Field-effect Charge Transport Research Articles

Pyrazinoquinoxaline derivatives for flexible electronic devices: effect of the mechanical properties of the crystals on device durability.

Understanding the interplay between the molecular structure and material properties of emerging p-type organic semiconductors marks a significant stride in the advancement of molecular electronics. Among the array of promising materials, mechanically flexible single crystals of π-conjugated molecules stand out due to their potential for cutting-edge applications in organic electronics. Notably, derivatives of pyrazinoquinoxaline (PQ) are recognized as versatile building blocks for constructing π-conjugated systems, showcasing good semiconductor performance in organic field-effect transistors (OFETs). In this study, we present an exploration into the p-type charge transport and mechanical characteristics of two newly synthesized PQ derivatives: 5,10-diphenyl-2,3,7,8-tetra(thiophen-2-yl)pyrazino[2,3-g]quinoxaline (DPTTQ) and 2,3,5,7,8,10-hexa(thiophen-2-yl)pyrazino[2,3-g]quinoxaline (HTPQ). HTPQ crystals exhibit flexural behaviour under applied stress, effortlessly returning to their initial configuration upon relaxation. Conversely, two polymorphic forms of DPTTQ crystals display brittle fracture when subjected to a similar stress. Specifically, DPTTQ molecules adopt a β-sheet packing, while HTPQ presents a γ-packing with a corrugated arrangement. Field-effect charge transport measurements reveal p-type charge transport in both DPTTQ and HTPQ, with HTPQ showcasing hole mobility up to 0.01 cm2 V-1 s-1, while DPTTQ exhibits mobility that is at least one order of magnitude lower. This variance in the field effect mobility can be directly correlated to the difference in crystal packing, highlighting a clear structure-property correlation. Moreover, taking advantage of the flexural nature of the HTPQ crystals, we fabricated durable electronic devices, which retain their conductivity for over 60 cycles of strain, indicating the efficacy of our chemical design in demonstrating high-performance flexible devices. These findings underscore the promise of semiconducting organics with γ-packing for achieving both better mobility and elasticity for integration into organic electronic devices.

Beyond the two-dimensional field-effect charge transport paradigm in molecular thin film transistors

Beyond the two-dimensional field-effect charge transport paradigm in molecular thin film transistors

Multi-State Heterojunction Transistors Based on Field-Effect Tunneling-Transport Transitions.

A monolithic ternary logic transistor based on a vertically stacked double n-type semiconductor heterostructure is presented. Incorporation of the organic heterostructure into the conventional metal-oxide-semiconductor field-effect transistor (MOSFET) architecture induces the generation of stable multiple logic states in the device; these states can be further optimized to be equiprobable and distinctive, which are the most desirable and requisite properties for multivalued logic devices. A systematic investigation reveals that the electrical properties of the device are governed by not only the conventional field-effect charge transport but also the field-effect charge tunneling at the heterointerfaces, and thus, an intermediate state can be finely tuned by independently controlling the transition between the onsets of these two mechanisms. The achieved device performance agrees with the results of a numerical simulation based on a pseudo-metal-insulator-metal model; the obtained findings therefore provide rational criteria for material selection in a simple energetic perspective. The operation of various ternary logic circuits based on the optimized multistate heterojunction transistors, including the NMIN and NMAX gates, is also demonstrated.

Field-Effect Charge Transport in Doped Polymer Semiconductor-Insulator Alternating Bulk Junctions with Ultrathin Transport Layers.

Conjugated-polymer field-effect transistors are attractive for flexible electronics. However, relatively high chemical doping (oxidation) concentration of p-type polymer semiconductors is usually not compatible with good transistor performance, due to poor switching-off capability and short-channel performance. Here, we propose a combined simulation and experimental investigation on charge transport in a semiconductor-insulator alternating bulk junction composed of repeating semiconductor and insulator regions, which shows better transistor performance at higher doping levels, as compared with traditional planar transistors. Moreover, the doped semiconductor transport layers in the junction should be less than 2 nm thick to ensure sufficient pinch-off capability. Using some semiconductors including poly(3-hexylthiophene), we utilize a fast solvent evaporation approach to obtain semiconductor-insulator alternating bulk junctions with ultrathin (thickness < 2 nm) semiconductor crystallites and with vertical gradients of both morphology and electronic properties. Doping with a concentration of up to 1019 cm-3 simultaneously induces the improvement of field-effect mobility, on/off ratio, and subthreshold swing, which leads to long-term (>1 year) stability, without lowering the short-channel performance. Moreover, these heterojunctions are optically transparent, nearly colorless, and flexible, thus could be exploited for wide electronic and photonic applications.

