Preparation and optoelectronic properties of multi-color high-efficiency CH3NH3Pb(BrxI1-x)3 perovskite light-emitting diodes

Multi-color light-emitting materials are essential lighting and displays. In this study, mixed halide system was applied to precisely tune the bandgap of CH3NH3Pb(Br x I1-x )3, thus regulating the emission wavelength. PEABr was employed to change the phase structure and morphology of CH3NH3Pb(Br x I1-x )3 perovskite thin films and improve the performance of multi-color perovskite light-emitting diodes (PeLEDs). Theoretical simulations through first-principles calculations and experiments demonstrate that multi-color PeLEDs can be achieved by adjusting the ratio of bromine (Br) and iodine (I) atoms in the CH3NH3Pb(Br x I1-x )3 perovskite. The maximum luminance of PEABr-modified green PeLEDs reached 7108 cd m−2, with a maximum current efficiency of 8.25 cd A−1 and a maximum external quantum efficiency (EQE) of 1.62%, which were greatly improved compared to the reference device without PEABr. In addition, the luminance of orange-yellow and red mixed-halide PeLEDs both exceed 100 cd m−2. The results demonstrate that the use of PEABr additive can effectively control the morphology of CH3NH3Pb(Br x I1-x )3 crystals, and high-performance multi-color light-emitting devices can be achieved by combining with mixed halide system. The electroluminescence spectra showed that the emission range of the devices covered the wavelength region of 520–720 nm, demonstrating their good application prospects in the field of multi-color displays.


Introduction
Organic-inorganic perovskite materials are a type of halide with the molecular formula ABX 3 . In addition to their excellent optoelectronic properties, another attractive feature is that their energy bandgap (E g ) can be continuously tuned by various methods, such as using formamidinium (FA) to replace the organic molecule methylammonium (MA) [1], replacing the divalent Pb ion with other metal ions such as Sn or Ge [2], or mixing halogen elements [3]. This feature has attracted widespread attention in areas such as light-emitting diodes (LEDs), solar cells, and photodetectors [4][5][6][7][8][9].
Mixed halide system is considered as a common method to achieve multi-color perovskite light-emitting diodes (PeLEDs). Protesescu et al used a mixed halide system (Cl/Br and Br/I) of perovskite materials to prepare photoluminescent nanocrystals with narrow emission linewidth, wide color gamut, high quantum yield, and long radiative lifetime [10]. Wang et al prepared a high-performance green PeLED with a simple structure of MAPbBr 3 through precise morphology engineering, and also obtained some bright halide-hybrid PeLEDs with narrow and clean emission bands in a wide color gamut by adjusting the halide ratio in halide perovskites at room temperature [11]. Chun et al prepared high-performance and air-stable MAPbI 3 by controlling the solvent volume and ligand, and used the same method to prepare color-tunable MAPbX 3 (X=Cl, Br, I) [12]. Although some literatures are available, the device performance still needs further optimization.
Compared with three-dimensional (3D) perovskites, two-dimensional (2D) perovskites show some advantages, such as excellent thermal stability, natural quantum-well structures, and large exciton binding Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. energy. Formation of 2D structures is an effective method to enhance the device performance of PeLEDs. Lee et al incorporated phenethylammonium iodide (PEAI) into perovskite, which can increase the radiation recombination rate and improve the performance of PeLEDs [13]. The introduction of PEAI greatly limited the growth of perovskite crystals during film formation, and perovskite films with small-sized microcrystals, reduced film roughness and decreases pinholes were obtained. Zhang et al used phenethylammonium bromide (PEABr) as a ligand and chloroform as an anti-solvent to fabricate blue perovskite nanocrystal films [14]. The quality of the films was greatly improved, and the performance of blue PeLEDs devices was more efficient and stable.
As mentioned above, the use of PEABr additive can effectively control the morphology of CH 3 NH 3 Pb(Br x I 1-x ) 3 crystals, and adjust the bandgap through forming two-dimensional structures. The synergistic effects of PEABr and halide hybridization are expected to further widen the emission wavelength of multi-color light-emitting devices. In this paper, PEABr was employed to change the phase structure and morphology of CH 3 NH 3 Pb(Br x I 1-x ) 3 perovskite thin films and enhance the performance of multi-color devices. The structure, electronic, and optical properties of mixed-halide perovskite CH 3 NH 3 Pb(Br x I 1-x ) 3 were investigated theoretically using first-principles calculations. Furthermore, CH 3 NH 3 Pb(Br x I 1-x ) 3 perovskite thin films and PeLED devices were fabricated and studied experimentally through modifying the type and proportion of mixed halide atoms.

