Yuichiro Yanagi

Flexible PTB7:PC71BM bulk heterojunction solar cells with a LiF buffer layer

Bulk heterojunction solar cells were fabricated using poly[4,8-bis[(2-ethylhexyl)oxy]benzo [1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]-thiophenediyl] (PTB7) and [6,6]-phenyl C71 butyric acid methyl ester (PC71BM) after a layer of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) was deposited on a flexible indium tin oxide (ITO)-coated polyethylene terephthalate substrate. The fabricated structures were Al/LiF/PTB7:PC71BM/PEDOT:PSS/ITO with or without a lithium fluoride (LiF) buffer layer, and the effect of the LiF buffer layer on the performance of the solar cells was investigated. The LiF layer significantly increased the open-circuit voltages and fill factors of the solar cells, presumably because of the work function shift of the aluminum cathode. As a result, the conversion efficiency increased from 2.31 to 4.02% owing to the presence of the LiF layer. From the results of a stability test, it was concluded that the inserted LiF layer acted as a shielding and scavenging protector, which prevented the intrusion of some chemical species into the active layer, thereby improving the lifetime of the unpakcaged devices.

Performance improvement of flexible bulk heterojunction solar cells using PTB7:PC71BM by optimizing spin coating and drying processes

Bulk-heterojunction solar cells were fabricated using a dichlorobenzene solution of poly[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b?]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]-thiophenediyl] (PTB7):[6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) on a flexible indium?tin-oxide-coated polyethylene terephthalate substrate. It was found that the performance of the solar cells could be markedly improved by minimizing the spin coating time of a blend of PTB7 and PC71BM to 10 s and maximizing the successive drying and solidification time up to 30 min in a confined Petri dish. As a result, a short-circuit current density of 14.5 mA/cm2, an open-circuit voltage of 0.62 V, and a power conversion efficiency of 3.67% were obtained. These improvements are attributed to the growth of favorable nanostructures during the slow drying process that increased the photocarrier collection efficiency while simultaneously increasing the performance fluctuations of each device.

Flexible Organic Solar Cells Based on Spin-Coated Blend Films of a Phenylene-Thiophene Oligomer Derivative and PCBM

Flexible organic solar cells were fabricated on a polyethylene terephthalate (PET) substrate using a phenylene–thiophene oligomer, 3,7-bis[5-(4-n-hexylphenyl)-2-thienyl]dibenzothiophene-5,5-dioxide (37HPTDBTSO), as a photoactive donor. In bulk-heterojunction (BHJ) solar cells, many factors such as blend weight ratio, and solution and substrate temperatures have great effects on the photovoltaic performances. The flexible solar cell fabricated from the blend solution with a weight ratio of 1:2 of 37HPTDBTSO to phenyl-C61-butyric acid methyl ester (PCBM) and a temperature of 50°C, exhibited a high open-circuit voltage of 0.74 V and the highest power conversion efficiency of 0.26%.

Phenylene–Thiophene Oligomer Derivatives for Thin-Film Transistors: Structure and Semiconductor Performances

Two phenylene–thiophene oligomer derivatives, 2,8-bis[5-(4-n-hexylphenyl)-2-thienyl]dibenzothiophene (28HPTDBT) and 3,7-bis[5-(4-n-hexylphenyl)-2-thienyl]dibenzothiophene (37HPTDBT), were used as active materials in thin-film organic field-effect transistors (OFETs). Although the two molecules have similar structures, they exhibited obvious differences in photophysical, crystal, π-stacking, and electrical properties. 28HPTDBT is an amorphous material and hence showed no semiconductor characteristics in its thin-film OFETs, while 37HPTDBT exhibited high crystallinity and strong π-stacking in the solid state, thus resulting in high charge carrier mobilities. The effects of gate insulators and annealing treatment on transistor performances were also investigated. Thin-film OFETs based on 37HPTDBT with an octadecanyltrichlorosilane (OTS)-treated SiO2 gate insulator exhibited excellent field-effect performances with a maximum mobility of 0.3 cm2 V-1 s-1 and a high Ion/Ioff current ratio of 1.5×105. Although annealing treatment improved the crystallinity of the thin films, the appearance of voids (cracks) resulted in a decrease in the charge carrier mobilities in the OFETs.

Application of highly conductive DMSO-treated PEDOT:PSS electrodes to flexible organic solar cells

In this paper, we report on bulk heterojunction organic solar cells fabricated on highly conductive poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOTPSS) electrodes directly deposited on a flexible polyethylene terephthalate (PET) substrate. The PEDOTPSS layers were deposited using a neutralized (pH~7) solution. The sheet resistance of the PEDOTPSS films became more than two orders of magnitudes smaller after two step treatments of the PEDOTPSS than that without the treatments. The treatments include 1) spin coating of the neutralized PEDOTPSS solution containing 5% of dimethyl sulfoxide (DMSO), and 2) dipping the deposited PEDOTPSS films in pure DMSO for 30 min As a result, a PEDOTPSS layer on a PET substrate with a sheet resistance of 158.5 Ω/□ and optical transmittance of 58.7% at the light wave length of 600 nm was realized. The organic solar cell based on poly[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b] dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]-thiophenediyl]: [6,6]-phenyl-C71-butyric acid methyl ester fabricated on the PEDOTPSS substrate exhibited power conversion efficiency of 2.51% that was larger than that of a solar cell fabricated on an indium tin oxide (ITO) coated PET substrate with a similar structure. The PEDOTPSS substrate was confirmed to be much more flexible than the ITO substrate from bending tests, and it showed no degradation in sheet resistance up to the radius of curvature of 3 mm.

Novel Thienyl-dibenzothiophene Oligomers End-capped by Hexylphenyl Groups as Potential Organic Semiconductor Materials

Novel thienyl-dibenzothiophene oligomers end-capped by hexylphenyl groups, 2,8-bis[5-(4-n-hexylphenyl)-2-thienyl]dibenzothiophene (28HPTDBT) and 3,7-bis[5-(4-n-hexylphenyl)-2-thienyl]dibenzothiophene (37HPTDBT), have been synthesized using Stille cross-coupling reactions. Newly synthesized oligomers were investigated by Ultraviolet-visible (UV-vis) and photoluminescence (PL) spectroscopy, cyclic voltammogram, and thermal analysis. Strong intermolecular π-π interactions, appropriate energy band gaps and low HOMO energy levels imply that the two oligomers are promising candidate materials for durable organic electronic devices.

Luminescence Gap in Regioregular Poly(3-hexylthiophene) Film

The luminescence gap in the regioregular poly(3-hexylthiophene) film is shown to be 1.93 eV at 16 K from the excitation energy dependence of the luminescence spectrum. The self-trapped exciton, of which the luminescence is due to the recombination, is suggested to consist of a self-trapped hole and an electron bound around the hole. The power law decay described by t−1.14 is observed, which is explained from the diffusion in the fractal structure of the electron bound around the self-trapped hole.

Bulk-Heterojunction Solar Cells Based on Poly(3-hexylthiophene) and (6,6)-phenyl-C61-butyric-acid Methyl Ester on Polyethylene Terephthalate Substrates

Bulk-heterojunction solar cells were fabricated based on poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) on an indium tin oxide (ITO) coated flexible polyethylene terephthalate (PET) substrate. Performance improvements of the flexible solar cells by optimizing post thermal annealing conditions are reported. The solar cells annealed at 150 oC showed the minimal deformation of the PET substrate, and the resulted conversion efficiency was 1.35% under the light irradiation conditions of the Superscript textAM1.5 simulated solar intensity of 100 mW/cm2.