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ABSTRACT: Device performance and photoinduced charge transfer are studied in donor/acceptor blends of the oxidation-resistant conjugated polymer poly[(4,8-bis(2-hexyldecyl)oxy)benzo[1,2-b:4,5-b′]dithiophene)-2,6-diyl-alt-(2,5-bis(3-dodecylthiophen-2-yl)benzo[1,2-d;4,5-d′]bisthiazole)] (PBTHDDT) with the following fullerene acceptors: [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM); [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM); and the indene-C60 bis-adduct IC60BA). Power conversion efficiency improves from 1.52% in IC60BA-based solar cells to 3.75% in PC71BM-based devices. Photoinduced absorption (PIA) of the PBTHDDT:fullerene blends suggests that exciting the donor polymer leads to long-lived positive polarons on the polymer and negative polarons on the fullerene in all three polymer fullerene blends. Selective excitation of the fullerene in PC71BM or PC61BM blends also generates long-lived polarons. In contrast, no discernible PIA features are observed when selectively exciting the fullerene in a PBTHDDT/IC60BA blend. A relatively small driving force of ca. 70 meV appears to sustain charge separation via photoinduced hole transfer from photoexcited PC61BM to the polymer. The decreased driving force for photoinduced hole transfer in the IC60BA blend effectively turns off hole transfer from IC60BA excitons to the host polymer, even while electron transfer from the polymer to the IC60BA remains active. Suppressed hole transfer from fullerene excitons is a potentially important consideration for materials design and device engineering of organic solar cells.
Advanced Functional Materials 10/2012; · 10.18 Impact Factor
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ABSTRACT: We report the solution-phase self-assembly of nanowires of a series of six oligothiophene-functionalized naphthalene diimides (NDI-nTH and NDI-nT) and their use as electron acceptors in all-nanowire bulk heterojunction organic solar cells. The dimensions and detailed morphology of the nanowires varied greatly with molecular structure of the NDI molecules and the solution concentration and solvent composition. Nanowires of NDI-3TH have typical dimensions of 80–250 nm in width, 2–10 μm in length, and 14.2–15.3 nm in thickness. Significant red-shift and enhancement of optical absorption in the low energy region (>600 nm) were observed in the nanowire suspension compared with solution and thin film, suggesting an increased supramolecular order in the nanowires. All-nanowire bulk heterojunction solar cells fabricated using NDI-3TH nanowires as electron acceptor and poly(3-hexylthiophene) nanowires as donor had a power conversion efficiency of 1.15%. External quantum efficiency of the photovoltaic cells showed that the n-type organic semiconductor nanowires contributed significantly to light harvesting. Our results represent the first demonstration of all-nanowire organic solar cells and show that non-fullerene n-type organic semiconductor nanowires can be effectively incorporated into bulk heterojunction solar cells.
Journal of Materials Chemistry 07/2012; 22:24373-24379. · 5.97 Impact Factor
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ABSTRACT: The performance of bulk heterojunction solar cells made from blends of a non-fullerene acceptor, N,N′-bis(2-ethylhexyl)-2,6-bis(5″-hexyl-[2,2′;5′,2″]terthiophen-5yl)-1,4,5,8-naphthalene diimide (NDI-3TH), and poly(3-hexylthiophene) (P3HT) donor is enhanced 10-fold by using a processing additive in conjunction with an electron-blocking and a hole-blocking buffer layers. The power conversion efficiency of P3HT:NDI-3TH solar cells improves from 0.14% to 1.5% by using a processing additive (1,8-diiodooctane) at an optimum concentration of 0.2 vol%, which is far below the 2-3 vol% optimum concentrations found in polymer/fullerene systems. TEM and AFM imaging show that the size and connectivity of the NDI-3TH domains in the phase-separated P3HT:NDI-3TH blends vary strongly with the concentration of the processing additive. These results demonstrate, for the first time, that processing additives can be effective in the optimization of the morphology and performance of bulk heterojunction polymer solar cells based on non-fullerene acceptors.
Advanced Energy Materials. 10/2011; 1(5):946–953.
