Letter pubs.acs.org/JPCL
Modified Conducting Polymer Hole Injection Layer for HighEfficiency Perovskite Light-Emitting Devices: Enhanced Hole Injection and Reduced Luminescence Quenching Xue-Feng Peng,† Xiao-Yan Wu,‡ Xia-Xia Ji,† Jie Ren,† Qi Wang,† Guo-Qing Li,† and Xiao-Hui Yang*,† †
School of Physical Science and Technology, Southwest University, Chongqing 400715, China Institute of Fluid Physics, Mianyang, China Academy of Engineering Physics, Mianyang 621900, China
‡
S Supporting Information *
ABSTRACT: Modification of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) with sodium-poly(styrenesulfonate) leads to a ca. 0.3 eV increase in the work function and 15 times enhancement in the photoluminescence intensity of the overlying perovskite layer, which is closely correlated with the formation of a highly PSSenriched top layer. As a direct result, the hybrid halide perovskite light-emitting devices with a modified PEDOT:PSS layer show the maximum external quantum efficiency of 7.2% and power efficiency of 19.0 lm W−1, which are 14−20 times those of the analogous devices using a pristine PEDOT:PSS layer and among the best reported values for the light-emitting devices using a neat perovskite emission layer. Our results illustrate that insufficient hole injection and luminescence quenching at the PEDOT:PSS anode are among the most important factors limiting the external quantum efficiencies of inverted perovskite light-emitting devices.
H
having a PFI-modified PEDOT:PSS hole injection contact with the groundbreaking EQE of 8.53% by using a molecular additive to restrict the crystallization of MAPbBr3 and manipulating the ratio of organic to inorganic content in the precursor solution.14 The utilization of a PEDOT:PSS and MoO3 composite HIL led to the superior EQE of the OIHP LEDs (ca. 0.2%) compared with that of the analogous devices with a PEDOT:PSS layer, which was attributed to enhanced hole injection and improved crystallinity of the OIHP layer.15 It is known that the involvement of vertical phase segregation in PEDOT:PSS films leads to the formation of a PSS-enriched top layer, which varies its work function and interaction with the adjacent layer.19 Hwang et al.20 reported that the removal of the PSS overlayer resulted in a ca. 0.4 eV decrease in PEDOT:PSS work function and intensified interactions with the adjacent layer due to the large overlap of the valence states across the interface. Modification of the interfaces between the electrode and OIHP layer is very important for the development of highperformance OIHP LEDs.1−3 Nevertheless, the effects of interfacial engineering on device performances remain largely undefined as the variations in the morphology and crystallinity of the OIHP layers induced by interface modifications often lead to the substantial changes in the charge transport and charge recombination dynamics of the OIHP layers. Huang et al.6 reported that the incorporation of a cross-linkable hole
ybrid organic−inorganic halide perovskites (OIHPs) are actively studied for light-emitting devices (LEDs) due to their advantages of solution-processability, high carrier mobility, tunable band gap, and good color purity.1−18 The Friend group first reported RT-operating green OIHP LEDs with the maximum external quantum efficiency (EQE) of 0.1%.4 In this pioneering report, both conventional and inverted devices employing the respective metal oxide and organic charge injection layer were discussed. Inverted devices comprising indium tin oxide (ITO)/hole injection layer (HIL)/OIHP emission layer (EML)/electron transport layer/low-workfunction metal cathode are easy to prepare and can be built on the flexible substrates, as the devices exclude a metal oxide layer such as TiO 2 or ZnO layer processed at high temperatures.1−3 One of the most commonly used HILs in such devices is poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) due to the advantages of planarization of ITO substrates to reduce the leakage current and relatively high work function to promote hole injection.19 Nevertheless, PEDOT:PSS interacts intensively with the overlying OIHP layer, quenching its light emission.5 In addition, the work function of PEDOT:PSS is not high enough to match the valence band maximum of MAPbBr3 located at ca. −5.7 eV below the vacuum level,1−3 restricting hole injection into MAPbBr3. To resolve the issues, Lee et al. reported that the addition of tetrafluoroethylene-perfluoro-3,6-dioxa-4-methyl-7-octene-sulfonic acid copolymer (PFI) increased the work function of PEDOT:PSS and reduced luminescence quenching, resulting in enhanced device EQE of ca. 0.125%.5 In the subsequent work, the same group described the OIHP LEDs © XXXX American Chemical Society
Received: July 31, 2017 Accepted: September 13, 2017
