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Nearly 100% Efficiency Enhancement of CHNHPbBr Perovskite Light-Emitting Diodes by Utilizing Plasmonic Au Nanoparticles Ping Chen, Ziyang Xiong, Xiaoyan Wu, Ming Shao, Yan Meng, Zu-hong Xiong, and Chunhong Gao J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b01562 • Publication Date (Web): 08 Aug 2017 Downloaded from http://pubs.acs.org on August 9, 2017

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The Journal of Physical Chemistry Letters

Nearly 100% Efficiency Enhancement of CH3NH3PbBr3 Perovskite Light-Emitting Diodes by Utilizing Plasmonic Au Nanoparticles

Ping Chen,1† Ziyang Xiong,1† Xiaoyan Wu, 2†* Ming Shao,3 Yan Meng,1 Zu-hong Xiong1 and Chunhong Gao1*

1

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

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

3

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

Corresponding Authors: 1*

E-mail: [email protected], Tel: +86-18323426052 (Chunhong Gao).

2*

E-mail: [email protected], Tel: +86-13628089840 (Xiaoyan Wu).

ACS Paragon Plus Environment

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ABSTRACT: Organic-inorganic hybrid perovskites have drawn considerable attentions due to their great potentials in lighting and displaying. Despite great progress has been demonstrated in perovskites light-emitting diodes (PeLEDs), the commercialization of PeLEDs was still limited by their low efficiencies and poor device stabilities. Utilizing the metallic nanoparticles was a feasible way to further improve the efficiencies of PeLEDs. Herein, substantially enhanced electroluminescent performance of CH3NH3PbBr3-based PeLEDs were first demonstrated by incorporating plasmonic gold nanoparticles (Au NPs) into the hole injection layer of poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). Compared to the reference device without Au NPs, 109% enhancement in maximum luminance and 97% enhancement in maximum EQE were achieved upon 9 vol.% Au NPs doping. Such enhancements can be ascribed to the localized surface plasmon resonance between Au NPs and CH3NH3PbBr3 excitons, as well as the enhanced electrical conductivity of modified PEDOT:PSS. Our studies indicated great potential of Au NPs in developing highly efficient PeLEDs.

TOC:


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Solution-processed organic-inorganic hybrid perovskites are promising candidates for next generation light-emitting materials owing to their unique optoelectronic properties such as low cost, extremely high photoluminescence quantum yield (PLQY),1 easily tunable band gap2-4 and narrow emission characteristics5. However, the commercialization of the perovskites lightemitting diodes (PeLEDs) is still limited because of their low efficiencies and poor device stabilities. Much efforts have been devoted to improving the current efficiency of PeLEDs including realization of pinhole-free perovskite film to suppress leakage currents6-21 and optimization of device structures with minimum injection barriers that guarantee the balanced charge injection into perovskites.7-9, 17, 21-29 Besides these attempts, one simple and efficient approach is to utilize the plasmonic metallic nanoparticles (MNPs).30-39 The MNPs exhibited impressive optical and electrical properties in improving the performance of LEDs. One of the most important property of MNPs is the localized surface plasmon resonance (LSPR)30-36 arising from the strong interaction between resonant photons and MNPs, through which photon-induced collective oscillations will take place among free electrons. The LSPR can accelerate the radiative decay rate of nearby excitons through nearfield coupling, facilitating light emission and thus enhancing the performance of the LEDs. In addition, the light scattering caused by the MNPs can help extract photons trapped within the substrate or waveguide modes, which increases the light extraction efficiency of such LEDs.37, 38 Finally, the enhanced electrical property of optoelectronic devices containing MNPs has been experimentally verified by researchers.39,

40

Despite the significant improvements have been


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achieved in organic LEDs incorporating MNPs, the effects of MNPs on PeLEDs have been rarely examined. Recently, S. K. Balakrishnan et al.35 explored a CsPbBr3 perovskite-gold (Au) hybrid nanostructure by adding Au salts (AuBr3 or AuCl3) into the solution of CsPbBr3 nanocrystals (NCs). They observed the formations of Au nanoparticles (NPs) on the corners of CsPbBr3 NCs, resulting in enhanced PL intensity and reduced PLQY of CsPbBr3 NCs. Following this study, plasmonic PeLEDs was developed by X. Zhang et al.36 They inserted the Ag nanorods: N,N’bis(1-naphthalenyl)-N,N’-bis(phenylbenzidine)

