Solution-processed Organic Semiconductors Research Articles

A Novel Solution Processable Electron Acceptor, C60(CN)2, for Bulk Heterojunction Photovoltaic Applications

AbstractPhotovoltaic devices based on soluble conjugated polymers have gained great interest in recent years because of the potential low cost of production and ease of fabrication. PCBM ([6,6]-phenyl-C61-butyric acid methyl ester), a fullerene derivative, has been extensively investigated as a solution processable electron acceptor for bulk-heterojunction (BHJ) photovoltaic devices blended with conjugate polymers like P3HT [poly(3-hexylthiophene)]. Here, we investigated a novel solution processable organic semiconductor, C60(CN)2, as an electron acceptor for bulk heterojunction photovoltaic applications. Optical and electrical properties of C60(CN)2 are studied and compared with PCBM. Blend devices with P3HT and C60(CN)2 have been fabricated and compared with P3HT-PCBM devices. The effect of thermal annealing on the device performance is evaluated. Open circuit voltage, short circuit current, fill factor and total efficiency data are compared with PCBM based devices.

Solution Processed Self-Assembled Monolayer Gate Dielectrics for Low-Voltage Organic Transistors

AbstractLow-voltage organic transistors are sought for implementation in high volume low-power portable electronics of the future. Here we assess the suitability of three phosphonic acid based self-assembling molecules for use as ultra-thin gate dielectrics in low-voltage solution processable organic field-effect transistors. In particular, monolayers of phosphonohexadecanoic acid in metal-monolayer-metal type sandwich devices are shown to exhibit low leakage currents and high geometrical capacitance comparable to previously demonstrated self-assembled monolayer (SAM) type dielectrics but with a higher surface energy. The improved surface energy characteristics enable processing of a wider range of organic semiconductors from solution. Transistors based on a number of solution-processed organic semiconductors with operating voltages below 2 V are also demonstrated.

Parylene-C and High-k Polymer Bilayer Gate Dielectric for Low-Operating Voltage Organic Field-Effect Transistors

We propose a way to fabricate polymer insulators with a high gate capacitance for low-operating voltage organic field-effect transistors (OFETs). The insulator consists of spin-coated cyanoethylpullulan as a high-k polymer and chemical vapor deposited parylene-C as a covering layer. Parylene-C layer is insoluble in a common organic solvent, so the dielectric system can be fabricated on a layer of solution processable organic semiconductors such as poly(9,9-dioctylfluorene-co-bithiophene) (F8T2). The OFET shows a field-effect mobility of 3.4 × 10−3 cm2/Vs,a threshold voltage of − 1 V, and an on/off current ratio of 5.9 × 102. We successfully observed low-voltage operation in OFETs with this dielectric system.

Large-area patterning of a solution-processable organic semiconductor to reduce parasitic leakage and off currents in thin-film transistors

We describe two techniques for patterning spin-cast thin films of a solution-processable organic semiconductor, triethylsilylethylnyl anthradithiophene (TES ADT), to eliminate parasitic leakage currents and to lower off currents in thin-film transistors. One technique utilizes UV light in the presence of solvent vapors to simultaneously define the active channel and to crystallize TES ADT. The second technique selectively removes TES ADT from the nonchannel regions of the thin-film transistors through direct contact with a poly(dimethylsiloxane) stamp. Both patterning techniques yield thin-film transistors with high charge-carrier mobility (⩾0.1cm2∕Vs), low off currents (10−10–10−11A), and minimal parasitic leakage.

