Three new fused thiophene semiconductors, end-capped with diperfluorophenylthien-2-yl (DFPT) groups (DFPT-thieno[2′,3′:4,5]thieno[3,2-b]thieno[2,3-d]thiophene (TTA), DFPT-dithieno[2,3-b:3′,2′-d]thiophenes (DTT), and DFPT-thieno[3,2-b]thiophene (TT)), are synthesized and characterized in organic thin film transistors. Good environmental stability of the newly developed materials is demonstrated via thermal analysis as well as degradation tests under white light. The molecular structures of all three perfluorophenylthien-2-yl end-functionalized derivatives are determined by single crystal X-ray diffraction. DFPT-TTA and DFPT-TT exhibit good n-type TFT performance, with mobilities up to 0.43 and 0.33 cm² V⁻¹ s⁻¹, respectively. These are among the best performing n-type materials of all fused thiophenes reported to date. The best thin film transistor device performance is achieved via an n-octadecyltrichlorosilane dielectric surface treatment on the thermally grown Si/SiO₂ substrates prior to vapor-phase semiconductor deposition. Within the DFPT series, carrier mobility magnitudes depend strongly on the semiconductor growth conditions and the gate dielectric surface treatment.

Solution-processed polymer-based logic circuits are typically associated with high operating voltage and slow switching speeds. Here, polymer field-effect transistors (PFETs) fabricated on hybrid self-assembled nanodielectric (SAND) structures are reported, the latter consisting of alternating organic–inorganic layers exhibiting low leakage current (≈10⁻⁹ A cm⁻²) and high capacitance (≈0.8 μF cm⁻²). Suitable device engineering, controllable dielectric parameters, and interface energetics enable PFET operation at ±1 V, field-effect mobility (μ _FET) > 2.0 cm² V⁻¹ s⁻¹, subthreshold swing ≈100 mV dec⁻¹, and switching response ≈150 ns. These performance parameters are orders of magnitude higher than similar devices fabricated from other polymer dielectrics. Inverter and NAND logic circuits fabricated from these SAND-based PFETs possess voltage gain up to 38 and maximum-frequency bandwidth of 2 MHz. A systematic study comparing different classes of dielectric and semiconducting material attributes the enhanced performance to improved relaxation dynamics of the SAND layer and tunable chemically functionalized interfaces.

The nature of charge transport and local structure are investigated in amorphous indium oxide-based thin films fabricated by spin-coating. The In–X–O series where X = Sc, Y, or La is investigated to understand the effects of varying both the X cation ionic radius (0.89–1.17 Å) and the film processing temperature (250–300 °C). Larger cations in particular are found to be very effective amorphosizers and enable the study of high mobility (up to 9.7 cm² V⁻¹ s⁻¹) amorphous oxide semiconductors without complex processing. Electron mobilities as a function of temperature and gate voltage are measured in thin-film transistors, while X-ray absorption spectroscopy and ab initio molecular dynamics simulations are used to probe local atomic structure. It is found that trap-limited conduction and percolation-type conduction mechanisms convincingly model transport for low- and high-temperature processed films, respectively. Increased cation size leads to increased broadening of the tail states (10–23 meV) and increased percolation barrier heights (24–55 meV) in the two cases. For the first time in the amorphous In–X–O system, such effects can be explained by local structural changes in the films, including decreased In–O and In–M (M = In, X) coordination numbers, increased bond length disorder, and changes in the MO _x polyhedra interconnectivity.

π-conjugated polymers based on the electron-neutral alkoxy-functionalized thienyl-vinylene (TVTOEt) building-block co-polymerized, with either BDT (benzodithiophene) or T2 (dithiophene) donor blocks, or NDI (naphthalenediimide) as an acceptor block, are synthesized and characterized. The effect of BDT and NDI substituents (alkyl vs alkoxy or linear vs branched) on the polymer performance in organic thin film transistors (OTFTs) and all-polymer organic photovoltaic (OPV) cells is reported. Co-monomer selection and backbone functionalization substantially modifies the polymer MO energies, thin film morphology, and charge transport properties, as indicated by electrochemistry, optical spectroscopy, X-ray diffraction, AFM, DFT calculations, and TFT response. When polymer P7 is used as an OPV acceptor with PTB7 as a donor, the corresponding blend yields TFTs with ambipolar mobilities of μ_e = 5.1 × 10⁻³ cm² V^–1 s^–1 and μ_h = 3.9 × 10⁻³ cm² V^–1 s^–1 in ambient, among the highest mobilities reported to date for all-polymer bulk heterojunction TFTs, and all-polymer solar cells with a power conversion efficiency (PCE) of 1.70%, the highest reported PCE to date for an NDI-polymer acceptor system. The stable transport characteristics in ambient and promising solar cell performance make NDI-type materials promising acceptors for all-polymer solar cell applications.

