Improved Efficiency of Perovskite Light-Emitting Diodes Using a Three-Step Spin-Coated CH3NH3PbBr3 Emitter and a PEDOT:PSS/MoO3-Ammonia Composite Hole Transport Layer

High efficiency perovskite light-emitting diodes (PeLEDs) using PEDOT:PSS/MoO3-ammonia composite hole transport layers (HTLs) with different MoO3-ammonia ratios were prepared and characterized. For PeLEDs with one-step spin-coated CH3NH3PbBr3 emitter, an optimal MoO3-ammonia volume ratio (0.02) in PEDOT:PSS/MoO3-ammonia composite HTL presented a maximum luminance of 1082 cd/m2 and maximum current efficiency of 0.7 cd/A, which are 82% and 94% higher than those of the control device using pure PEDOT:PSS HTL respectively. It can be explained by that the optimized amount of MoO3-ammonia in the composite HTLs cannot only facilitate hole injection into CH3NH3PbBr3 through reducing the contact barrier, but also suppress the exciton quenching at the HTL/CH3NH3PbBr3 interface. Three-step spin coating method was further used to obtain uniform and dense CH3NH3PbBr3 films, which lead to a maximum luminance of 5044 cd/m2 and maximum current efficiency of 3.12 cd/A, showing enhancement of 750% and 767% compared with the control device respectively. The significantly improved efficiency of PeLEDs using three-step spin-coated CH3NH3PbBr3 film and an optimum PEDOT:PSS/MoO3-ammonia composite HTL can be explained by the enhanced carrier recombination through better hole injection and film morphology optimization, as well as the reduced exciton quenching at HTL/CH3NH3PbBr3 interface. These results present a promising strategy for the device engineering of high efficiency PeLEDs.


Introduction
Taking advantage of high photoluminescence quantum yield (PLQY), excellent color purity, high carrier mobility and low-temperature solution-processing, organometal halide perovskites have been studied extensively for their applications in solution-processed light-emitting diodes (LEDs) [1,2]. Since Friend's group reported the first demonstration about room-temperature infrared and green light emission observed in LEDs with CH 3 NH 3 PbX 3 (X is I − , Bror Cl − ) perovskite emission layers (EML) in 2014 [3], organic-inorganic perovskite light-emitting diodes (PeLEDs) have attracted much attention

Device Fabrication
Our PeLEDs were prepared on pre-patterned ITO-coated glass substrates with the sheet resistance of~15 Ω/m 2 . The basic device structure is ITO/composite HTL/CH 3 NH 3 PbBr 3 /TPBi/LiF/Al. Typically, the substrates were cleaned ultrasonically in acetone, methanol and deionized water for 5 min sequentially. After drying with a nitrogen gun, the substrates were treated by oxygen plasma for 5 min in order to modify the work function of ITO effectively.
The PEDOT:PSS and PEDOT:PSS/MoO 3 -ammonia composite layers were spin-coated onto the substrates at 8000 rpm for 30 s, and then annealed at 150 • C for 15 min in a nitrogen atmosphere. For the one-step spin coating, the perovskite precursor solution was spin-coated at 8000 rpm for 30 s, and then annealed at 80 • C for 10 min. While for the three-step spin coating, the precursor was spin-coated by three times with sequential speeds of 2000, 4000 and 6000 rpm for 30 s, followed by annealing at 80 • C for 10 min for each-step spin coating. No anti-solvent and other additives were used in the spin coating of CH 3 NH 3 PbBr 3 layers.
Finally, the substrates were transferred to a physical vapor thermal evaporation system, in which a 30 nm TPBi, a 0.5 nm LiF and a 100 nm Al were deposited sequentially for electron transport layer (ETL), electron injection layer (EIL) and cathode in a base pressure of~3 × 10 −7 Torr respectively. Each substrate contains four devices with the active area of 0.1 cm 2 . All PeLEDs were encapsulated simply with cover glass slides in the glovebox and then tested immediately in ambient air.