Simulation
This article builds the perovskite crystal structure based on the tetragonal phase CH 3 NH 3 PbI 3 with the I4/mcm space group. A cell consisting of four CH 3 NH 3 PbI 3 molecules with 48 atoms and initial lattice constants using experimental data a = b = 0.880 nm and c = 1.269 nm [15]. Then, attempts were made to substitute the iodine (I) atoms in CH 3 NH 3 PbI 3 with bromine (Br) atoms, resulting in the crystal structure of CH 3 NH 3 Pb(Br x I 1−x ) 3 , until all I atoms were completely replaced by Br atoms, and then optimizing their geometrical structures, respectively [16]. All calculations were performed based on the first-principles of density functional theory (DFT) as implemented in the CASTEP code [17]. The first-principles plane wave super-soft pseudopotential method based on density functional theory was adopted in this work. The generalized gradient approximation (GGA) parametrized by Perdew-Burke-Ernzerhof (PBE) were ultilized for the electron exchange-correlation energy functional, in which the TS dispersion correction is ticked. A cutoff energy for the expansion of the plane waves was set to be 400 eV. The total energy tolerance, maximum force, maximum stress and maximum displacement were set to be 5.0 × 10 −5 eV/atom, 0.01 eV nm −1 , 0.2 GPa, and 5 × 10 4 nm to ensure convergence, respectively [15].

Preparation of perovskite precursor solution
0.4 mmol of MAX and 0.4 mmol of PbX 2 (X is Br or I), respectively, and PEABr were dissolved in 1 ml of DMF solvent to obtain 0.4 M of CH 3 NH 3 Pb(Br x I 1-x ) 3 precursor solution. 0.4 M of CH 3 NH 3 Pb(Br x I 1−x ) 3 precursor solution was obtained by adjusting the contents of MABr, MAI, PbBr 2 and PbI 2 , which was divided into four groups CH 3 NH 3 PbBr 3 , CH 3 NH 3 Pb(Br 0.67 I 0.33 ) 3 , CH 3 NH 3 Pb(Br 0.33 I 0.67 ) 3 , and CH 3 NH 3 PbI 3 (i.e., x = 1, 0.67, 0.33, 0), and the four groups of solutions were placed on a heating table at 60°C and stirred at 500 rpm with shading for 12 h [18][19][20]. m% PEABr ratio refers to the ratio of PEABr and the molar ratio between PbX 2 (i.e., mPEABr/mPbBr 2 = m%), and the concentration of the prepared chalcogenide precursor solution was 0.4 M.

Fabrication of PeLEDs
The device structure employed in this work is ITO/PEDOT:PSS/CH 3 NH 3 Pb(Br x I 1−x ) 3 /TPBi/LiF/Al. The sheet resistance of ITO is about 15 Ω m −2 , and the effective emission area is 0.1 cm 2 . Firstly, the ITO substrate was cleaned by ultrasonication with isopropanol, anhydrous ethanol, and deionized water successively for 10 min to prevent interference with subsequent spin-coating processes. Subsequently, the surface water on the substrate was blown off using a nitrogen gun, followed by oxygen plasma treatment for 20 min. After that, the ITO substrate was transferred to a N 2 -filled glove box for spin-coating. An appropriate amount of PEDOT:PSS was filtered through a 0.45 μm PTFE filter before use, then filtered again through a 0.2 μm PTFE filter, and was adequately shaken in an ultrasonic cleaner to prevent precipitation. The PEDOT:PSS was spin-coated onto the substrate at a speed of 8000 rpm for 40 seconds and then annealed for 15 min at 140°C. After cooling, the mixed perovskite precursor solution was spin-coated at 4000 rpm for 60 seconds on top of the PEDOT:PSS layer. At the 6th second, 300 μl of ethyl acetate antisolvent was added dropwise quickly and uniformly, and then the sample was annealed at 80°C for 10 min on a hot plate. Finally, the samples were transferred to a vacuum thermal evaporation chamber with a vacuum of 10 −7 Torr for deposition. The electron transport layer (ETL) was 40 nm TPBi, the electron injection layer was 0.5 nm LiF, and the cathode was 100 nm Al. All PeLEDs were tested immediately after deposition in ambient air.