(NPB)

mixed

layer

between

poly(3,4-

ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) and CsPbBr3 NCs. Although ~42% enhanced luminance and ~43% enhanced efficiency were demonstrated compared to the controlled PeLEDs without Ag nanorods, the performance of the plasmonic PeLEDs needs further improvement (the maximum current efficiency and the maximal external quantum efficiency (EQE) were only ~1.42 cd/A and ~0.43%, respectively), and the effects of MNPs’ concentration on the optical and the electrical properties of PeLEDs, as well as on the morphology of perovskites should be studied thoroughly. In this study, the synthesized Au NPs with a diameter of ~20 nm were first incorporated into the hole-injection layer (HIL) of PEDOT:PSS for highly efficient plasmonic PeLEDs, where mixed

perovskites

of

poly(9-vinylcarbazole):CH3NH3PbBr3:1,3,5-Tris(1-phenyl-1H-

benzimidazol-2-yl)benzene (PVK:MAPbBr3:TPBi), named as “M-Pero” film, was used as green emitting layer. By varying the volume ratio of Au NPs, the effects of Au NPs concentration on the


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optical and the electrical properties of PeLEDs, as well as on the morphology of PEDOT:PSS and perovskites films were thoroughly investigated. The optimal volume ratio of Au NPs was found to be 9 vol.%, leading to an efficient plasmonic PeLEDs with turn-on voltage of ~3.0 V, maximum luminance of ~16050 cd/m2 and maximum EQE of ~1.83% (corresponding to maximum current efficiency of ~7.64 cd/A). Compared to the reference device without Au NPs, substantially enhancements of 109% increase in maximum luminance and 97% increase in maximum EQE were achieved, respectively. It is believed that the plasmonic Au NPs have great potential in improving the performance of PeLEDs, and the solution processed Au NPs are a more suitable for large-area and low-cost fabrication. Au NPs Synthesis and preparation of Au NPs modified PEDOT:PSS. Au NPs aqueous solution was synthesized through Frens method.33 The detailed synthesis process was described in Supporting Information. Au NPs with ~20 nm diameter size was uniformly dispersed in the aqueous solution, giving total concentration about 1.2 pM. Then, the Au NPs aqueous solution was mixed with PEDOT:PSS (AI4083 CLEVIOS) at desired volume ratios. After three hours stirring at room temperature, the Au NPs modified PEDOT:PSS solution was obtained. Perovskites and Organic Materials. Methylammonium Bromide (MABr, > 99.99%) and Lead Bromide (PbBr2, > 99.99%) were purchased from Xi’an Polymer Light Technology Corp. Dimethylformamide (DMF) and chlorobenzene (anhydrous, > 99.9%) were purchased from AlfaAesar. PVK (99.99%, average Mw ≈ 100,000) was purchased from Alfa-Aesar. TPBi (> 99.0%)


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and cesium carbonate (Cs2CO3) (> 99.99%) was purchased from Suzhou Fangsheng Photoelectricity Shares Co, Ltd. All these materials were used as received. Perovskites Precursor Solution. The M-Pero precursor solution was prepared by introducing PVK:TPBi chlorobenzene solution into MAPbBr3 DMF solution, which were described in our previous work.21 The anti-solvent of chlorobenzene accelerated crystallization of perovskites during the film formation and enhanced the MAPbBr3 perovskites film morphology, while the PVK:TPBi matrix facilitated bipolar charge injection into the MAPbBr3. PeLEDs fabrication. The ITO glass (15 Ω/square) substrates were cleaned by ultrasonic treatment in ethanol, acetone and detergent water sequentially. Then, the substrates are treated with 5 min UV-Ozone plasma (120 W). Then, the Au NPs modified PEDOT:PSS solution was spin-coated onto the ITO substrate (2500 rpm, 40 s) and baked at 120 °C for 20 min under ambient conditions. The M-Pero film was used as the emitting layer and spin-coated onto Au NPs modified PEDOT:PSS film (4000 rpm, 60 s) in the glove-box containing less than 0.1 ppm oxygen and moisture. After that, the M-Pero film was annealed at 80 °C for 10 min. Subsequently, TPBi and Cs2CO3/Al were continuously thermal evaporated under high vacuum of ~5×10-5 Pa. The device active area was ~2 × 3 mm2. Characterizations. After fabrication, the PeLEDs were encapsulated immediately in the glove-box. The current density-luminance-voltage (J-L-V) characteristics were carried out by Keithley2400 source Meter and silicon photodiode calibrated by PR670 Spectra Colorimeter. The steady photoluminescence (PL) and electroluminescence (EL) spectra were measured by Hitachi


