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

N,N'-Diphenylperylene Diimide Functioning as a Sensitizing Light Absorber for Organic Thin-Film Solar Cells

ArticleORGANIC ELECTRONICS. 14(2):464-468 (2013)journal articl

Bipyridyl-substituted benzo[1,2,3]triazoles as a thermally stable electron transporting material for organic light-emitting devices

We developed new electron-transporting materials (ETMs) for organic light-emitting devices (OLEDs) based on benzo[1,2,3] triazole and two bipyridines. Four derivatives based on the same skeleton were synthesized with four different substituents: phenyl (BpyBTAZ-Ph), biphenyl (-BP), m-terphenyl (-mTP), and o-terphenyl (-oTP). These BpyBTAZ compounds have good thermal stabilities, and their decomposition temperatures were greater than 410 degrees C, which is significantly higher than that of tris(8-quinolinolato) aluminium (Alq), the conventional OLED material. BpyBTAZ compounds show preferable amorphous nature, and moreover, the glass transition temperatures (T(g)s) of both BpyBTAZ-TP compounds exceed 100 degrees C. Furthermore, BpyBTAZ-BP exhibits no melting point and is fully amorphous. The electron affinities of the materials are as large as 3.3 eV and their electron mobility is sufficiently high. These characteristics accounted for a reduction in the operational voltage of OLEDs with BpyBTAZ compounds compared with the reference device with Alq as an ETM. Specifically, the electron mobility of all the BpyBTAZ compounds exceeds 1 x 10(-4) cm(2) V(-1) s(-1) at an electric field of 1 MV cm(-1). In addition, it was revealed that both BpyBTAZ-TP-based devices showed longer luminous lifetimes and smaller voltage increases during continuous operation at 50 mA cm(-2), compared with the Alq reference device.ArticleJOURNAL OF MATERIALS CHEMISTRY. 21(32):11791-11799 (2011)journal articl

A highly efficient sublimation purification system using baffles with orifices

Here we report a highly efficient sublimation purification system using baffles with orifices in the sublimation tube. It is clearly demonstrated that the purity of materials is increased largely by introducing some pieces of baffles with orifices in the high-temperature region (sublimation region), which was confirmed by comparing the melting point (T-m) and the high-performance liquid chromatography (HPLC) purity with those of purified one by a conventional system. The driving voltages of electroluminescence (EL) devices were also compared and showed the same tendency with the T-m and HPLC purity data. Not only the purity but also the effective yield of purified materials was also increased by introducing baffles in low-temperature region. Some expected mechanisms of improving the purification efficiency by introducing baffles were also discussed.ArticleOrganic Electronics. 11(5):794-800 (2010)journal articl

Comparative Study of Soluble Naphthalene Diimide Derivatives Bearing Long Alkyl Chains as n-type Organic Thin-Film Transistor Materials

ArticleORGANIC ELECTRONICS. 14(2):516-522 (2013)journal articl

Thermal treatment effects on N-alkyl perylene diimide thin-film transistors with different alkyl chain

The authors report that thermal treatment effect on various N,N'-dialkyl-3,4,9,10-perylene tetracarbxylic diimides [PTCDI-Cn, alkyl-dodecyl (n = 12), butadecyl (n = 14), octadecyl (n = 18)] thin-film transistors (TFTs) depends on the substituted alkyl chain length. It is clearly demonstrated that there are two kinds of molecular movements during the thermal treatment on PTCDI films; molecular rearrangement in the same layer and molecular migration from the lower layer to the upper layer. The former is directly related to the grain growth and can be controllable by applying an external electric field. The latter is also related not only to the grain growth but also to the formation of cracks between grains. These two movements show opposite dependence on the alkyl chain length during the thermal treatment; the former is more active in longer alkyl chain, but the latter in shorter one. However, they also have opposite effect to TFT performance, and PTCDI films with longer alkyl chains have great advantage on TFT performance for the thermal treatment. Consequently, PTCDI-C18 TFTs show the highest electron mobility as large as 1.2 cm(2)/V s after the thermal treatment at 140 degrees C. (C) 2010 American Institute of Physics. [doi:10.1063/1.3525997]ArticleJOURNAL OF APPLIED PHYSICS. 108(12):124512 (2010)journal articl

Benzene substituted with bipyridine and terpyridine as electron-transporting materials for organic light-emitting devices

New electron-transporting materials for organic light-emitting devices (OLEDs) based on trisubstituted benzene with both bipyridine and terpyridine, 1,3-bisbipyridyl-5-terpyridylbenzene (BBTB) and 1-bipyridyl-3,5-bisterpyridylbenzene (BTBB), were developed. Glass transition temperatures of BBTB and BTBB were 93 degrees C and 108 degrees C, respectively, and BTBB was completely amorphous with no melting point. Electron mobilities of BTBB exceeded the order of 10(-4) cm(2) V-1 s(-1), while those of BBTB were very high and reached 10(-3) cm(2) V-1 s(-1) at an electric field of approximately 500 kV cm(-2). These high mobilities contributed to a low voltage operation. For example, in the case of the conventional aluminum trisquinolinol (Alq)-based fluorescent OLED with BTBB, current densities of 3.5 mA cm(-2) and 100 mA cm(-2) were reached at voltages of 3.0 V and 4.5 V, respectively. In addition, ionization potentials of BBTB (6.33 eV) and BTBB (6.50 eV) were sufficiently large to confine holes in common emissive layers.ArticleJOURNAL OF MATERIALS CHEMISTRY. 22(14):6765-6773 (2012)journal articl