Device Characterization
The thickness of PEDOT:PSS, PEDOT:PSS/MoO 3 -ammonia composite and perovskite films were recorded by an Alpha-Step D-600 stylus profiler (KLA Corporation, Milpitas, CA, USA). The absorption spectra, transmittance spectra and photoluminescence (PL) spectra were carried out with a HITACHI U-3900 ultraviolte/visible spectrophotometer and a HITACHI F-4600 luminescence spectrometer (Japan), respectively. The surface morphology of perovskite films were observed with a scanning electron microscopy (SEM, FEI Sirion FEG, FEI Corporation, Eindhoven, Netherlands). X-ray diffraction (XRD) patterns were measured with a PANalytical Empyrean X-ray diffractometer (PANalytical B. V., Almelo, Netherlands). The luminance-current density-voltage (L-J-V) characteristics of PeLEDs were tested using a Keithley 2400 source meter and a Keithley 2000 multimeter (Tektronix, Inc., Beaverton, OR, USA) coupled with a calibrated silicon photodetector (1 cm in diameter), which capture and convert photons emitted from the glass side. The electroluminescence (EL) spectra of the devices were monitored by an Ocean Optics fiber-optic spectrometer (Ocean Optics, Inc., Largo, FL, USA). Figure 1a shows the schematic structure of our PeLEDs, which consist of ITO as a transparent anode, the PEDOT:PSS/MoO 3 -ammonia composite as a HTL, CH 3 NH 3 PbBr 3 as an EML, TPBi as an ETL, LiF as an EIL, and Al as a cathode, respectively. To obtain good surface morphology of CH 3 NH 3 PbBr 3 films, they were prepared by one-step and three-step spin coating of a CH 3 NH 3 PbBr 3 precursor respectively. Figure 1b shows energy level diagrams of the PeLEDs with a pure PEDOT:PSS HTL. It is noted that the energy barrier between the PEDOT:PSS and CH 3 NH 3 PbBr 3 layers is~0.5 eV, which may result in low device efficiency. The doping of MoO 3 -ammonia in the PEDOT:PSS is expected to increase the work function of the PEDOT:PSS HTL and correspondingly reduce the contact barrier between the HTL and the CH 3 NH 3 PbBr 3 EML for efficient hole injection [25,26].

Results and Discussion
Micromachines 2019, 10, x 4 of 12 characteristics of PeLEDs were tested using a Keithley 2400 source meter and a Keithley 2000 multimeter (Tektronix, Inc., Beaverton, OR, USA) coupled with a calibrated silicon photodetector (1 cm in diameter), which capture and convert photons emitted from the glass side. The electroluminescence (EL) spectra of the devices were monitored by an Ocean Optics fiber-optic spectrometer (Ocean Optics, Inc., Largo, FL, USA). Figure 1a shows the schematic structure of our PeLEDs, which consist of ITO as a transparent anode, the PEDOT:PSS/MoO3-ammonia composite as a HTL, CH3NH3PbBr3 as an EML, TPBi as an ETL, LiF as an EIL, and Al as a cathode, respectively. To obtain good surface morphology of CH3NH3PbBr3 films, they were prepared by one-step and three-step spin coating of a CH3NH3PbBr3 precursor respectively. Figure 1b shows energy level diagrams of the PeLEDs with a pure PEDOT:PSS HTL. It is noted that the energy barrier between the PEDOT:PSS and CH3NH3PbBr3 layers is ~0.5 eV, which may result in low device efficiency. The doping of MoO3-ammonia in the PEDOT:PSS is expected to increase the work function of the PEDOT:PSS HTL and correspondingly reduce the contact barrier between the HTL and the CH3NH3PbBr3 EML for efficient hole injection [25,26].  (~30 nm for one-step spin coating, ~55 nm for three-step spin coating)], respectively. The effect of the small MoO3-ammonia amount on the surface morphology of CH3NH3PbBr3 film is not evident. As reported in the literature, multi-step spin coating is expected to improve the surface morphology of perovskite film [27,28]. As shown in Figure 2, uniform and compact perovskite films with enhanced crystallinity formed on increasing the coating times from one to three.  