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Highly oriented perovskites for efficient light-emitting diodes with balanced charge transport
Journal Pre-proof Highly oriented perovskites for efficient light-emitting diodes with balanced charge transport Haiyan Du, Wei Hui, Lianqi Tang, Jian Qiu, Tingting Niu, Yingguo Yang, Xingyu Gao, Yingdong Xia, Yonghua Chen, Wei Huang PII:
S1566-1199(19)30556-7
DOI:
https://doi.org/10.1016/j.orgel.2019.105529
Reference:
ORGELE 105529
To appear in:
Organic Electronics
Received Date: 24 May 2019 Revised Date:
26 September 2019
Accepted Date: 27 October 2019
Please cite this article as: H. Du, W. Hui, L. Tang, J. Qiu, T. Niu, Y. Yang, X. Gao, Y. Xia, Y. Chen, W. Huang, Highly oriented perovskites for efficient light-emitting diodes with balanced charge transport, Organic Electronics (2019), doi: https://doi.org/10.1016/j.orgel.2019.105529. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Highly Oriented Perovskites for Efficient Light-Emitting Diodes with Balanced Charge Transport Haiyan Du,1 Wei Hui,1 Lianqi Tang,1 Jian Qiu,1 Tingting Niu,1 Yingguo Yang,2 Xingyu Gao,2 Yingdong Xia,1,* Yonghua Chen,1 and Wei Huang1,3,4 1
Key Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials (IAM), Jiangsu National Synergistic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University (NanjingTech), 30 South Puzhu Road, Nanjing 211816, P. R. China. 2 Shanghai Synchrotron Radiation Facility, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201204, P. R. China. 3 Shaanxi Institute of Flexible Electronics (SIFE), Northwestern Polytechnical University (NPU), 127 West Youyi Road, Xi'an 710072, China. 4 Key Laboratory for Organic Electronics & Information Displays (KLOEID), and Institute of Advanced Materials (IAM), Nanjing University of Posts and Telecommunications, 9 Wenyuan Road, Nanjing 210023, China. E-mail: [email protected]
1
Abstract Ruddlesden-Popper (RP) perovskites have attracted interests due to high performance in perovskite light-emitting diodes (PeLEDs). However, the insulating organic spacers decrease carrier mobility, and increase charge accumulation and non-radiative recombination losses, which significantly undermine device performance. Herein, by blending NH3I(CH2)8NH3I (ODA) and INH3(CH2)2O(CH2)2O(CH2)2NH3I (EDBE) large organic spacers into the precursor solution, we achieve preferential orientation of perovskite crystals perpendicular to the substrate with conductive channels across the two injecting electrodes. Consequently, effective charge injection and balanced charge transport are achieved, which is beneficial to increase radiation recombination. A low turn-on voltage of 1.4 V is achieved for mixture-1.5 perovskite (ODA: EDBE=3:2, molar ratio) PeLEDs, and a maximum external quantum efficiency increases to 5.8% (EL peak ≈766 nm) at 2.0 V with a current density of 6.05 mA cm−2 compared to that of the pure ODA (2.4%) and EDBE (1.1%) devices. The findings may spur new developments in charge injection and electron-hole balance for realizing efficient PeLEDs.
2
1. Introduction Organometal halide perovskites have attracted significant attention in many fields [1-5]. Among them, light-emitting diodes (LEDs) have been demonstrated to have great potential due to their high color purity (full width at half maximum ~ 20 nm) irrespective of the crystal size, tunable optical bandgap from visible to infrared regions, and solution processability [6-13]. Many strategies have been reported to overcome the disadvantages of organic LEDs (e.g., complex synthesis, high cost, and poor color purity) [13] or inorganic LEDs (e.g., high cost of fabrication, and poor device flexibility) [14]. For perovskite LEDs (PeLEDs), structural engineering and energy manipulation have been put forward to realize high efficiency and stability. However, 3D perovskites usually suffer from long exciton diffusion length and small exciton binding energy, which limits the efficiency improvement by dissociating excitons and decreasing electron-hole capture rates for radiative recombination [7, 15-17]. Dimensionality reduction of 3D perovskites is an effective way for high performance PeLEDs. The Ruddlesden-Popper (RP) perovskites, which fall between 2D and 3D perovskites and combine each of their merits, can be obtained through slicing 3D perovskites across di erent planes by inserting large organic ammonium cations in-between inorganic components [18, 19]. The structure of RP perovskite can be recognized as a multiple quantum well in which the organic ammonium cations act as barriers and the inorganic perovskite sheets are as wells [17, 20, 21]. The large organic spacer cations are bonded to the inorganic layers by electrostatic interactions between the ammonium groups and the halide anions [22]. The charge carriers are confined within the organic components to ensure the radiative recombination at the low excitation density [23]. The high performance PeLEDs have been reported in infrared, red, green, and blue emission [5, 24-26]. However, the unique crystal structure of RP perovskites also suffers from an unfavorable issue that the insulating organic spacers decrease carrier mobility, increase charge accumulation and non-radiative recombination losses, and hence significantly undermine device performance [27, 28]. 3
Recently, Gao et al incorporated CH3NH3Cl additives into perovskite precursor solution to achieve vertically oriented perovskite crystals, which facilitated efficient charge injection and transport in the vertical direction without impediment by the insulating organic layers [29]. Similarly, Mohite et al proposed that the vertically aligned perovskite thin films can form conductive channels across the two injecting electrodes, leading to higher radiative recombination efficiency at the same current densities. Besides, they also proved that the imbalanced electron and hole injection in the high injection region would lead to low quantum efficiency at high voltage [14]. Therefore, further exploration of effective organic spacers to facilitate charge injection and balanced charge transport in RP perovskite films is essential for efficient PeLEDs. Many researches have been focusing on this issue. It is demonstrated that INH3(CH2)2O(CH2)2O(CH2)2NH3I (EDBE) can enforce the vertical growth of perovskite crystals by forming hydrogen bonds with another adjacent EDBE molecule [30]. Meanwhile, NH3I(CH2)8NH3I (ODA) has been shown to play a critical role in passivating perovskite surface and grain boundaries [31]. Moreover, introducing a second spacer cation into the precursor solution would promote the vertical growth of the perovskite film and improve the charge transport [32,33]. For example, when BA+ and PEA+ co-work to form [(BA0.5PEA0.5)2FA3Sn4I13] 2DRP perovskites, the intermediate phase which impedes the homogeneous and ordered nucleation of the crystal would be suppressed e ectively, thus enabling a high-quality perovskite film and improved crystal orientation [33]. Here, we demonstrated an effective approach to achieve efficient charge injection and balanced charge transport by mixing two large organic diammonium spacers (ODA and EDBE). Compared to pure ODA and EDBE based perovskite films, the mixture-1.5 perovskite films (ODA: EDBE=3:2, molar ratio) show preferential orientation with the perovskite frameworks perpendicular to the substrate, which facilitates efficient carrier injection and balancing charge transport. Additionally, the mixture-1.5 perovskite film also exhibits high surface coverage with less pinholes and smaller crystals, leading to decreased leakage current and strong photoluminescence. Finally, a highly efficient PeLED is achieved with a low turn-on voltage of 1.4 eV 4
