Byung Jun Jung

LADDER-TYPE OLIGO-P-PHENYLENE-CONTAINING COPOLYMERS WITH HIGH OPEN-CIRCUIT VOLTAGES AND AMBIENT PHOTOVOLTAIC ACTIVITY

Four ladder-type oligo-p-phenylene containing donor-acceptor copolymers were designed, synthesized, and characterized. The ladder-type oligo-p-phenylene was used as an electron donor unit in these copolymers to provide a deeper highest occupied molecular orbital (HOMO) level for obtaining polymer solar cells with a higher open-circuit voltage, while 4,7-dithien-2-yl-2,1,3-benzothiadiazole or 5,8-dithien-2-yl-2,3-diphenylquinoxaline was chosen as an electron acceptor unit to tune the electronic band gaps of the polymers for a better light harvesting ability. These copolymers exhibit field-effect mobilities as high as 0.011 cm(2)/(V s). Compared to fluorene containing copolymers with the same acceptor unit, these ladder-type oligo-p-phenylene containing copolymers have enhanced and bathochromically shifted absorption bands and much better solubility in organic solvents. Photovoltaic applications of these polymers as light-harvesting and hole-conducting materials are investigated in conjunction with [6,6]-phenyl-C61-butyric acid methyl ester (PC(61)BM) or [6,6]-phenyl-C71-butyric acid methyl ester (PC(71)BM). Without extensive optimization work, a power conversion efficiency (PCE) of 3.7% and a high open-circuit voltage of 1.06 V are obtained under simulated solar light AM 1.5 G (100 mW/cm(2)) from a solar cell with an active layer containing 20 wt % ladder-type tetra-p-phenylene containing copolymer (P3FTBT6) and 80 wt % PC(61)BM. Moreover, a high PCE of 4.5% was also achieved from a solar cell with an active layer containing 20 wt % P3FTBT6 and 80 wt % PC(71)BM.

Synthesis, Structural Characterization, and Unusual Field-Effect Behavior of Organic Transistor Semiconductor Oligomers: Inferiority of Oxadiazole Compared with Other Electron-Withdrawing Subunits

A new series of heterocyclic oligomers based on the 1,3,4-oxadiazole ring were synthesized. Other electron-deficient cores (fluorenone and fumaronitrile) were introduced to investigate the oligomers as n-channel materials. The physical properties, thin film morphologies, and field-effect transistor characteristics of the oligomers were evaluated. Thin films were deposited at different substrate temperatures and on variously coated Si/SiO(2) for device optimization. Contrary to our expectations, the thin film devices of 4 revealed p-channel behavior, and the average hole mobility was 0.14 cm(2) V(-1) s(-1) (maximum value 0.18 cm(2) V(-1) s(-1)). Compound 11 is the first example of an oxadiazole-containing organic semiconductor (OSC) oligomer in an n-channel organic field-effect transistor (OFET) and shows moderate mobilities. Non-oxadiazole-containing oligomers 9 and 12 showed n-channel OFET behavior on hexamethyldisilazane-treated and Cytop spin-coated SiO(2) in vacuum. These are the first fluorenone- and fumaronitrile-based n-OSCs demonstrated in transistors. However, oxadiazole-core materials 14 and 16 were inactive in transistor devices.

Field-effect-tuned lateral organic diodes

The operation of organic diodes in solar cells and light-emitting displays strongly depends on the properties of the interfaces between hole- and electron-carrying organic semiconductors. Such interfaces are difficult to characterize, as they are usually buried under the surface or exist as an irregular “bulk heterojunction.” Using a unique fluorinated barrier layer-based lithographic technique, we fabricated a lateral organic p-n junction, allowing the first observation of the potential at an organic p-n interface simultaneously with the charge transport measurements. We find that the diode characteristics of the device (current output and rectification ratio) are consistent with the changes in the surface potentials near the junction, and the current-voltage curves and junction potentials are strongly and self-consistently modulated by a third, gate electrode. The generality of our technique makes this an attractive method to investigate the physics of organic semiconductor junctions. The lithographic technique is applicable to a wide variety of soft material patterns. The observation of built-in potentials makes an important connection between organic junctions and textbook descriptions of inorganic devices. Finally, these kinds of potentials may prove to be controlling factors in charge separation efficiency in organic photovoltaics.