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F-2500 spectrometer and PR670 Spectra Colorimeter, respectively. Absorption of Au NPs solution was measured on a Shimadzu UV-2600 spectrophotometer. The time-resolved PL spectra were recorded by using a time-correlated single photon counting system (FL-TCSPC, Horiba Jobin Yvon) with ~375 nm pico-second pulsed laser. Transmission electron microscopy (TEM, JEM-100CXП) was used to characterize the topography of Au NPs, and the M-Pero film morphology of was investigated by scanning electron microscopy (SEM, JEOL JSM-7100F) and atomic force microscopy (AFM, MultiMode Nanoscope III). The crystalline characterizations were determined by X-ray diffraction (XRD) patterns characterized by Shimadzu XRD-7000. The impedance measurements were examined in acetonitrile solution using CHI 760D Electrochemical Workstation impedance analyzer with 5 mV perturbation oscillation signal at a frequency of 0.1~100 kHz. To evaluate the work-function of Au NPs modified PEDOT:PSS film, scanning Kelvin Probe measurements were performed (SKP 5050, KP Technology) with contact potential difference referred to a nominal gold work function of ~5.1 eV.


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Figure 1. (a) The device structure of the PeLEDs incorporating Au NPs in the PEDOT:PSS layer (plasmonic PeLEDs). The volume ratio of Au NPs was 9 vol.%. The inset ① shows the high-resolution TEM images of the Au NPs, and the inset ② shows the morphology of M-Pero film on the Au NPs modified PEDOT:PSS layer. (b) FDTD stimulation of the electromagnetic field distributed around the Au NPs. (c) The energy level diagram of the plasmonic PeLEDs. (d) The absorption spectra of Au NPs solution, PL and EL spectra of M-Pero film. The inset ③ shows the photograph of the plasmonic PeLEDs working at 7 V.

A multilayered structure of ITO/PEDOT:PSS with 9 vol. % Au NPs/PVK:MAPbBr3:TPBi (M-Pero)/TPBi/Cs2CO3/Al was created as the plasmonic PeLEDs, as shown in Figure 1 (a). According to the high-resolution TEM images displayed in the inset ①, Au NPs were spherical in shape and uniformly dispersed with an average diameter of ~20 nm. However, such bare Au NPs cannot be directly used in the emitting layer because exciton quenching will take place via


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nonradiative energy transfer process at the surfaces of the Au NPs.33, 35, 41, 42 Therefore, “capping” was usually required for Au NPs.33, 34, 36, 41, 42 In this study, Au NPs were capped by PEDOT:PSS via dispersing its aqueous solution to PEDOT:PSS, and co-deposited. When the volume ratio of Au NPs was 9 vol.%, the Au NPs (~20 nm) were fully embedded in ~35 nm thick PEDOT:PSS layer (please see cross-section SEM images of PEDOT:PSS layers without and with Au NPs in Figure S1). Furthermore, AFM images of PEDOT:PSS layer without and with 9 vol.% Au NPs were shown in Figure S2. The surface of PEDOT:PSS kept almost identical after 9 vol.% Au NPs doping. The root-mean-squared roughness of 1.01 nm and 1.10 nm were obtained for PEDOT:PSS without and with 9 vol.% Au NPs, respectively. Such smooth surface benefitted the formation of high-quality perovskites emitting layer. As shown in the inset