and a high external quantum efficiency (EQE) of 5.8% (EL peak ≈766 nm). 2. Experimental Section 2.1 Preparation of perovskite precursor solutions According to structural formula of (NH3RNH3)(MA)n-1PbnX3n+1, the perovskite precursor solution was prepared by mixing PbI2, MAI and different molar ratios of ODA and EDBE (molar ratio, 0:1, 3:2, 1:0) in methylamine acetate with a concentration of 180 mg mL-1. The methylamine acetate is an environmentally friendly room-temperature molten salt, which has been reported to be an alternative to traditional toxic and highly coordinating solvents for preparing high-quality perovskite thin films. [34] The solution is clear and colorless after stirring at 60 °C for 2 h (Figure S1). The EDBE was synthesized by stirring Hydriodic acid and 2,2-(ethylenedioxy)bis(ethylammonium) with a molar ratio of 2:1 in an ice bath for 2 h. The solution was then evaporated at 65 °C to obtain the EDBE precipitate, which was washed three times with diethyl ether and then dried under vacuum. 2.2 Devices Fabrication The indium tin oxide (ITO) substrates were cleaned sequentially with detergent, de-ionized water, acetone, and iso-propanol, followed by drying with N2 flow and UV-ozone treatment for 20 min. ZnMgO nanocrystals (purchased from Guangdong Poly OptoElectronics Co., Ltd) were spin-coated onto the ITO-coated glass substrates at 3000 rpm for 30 s, and then annealed in air at 120 °C for 30 min. Perovskite precursor solution was spin-coated at 4000 rpm for 20 s at a constant substrate temperature during the whole spin-coating process. Note that the whole preparation process was under ambient conditions, followed by annealing at heating table. The thickness of the three perovskite films is close to 180 nm. The TFB layers were deposited at 2000 rpm for 40 s from an m-xylene solution (14 mg ml-1) in a glovebox. Finally, the device was prepared by thermal evaporation of MoO3 and Au at a pressure of 1×10-4 Pa. The area of each device is 0.05 cm2, as defined by the overlap of the ITO and the thermally evaporated Au. 2.3
Characterization:
All devices were tested in an N2-filled glovebox at room temperature using a Keithley 5
2400 source meter and a QE65 Pro spectrometer. A field emission SEM (Hitachi S-4300) was used to acquire SEM images. The instrument uses an electron beam accelerated at 10-30 kV, enabling operation at a variety of currents. The crystal structure of the perovskite films was analyzed using an X-ray diffractometer, Bruker AXS D8, with CuKa radiation. The Grazing incidence wide angle x-ray scattering (GIWAXS) data were obtained with beamline BL14B1 at Shanghai Synchrotron Radiation Facility (SSRF). Ultraviolet-visible spectra were measured by a SHIMADZU ultraviolet-3600 spectrophotometer. The TRPL was collected using an Optronis Optoscope streak camera system which has an ultimate temporal resolution of ∼10 ps. Ultraviolet–visible absorbance spectra were recorded on an ultraviolet– visible spectrophotometer with an integrating sphere (Cary 5000, Agilent). PL spectra of the perovskite films were measured at room temperature using a fluorescent spectrophotometer (F-4600, HITACHI) with a 200 W Xe lamp as an excitation source. SCLC measurements were carried out with scan range from 0 to 5.0 V. 3. Results and discussion We mixed MAI, PbI2 and different molar ratios of ODA and EDBE (0:1, 3:2, 1:0) to prepare the perovskite precursor solutions with the stoichiometric molar ratio of 3:4:2. The
obtained
perovskites
of
(EDBE)Pb4MA3I13,
(ODA)Pb4MA3I13,
and
[(ODA)0.6(EDBE)0.4]Pb4MA3I13 are abbreviated as EDBE, ODA, and mixture-1.5, respectively. In this work, all perovskite films were fabricated by hot-casting method. The vertical orientation of the perovskite film is essential for highly efficient PeLEDs for the fact that the insulating nature of the organic layers would inhibit the charge carrier transport. Grazing-incidence wide-angle X-ray scattering (GIWAXS) measurements were first performed to measure the orientation of the perovskite thin films, as shown in Fig 1a-c. The ODA perovskite films clearly show the Debye-Scheler rings with a few Bragg spots, indicating that the grain orientation is random with a few grains perpendicular to the substrate. The three low-angle diffraction peaks between qz = 0-10 nm are observed, which demonstrates the formation of the 2D phases [35]. In contrast, sharp and discrete Bragg spots are observed along certain extended arc segments in the EDBE and mixture-1.5 films, 6
indicating a strong vertical orientation of the grains with respect to the substrate [29, 36, 37]. Much stronger and sharper discrete Bragg spots are achieved in mixture-1.5 perovskite film than that of the EDBE film, suggesting that the highly oriented crystal growth can be controlled by mixing the two large cations. We also observe a transformation from the ring-like Debye Scherer patterns (Figure S2) at room temperature to discrete Bragg spots under hot-casting deposition, which is consistent with previous reports [14] that the temperature is a key factor to assist preferential crystal orientation of the thin film. It should be noted that the 2D phases are observed in both EDBE and mixture-1.5 perovskite films, which implies a challenge to obtain pure RP perovskites with fixed layers. Furthermore, a pole figure is presented in Figure S3 where the mixture-1.5 perovskite film only shows discrete Bragg spots at 90° without noticeable peaks along the same rings, demonstrating the strong preferential grain orientation. The efficient charge injection and transport can be expected in the vertical direction by the formation of continuous charge-transport channels through the conducting inorganic layers in mixture-1.5 perovskite film over the pure ODA and EDBE perovskite films.