Response diversity and dual response mechanism of organic field-effect transistors with dinitrotoluene vapor

We employ an assortment of n- and p-channel organic semiconductors to fabricate organic field-effect transistors (OFETs) that respond in different ways to dinitrotoluene (DNT) vapor. Different responses are obtained from devices with various semiconductor materials and receptor functionality. Differences are noted in the magnitude and direction of current changes, and also the device parameters (mobility and threshold voltage) affected. The pattern of responses includes sufficient diversity to set it apart from other vapors that might give similar responses to some OFETs in the set, but not all of them. This gives rise to the possibility of distinguishing DNT from other vapors in a manner that could be further developed using mechanistic principles elucidated in this work.

Tunability of Mobility and Conductivity over Large Ranges in Poly(3,3′′′-didodecylquaterthiophene)/Insulating Polymer Composites

Semiconducting polymers are currently being considered as active layers in field-effect transistors, in which high charge carrier mobility and low off conductivity are important. For other applications, such as certain spintronic mechanisms, the opposite characteristics are desirable. Blending such polymers with insulating polymers would be expected to lower the mobility. In this paper, we report that the use of hydrocarbon polymers such as polystyrene as insulators generally raises the mobility when the semiconducting polymer is poly(bisdodecylquaterthiophene). A high mobility value of nearly 0.1 cm(2)/V.s was obtained for an optimal blend. While this is counterintuitive, it is consistent with a few other recent reports. In order to lower the mobility significantly, a much more polar and irregular blending agent is needed. The further addition of tetrafluorotetracyanoquinodimethane as a dopant gave a rare low mobility/high conductivity combination of properties, with a charge carrier density on the order of 10(19) cm(-3). Thus, mobility and conductivity were tuned somewhat independently over 3 and 4 orders of magnitude, respectively.

Overcoming tradeoff between mobility and bias stability in organic field-effect transistors according to the self-assembled monolayer chain lengths

This study examined the relationship between the mobility and bias stability of pentacene-based organic field-effect transistors (OFETs) regarding a self-assembled monolayer (SAM) treatment. For this systematic study, four types of silazane-based SAMs with different alkyl chain lengths in the range of 1–8 were used. Silazane-based SAMs have an advantage of processability due to the mild reaction conditions. The mobility was increased from 0.29 without SAM to 0.46, 0.61, 0.65, and 0.84 cm2/V s after the SAM-treatment with an alkyl chain length of 1, 3, 4, and 8, respectively. On the other hand, inverse proportional relationship was observed between the bias stability and SAM alkyl chain length. Under high gate bias stress (equivalent to electric field of 3 MV/cm) for 2 h, the threshold voltage shift of the OFET was decreased from 12.19 V without SAM to 5.69 V with a short SAM-treatment (alkyl chain length of 1) and 7.14 V with a long SAM-treatment (alkyl chain length of 8). This is the significant finding ...

Naphthalenetetracarboxylic Diimide Layer-Based Transistors with Nanometer Oxide and Side Chain Dielectrics Operating below One Volt