Figure 2 shows the SEM images of CH 3 NH 3 PbBr 3 films prepared on pure PEDOT:PSS and PEDOT:PSS/MoO 3 -ammonia composite HTLs [glass/ITO/composite HTL (~40 nm)/CH 3 NH 3 PbBr 3 (~30 nm for one-step spin coating,~55 nm for three-step spin coating)], respectively. The effect of the small MoO 3 -ammonia amount on the surface morphology of CH 3 NH 3 PbBr 3 film is not evident. As reported in the literature, multi-step spin coating is expected to improve the surface morphology of perovskite film [27,28]. As shown in Figure 2, uniform and compact perovskite films with enhanced crystallinity formed on increasing the coating times from one to three. Figure 3 shows XRD patterns of one-step and three-step coated CH 3 NH 3 PbBr 3 films (glass/composite HTL (~40 nm)/CH 3 NH 3 PbBr 3 (~30 nm for one-step spin coating,~55 nm for three-step spin coating)). All the XRD patterns show two characteristic peaks at 15 • and 30 • , assigned to (100) and (200) crystal planes respectively, suggesting the crystal growth orientation along (100) planes. As shown in Figure 3a, the intensity of diffraction peaks was enhanced in three-step spin-coated CH 3 NH 3 PbBr 3 film compared with one-step spin-coated CH 3 NH 3 PbBr 3 film on pure PEDOT:PSS film, suggesting a better crystallization on increasing the coating time from one to three. As shown in Figure 3b, on increasing the ratio of MoO 3 -ammonia from 0 to 0.03, the intensity of diffraction peaks of three-step spin-coated CH 3 NH 3 PbBr 3 film increases monotonically. The similar trend is also found in the one-step spin-coated CH 3 NH 3 PbBr 3 films with different MoO 3 -ammonia ratios. This result is well consistent with previously reported results [25,26], which may be explained by that the MoO 3 particles can act as crystal nuclei for the growth of spin-coated perovskite film. Micromachines 2019, 10, x 5 of 12  Figure 3 shows XRD patterns of one-step and three-step coated CH3NH3PbBr3 films (glass/composite HTL (~40 nm)/CH3NH3PbBr3 (~30 nm for one-step spin coating, ~55 nm for threestep spin coating)). All the XRD patterns show two characteristic peaks at 15° and 30°, assigned to (100) and (200) crystal planes respectively, suggesting the crystal growth orientation along (100) planes. As shown in Figure 3a, the intensity of diffraction peaks was enhanced in three-step spincoated CH3NH3PbBr3 film compared with one-step spin-coated CH3NH3PbBr3 film on pure PEDOT:PSS film, suggesting a better crystallization on increasing the coating time from one to three. As shown in Figure 3b, on increasing the ratio of MoO3-ammonia from 0 to 0.03, the intensity of diffraction peaks of three-step spin-coated CH3NH3PbBr3 film increases monotonically. The similar trend is also found in the one-step spin-coated CH3NH3PbBr3 films with different MoO3-ammonia ratios. This result is well consistent with previously reported results [25,26], which may be explained by that the MoO3 particles can act as crystal nuclei for the growth of spin-coated perovskite film.     