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DOI: 10.1021/acs.jpclett.7b01992 J. Phys. Chem. Lett. 2017, 8, 4691−4697
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The Journal of Physical Chemistry Letters
eter using the excitation wavelength of 425 nm. The structure analysis of the OIHP films was conducted with a Rigaku D/ Max-B X-ray diffractometer (XRD) equipped with Cu Kα radiation source. The morphologies of the PEDOT:PSS and SPSS composite and MAPbBr3 layers were studied using a Hitachi atomic force microscope (AFM) and Jeon scanning electron microscope (SEM), respectively. Monochromatic Al Kα (1486.6 eV) and He I (21.2 eV) sources of ESCALAB 250Xi systems were used in the X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) measurements. The S-PSS and PEDOT:PSS mixed solutions are prepared by blending the aqueous solutions of S-PSS and PEDOT:PSS with the respective S-PSS/PEDOT:PSS ratio of 0.5, 2.5, 5, and 7.5, and the HILs with various S-PSS contents, denoted as HIL-0, -0.5, -2.5, -5, and -7.5, accordingly, are deposited onto the ITO substrates. The surface morphology of HIL-5 measured by AFM is shown in Figure 1a, and the AFM images of the other
injection layer resulted in an eight-fold increase in device luminance efficiency, which was attributed to the improvements in both charge balance of the devices and surface coverage of the OIHP layers. To clearly illustrate the effects of interface modifications and identify the factors limiting device efficiency, it is necessary to design the configuration where the parameters such as the charge balance factor are decoupled with the morphology and crystallinity of the OIHP layers upon interfacial modifications. We study the morphology, surface composition, and work f u n c t i o n o f t h e P E D O T : P S S a n d s o d i u m - p o ly (styrenesulfonate) (S-PSS) composite HILs with various SPSS contents as well as the morphology, crystal structure, and photoluminescence (PL) property of the overlying OIHP layers. Modification of PEDOT:PSS with S-PSS leads to a large reduction in OIHP luminescence quenching and the concomitant enhancement in the hole injection efficiency, which translates to high-efficiency OIHP LEDs with the maximum EQE of 7.2% and power efficiency (PE) of 19.0 lm W−1, more than one order of magnitude higher than those of the analogous devices with a pristine PEDOT:PSS layer. In addition, the morphology, composition, and crystal structure of the OIHP layers on top of different HILs are nearly unaltered, allowing us to establish a clear correlation between interfacial modifications and device performances and to identify the critical limiting factors for device efficiency. Materials and Solution Preparation. Methylammonium bromide (MABr), lead(II) bromide (PbBr2), and 1,3,5-tri(m-pyrid3-yl-phenyl) benzene (TmPyPB) were purchased from Xi’an Polymer Technology (China):PEDOT:PSS (AI 4083) and sodium-poly(styrenesulfonate) (S-PSS) was obtained from Heraeus (Germany) and Sigma-Aldrich. All of the materials were used as received. 63 mg MABr (0.53 mmol) and 137 mg PbBr2 (0.37 mmol) were codissolved into 1 mL of N,Ndimethylformamide (DMF) (Figure S1 in the Supporting Information (SI)). The mixed PEDOT:PSS and S-PSS solution was prepared by blending the aqueous S-PSS solution and PEDOT:PSS dispersion with the respective S-PSS/PEDOT:PSS volume ratio of 0.5, 2.5, 5, and 7.5. The S-PSS/ PEDOT:PSS weight ratio for HIL-5 (the S-PSS/PEDOT:PSS volume ratio of 5:1) is 4.4 to 5.8 according to the product information that the solid content for PEDOT:PSS AI4083 is 1.3 to 1.7%. The mixed PEDOT:PSS and S-PSS solutions were stirred for 4 h. Device Preparation and Measurements. Indium tin oxide (ITO) substrates were sequentially cleaned with deionized water and organic solvents and then treated with UV-ozone for 20 min immediately prior to device preparation. Various HILs were spin-coated onto the ITO substrates from the aqueous solutions, which were thermally treated at 170 °C for 10 min under ambient conditions to remove the moisture. The MAPbBr3 precursor solution was spin-coated onto the HILs at 3000 rpm for 5 s; afterward, 350 μL of chlorobenzene was dripped onto the spinning sample surface to expedite the crystallization process.21 The samples were thermally treated at 60 °C for 20 min to induce MAPbBr3 crystallization. Finally, 60 nm TmPyPB, 1 nm CsF, and 200 nm Al were successively evaporated. The characteristics of the devices were measured by a Keithley 2400 Source Measure Unit and Konica Minolta Chroma Meter CS-100A, which were controlled by a homemade Labview program. The EL spectra were recorded with an Ocean Optics USB4000 UV−vis spectrometer. The PL spectra were measured with a Hitachi F4600 fluorospectrom-
Figure 1. (a) 3 μm × 3 μm AFM image of HIL-5. (b) Top-view (inset: the high-resolution image) and (c) cross-section SEM images of EML5.