Chalcogen Bridged Thieno- and Selenopheno[2,3-d:5,4-d′]bisthiazole and Their Diketopyrrolopyrrole Based Low-Bandgap Copolymers

We report the synthesis and characterization of four novel small bandgap copolymers incorporating the electron-deficient thieno[2,3-d:5,4-d′]bisthiazole and selenopheno[2,3-d:5,4-d′]bisthiazole building blocks with a series of electron-deficient diketopyrrolopyrole units. The four resultant copolymers were synthesized via palladium Stille cross-coupling reaction, and their optical, thermal stability, electrochemical, and field-effect charge transport properties were investigated. All copolymers showed low optical bandgaps (1.53–1.56 eV); in addition, X-ray diffraction on solution-cast films revealed that the selenium-containing copolymers exhibit higher crystallinity compared to their thiophene counterparts.

Metallopolyyne polymers containing naphthalene diimide-oligothiophene moieties and their applications in organic field-effect transistors

Two novel platinum(II) donor-acceptor (D-A) naphthalene diimide (NDI)-based copolymers have been designed and synthesized. The effects of thiophene numbers on the thermal, optical, electronic and charge transport properties of the polymers have been investigated. These solution processable polymers exhibit p-channel field-effect charge transport characteristics, unlike other organic NDI-based polymeric analogues. All the results indicated that they have potential applications in high-performance organic field-effect transistors (OFETs).

Nanoscale Charge Percolation Analysis in Polymer-Sorted (7,5) Single-Walled Carbon Nanotube Networks.

The current percolation in polymer-sorted semiconducting (7,5) single-walled carbon nanotube (SWNT) networks, processed from solution, is investigated using a combination of electrical field-effect measurements, atomic force microscopy (AFM), and conductive AFM (C-AFM) techniques. From AFM measurements, the nanotube length in the as-processed (7,5) SWNTs network is found to range from ≈100 to ≈1500 nm, with a SWNT surface density well above the percolation threshold and a maximum surface coverage ≈58%. Analysis of the field-effect charge transport measurements in the SWNT network using a 2D homogeneous random-network stick-percolation model yields an exponent coefficient for the transistors OFF currents of 16.3. This value is indicative of an almost ideal random network containing only a small concentration of metallic SWNTs. Complementary C-AFM measurements on the other hand enable visualization of current percolation pathways in the xy plane and reveal the isotropic nature of the as-spun (7,5) SWNT networks. This work demonstrates the tremendous potential of combining advanced scanning probe techniques with field-effect charge transport measurements for quantification of key network parameters including current percolation, metallic nanotubes content, surface coverage, and degree of SWNT alignment. Most importantly, the proposed approach is general and applicable to other nanoscale networks, including metallic nanowires as well as hybrid nanocomposites.

Naphthobisthiazole diimide-based n-type polymer semiconductors: synthesis, π-stacking, field-effect charge transport, and all-polymer solar cells

