Yasuhiko Shirota

High performance organic photovoltaic devices using amorphous molecular materials with high charge-carrier drift mobilities

pn-Heterojunction organic photovoltaic devices (OPVs) using amorphous molecular materials with high hole drift mobilities of 10−2u2002cm2u2009V−1u2009s−1, tris[4-(2-thienyl)phenyl]amine and tris[4-(5-phenylthiophen-2-yl)phenyl]amine, as electron donors and fullerene as an electron acceptor were fabricated. In spite of the use of amorphous materials instead of crystalline materials, the devices exhibited high performance with fill factors of 0.62–0.71 and power conversion efficiencies of 1.5%–1.7% under air-mass 1.5 G illumination at an intensity of 100u2002mWu2009cm−2.

Surface relief grating formation on a single crystal of 4-(dimethylamino)azobenzene

Surface relief grating (SRG) formation on an organic single crystal by irradiation with two coherent laser beams has been demonstrated by using 4-(dimethylamino)azobenzene (DAAB). It was found that the SRG formation was greatly depending upon both the coordination of the crystal and the polarization of the writing beams. The dependence of the polarization of writing beams on the SRG formation using the single crystal was found to be quite different from that reported for amorphous polymers and photochromic amorphous molecular materials, suggesting that the mechanism of the SRG formation on the organic crystal is somewhat different from that on amorphous materials.

Combinatorial study of exciplex formation at the interface between two wide band gap organic semiconductors

Combinatorial screening of exciplex formation in blends of 4,4′,4″-tris[2-naphthyl (phenyl)-amino] triphenylamine (2-TNATA), and 2,2′,7,7′-tetrakis(2,2′-diphenylvinyl) spiro-9,9′-bifluorene (spiro-DPVBi) is described. The blended layer was incorporated in ITO/[2-TNATA]/[1:1 2-TNATA:spiro-DPVBi]/[N,N′ - diphenyl - N,N′ - bis (1-naphthylphenyl) - 1,1′ - bi-phenyl - 4,4′-diamine (NPB)]/[spiro-DPVBi]/[tris(8-hydroxy quinoline) Al]/CsF∕Al organic light-emitting devices; the thickness of the blend and NPB layers were varied systematically. The electroluminescence quantum yield decreased as the blended layer thickness increased. The NPB spacer layer reduced the exciplex formation; an 8-nm-thick layer completely suppressed it.

Surface Relief Grating Formation Using a Novel Azobenzene-based Photochromic Amorphous Molecular Material, Tris[4-(phenylazo)phenyl]amine

ABSTRACT A novel azobenzene-based photochromic amorphous molecular material, tris[4-(phenylazo)phenyl]amine (TPAPA), was designed and synthesized. TPAPA was found to readily form an amorphous glass with a glass-transition temperature of 67°C and to exhibit photochromism as amorphous film. Irradiation of amorphous film of TPAPA with two coherent Ar+ laser beams led to the formation of surface relief grating with a diffraction efficiency of 2 ≈ 3% and a modulation depth of ca. 100 nm.

MoO3 as a Cathode Buffer Layer Material for the Improvement of Planar pn-Heterojunction Organic Solar Cell Performance

The use of MoO3 as a cathode buffer layer inserted between LiF and Al improved the power conversion efficiency (PCE) of planar pn-heterojunction organic solar cells (OSCs) by reducing exciton quenching at the interface between the n-type organic active layer and the electrode. The cell using an amorphous molecular material, tris[4-(5-phenylthiophen-2-yl)phenyl]amine, as a p-type organic semiconductor, C70 as an n-type organic semiconductor and MoO3 as a cathode buffer layer exhibited a PCE of 3.3% under AM1.5G illumination (100 mW cm-2), which is of the highest level among those for planar pn-heterojunction OSCs using amorphous molecular materials as donor materials.