②

of Figure 1 (a), dense and

uniform M-Pero film was obtained on the surface of 9 vol.% Au NPs modified PEDOT:PSS layer, which was critical to suppress the leaking current loss for highly efficient PeLEDs. Moreover, it should be mentioned that the LSPR is a “near-field effect” within the range of tens of nanometers,30-36 and in our case exciton formation area can fall into the electromagnetic field of Au NPs, which enables surface plasmon coupling. This argument can be understood from the following three aspects. First, the ~20 nm Au NPs were just embedded in PEDOT:PSS in our work. Second, the finite-different time-domain (FDTD) simulation of LSPR intensity profile at axis line (x=0) in Figure 1 (b) indicated the effective range of LSPR was ~10 nm. Third, the exciton recombination zone would be very close to the PEDOT:PSS/CH3NH3PbBr3 interface, which can be deduced from the energy level diagram of the plasmonic PeLEDs, as shown in


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Figure 1 (c). The energy level values were taken from Ref. [43]. It can be seen that the injection of holes from PEDOT:PSS into MAPbBr3 layer was partly prohibited due to the large energy barrier of ~0.5-0.6 eV, while electrons can be easily injected into MAPbBr3 from TPBi. As a result, the exciton recombination zone would be very close to the PEDOT:PSS/ M-Pero interface. This assertion was clarified by Y. H. Kim et al.5 and D. B. Kim et al.23 whom further modified the PEDOT:PSS with high work function (WF) materials of perfluorinated ionomer (PFI) and MoO3 respectively to enhance the hole injection efficiency. In our case, the fact that exciton recombination zone were located closely to the PEDOT:PSS/M-Pero interface indicated that the exciton formation area can fall into the electromagnetic fields of Au NPs, enabling the LSPR coupling in spatial. Another prerequisite for LSPR effect was sufficient overlap between the absorption of Au NPs and the PL (and/or EL) emission of M-Pero film.35, 36 The absorption of Au NPs solution was plotted in Figure 1 (d), meanwhile the PL spectra of M-Pero film and EL spectra of PeLEDs were displayed for comparison. It was shown that the absorption peak of Au NPs was located at ~520 nm, which matched well with the emission peaks of M-Pero film (the PL and EL peaks were both located at ~534 nm). The large-scale overlap between the absorption of Au NPs and the PL (and/or EL) of M-Pero film will lead to emission enhancement of MAPbBr3 perovskites.


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Figure 2. (a) The J-V characteristics, (b) L-V characteristics, (c) current efficiency and (d) EQE versus voltage of the of the plasmonic PeLEDs with different volume ratios of Au NPs.

Table 1. Summary of performances for plasmonic PeLEDs with different volume ratios of Au NPs. Volume ratio Turn-on voltage Maximum Maximum Current (vol. %) (V) Luminance (cd/m2) Efficiency (cd/A)

Maximum EQE (%)

0

3.2

7673

3.91

0.93

5

3.0

8087

4.97

1.19

9

3.0

16050

7.64

1.83

17

3.0

12668

7.57

1.81

33

3.2

9921

6.19

1.48

50

3.4

1545

0.98

0.23

66

3.2

779

0.45

0.11


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The J-L-V characteristics of the plasmonic PeLEDs were illustrated in Figure 2. More detailed parameters were summarized in Table 1. The J-V characteristics in Figure 2 (a) indicated the device currents increased monotonously with increasing the volume ratio of Au NPs. The L-V characteristics of the plasmonic PeLEDs were displayed in Figure 2 (b). Different to the monotonic trend of J-V characteristics with the volume ratio of Au NPs, two different evolutions of the maximum luminance were observed. When the Au NPs volume ratio was lower than 9 vol.%, the maximum luminance increased by increasing the Au NPs volume ratio. The optimized maximum luminance was ~16050 cd/m2 when 9 vol.% Au NPs was used, which was 109% enhancement as compared to the maximum luminance of PeLEDs without Au NPs (only ~7673 cd/m2). Moreover, the turn-on voltage (defined as the driving voltage at luminance of ~1 cd/m2) of PeLEDs was slightly lowered from ~3.2 V to ~3.0 V. In the higher range of Au NPs volume ratio > 9 vol.%, the maximum luminance decreased continually with the Au NPs volume ratio, and finally dropped below the level of PeLEDs without Au NPs if the volume ratio was higher than 33 vol.%. The current efficiency and EQE of the plasmonic PeLEDs were shown in Figure 2 (c) and (d), respectively. Very similar to the trend of L-V characteristics, the maximum current efficiency and EQE first increased with Au NPs in the low volume ratio of 0< Au NPs volume ratio ≤ 9 vol.%, and the optimized maximum current efficiency (~7.64 cd/A) and EQE (~1.83%) were achieved at 9 vol.% Au NPs. Compared to the maximum current efficiency and EQE of PeLEDs without Au NPs, substantial enhancements of 97% were obtained. By further increasing the volume ratio of Au NPs, both current efficiency and EQE started to decrease. The EL spectra of