New n-type conjugated polymer semiconductors bearing an electron-deficient naphthobisthiazole diimide (NBTDI) moiety have been synthesized and their electronic energy levels, solid-state morphology, field-effect charge transport and photovoltaic properties were investigated. Stille coupling polymerization of the new monomer 5,11-bis(4-bromophenyl)-2,8-bis(2-decyltetradecyl)-benzothiazolo[4,5,6,7-lmn]thiazolo[5,4-f][3,8]phenanthroline-1,3,7,9(2H, 8H)-tetrone with distannyl derivatives of the arylene moiety (1,4-phenylene/2,5-thienylene/vinylene) yielded poly(naphthobisthiazole diimide)s (PNBTDIs) with number-average molecular weights of 73–89 kDa and polydispersity indexes of 2.46–3.08. Thin films of the PNBTDIs have strong absorption bands in the visible region, resulting in an absorption edge optical band gap of 1.73–2.02 eV. X-ray diffraction analysis of neat films of PNBTDIs revealed lamellar crystalline materials with a rather short intermolecular π–π stacking distance of 0.34–0.35 nm. In thin film transistors, the PNBTDIs showed unipolar n-channel transport with electron mobility as high as 1.5 × 10−2 cm2 V−1 s−1. All-polymer solar cells incorporating the PNBTDIs as an electron acceptor and thiazolothiazole copolymer (PSEHTT) as a donor had power conversion efficiencies of 1.1–1.5%. The results demonstrate that poly(naphthobisthiazole diimide)s are promising n-type materials for field-effect transistors and all-polymer solar cells.

Thiazolothiazole Donor–Acceptor Conjugated Polymer Semiconductors for Photovoltaic Applications

A series of eight new donor–acceptor conjugated copolymers based on thiazolo[5,4-d]thiazole (TT) were synthesized by Suzuki and Stille polycondensations, and their optical, electronic structure, field-effect charge transport, and photovoltaic properties were investigated. The number-average molecular weight was moderate to high (20–103 kDa) for four of the new copolymers and low (<10 kDa) for the rest. The optical band gap (1.7–2.2 eV) and the ionization potential (4.9–5.7 eV) of the copolymers could be tuned by varying the strength of the electron-donating comonomers. X-ray diffraction analysis of the neat TT polymer thin films showed that they exhibit a lamellar crystalline morphology with a (100) d-spacing of 1.449–1.881 nm. The polymers had field-effect hole mobilities of 2 × 10–3–3 × 10–2 cm2/(V s). Although fullerene (PC71BM)-based bulk heterojunction solar cells showed that most of the new polymers had low photovoltaic power conversion efficiency (<2.4%), the standout polymer PDEHTT had a high effi...

Annealing temperature dependence of the efficiency and vertical phase segregation of polymer/polymer bulk heterojunction photovoltaic cells

We report observation of annealing temperature-induced simultaneous vertical phase segregation and large enhancement of power conversion efficiency (PCE) of all-polymer bulk heterojunction (BHJ) solar cells composed of a poly(3-hexylthiophene) (P3HT) donor and a naphthalene diimide-selenolo[3,2-b]selenophene copolymer (PNDISS) acceptor. The PCE of P3HT:PNDISS BHJ devices increased over 50-fold from 0.04% to 2.03% when the annealing temperature was increased from 50 to 150 °C. Absorption spectroscopy and photoluminescence quenching experiments provide evidence of increasing phase segregation of the polymer/polymer blend films with increasing annealing temperature. Field-effect charge transport, contact angle, surface energy, and variable angle ellipsometry measurements on the P3HT:PNDISS blend films showed that thermal annealing induced vertical phase segregation, whereby the low surface energy polymer (P3HT) migrated to the bulk, while the high surface energy polymer (PNDISS) enriches at the substrate/blend interface.

Charge Transport in Ordered and Disordered Regions in Pristine and Sonicated-Poly(3-hexylthiophene) Films

Spectral trap density of states (DOS) and intrinsic mobilities in pristine and sonicated P3HT films were calculated and compared using temperature-dependent field-effect charge transport measurements. With sonication of P3HT solution, the field effect transistor (FET) hole mobility in its film state was improved remarkably in comparison with that in a pristine-poly(3-hexylthiophene) (P3HT) film. Spectral trap DOS were calculated through gate voltage-dependent activation energy measurements using the bottom-contact field effect transistors (FETs). For the pristine-P3HT films, the width of the shallow trap DOS was increased, indicating increased disorder in comparison with that for the sonicated-P3HT films. Further, the intrinsic mobility, 0.155 cm2/(V s), in the sonicated-P3HT film was far larger than that, 0.018 cm2/(V s), in the pristine one, suggesting that the structural properties of the P3HT films in the ordered region close to the gate dielectric are different. This reveals that a higher mobility in the sonicated-P3HT film originates from more efficient hole transport in the ordered P3HT region as well as less trap densities in the disordered region.