7
Figure 1. GIWAXS maps and schematic figures of the crystalline orientation for (a) ODA, (b) mixture-1.5 and (c) EDBE films.
Poor surface morphology and coverage cause non-radiative recombination losses, which is one of the critical problems that degrade the luminescent efficiency in planar PeLEDs, especially when the perovskite layers are deposited using a solution process [38]. The ODA, EDBE and mixture-1.5 perovskite films are deposited on ITO/ZnMgO substrate and investigated by scanning electron microscopy (SEM), as shown in Figure 2a-c. Numerous pinholes are observed on the ODA perovskite film surface, which may result in a direct contact between electron and hole transport layers, leading to severe current leakage and significant degradation of the device efficiency. For EDBE perovskite film, besides large pinholes, the surface is uneven with accumulating crystals, which can be acted as charge injection barriers due to the poor contact. In comparison, a dense and even perovskite film with less pinholes is obtained by mixing ODA and EDBE spacers (mixture-1.5), which is beneficial to spatially confine electrons and holes to promote bimolecular radiative recombination 8
and achieve high performance device [39-40].
Figure 2. Top-view SEM images of the perovskite films based on (a) ODA, (b) mixture-1.5 and (c) EDBE.
To further justify the crystal formation, X-ray di raction (XRD) was conducted (Figure 3a). For all perovskite films, two dominant planes are observed at diffraction angles (2θ) of 14.20° and 28.48°, which indicate the diffraction planes (111) and (202). The diffraction planes (111) and (202) have been reported to indicate the highly oriented inorganic layers perpendicular to the substrate in the RP perovskites [41, 42]. Combined with the GIWAXS results and the applied hot-casting method, we can confirm the vertical crystal growth of the perovskites on the substrate. The importance of structural engineering to control the optoelectronic properties of perovskite materials was further determined by steady and time-resolved photoluminescence (PL) spectra. The thickness of the three perovskite films are the same (~180 nm). Figure 3b shows that the absorption edges of the three perovskite films remain unchanged, and the mixture-1.5 perovskite film presents very bright PL emission compared to that of the pure ODA and EDBE perovskite films, which means that the nonradiative recombination of mixture-1.5 perovskite film is significantly suppressed. Therefore, the huge increase of PL is due to the reduction of excitons quenching mainly caused by the non-radiative defects at the grain boundaries. In order to further examine the kinetics of excitons and the recombination process, time-resolved PL (TRPL) spectra of ODA, EDBE and mixture-1.5 perovskite films are characterized, as shown in Figure 3c. The data are fitted from a triexponential function, where τi is the lifetime of the components, and fi is the fraction of the components. As previously reported [43], 9
the PL decay of perovskite films occurs through three pathways where the longer lifetime τ3 (slow) represents the intrinsic trap-free charge recombination inside the grains, and the shorter lifetimes τ1 (fast) and τ2 (middle) are related to two kinds of trap-assisted recombination at grain boundaries. As shown in Table S1, the τ3 is significantly increased by the sequence of 64.44, 104.94 and 105.82 ns for ODA, EDBE and mixture-1.5 perovskite films, demonstrating the reduced recombination in the mixture-1.5 perovskite films. Moreover, the short average PL lifetime for the ODA perovskite film is 33.7 ns, while similar long PL lifetimes are 66.1 and 67.2 ns for mixture-1.5 and EDBE perovskite films. This can be attributed to the high-quality film with less defects and the balanced electrons and holes transport.
Figure 3. (a) XRD patterns, (b) UV-Vis absorption and PL spectra, and (c) TRPL spectra of ODA, EDBE and mixture-1.5 perovskite films.
The high-quality and highly oriented mixture-1.5 perovskite film is desirable to fabricate efficient PeLEDs. Therefore, we further investigate the electroluminescence (EL) properties of the PeLEDs with a device configuration of ITO/ZnMgO (20 nm)/perovskites/poly(9,9 dioctyl fluorene co N (4 butylphenyl)diphenylamine) (TFB)(25 nm)/MoO3(8 nm)/Au(100 nm) (Figure 4a). First, the charge injection and transport are studied by measuring the J-V curves of electron-only devices (ITO/ZnMgO(20 nm)/perovskite/PCBM(50 nm)/MoO3(8 nm)/Al(100 nm)) and hole-only devices (ITO/PEDOT(30 nm)/perovskite/TFB(25 nm)/LiF(5 nm)/Al(100 nm)). As shown in Figure 4b, the device with mixture-1.5 perovskite film exhibits higher current density at the same voltages than that of the device with pure ODA and 10
EDBE perovskite films. This can be attributed to the vertical orientation growth of the perovskite slabs as we confirmed in the results of GIWAXS, and the efficient charge injection and transport are achieved in the devices with mixture-1.5 perovskite over the devices with pure ODA and EDBE perovskites. Furthermore, balanced electron and hole charge transport is observed in the mixture-1.5 perovskite based devices, which would increase the radiative recombination, enabling high quantum efficiency for the PeLEDs. To understand the mechanism of achieving charge balance via inter-mixing of ODA and EDBE, we further measured the trap-state density of holes and electrons by fabricating hole-only and electron-only devices based on EDBE, ODA and mixture-1.5 perovskites. The trap-state densities (Nt) can be calculated by the following equation [44]
Nt =
2ε0 εr VTFL qL2
(1)
where εr is relative dielectric constant of perovskite (εr = 25.7) [45], ε0 is the vacuum permittivity, L is the thickness of the perovskite film (~180 nm), q is the elemental charge, and VTFL is the onset voltage of trap-filled limit region. As shown in Figure S4, the hole trap-state density of 1.5-mixture perovskite is 5.09 × 10
which
is 40% and 37% times lower than that of ODA and EDBE perovskites respectively (Figure S4 (c)-(e)), while the electron trap-state density of 1.5-mixture perovskite is 5.88 × 10
which is only 11% and 17% times lower than that of ODA and
EDBE perovskites respectively (Figure S4 (f)-(h)). It is clear that the relative reduction of hole trap-state density is more than that of electron trap-state density for 1.5-mixture perovskite, indicating that the passivation of hole trap-state is more effective than that of the electron trap-state by mixing ODA and EDBE spacers in perovskites. This may compromise the result from Figure 4b that the electron current is larger than the hole current in both ODA and EDBE cases, leading to charge balance in 1.5-mixture perovskite. The current density and radiance of the PeLEDs as a function of voltage are presented in Figure 4c and 4d. The devices with different molar ratios of ODA and EDBE are also shown as a comparison (Figure S5). In contrast to Figure 4b, the 11
current density of the mixture-1.5 PeLED is the smallest among all the devices (Figure 4c), which indicates increased radiative recombination of holes and electrons and reduced leakage current, making it favorable for efficient PeLEDs. The EL starts to shoot up at about 1.4 V in the mixture-1.5 PeLED, which is lower than the bandgap of MAPbI3 perovskite (1.6 V) [46], thereby indicating efficient charge injections into the mixture-1.5 perovskite emitters and balanced charge carrier transport. This below-bandgap turn-on voltage could be explained by Auger-mediated energy up-conversion process [10, 47], where there is energy released from the Auger recombination of electrons and holes to facilitate the charge injection into the emitting layer. The radiance of the mixture-1.5 device is as high as 6.5 W sr-1 m-2 at 3.1 V compared to EDBE (5.5 W sr-1 m-2) and ODA (4.5 W sr-1 m-2) devices. As shown in Figure 4e, compared to the low values of EQE of pure ODA and EDBE PeLEDs, the maximum EQE achieves 5.8% at 2.0 V with a current density of 6.05 mA cm−2 for the mixture-1.5 device. This can be attributed to the improved charge injection and balanced charge transport by mixing ODA and EDBE large spacers. The EL spectra are shown in Figure 4f at a current density of 60 mA cm-2. It is remarkable that the highest EL intensity is observed in mixture-1.5 based device and the EL position (766 nm) corresponds to that of the PL spectra shown in Figure 3b, which indicates the superior surface trap passivation by the strong interaction between the amine group of 3:2 ratio-based organic diammonium spacers and the surface of the perovskite.