Figure 3 shows XRD patterns of one-step and three-step coated CH3NH3PbBr3 films (glass/composite HTL (~40 nm)/CH3NH3PbBr3 (~30 nm for one-step spin coating, ~55 nm for threestep spin coating)). All the XRD patterns show two characteristic peaks at 15° and 30°, assigned to (100) and (200) crystal planes respectively, suggesting the crystal growth orientation along (100) planes. As shown in Figure 3a, the intensity of diffraction peaks was enhanced in three-step spincoated CH3NH3PbBr3 film compared with one-step spin-coated CH3NH3PbBr3 film on pure PEDOT:PSS film, suggesting a better crystallization on increasing the coating time from one to three. As shown in Figure 3b, on increasing the ratio of MoO3-ammonia from 0 to 0.03, the intensity of diffraction peaks of three-step spin-coated CH3NH3PbBr3 film increases monotonically. The similar trend is also found in the one-step spin-coated CH3NH3PbBr3 films with different MoO3-ammonia ratios. This result is well consistent with previously reported results [25,26], which may be explained by that the MoO3 particles can act as crystal nuclei for the growth of spin-coated perovskite film.    Figure 4 shows steady-state PL spectra of one-step and three-step coated CH 3 NH 3 PbBr 3 films [glass/ITO/composite HTL (~40 nm)/CH 3 NH 3 PbBr 3 (~30 nm for one-step spin coating,~55 nm for three-step spin coating)] conducted by using a luminescence spectrometer with an excitation wavelength of 315 nm. All the PL spectra show a well-defined peak at~528 nm. As shown in Figure 4a, PL intensity was enhanced in three-step spin-coated CH 3 NH 3 PbBr 3 film compared with one-step spin-coated CH 3 NH 3 PbBr 3 film on pure PEDOT:PSS film. It can be explained by that the amount and morphology of perovskite material affect the PL intensity, namely more excitons will be generated by increasing the amount of perovskite particles, leading to the enhancement of the PL intensity when the thickness of perovskite film increases from~30 nm (one-step coating) tõ 55 nm (three-step coating) shown in Figure 4a. As shown in Figure 4b, on increasing the ratio of MoO 3 -ammonia from 0 to 0.02, the PL intensity of three-step spin-coated CH 3 NH 3 PbBr 3 film gradually increases, while the further increase of the amount of MoO 3 -ammonia leads to the decrease of the PL intensity. The similar trend is also found in the one-step spin-coated CH 3 NH 3 PbBr 3 films with different MoO 3 -ammonia ratios. It is suggested that the optimal MoO 3 -ammonia ratio is beneficial for blocking the exciton quenching at the HTL/CH 3 NH 3 PbBr 3 interface, while the excessive MoO 3 -ammonia ratio is unfavorable. These results may be due to the increase of MoO 3 on top of the HIL separating excitons generated in the CH 3 NH 3 PbBr 3 EML from the quenching of PEDOT:PSS. However, on increasing the MoO 3 -ammonia amount, dopant aggregation or trap states may also occur at the HTL/EML interface, leading to the decay of photoluminescence. Figure 4c shows the transmittance spectra of PEDOT:PSS and PEDOT:PSS/MoO 3 -ammonia composite films with different MoO 3 -ammonia ratios (glass/ITO/composite HTL (~40 nm)). As shown in transmittance spectra, a small amount of MoO 3 has little effect on the transmittance of PEDOT:PSS/MoO 3 -ammonia composite films in the visible range. The transmittances of four samples are near-identical, indicating that the doping of MoO 3 -ammonia in PEDOT:PSS HTL cannot impede the light passing through the HTL in this work. Figure 4d shows the absorption spectra of one-step and three-step spin-coated perovskite films on pure PEDOT:PSS HTL. Both two absorption spectra show a well-defined peak at~526 nm. Furthermore, the absorption intensity was enhanced on increasing the coating time from one to three, which can be attributed to the increase of the thickness of perovskite film from~30 nm (one-step coating) to~55 nm (three-step coating). morphology of perovskite material affect the PL intensity, namely more excitons will be generated by increasing the amount of perovskite particles, leading