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ABSTRACT: This feature article focuses on the relationship between the interfacial structures constructed by molecular self-organization and the properties of organic photovoltaic devices. The use of self-assembled monolayers (SAMs) is reviewed for metal and metal oxide/organic interfaces, while surface-segregated monolayers (SSMs) are introduced as a new method for the modification of organic/organic interfaces. Research up to now has clearly demonstrated the effectiveness of the control of energy levels and other properties at the interfaces to enhance photovoltaic performance. The possibility of more precise control of the interfacial structures is also discussed.
Physical Chemistry Chemical Physics 02/2012; 14(11):3713-24. · 3.57 Impact Factor
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ABSTRACT: Copolymers with an alternating structure of regioregular oligo(3-hexylthiophene) (O3HT) with different lengths and 2,5-dibutyl-3,6-di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (DPP) were synthesized through Stille coupling reaction. The light absorption of the copolymers can be rationally tuned to have a broad spectrum across the visible region by adjusting the length of O3HT. Organic solar cells fabricated with the copolymers and PCBM showed a broad photoresponse and a comparable efficiency to that of poly(3-hexylthiophene) (P3HT):PCBM cells. The external quantum efficiency and fluorescence spectra suggested that the intrachain energy transfer from the O3HT block to the vicinity of the DPP unit could limit the photovoltaic performance of the copolymers.
Macromolecular Rapid Communications 02/2012; 33(8):658-63. · 4.60 Impact Factor
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ABSTRACT: In an ideal model, a p-n junction is formed by two stacked slabs of semiconductors. Although the construction of actual devices is generally more complex, we show that such a simple method can in fact be applied to the formation of organic heterojunctions. Two films of the organic semiconductors poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C(61)-butyric acid methyl ester (PCBM) can be connected by a simple film-transfer method without disturbing their flat surfaces. Each film can further be modified with a surface-segregated monolayer to tune the strength and direction of the surface dipole moment. Using this method, we fabricated bilayer organic photovoltaic devices with interfacial dipole moments that were selected to align the energy levels at the heterojunction. The open-circuit voltages of the P3HT/PCBM devices could be tuned over a wide range between 0.3 and 0.95 V, indicating that, even if the same combination of bulk materials is used, the interfacial properties drastically alter the performance of organic photovoltaic devices.
Nature Material 06/2011; 10(6):450-5. · 32.84 Impact Factor
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Angewandte Chemie International Edition 03/2011; 50(12):2799-803. · 13.45 Impact Factor
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ABSTRACT: We studied how the layers with different transport properties buried inside a semiconductor material affect the characteristics of organic thin film transistors (OTFT) using a well-defined multilayered structure fabricated by a contact film transfer method that we recently developed (ACS Appl. Mater. Interfaces 1, 2660 (2009)). A simple model with the charge distribution in the organic semiconductor film, the mobility dependence on the charge density, and the buried surface layers with a high mobility successfully reproduced the experimental mobility dependence on the thickness of the spin-coated films and the gate voltage. These results demonstrated that charge transport layers located far from the dielectric interface could contribute significantly to the total current in OTFTs.
ACS Applied Materials & Interfaces 01/2011; 3(2):139-42. · 4.53 Impact Factor
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ABSTRACT: The effects of molecular packing on electrical instability in poly(3-hexylthiophene) thin-film transistors were investigated by comparing two devices prepared using the same polymer solution and dielectric layer but by different processes. One device, which was prepared by a contact film transfer method by Wei etal [ACS Appl. Mater. Interfaces 1, 2660 (2009)], utilized the structure formed at the air/solution interface and exhibited high performance, the absence of hysteresis and excellent electrical stability under prolonged continuous bias stress, in contrast to the other device prepared by conventional spin coating, which utilized structures formed at the solution/dielectric interface. Neither thermal nor solvent annealing was applied to control the molecular packing, and thus these results indicate that the highly ordered molecular packing formed at the air/solution interface can simultaneously contribute to enhanced electrical stability.
Applied Physics Letters 07/2010; · 3.84 Impact Factor
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ABSTRACT: Two kinds of thieno[3,4-b]pyrazine-based monomers, 2,3-dimethyl-5,7-di(2-bromothien-5-yl)-thieno[3,4-b]pyrazine and 2,3-diphenyl-5,7-di(2-bromothien-5-yl)-thieno[3,4-b]pyrazine, were synthesized via an improved synthetic route. These two monomers and 4,7-di(2-bromothien-5-yl)-2,1,3-benzothiadiazole were copolymerized with three donor segments (fluorene, carbazole, and indolocarbazole) separately by a Suzuki cross-coupling reaction to give six types of 5,7-dithien-2-yl-thieno[3,4-b]pyrazine (DTTP)-based donor−acceptor (D−A) copolymers (TP1-6) and three types of 5,7-dithien-2-yl-2,1,3-benzothiadiazole (DTBT)-based D−A copolymers (PF-DTBT, PC-DTBT, and PIC-DTBT). The optical properties, electrochemical behavior, and energy levels of these nine copolymers were investigated. The photovoltaic performance of the copolymers was compared and discussed considering their energy levels.
