Xiong Gong

Nanowire crystals of a rigid rod conjugated polymer.

In this paper, we show that well-defined, highly crystalline nanowires of a rigid rod conjugated polymer, a poly(para-phenylene ethynylene)s derivative with thioacetate end groups (TA-PPE), can be obtained by self-assembling from a dilute solution. Structural analyses demonstrate the nanowires with an orthorhombic crystal unit cell wherein the lattice parameters are a approximately = 13.63 A, b approximately = 7.62 A, and c approximately = 5.12 A; in the nanowires the backbones of TA-PPE chains are parallel to the nanowire long axis with their side chains standing on the substrate. The transport properties of the nanowires examined by organic field-effect transistors (OFETs) suggest the highest charge carrier mobility approaches 0.1 cm(2)/(V s) with an average value at approximately 10(-2) cm(2)/(V s), which is 3-4 orders higher than that of thin film transistors made by the same polymer, indicating the high performance of the one-dimensional polymer nanowire crystals. These results are particular intriguing and valuable for both examining the intrinsic properties of PPEs polymer semiconductors and advancing their potential applications in electronic devices.

High-Detectivity Polymer Photodetectors with Spectral Response from 300 nm to 1450 nm

Polymer Photodetectors Optical sensing is used in a wide range of applications, such as low-light detection systems in cars and cameras. Most photodetectors have a limited spectral range and can only detect a narrow range of wavelengths. Gong et al. (p. 1665, published online 13 August) developed polymer photodetectors with extremely broad spectral response and exceptionally high sensitivity that can exceed the response of an inorganic semiconductor detector at liquid helium temperature. A key aspect in the device design is the inclusion of blocking layers to reduce significantly the dark current or noise in the devices. Well-designed polymer photodetectors show performance comparable with the best inorganic devices. Sensing from the ultraviolet-visible to the infrared is critical for a variety of industrial and scientific applications. Today, gallium nitride–, silicon-, and indium gallium arsenide–-based detectors are used for different sub-bands within the ultraviolet to near-infrared wavelength range. We demonstrate polymer photodetectors with broad spectral response (300 to 1450 nanometers) fabricated by using a small-band-gap semiconducting polymer blended with a fullerene derivative. Operating at room temperature, the polymer photodetectors exhibit detectivities greater than 1012 cm Hz1/2/W and a linear dynamic range over 100 decibels. The self-assembled nanomorphology and device architecture result in high photodetectivity over this wide spectral range and reduce the dark current (and noise) to values well below dark currents obtained in narrow-band photodetectors made with inorganic semiconductors.

Efficient, Air-Stable Bulk Heterojunction Polymer Solar Cells Using MoOx as the Anode Interfacial Layer

The use of molybdenum oxide as the anode interfacial layer in conventional bulk heterojunction polymer solar cells leads to an improved power conversion efficiency and also dramatically increases the device stability. This indicates that the engineering of improved anode interface materials is an important method by which to fabricate efficient and stable polymer solar cells.

Donor–Acceptor Conjugated Polymer Based on Naphtho[1,2-c:5,6-c]bis[1,2,5]thiadiazole for High-Performance Polymer Solar Cells

Donor-acceptor conjugated polymers PBDT-DTBT and PBDT-DTNT, based on 2,1,3-benzothiadiazole (BT) and naphtho[1,2-c:5,6-c]bis[1,2,5]thiadiazole (NT), have been designed and synthesized for polymer solar cells. NT contains two fused 1,2,5-thiadiazole rings that lower the band gap, enhance the interchain packing, and improve the charge mobility of the resulting polymer. Consequently, the NT-based polymer PBDT-DTNT exhibited considerably better photovoltaic performance with a power conversion efficiency (PCE) of 6.00% when compared with the BT-based polymer PBDT-DTBT, which gave a PCE of 2.11% under identical device configurations.

Inverted polymer solar cells with 8.4% efficiency by conjugated polyelectrolyte

Bulk heterojunction (BHJ) polymer solar cells (PSCs) that can be fabricated by solution processing techniques are under intense investigation in both academic institutions and industrial companies because of their potential to enable mass production of flexible and cost-effective alternative to silicon-based solar cells. A combination of novel polymer development, nanoscale morphology control and processing optimization has led to over 8% power conversion efficiencies (PCEs) for BHJ PSCs with a conventional device structure. Attempts to develop PSCs with an inverted device structure as required for achieving high PECs and good stability have, however, met with limited success. Here, we report that a high PCE of 8.4% under AM 1.5G irradiation was achieved for BHJ PSCs with an inverted device structure. This high efficiency was obtained through interfacial engineering of solution-processed electron extraction layer, leading to facilitate electron transport and suppress bimolecular recombination. These results provided an important progress for solution-processed PSCs, and demonstrated that PSCs with an inverted device structure are comparable with PSCs with the conventional device structure.

Colorimetric detection of DNA, small molecules, proteins, and ions using unmodified gold nanoparticles and conjugated polyelectrolytes

We have demonstrated a novel sensing strategy employing single-stranded probe DNA, unmodified gold nanoparticles, and a positively charged, water-soluble conjugated polyelectrolyte to detect a broad range of targets including nucleic acid (DNA) sequences, proteins, small molecules, and inorganic ions. This nearly “universal” biosensor approach is based on the observation that, while the conjugated polyelectrolyte specifically inhibits the ability of single-stranded DNA to prevent the aggregation of gold-nanoparticles, no such inhibition is observed with double-stranded or otherwise “folded” DNA structures. Colorimetric assays employing this mechanism for the detection of hybridization are sensitive and convenient—picomolar concentrations of target DNA are readily detected with the naked eye, and the sensor works even when challenged with complex sample matrices such as blood serum. Likewise, by employing the binding-induced folding or association of aptamers we have generalized the approach to the specific and convenient detection of proteins, small molecules, and inorganic ions. Finally, this new biosensor approach is quite straightforward and can be completed in minutes without significant equipment or training overhead.