HILs are presented in Figure S2. All of the samples show uniform and smooth morphologies with the root-mean-square (RMS) roughness values in the range of 0.7 to 1.1 nm. It is commonly recognized that PEDOT:PSS films show vertical phase segregation, leading to the formation of a PSSenriched top surface,19,20,22,23 as schematically illustrated in Figure 2a. The surface chemical compositions of the S-PSSmodified PEDOT:PSS layers are investigated by XPS. S 2p core-level spectra of the HILs consist of two signed peak regions at 170 to 168.4 and 165.6 to 164.5 eV, as shown in Figure 2b and Figure S3, originating from the sulfur atoms of PSS and PEDOT moieties, respectively.20,22,23 The ratio of the PEDOT to PSS content can be estimated by using the area ratio between the peak regions at 165.6 to 164.5 and 170 to 168.4 eV.22 The ratio of the PEDOT to PSS content for HIL-0 is 0.12, consistent with the previous reports.22,23 The ratio decreases from 0.087 for HIL-0.5 to 0.026, 0.008, and 0.002 for the respective HIL-2.5, -5, and -7.5, as shown in Figure 2c, which indicates the formation of a highly PSS-enriched overlayer. A direct consequence of the PSS enrichment on the film top surface is the work-function alteration.22,23 In this regard, the work-function values of HILs-0, -5, and -7.5 are measured by UPS. As shown in Figure 2d, the work function of HIL-0 is 4.9 eV, in accordance with the previously reported value.19,24 HIL-5 shows enlarged work function of 5.2 eV, which can be attributed to the reduction of filled state densities near the Fermi level caused by the PSS enrichment on the HIL top surface.23 The work function of HIL-7.5 is similar to that of HIL-5. Therefore, the incorporation of the S-PSS-modified PEDOT:PSS HILs between ITO electrode and MAPbBr3 EML is expected to reduce the hole injection barrier, resulting in enhanced hole injection. 4692
DOI: 10.1021/acs.jpclett.7b01992 J. Phys. Chem. Lett. 2017, 8, 4691−4697
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Figure 2. (a) Scheme of forming a PSS-rich layer on the top surface of the S-PSS modified PEDOT:PSS films due to vertical phase-segregation. (b) XPS S 2p core level spectrum of HIL-0. (c) Ratios of the PEDOT to PSS content as determined from the XPS spectra for various HILs. (d) Secondary electron cutoff region of the UPS spectra for HIL-0, -5, and -7.5.
Figure 3. (a) Current density−voltage characteristics of the hole-dominated devices with the structure of ITO/HILs/MAPbBr3/Al. (b) PL spectra of the MAPbBr3 EMLs on top of the HILs with various S-PSS contents under the excitation of 425 nm. (c) Time-resolved luminescence decay measurements of EMLs-0 and -5.
