A Study on Efficient Charge Transport Layer for High-Performance Perovskite Based Light-Emitting

APS and SPV measurements for quasi-2D perovskite (35 nm) with n = 3 deposited on NiOx and PEDOT:PSS. a) APS spectra for 35-nm thick perovskite films deposited on NiOx and PEDOT:PSS. SEM images of quasi-2D perovskite with n = 3 deposited on a) NiOx, b) pH acidic, c) pH neutral, and d) pH basic PEDOT:PSS.

Introduction

Perovskite Light-emitting materials

• High color purity of perovskite
• Bandgap tunability of Perovskite
• Influence of A cation
• Influence of B metal cation
• Influence of X halide anion
• PLQY characteristics of perovskite
• Strategies for improving the luminescence of perovskite
• Layered perovskite
• Grain size control

The emission wavelength of the perovskite can cover the visible light region with compositional control of perovskite. The high color purity of the perovskite is also suitable as an emerging super-high-definition display material.

Figure 1.2. Perovskite structure

Perovskite based Light-emitting Diodes

• Structure of PeLEDs
• Characterization of PeLEDs
• Progress in PeLED efficiencies
• Stability issues of PeLED operation

If the intensity of the emitted light is counted against the injected current, the efficiency also increases. With the same lighting, the luminous flux and energy consumption are constant. The FWHM is expressed in terms of energy (meV) and in wavelength (nm), and the half-width of the perovskite is usually known as 100 meV.

Therefore, it is also important to increase the raiative efficiency of the perovskite film to realize a high-efficiency PeLED, but it is also important to increase the charge balance due to the effective charge injection from the contact layer. However, this structure did not show a successful structure efficiency due to the difficulty of charge injection into green or blue perovskites before the surface modification of the metal oxide layer. This improved the morphology of the perovskite layer and reduced the energy barrier of the perovskite layer to electrons.

Compared with conventional PeLEDs with inverted structure, the schematic diagram of the structure of normal type PeLEDs is shown in Figure 1.21. In contrast, the lifespan of a more recent commercialized OLED is between 1,000 and 100,000 hours. In early generation OLEDs, instability was caused by burn-in due to crystallization of organic material, but device encapsulation and manufacturing optimization overcome stability problems.

Figure 1.22. Electroluminescence spectra and image of perovskite LEDs [16]

Improved performance of perovskite light-emitting diodes using a PEDOT:PSS

• Research back ground
• Experimental
• Result and discussion
• Conclusion
• References

To observe the morphology of MAPbBr3 films on the PEDOT:PSS and PEDOT:MoO3 composite layer, they were measured via scanning electron microscopy. The surface morphology of PEDOT:PSS and PEDOT:MoO3 composite layers was measured using atomic force microscopy (DI-3100, Veeco Co.). Moreover, the adsorption of MoO3 into the PEDOT:PSS of the PEDOT:MoO3 composite layer was confirmed via X-ray photoelectron spectra measurements by monitoring the position and shape of the Mo 3d5/2.

The lower surface coverage of the CH3NH3PbBr3 film prepared on the PEDOT:MoO3 (0.7 wt%) composite layer may originate from the unfavorable morphology of the PEDOT:MoO3 composite layer with high RMS roughness. Thus, the optimum wt% of MoO3 in the PEDOT:MoO3 composite was required to obtain highly efficient PeLEDs. XRD patterns of the CH3NH3PbBr3 films prepared on different conditions of PEDOT:MoO3.

This result confirmed that the hole injection in the PeLEDs with the PEDOT:MoO3 composite layer was improved by reducing the contact barrier at the CH3NH3PbBr3/HTL interface. S3 (ESI†), the morphologies of CH3NH3PbBr3 films grown on PEDOT:MoO3 composite layer with different amounts of MoO3 powder were observed using SEM. As the amount of MoO3 in the PEDOT:MoO3 composite layer increased, non-uniform perovskite films with pinholes (red circle) were created by the unfavorable growth of the perovskite crystal on MoO3.

Figure 2.1 schematic of (a) PeLEDs device structure and (b) the energy levels diagram of each layer

Control of Interface Defects for Efficient and Stable Quasi‐2D Perovskite Light‐

Research back ground

Controlling Interface Defects for Efficient and Stable Quasi-2D Perovskite Light-Emitting Diodes Using a Nickel Oxide Hole Injection Layer. Furthermore, crystalline metal oxides can facilitate the growth of highly crystalline perovskite and improve the interface quality between metal oxides and perovskite, [34,35] leading to a reduced non-radiative recombination pathway. In this study, we control perovskite trap density through compositional, dimensional, and interfacial modulations, which substantially increase bimolecular radiative recombination by surpassing trap-mediated nonradiative recombination.

Due to the low trap density, long-lived free carriers of formamidinium (FA)-based perovskite are allowed to recombine into small radiative domains by modulating efficient and fast energy transfer through the dimensionality. Moreover, we investigate interfacial defects of perovskite deposited on NiOx and PEDOT:PSS by ambient pressure air photoemission spectroscopy (APS), the Kelvin probe (KP) and surface photovoltage (SPV) for perovskite films of different thicknesses. Crystalline NiOx enables the growth of highly crystalline perovskite with fewer interface defects, which improves the optical properties and photostability of perovskite as well as the operational stability of PeLEDs.

By effectively controlling the perovskite trap density, we demonstrate efficient and stable green emission PeLEDs with a maximum luminance of 24,100 cd m−2, a maximum current efficiency (CE) of 62.4 cd A−1, and a maximum EQE of 14.6%.