Oligothiophene‐Bridged Bis(arylene ethynylene) Small Molecules for Solution‐Processible Organic Solar Cells with High Open‐Circuit Voltage

A new series of conjugated oligothiophene-bridged bis(arylene ethynylene) small molecules have been designed, synthesized, and characterized by photophysical, electrochemical and computational methods. These compounds were found to have optimal LUMO levels that ensure effective charge transfer from these compounds to [6,6]-phenyl-C71-butyric acid methyl ester (PC70BM). They were utilized as good electron-donor materials that can be blended with electron-acceptor PC70BM in the fabrication of solution-processed molecular bulk heterojunction (BHJ) solar cells. All of these BHJ devices showed very high open-circuit voltage (V(oc)) of 0.90-0.97 V, and the best power conversion efficiency achieved was 3.68%. The high V(oc) is consistent with the deeper low-lying HOMO level and is relatively insensitive to the donor : acceptor blend ratio. The spin-coated thin films of these small molecules showed p-channel field-effect charge transport with the hole mobilities of up to 2.04×10(-4) cm(2) V(-1) s(-1). These compounds illuminate the potential of solution-processible small-molecular aryl acetylide compounds for efficient power generation in photovoltaic implementation.

Platinum‐Based Poly(Aryleneethynylene) Polymers Containing Thiazolothiazole Group with High Hole Mobilities for Field‐Effect Transistor Applications

Two solution-processable platinum-acetylide polymers functionalized with the electron-deficient thiazolothiazole spacer are synthesized and show absorption features spanning from 320 to 600 nm and optical bandgaps of 2.15 and 2.05 eV. The spin-coated polymer thin films of both materials exhibit p-channel field-effect charge transport characteristics with impressive peak field-effect charge-carrier mobilities of (2.1-2.8) × 10(-2) cm(2) V(-1) s(-1) and on/off ratios of (0.8-1.0) × 10(5) for the holes. The high hole mobility value reported for one of the polymers is among the highest reported for metallopolyynes to date. It is also shown that the hole mobility can be notably increased by extending the conjugation length of the chain from the monothienyl to the bithienyl segment on each side of the thiazolothiazole ring.

Platinum(II)–Bis(aryleneethynylene) Complexes for Solution‐Processible Molecular Bulk Heterojunction Solar Cells

Four new solution-processible small-molecular platinum(II)-bis(aryleneethynylene) complexes consisting of benzothiadiazole as the electron acceptor and triphenylamine and/or thiophene as the electron donor were conveniently synthesized and characterized by physicochemical and computational methods, and utilized as the electron-donor materials in the fabrication of solution-processed bulk heterojunction (BHJ) solar cells. The effect of different electron-donor groups in these small molecules on the optoelectronic and photovoltaic properties was also examined. The optical and time-dependent density functional theory studies showed that the incorporation of stronger electron-donor groups significantly enhanced the solar-absorption abilities of the complexes. These molecular complexes can serve as good electron donors for fabricating BHJ devices by blending them with the [6,6]-phenyl-C(71)-butyric acid methyl ester (PC(70)BM) as the electron acceptor. The best power conversion efficiency of 2.37% was achieved with the open-circuit voltage of 0.83 V, short-circuit current density of 7.10 mA cm(-2) and fill factor of 0.40 under illumination of an AM 1.5 solar-cell simulator. The spin-coated thin films showed p-channel field-effect charge transport with hole mobilities of up to 2.4×10(-4) cm(2) V(-1) s(-1) for these molecules. The present work illuminates the potential of well-defined organometallic complexes in developing light-harvesting small molecules for efficient power generation in organic photovoltaics implementation.

