Elucidating the Role of Substrates on Domain Distribution of Quasi-2D Perovskites for Blue Light-Emitting Diodes

Abstract

Perovskite light-emitting diodes (PeLEDs) have been developed at a rapid pace with high external quantum efficiencies (EQEs) surpassing 20% for red, infrared, and green emissions; nevertheless, the performance of their blue-emitting cousins remains relatively quite low. Among various crucial factors that could impact these solution-processed devices, selection of a suitable underlayer (usually a hole-transport layer) is critical to the quality of resulting perovskite films. Particularly, the role of the substrate is highly important in the case of quasi-2D perovskite films where the emission occurs as a result of cascade energy transfer among various perovskite domains with different numbers of lead-halide octahedral layers (n) sandwiched between long-chain organic molecules. Herein, we explored the impact of the substrate on domain distribution of quasi-2D perovskite films, and a series of efficient and color-stable blue PeLEDs is attained through precise stoichiometric engineering. By employing a range of characterization tools, we thoroughly investigated the crystallization mechanism of quasi-2D perovskites on different substrates, including poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and poly[(9,9-dioctylfluorenyl-2,7-diyl)-co -(4,4′-( N -(4-sec-butylphenyl)diphenylamine)] (TFB). Perovskite films spin-coated on PEDOT:PSS exhibited cyan emission from the n ≥ 4 quasi-2D phase with uniformly distributed small grains. In contrast, perovskite layers formed on TFB featured an island morphology and showed blue emission from the quasi-2D phase with n-values between 2 and 4. In addition, we examined how the variation of A-site cations affects the optical properties of quasi-2D perovskites. Consequently, PEDOT:PSSbased champion PeLED exhibited cyan emission at 495 nm with a maximum EQE of 6.73%, whereas TFB-based counterparts manifested blue emission at 486 nm with a peak EQE of 3.95%. We anticipate that our work will provide a fundamental guideline to rationally selecting the substrates for the optimized growth of quasi-2D perovskites toward high-efficiency PeLEDs.

3D and 2D Metal Halide Perovskites for Blue Light-Emitting Diodes

“…Another approach to controlling the distribution of the 2D and 3D phases in the films is proper selection of the underlying layer. PEDOT:PSS, poly[(9,9-dioctylfluorenyl-2,7diyl)-co-(4,4-(N-(4-sec-butylphenyl)diphenylamine)] (TFB), poly(9-vinylcarbazole) (PVK), and poly[bis(4-phenyl)(4-butylphenyl)amine] (poly-TPD) have been used as HTLs in LED device architectures [69][70][71][72]. Moreover, most of the MHP layers were directly deposited on the PEDOT:PSS layer through solution processing.…”

“…Moreover, most of the MHP layers were directly deposited on the PEDOT:PSS layer through solution processing. Therefore, the crystallization kinetics of MHP crystal growth during solution processing was significantly affected by the underlying layer [69,70]. Cai et al reported that when TFB was used as the HTL, lower-dimensional 2D phases (2 ≤ n ≤ 4) were formed more favorably than when PE-DOT:PSS was used as the HTL [69].…”

“…Therefore, the crystallization kinetics of MHP crystal growth during solution processing was significantly affected by the underlying layer [69,70]. Cai et al reported that when TFB was used as the HTL, lower-dimensional 2D phases (2 ≤ n ≤ 4) were formed more favorably than when PE-DOT:PSS was used as the HTL [69]. The microstructure images of the quasi-2D MHP films deposited on the PEDOT:PSS and TFB layers showed different morphological characteristics (Figure 9a,b).…”

Efficient NIR Perovskite Light-Emitting Diodes Enabled by Incorporating an Anthracene Derivative as a Bifunctional Electron Transport Layer

“…Particularly, the hole mobility is higher than the electron mobility in such layered perovskite films. , For instance, Ahn et al introduced polyvinylpyrrolidone (PVP) as an insulating layer to lower the hole-current and achieve better charge-carrier balance, which resulted in Q-2D green PeLEDs with improved performance . Notably, the electron mobility of commonly used electron transport material 2,2′,2″-(1,3,5-benzinetriyl)-tris­(1-phenyl-1- H -benzimidazole) (TPBi) is only 3.3 × 10 –5 cm 2 V –1 s –1 , which is much inferior to the mobility of the widely used hole transport material poly­[(9,9-dioctylfluorenyl-2,7-diyl)- alt -(4,4′-( N -(4-butylphenyl)] (TFB); this difference in electron–hole mobilities consequently exacerbates the imbalance between electron and hole currents in the device. ,− Such imbalanced charge injection in PeLEDs results in the formation and accumulation of excitons at the perovskite/ETL interface, which can be quenched by the cathode or be transformed to joule heat, thereby contributing to the low efficiency with severe roll-off and operational instability. Moreover, the interfaces between EML and the charge transporting layers are vulnerable to various defects that further deteriorate the performance of PeLEDs.…”