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ABSTRACT: Six new donor–acceptor copolymers based on benzobisthiazole and various donor moieties (dithienosilole, dithienopyrrole, cyclopentadithiophene, carbazole, benzodithiophene, and bithiophene) were synthesized, characterized, and used in field-effect transistors and solar cells. The series of polybenzobisthiazoles with donor–acceptor architecture have optical band gaps of 1.83–2.18 eV, have identical LUMO energy levels (−3.3 eV), and have a HOMO energy level that varied from −4.79 eV in PBTDTP to −5.71 eV in PBTHDDT. X-ray diffraction of the polybenzobisthiazole films showed a lamellar crystalline structure with an interlayer d-spacing of 1.56 nm in PBTOT to 1.83 nm in PBTDTP and 2.12 nm in PBTHDDT and a short π-stacking distance (0.353–0.378 nm). The highly crystalline nature of the polybenzobisthiazoles facilitated high field-effect carrier mobility (up to 0.011 cm2/(V s)), which remained very stable under ambient conditions for 2 years. Bulk heterojunction solar cells made from one of the benzobisthiazole-based copolymers gave a power conversion efficiency of up to 3.0% under 100 mW/cm2 AM1.5 sunlight illumination in air.
08/2011;
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Advanced Materials 11/2010; 22(42):4744-8. · 13.88 Impact Factor
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ABSTRACT: We demonstrate the use of n/p polymer/polymer heterojunctions deposited by sequential solution processing to fabricate ambipolar field-effect transistors and complementary logic circuits. Electron and hole mobilities in the transistors were ∼0.001-0.01 cm(2)/(V s) in air without encapsulation. Complementary circuits integrating multiple ambipolar transistors into NOT, NAND, and NOR gates were fabricated and shown to exhibit sharp signal switching with a high voltage gain.
ACS Applied Materials & Interfaces 10/2010; 2(11):2974-7. · 4.53 Impact Factor
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ABSTRACT: A novel π-conjugated heptacyclic framework has been synthesized via a new one-step cyclization that results in a new class of n-type organic semiconductors. Single-crystal structures of bisindenoanthrazolines DADF and DADK showed that the π-conjugated heptacyclic framework is planar and leads to a slipped face-to-face π-stacking with short intramolecular distances (3.39 Å and 3.56 Å, respectively). The series of bisindenoanthrazolines have a formal reduction potential of −0.68 to −0.70 V (vs SCE) and an estimated electron affinity (LUMO level) of 3.65−3.72 eV. Electron mobility in evaporated thin films of the bisindenoanthrazolines, measured by the space-charge limited current method, was as high as 3.84 × 10−4 cm2/(V s) under ambient air conditions. Organic light-emitting diodes based on DADA as the emissive material gave the best performance among the four molecules with a maximum brightness of 7610 cd/m2, and maximum efficiency of 6.6 cd/A with EQE of 2.0% at a brightness of 936 cd/m2. Phosphorescent organic light emitting diodes with fac-tris(2-phenylpyridine)iridium (Ir(ppy)3) as the green triplet emitter and a bisindenoanthrazoline as the electron transport layer showed a brightness of 62 000 cd/m2 and luminous efficiency of 39.2 cd/A at a brightness of 4270 cd/m2. Nanowires of DADF and DADK self-assembled from solution were found to be single-crystalline and their morphology was further investigated by electron microscopy techniques. These results demonstrate the potential of bisindenoanthrazolines as new n-type semiconductors for organic electronics and optoelectronics.
09/2010;
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Macromolecules 10/2009; 42:8615-8618. · 5.17 Impact Factor
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ABSTRACT: Spin-coated polymer-based phosphorescent organic light-emitting diodes (PhOLEDs) were found to be substantially brighter and more efficient when 4,9-diphenylbisindenoanthorazoline (DADA) was used as an electron-transport material compared to widely used Alq3. Devices with fac-tris(2-phenylpyridine)iridium (Ir(ppy)3) dispersed in poly(N-vinylcarbazole) as the emissive layer and DADA as the electron-transport layer showed a maximum brightness of 73 600 cd/m2 and a maximum luminous efficiency of 48.1 cd/A at a brightness of 5640 cd/m2. The high electron affinity (3.67 eV) and electron mobility (3.1 × 10−5 cm2/V·s) of DADA explain its effectiveness as an electron-transport material in PhOLEDs. These results demonstrate the use of an electron-transport material having a high electron affinity as a promising strategy for improving the charge injection and overall performance of PhOLEDs.
10/2009;
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Journal of the American Chemical Society 02/2008; 130(4):1118-9. · 9.91 Impact Factor