Fine-Patterned Organic Thin Film Transistors using Solution Organic Semiconductor Materials

To integrate solution-processible organic semiconductors in the array of thin-film transistors, fine-patterned organic thin-film transistors were fabricated by photolithography. Gold, which has a high work function, was used as a source–drain electrode to reduce the energy barrier between the organic semiconductor and the source–drain. Stacked double layers of gold and chromium were deposited and etched on a glass substrate to enhance the poor adhesion of gold on glass, and then a self-assembled monolayer was formed on them. Solutions of an organic semiconductor and a low-k organic gate insulator were spun by a spin coater and patterned by dry etching using a gate metal as a mask. The devices fabricated by this method show good electrical properties, average field effect mobilities of 0.2–0.3 cm2 V-1 s-1, and an on/off current ratio of over 105 in device dimensions of W/L=84/6 µm. The leakage current through the gate insulator between the gate and the source–drain was similar to that of SiNx in amorphous silicon thin-film transistor (a-Si TFT), which is lower than 10 pA. When the bias-stress stability of organic thin film transistors (OTFTs) was compared with that of a-Si TFT, the threshold voltage shift of OTFT was comparable to that of a-Si TFT, which is smaller than 1 V. However, the stability of OTFT should be further improved and discussed.

Correlation of molecular structure, packing motif and thin-film transistor characteristics of solution-processed n-type organic semiconductors based on dodecyl-substituted C 60 derivatives

We report the performance of solution-processed n-type organic thin-film transistors (OTFTs) based on four different types of dodecyl-substituted C 60 derivatives. Crystallinity and morphology of the spin-coated films highly depend on the compounds. C60-fused N-methylpyrrolidine- meta-C12 phenyl (C60MC12) exhibited the highest crystallinity and mobility of 0.09 cm 2/Vs among the compounds investigated. We found that not only chain-length but also chain-orientation play an important role for fabrication of highly ordered crystalline film, leading to high electron mobility in solution-processed n-type OTFTs.

Dual-gate organic thin-film transistors

A dual-gate organic thin-film transistor is realized using solution-processed organic semiconductor and insulator layers. Electrodes are made from gold. Compared to conventional single-gate transistors, this device type has a higher on current and steeper subthreshold slope. We show that the improved performance is the result of a nonconstant threshold voltage rather than formation of a second accumulation channel. Formation of a second accumulation channel does occur but the field-effect mobility associated with this channel is a factor 104 lower than the primary channel due to the relatively rough insulator-semiconductor interface.

Poly(3,3‘ ‘-dialkylterthiophene)s: Room-Temperature, Solution-Processed, High-Mobility Semiconductors for Organic Thin-Film Transistors

Most solution-processed organic semiconductors for organic thin film transistor applications require high-temperature annealing and/or fabrication in an inert atmosphere to achieve high mobility. Solution-processed poly(3,3‘ ‘-dialkylterthiophene) 3 was found to provide functionally useful transistor properties even when fabricated at room temperature under ambient conditions.

Alkylidene Fluorene Liquid Crystalline Semiconducting Polymers for Organic Field Effect Transistor Devices

Organic electronic devices comprising arrays of organic field effect transistors (OFETs) are expected to create a range of novel applications for which the ability to be fabricated in large areas, on flexible substrates, with nonconventional shapes, and at low cost are key enabling factors. To improve the electrical performance of such devices, new solution processable organic semiconductors are required with high charge carrier mobilities and environmental stability. This work describes the molecular design of a p-type charge transport liquid crystalline polymer, in an attempt to control the factors responsible for both mobility and stability. Molecules were designed that were able to exhibit closely packed, π stacked morphologies, which can result in efficient intermolecular charge hopping and hence high mobility. Molecular manipulation of the conjugated π electron system was required to optimize the HOMO energy level, to both resist oxidation and be able to readily accept holes from a source electrode.

High-performance, solution-processed organic thin film transistors from a novel pentacene precursor.

The Lewis acid-catalyzed Diels-Alder reaction of the organic semiconductor pentacene with N-sulfinylacetamide yields a soluble adduct. Spin-coated thin films of this adduct undergo solid-phase conversion to form thin films of pentacene at moderate temperatures. Organic thin film transistors fabricated by spin-coating this adduct, followed by thermal conversion to pentacene, exhibit the highest mobility reported to date for a solution-processed organic semiconductor.