New Poly(arylene vinylene)s Based on Diketopyrrolopyrrole for Ambipolar Transistors

Three diketopyrrolopyrrole-based donor–acceptor copolymer semiconductors were synthesized by Stille coupling polymerization and their structural, photophysical, electrochemical, and field-effect charge transport properties were characterized. The new copolymers, poly[3,6-(2,5-bis(2-hexyldecyl)pyrrolo[3,4-c]pyrrole-1,4-dione)-alt-1,2-bis-(2′-thienyl)vinyl-5′,5″-diyl] (HD-PPTV), poly[3,6-(2,5-bis(2-hexyldecyl)pyrrolo[3,4-c]pyrrole-1,4-dione)-alt-1,2-bisphenylvinyl-4′,4″-diyl] (HD-PPPV), and a 50:50 random copolymer (PPTPV), have moderate number-average molecular weights (21800–88800 g/mol). The copolymers have optical absorption bands in the visible to near-IR regions with optical bandgaps that varied from 1.22 eV for HD-PPTV to 2.0 eV for HD-PPPV. Ionization potentials of 5.14–5.48 eV and electron affinities of 3.13–3.34 eV were derived from cyclic voltammetry. Ambipolar charge transport was seen in transistors based on HD-PPTV and PPTPV, whereas only hole transport was observed in HD-PPPV. Field-effect tr...

Low bandgap copolymer of 1,4-diketopyrrolo[3,4-c]pyrrole and thieno[3,2-b]thiophene: Synthesis and applications in polymer solar cells and field-effect transistors

A new conjugated polymer, poly(3,6-dihexyl-thieno[3,2-b]thiophenyl-2,5-diyl-alt-2,5-di(2-ethylhexyl)-3,6-bis(thien-2,5-diyl)-pyrrolo[3,4-c]-pyrrole-1,4-dione) (DH-PTTDPP) was synthesized by Pd-catalyzed Stille-coupling method. The thermal, absorption, electrochemical, photovoltaic properties, and field-effect charge transport of the polymer were examined. The polymer is soluble in common organic solvents and possesses a broad absorption band covering from 500nm to 900nm with an optical bandgap of 1.33eV. Field-effect transistors (FETs) based on DH-PTTDPP thin film showed a hole mobility of 2.5×10−3cm2V−1s−1 with an on/off ratio of 105, which is among the high values for the conjugated polymers. Polymer solar cells (PSCs) based on DH-PTTDPP/PC70BM (1:1.5, w/w) demonstrated a power conversion efficiency of 1.90% under the illumination of AM 1.5, 100mWcm−2. These results indicate that DH-PTTDPP is an attractive polymer semiconductor for the applications in FETs and PSCs.

Platinum–Acetylide Polymers with Higher Dimensionality for Organic Solar Cells

A new series of platinum(II)-acetylide polymers P1-P3 containing thiophene-triarylamine chromophores of different dimensions were synthesized and their electronic band structures, field-effect charge transport, and application in bulk heterojunction solar cells were evaluated. These materials are soluble in polar organic solvents and show strong absorptions in the solar spectra (with the highest absorption coefficient of 1.59×10(5) cm(-1) from thin films), thus rendering them excellent candidates for bulk heterojunction polymer solar cells. The spin-coated polymer thin films showed p-channel field-effect charge transport with hole mobilities of 1.90×10(-5) to 7.86×10(-5) cm(2) V(-1) s(-1) for P1-P3 and an improved charge carrier transport was found for P2 with higher molecular dimensionality than P1. The dependence of their photovoltaic properties and dimensionality was also investigated. Even if the polymers possess relatively high bandgaps and narrow absorption bandwidths, the highest power conversion efficiency of 2.24 % can be obtained based on blends of P3 with [6,6]phenyl-C(61)-butyric acid methyl ester (PCBM) (1:5, w/w) under AM1.5 simulated solar illumination. The present work indicates that multidimensional polymers exhibit a better photovoltaic performance over the linear polymers under the same measurement conditions and can provide an attractive approach to developing highly efficient conjugated metallopolymers for efficient power generation.