02/2010;
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ABSTRACT: A highly effective photovoltaic polymer with near-infrared response, poly{N-[1-(2-ethylhexyl)-3-ethylheptanyl]-dithieno[3,2-b:2′,3′-d]pyrrole-3,6-dithien-2-yl-2,5-dibutylpyrrolo[3,4-c]pyrrole-1,4-dione-5′,5′′-diyl} {PDTP-DTDPP(Bu)}, was synthesized. The polymer has high molecular weight, good solubility, and a broad absorption spectrum in the range of 500−1100 nm. Field effect transistor charge mobility of PDTP-DTDPP(Bu) arrived 0.05 cm2 V−1 s−1. Bulk heterojunction type polymer solar cells based on PDTP-DTDPP(Bu) and PC70BM have broad photocurrent response wavelength range from 300 nm to 1.1 μm. High short-circuit current (14.87 mA/cm2) and power conversion efficiency (2.71%) were achieved, which is a significant advance for efficient photovoltaic polymers that respond to the whole range of the solar spectrum.
12/2009;
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ABSTRACT: The charge-carrier transport in the structures formed at the surface of various conjugated polymer films is investigated by constructing organic thin-film transistors using a novel and simple contact film transfer method. Thin-film transistors prepared by this transfer process have higher field-effect mobility values compared with conventional spin-coated devices for all the studied polymers. In contrast to previous reports, the hole mobility in regioregular poly(3-alkylthiophene)s does not depend on the length of the alkyl chain when the contact film transfer method is used. These results suggest that the thiophene rings adopt a highly ordered edge-on orientation and strong interchain pi-pi interactions spontaneously form at the polymer/air interface during the spin coating.
ACS Applied Materials & Interfaces 11/2009; 1(11):2660-6. · 4.53 Impact Factor
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ABSTRACT: We report a new type of ordered monolayer for the surface modification of organic semiconductors. Fullerene derivatives with fluorocarbon chains ([6,6]-phenyl-C(61)-buryric acid 1H,1H-perfluoro-1-alkyl ester or FC(n)) spontaneously segregated as a monolayer on the surface of a [6,6]-phenyl-C(61)-butyric acid methyl ester (PCBM) film during a spin-coating process from the mixture solutions, as confirmed by X-ray photoelectron spectroscopy (XPS). Ultraviolet photoelectron spectroscopy (UPS) showed the shift of ionization potentials (IPs) depending on the fluorocarbon chain length, indicating the formation of surface dipole moments. Surface-sensitive vibrational spectroscopy, sum frequency generation (SFG) revealed the ordered molecular orientations of the C(60) moiety in the surface FC(n) layers. The intensity of the SFG signals from FC(n) on the surface showed a clear odd-even effect when the length of the fluorocarbon chain was changed. This new concept of the surface-segregated monolayer provides a facile and versatile approach to modifying the surface of organic semiconductors and is applicable to various organic optoelectronic devices.
Journal of the American Chemical Society 11/2009; 131(48):17597-604. · 9.91 Impact Factor
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ABSTRACT: Ambipolar organic thin-film transistors with a bilayer structure of poly(3-hexylthiophene) and [6,6]phenyl C(61) butyric acid methyl ester were fabricated using a simple solution-based, contact-film-transfer method. The transistors exhibited balanced electron and hole mobilities of 2.1 x 10(-2) and 1.1 x 10(-2) cm(2) V(-1) s(-1), respectively. Complementary inverters based on two identical ambipolar transistors showed good performance with a gain of 14.
ACS Applied Materials & Interfaces 09/2009; 1(9):1865-8. · 4.53 Impact Factor
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10/2008;
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Advanced Materials 05/2008; 20(11):2211 - 2216. · 13.88 Impact Factor
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ABSTRACT: We have developed a new method to fabricate an array of TiO2 nanorod assemblies on a flat TiO2 surface. The TiO2 nanostructures with a height of approximately 40 nm were synthesized via a low temperature sol−gel reaction in a reversed micelle system. Cross-sectional transmission electron microscopy observation revealed that each nanostructure is an assembly of nanorods with a diameter and length of approximately 4 nm and approximately 40 nm, respectively. The fabricated array of TiO2 nanorods was applied to construct photovoltaic devices combined with a semiconducting polymer. When the nanostructured TiO2 substrate is used, photovoltaic performance reaches a power conversion efficiency of 0.39% and a fill factor of 0.47 under AM 1.5 illumination. Dependence of external quantum efficiency on polymer thickness showed the maximum efficiency in a thicker polymer film condition in the presence of the TiO2 nanorod assembly, indicating the nanostructure functions as an efficient exciton collector.
09/2006;