Characterization
An Alpha-Step D-600 step profiler was used to measure the thickness of PEDOT:PSS and perovskite films. A Hitachi U-3900 UV-vis spectrophotometer was used to test the absorption spectra. A FLS1000 spectrometer was used to measure steady-state photoluminescence (PL) spectra and time-resolved fluorescence spectra (TRPL) of the perovskite films at an excitation wavelength of 370 nm. The surface morphology was observed by scanning electron microscopy (FEI SIRION-200). a PANalytical Empyrean x-ray diffractometer was used to characterize the structure of perovskite films. The luminance-current density-voltage (L-J-V) characteristics of the devices were tested using a Keithley 2400 source meter and a Keithley 2000 multimeter coupled to a calibrated silicon photodetector (1 cm diameter), which captures and converts photons emitted from the glass side. An Ocean Optics fiber-optic spectrometer was used to record the electroluminescence (EL) spectra.

Results and discussion
In this study, the crystal structure of CH 3 NH 3 PbI 3 in the I4/mcm space group was used as the basis to construct the perovskite crystal structure. Bromine (Br) atoms were then substituted for iodine (I) atoms in CH 3 NH 3 PbI 3 to form the crystal structure of CH 3 NH 3 Pb(Br x I 1-x ) 3 , until all iodine (I) atoms were completely replaced by bromine (Br) atoms. Here, the energy band structures were calculated in the reciprocal space along the high symmetry lines. Accordingly, the energy band structures of CH 3 NH 3 Pb(Br x I 1-x ) 3  The band gaps were also determined experimentally by the PL and absorption measurement (figure 8), and the values were found to be 1.62 eV, 1.81 eV, 2.01 eV, and 2.34 eV for x = 0, 0.33, 0.67, and 1, respectively. The theoretical values were slightly lower than the experimental values, which is because the first-principles calculations used an approximate function and generally underestimate the band gap [21]. Figure 2 shows the calculated electronic density of states (DOS) for CH 3 NH 3 Pb(Br x I 1-x ) 3 . From figures 2(b)-(c), it can be seen that the valence band maximum (VBM) energy level of CH 3 NH 3 Pb(Br x I 1−x ) 3 with Br doping content (x = 0.33 and 0.67) mainly originates from the combination of Br-4p, I-5p, Pb-6s and Pb-6p states. On the other hand, the conduction band minimum (CBM) energy level mainly comes from the Pb-6p state, and the introduction of Br into Br-4p state in VBM is the result of the combined effect of I-5p and Br-4p states. Additionally, the contribution of Pb-6p to CBM remains constant in all calculated systems. The energy levels of CH 3 NH 3 + ion are mainly distributed below −4 eV, and its contribution to the density of states near the Fermi level is almost zero, indicating that there is no significant covalent interaction between the organic ion CH 3 NH 3 + and Pb or I atoms [22]. Through simulation calculations, we studied the influence of halide ions on the bandgap of CH 3 NH 3 Pb(Br x I 1-x ) 3 structure. The calculated band structure and density of states indicate that the bandgap of CH 3 NH 3 Pb(Br x I 1-x ) 3 perovskite can be adjusted by designing the types and proportions of halide atoms on the X site, thereby regulating the emission wavelength. Subsequently, CH 3 NH 3 Pb(Br x I 1-x ) 3 perovskite light-emitting diodes (PeLEDs) were prepared by solution spin-coating method, and their optoelectronic properties were further studied.
In this work, PEABr was used as an additive to modify the surface morphology, crystal structure, and optical properties of perovskite films. The influence of different concentrations of PEABr on the crystallization process of CH 3 NH 3 PbBr 3 perovskite films was characterized using SEM, as shown in figure 3. The film prepared without PEABr has many pinholes, and the perovskite grains are large. With adding PEABr, the perovskite grains gradually decreased in size, the pinholes on the film surface became significantly less, and the film became denser. This can be caused by the bulk cations in PEABr, which inhibit the growth of perovskite grains [23].