to the enhancement of the PL intensity when the thickness of perovskite film increases from ~30 nm (one-step coating) to ~55 nm (three-step coating) shown in Figure 4a. As shown in Figure 4b, on increasing the ratio of MoO3-ammonia from 0 to 0.02, the PL intensity of three-step spin-coated CH3NH3PbBr3 film gradually increases, while the further increase of the amount of MoO3-ammonia leads to the decrease of the PL intensity. The similar trend is also found in the one-step spin-coated CH3NH3PbBr3 films with different MoO3-ammonia ratios. It is suggested that the optimal MoO3-ammonia ratio is beneficial for blocking the exciton quenching at the HTL/CH3NH3PbBr3 interface, while the excessive MoO3-ammonia ratio is unfavorable. These results may be due to the increase of MoO3 on top of the HIL separating excitons generated in the CH3NH3PbBr3 EML from the quenching of PEDOT:PSS. However, on increasing the MoO3-ammonia amount, dopant aggregation or trap states may also occur at the HTL/EML interface, leading to the decay of photoluminescence. Figure 4c shows the transmittance spectra of PEDOT:PSS and PEDOT:PSS/MoO3-ammonia composite films with different MoO3-ammonia ratios (glass/ITO/composite HTL (~40 nm)). As shown in transmittance spectra, a small amount of MoO3 has little effect on the transmittance of PEDOT:PSS/MoO3-ammonia composite films in the visible range. The transmittances of four samples are near-identical, indicating that the doping of MoO3ammonia in PEDOT:PSS HTL cannot impede the light passing through the HTL in this work. Figure  4d shows the absorption spectra of one-step and three-step spin-coated perovskite films on pure PEDOT:PSS HTL. Both two absorption spectra show a well-defined peak at ~526 nm. Furthermore, the absorption intensity was enhanced on increasing the coating time from one to three, which can be attributed to the increase of the thickness of perovskite film from ~30 nm (one-step coating) to ~55 nm (three-step coating).   These four devices are labelled as S1, S2, S3, and S4 for clarity respectively. The detailed device parameters of the PeLEDs (S1, S2, S3, S4) are summarized in Table 1. Figure 5a, on increasing the MoO 3 -ammonia ratio from 0 to 0.03, the turn-on voltage, which are defined as the driving voltage at ∼1 mA/cm 2 , decreases monotonically from 4.3 V to 4.14 V. Besides, the current density of the PeLEDs increases on increasing the MoO 3 -ammonia amount, suggesting a reduced energy barrier at HTL/EML interface, inducing more efficient hole injection into CH 3 NH 3 PbBr 3 layer [25,26]. As described in the L-J, CE-J, and EQE-J characteristics, a maximum luminance of 1082 cd/m 2 , a maximum CE of 0.7 cd/A and a maximum EQE of 0.11% were observed in the device with the MoO 3 -ammonia ratio of 0.02 (device S3), indicating the optimal volume ratio, while for the control device with pure PEDOT:PSS HTL (device S1), the maximum luminance of 593 cd/m 2 and maximum CE of 0.36 cd/A were obtained. Therefore, the optimized device shows a 82% enhancement in the maximum luminance and 94% enhancement in the maximum CE respectively.   