Strain and Hückel Aromaticity: Driving Forces for a Promising New Generation of Electron Acceptors in Organic Electronics

The tangible possibility of fabricating flexible, lightweight organic photovoltaic devices (OPVs) by using roll-to-roll coaters, similar to those used in the production of print magazines and newspapers, renders this technology a valid alternative to expensive crystalline silicon photovoltaic cells. The most widely used active layer for these OPVs, the so-called bulk heterojunction (BHJ), 5] is based on photoinduced charge transfer from an electron-donating material, such as a light-absorbing and hole-conducting polymer, to an electron-accepting component, typically fullerene[60] and its derivative 1-(3-methoxycarbonyl) propyl-1phenyl-[6,6]-C61 ([C60]PCBM). [6, 7] Several research groups have reported a wide range of new polymeric donor structures that absorb light over a broad wavelength range, and have a narrow energy gap and increased charge transport and collection at the electrode. However, there have been fewer reports on new structures of acceptor components that do not contain fullerene derivatives. C60 and C70 PCBMs are currently considered the most successful acceptor architectures, despite only slight improvements when modifying these functionalized fullerenes. For example, the insertion of electron-donating groups on the phenyl ring of the [C60]PCBM to tune the lowest unoccupied molecular orbital (LUMO) energy levels improved the open-circuit voltage (Voc), while maintaining a relatively unchanged efficiency. Furthermore, [C70]PCBM, which absorbs a wider range of wavelengths than [C60]PCBM, [16] was employed with low-band-gap polymers such as poly[2,6-(4,4-bis-(2-ethylhexyl-4H-cyclopenta[2,1b;3,4b’]-dithiophene)-alt-4,7-(2,1,3-benzothiadizole)] (PCPDTBT), to broaden the photocurrent spectral range. Although encouraging photocurrent and photovoltage values were obtained, a low overall efficiency, which arises from loss mechanisms, was observed. Despite the wide use of these fullerene derivatives, the synthesis of new acceptors with energy levels significantly different from those of current C60 derivatives, and wide versatility in terms of derivatization and functionalization is urgently required. Herein, we report the inherent potential of a new generation of acceptor compounds based on the 9,9’-bifluorenylidene (99’BF) backbone. 99’BF could be considered a tetrabenzofulvalene with an atom numbering that reflects fluorene linked by a double bond between the 9 and 9’ carbon atoms. In the ground state, 99’BF is forced to be coplanar because of the presence of the double bond, but the repulsive interaction between the H1– H1’ and H8–H8’ protons twists the structure of the dimer. The addition of one electron across the C9–C9’ bond is highly favorable for two main reasons: steric (“twist”) strain relief and gain in aromaticity to a 14-p-electron system (Scheme 1).

Biosensors from conjugated polyelectrolyte complexes

A charge neutral complex (CNC) was formed in aqueous solution by combining an orange light emitting anionic conjugated polyelectrolyte and a saturated cationic polyelectrolyte at a 1:1 ratio (per repeat unit). Photoluminescence (PL) from the CNC can be quenched by both the negatively charged dinitrophenol (DNP) derivative, (DNP-BS−), and positively charged methyl viologen (MV2+). Use of the CNC minimizes nonspecific interactions (which modify the PL) between conjugated polyelectrolytes and biopolymers. Quenching of the PL from the CNC by the DNP derivative and specific unquenching on addition of anti-DNP antibody (anti-DNP IgG) were observed. Thus, biosensing of the anti-DNP IgG was demonstrated.

Bulk Heterojunction Solar Cells with Large Open-Circuit Voltage: Electron Transfer with Small Donor-Acceptor Energy Offset

e 99BF 99BF(.) Power conversion effi ciencies (PCEs) (in response to solar AM1.5 radiation) as high as 6–8% have been reported for bulk heterojunction (BHJ) polymer solar cells. [ 1 , 2 ] In order to attain PCEs over 10%, BHJ materials capable of generating larger open circuit voltage (V oc ) are required. [ 3 , 4 ] One approach to increase V oc is to develop low-bandgap semiconducting polymers with deeper highest occupied molecular orbital (HOMO) energies. An alternative approach is to develop new electron acceptors with higher lowest unoccupied molecular orbital (LUMO) energies. The pathway to low-bandgap semiconducting polymers with deeper HOMOs is well understood, and BHJ solar cells fabricated by semiconducting polymers with deeper HOMOs have successfully exhibited larger V oc . [ 5 ] However, the discovery of new (non-fullerene) electron acceptors with higher LUMOs remains undeveloped. [ 6 ]

High and Balanced Hole and Electron Mobilities from Ambipolar Thin-Film Transistors Based on Nitrogen-Containing Oligoacences

We demonstrate a strategy for designing high-performance, ambipolar, acene-based field-effect transistor (FET) materials, which is based on the replacement of C-H moieties by nitrogen atoms in oligoacenes. By using this strategy, two organic semiconductors, 6,13-bis(triisopropylsilylethynyl)anthradipyridine (1) and 8,9,10,11-tetrafluoro-6,13-bis(triisopropylsilylethynyl)-1-azapentacene (3), were synthesized and their FET characteristics studied. Both materials exhibit high and balanced hole and electron mobilities, 1 having μ(h) and μ(e) of 0.11 and 0.15 cm(2)/V·s and 3 having μ(h) and μ(e) of 0.08 and 0.09 cm(2)/V·s, respectively. The successful demonstration of high and balanced ambipolar FET properties from nitrogen-containing oligoacenes opens up new opportunities for designing high-performance ambipolar organic semiconductors.