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plasmonic PeLEDs with 9 vol.% Au NPs were exhibited in Figure S3. After normalization, all EL spectra were identical to each other, indicating the emissions merely came from MAPbBr3 excitons, and the incorporation of Au NPs will not change the color purity of PeLEDs. It was expected that Au NPs will not only cause substantially modified optical and electrical properties of M-Pero film, but also cause notable morphology changes to PEDOT:PSS and MPero film. The final EL performance of plasmonic PeLEDs should be synergetic contributions from (i) optical effects, (ii) electrical effects and (iii) morphology changes induced by Au NPs, which will be discussed carefully in the following parts.

Figure 3. (a) The time-resolved PL spectra and (b) the steady PL spectra of M-Pero film with different volume ratio of Au NPs. (a) and (b) shared the same labels of curves. The insets ① and ② in (b) show the photograph of M-Pero films with and without Au NPs modified PEDOT:PSS before and after ~365 nm UV lamp irradiation, respectively. (c) The curves of PL intensity at ~534 nm peak and average lifetime deduced from time-resolved PL spectra versus the volume ratio of Au NPs.

The influences of Au NPs on the optical properties of M-Pero film were shown in Figure 3. Time-resolved PL spectra and steady PL measurements can provide direct evidence of accelerated


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radiative decay of excitons and PL enhancement of M-Pero film in the presence of Au NPs, respectively. As shown in Figure 3 (a), compared to the PL decay of M-Pero film without Au NPs, PL decayed faster when Au NPs were incorporated into the PEDOT:PSS. Moreover, the higher volume ratio of Au NPs was incorporated, more remarkable PL decays can be seen. By fitting the PL decay curves with a tri-exponential decay model,21, 44 three decay components were obtained, which can be denoted as τ1 (fast), τ2 (middle) and τ3 (slow). The τ3 (slow) component was related to “radiative recombination inside the grains”, while the τ1 (fast) and τ2 (middle) components were attributed to “two kinds of trap-assisted recombination at grain boundaries.” The fitting parameters with different volume ratio of Au NPs were summarized in Table S1. It is very useful to discuss how each component changes by Au NPs. Typically, a shorter lifetime indicated the acceleration of the corresponding recombination process. Compared to the M-Pero film without Au NPs, the M-Pero film using 9 vol.% Au NPs exhibited shorter τ3 and larger A3, verifying the radiative recombination inside the grains is accelerated by LSPR effect. In contrast, τ1 and τ2 became longer, but A1 and A2 became smaller upon 9 vol.% Au NPs doping. Thus, the trap-assisted recombination was suppressed. Therefore, the PL intensity was enhanced. By further increasing the concentration of Au NPs to 33 vol.%, τ3 started to increase but τ1 and τ2 started to decrease, indicating the recovery of trap-assisted recombination in this condition. This is because the Au NPs were not completely embedded in the PEDOT:PSS (discussed later in Figure 5), forming non-radiative channels for perovskite excitons.


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The average lifetime (τavg) was calculated according to:

τ avg =

A1τ 1 + A2τ 2 + A3τ 3 A1 + A2 + A3

(1)

