Elucidating the Crucial Role of Hole Injection Layer in Degradation of Organic Light-Emitting Diodes

Although the luminous efficiency has been significantly improved in multilayered organic light-emitting diodes (OLEDs), understanding the major factors that influence degradation of OLEDs remains a major challenge due to their complex device structure. In this regard, we elucidate the crucial role of hole injection layer (HIL) in degradation of OLEDs by using systematically controlled hole injection interfaces. To analyze charge injection dependent degradation mechanism of OLEDs, we fabricate multilayered small-molecule OLEDs with molecularly controlled HILs. Although a reduced hole injection energy barrier greatly improves both a luminous efficiency and an operational lifetime (>10 times) of the OLEDs at the same time, large hole injection energy barrier increasingly aggravates its charge injection and transport during device operation. By using various kinds of nondestructive analyses at gradual stages of degradation, we demonstrate that accumulated charges at interfaces due to inefficient charge injection accelerates rate of device degradation

Synergetic Influences of Mixed-Host Emitting Layer Structures and Hole Injection Layers on Efficiency and Lifetime of Simplified Phosphorescent Organic Light-Emitting Diodes

We used various nondestructive analyses to investigate various host material systems in the emitting layer (EML) of simple-structured, green phosphorescent organic light-emitting diodes (OLEDs) to clarify how the host systems affect its luminous efficiency (LE) and operational stability. An OLED that has a unipolar single-host EML with conventional poly­(3,4-ethylenedioxythiophene):poly­(styrenesulfonate) (PEDOT:PSS) showed high operating voltage, low LE (∼26.6 cd/A, 13.7 lm/W), and short lifetime (∼4.4 h @ 1000 cd/m²). However, the combined use of a gradient mixed-host EML and a molecularly controlled HIL that has increased surface work function (WF) remarkably decreased operating voltage and improved LE (∼68.7 cd/A, 77.0 lm/W) and lifetime (∼70.7 h @ 1000 cd/m²). Accumulated charges at the injecting interfaces and formation of a narrow recombination zone close to the interfaces are the major factors that accelerate degradation of charge injection/transport and electroluminescent properties of OLEDs, so achievement of simple-structured OLEDs with high efficiency and long lifetime requires facilitating charge injection and balanced transport into the EML and distributing charge carriers and excitons in EML

Interaction of UL48 with RIP1 in CoIP assays.

<p>(A and B) 293T cells were co-transfected with plasmid expressing HA-UL48 and Flag-RIP1 as indicated. At 24 h after transfection, total cell lysates were immunoprecipitated with anti-HA (A) or anti-Flag (B) antibody, followed by immunoblotting with anti-flag (A) or anti-HA (B) antibody. The protein levels of HA-UL48 and Flag-RIP1 proteins in total cell lysates were also determined by immunoblotting. (C) The domain structure of RIP1 consists of the N-terminal kinase domain (KD), the intermediate domain (ID), and the C-terminal death domain (DD). The TNFα-induced K63-linked polyubiquitination site (K377) and the RHIM within the internal domain are indicated. (D to G) 293T cells were co-transfected with plasmids expressing Myc-UL48 and HA-RIP1 (wild-type or mutants). At 24 h after transfection, total cell lysates were immunoprecipitated with anti-Myc antibody, followed by immunoblotting with anti-HA antibody. The levels of Myc-UL48 and HA-RIP1 proteins in total cell lysates were also determined by immunoblotting. The amounts of wild-type and mutant HA-RIP1 proteins co-immunoprecipitated over the input amounts of proteins in (D) were quantitated by counting using ImageJ (NIH) and the relative binding efficiency is shown as a graph in (E). RIP1 ΔKD appeared as a doublet and the ratio was dependent on the conditions of cell lysate preparation.</p

