4000 rpm for 20 s, and annealing at 75 °C for 10 min. Finally, the electron transport layer TPBi, the modified layer LiF and the electrode Al were evaporated on the perovskite film to the thickness of 45 nm, 2 nm and 100 nm, in the speed of 1 Ȧ/s, 0.14 Ȧ/s and 7 Ȧ/s, respectively, under the pressure of 5 × 10−4 Pa.
3. Characterization
PeLED device was measured at room temperature in N2-filled glove-box, and tested on the top of integration sphere, which collected the forward light emission. The fiber integration sphere (FOIS-1) coupled with a QE65 Pro spectrometer was used to characterize the emission light. The dependence of bias voltage and current density was measured by Keithley 2400 source meter.
The SEM (JEOL, JSM-7800F) and AFM (non-contact mode, Park XE7) measurement was performed based on the structure of ITO/PEDOT:PSS/PVK/perovskite. The XRD (Smartlab (3KW)) measurement was performed based on Glass/perovskite. UV-visible absorption spectrum of perovskite films on quartz glasses was collected by an ultraviolet spectrophotometer (UV-1750, SHIMADZU) in room temperature. PL spectrum was measured by a fluorescence spectrophotometer (F-4600, HITACHI) with a 200 W Xe lamp as an excitation source.
4. Result and discussion
The organic amine studied in this work was shown in Fig. 1(a). Figure 1(b) gave the schematic diagrams of unit cell structure of quasi-2D layered perovskites according to the chemical formula of TFA2MAn-1PbnBr3n + 1 (n = 2-5) in our case, where the n values is equal to the number of octahedral inorganic lead bromide layers separated by the two TFABr layers. Generally, in order to obtain the perovskites with different layers, different molar ratio of raw materials was prepared. To the best of our knowledge, it should be noted that it is extremely difficult to obtain the specific layered perovskite along with the various n values even using accurately controlled synthetic method [15].
Fig. 1 a) Structures of TFA molecule. b) Schematic diagrams of unite cell structure of (TFA)2MAn-1PbnBr3n + 1 quasi-2D layered perovskite, where n refers to the number of layers from n = 2 to n = 5, The TFA spacer layers terminated the perovskite layers. c) SEM and d) AFM images of perovskite films with different layers from n = 2 to n = 5.
Figures 1(c) and 1(d) show the scanning electron microscopy (SEM) and the corresponding atomic force microscopy (AFM) images of layered perovskite films with different n values. With small n value (n = 2 and n = 3), the perovskite film possesses good film quality with high film coverage and small surface roughness. The implication of lower number layers infers that more TFABr was added into the precursor solution. Meanwhile, the grain size of the perovskite film decreases with the n value. The grain size of the n = 2 film is much smaller than that of the n = 5 film (Figs. 1(c) and 1(d)), which is similar to the 3D analogues (Figs. 2(a) and 2(b)). The root-mean-square (r.m.s) roughness was determined to be 4.038, 7.591, 10.095, 15.100 and 27.200 for the film with n value of 2, 3, 4, 5, and (3D), respectively. We speculate that the addition of TFABr into precursor solution may have strong interaction with the methylamine bromide (CH3NH3Br, MABr) and lead bromide (PbBr2), accelerating the crystallization, and thus leading to the small grain size. Finally, the light emission of the films would be enhanced because of the quantum confinement effect [16].