12
Figure 4. (a) Schematic structure of the perovskite device with an emissive perovskite layer sandwiched between an electron injector (ZnMgO) and a hole injector (TFB). (b) Current density-voltage (J-V) curves of electron-only and hole-only devices. Characterization of the 3D, EDBE, mixture-1.5 and ODA based PeLEDs with (c) J-V curves, (d) Radiance versus voltage curves, (e) EQE versus voltage curves, and (f) EL spectra.
13
Table 1. Performance summary of the PeLEDs based on 3D, EDBE, mixture-1.5 and ODA perovskite films. Perovskites 3D EDBE mixture-1.5 ODA
Turn-on voltage
Radiancemax
EQEmax
(V)
(W sr-1 m-2)
(%)
1.9 1.7 1.4 1.5
0.06 5.5 6.5 4.5
0.07 1.1 5.8 2.4
4. Conclusions We demonstrate an efficient PeLED based on vertical aligned growth with respect to the substrate by employing mixture-1.5 perovskite (ODA: EDBE=3:2, molar ratio). The merits of such design are i) charge carriers can be effectively injected and transport in the formed conductive channels without impediment by the insulating organic layers; ii) the balanced electron and hole mobility would facilitate radiative recombination. As a result, the mixture-1.5 perovskite device shows greatly improved device performance with a low turn-on voltage of 1.4 V and a high EQE up to 5.8%. These findings may provide guidance for designing new perovskites and realizing efficient PeLEDs.
Acknowledgements This work was financially supported by the National Key R&D Program of China (Grant 2017YFB1002900), the National Basic Research Program of ChinaFundamental Studies of Perovskite Solar Cells (2015CB932200), the Natural Science Foundation of China (51602149, 61705102, and 91733302), Natural Science Foundation
of Jiangsu
Province,
China (BK20150064,
BK20161011,
and
BK20161010), Jiangsu Specially-Appointed Professor program, Young 1000 Talents Global Recruitment Program of China, “Six talent peaks” Project in Jiangsu Province China.
14
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Structurally
Engineered
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Dynamics
in
Hybrid
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20
Highly Oriented Perovskites for Efficient Light-Emitting Diodes with Balanced Charge Transport Haiyan Du,1 Wei Hui,1 Yiting Zheng,1 Lianqi Tang,1 Jingjing Qiu,1 Jian Qiu,1 Tingting Niu,1 Yingguo Yang,2 Xingyu Gao,2 Yingdong Xia,1,* Yonghua Chen,1 and Wei Huang1,3,4 1
Key Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials (IAM), Jiangsu National Synergistic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University (NanjingTech), 30 South Puzhu Road, Nanjing 211816, P. R. China. 2 Shanghai Synchrotron Radiation Facility, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201204, P. R. China. 3 Shaanxi Institute of Flexible Electronics (SIFE), Northwestern Polytechnical University (NPU), 127 West Youyi Road, Xi'an 710072, China. 4 Key Laboratory for Organic Electronics & Information Displays (KLOEID), and Institute of Advanced Materials (IAM), Nanjing University of Posts and Telecommunications, 9 Wenyuan Road, Nanjing 210023, China. E-mail: [email protected]
21
Figure S1. Picture of the perovskite precursor solutions of mixture-1.5, ODA and EDBE in methylamine acetate with a concentration of 180 mg mL-1.
22
Figure S2. GIWAXS map of the mixture-1.5 perovskite thin films deposited at room temperature.
23
Figure S3. The pole figures showing the differences in broadness of the Azimuth angle.
24
Table S1. PL lifetime obtained from TRPL of perovskite films Perovskites τ1 (ns) f1 (%) τ2 (ns) f2 (%) τ3 (ns) EDBE
5.99
6.59
ODA
6.5
11.06 21.44 49.97
mixture-1.5 11.84 10.28
28.19 40.73 104.94
37.3
f3 (%)
tave(ns)
52.68
67.2
64.44
33.97
33.7
43.85 105.82
45.87
66.1
PL lifetime curves obtained from time-correlated single photon counting measurement of perovskite films. τave = Σ( fiτi ), where fi are fractional intensities and τi are measured lifetimes.
25
Figure S4. (a) hole-only device. (b) electron-only device. Current density-voltage (J-V) curves of hole-only devices based on (c) ODA; (d) mixture-1.5; (e) EDBE and electron-only devices based on (f) ODA; (g) mixture-1.5; (h) EDBE.
26
Figure S5. (a) Current density versus voltage, (b) radiance versus voltage, (c) EQE versus voltage, and (d) EL spectra of perovskites with different molar ratios of ODA and EDBE (3:7, 1:1, 3:2, and 7:3).