Then, the curve of τavg versus the volume ratio of Au NPs was plotted in Figure 3 (c). The τavg of M-Pero film without Au NPs was ~6.12 ns, whereas it decreased to ~3.98 ns when the volume ratio of Au NPs increased to 9 vol.%. By further increasing the volume ratio of Au NPs to 66 vol.%, τavg continuously decreased to ~0.29 ns. It has been demonstrated that the variation of τavg by Au NPs could strongly influence the PL intensity.30-36 The steady PL spectra of M-Pero film with different volume ratios of Au NPs were exhibited in Figure 3 (b). To identify the relationship between τavg and PL intensity, the curve of PL intensity at ~534 nm peak versus the volume ratio of Au NPs was also plotted in Figure 3 (c) for comparison. In the range of 0 to 66 vol.%, it exhibited a monotonously increase of PL intensity with increasing the volume ratio of Au NPs. The maximal PL intensity was found to be ~1.85× 10-2 W/sr·m2 in 66 vol.% Au NPs modified M-Pero film which was 4.2 fold stronger as the PL intensity of the film without Au NPs modifying (only of ~4.39×10-3 W/sr·m2). Opposite trends of τavg and PL intensity versus volume ratios of Au NPs indicated the enhancement of PL intensity was strongly correlated to the decreased τavg, which can be explained as accelerated radiative decay of excitons by Au NPs induced LSPR effects.30-36 The accelerated radiative decay rate will prevent excitons from non-radiatively loss at MAPbBr3 perovskites boundaries43 and/or at PEDOT:PSS/M-Pero interface5, 23, and thus enhance the PL intensity.


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The light scattering effect of Au NPs may be another mechanism responsible for the PL enhancement of M-Pero film. The angular emission test is helpful in examining the scattering effect of Au NPs. 37 We conducted this experiment, and the results were shown in Figure S4. The PL intensity data were chosen using the average value obtained from three measurements. As can be seen, within the angle of 10° - 90°, both of the devices with and without Au NPs have the almost identical emission patterns approaching to Lambertian emission profile, and the emission intensity is decreased gradually with the increase of viewing angle. If the scattering effect was the dominant mechanism, the normalized PL intensity will have more distribution at large angles, thus higher PL intensity will be observed in M-Pero films with 9 vol.% Au NPs. However, Figure S4 shows that at large angles (> 0°) the normalized PL intensity was even lower after 9 vol.% Au NPs doping. Thus, the scattering effect was insignificant in our case. Finally, the concentration of Au NPs was very low (only 9 vol.%) and size of Au NPs was very small (only ~20 nm), which also weakening the scattering effects. Therefore, we concluded that the light scattering effect did not play an important role in the PL enhancement of M-Pero film.


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Figure 4. (a) The J-V characteristics of hole-dominant devices of ITO/ Au NPs modified PEDOT:PSS / PVK:MAPbBr3:TPBi/ Al with different volume ratios of Au NPs. (b) The work function and (c ) the impedance spectra of Au NPs modified PEDOT:PSS films with different volume ratios of Au NPs.

Besides optical effects on M-Pero film, the Au NPs will cause notable electrical changes of PeLEDs. The J-V characteristics of hole-dominated devices of ITO/Au NPs modified PEDOT:PSS /PVK:MAPbBr3:TPBi/Al were used to examine the role of Au NPs in hole injection of the PeLEDs, as shown in Figure 4 (a). Compared to the reference device without Au NPs, the hole-dominated current increased apparently when Au NPs were incorporated into PEDOT:PSS. Moreover, the higher volume ratio of Au NPs was incorporated, the higher hole-dominated current density can be seen. In order to understand the origin of such Au NPs-enhanced hole injection, work functions and electrical conductivity of Au NPs modified PEDOT:PSS films were measured, respectively. First, the work functions of Au NPs modified PEDOT:PSS films were determined by scanning Kelvin Probe techniques. The average of work function data and standard deviations were obtained from 10 measurements. As shown in Figure 4 (b), the work function was around


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~5.18 ± 0.04 eV, and changed very little with different volume ratios of Au NPs. Our observations were coincident with the ultraviolet photoelectron spectroscopy (UPS) measurements of Au NPs modified PEDOT:PSS carried out by Y. Xiao et al.34 They also observed the independence of the work function on the volume ratio of Au NPs, and explained it by the similarity between the work function of Au NPs (~5.1 eV) and the HOMO value of PEDOT:PSS (~5.2 eV). This result indicated that the work function variation of Au NPs modified PEDOT:PSS was not the main cause for enhanced hole injection efficiency. Then, the impedance measurements were performed to evaluate the conductivity of Au NPs modified PEDOT:PSS films. As shown in Figure 4 (c), the resistance of Au NPs modified PEDOT:PSS films decreased continually by increasing the volume ratio of Au NPs, which means higher electrical conductivity of PEDOT:PSS film upon Au NPs doping. Obviously, enhancing the electrical conductivity by Au NPs was beneficial to the hole transporting in PEDOT:PSS film, and ultimately improved the hole injection efficiency into the M-Pero film.