The reduced grain size of the layered perovskite films with smaller n value can also be confirmed from the X-ray diffraction (XRD) patterns, as show in Figs. 3(a) and 3(b). From the FWHM (n = 2, 0.22°; n = 3, 0.18°; n = 4, 0.13°; n = 5, 0.12°) of the prominent diffraction peaks in Fig. 3(a) or the enlarged curved of the Fig. 3(b), the average grain size of ~36, ~44, ~61, and ~66 nm for perovskite films with layer of n = 2, n = 3, n = 4, and n = 5, respectively, were estimated by the Scherrer equation [17, 18]. The result was consistent with the conclusion from SEM and AFM. In addition, the XRD measurement was carried to investigate the formation of the perovskite films. Two obvious diffraction peaks was found at 15.0 ± 0.1° and 30.2 ± 0.1°, which can be assigned to the (100) and (200) planes, respectively. And two sensibly less intense diffraction peaks at 34 ± 0.1° and 46° could be associated with the (210) and (300) lattice planes, respectively. These observations are nearly in agreement with the conditions of 3D analogues in previous report [19]. In the case of pure 2D perovskite (TFA2PbBr4), we also perceived the obvious (100) plane and (200) plane at 15.02° and 30.26°, respectively (Fig. 4). These could be due to the strong interaction between the TFA+ and inorganic cages, affecting the distortion of the Pb-I framework, which induced the crystal growth in the flat of <110>-oriented zigzag structure, which was reported previously [20]. As a result, an intensive competition between the TFA+ and MA+ in the crystallization process of layered perovskites. Therefore, we inferred that there were not only one phases in the layered perovskite films, which can be proved by the tiny diffraction peaks (where the red arrows point in Fig. 3(a)), which gradually disappeared with the increase in the number of layers. Two main XRD peaks of (100) and (200) was magnified in Fig. 3(b) to show the shift of diffraction angle with the changes of the number of layers (The dashed line in Fig. 3(b) is the vertical line from the peak to the X axis.). Along with the decrease in the number of layers, the peaks migrate to the left, indicating the increase of lattice constant, which is consistent with the more TFA in the unit cells. Most importantly, the stability of layered perovskite films was remarkably improved compared to 3D analogues because of the hydrophobic property of the F element. The layered perovskite films of n = 3 was stored in ambient for more than 4 months under the humidity of 60% ( ± 10%) and temperature of 30 °C ( ± 10 °C). There are no any diffraction peaks of primitive MABr and PbBr2 four months later. Together with the hydrophobicity of F atom and the stronger bond of C-F than C-H, we proposed that F substitution and the formation of intermolecular hydrogen bond C-H—F-C can significantly stabilize the crystal structure, which improve the stability of the layered perovskite films. The UV-vis absorption spectra of the layered perovskite films from n = 2 to n = 5 were shown in Fig. 3(d). The UV-vis absorption spectra measured in room temperature showed two distinct absorption peaks at about 325 nm and 525 nm, respectively, which implied the existence of layered perovskite or the mixture of 2D and layered perovskite by the reason of the characteristic absorption peak at about 325 nm for n = 1 (Fig. 4).
Fig. 3 a) XRD patterns of the layered perovskites with different n values from n = 2 to 5. The (100), (200), (210) and (300) labels correspond to preferred diffraction planes. b) Magnification of XRD profile based in the (100) and (200) characteristic peaks. c) Stability measurements on (TFA)2MA2Pb3Br10 (n = 3) layered perovskite films, which was fabricated in glove box and placed in the atmosphere for more than four months at the temperature of 30 °C ( ± 10 °C) and the humidity of 60% ( ± 10%). d) UV-vis absorption of layered perovskite films on quartz substrate from n = 2 to n = 5.
Our perovskite light emitting diodes (PeLED) devices with green emission were fabricated with a typical planar heterojunction structure (Fig. 5(a)) of glass/indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) /Poly(9-vinylcarbazole)(PVK)/(TFA)2MAn-1PbnBr3n+1/1,3,5-Tris(1-phenyl-1H-benzimidazol-2-yl)benzene (TPBi) (40 nm)/LiF (1 nm)/Al (100 nm). The perovskite film was sandwiched between two semiconductor materials PVK and TPBi with large bandgap to confine the injected charges and then enhance the light emission. PEDOT:PSS was deposited onto an ITO-coated glass substrate by spin-coating from solution, sever as an efficient hole injector. Afterwards, the PVK was deposited on the top of PEDOT:PSS used to transport holes, as well as blocking electron in the PeLED. The thin perovskite film was spin coated onto the PVK by the solution process after twice rinsed by DMF as green light emitter. The perovskite film was covered with a 40 nm layer of TPBi which was chosen to work as an electron transport layer. The LiF sever as the buffer layer of electrode to enhance the carrier injection equilibrium and device stability. The energy level diagram corresponding to the device structure was shown in Fig. 5(b). The energy level of the different layer perovskite may change because of the TFABr incorporation whereas these changes are expected to be small.