27
Highlights 1. Preferential orientation of perovskite crystals can be achieved by blending two
large organic spacers (ODA and EDBE) into the precursor solution. 2. Effective charge injection and balanced charge transport are achieved. 3. A high EQE of 5.8% was achieved compared to pure ODA (2.4%) and EDBE (1.1%)
PeLEDs.
S1566-1199(19)30556-7
DOI:
https://doi.org/10.1016/j.orgel.2019.105529
Reference:
ORGELE 105529
To appear in:
Organic Electronics
Received Date: 24 May 2019 Revised Date:
26 September 2019
Accepted Date: 27 October 2019
Please cite this article as: H. Du, W. Hui, L. Tang, J. Qiu, T. Niu, Y. Yang, X. Gao, Y. Xia, Y. Chen, W. Huang, Highly oriented perovskites for efficient light-emitting diodes with balanced charge transport, Organic Electronics (2019), doi: https://doi.org/10.1016/j.orgel.2019.105529. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Highly Oriented Perovskites for Efficient Light-Emitting Diodes with Balanced Charge Transport Haiyan Du,1 Wei Hui,1 Lianqi Tang,1 Jian Qiu,1 Tingting Niu,1 Yingguo Yang,2 Xingyu Gao,2 Yingdong Xia,1,* Yonghua Chen,1 and Wei Huang1,3,4 1
Key Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials (IAM), Jiangsu National Synergistic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University (NanjingTech), 30 South Puzhu Road, Nanjing 211816, P. R. China. 2 Shanghai Synchrotron Radiation Facility, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201204, P. R. China. 3 Shaanxi Institute of Flexible Electronics (SIFE), Northwestern Polytechnical University (NPU), 127 West Youyi Road, Xi'an 710072, China. 4 Key Laboratory for Organic Electronics & Information Displays (KLOEID), and Institute of Advanced Materials (IAM), Nanjing University of Posts and Telecommunications, 9 Wenyuan Road, Nanjing 210023, China. E-mail: [email protected]
1
Abstract Ruddlesden-Popper (RP) perovskites have attracted interests due to high performance in perovskite light-emitting diodes (PeLEDs). However, the insulating organic spacers decrease carrier mobility, and increase charge accumulation and non-radiative recombination losses, which significantly undermine device performance. Herein, by blending NH3I(CH2)8NH3I (ODA) and INH3(CH2)2O(CH2)2O(CH2)2NH3I (EDBE) large organic spacers into the precursor solution, we achieve preferential orientation of perovskite crystals perpendicular to the substrate with conductive channels across the two injecting electrodes. Consequently, effective charge injection and balanced charge transport are achieved, which is beneficial to increase radiation recombination. A low turn-on voltage of 1.4 V is achieved for mixture-1.5 perovskite (ODA: EDBE=3:2, molar ratio) PeLEDs, and a maximum external quantum efficiency increases to 5.8% (EL peak ≈766 nm) at 2.0 V with a current density of 6.05 mA cm−2 compared to that of the pure ODA (2.4%) and EDBE (1.1%) devices. The findings may spur new developments in charge injection and electron-hole balance for realizing efficient PeLEDs.
2
1. Introduction Organometal halide perovskites have attracted significant attention in many fields [1-5]. Among them, light-emitting diodes (LEDs) have been demonstrated to have great potential due to their high color purity (full width at half maximum ~ 20 nm) irrespective of the crystal size, tunable optical bandgap from visible to infrared regions, and solution processability [6-13]. Many strategies have been reported to overcome the disadvantages of organic LEDs (e.g., complex synthesis, high cost, and poor color purity) [13] or inorganic LEDs (e.g., high cost of fabrication, and poor device flexibility) [14]. For perovskite LEDs (PeLEDs), structural engineering and energy manipulation have been put forward to realize high efficiency and stability. However, 3D perovskites usually suffer from long exciton diffusion length and small exciton binding energy, which limits the efficiency improvement by dissociating excitons and decreasing electron-hole capture rates for radiative recombination [7, 15-17]. Dimensionality reduction of 3D perovskites is an effective way for high performance PeLEDs. The Ruddlesden-Popper (RP) perovskites, which fall between 2D and 3D perovskites and combine each of their merits, can be obtained through slicing 3D perovskites across di erent planes by inserting large organic ammonium cations in-between inorganic components [18, 19]. The structure of RP perovskite can be recognized as a multiple quantum well in which the organic ammonium cations act as barriers and the inorganic perovskite sheets are as wells [17, 20, 21]. The large organic spacer cations are bonded to the inorganic layers by electrostatic interactions between the ammonium groups and the halide anions [22]. The charge carriers are confined within the organic components to ensure the radiative recombination at the low excitation density [23]. The high performance PeLEDs have been reported in infrared, red, green, and blue emission [5, 24-26]. However, the unique crystal structure of RP perovskites also suffers from an unfavorable issue that the insulating organic spacers decrease carrier mobility, increase charge accumulation and non-radiative recombination losses, and hence significantly undermine device performance [27, 28]. 3
Recently, Gao et al incorporated CH3NH3Cl additives into perovskite precursor solution to achieve vertically oriented perovskite crystals, which facilitated efficient charge injection and transport in the vertical direction without impediment by the insulating organic layers [29]. Similarly, Mohite et al proposed that the vertically aligned perovskite thin films can form conductive channels across the two injecting electrodes, leading to higher radiative recombination efficiency at the same current densities. Besides, they also proved that the imbalanced electron and hole injection in the high injection region would lead to low quantum efficiency at high voltage [14]. Therefore, further exploration of effective organic spacers to facilitate charge injection and balanced charge transport in RP perovskite films is essential for efficient PeLEDs. Many researches have been focusing on this issue. It is demonstrated that INH3(CH2)2O(CH2)2O(CH2)2NH3I (EDBE) can enforce the vertical growth of perovskite crystals by forming hydrogen bonds with another adjacent EDBE molecule [30]. Meanwhile, NH3I(CH2)8NH3I (ODA) has been shown to play a critical role in passivating perovskite surface and grain boundaries [31]. Moreover, introducing a second spacer cation into the precursor solution would promote the vertical growth of the perovskite film and improve the charge transport [32,33]. For example, when BA+ and PEA+ co-work to form [(BA0.5PEA0.5)2FA3Sn4I13] 2DRP perovskites, the intermediate phase which impedes the homogeneous and ordered nucleation of the crystal would be suppressed e ectively, thus enabling a high-quality perovskite film and improved crystal orientation [33]. Here, we demonstrated an effective approach to achieve efficient charge injection and balanced charge transport by mixing two large organic diammonium spacers (ODA and EDBE). Compared to pure ODA and EDBE based perovskite films, the mixture-1.5 perovskite films (ODA: EDBE=3:2, molar ratio) show preferential orientation with the perovskite frameworks perpendicular to the substrate, which facilitates efficient carrier injection and balancing charge transport. Additionally, the mixture-1.5 perovskite film also exhibits high surface coverage with less pinholes and smaller crystals, leading to decreased leakage current and strong photoluminescence. Finally, a highly efficient PeLED is achieved with a low turn-on voltage of 1.4 eV 4