The molecular packing motifs within crystalline domains should be a key determinant of charge transport in thin-film transistors (TFTs) based on small organic molecules. Despite this implied importance, detailed information about molecular organization in polycrystalline thin films is not available for the vast majority of molecular organic semiconductors. Considering the potential of fused thiophenes as environmentally stable, high-performance semiconductors, it is therefore of interest to investigate their thin film microstructures in relation to the single crystal molecular packing and OTFT performance. Here, the molecular packing motifs of several new benzo[d,d′]thieno[3,2-b;4,5-b′]dithiophene (BTDT) derivatives are studied both in bulk 3D crystals and as thin films by single crystal diffraction and grazing incidence wide angle X-ray scattering (GIWAXS), respectively. The results show that the BTDT derivative thin films can have significantly different molecular packing from their bulk crystals. For phenylbenzo[d,d′]thieno[3,2-b;4,5-b′]dithiophene (P-BTDT), 2-biphenylbenzo[d,d′]thieno-[3,2-b;4,5-b′]dithiophene (Bp-BTDT), 2-naphthalenylbenzo[d,d′]thieno[3,2-b;4,5-b′]dithiophene (Np-BTDT), and bisbenzo[d,d′]thieno[3,2-b;4,5-b′]dithiophene (BBTDT), two lattices co-exist, and are significantly strained versus their single crystal forms. For P-BTDT, the dominance of the more strained lattice relative to the bulk-like lattice likely explains the high carrier mobility. In contrast, poor crystallinity and surface coverage at the dielectric/substrate interface explains the marginal OTFT performance of seemingly similar PF-BTDT films.

Facile one-pot [1 + 1 + 2] and [2 + 1 + 1] syntheses of thieno[3,2-b]thieno[2′,3′:4,5]thieno[2,3-d]thiophene (tetrathienoacene; TTA) semiconductors are described which enable the efficient realization of a new TTA-based series for organic thin-film transistors (OTFTs). For the perfluorophenyl end-functionalized derivative DFP-TTA, the molecular structure is determined by single-crystal X-ray diffraction. This material exhibits n-channel transport with a mobility as high as 0.30 cm²V⁻¹s⁻¹ and a high on-off ratio of 1.8 × 10⁷. Thus, DFP-TTA has one of the highest electron mobilities of any fused thiophene semiconductor yet discovered. For the phenyl-substituted analogue, DP-TTA, p-channel transport is observed with a mobility as high as 0.21 cm²V⁻¹s⁻¹. For the 2-benzothiazolyl (BS-) containing derivative, DBS-TTA, p-channel transport is still exhibited with a hole mobility close to 2 × 10⁻³ cm²V⁻¹s⁻¹. Within this family, carrier mobility magnitudes are strongly dependent on the semiconductor growth conditions and the gate dielectric surface treatment.

The performance of bottom-contact thin-film transistor (TFT) structures lags behind that of top-contact structures owing to the far greater contact resistance. The major sources of the contact resistance in bottom-contact TFTs are believed to reflect a combination of non-optimal semiconductor growth morphology on the metallic contact surface and the limited available charge injection area versus top-contact geometries. As a part of an effort to understand the sources of high charge injection barriers in n-channel TFTs, the influence of thiol metal contact treatment on the molecular-level structures of such interfaces is investigated using hexamethyldisilazane (HMDS)-treated SiO₂ gate dielectrics. The focus is on the self-assembled monolayer (SAM) contact surface treatment methods for bottom-contact TFTs based on two archetypical n-type semiconductors, α,ω-diperfluorohexylquarterthiophene (DFH-4T) and N,N′bis(n-octyl)-dicyanoperylene-3,4:9,10-bis(dicarboximide) (PDI-8CN₂). TFT performance can be greatly enhanced, to the level of the top contact device performance in terms of mobility, on/off ratio, and contact resistance. To analyze the molecular-level film structural changes arising from the contact surface treatment, surface morphologies are characterized by atomic force microscopy (AFM) and scanning tunneling microscopy (STM). The high-resolution STM images show that the growth orientation of the semiconductor molecules at the gold/SAM/semiconductor interface preserves the molecular long axis orientation along the substrate normal. As a result, the film microstructure is well-organized for charge transport in the interfacial region.