Graphenes Converted from Polymers

Because the direct formation of large, patterned graphene layers on active electronic devices without any physical transfer process is an ultimate important research goal for practical applications, we first developed a cost-effective, scalable, and sustainable process to form graphene films from solution-processed common polymers directly on a SiO₂/Si substrate. We obtained few-layer graphene by heating the thin polymer films covered with a metal capping layer in a high-temperature furnace under low vacuum in an Ar/H₂ atmosphere. We find that the metal capping layer appears to have two functions: prevention of vaporization of dissociated molecules and catalysis of graphene formation. We suggest that polymer-derived graphene growth directly on inert substrates in active electronic devices will have great advantages because of its simple, inexpensive, and safer process

Solution-Processed n‑Type Graphene Doping for Cathode in Inverted Polymer Light-Emitting Diodes

n-Type doping with (4-(1,3-dimethyl-2,3-dihydro-1H-benzoimidazol-2-yl)­phenyl) dimethylamine (N-DMBI) reduces a work function (WF) of graphene by ∼0.45 eV without significant reduction of optical transmittance. Solution process of N-DMBI on graphene provides effective n-type doping effect and air-stability at the same time. Although neutral N-DMBI act as an electron receptor leaving the graphene p-doped, radical N-DMBI acts as an electron donator leaving the graphene n-doped, which is demonstrated by density functional theory. We also verify the suitability of N-DMBI-doped n-type graphene for use as a cathode in inverted polymer light-emitting diodes (PLEDs) by using various analytical methods. Inverted PLEDs using a graphene cathode doped with N-DMBI radical showed dramatically improved device efficiency (∼13.8 cd/A) than did inverted PLEDs with pristine graphene (∼2.74 cd/A). N-DMBI-doped graphene can provide a practical way to produce graphene cathodes with low WF in various organic optoelectronics

Vapor-Assisted Ex-Situ Doping of Carbon Nanotube toward Efficient and Stable Perovskite Solar Cells

Single-walled carbon nanotubes (CNTs) has been considered as a promising material for a top electrode of perovskite solar cells owing to its hydrophobic nature, earth-abundance, and mechanical robustness. However, its poor conductivity, a shallow work function, and nonreflective nature have limited further enhancement in power conversion efficiency (PCE) of top CNT electrode-based perovskite solar cells. Here, we introduced a simple and scalable method to address these issues by utilizing an ex-situ vapor-assisted doping method. Trifluoromethanesulfonic acid (TFMS) vapor doping of the free-standing CNT sheet enabled tuning of conductivity and work function of the CNT electrode without damaging underneath layers. The sheet resistance of the CNT sheet was decreased by 21.3% with an increase in work function from 4.75 to 4.96 eV upon doping of TFMS. In addition, recently developed 2D perovskite-protected Cs-containing formamidium lead iodide (FACsPbI3) technology was employed to maximize the absorption. Because of the lowered resistance, better energy alignment, and improved absorption, the CNT electrode-based PSCs produced a PCE of 17.6% with a JSC of 24.21 mA/cm2, VOC of 1.005 V, and FF of 0.72. Furthermore, the resulting TFMS-doped CNT-PSCs demonstrated higher thermal and operational stability than bare CNT and metal electrode-based devices

Tuning Molecular Interactions for Highly Reproducible and Efficient Formamidinium Perovskite Solar Cells via Adduct Approach

The Lewis acid–base adduct approach has been widely used to form uniform perovskite films, which has provided a methodological base for the development of high-performance perovskite solar cells. However, its incompatibility with formamidinium (FA)-based perovskites has impeded further enhancement of photovoltaic performance and stability. Here, we report an efficient and reproducible method to fabricate highly uniform FAPbI₃ films via the adduct approach. Replacement of the typical Lewis base dimethyl sulfoxide (DMSO) with <i>N</i>-methyl-2-pyrrolidone (NMP) enabled the formation of a stable intermediate adduct phase, which can be converted into a uniform and pinhole-free FAPbI₃ film. Infrared and computational analyses revealed a stronger interaction between NMP with the FA cation than DMSO, which facilitates the formation of a stable FAI·PbI₂·NMP adduct. On the basis of the molecular interactions with different Lewis bases, we proposed criteria for selecting the Lewis bases. Owed to the high film quality, perovskite solar cells with the highest PCE over 20% (stabilized PCE of 19.34%) and average PCE of 18.83 ± 0.73% were demonstrated