Experimental

Device Characterization: The J-V-L and device efficiency of the encapsulated PeLEDs were obtained using a computer-controlled Keithley 2400 Source Meter and a Konica Minolta spectroradiometer (CS-2000, Minolta) under ambient conditions. The time-resolved PL signal was obtained using a TCSPC module (PicoHarp) with a photomultiplier tube (PMA-C 182-N-M). PLQY measurement: The PLQY of the perovskite films was obtained using an integrating sphere method.

Confocal Fluorescence Imaging: Confocal PL images of the perovskite films were measured using an LSM 780 NLO laser scanning confocal microscope (Carl Zeiss) with a 100x oil immersion objective (Plan-APO, NA = 1.46). EL Microscopy: EL microscopy images of PeLED samples fabricated with a quasi-2D perovskite film with n = 3 deposited on NiOx and PEDOT:PSS were obtained using an inverted microscope (IX81, Olympus). For APS measurements, the sample was illuminated with UV light from a monochromatic deuterium lamp source (4–7 eV).

The raw photoemission data were corrected for offset and the square root was taken, with the HOMO value found from the extrapolated intersection of the straight-line fit of the square root of the photoemission with the baseline. KP WF measurements were taken using a 2-mm gold alloy-coated vibrating tip above the surface of the sample, with the resulting contact potential difference between the tip and the sample at the. For SPV measurements, the sample was illuminated with a 150-W quartz halogen lamp connected by optical fiber and focused on the sample.

Results and discussion

To investigate defects at the interface of the perovskite deposited on NiOx and PEDOT:PSS, APS, KP and SPV were performed on the perovskite films (at 15 nm, 35 nm and 120 nm thicknesses) deposited on NiOx and PEDOT:PSS. The photoemission spectra of the perovskite films deposited on NiOx and PEDOT:PSS at thicknesses of 15 nm and 120 nm are shown in Figure 3.6 a,b. APS spectra for a) 15 nm thick and b) 120 nm thick perovskite films deposited on NiOx and PEDOT:PSS.

The morphologies of the perovskite films deposited on NiOx and PEDOT:PSS were observed by SEM (Figure 3.8). The morphologies of perovskite films deposited on PEDOT:PSS at different pH values ​​were compared to investigate the effect of their acidity on perovskite film growth. The morphologies of perovskite films deposited on PEDOT:PSS with different pH values ​​were compared to investigate the effect of its acidity on perovskite film growth.

Optical properties and stability of perovskite films deposited on NiOx and PEDOT:PSS. a) Time-resolved PL spectra, and b) steady-state PL spectra of quasi-2D perovskite with n = 3 deposited on NiOx and PEDOT:PSS. Device performance of PeLEDs fabricated with 3D FAPbBr3 and quasi-2D perovskites with n = 2, 3 and 5 deposited on NiOx and quasi-2D perovskite with n = 3 deposited on PEDOT:PSS. Summary of device performance of PeLEDs fabricated with 3D FAPbBr3 and quasi-2D perovskites with n = 2, 3 and 5 deposited on NiOx and quasi-2D perovskite with n = 3 deposited on PEDOT:PSS.

Figure 3.2. Schematic of device structure, chemical structure, cross-sectional SEM image, XRD, absorption and PL

Conclusion

67 . fabricated with PEDOT:PSS revealed large dark regions considered as defects and, in particular, brighter emissions near the dark regions. The brightness at the boundaries of the dark areas decreased slowly compared to other areas over time. Although the exact mechanism of the phenomenon is challenging to investigate and is still under investigation, defects in the perovskite obviously affect the operation stability of PeLED.

Operational stability and EL microscopic images of PeLEDs fabricated with quasi-2D perovskite with n = 3 deposited on NiOx and PEDOT:PSS. a) Normalized luminance and b) EL microscope images of encapsulated PeLEDs fabricated with quasi-2D perovskite with n = 3 deposited on NiOx and PEDOT:PSS as a function of operating time under ambient conditions. EL spectral stability of PeLEDs fabricated with NiOx as a function of operating time under ambient conditions. a) Normalized EL spectra and b) CIE coordinates of PeLEDs fabricated with NiOx. such as over operating time.

Figure 3.15. Operational stability and EL microscopic images of PeLEDs fabricated with quasi-2D perovskite with n = 3 deposited on NiO x and PEDOT:PSS

Efficient Large-Area Cesium-Based Quasi-2D Perovskite Light-Emitting Diodes

• Research back ground
• Experimental
• Results and discousions
• Conclusion
• Reference

One of the methods of making perovskite films, a hot casting method using preheating substrate, was used to produce uniform and complete coverage of the perovskite film morphology on the PVK interlayer. The hot casting method improved the morphology of the perovskite film and increased the crystallinity and preferential orientation of perovskite crystals in the vertical direction. Quasi-2d perovskite films (~70 nm) as emissive layer were fabricated by controlling the stoichiometric amounts of cesium bromide (CsBr), lead bromide (PbBr2), and benzyl ammonium bromide (BABr).

The illustration of how the perovskite solution is coated on a wetting NiOx layer (Fig. 1e) and non-wetting PVK coated NiOx (Figure 4.6) is shown. However, the perovskite films grown on the substrate at 120℃ exhibit relatively low PL intensity due to poorer morphology (Figure 4.10). Time-resolved photoluminescence (TRPL) of perovskite film at different substrate temperatures was measured to investigate the lifetime of the excited carriers.

The lifetime extension in perovskite on a preheated substrate is due to the formation of perovskite crystals with a low defect density (Figure 4.11). In addition, the PVK layer coated between the NiOx and the perovskite film has the effect of improving the PeLED performance due to better hole injection (Figure 4.14). The stability of PeLED operation with respect to different methods of manufacturing perovskite films is compared in Figure 4.15 d.

Figure 4.1. Cross section SEM images of Cs based quasi-2D PeLEDs

Acknowledgements (감사의 글)