We further studied the effect of PEABr additive on the optoelectronic performance of CH 3 NH 3 PbBr 3 PeLEDs. The device structure is: ITO/PEDOT:PSS/CH 3 NH 3 PbBr 3 +PEABr/TPBi/LiF/Al. Figure 4(a) is a schematic diagram of the PeLED device structure, where CH 3 NH 3 PbBr 3 serves as the emission layer (EML). Figure 4(b) shows the energy level diagram of green CH 3 NH 3 PbBr 3 PeLEDs. Figure 5 shows the current density-voltage density (J-V), luminance-current density (L-J), current efficiency-current density (CE-J), and external quantum efficiency-current density (EQE-J) characteristic curves of PeLEDs prepared with different concentrations of PEABr. The turn-on voltage is determined with the operating voltage when the current density is 1 mA cm −2 . The detailed parameters of the PeLEDs device are shown in table 1. As shown in the J-V curve of figure 5(a), the turn-on voltage of the PeLEDs without PEABr is 3.34 V, which decreased to 3.28 V with the introduction of PEABr. The addition of PEABr can reduce the pinholes and decrease the carrier losses, which is beneficial for carrier transport and thus increases the current density. However, the conductivity of PEA + ions is poor, and excessive PEABr will also reduce the current density. Figure 5(b) shows the luminance-current density curves of PeLEDs prepared with different concentrations of PEABr. When the PEABr concentration is 30%, the optimal device shows a maximum luminance of 7108 cd m −2 . Current efficiency and EQE are important parameters for evaluating the luminescence performance of PeLEDs. The maximum current efficiency of PeLEDs with PEABr doping concentrations of 30% and 40% are 8.25 cd A −1 and 8.92 cd A −1 ( figure 5(c)), respectively, and the maximum EQE is 1.62% and 1.75% ( figure 5(d)), respectively, which are significantly higher than the control device without PEABr. However, the maximum luminance is only 436 cd m −2 for the device with 40% PEABr, which is much lower than that with 30% PEABr. This indicates that the appropriate addition of PEABr can significantly improve the surface morphology of the perovskite emitting layer, which is essential for carrier radiative recombination [23]. Furthermore, the large binding energy in two-dimensional perovskite phase can lead to the increased luminous efficiency of devices. However, excessive PEABr will lead to a weakening of the carrier transport performance, thereby affecting the luminescence performance of PeLEDs [14].
In addition to green PeLEDs, we also investigated the effect of PEABr additives on the optoelectronic properties of CH 3 NH 3 Pb(Br x I 1-x ) 3 perovskite PeLEDs devices with different halide ratios. The similar enhancement of luminous efficiency for multi-color devices were obtained when the PEABr concentration was 30%. In the rest of this work, we analyzed and compared the optoelectronic properties of CH 3 NH 3 Pb(Br x I 1-x ) 3   perovskite films and PeLEDs prepared with different proportions of bromine (Br) and iodine (I) atoms when the PEABr doping concentration was 30%.
First, we investigated the structure and phase variation of CH 3 NH 3 Pb(Br x I 1-x ) 3 perovskite films with different halogen atom ratios. Figure 6(a) shows the XRD patterns of hybrid halide CH 3 NH 3 Pb(Br x I 1-x ) 3   perovskite films with 30% PEABr concentration. The typical peaks at 15°and 30°are ascribed to the (100) and (200) face of three-dimensional CH 3 NH 3 PbBr 3 crystals, respectively. With increasing the PEABr concentration, a new diffraction peak appears below 10°, indicating the formation of two-dimensional perovskite phase [24]. The large binding energy in two-dimensional perovskite phase can lead to the increased luminous efficiency of devices. Figure 6(b) shows the magnified XRD patterns of CH 3 NH 3 Pb(Br x I 1-x ) 3 perovskite films in the range of 2-Theta = 27°−31°. It can be seen that CH 3 NH 3 PbBr 3 has a cubic phase structure with the Pm3m space group, and with the increase of iodine content in the bromine-iodine ratio, the diffraction peaks of CH 3 NH 3 Pb(Br x I 1-x ) 3 films systematically move to the low-angle diffraction region. During the process of continuous replacement of bromine atoms with iodine atoms, the main peak corresponding to the (200) crystal plane of the 3D perovskite gradually transforms into the main peak corresponding to the (004) and (220) crystal planes of the 3D perovskite, which correspond to the tetragonal phase structure with the I4/mcm space group of CH 3 NH 3 PbI 3 . The appearance of a new diffraction peak at around 2-Theta=5.5°in the perovskite film indicates the formation of low-dimensional perovskite phases after adding PEABr [24]. Figure 7 shows SEM images of CH 3 NH 3 Pb(Br x I 1-x ) 3 thin films with different halogen ratios after the addition of PEABr. The morphology of the active layer of the perovskite has a significant impact on its optoelectronic properties. The change in the composition of the perovskite inevitably bring about changes in the corresponding crystal growth dynamics during the perovskite crystallization process, which in turn affects the film morphology. As can be seen from figure 7, the CH 3 NH 3 Pb(Br x I 1-x ) 3 thin films with PEABr is relatively flat and dense, while few pinholes can be found. It is indicated that PEABr can facilitate the high-quality CH 3 NH 3 Pb(Br x I 1-x ) 3 thin films.