MAPbBr3 EMLs are spin-coated onto various HILs, which are denoted as EML-0, -0.5, -2.5, -5, and -7.5, respectively. All of the top-view SEM images of the HTLs show similar closely packed grains, as presented in Figure 1b for EML-5 and Figure S4 for the other EMLs, indicating that modification of PEDOT:PSS with S-PSS has negligible influences on the morphologies of the MAPbBr3 EMLs. The cross-section SEM image of EML-5 presented in Figure 1c reveals that a compact MAPbBr3 layer with full surface coverage is formed atop of HIL-5. The composition and crystal structure of the MAPbBr3
EMLs are analyzed by XRD, as shown in Figure S5. X-ray diffractograms of the MAPbBr3 EMLs show the peaks at 15.02, 30.04, and 45.06° with similar intensities and full width at halfmaximum (fwhm) values, assigned to the (100), (200), and (300) planes of MAPbBr3 cubic phase,25 respectively, which indicates complete MAPbBr3 formation and the MAPbBr3 EMLs with similar crystal orientation and crystallinity. In summary, the morphology, composition, and crystal structure of the overlying MAPbBr3 EMLs are nearly unaltered upon 4693
DOI: 10.1021/acs.jpclett.7b01992 J. Phys. Chem. Lett. 2017, 8, 4691−4697
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The Journal of Physical Chemistry Letters Table 1. Detailed Properties of the Time-Resolved PL Decay of EMLs-0 and -5 EML-0 EML-5
τ1 (ns)
A1 (%)
τ2 (ns)
A2 (%)
τ3 (ns)
A3 (%)
χ2
τave (ns)
0.44 0.72
61.05 49.81
4.79 9.41
29.82 36.23
45.38 68.25
9.13 13.76
1.14 1.17
5.83 13.16
Figure 4. (a) Schematic architecture and (b) energy level diagram of the OIHP LEDs with the structure of ITO/HILs/MAPbBr3/TmPyPB/CsF/Al.
Figure 5. Properties of the OIHP LEDs with the structure of ITO/HILs/MAPbBr3/TmPyPB/CsF/Al including EQE−J curves (a), EL spectra (b), J−V (c), L−V (d), and PE−J plots (e) and the histograms of device EQEs (f).
and -7.5, respectively, are accessed. The devices do not emit light in the measurements, validating their hole-dominant feature. The current density of the hole-dominated devices first increases with increasing the S-PSS/PEDOT:PSS ratio from 0 to 5, as shown in Figure 3a, then starts to decrease with a further increase in the ratio to 7.5. The hole injection barrier at the HIL and MAPbBr3 EML interface is the key factor to
using various HILs, allowing us to establish a clear correlation between interface modifications and device performances. Having enhanced work function of the S-PSS modified PEDOT:PSS films in mind, we proceed to study the hole injection capability of various HILs. For this purpose, the holedominated devices with the structure of ITO/HILs/MAPbBr3 EML/Al, denoted as the hole-dominated device-0, -0.5, -2.5, -5, 4694
DOI: 10.1021/acs.jpclett.7b01992 J. Phys. Chem. Lett. 2017, 8, 4691−4697
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The Journal of Physical Chemistry Letters determine the hole current density.5 The utilization of the SPSS modified PEDOT:PSS HILs with enlarged work function can reduce the hole injection barrier at the interface, resulting in increased hole current density. Meanwhile, the enrichment of the ionic S-PSS on the top surface of the HIL may change the electrostatic interaction at the HIL/OIHP EML interface, promoting hole injection as well. Reduction of the hole current density for the hole-dominated device-7.5 may be associated with the formation of a thick low-conductivity S-PSS layer, which impedes hole injection.24,26 Besides charge balance, the PL efficiency of the OIHP EMLs is essential to achieve high EQE for OIHP LEDs. To investigate the effects of the S-PSS and PEDOT:PSS layer addition on the PL characteristics of the overlying MAPbBr3 EMLs, we carry out the steady-state PL measurements on the samples with the configurations of Glass/MAPbBr3 (EML-G), Glass/S-PSS/ MAPbBr 3 (EML-S), and Glass/PEDOT:PSS/MAPbBr 3 (EML-0). As shown in Figure 3b, the PL intensity of