and a high external quantum efficiency (EQE) of 5.8% (EL peak ≈766 nm). 2. Experimental Section 2.1 Preparation of perovskite precursor solutions According to structural formula of (NH3RNH3)(MA)n-1PbnX3n+1, the perovskite precursor solution was prepared by mixing PbI2, MAI and different molar ratios of ODA and EDBE (molar ratio, 0:1, 3:2, 1:0) in methylamine acetate with a concentration of 180 mg mL-1. The methylamine acetate is an environmentally friendly room-temperature molten salt, which has been reported to be an alternative to traditional toxic and highly coordinating solvents for preparing high-quality perovskite thin films. [34] The solution is clear and colorless after stirring at 60 °C for 2 h (Figure S1). The EDBE was synthesized by stirring Hydriodic acid and 2,2-(ethylenedioxy)bis(ethylammonium) with a molar ratio of 2:1 in an ice bath for 2 h. The solution was then evaporated at 65 °C to obtain the EDBE precipitate, which was washed three times with diethyl ether and then dried under vacuum. 2.2 Devices Fabrication The indium tin oxide (ITO) substrates were cleaned sequentially with detergent, de-ionized water, acetone, and iso-propanol, followed by drying with N2 flow and UV-ozone treatment for 20 min. ZnMgO nanocrystals (purchased from Guangdong Poly OptoElectronics Co., Ltd) were spin-coated onto the ITO-coated glass substrates at 3000 rpm for 30 s, and then annealed in air at 120 °C for 30 min. Perovskite precursor solution was spin-coated at 4000 rpm for 20 s at a constant substrate temperature during the whole spin-coating process. Note that the whole preparation process was under ambient conditions, followed by annealing at heating table. The thickness of the three perovskite films is close to 180 nm. The TFB layers were deposited at 2000 rpm for 40 s from an m-xylene solution (14 mg ml-1) in a glovebox. Finally, the device was prepared by thermal evaporation of MoO3 and Au at a pressure of 1×10-4 Pa. The area of each device is 0.05 cm2, as defined by the overlap of the ITO and the thermally evaporated Au. 2.3
Characterization:
All devices were tested in an N2-filled glovebox at room temperature using a Keithley 5
2400 source meter and a QE65 Pro spectrometer. A field emission SEM (Hitachi S-4300) was used to acquire SEM images. The instrument uses an electron beam accelerated at 10-30 kV, enabling operation at a variety of currents. The crystal structure of the perovskite films was analyzed using an X-ray diffractometer, Bruker AXS D8, with CuKa radiation. The Grazing incidence wide angle x-ray scattering (GIWAXS) data were obtained with beamline BL14B1 at Shanghai Synchrotron Radiation Facility (SSRF). Ultraviolet-visible spectra were measured by a SHIMADZU ultraviolet-3600 spectrophotometer. The TRPL was collected using an Optronis Optoscope streak camera system which has an ultimate temporal resolution of ∼10 ps. Ultraviolet–visible absorbance spectra were recorded on an ultraviolet– visible spectrophotometer with an integrating sphere (Cary 5000, Agilent). PL spectra of the perovskite films were measured at room temperature using a fluorescent spectrophotometer (F-4600, HITACHI) with a 200 W Xe lamp as an excitation source. SCLC measurements were carried out with scan range from 0 to 5.0 V. 3. Results and discussion We mixed MAI, PbI2 and different molar ratios of ODA and EDBE (0:1, 3:2, 1:0) to prepare the perovskite precursor solutions with the stoichiometric molar ratio of 3:4:2. The
obtained
perovskites
of
(EDBE)Pb4MA3I13,
(ODA)Pb4MA3I13,
and
[(ODA)0.6(EDBE)0.4]Pb4MA3I13 are abbreviated as EDBE, ODA, and mixture-1.5, respectively. In this work, all perovskite films were fabricated by hot-casting method. The vertical orientation of the perovskite film is essential for highly efficient PeLEDs for the fact that the insulating nature of the organic layers would inhibit the charge carrier transport. Grazing-incidence wide-angle X-ray scattering (GIWAXS) measurements were first performed to measure the orientation of the perovskite thin films, as shown in Fig 1a-c. The ODA perovskite films clearly show the Debye-Scheler rings with a few Bragg spots, indicating that the grain orientation is random with a few grains perpendicular to the substrate. The three low-angle diffraction peaks between qz = 0-10 nm are observed, which demonstrates the formation of the 2D phases [35]. In contrast, sharp and discrete Bragg spots are observed along certain extended arc segments in the EDBE and mixture-1.5 films, 6
indicating a strong vertical orientation of the grains with respect to the substrate [29, 36, 37]. Much stronger and sharper discrete Bragg spots are achieved in mixture-1.5 perovskite film than that of the EDBE film, suggesting that the highly oriented crystal growth can be controlled by mixing the two large cations. We also observe a transformation from the ring-like Debye Scherer patterns (Figure S2) at room temperature to discrete Bragg spots under hot-casting deposition, which is consistent with previous reports [14] that the temperature is a key factor to assist preferential crystal orientation of the thin film. It should be noted that the 2D phases are observed in both EDBE and mixture-1.5 perovskite films, which implies a challenge to obtain pure RP perovskites with fixed layers. Furthermore, a pole figure is presented in Figure S3 where the mixture-1.5 perovskite film only shows discrete Bragg spots at 90° without noticeable peaks along the same rings, demonstrating the strong preferential grain orientation. The efficient charge injection and transport can be expected in the vertical direction by the formation of continuous charge-transport channels through the conducting inorganic layers in mixture-1.5 perovskite film over the pure ODA and EDBE perovskite films.
7
Figure 1. GIWAXS maps and schematic figures of the crystalline orientation for (a) ODA, (b) mixture-1.5 and (c) EDBE films.