We designed a new naphthalenetetracarboxylic diimide (NTCDI) semiconductor molecule with long fluoroalkylbenzyl side chains. The side chains, 1.2 nm long, not only aid in self-assembly and kinetically stabilize injected electrons but also act as part of the gate dielectric in field-effect transistors. On Si substrates coated only with the 2 nm thick native oxide, NTCDI semiconductor films were deposited with thicknesses from 17 to 120 nm. Top contact Au electrodes were deposited as sources and drains. The devices showed good transistor characteristics in air with 0.1-1 μA of drain current at 0.5 V of V(G) and V(DS) and W/L of 10-20, even though channel width (250 μm) is over 1000 times the distance (20 nm) between gate and drain electrodes. The extracted capacitance-times-mobility product, an expression of the sheet transconductance, can exceed 100 nS V(-1), 2 orders of magnitude higher than typical organic transistors. The vertical low-frequency capacitance with gate voltage applied in the accumulation regime reached as high as 650 nF/cm(2), matching the harmonic sum of capacitances of the native oxide and one side chain and indicating that some gate-induced carriers in such devices are distributed among all of the NTCDI core layers, although the preponderance of the carriers are still near the gate electrode. Besides demonstrating and analyzing thickness-dependent NTCDI-based transistor behavior, we also showed <1 V detection of dinitrotoluene vapor by such transistors.

Improved morphology and performance from surface treatments of naphthalenetetracarboxylic diimide bottom contact field-effect transistors.

We report bottom contact organic field-effect transistors (OFETs) with various surface treatments based on n-channel materials, specifically, 1,4,5,8-naphthalene-teracarboxylic diimides (NTCDIs) with three different fluorinated N-substituents, systematically studied with a particular emphasis on the interplay between the morphology of the organic semiconductor films and the electrical device properties. The morphological origins of the improvements were directly and dramatically visualized at the semiconductor-contact interface. As a result of a series of treatments, a large range of performances of bottom contact side-chain-fluorinated NTCDI OFETs (mobility from 1 x 10(-6) to 8 x 10(-2) cm(2)/(V s), on/off ratio from 1 x 10(2) to 1 x 10(5)) were obtained. The surface treatments enabled systems that had shown essentially no OFET activity without electrode modification activity to perform nearly as well as top contact devices made from the same materials. In addition, for the fresh bottom contact NTCDI device, the effect of gate bias stress on the tens-of-minutes time scale, during which the threshold voltage (V(t)) shifted and relaxed with similar time constants, was observed.

Simultaneous improvement of performance and stability in PEDOT:PSS–sorbitol composite based flexible thermoelectric modules by novel design and fabrication process

A flexible thermoelectric (TE) module was fabricated by merging two distinct highly flexible polyethersulfone (PES) substrates having either poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS)-D-sorbitol composite as an active layer or thin gold electrodes for interconnection, respectively. Two substrates were well glued together in a general lamination method owing to the sticky nature of sorbitol, without any other adhesives. In addition, the PEDOT:PSS film was thermally annealed and encapsulated simultaneously by the merging process, which can reduce the TE module fabrication steps. The merged module consisting of four uni-legs exhibited a Seebeck voltage of 40.7 µV/K, which were higher than the values of the TE modules prepared using a well-known additives such as dimethyl sulfoxide (DMSO) or etheylene glycol (EG) (∼30–40 µV/K).

RCA-Based Biosensor for Electrical and Colorimetric Detection of Pathogen DNA

For the diagnosis and prevention of diseases, a range of strategies for the detection of pathogens have been developed. In this study, we synthesized the rolling circle amplification (RCA)-based biosensor that enables detection of pathogen DNA in two analytical modes. Only in the presence of the target DNA, the template DNA can be continuously polymerized by simply carrying out RCA, which gives rise to a change of surface structure of Au electrodes and the gap between the electrodes. Electrical signal was generated after introducing hydrogen tetrachloroaurate (HAuCl4) to the DNA-coated biosensor for the improvement of the conductivity of DNA, which indicates that the presence of the pathogen DNA can be detected in an electrical approach. Furthermore, the existence of the target DNA was readily detected by the naked eyes through change in colors of the electrodes from bright yellow to orange-red after RCA reaction. The RCA-based biosensor offers a new platform for monitoring of pathogenic DNA with two different detection modes in one system.