EML-S is slightly lower than that of EML-G, and both of EMLs-G and -S show more than 10 times larger PL intensities with respect to EML-0. The result corroborates with the previous reports5,24 that PEDOT:PSS strongly quenches light emission of MAPbBr3 while S-PSS does not. As shown in Figure 3b, the PL intensities of EMLs-0.5, -2.5, -5, and -7.5, which are intermediate between those of EMLs-0 and -S, increase by a factor of ca. 10 with increasing the S-PSS content. The above results indicate that modification of PEDOT:PSS with S-PSS reduces luminescence quenching, resulting in largely enhanced MAPbBr3 PL intensity. The time-resolved luminescence decay measurements of EMLs-0 and -5 are shown in Figure 3c. PL decay traces of EMLs-0 and -5 can be well fitted with the following triexponential decay model:27I0 = A1e−t/τ1 + A2e−t/τ2 + A3e−t/τ3, where I0 is the normalized intensity, A1, A2, and A3 are the fractions of the components, and τ1, τ2, and τ3 are the characteristic lifetimes for the components. According to Zheng et al.,27 the τ3 component describes the radiative recombination process. The fitting parameters for the model are presented in Table 1. EML-5 shows the larger A3 and τ3 values compared with EML-0, which is in line with the results of the steady-state PL measurements, indicating alleviated PL quenching. OIHP LEDs using different HILs with the structure of ITO/ HILs/MAPbBr3/TmPyPB/CsF/Al, which are denoted as device-0, -0.5, -2.5, -5, and -7.5, respectively, are prepared. The schematic architecture and energy level diagram of the devices are shown in Figure 4a,b. The characteristics of the devices are presented in Figure 5. The maximum EQE and PE of device-0 are 0.5% and 1.0 lm W−1, as shown in Figure 5a,e, in concord with the previously reported values.13,28 The EL spectrum of device-0 has the maximum at 529 nm and a narrow fwhm of 21 nm, as shown in Figure 5b. The EQE, as well as the current density, luminance, and PE of the devices, increases sharply with increasing the SPSS/PEDOT:PSS ratio from 0 to 5, as shown in Figure 5a,c−e, respectively. More specifically, the maximum EQE increases from 0.5% for device-0 to 1.5, 3.7, and 7.2% for the respective device-0.5, -2.5, and -5, as shown in Figure 5a. Notably, device5 shows the maximum luminance of 17 630 cd m−2, EQE of 7.2%, and PE of 19.0 lm W−1. The maximum EQE and PE of device-5 are 14−20 times those of device-0 and are among the best reported values for the LEDs using a neat MAPbBr3 EML (Table 2). A large increase in device EQE and PE can be attributed to enhanced hole injection and MAPbBr3 PL intensity. Compared with device-5, device-7.5 shows the
Table 2. Summary of the Properties of Inverted LEDs Based on a Neat MAPbBr3 Layera device architecture ITO/PEDOT:PSS:MoO3/ MAPbBr3/SPB-02/LiF/ Ag15 PEDOT:PSS:PFI/MAPbBr3/ TPBI/LiF/Al14 ITO/PEDOT:PSS/ MAPbBr3/Bphen/LiF/Al7 ITO/PEDOT:PSS/HTL/ MAPbBr3/PCBM/Ag6 ITO/PEDOT:PSS/ MAPbBr3/TPBi/Ca/Al11 ITO/NiOx/MAPbBr3/TPBi/ LiF/Al17 ITO/PEDOT:PSS/ MAPbBr3/TmPyPB/LiF/ Al16 ITO/PEDOT:PSS/ MAPbBr3/TPBi/LiF/Ag13 ITO/m-PEDOT:PSS/ MAPbBr3/TmPyPB/CsF/ Alb a+
EQE+ (%)
LE+ (cd A−1)
PE+ (lm W−1)
0.20
1.0
0.3
21.4
∼5.0
luminance+ (cd m−2) 9800 >10 000
0.15
0.5
3868
0.27
0.9
∼2000
5.1
8794
15.9
65300
3.38
15.3
0.71
3.3
7.26
29.0
6.0
6124 14 460
19.0
17 630
, maximum value. bPresent work.
decreased current density at a given voltage (Figure 5c), in line with the observation of the reduced hole current density of the hole-dominated device-7.5 (Figure 3a). The maximum EQE of device-7.5 is ca. 4.9%, lower than that of device-5, which may be associated with decreased hole injection. Detailed device properties are summarized in Table 3. Table 3. Detailed Properties of the OIHP LEDs with Various HILsa device-0 device-0.5 device-2.5 device-5 device-7.5 a+
EQE+ (%)
PE+ (lm W−1)
LE+ (cd A−1)
0.5 1.5 3.7 7.2 4.9
1.0 3.0 8.8 19.0 12.1
2.0 6.2 15.0 29.0 19.7
, maximum value.