Poor surface morphology and coverage cause non-radiative recombination losses, which is one of the critical problems that degrade the luminescent efficiency in planar PeLEDs, especially when the perovskite layers are deposited using a solution process [38]. The ODA, EDBE and mixture-1.5 perovskite films are deposited on ITO/ZnMgO substrate and investigated by scanning electron microscopy (SEM), as shown in Figure 2a-c. Numerous pinholes are observed on the ODA perovskite film surface, which may result in a direct contact between electron and hole transport layers, leading to severe current leakage and significant degradation of the device efficiency. For EDBE perovskite film, besides large pinholes, the surface is uneven with accumulating crystals, which can be acted as charge injection barriers due to the poor contact. In comparison, a dense and even perovskite film with less pinholes is obtained by mixing ODA and EDBE spacers (mixture-1.5), which is beneficial to spatially confine electrons and holes to promote bimolecular radiative recombination 8
and achieve high performance device [39-40].
Figure 2. Top-view SEM images of the perovskite films based on (a) ODA, (b) mixture-1.5 and (c) EDBE.
To further justify the crystal formation, X-ray di raction (XRD) was conducted (Figure 3a). For all perovskite films, two dominant planes are observed at diffraction angles (2θ) of 14.20° and 28.48°, which indicate the diffraction planes (111) and (202). The diffraction planes (111) and (202) have been reported to indicate the highly oriented inorganic layers perpendicular to the substrate in the RP perovskites [41, 42]. Combined with the GIWAXS results and the applied hot-casting method, we can confirm the vertical crystal growth of the perovskites on the substrate. The importance of structural engineering to control the optoelectronic properties of perovskite materials was further determined by steady and time-resolved photoluminescence (PL) spectra. The thickness of the three perovskite films are the same (~180 nm). Figure 3b shows that the absorption edges of the three perovskite films remain unchanged, and the mixture-1.5 perovskite film presents very bright PL emission compared to that of the pure ODA and EDBE perovskite films, which means that the nonradiative recombination of mixture-1.5 perovskite film is significantly suppressed. Therefore, the huge increase of PL is due to the reduction of excitons quenching mainly caused by the non-radiative defects at the grain boundaries. In order to further examine the kinetics of excitons and the recombination process, time-resolved PL (TRPL) spectra of ODA, EDBE and mixture-1.5 perovskite films are characterized, as shown in Figure 3c. The data are fitted from a triexponential function, where τi is the lifetime of the components, and fi is the fraction of the components. As previously reported [43], 9
the PL decay of perovskite films occurs through three pathways where the longer lifetime τ3 (slow) represents the intrinsic trap-free charge recombination inside the grains, and the shorter lifetimes τ1 (fast) and τ2 (middle) are related to two kinds of trap-assisted recombination at grain boundaries. As shown in Table S1, the τ3 is significantly increased by the sequence of 64.44, 104.94 and 105.82 ns for ODA, EDBE and mixture-1.5 perovskite films, demonstrating the reduced recombination in the mixture-1.5 perovskite films. Moreover, the short average PL lifetime for the ODA perovskite film is 33.7 ns, while similar long PL lifetimes are 66.1 and 67.2 ns for mixture-1.5 and EDBE perovskite films. This can be attributed to the high-quality film with less defects and the balanced electrons and holes transport.
Figure 3. (a) XRD patterns, (b) UV-Vis absorption and PL spectra, and (c) TRPL spectra of ODA, EDBE and mixture-1.5 perovskite films.
The high-quality and highly oriented mixture-1.5 perovskite film is desirable to fabricate efficient PeLEDs. Therefore, we further investigate the electroluminescence (EL) properties of the PeLEDs with a device configuration of ITO/ZnMgO (20 nm)/perovskites/poly(9,9 dioctyl fluorene co N (4 butylphenyl)diphenylamine) (TFB)(25 nm)/MoO3(8 nm)/Au(100 nm) (Figure 4a). First, the charge injection and transport are studied by measuring the J-V curves of electron-only devices (ITO/ZnMgO(20 nm)/perovskite/PCBM(50 nm)/MoO3(8 nm)/Al(100 nm)) and hole-only devices (ITO/PEDOT(30 nm)/perovskite/TFB(25 nm)/LiF(5 nm)/Al(100 nm)). As shown in Figure 4b, the device with mixture-1.5 perovskite film exhibits higher current density at the same voltages than that of the device with pure ODA and 10
EDBE perovskite films. This can be attributed to the vertical orientation growth of the perovskite slabs as we confirmed in the results of GIWAXS, and the efficient charge injection and transport are achieved in the devices with mixture-1.5 perovskite over the devices with pure ODA and EDBE perovskites. Furthermore, balanced electron and hole charge transport is observed in the mixture-1.5 perovskite based devices, which would increase the radiative recombination, enabling high quantum efficiency for the PeLEDs. To understand the mechanism of achieving charge balance via inter-mixing of ODA and EDBE, we further measured the trap-state density of holes and electrons by fabricating hole-only and electron-only devices based on EDBE, ODA and mixture-1.5 perovskites. The trap-state densities (Nt) can be calculated by the following equation [44]
Nt =
2ε0 εr VTFL qL2
(1)
where εr is relative dielectric constant of perovskite (εr = 25.7) [45], ε0 is the vacuum permittivity, L is the thickness of the perovskite film (~180 nm), q is the elemental charge, and VTFL is the onset voltage of trap-filled limit region. As shown in Figure S4, the hole trap-state density of 1.5-mixture perovskite is 5.09 × 10
which
is 40% and 37% times lower than that of ODA and EDBE perovskites respectively (Figure S4 (c)-(e)), while the electron trap-state density of 1.5-mixture perovskite is 5.88 × 10
which is only 11% and 17% times lower than that of ODA and
EDBE perovskites respectively (Figure S4 (f)-(h)). It is clear that the relative reduction of hole trap-state density is more than that of electron trap-state density for 1.5-mixture perovskite, indicating that the passivation of hole trap-state is more effective than that of the electron trap-state by mixing ODA and EDBE spacers in perovskites. This may compromise the result from Figure 4b that the electron current is larger than the hole current in both ODA and EDBE cases, leading to charge balance in 1.5-mixture perovskite. The current density and radiance of the PeLEDs as a function of voltage are presented in Figure 4c and 4d. The devices with different molar ratios of ODA and EDBE are also shown as a comparison (Figure S5). In contrast to Figure 4b, the 11
current density of the mixture-1.5 PeLED is the smallest among all the devices (Figure 4c), which indicates increased radiative recombination of holes and electrons and reduced leakage current, making it favorable for efficient PeLEDs. The EL starts to shoot up at about 1.4 V in the mixture-1.5 PeLED, which is lower than the bandgap of MAPbI3 perovskite (1.6 V) [46], thereby indicating efficient charge injections into the mixture-1.5 perovskite emitters and balanced charge carrier transport. This below-bandgap turn-on voltage could be explained by Auger-mediated energy up-conversion process [10, 47], where there is energy released from the Auger recombination of electrons and holes to facilitate the charge injection into the emitting layer. The radiance of the mixture-1.5 device is as high as 6.5 W sr-1 m-2 at 3.1 V compared to EDBE (5.5 W sr-1 m-2) and ODA (4.5 W sr-1 m-2) devices. As shown in Figure 4e, compared to the low values of EQE of pure ODA and EDBE PeLEDs, the maximum EQE achieves 5.8% at 2.0 V with a current density of 6.05 mA cm−2 for the mixture-1.5 device. This can be attributed to the improved charge injection and balanced charge transport by mixing ODA and EDBE large spacers. The EL spectra are shown in Figure 4f at a current density of 60 mA cm-2. It is remarkable that the highest EL intensity is observed in mixture-1.5 based device and the EL position (766 nm) corresponds to that of the PL spectra shown in Figure 3b, which indicates the superior surface trap passivation by the strong interaction between the amine group of 3:2 ratio-based organic diammonium spacers and the surface of the perovskite.