Figure 6 shows J-V, L-J, CE-J, and EQE-J curves for the three-step CH3NH3PbBr3 PeLEDs with pure PEDOT:PSS and PEDOT:PSS/MoO3-ammonia (1:0.01, 1:0.02, 1:0.03) composite HTLs, which are labelled as T1, T2, T3, and T4 for clarity respectively. The detailed device parameters of the PeLEDs (T1, T2, T3, T4) are summarized in Table 2. As shown in Figure 6a, on increasing the MoO3-ammonia ratio from 0 to 0.03, the turn-on voltage decreases from 4.08 V to 3.68 V. Besides, the current density of the PeLEDs increases on increasing the MoO3-ammonia amount, which is similar to the trend observed in one-step devices (S1, S2, S3, S4). As shown in Figure 6c and Figure 6d, the CE and EQE increase at low current densities because of the rapidly increased luminance (or the number of These results suggest that the hole injection in the PeLEDs with the PEDOT:PSS/MoO 3 -ammonia composite layer can be improved by reducing the contact barrier [25,26] and blocking the exciton quenching at the HTL/CH 3 NH 3 PbBr 3 interface [26]. However, the device efficiency decreases when an excessive MoO 3 -ammonia amount was doped in the PEDOT:PSS/MoO 3 -ammonia composite HTL (0.03), possibly due to the trap states formed at HTL/EML interface after the doping of the excessive MoO 3 -ammonia. The inset in Figure 5a shows the normalized electroluminescence (EL) spectra of CH 3 NH 3 PbBr 3 PeLEDs using PEDOT:PSS/MoO 3 -ammonia composite HTLs with different amounts of MoO 3 -ammonia, indicating that the EL spectra of PeLEDs with composite HTLs (S2, S3, S4) are nearly identical to that with pure PEDOT:PSS HTL (S1). It is suggested that the MoO 3 -ammonia doping cannot modify the emission profiles of CH 3 NH 3 PbBr 3 PeLEDs, which have an EL peak at ∼528 nm. Figure 6 shows J-V, L-J, CE-J, and EQE-J curves for the three-step CH 3 NH 3 PbBr 3 PeLEDs with pure PEDOT:PSS and PEDOT:PSS/MoO 3 -ammonia (1:0.01, 1:0.02, 1:0.03) composite HTLs, which are labelled as T1, T2, T3, and T4 for clarity respectively. The detailed device parameters of the PeLEDs (T1, T2, T3, T4) are summarized in Table 2. As shown in Figure 6a, on increasing the MoO 3 -ammonia ratio from 0 to 0.03, the turn-on voltage decreases from 4.08 V to 3.68 V. Besides, the current density of the PeLEDs increases on increasing the MoO 3 -ammonia amount, which is similar to the trend observed in one-step devices (S1, S2, S3, S4). As shown in Figure 6c,d, the CE and EQE increase at low current densities because of the rapidly increased luminance (or the number of excitons). On further increasing the current density, the luminance increases more slowly or decreases, leading to the decreased CE and EQE, namely the efficiency roll-off. As reported, the efficiency roll-off of OLEDs is mainly caused by charge imbalance and quenching processes [10,29,30]. Similarly, in this work, higher CE found in the device with the MoO 3 -ammonia ratio of 0.02 can be explained by the balance of electrons and holes in the EM, as well as the reduced exciton quenching. From the L-J, CE-J, and EQE-J characteristics, a maximum luminance of 5044 cd/m 2 , a maximum CE of 3.12 cd/A and a maximum EQE of 0.5% were also observed in the device with the optimal MoO 3 -ammonia ratio of 0.02 (device T3), while for the device with pure PEDOT:PSS HTL (device T1), the maximum luminance of 2309 cd/m 2 and maximum CE of 1.47 cd/A were obtained. Thus, the optimized device shows a 118% enhancement in the maximum luminance and 112% enhancement in the maximum CE respectively. Compared with the control device with pure PEDOT:PSS HTL and one-step spin-coated CH 3 NH 3 PbBr 3 film (device S1), a 750% enhancement in the maximum luminance and 767% enhancement in the maximum CE were obtained for the optimized device (device T3). The inset in Figure 6a shows the normalized EL spectra of three-step CH 3 NH 3 PbBr 3 PeLEDs using PEDOT:PSS/MoO 3 -ammonia composite HTLs with different amounts of MoO 3 -ammonia, in which all PeLEDs have an EL peak at ∼528 nm. Figure 7 shows the EL curves measured at different current densities. The results indicate that all PeLEDs have an EL peak at~528 nm, suggesting the color stability of our devices.