All of the devices show the characteristic EL spectrum of MAPbBr3, as shown in Figure 5b, and the Commission Internationale de I’Eclairage (CIE) coordinates of (0.18, 0.76), indicating good carrier confinement within the MAPbBr3 EML (Figure 4b). The properties of device-5 are rather insensitive to the HIL thickness (Figure S6). The histograms of the EQEs for several batches of 30 independently fabricated OIHP LEDs are shown in Figure 5f. The EQEs of the devices are well reproducible. The stability of the devices is inferior to that of the analogous devices with a CuSCN HIL, as shown in Figure S7. Excellent EQE of device-5, together with a large variation of device EQE by a factor of 14 with the changes of the S-PSS content in the HILs, indicates that the present approach tackles the critical problems limiting device EQEs. Thus our results clearly illustrate that insufficient hole injection and luminescence quenching at the PEDOT:PSS anode are among the most important factors limiting the EQEs of inverted OIHP LEDs. In conclusion, a highly PSS-enriched layer is formed on the top surface of the S-PSS modified PEDOT:PSS layers, which leads to an increase in the work function from 4.9 to 5.2 eV and 4695
DOI: 10.1021/acs.jpclett.7b01992 J. Phys. Chem. Lett. 2017, 8, 4691−4697
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Light-Emitting Diodes. ACS Appl. Mater. Interfaces 2016, 8, 27006− 27011. (7) Jiao, B.; Zhu, X.; Wu, W.; Dong, H.; Xia, B.; Xi, J.; Lei, T.; Hou, X.; Wu, Z. A Facile One-Step Solution Deposition Via Non-Solvent/ Solvent Mixture for Efficient Organometal Halide Perovskite LightEmitting Diodes. Nanoscale 2016, 8, 11084−11090. (8) Li, J.; Bade, S. G.; Shan, X.; Yu, Z. Single-Layer Light-Emitting Diodes Using Organometal Halide Perovskite/Poly(Ethylene Oxide) Composite Thin Films. Adv. Mater. 2015, 27, 5196−5202. (9) Sadhanala, A.; Ahmad, S.; Zhao, B.; Giesbrecht, N.; Pearce, P. M.; Deschler, F.; Hoye, R. L.; Godel, K. C.; Bein, T.; Docampo, P.; et al. Blue-Green Color Tunable Solution Processable Organolead ChlorideBromide Mixed Halide Perovskites for Optoelectronic Applications. Nano Lett. 2015, 15, 6095−6101. (10) Wang, J.; Wang, N.; Jin, Y.; Si, J.; Tan, Z. K.; Du, H.; Cheng, L.; Dai, X.; Bai, S.; He, H.; et al. Interfacial Control Toward Efficient and Low-Voltage Perovskite Light-Emitting Diodes. Adv. Mater. 2015, 27, 2311−2316. (11) Wang, Z.; Cheng, T.; Wang, F.; Dai, S.; Tan, Z. Morphology Engineering for High-Performance and Multicolored Perovskite LightEmitting Diodes with Simple Device Structures. Small 2016, 12, 4412−4420. (12) Ji, X.; Peng, X.; Lei, Y.; Liu, Z.; Yang, X. Multilayer Light Emitting Devices with Organometal Halide Perovskite: Polymer Composite Emission Layer: The Relationship of Device Performance with the Compositions of Emission Layer and Device Configurations. Org. Electron. 2017, 43, 167−174. (13) Yu, J. C.; Kim, D. W.; Kim, D. B.; Jung, E. D.; Lee, K. S.; Lee, S.; Nuzzo, D. D.; Kim, J. S.; Song, M. H. Effect of the Solvent Used for Fabrication of Perovskite Films by Solvent Dropping on Performance of Perovskite Light-Emitting Diodes. Nanoscale 2017, 9, 2088−2094. (14) Cho, H.; Jeong, S. H.; Park, M. H.; Kim, Y. H.; Wolf, C.; Lee, C. L.; Heo, J. H.; Sadhanala, A.; Myoung, N.; Yoo, S.; et al. Overcoming the Electroluminescence Efficiency Limitations of Perovskite LightEmitting Diodes. Science 2015, 350, 1222−1225. (15) Kim, D. B.; Yu, J. C.; Nam, Y. S.; Kim, D. W.; Jung, E. D.; Lee, S. Y.; Lee, S.; Park, J. H.; Lee, A.