Investigation of Using Contact and Non-contact Printing Technologies For Organic Transistor Fabrication

AbstractLow cost, high volume manufacturing processes are envisioned for solution processable organic semiconductor integrated circuits (IC) fabrication. The organic IC may be the low cost solution for driving electronic devices, i.e. smart cards, RFID tags, flexible displays, personal area networks, and body area networks. This study investigated the manufacturability of organic electronics (organic field effect transistors (OFETs), organic light emitting diodes (OLEDs), etc.) using commercially available printing technologies and materials systems qualified for use in microelectronic products. The evaluated contact printing technologies were pad printing and screen-printing; the non-contact printing technologies were ink jetting and micro dispensing. The material system selection for transistor structures and active layers was based on printing technology requirements and commercial availability. The materials were polymer thick film conductors and insulators, conductive nano-particle suspensions, and organic polymer systems. A series of material property characterization and printing process development studies were conducted. Several OFET designs were created and functional all printed organic transistors were demonstrated. The device electrical performance was characterized.

High-performance all-polymer integrated circuits

In this letter, we demonstrate the integration of all-polymer field-effect transistors in fully functional integrated circuits with operating frequencies of several kHz. One of the key items is an approach to incorporate low-Ohmic vertical interconnects compatible with an all-polymer approach. Inverters, NAND gates, and ring oscillators with transistor channel lengths down to 1 μm have been constructed. Inverters show voltage amplification at moderate biases and pentacene seven-stage ring oscillators show switching frequencies of a few kHz. The potential to realize large integrated circuits is demonstrated by a 15 bit code generator circuit using several hundreds of devices. The proposed concept was evaluated for three solution-processable organic semiconductors.

Organic field-effect transistors and all-polymer integrated circuits

Electrical properties of field-effect transistors made of different solution processable organic semiconductors are described. The temperature and gate-voltage dependence of the mobility is shown and theoretically described using a model based on the variable-range hopping of charge carriers in an exponential density of states. Furthermore, a technology has been developed to make all-polymer integrated circuits. It involves reproducible fabrication of field-effect transistors on flexible substrates, where the semiconducting, conducting and insulating parts are all made of polymers. Integrated circuits consisting of more than 300 field-effect transistors are demonstrated.

Field-effect transistors made from solution-processed organic semiconductors

We present results on metal-insulator-semiconductor field-effect transistors using conjugated organic semiconductors which can be processed from solution. The polymer poly(β′-dodecyloxy(-α,α′-α,α″-)terthienyl) is processed directly from solution whilst the polymer poly(thienylene vinylene) and the molecule pentacene are processed via soluble precursors. The operation mechanism of the transistors has been explained and expressions to extract mobility values and to interpret on/off ratios have been presented. The obtained transistor characteristics are explained in relation to the measured dopant concentrations, bulk conductivities and field-effect mobilities. All characteristics can be simulated using as input the transistor sheet conductivity as a function of the gate bias. Within a simple model the bulk conductivities and field-effect mobilities along with their temperature dependences are explained using variable-range hopping for heavily doped systems and polaronic thermally activated transport for lightly doped systems. This is a consequence of the density of states of conjugated systems which dynamically changes upon introduction of charge either by a field effect or through doping. The observed relationships for the various organic semiconductors are rationalized in a schematic mobility-conductivity plot. Transistors constructed from semiconductors processed from precursors display characteristics that allow for the construction of simple logic gates. Results from inverter, NOR and NAND gates and a simple ring oscillator circuit are shown. Switching frequencies of a few kHz have been achieved. The shelf-lives of devices are encouraging. Under stress operation, however, it is found that relaxation processes give rise to reversible current loss. This relaxation effect is intrinsic to the semiconductor. The origin may be related to the density of states for conjugated molecules being dynamic upon doping.