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Highlights: [•] The doping effects of cesium compounds on device characteristics of OLEDs are studied. [•] The charge transport property is highly dependent on doping constituent materials. [•] The operational stability of OLEDs replies on the doping mechanism. [Copyright &y& Elsevier]

Abstract An electron-rich moiety, pyrrolo[3,2-b]pyrrole, has been used as a new donor block in organic lighting-emitting diodes (OLEDs). Combined with a strong electron-withdrawing group of 2,1,3-benzothiadiazole, a novel red fluorophore BTZPP was synthesized. The solution-processed red-emitting device based on BTZPP showed encouraging performances with external quantum efficiency of up to 3.4%. This evidently indicates that the pyrrolo[3,2-b]pyrrole group can be a promising electron-donating moiety for enriching the material types and promoting the development of OLEDs. Graphical abstract Image 1 Highlights • Pyrrolo[3,2-b]pyrrole was used as a new donor block in OLEDs. • A novel red fluorophore BTZPP was synthesized basing on pyrrolo[3,2-b]pyrrole. • Using BTZPP, a simple OLED device was realized by an economical wet process. • The red emitting device demonstrated a high external quantum efficiency of 3.4%. [ABSTRACT FROM AUTHOR]

We report computational and experimental studies (i) on the Fermi level (E F ) pinning phenomena in weakly interacting electrode-organic semiconductor systems with pentacene, N, N-Di(naphthalene-l-yl)-N, N’-diphenyl-benzidine (α-NPD) and Poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT)], and (ii) to elucidate a common reason of the pinning phenomena which have been widely observed for various organic materials on inert electrode surfaces. For (i), the computed results on the electrode-dependence of E F -HOMO and E F -LUMO distance agreed excellently with UPS and Kelvin-probe results. For (ii) we found theoretically that the pinning phenomena occur at systems even without any electronic states in the HOMO-LUMO gap, and thereby indicate that this is a universal Fermi-level pinning-like phenomena for various band gap materials, even if specific interface states do not exist upon contact. We further obtained for Gaussian-distributed HOMO and LUMO that the minimal hole and electron injection barriers are quantitatively determined by degree of standard deviations of HOMO and LUMO bands, which are in excellent agreement with the experimental results. Furthermore, our results provide a guideline for approaching the “zero” injection barriers. [ABSTRACT FROM AUTHOR]

The role of transition metal oxides (TMOs) based intermediate connectors in tandem organic light emitting diodes (OLEDs) has been studied via capacitance-voltage and current-voltage characteristics, in order to elucidate the dynamic processes of charges generation and transport within externally applied voltages. The TMO-based intermediate connectors are composed of molybdenum trioxide (MoO 3 ) and cesium azide (CsN 3 )-doped-4, 7–diphenyl-1, 10-phenanthroline (BPhen) layers, where MoO 3 and CsN 3 are used due to low deposition temperatures. From the obtained results of capacitance and current density, charges generation in CsN 3 :BPhen/MoO 3 /NPB is proposed to the defect states in thermally evaporated MoO 3 , which offers a minimal energy offset for charges generation. Moreover, our results clearly indicate that charges generation efficiency is not only relying on the MoO 3 -NPB interface, but also influenced by CsN 3 :BPhen-MoO 3 interface. CsN 3 doped BPhen layer further improves charges separation efficiency, which finally results in favorable charges transport into the adjacent layers and ensure to function efficiently for tandem OLEDs. [ABSTRACT FROM AUTHOR]

Highlights: [•] Solution-processed Au nanoparticles (NPs):MoO X was used as an inter-layer in OLEDs. [•] The interface properties and good device stability were investigated. [•] The improved electrical properties were achieved due to modification of interface. [•] Localized surface plasmon induced efficiency enhancement was realized due to Au NPs. [Copyright &y& Elsevier]

Abstract: We report on the role of cesium fluoride (CsF) doping on the enhanced electron transport properties of tris-(8-hydroxyquinolin) aluminum (Alq3) for organic light-emitting diodes. The electronic structures of CsF-doped Alq3 layers with various doping concentration are characterized by in situ ultraviolet and X-ray photoelectron spectroscopies, showing an n-type electrical doping effect with Fermi level shift towards unoccupied molecular orbital and the formation of chemistry-induced gap-states. The increase in conductivity and reduction in electron injection barrier in CsF-doped Alq3 layer with optimal doping concentration lead to the enhanced electron injection and transport, which are consistent with the improved electrical characteristics of OLEDs. [Copyright &y& Elsevier]

Abstract: We report on a high-quality hybrid intermediate connector (IC) used in tandem organic light-emitting diodes (OLEDs), which is composed of an ultrathin MoO3 interlayer sandwiched between a n-type Cs2CO3-doped 4,7-diphenyl-1,10-phenanthroline (BPhen) layer and a p-type MoO3-doped N,N′-diphenyl-N,N′-bis(1-naphthyl)-(1,1′-biphenyl)-4,4′-diamine (NPB) layer. The charge generation characteristics for light emission in tandem OLEDs have been identified by studying the interfaces and the corresponding devices. The hybrid IC structure exhibits superior charge generation capability, and its interfacial electronic structures are beneficial to the generation and injection of electrons and holes into bottom and top emission units, respectively. Compared to the organic-TMO bilayer and doped p–n junction structures, the hybrid IC structure combining MoO3-based interlayer and p-type doping can effectively decrease the driving voltage and improve the current efficiency of tandem devices due to the increased bulk heterojunction-like charge generation interfaces. Our results indicate that the TMO-based hybrid IC structure can be a good structure in the fabrication of high-efficiency tandem OLEDs. [Copyright &y& Elsevier]

Three D-σ-A type thermally activated delayed fluorescent materials (TADF), AQ-S-Cz, AQ-S-DPA and AQ-S-DMAC, with sulfur atom connect donors and acceptor, were designed and synthesized. All of the emitters exhibit small ΔE ST (<0.2 eV) and the limited overlap of the molecular frontier orbitals because of the C–S σ bond and the distorted molecular structure. AQ-S-Cz and AQ-S-DMAC with rigid donor obtained higher photoluminescence quantum yields than AQ-S-DPA. The three emitters have excellent TADF performance and short delayed fluorescence lifetimes (0.32 μs for AQ-S-Cz, 0.15 μs for AQ-S-DPA, 0.28 μs for AQ-S-DMAC). The experiment results show that the three emitters have intra-molecular charge transition (intra-CT), especially inter-molecular charge transition (inter-CT) effect. The luminescence of these emitters can be efficiently regulated by intra-CT, especially inter-CT and realize prominent red shift from yellow (556 nm) to red emission (657 nm). Compared with that of AQ-S-Cz and AQ-S-DPA, the device based on AQ-S-DMAC obtained maximum external quantum efficiencies of 7.17% with turn-on voltage of 3.5 V. This work enriches the D-σ-A type orange-red TADF materials and provides a perspective approach of developing long wavelength TADF emitters. [Display omitted] • Three D-σ-A type TADF materials with C–S σ bond were designed and synthesized. • The luminescence of three emitters can be efficiently adjusted by intra-CT, especially inter-CT, and realize obvious red shift from yellow to red emission. • The smaller ΔE ST s can be obtained by C–S bond blocking the conjugate between donor and acceptor, meanwhile the rigid donor suppress the non-radiation process to achieve higher PLQYs. • The AQ-S-DMAC-based TADF-OLED emits orange-red fluorescence and obtains EQE max of 7.17%. [ABSTRACT FROM AUTHOR]