-Y.; Lee, B. R.; et al. Improved Performance of Perovskite Light-Emitting Diodes Using a PEDOT:PSS and MoO3 Composite Layer. J. Mater. Chem. C 2016, 4, 8161−8165. (16) Zhao, X.; Zhang, B.; Zhao, R.; Yao, B.; Liu, X.; Liu, J.; Xie, Z. Simple and Efficient Green-Light-Emitting Diodes Based on Thin Organolead Bromide Perovskite Films Via Tuning Precursor Ratios and Postannealing Temperature. J. Phys. Chem. Lett. 2016, 7, 4259− 4266. (17) Chih, Y. K.; Wang, J. C.; Yang, R. T.; Liu, C. C.; Chang, Y. C.; Fu, Y. S.; Lai, W. C.; Chen, P.; Wen, T. C.; Huang, Y. C.; et al. NiOx Electrode Interlayer and CH3NH2/CH3NH3PbBr3 Interface Treatment to Markedly Advance Hybrid Perovskite-Based Light-Emitting Diodes. Adv. Mater. 2016, 28, 8687−8694. (18) Kumawat, N. K.; Jain, N.; Dey, A.; Narasimhan, K. L.; Kabra, D. Quantitative Correlation of Perovskite Film Morphology to Light Emitting Diodes Efficiency Parameters. Adv. Funct. Mater. 2017, 27, 1603219. (19) Groenendaal, B. L.; Jonas, F.; Freitag, D.; Pielartzik, H.; Reynolds, J. R. Poly(3,4-Ethylenedioxythiophene) and Its Derivatives: Past, Present, and Future. Adv. Mater. 2000, 12, 481−494. (20) Hwang, J.; Amy, F.; Kahn, A. Spectroscopic Study on Sputtered PEDOT Center Dot PSS: Role of Surface PSS Layer. Org. Electron. 2006, 7, 387−396. (21) Xiao, M.; Huang, F.; Huang, W.; Dkhissi, Y.; Zhu, Y.; Etheridge, J.; Gray-Weale, A.; Bach, U.; Cheng, Y. B.; Spiccia, L. A Fast Deposition-Crystallization Procedure for Highly Efficient Lead Iodide Perovskite Thin-Film Solar Cells. Angew. Chem., Int. Ed. 2014, 53, 9898−9903. (22) Greczynski, G.; Kugler, T.; Keil, M.; Osikowicz, W.; Fahlman, M.; Salaneck, W. R. Photoelectron Spectroscopy of Thin Films of PEDOT-PSS Conjugated Polymer Blend: A Mini-Review and Some New Results. J. Electron Spectrosc. Relat. Phenom. 2001, 121, 1−17.
15 times enhancement in the PL intensity of the overlying MAPbBr3 layer. Meanwhile, the morphology, composition, and crystal structure of the MAPbBr3 overlayers are nearly unaltered. MAPbBr3 LEDs with a S-PSS-modified PEDOT:PSS HIL show the maximum EQE of 7.2% and PE of 19.0 lm W−1, which are 14−20 times those of the analogous devices with a pristine PEDOT:PSS layer and among the best reported values for the LEDs with a neat MAPbBr3 emission layer. Significant enhancement in device performances can be attributed to the synergistic effects of promoting hole injection and reducing luminescence quenching. A clear correlation between the utilization of various HILs and device performances underlines that insufficient hole injection and luminescence quenching at the PEDOT:PSS anode are among the most important factors limiting the EQEs of inverted OIHP LEDs.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.7b01992. Properties of the OIHP LEDs prepared with different MABr/PbBr2 molar ratios; the AFM images of the HILs; XPS S 2p core-level spectra of the HILs; the top-view SEM images and XRD patterns of the MAPbBr3 layers on top of various HILs; properties of the OIHP LEDs with different HIL thicknesses; and the stability measurements of the OIHP LEDs with different HIL layers. (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Xiao-Hui Yang: 0000-0002-0753-4385 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Prof. Dieter Neher from University of Potsdam for valuable discussion. This work was supported by the National Natural Science Foundation of China (grant nos. 61177030 and 11474232).
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REFERENCES
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