Measurement of Exciton Diffusion Lengths of Phthalocyanine Derivatives Based on Interlayer Excitation Transfer

The exciton diffusion length (L-D) of several phthalocyanines (Pcs) was investigated by use of organic photovoltaic cells, where two different p-type organic semiconductor materials, including a Pc layer, were laminated, and interlayer excitation energy transfer occurred between them. The distribution of excitons in the Pc layer that accept energy from the other p-type semiconductor material by excitation transfer should be much narrower than that of excitons created by light absorption directly. This delta-function-like generation of excitons allowed the simple analysis of exciton diffusion. The results of the analysis indicated that the planar Pcs (copper Pc and metal-free Pc) showed shorter L(D)s of around 5 nm, as compared to the 18.5 +/- 4.8 nm shown by the pyramidal-shaped Pc (tin Pc).ArticleTHIN SOLID FILMS. 527:239-243 (2013)journal articl

A molecular thermometer based on luminescence of copper(II) tetraphenylporphyrin

The photoluminescence (PL) spectra from solid samples consisting of polystyrene and 5,10,15,20-tetraphenyl-21H,23H-porphine copper(II) (CuTPP) revealed two sharp peaks at 653 and 720 nm, and indicated another broad peak at around 780 nm, which could not be observed in CuTPP solutions. The intensity ratio of the two peaks appearing at 653 nm and at around 780 nm demonstrated clear dependence on temperature. This dependence of the PL intensity ratio was observed under the atmosphere and even in water. The findings mean that we were able to utilize PL from CuTPP as a thermometer under various conditions. We measured the temperatures of an organic light-emitting diode (OLED) during operation based on its electroluminescence, where the OLED contained CuTPP as a light-emitting dopant, to demonstrate the CuTPP thermometer.ArticleThin Solid Films. 518(2):563-566 (2009)journal articl

Wet-processed n-type OTFTs utilizing highly-stable colloids of a perylene diimide derivative

Here we propose and demonstrate a new approach to wet-processed organic thin-film transistors utilizing highly-stable colloids of small molecular organic semiconductors prepared by laser ablation technique. Highly stable N,N′-ditridecyl-3,4,9,10-perylenetetracarboxylic diimide (PTCDI-C13) colloids of 0.1 wt% concentration were prepared successfully by laser ablation technique in acetonitrile. Mean particle size of the prepared PTCDI-C13 colloid measured by dynamic light scattering method was smaller than 50 nm which is also confirmed by a transmission electron microscope observation. By simple drop-casting of prepared PTCDI-C13 colloids, typical n-type OTFTs with good saturation properties were fabricated successfully. The highest electron mobility was 1.1 × 10−3 cm2/Vs and increased up to 0.027 cm2/Vs by post-annealing treatment.ArticleORGANIC ELECTRONICS. 14(1):19-25 (2013)journal articl

Sensitization of organic photovoltaic cells based on interlayer excitation energy transfer

We incorporated an additional p-type organic semiconductor layer (APL) between the anode and the phthalocyanine layer, which is an indispensable p-type semiconductor layer (IPL) in forming a p/n junction with a fullerene C-60 layer. We used two thiophene/phenylene co-oligomers as the APL. The incorporation increases the short-circuit current density (J(SC)) and enhances incident photon-current conversion efficiency (IPCE) over the wavelength region where the APL shows strong absorption. Combined dependence of the APL/IPL implies that Forster resonance excitation transfer is the main factor in J(SC) and IPCE enhancements. We demonstrate clearly that the 'positive' hole injection barrier at the interface between the APL and the IPL impacts the smooth transportation of holes to the indium-tin-oxide anode. However, the small positive hole barrier of 0.1 eV has no noticeable influence on the fill factor of the current density versus voltage characteristic under photoirradiation, or on those devices with 'negative' hole barriers.ArticleOrganic Electronics. 11(4):700-704 (2010)journal articl

Cascade-Type Excitation Energy Relay in Organic Thin-Film Solar Cells

ArticleORGANIC ELECTRONICS. 14(3):814-820 (2013)journal articl

Enhancing spectral contrast in organic red-light photodetectors based on a light-absorbing and exciton-blocking layered system

This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. The following article appeared in JOURNAL OF APPLIED PHYSICS. 108(3):034502 (2010) and may be found at https://doi.org/10.1063/1.3466766 .We demonstrated a highly sensitive red-light photodetector based on a mixed copper phthalocyanine (CuPc) and fullerene C-60 photoactive layer, similar to a so-called bulk heterojunction structure usually used in the field of organic photovoltaics. We incorporated an additional set of organic layers that was composed of two organic p-type semiconductors to reduce the blue-light sensitivities of CuPc- and C-60-based organic photodetectors. We used alpha, omega-diphenyl sexi-thiophene (P6T) and alpha, omega-bis(biphenyl-4-yl)ter-thiophene (BP3T), which are thiophene-based materials and usually have good hole-transporting properties. A thick (>100 nm) P6T layer absorbed blue light, preventing it from reaching the photoactive layer, and a thin (similar to 20 nm) BP3T layer whose band gap was larger than that of P6T blocked excitation energy transfer from P6T to CuPc. Thus, we successfully demonstrated a red-light photodetector with high peak sensitivity and whose current-voltage characteristics did not worsen. The optimal device showed a peak incident photon-current conversion efficiency of 51.7% at 620 nm and a specific detectivity of 4.0 X 10(11) cm Hz(1/2)/W.ArticleJOURNAL OF APPLIED PHYSICS. 108(3):034502 (2010)journal articl