Fig. 5 a) Schematic illustrations of device architecture. Al, aluminum; LiF, lithium fluoride; TPBi, 1,3,5-Tris(1-phenyl-1H-benzimidazol-2-yl)benzene; PVK, Poly(9-vinylcarbazole); PEDOT:PSS, poly(3,4-ethylenedioxythiophene) polystyrene sulfonate; ITO, Indium tin oxide. b) Energy band diagram of PeLEDs. The energy level values of the layers were taken from the literatures [5,21].
The current density-voltage-luminance (J-V-L) characteristics of the PeLEDs based on the layered perovskite from n = 2 to n = 5 which contrast with the 3D perovskite were presented in Fig. 6(a). It is obviously that the current density of the 3D perovskite is much larger than the layered perovskite, which is attributed to the small binding energy, better carrier mobility, and poor film coverage of the 3D perovskite. The turn on voltages of the layered PeLED devices were approximately at about 3.2 - 3.8 V, which may be attributed to the composition of layered perovskite with different interface properties. The luminance increases rapidly with increasing the voltage. A maximum value of 1200 cd/m2 was achieved under the drivingvoltage of 9 V (n = 3). The highest current efficiency corresponding to the maximum luminance achieved to 0.3 cd/A (Fig. 6(b)). Further increasing the layers resulted in the decay of device performance, which due to the poor coverage of the perovskite films and large amount of pin holes, resulting in an obvious leakage current. The EL spectra were shown in Fig. 6(c), centered at about 532 nm with especially narrow FWHM (full width at half maximum) of 22 nm. It is noted that the EL is red shift as the increase of layers. Besides, no any distinct EL derived from the transport layer is detected in the whole EL spectrum under different voltage. The color coordinates of the Commission Internationale de l’ Eclairage (CIE) is (0.21,0.73), exhibited relatively saturated and pure color, as show in Fig. 6(d).
Fig. 6 Optoelectronic characteristics of layered PeLED. a) J-V-L characteristics. b) Current efficiency versus voltage characteristics. c) EL spectra of devices. d) Color coordinates of the EL of n = 3 PeLEDs in the CIE chromaticity diagram.
5. Conclusion
In summary, we have investigated layered perovskites with TFA as a space layer for improved stability in the atmosphere and high performance PeLEDs. The grain size of the layered perovskite crystallites was dramatically reduced, and the quality of the layered perovskite films was sharply improved, which finally enhanced the performance of the corresponding devices. The maximum luminance of 1200 cd/m2 and the current efficiency of 0.3 cd/A were achieved. The device performance can be further improved regarding to the interface engineering. It is worth to note that fluorine-containing organics for layered perovskites can improve the air stability and reduced the roughness of the perovskites. We believe that the excellent film stability using fluorine-containing organics offers a new pathway for the development of high-performance and stable layered PeLEDs.
Funding
National Key R&D Program of China (Grant 2017YFB1002900); the National Basic Research Program of China- Fundamental Studies of Perovskite Solar Cells (2015CB932200); the Natural Science Foundation of China (Grant 51035063, 61605073),; Natural Science Foundation of Jiangsu Province, China (Grants 55135039 and 55135040); Jiangsu Specially-Appointed Professor program (Grant 54907024); Startup from Nanjing Tech University (Grants 3983500160, 3983500151, and 44235022); Young 1000 Talents Global Recruitment Program of China, “Six talent peaks” Project in Jiangsu Province China (Grant No. 51235014); the Science and Technology Development Fund from Macau SAR (FDCT-116/2016/A3, FDCT-091/2017/A2); Start-up Research Grant (SRG2016-00087-FST) from Research & Development Office at University of Macau.
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