12
Figure 4. (a) Schematic structure of the perovskite device with an emissive perovskite layer sandwiched between an electron injector (ZnMgO) and a hole injector (TFB). (b) Current density-voltage (J-V) curves of electron-only and hole-only devices. Characterization of the 3D, EDBE, mixture-1.5 and ODA based PeLEDs with (c) J-V curves, (d) Radiance versus voltage curves, (e) EQE versus voltage curves, and (f) EL spectra.
13
Table 1. Performance summary of the PeLEDs based on 3D, EDBE, mixture-1.5 and ODA perovskite films. Perovskites 3D EDBE mixture-1.5 ODA
Turn-on voltage
Radiancemax
EQEmax
(V)
(W sr-1 m-2)
(%)
1.9 1.7 1.4 1.5
0.06 5.5 6.5 4.5
0.07 1.1 5.8 2.4
4. Conclusions We demonstrate an efficient PeLED based on vertical aligned growth with respect to the substrate by employing mixture-1.5 perovskite (ODA: EDBE=3:2, molar ratio). The merits of such design are i) charge carriers can be effectively injected and transport in the formed conductive channels without impediment by the insulating organic layers; ii) the balanced electron and hole mobility would facilitate radiative recombination. As a result, the mixture-1.5 perovskite device shows greatly improved device performance with a low turn-on voltage of 1.4 V and a high EQE up to 5.8%. These findings may provide guidance for designing new perovskites and realizing efficient PeLEDs.
Acknowledgements This work was financially supported by the National Key R&D Program of China (Grant 2017YFB1002900), the National Basic Research Program of ChinaFundamental Studies of Perovskite Solar Cells (2015CB932200), the Natural Science Foundation of China (51602149, 61705102, and 91733302), Natural Science Foundation
of Jiangsu
Province,
China (BK20150064,
BK20161011,
and
BK20161010), Jiangsu Specially-Appointed Professor program, Young 1000 Talents Global Recruitment Program of China, “Six talent peaks” Project in Jiangsu Province China.
14
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20
Highly Oriented Perovskites for Efficient Light-Emitting Diodes with Balanced Charge Transport Haiyan Du,1 Wei Hui,1 Yiting Zheng,1 Lianqi Tang,1 Jingjing Qiu,1 Jian Qiu,1 Tingting Niu,1 Yingguo Yang,2 Xingyu Gao,2 Yingdong Xia,1,* Yonghua Chen,1 and Wei Huang1,3,4 1
Key Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials (IAM), Jiangsu National Synergistic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University (NanjingTech), 30 South Puzhu Road, Nanjing 211816, P. R. China. 2 Shanghai Synchrotron Radiation Facility, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201204, P. R. China. 3 Shaanxi Institute of Flexible Electronics (SIFE), Northwestern Polytechnical University (NPU), 127 West Youyi Road, Xi'an 710072, China. 4 Key Laboratory for Organic Electronics & Information Displays (KLOEID), and Institute of Advanced Materials (IAM), Nanjing University of Posts and Telecommunications, 9 Wenyuan Road, Nanjing 210023, China. E-mail: [email protected]
21
Figure S1. Picture of the perovskite precursor solutions of mixture-1.5, ODA and EDBE in methylamine acetate with a concentration of 180 mg mL-1.
22
Figure S2. GIWAXS map of the mixture-1.5 perovskite thin films deposited at room temperature.
23
Figure S3. The pole figures showing the differences in broadness of the Azimuth angle.
24
Table S1. PL lifetime obtained from TRPL of perovskite films Perovskites τ1 (ns) f1 (%) τ2 (ns) f2 (%) τ3 (ns) EDBE
5.99
6.59
ODA
6.5
11.06 21.44 49.97
mixture-1.5 11.84 10.28
28.19 40.73 104.94
37.3
f3 (%)
tave(ns)
52.68
67.2
64.44
33.97
33.7
43.85 105.82
45.87
66.1
PL lifetime curves obtained from time-correlated single photon counting measurement of perovskite films. τave = Σ( fiτi ), where fi are fractional intensities and τi are measured lifetimes.
25
Figure S4. (a) hole-only device. (b) electron-only device. Current density-voltage (J-V) curves of hole-only devices based on (c) ODA; (d) mixture-1.5; (e) EDBE and electron-only devices based on (f) ODA; (g) mixture-1.5; (h) EDBE.
26
Figure S5. (a) Current density versus voltage, (b) radiance versus voltage, (c) EQE versus voltage, and (d) EL spectra of perovskites with different molar ratios of ODA and EDBE (3:7, 1:1, 3:2, and 7:3).
27
Highlights 1. Preferential orientation of perovskite crystals can be achieved by blending two
large organic spacers (ODA and EDBE) into the precursor solution. 2. Effective charge injection and balanced charge transport are achieved. 3. A high EQE of 5.8% was achieved compared to pure ODA (2.4%) and EDBE (1.1%)
PeLEDs.