Organic wrinkles embedded in high-index medium as planar internal scattering structures for organic light-emitting diodes

Organic wrinkle structures are investigated as an internal scattering layer for the fabrication of highly efficient organic light-emitting diodes (OLEDs). The solution-processed wrinkle structures are planarized with a high-index layer, resulting in a reduced surface roughness and optical haze simultaneously. The OLED device including the wrinkle structures achieved external quantum efficiency (EQE) of 24.2%, corresponding to a 1.24 time enhancement with respect to that of a control device having no internal scattering layer. By attaching a micro lens array (MLA) or a half ball lens (HBL) on the external surface of the substrate, the proposed device exhibits an EQE of 30.5% (1.56 time enhancement) and 41.8% (2.14 time enhancement), respectively. Randomness of the distributed wrinkle structures is shown to provide additional benefits of reduced angular spectral distortion and Lambertian-like emission properties. Trans-scale optical simulation combining wave-optics and the geometrical effect are used for the quantitative analysis of the emission characteristics of the device.     

Energetic distribution of interface states extracted from photo-conductance of organic thin film transistors

In this paper, a new method is introduced to obtain the energetic distribution of the interface states (density of states; DOS) extracted from the photo-conductance of organic thin film transistors (OTFTs) which exhibit varied transfer characteristics under illumination with different photon energies. The method was applied to pentacene OTFTs, and the results were compared with existing data. The major findings were not only the existence of the well-known peaks of DOS at 1.82 eV (free exciton of pentacene), and at 1.49 eV (extrinsic exciton due to dihydropentacene) but also new peaks were found at 1.25 eV, 1.29 eV, 1.31 eV, and 1.35 eV in the mid-gap. The new peaks were strongly enhanced under exposure to oxygen, and thus seem to be related to the defects associated with the presence of oxygen.     

Solution processable PEDOT:PSS based hybrid electrodes for organic field effect transistors

We report high performance solution processed conductive inks used as contact electrodes for printed organic field effect transistors (OFETs). Poly(3,4-ethylenedioxythiophene): polystyrene sulfonate (PEDOT:PSS) electrodes show highly improved very low sheet resistance of 65.8 ± 6.5 Ω/square (Ω/□) by addition of dimethyl sulfoxide (DMSO) and post treatment with methanol (MeOH) solvent. Sheet resistance was further improved to 33.8 ± 8.6 Ω/□ by blending silver nanowire (AgNW) with DMSO doped PEDOT:PSS. Printed OFETs with state of the art diketopyrrolopyrrole-thieno[3,2-b]thiophene (DPPT-TT) semiconducting polymer were demonstrated with various solution processable conductive inks, including bare, MeOH treated PEDOT:PSS, single wall carbon nanotubes, and hybrid PEDOT:PSS-AgNW, as the source and drain (S/D) electrode by spray printing using a metal shadow mask. The highest field effect mobility, 0.49 ± 0.03 cm² V⁻¹ s⁻¹ for DPPT-TT OFETs, was obtained using blended AgNW with DMSO doped PEDOT:PSS S/D electrode.     

Development of new high‐performance visible light photoinitiators based on carbazole scaffold and their applications in 3d printing and photocomposite synthesis

In this article, new compounds based on the carbazole scaffold (DMs = DM1 and DM2, constituted by a carbazole unit connected on positions 3 and 6 to a two 4,4′‐dimethoxydiphenylamine groups and differing by the substituent present on the nitrogen heteroatom of the carbazole core) were synthesized and proposed as high‐performance visible light photoinitiators/photosensitizers for both the free‐radical polymerization of methacrylates and the cationic polymerization of epoxides upon visible light exposure using LED@405 nm. Remarkably, DM2 leads to higher final conversions than DM1. In order to study the photophysical and photochemical properties of the carbazole derivatives, different parameters were taken into account such as the light absorption, the steady‐state photolysis, and the fluorescence spectroscopy. Using different techniques such as fluorescence quenching, redox behavior, and cyclic voltammetry, we are able to discuss the photosensitization/photoinitiation reactions providing a full coherent picture of the involved chemical mechanisms. The photosensitization of the carbazole derivatives occurred predominantly via singlet excited states at the rate of the diffusion limit. Upon exposure to laser diode at 405 nm, DMs show high performance in initiating systems for 3D resins. Remarkably, DM2 can also be used in photocomposite synthesis using light‐emitting diode conveyor. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019, 57, 2081–2092     

Cyclodimers and Cyclotrimers of 2,3 -Bisalkynylated Anthracenes,Phenazines and Diazatetracenes

The synthesis of novel (N−)acene-based cyclooligomers is reported. Glaser-Hay coupling of the bisethynylated monomers results in cyclodimers and cyclotrimers that are separable by column and gel-permeation chromatographies. For the diazatetracene, the use of sec-butyl-silylethynyl groups is necessary to achieve solubility. Diazatetracene-based cyclodimers and cyclotrimers were used as semiconductors in thin-film transistors. Although their optoelectronic properties are quite similar, their electron mobilities in proof-of-concept thin-film transistors differ by an order of magnitude.     

Dye mixture promoted light harvesting for organic dye-sensitized solar cells using triphenylamine dyes with various numbers of anchoring groups

Co-sensitizers and co-adsorbents are promising materials to enhance the light harvesting efficiency and reduce the un-expected back transfer reaction (recombination) of dye-sensitized solar cells (DSSCs). In this study, three sensitizers with triphenylamine as an electron donor, thiophene as a bridge and various numbers of acceptors/anchors cyanoacetic acid (TPA3T1A, TPA3T2A and TPA3T3A) were synthesized, and TPA3T1A and TPA3T2A were used as co-adsorbents with TPA3T3A. The results showed that co-adsorption on the TiO₂ surface at the following percentages, TPA3T3A 73%, TPA3T1A 17% and TPA3T2A 10%, resulted in an increase in the photovoltaic performance of the DSSCs from 5.27% to 5.83% compared to that of a single TPA3T3A sensitizer due to the increasing J_SC and V_OC. This enhancement might be due to improved light absorption and decreasing recombination by the co-sensitizers, TPA3T1A and TPA3T2A, occupying all the empty plases on the TPA3T3A-adsorbed TiO₂ surface.