Near-infrared electroluminescence based on perylenediimide-doped tris(8-quinolinolato) aluminum

For the purpose of exploring near-infrared emission, the photoluminescence (PL) and electro- luminescence (EL) of N,N′-bis(neopentyl)-3,4:9,10-perylenebis(dicarboximide) (BNPTCD): tris(8-quinolinolato)aluminum (Alq3) mixed films were investigated. It was found that BNPTCD:Alq3 mixed films exhibit PL in the near-infrared wavelength region and that organic light-emitting diodes using Alq3 highly doped with BNPTCD as an emitting layer exhibit near-infrared EL with a peak at 805 nm originating from BNPTCD aggregates.

Intermolecular Interaction by Apical Oxygen in Titanyl Phthalocyanine

Local charge distribution around the titanium atom of amorphous and α-crystalline TiOPc films was investigated using Ti 2 p X-ray absorption and emission spectroscopy. The X-ray emission spectra show two characteristic features that correspond to charge transfer states that result from in-plane Ti–N and apical Ti–O hybridizations. In the amorphous phase, the Ti–O charge transfer state has both localized and delocalized properties, whereas in the α-crystalline phase it has only the delocalized property. These different properties evidence the role of the oxygen atom in forming a network with a neighboring molecule and in balancing the total dipole moment in the α-crystalline phase.

Charge transport in amorphous molecular materials

Charge carrier drift mobilities of a series of hole-transporting amorphous molecular materials have been determined by a time-of-flight method. Electric-field and temperature dependencies of carrier mobilities have been analyzed in terms of the disorder formalism, and charge transport in amorphous molecular materials is discussed in relation to molecular structures. Hole-transporting amorphous molecular materials with high mobilities of the order of 10-2cm2V-1s-1 have been developed.

Photochromic Amorphous Molecular Material: Polarized-Light Induced Dichroism of Amorphous Film of 4-[Bis(9,9-dimethylfluoren-2-yl)amino]azobenzene

ABSTRACT Dichroism of amorphous film of an azobenzene-based photochromic amorphous molecular material, 4-[bis(9,9-dimethylfluoren-2-yl)amino]azobenzene (BFlAB), induced by irradiation with linearly polarized Ar+ laser light has been investigated.

Organic Photovoltaic Devices Using an Amorphous Molecular Material With High Hole Drift Mobility, Tris[4-(2-thienyl)phenyl]amine

A planar p-n heterojunction organic photovoltaic (OPV) device using an amorphous molecular material with a high hole drift mobility of 1.1 × 10-2 cm2/V·s at an electric field of 1.0 × 105 V/cm at 293 K, tris[4-(2-thienyl)phenyl]amine (TTPA), as an electron donor, and C60, as an electron acceptor, indium-tinoxide (ITO)/poly(3,4-ethylenedioxythiophene) doped with poly(4-styrene sulfonate) (PEDOT:PSS) (ca.30 nm)/TTPA (30 nm)/C60 (40 nm)/LiF (0.1 nm)/Al (150 nm), exhibited high performance with a fill factor of 0.62 and a power conversion efficiency (PCE) of 1.5 % under air-mass 1.5G illumination at an intensity of 100 mW/cm2. A p-i-n-type OPV device having a mixed interlayer of TTPA and C60, ITO/PEDOT (ca.30 nm)/TTPA (27 nm)/TTPA:C60 (1:4 molar ratio, 20 nm)/C60 (23 nm)/LiF (0.1 nm)/Al (100 nm), exhibited higher performance with a PCE of 1.8% under the same irradiation conditions. A bulk p-n heterojunction OPV devices fabricated by spin coating from solution of TTPA and [6,6]-phenyl-C61-butyric acid methyl ester ([6,6]-PCBM), ITO/PEDOT:PSS (ca. 30 nm)/TTPA:[6,6]-PCBM (1:4 molar ratio, ca. 73 nm)/LiF (0.1 nm)/Al (100 nm), exhibited a PCE of 1.3%. The high performance of the present devices is attributed to the high charge-carrier mobilities of the materials and the relatively high ionization potential of TTPA.