Effects of volatile additives in solutions used to prepare polythiophene-based thin-film transistors

We investigate the effects of volatile additives in solutions used to prepare thin-film transistors (TFTs) of regioregular poly(3-hexylthiophene) (P3HT). We use the additives trifluoromethylbenzene (TFMB) and methylcyclohexane (MCH) because they are poor solvents for P3HT. The additives improve the performance of the resulting TFTs when the boiling point (T(b)) of the major solvent, carbon tetrachloride, is lower than that of the additive. The maximum mobility is (4.0 +/- 60.9) x 10(-2) cm(2)V(-1)s(-1), which is 6.1 times larger than that of TFTs prepared without TFMB or MCH added to the solution; the on/off ratio and the subthreshold slope were also improved. The relative T(b) of the solvent and the additive affected the film formation with the amount of TFMB or MCH remaining at the final stage of thin film deposition influencing the precipitation of P3HT aggregates. (C) 2011 American Institute of Physics. [doi:10.1063/1.3553878]ArticleJOURNAL OF APPLIED PHYSICS. 109(5):54504 (2011)journal articl

Discotic liquid crystals of transition metal complexes 50(dagger): spiranthes-like supramolecular structure of phthalocyanine-fullerene dyads

We have synthesized novel liquid crystalline Pc-C-60 dyads (CnS)(6)PcCu-C-60 (n = 14, 16, 18: 1a-1c) by using our developed synthetic method in order to investigate the mesomorphism and alignment behavior. Each of the (CnS)(6)PcCu-C-60 dyads shows perfect homeotropic alignment in the Col(ho) mesophase between two glass plates for n = 14, 16, 18 and also on a glass plate for n = 14, although none of the parent Pc compounds (CnS)(8)PcCu and the Pc precursors (CnS)(6)PcCu-OH and (CnS)(6)PcCu-OFBA shows homeotropic alignment. It may be attributed to the strong affinity between fullerene and glass surface. Although the reason is not so clear at the present time, this is very useful guideline for the molecular design to prepare homeotropic alignment-showing discotic liquid crystals. Very interestingly, the spherical C-60 parts form a helical structure around the column formed by the disk-like Pc parts. This supramolecular structure very resembles spiranthes. The spiranthes-like supramolecular structure is compatible with one-dimensional nano-array expecting the high conversion efficiency of solar cells.ArticleJOURNAL OF PORPHYRINS AND PHTHALOCYANINES. 18(5):366-379 (2014)journal articl

Chemiluminescence behavior of fluorescent aromatics tethered 9-methylidene-10-methylacridans involving chemiluminescence resonance energy transfer (CRET) quenching

The chemiluminescence (CL) behavior, observed in the singlet oxygenation of three fluorescent aromatics tethered 9-methylidene-10-methylacridans bearing the pyrene, perylene, and stilbene moieties, was investigated. The CL spectrum of 9-(perylen-3′-ylidene)-10-methylacridan displayed a red-shifted emission different from the fluorescent products, while 9-(pyren-1′-ylidene)-10-methylacridan and 9-(4′-styrylbenzylidene)-10-methylacridan produced very weak CLs. A chemiluminescence resonance energy transfer (CRET) quenching of the excited aromatic aldehydes by the acridans remaining as the unreacted reactants was found to result in these unexpected CL behaviors.ArticleTetrahedron Letters. 54(11):1338-1343 (2013)journal articl

Bi- or ter-pyridine tris-substituted benzenes as electron-transporting materials for organic light-emitting devices

We demonstrated that 1,3,5-tris([2',2 '']bipyridin-6'-yl) benzene (BpyB) and 1,3,5-tris([ 2',2 '',6 '',2''']terpyridin-6'-yl)benzene (TpyB) are good electron-transport (ET) layer materials for organic light-emitting devices (OLEDs). The new materials exhibit high electron mobilities of around 10 (4) cm(2) V (1) s (1), and OLEDs comprising the materials operate at lower voltage than the OLEDs with tris(8-hydroxyquinolinato) aluminum (Alq) as an ET material. The new materials are also useful for phosphorescent OLEDs in blue-, green-, and red-light-emitting devices, where they function as both electron-transporting and hole-blocking materials. Phosphorescent OLEDs with the new materials operate at lower voltage (while maintaining comparable external quantum efficiencies of electroluminescence) than does the corresponding reference device with bathocuproine/Alq as electron-transporting and hole-blocking layers.ArticleORGANIC ELECTRONICS. 11(12):1966-1973 (2010)journal articl