The steady-state PL spectra of CH 3 NH 3 Pb(Br x I 1-x ) 3 perovskite thin films were measured under excitation at 370 nm, as shown on the right side of figure 8. The PL peak of CH 3 NH 3 PbBr 3 perovskite thin film was located at 520.6 nm. With the increase of iodine content, the PL peak shifted to longer wavelengths, and the FWHM of the peak width also increased. The wavelength of PL peak is 582.2 nm, 662.6 nm, 720.0 nm for the Br:I molar ratio of 2:1, 1:2, 3:0, respectively. After adding PEABr, two small PL peaks appeared at around 410 nm and 435 nm, which corresponded to PL peaks of n = 1 and n = 2 perovskite phases, respectively [25,26]. On the left side of figure 8 is the UV-vis absorption spectrum of CH 3 NH 3 Pb(Br x I 1-x ) 3 thin films, which shows the same evolution pattern as the PL peak. The absorption edge shifted to longer wavelengths with the increase of iodine content. Figure 9 shows the time-resolved photoluminescence spectra (TRPL) of CH 3 NH 3 Pb(Br x I 1−x ) 3 thin film perovskites with different halide ratios. Through bi-exponential decay fitting of TRPL data, the obtained average decay time (τavg) gradually increases from 2.17 ns to 16.39 ns as the Br content in the hybrid perovskite increases. This prolonged radiative lifetime indicates that the pure bromine-based perovskite film after introducing PEABr has fewer defects and lower non-radiative recombination rate compared to mixed-halide perovskite films, resulting in a longer PL lifetime. It may be originated from the perovskite phase segregation, which is caused by ion migration in mixed halide system.  Figure 10 shows the current density-voltage (J-V), luminance-current density (L-J), external quantum efficiency-current density (EQE-J) curves, and normalized electroluminescence (EL) spectra of PeLEDs with different halide ratios of CH 3 NH 3 Pb(Br x I 1−x ) 3 . The detailed parameters of PeLEDs are summarized in table 2. The maximum luminance and EQE of the green PeLEDs were 7108 cd m −2 and 1.62%, respectively. The maximum luminance and EQE of the orange-yellow PeLEDs were 292 cd m −2 and 0.22%, respectively. The maximum luminance and EQE of the pink PeLEDs were 60 cd m −2 and 0.36%, respectively. The maximum luminance and EQE of the red PeLEDs were 110 cd m −2 and 1.20%, respectively. In mixed halide system, the perovskite phase segregation caused by ion migration may lead to the low luminous efficiency of PeLEDs. Thermal quenching may be another reason for the inferior performance of mix-halide PeLEDs [11]. Figure 10(d) shows the normalized EL spectra of PeLEDs using mix-halide CH 3 NH 3 Pb(Br x I 1-x ) 3 . It can be    observed that all peak positions are almost identical to the PL spectra, and the inset depicts a comparison of the emission colors of PeLEDs with different halide ratios of CH 3 NH 3 Pb(Br x I 1-x ) 3 .

Conclusion
In summary, we successfully prepared surface-dense perovskite films and reduced the size of perovskite particles by introducing PEABr additive into the precursor solution, which enhanced the radiative recombination probability of the perovskite film through carrier confinement. Theoretical simulations and experiments demonstrate that high-brightness PeLEDs devices with PL emission spectra covering the range of 520 nm ∼ 720 nm were achieved by adjusting the ratio of bromine (Br) and iodine (I) atoms in the CH 3 NH 3 Pb(Br x I 1-x ) 3 perovskite. When the PEABr content was 30%, the light-emitting diode devices showed the best performance, with the maximum luminance of 7108 cd m −2 and the maximum EQE of 1.62% for green perovskite lightemitting diodes, the maximum luminance of 292 cd m −2 and the maximum EQE of 0.22% for orange-yellow devices, the maximum luminance of 60 cd m −2 and the maximum EQE of 0.36% for pink devices, and the maximum luminance of 110 cd m −2 and the maximum EQE of 1.20% for red devices. The results demonstrate that the use of PEABr additive can effectively control the morphology of CH 3 NH 3 Pb(Br x I 1-x ) 3 crystals, and combined with halide hybridization, high-performance multi-color light-emitting devices can be achieved. Table 2. Device parameters of PeLEDs with CH 3 NH 3 Pb(Br x I 1-x ) 3 (x = 1, 0.67, 0.33, 0). CH 3 NH 3 Pb(Br x I 1-x ) 3 L max (cd/m 2 ) EQE max (%) Turn-onvoltage (V) EL-peak (nm)