Jeremy W. Ward

Effect of Acene Length on Electronic Properties in 5‐, 6‐, and 7‐Ringed Heteroacenes

Interest in organic semiconductors is motivated by their promise to offer a viable route to fabricating low-cost electronic devices on arbitrary substrates, and by the versatility of their chemical structures and physical properties, accomplished by means of molecular engineering. [ 1–3 ] Molecular modifi cations can yield soluble semiconductors that allow reduced complexity device fabrication using methods such as spin-coating, ink-jet printing, roll-to-roll processing and spray-deposition. [ 4–7 ]

Direct Structural Mapping of Organic Field‐Effect Transistors Reveals Bottlenecks to Carrier Transport

O N Organic fi eld-effect transistors (OFETs) continue to attract considerable attention due to the steady improvement of their performance over the past decade and their increasingly competitive position with respect to inorganic technologies, such as a-Si. The availability of solution-processable high-performance organic semiconductors makes this a particularly attractive technology for use in low-cost, fl exible and lightweight electronic devices. [ 1–4 ] However, it is often the case that the microstructure of organic semiconductors in devices is far from ideal and very challenging to understand and control when fi lms are deposited on top of a substrate patterned with electrodes. The organic fi lm can present heterogeneities across multiple length scales, this being a serious limitation for the performance and reproducibility of corresponding devices. In particular, it has been shown that the microstructure and morphology of the organic semiconductor, including its crystallinity, [ 5 , 6 ] polymorphism and texture, [ 7 , 8 ] grain and domain sizes, [ 9 , 10 ] grain boundary density and misorientation, [ 11 , 12 ] , as well as surface coverage and wettability of the substrate play pivotal roles in mediating device performance. [ 13 , 14 ] For instance, in the case of fl uorinated 5,11-bis(triethylsilylethynyl) anthradithiophene (diF-TES-ADT), treatment of the bottom Au contacts with pentafl uorobenzene thiol (PFBT) has been shown to dramatically improve carrier transport in bottom-contact OFETs. [ 15 , 16 ] The surface treatment prevents the < 111 > -textured crystallites exhibiting poor in-plane π -stacking from nucleating. It was shown that < 001 > grains grow on the treated electrodes and extend laterally several microns from their edges, thus preventing growth of the undesirable crystallite orientations in

Tailored interfaces for self-patterning organic thin-film transistors

Patterning organic thin-film transistors (OTFTs) is critical in achieving high electronic performance and low power consumption. We report on a high-yield, low-complexity patterning method based on exploiting the strong tendency of halogen-substituted organic semiconductors to crystallize along chemically tailored interfaces. We demonstrate that the organic semiconductor molecules self-align on the contacts, when the halogen–halogen interaction is allowed by the chemical structures and conformations of the self-assembled monolayer and organic semiconductor. The ordered films exhibit high mobilities and constrain the current paths. The regions surrounding the devices, where the interaction is inhibited, consist of randomly oriented molecules, exhibiting high-resistivity and electrically insulating neighboring devices. To identify the role of F–F interactions in the development of crystalline order, we investigate OTFTs fabricated on mono-fluorinated benzene thiol treated contacts, which allows us to isolate the interactions between the F originating from the organic semiconductor and the F in each position on the benzene ring of the thiol, and to selectively study the role of each interaction. Combining the results obtained from quantitative grazing incidence X-ray diffraction and Kelvin probe measurements, we show that the surface treatments induce structural changes in the films, but also alter the injection picture as a result of work function shifts that they introduce. We show that both effects yield variations in the field-effect transistor characteristics, and we are able to tune the field-effect mobility more than two orders of magnitude in the same material.

Versatile Organic Transistors by Solution Processing

A selection of the latest developments in organic electronic materials and organic field-effect transistor (OFET) devices is reviewed here with an emphasis on the synthetic and manufacturing versatility, ease of processing, and low cost offered by solution processability. At the heart of these benefits is the nature of the weak van der Waals intermolecular interactions inherent to organic compounds. This allows processability with a relatively small amount of energy investment. Material solubility, in particular, creates unique pathways for film fabrication and the design of new device architectures, while presenting new manufacturing challenges to explore. In this review we provide a chronological presentation of the important developments in the solution-deposited organic small-molecule semiconductor, dielectric, and electrode materials used in OFETs, making specific note of current benchmarks. Organic device architectures and fabrication methods that are characterized by reduced complexity and ease of implementation are discussed.

Quantitative analysis of the density of trap states at the semiconductor-dielectric interface in organic field-effect transistors

The electrical properties of organic field-effect transistors are governed by the quality of the constituting layers, and the resulting interfaces. We compare the properties of the same organic semiconductor film, 2,8-difluoro- 5,11-bis (triethylsilylethynyl) anthradithiophene, with bottom SiO2 dielectric and top Cytop dielectric and find a 10× increase in charge carrier mobility, from 0.17 ± 0.19 cm2 V−1 s−1 to 1.5 ± 0.70 cm2 V−1 s−1, when the polymer dielectric is used. This results from a significant reduction of the trap density of states in the semiconductor band-gap, and a decrease in the contact resistance.