Herein, we report a new family of naphthaleneamidinemonoimide-fused oligothiophene semiconductors designed for facile charge transport in organic field-effect transistors (OFETs). These molecules have planar skeletons that induce high degrees of crystallinity and hence good charge-transport properties. By modulating the length of the oligothiophene fragment, the majority carrier charge transport can be switched from n-type to ambipolar behavior. The highest FET performance is achieved for solution-processed films of 10-[(2,2′-bithiophen)-5-yl]-2-octylbenzo[lmn]thieno[3′,4′:4,5]imidazo[2,1-b][3,8]phenanthroline-1,3,6(2 H)-trione (NDI-3 Tp), with optimized film mobilities of 2×10⁻² and 0.7×10⁻² cm² V⁻¹ s⁻¹ for electrons and holes, respectively. Finally, these planar semiconductors are compared with their twisted-skeleton counterparts, which exhibit only n-type mobility, in order to understand the origin of the ambipolarity in this new series of molecular semiconductors.

The temperature dependence of field-effect transistor (FET) mobility is analyzed for a series of n-channel, p-channel, and ambipolar organic semiconductor-based FETs selected for varied semiconductor structural and device characteristics. The materials (and dominant carrier type) studied are 5,5′′′-bis(perfluorophenacyl)-2,2′:5′,2″:5″,2′′′-quaterthiophene (1, n-channel), 5,5′′′-bis(perfluorohexyl carbonyl)-2,2′:5′,2″:5″,2′′′-quaterthiophene (2, n-channel), pentacene (3, p-channel); 5,5′′′-bis(hexylcarbonyl)-2,2′:5′,2″:5″,2′′′-quaterthiophene (4, ambipolar), 5,5′′′-bis-(phenacyl)-2,2′: 5′,2″:5″,2′′′-quaterthiophene (5, p-channel), 2,7-bis((5-perfluorophenacyl)thiophen-2-yl)-9,10-phenanthrenequinone (6, n-channel), and poly(N-(2-octyldodecyl)-2,2′-bithiophene-3,3′-dicarboximide) (7, n-channel). Fits of the effective field-effect mobility (µ_eff) data assuming a discrete trap energy within a multiple trapping and release (MTR) model reveal low activation energies (E_As) for high-mobility semiconductors 1–3 of 21, 22, and 30 meV, respectively. Higher E_A values of 40–70 meV are exhibited by 4–7-derived FETs having lower mobilities (µ_eff). Analysis of these data reveals little correlation between the conduction state energy level and E_A, while there is an inverse relationship between E_A and µ_eff. The first variable-temperature study of an ambipolar organic FET reveals that although n-channel behavior exhibits E_A = 27 meV, the p-channel regime exhibits significantly more trapping with E_A = 250 meV. Interestingly, calculated free carrier mobilities (µ₀) are in the range of ∼0.2–0.8 cm² V⁻¹ s⁻¹ in this materials set, largely independent of µ_eff. This indicates that in the absence of charge traps, the inherent magnitude of carrier mobility is comparable for each of these materials. Finally, the effect of temperature on threshold voltage (V_T) reveals two distinct trapping regimes, with the change in trapped charge exhibiting a striking correlation with room temperature µ_eff. The observation that E_A is independent of conduction state energy, and that changes in trapped charge with temperature correlate with room temperature µ_eff, support the applicability of trap-limited mobility models such as a MTR mechanism to this materials set.