As shown in
These results indicate that the hole injection in the PeLEDs with the PEDOT:PSS/MoO 3 -ammonia composite layer can be improved by reducing the contact barrier [25,26] and suppressing the exciton quenching at the HTL/CH 3 NH 3 PbBr 3 interface [26]. Besides, three-step spin coating method can improve the surface morphology of the CH 3 NH 3 PbBr 3 perovskite film shown in Figure 2. Furthermore, the as-obtained perovskite layer exhibited a stronger PL intensity shown in Figure 4. These factors induce the significant improvement on luminous performance of our PeLEDs. Therefore, the significantly improved efficiency of PeLEDs using three-step spin-coated CH 3 NH 3 PbBr 3 film and an optimum PEDOT:PSS/MoO 3 -ammonia composite HTL can be explained by the enhanced carrier recombination through better hole injection and film morphology optimization, as well as the reduced exciton quenching at HTL/CH 3 NH 3 PbBr 3 interface. the maximum luminance and 767% enhancement in the maximum CE were obtained for the optimized device (device T3). The inset in Figure 6a shows the normalized EL spectra of three-step CH3NH3PbBr3 PeLEDs using PEDOT:PSS/MoO3-ammonia composite HTLs with different amounts of MoO3-ammonia, in which all PeLEDs have an EL peak at ∼528 nm. Figure 7 shows the EL curves measured at different current densities. The results indicate that all PeLEDs have an EL peak at ~528 nm, suggesting the color stability of our devices. The inset is normalized EL spectra of the PeLED devices at 20 mA/cm 2 , which were shifted vertically for clarity.   These results indicate that the hole injection in the PeLEDs with the PEDOT:PSS/MoO3-ammonia composite layer can be improved by reducing the contact barrier [25,26] and suppressing the exciton quenching at the HTL/CH3NH3PbBr3 interface [26]. Besides, three-step spin coating method can improve the surface morphology of the CH3NH3PbBr3 perovskite film shown in Figure 2. Furthermore, the as-obtained perovskite layer exhibited a stronger PL intensity shown in Figure 4. These factors induce the significant improvement on luminous performance of our PeLEDs. Therefore, the significantly improved efficiency of PeLEDs using three-step spin-coated CH3NH3PbBr3 film and an optimum PEDOT:PSS/MoO3-ammonia composite HTL can be explained by the enhanced carrier recombination through better hole injection and film morphology optimization, as well as the reduced exciton quenching at HTL/CH3NH3PbBr3 interface. EL spectra of the PeLED devices with a three-step spin-coated emitter and a PEDOT:PSS/MoO 3 -ammonia (1:0.02) composite HTL at different current densities. The inset is a luminescence image of the device at 50 mA/cm 2 .

Conclusions
In summary, we demonstrated improved performance of PeLEDs using a PEDOT:PSS/MoO 3 -ammonia composite HTL by reducing the energy barrier and blocking the exciton quenching at the HTL/CH 3 NH 3 PbBr 3 interface. For PeLEDs with one-step spin-coated CH 3 NH 3 PbBr 3 film, an enhancement of 82% in the maximum luminance and 94% in the maximum CE was found in PeLED with an optimal MoO 3 -ammonia volume ratio (0.02) in PEDOT:PSS/MoO 3 -ammonia composite HTL compared with the control device with pure PEDOT:PSS HTL respectively. Three-step spin coating method was further used to obtain uniform and dense CH 3 NH 3 PbBr 3 films, which lead to a maximum luminance of 5044 cd/m 2 and maximum CE of 3.12 cd/A, which are 750% and 767% larger than those of the control device respectively. The significantly improved efficiency of PeLEDs using three-step spin-coated CH 3 NH 3 PbBr 3 film and an optimum PEDOT:PSS/MoO 3 -ammonia composite HTL can be originated from the enhanced carrier recombination through better hole injection and film morphology optimization, as well as the reduced exciton quenching at HTL/CH 3 NH 3 PbBr 3 interface. These results suggest a promising clue for the device engineering of high efficiency PeLEDs.