Low-voltage polymer/small-molecule blend organic thin-film transistors and circuits fabricated via spray deposition

Organic thin-film electronics have long been considered an enticing candidate in achieving high-throughput manufacturing of low-power ubiquitous electronics. However, to achieve this goal, more work is required to reduce operating voltages and develop suitable mass-manufacture techniques. Here, we demonstrate low-voltage spray-cast organic thin-film transistors based on a semiconductor blend of 2,8-difluoro- 5,11-bis (triethylsilylethynyl) anthradithiophene and poly(triarylamine). Both semiconductor and dielectric films are deposited via successive spray deposition in ambient conditions (air with 40%–60% relative humidity) without any special precautions. Despite the simplicity of the deposition method, p-channel transistors with hole mobilities of >1 cm2/Vs are realized at −4 V operation, and unipolar inverters operating at −6 V are demonstrated.

Solution-Processed Organic and Halide Perovskite Transistors on Hydrophobic Surfaces

Solution-processable electronic devices are highly desirable due to their low cost and compatibility with flexible substrates. However, they are often challenging to fabricate due to the hydrophobic nature of the surfaces of the constituent layers. Here, we use a protein solution to modify the surface properties and to improve the wettability of the fluoropolymer dielectric Cytop. The engineered hydrophilic surface is successfully incorporated in bottom-gate solution-deposited organic field-effect transistors (OFETs) and hybrid organic-inorganic trihalide perovskite field-effect transistors (HTP-FETs) fabricated on flexible substrates. Our analysis of the density of trapping states at the semiconductor-dielectric interface suggests that the increase in the trap density as a result of the chemical treatment is minimal. As a result, the devices exhibit good charge carrier mobilities, near-zero threshold voltages, and low electrical hysteresis.

Low-temperature phase transitions in a soluble oligoacene and their effect on device performance and stability

The use of organic semiconductors in high-performance organic field-effect transistors requires a thorough understanding of the effects that processing conditions, thermal, and bias-stress history have on device operation. Here, we evaluate the temperature dependence of the electrical properties of transistors fabricated with 2,8-difluoro-5,11-bis(triethylsilylethynyl)anthradithiophene, a material that has attracted much attention recently due to its exceptional electrical properties. We have discovered a phase transition at T = 205 K and discuss its implications on device performance and stability. We examined the impact of this low-temperature phase transition on the thermodynamic, electrical, and structural properties of both single crystals and thin films of this material. Our results show that while the changes to the crystal structure are reversible, the induced thermal stress yields irreversible degradation of the devices.

Variation of the Side Chain Branch Position Leads to Vastly Improved Molecular Weight and OPV Performance in 4,8-dialkoxybenzo[1,2-b:4,5-b′]dithiophene/2,1,3-benzothiadiazole Copolymers

Through manipulation of the solubilizing side chains, we were able to dramatically improve the molecular weight (𝑀𝑤) of 4,8-dialkoxybenzo[1,2-b:4,5-b′]dithiophene (BDT)/2,1,3-benzothiadiazole (BT) copolymers. When dodecyl side chains (P1) are employed at the 4- and 8-positions of the BDT unit, we obtain a chloroform-soluble copolymer fraction with 𝑀𝑤 of 6.3 kg/mol. Surprisingly, by moving to the commonly employed 2-ethylhexyl branch (P2), 𝑀𝑤 decreases to 3.4 kg/mol. This is despite numerous reports that this side chain increases solubility and 𝑀𝑤. By moving the ethyl branch in one position relative to the polymer backbone (1-ethylhexyl, P3), 𝑀𝑤 is dramatically increased to 68.8 kg/mol. As a result of this 𝑀𝑤 increase, the shape of the absorption profile is dramatically altered, with 𝜆max = 637 nm compared with 598 nm for P1 and 579 nm for P2. The hole mobility as determined by thin film transistor (TFT) measurements is improved from ∼1×10−6 cm2/Vs for P1 and P2 to 7×10−4 cm2/Vs for P3, while solar cell power conversion efficiency in increased to 2.91% for P3 relative to 0.31% and 0.19% for P1 and P2, respectively.

Tuning the microstructure and electronic performance in organic thin-film transistors using chemical modifications at interfaces

A challenge with the study of organic thin-film transistors rests in understanding the mechanism behind microstructure formation and its effects on the charge carrier mobility. Often, increased order in the molecular packing of the organic semiconductor film results in a superior mobility. The microstructure of the organic thin-films can be tuned by chemically modifying the surfaces using treatments, such as self-assembled monolayers (SAMs). In 2,8-difluoro-5,11-bis(triethylsilylethynyl) (diF-TES ADT) devices with pentafluorobenzenethiol (PFBT) treated contacts, for example, high mobility regions are produced on the gold contacts, due to large grain growth, while low mobility regions are formed off the contacts, as a result of small grain growth. To explore the mechanism driving this effect, we selectively choose fluorinated contact treatments which are able to introduce targeted interactions at interface between organic semiconductor and SAM-treated contacts. This allows us to isolate key mechanisms behind microstructure formation. Through selecting a specific number of fluorine atoms and placing them into key locations on a benzene thiol base, we can tune the field-effect mobility from 10–3 cm2V-1s-1 to 0.1 cm2V-1s-1 in this organic semiconductor. We combine FET measurements with GIXD data to correlate thin-film microstructure with electronic properties. To investigate the mechanism responsible for these differences in device performance, we perform first-principle density functional theory calculations, which allow us to quantify the interactions at the interfaces between the organic semiconductor and the various contact treatments. We find that Fluorine-Fluorine interactions between the organic semiconductor and the Fluorinated SAM used for contact treatment induce this unique microstructure. Through this knowledge one can selectively choose processing parameters to attain desired film microstructure and improve device performance.