전도성 고분자 및 MXene을 계면층 및 전극으로 이용한 유기 및 페로브스카이트 발광 다이오드의 효율 향상 연구

Abstract

Organic/inorganic hybrid perovskites (OHPs) rapidly rose as a promising light-emitters because they have plenty of advantages such as high color purity with a narrow emission spectrum (full width at half maximum (FWHM) < 20 nm), simple material synthesis, low material cost, solution-processability, and easy device fabrication. In perovskite light-emitting diodes (PeLEDs), interfacial engineering between the transparent conducting electrode (TCE) and the light-emitting layer (EML) is very important to achieve high electroluminescence (EL) efficiency and device lifetime. The primary role of the interfacial layer between the anode and the EML is to reduce the hole injection barrier between them by modifying the interface energetics between the anode and the EML. Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) is the most widely-used anode interfacial layer, i.e., hole injection layer (HIL), because it is solution-processable, highly transparent, and mechanically flexible. However, the work function (WF) of PEDOT:PSS (~ 5.0–5.2 eV) is not high enough to facilitate the holes from the anode (WF of 4.7–4.9 eV for ITO) to the EML (highest occupied molecular orbital (HOMO) energy level of ~5.4–5.9 eV for organic emitters, valence band maximum (VBM) of ~5.4–6.0 eV for perovskite materials) and it also has drawbacks such as strong acidity and hygroscopic property. Especially, PEDOT:PSS etches indium tin oxide (ITO) and thereby metallic species (e.g., In and Sn) are etched out from ITO. Those metallic species can diffuse toward overlying semiconducting layers and act as luminescence quenching sites, leading to severe loss in EL efficiency. Therefore, several material engineering approaches to overcome those limitations of the conventional polymeric interfacial layer are discussed. Since ITO has another inherent problem such as brittleness, increasing cost of In and low WF, efforts to develop new TCE materials are also required. In chapter 2, a low-acidity and polar solvent-soluble self-doped conducting polymer poly(styrene sulfonic acid)-grafted-polyaniline (PSS-g-PANI), is used as a HIL of PeLEDs. By replacing highly acidic PEDOT:PSS to PSS-g-PANI, the amount of diffused metallic species, especially In, from ITO is significantly reduced. Also, the polar solvent-soluble character of PSS-g-PANI facilitated fine control of crystallization of methylammonium bromide (MAPbBr3) perovskite, leading to the formation of a uniform and pinhole-free MAPbBr3 film with closely packed small grains. Consequently, the device efficiency was doubled from 7.07 cd A-1 to 14.3 cd A-1 by using PSS-g-PANI, instead of PEDOT:PSS. Furthermore, formamidinium lead bromide (FAPbBr3) nanoparticle (NP) LEDs with PSS-g-PANI achieved significantly high current efficiency (CE) of 37.6 cd A−1. In chapter 3, the acidity of PEDOT:PSS is reduced by using weak base additives and the WF of PEDOT:PSS is increased by introducing high WF perfluorinated ionomer (PFI). The strong acidity and hygroscopic property of PEDOT:PSS originates from free poly(styrene sulfonate) (PSS-) ions which are not bound to PEDOT+. The weak base interacts with free PSS- ions and thereby reduce the acidity of PEDOT:PSS. Since PFI has a high ionization potential, self-organization occurs in the HIL. Therefore, the HIL with PFI has a gradient WF from the bottom (ca. 5.2 eV) to top surface (ca. 5.95 eV) of the HIL, so it is named as GraHIL. However, introducing a weak base additive into the GraHIL lowered the WF of the HIL because of conformational changes (i.e., phase separation and vertical segregation) of polymer chains. Aniline has a relatively small dipole moment among other base additives (i.e., imidazole and 2,3-dihydroxypyridine (DOH)), affecting less on the conformation of the HIL. The neutralized GraHIL (named as n-GraHIL) has a low acidity (pH<6) and high WF (>5.8 eV), leading to reduced luminescence quenching in MAPbBr3 polycrystalline (PC) perovskite. Also, the n-GraHIL achieved high current efficiency (CE) in PeLEDs, independently from perovskite types (17.16 cd A-1 in PeLEDs based on MAPbBr3 PC films, and 52.55 cd A-1 in PeLEDs based on FAPbBr3 NPs). In chapter 4, a newly-discovered two-dimensional (2D) transition metal carbide called MXene is used as a transparent conducting electrode (TCE) of phosphorescence organic light-emitting diodes (OLEDs) and a suitable interfacial layer is used as a HIL between the anode and the overlying semiconducting layer. MXene has great advantages of simple synthesis, solution processability and high electrical conductivity, etc. Here, titanium carbide (Ti3C2) MXene is used as TCEs of OLEDs. We found that the Ti3C2 has a higher WF (ca. 4.9 eV) than ITO (ca. 4.7 eV). Also, We discovered that the conventional water-dispersed PEDOT:PSS delaminates the Ti3C2 layer, while the alcohol-based n-GraHIL was uniformly formed on the Ti3C2. Also, the n-GraHIL prevented the oxidation of Ti3C2 during processing and storage. High-efficiency Ir(ppy)2(acac)-based phOLEDs (CE of 101.9 cd A-1 and EQE of 28.5%) were achieved using Ti3C2 as a TCE and n-GraHIL as a HIL. Furthermore, flexible OLEDs were demonstrated by employing this Ti3C2/n-GraHIL on PET substrates.