Electron-transporting organic semiconductors (n-channel) for field-effect transistors (FETs) that are processable in common organic solvents or exhibit air-stable operation are rare. This investigation addresses both these challenges through rational molecular design and computational predictions of n-channel FET air-stability. A series of seven phenacyl–thiophene-based materials are reported incorporating systematic variations in molecular structure and reduction potential. These compounds are as follows: 5,5′′′-bis(perfluorophenylcarbonyl)-2,2′:5′,- 2′′:5′′,2′′′-quaterthiophene (1), 5,5′′′-bis(phenacyl)-2,2′:5′,2′′: 5′′,2′′′-quaterthiophene (2), poly[5,5′′′-(perfluorophenac-2-yl)-4′,4′′-dioctyl-2,2′:5′,2′′:5′′,2′′′-quaterthiophene) (3), 5,5′′′-bis(perfluorophenacyl)-4,4′′′-dioctyl-2,2′:5′,2′′:5′′,2′′′-quaterthiophene (4), 2,7-bis((5-perfluorophenacyl)thiophen-2-yl)-9,10-phenanthrenequinone (5), 2,7-bis[(5-phenacyl)thiophen-2-yl]-9,10-phenanthrenequinone (6), and 2,7-bis(thiophen-2-yl)-9,10-phenanthrenequinone, (7). Optical and electrochemical data reveal that phenacyl functionalization significantly depresses the LUMO energies, and introduction of the quinone fragment results in even greater LUMO stabilization. FET measurements reveal that the films of materials 1, 3, 5, and 6 exhibit n-channel activity. Notably, oligomer 1 exhibits one of the highest μ_e (up to ≈0.3 cm² V⁻¹ s⁻¹) values reported to date for a solution-cast organic semiconductor; one of the first n-channel polymers, 3, exhibits μ_e≈10⁻⁶ cm² V⁻¹ s⁻¹ in spin-cast films (μ_e=0.02 cm² V⁻¹ s⁻¹ for drop-cast 1:3 blend films); and rare air-stable n-channel material 5 exhibits n-channel FET operation with μ_e=0.015 cm² V⁻¹ s⁻¹, while maintaining a large I_on:off=10⁶ for a period greater than one year in air. The crystal structures of 1 and 2 reveal close herringbone interplanar π-stacking distances (3.50 and 3.43 Å, respectively), whereas the structure of the model quinone compound, 7, exhibits 3.48 Å cofacial π-stacking in a slipped, donor-acceptor motif.

Principal goals in organic thin-film transistor (OTFT) gate dielectric research include achieving: (i) low gate leakage currents and good chemical/thermal stability, (ii) minimized interface trap state densities to maximize charge transport efficiency, (iii) compatibility with both p- and n- channel organic semiconductors, (iv) enhanced capacitance to lower OTFT operating voltages, and (v) efficient fabrication via solution-phase processing methods. In this Review, we focus on a prominent class of alternative gate dielectric materials: self-assembled monolayers (SAMs) and multilayers (SAMTs) of organic molecules having good insulating properties and large capacitance values, requisite properties for addressing these challenges. We first describe the formation and properties of SAMs on various surfaces (metals and oxides), followed by a discussion of fundamental factors governing charge transport through SAMs. The last section focuses on the roles that SAMs and SAMTs play in OTFTs, such as surface treatments, gate dielectrics, and finally as the semiconductor layer in ultra-thin OTFTs.

This contribution presents an electrochemical, Raman spectroscopic, and theoretical study probing the differences in molecular and electronic structure of two quinoidal oligothiophenes (3′,4′-dibutyl-5,5″-bis(dicyanomethylene)-5,5″-dihydro-2,2′:5′,2″-terthiophene and 5,5′-bis(dicyanomethylene)-3-hexyl-2,5-dihydro-4,4′-dihexyl-2,2′,5,5′-tetrahydro-tetrathiophene) with terminal tetracyanomethylene functionalization and aromatic oligothiophenes where acceptor moieties are positioned at lateral positions along the conjugated chain (6,6′-dibutylsulfanyl-[2,2′-bi-[4-dicyanovinylene-4H-cyclopenta[2,1-b:3,4-b′]dithiophene]). In this way, the consequences of linear and cross conjugation are compared and contrasted. From this analysis, it is apparent that organic field-effect transistors fabricated with cross-conjugated tetrathiophene semiconductors should combine the benefits of an electron-donor aromatic chain with strongly electron-accepting tetracyanomethylene substituents. The corresponding organic field-effect transistors exhibit ambipolar transport with rather similar hole and electron mobilities. Moreover, n-channel conduction is enhanced to yield one of the highest electron mobilities found to date for this type of material.