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Figure 5. SEM images of (a)-(d) PEDOT:PSS and (e)-(h) M-Pero film on PEDOT:PSS, with different volume ratios of Au NPs.

The influence of the Au NPs volume ratio on the morphology of PEDOT:PSS and M-Pero film was shown in Figure 5 (a)-(d) and Figure 5 (e)-(f), respectively. As shown in Figure 5 (a) and (b), Au NPs can be fully embedded in PEDOT:PSS after 9 vol.% Au NPs doping, and the surface morphology of PEDOT:PSS remained almost identical as compared to that of PEDOT:PSS without Au NPs doping. Nevertheless, when volume ratios of Au NPs increased from 9 vol.% to 33 vol.%, Au NPs started to appear on the surface of PEDOT:PSS, as shown in Figure 5 (c). By further increasing the volume ratios of Au NPs to 66 vol.%, as shown in Figure 5 (d), we observed more and more exposed Au NPs. One important reason for Au NPs exposure was that the PEDOT:PSS was too thin to cap Au NPs completely due to the low concentration of PEDOT:PSS. In this situation, Au NPs will directly contact with subsequent MAPbBr3 perovskites, and the nonradiative decay of exciton formed on perovskite through energy transfer will take place at Au


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NPs surface.33,

35, 40, 41

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Such nonradiative decay induced by Au NPs will regress the EL

performance of plasmonic PeLEDs. Additionally, the morphology of M-Pero film was also found to be deteriorated upon high volume ratios of Au NPs doping. When M-Pero film was deposited on the pure PEDOT:PSS containing no Au NPs , nearly full coverage of MAPbBr3 crystalline film was obtained with grain size of ~300 to 800 nm, which was consistent with our previous results.21 After adding small amount of Au NPs (e.g., 9 vol.% case), the film coverage was still very high with uniform and dense stacking of MAPbBr3 crystals, as shown in Figure 5 (f). Such high quality film was critical to suppress the leaking current loss in the PeLEDs, and meanwhile the MAPbBr3 perovskites can take good advantages of unique optical and electrical properties of Au NPs. When the volume ratio of Au NPs kept increasing from 33 vol.% to 66 vol.%, the morphology of M-Pero film became poor, leaving observable pinholes accounting for large leaking current. As shown in Figure 5 (g) and (h), many isolated MAPbBr3 cubic grains with larger size ranging from ~500 to 1500 nm aggregated separately on the top surface of Au NPs highly doped PEDOT:PSS layer, thus further enlarging the areas of pinholes. Therefore, it was believed that highly Au NPs doping was unfavorable for good morphology formation of M-Pero film, and the leaking current resulted in the high device current density in Plasmonic PeLEDs with 33 vol.% and 66 vol.% Au NPs, as shown in Figure 2 (a). It has been shown that PL intensity kept increasing when the Au NPs doping rate increased from 0 to 66 vol.%. Although Au NPs started to expose on the surface of PEDOT:PSS when high


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concentration of Au NPs (e.g., 33% and 66%) was used, it should be noted that the LSPR effects were also enhanced with larger amount of Au NPs, which overwhelms the negative effects of Au NPs exposing (evidenced by A3 value > 84%). Therefore, the PL intensity kept increasing when the doping rate increased from 0 to 66%. But for the EL condition, the serious morphology destroy should be taken into account, resulting in large leaking current in the device which will reduce the efficiency of PeLEDs. By considering all these effects, the EL performance decreased when concentration of Au NPs was higher than 9%. To rule out the influence of crystallizations of MAPbBr3 perovskites by Au NPs, XRD patterns of M-Pero film on the top of PEDOT:PSS with different volume ratios of Au NPs were characterized, as shown in Figure S5. All XRD patterns were identical to each other, giving same diffraction peaks at 14.90°, 21.08°, 29.98°, 33.62°, 36.92°, 42.94° and 45.68° which were assigned to the (100), (110), (200), (210), (211), (220) and (300) planes of perovskite structure, respectively.21 This result indicated that the crystallization properties of MAPbBr3 perovskites were not influenced by the incorporation of Au NPs. From above discussions, it can be seen that the incorporation of Au NPs into PEDOT:PSS can cause “three prominent effects” in plasmonic PeLEDs, which governed the final EL performance of plasmonic PeLEDs. Specifically, “three prominent effects” were LSPR coupling between Au NPs and MAPbBr3 excitons, enhanced electrical conductivity of Au NPs modified PEDOT:PSS and morphology controls of both PEDOT:PSS and M-Pero film. Obviously, LSPR coupling and enhanced electrical conductivity were positive effects for improvements in EL


21


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performance of plasmonic PeLEDs, while the deteriorated morphology including Au NPs exposing and large pinholes among MAPbBr3 perovskites crystals were considered as negative effects responsible for the EL performance regression of plasmonic PeLEDs. In the lower range of Au NPs volume ratios ≤ 9 vol.%, the MAPbBr3 perovskites film kept almost full coverage and Au NPs were fully embedded in PEDOT:PSS, thus the positive effects were dominant in plasmonic PeLEDs. As a result, increasing the Au NPs volume ratio will enhance the positive effects of Au NPs, which was beneficial to the EL performance of plasmonic PeLEDs. While in the higher range of Au NPs volume ratios > 9 vol.%, the EL performance of plasmonic PeLEDs started to decrease. This is because the Au NPs were exposed on the PEDOT:PSS surface and the morphology of M-Pero film became poor. When negative effects overwhelmed the positive effects, the EL performance of plasmonic PeLEDs will drop below the level of referenced PeLEDs without Au NPs, as evidenced by very poor EL efficiency of the plasmonic PeLEDs at Au NPs volume ratio of 50 vol.% and 66 vol.% (Figure 2 (c)). In summary, we have demonstrated an efficient plasmonic PeLEDs by simply incorporating chemically synthesized Au NPs with a diameter of ~20 nm into the spin-coated PEDOT:PSS film. The plasmonic PeLEDs gave efficient emissions from MAPbBr3 perovskites with turn-on voltage of ~3.0 V, maximum luminance of ~16050 cd/m2 and maximum EQE of ~1.83% (corresponding to maximum current efficiency of ~7.64 cd/A). Compared to the reference PeLEDs without Au NPs, 109% enhancement in maximum luminance and 97% enhancement in maximum EQE were achieved without spectral shifting. Such substantial enhancements of EL performance can be


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ascribed to the LSPR coupling between Au NPs and MAPbBr3 excitons, as well as the enhanced hole injection by incorporating higher electrical conductivity of Au NPs into PEDOT:PSS. However, the morphology of PEDOT:PSS and M-Pero film will be seriously destroyed upon highly Au NPs doping, causing Au NPs exposure and large leaking current loss in the plasmonic PeLEDs. Therefore, EL performance of plasmonic PeLEDs regressed. Our results showed that plasmonic Au NPs have great potential in developing highly efficient PeLEDs.


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AUTHOR INFORMATION Corresponding Authors: 1* E-mail: [email protected], Tel: +86-18323426052 (Chunhong Gao) 2*

E-mail: [email protected], Tel: +86-13628089840 (Xiaoyan Wu)

† P. Chen, Z. Xiong, and X. Wu contributed equally to this work. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was financially supported by National Natural Science Foundation (NSF) of China (Grant Nos. 61404108, 11504300 and 11374242), NSFA of China (Grant Nos. U1630125) Natural Science Foundation Project of CQ CSTC (Grant Nos. cstc2015jcyjA50002), and Fundamental Research Funds for the Central Universities (Grant Nos. XDJK2017D140).

SUPPORTING INFORMATION (1) Synthesis of Au NPs aqueous solution, (2) Cross-section SEM images of PEDOT:PSS layer without and with 9 vol.% Au NPs, (3) AFM images of PEDOT:PSS layer without and with 9 vol.% Au NPs, (4) The normalized electroluminescent intensity of PEDOT:PSS layer without and with 9 vol.% Au NPs, (5) Fitting parameters of transient PL decay curves, (6) The normalized angular photoluminescence intensity of M-Pero film with and without 9 vol.% Au NPs, (7) XRD patterns of M-Pero films deposited on Au NPs modified PEDOT:PSS with different volume ratios of Au NPs.


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