Ladder-type heteroacenepolymers bearing carbazole and thiophene ring units and their use in field-effect transistors and photovoltaic cells

A family of ladder-type π-excessive conjugated monomer (dicyclopentathienocarbazole (DCPTCz)) integrating the structural components of carbazole and thiophene into a single molecular entity is synthesized and polymerized by oxidative coupling to yield poly(dicyclopentathienocarbazole) (PDCPTCz). Moreover, through the careful selection of 2,1,3-benzothiadiazole unit as a π-deficient building block, the dicyclopentathienocarbazole-based donor–acceptor copolymer (poly(dicyclopentathienocarbazole-alt-2,1,3-benzothiadiazole) (PDCPTCz-BT)) is prepared by Suzuki polycondensation. The optical, electrochemical, and field-effect charge transport properties of the resulting polymers (PDCPTCz and PDCPTCz-BT) are not only characterized in detail but also their bulk-heterojunction (BHJ) solar cell in combination with PC71BM are evaluated. The values of field-effect mobility (µ) for PDCPTCz and PDCPTCz-BT are 8.7 × 10−6 cm2 V−1s−1 and 2.7 × 10−4 cm2 V−1s−1, respectively. A power conversion efficiency (PCE) of 1.57% is achieved on the PDCPTCz-BT/PC71BM device, implying that the push–pull copolymers based on ladder-type dicyclopentathienocarbazole as an electron-donating moiety are promising for organic electronic devices.

Regioregular Poly(3-pentylthiophene): Synthesis, Self-Assembly of Nanowires, High-Mobility Field-Effect Transistors, and Efficient Photovoltaic Cells

We report the synthesis, characterization, solution-phase assembly of nanowires, field-effect charge transport, and photovoltaic properties of regioregular poly(3-pentylthiophene) (P3PT), the first regioregular poly(3-alkylthiophene) with an odd-numbered alkyl side chain length to be so investigated. Two samples of P3PT with weight-average molecular weights of 61 800 and 77 000 and 1.4−1.5 polydispersity index have comparable solubility and processability in organic solvents as regioregular poly(3-hexylthiophene) (P3HT) but have a higher melting transition at 259 °C. X-ray diffraction of P3PT films revealed a lamellar structure with an interlayer d100 spacing of 1.51 nm and a π-stacking d010 spacing of 0.374 nm, both smaller than in P3HT. Crystalline nanowires of 16−17 nm width and aspect ratios as high as 465 were assembled from P3PT solution. Field-effect transistors fabricated from P3PT thin films showed a mobility of holes of up to 0.10 cm2/(V s), and the carrier mobility increased with molecular weight. Bulk heterojunction solar cells based on P3PT/fullerene (PC71BM) blend thin films had a power conversion efficiency of 3.70% under 100 mW/cm2 AM1.5 solar illumination in air and a maximum external quantum efficiency of 69%. Similarly illuminated solar cells based on nanocomposites of P3PT nanowires/PC71BM had a 3.33% power conversion efficiency. These results demonstrate that P3PT is an attractive polymer semiconductor suitable for applications in thin-film and nanowire-based field-effect transistors and bulk heterojunction solar cells.

Organometallic Donor−Acceptor Conjugated Polymer Semiconductors: Tunable Optical, Electrochemical, Charge Transport, and Photovoltaic Properties

A series of 11 organometallic conjugated polymer semiconductors bearing a platinum (Pt) center have been synthesized and their electronic band structures, field-effect charge transport, and use in bulk heterojunction solar cells were evaluated. The Pt-bridged donor−acceptor conjugated poly(aryleneethynylene)s were synthesized by Sonogashira-type polycondensation and are exemplified by poly[4-(5′-trans-bis(tributylphosphine)platinum ethynyl-thiophen-2′-yl)-7-(5′′-ethynyl-thiophen-2′′-yl)-benzo[1,2,5]thiadiazole] (BT-BTPt), poly[5-(5′-trans-bis(tributylphosphine)platinum ethynyl-thiophen-2′-yl)-8-(5′′-ethynyl-thiophen-2′′-yl)-2,3-diheptyl-pyrido[3,4-b]pyrazine] (HPP-BTPt), and poly[5-(5′-trans-bis(tributylphosphine)platinum ethynyl-thiophen-2′-yl)-7-(5′′-ethynyl-thiophen-2′′-yl)-thieno[3,4-b]pyrazine] (TP-BTPt). The Pt-bridged polymers had reversible electrochemical reduction waves from which electron affinities were found to be 2.95 to 3.28 eV. From the onset oxidation potentials of the polymers, we similarly determined ionization potentials to be 4.82 to 5.23 eV. Optical bandgaps of the donor−acceptor polymers were 1.49 to 1.97 eV. The spin coated polymer thin films showed p-channel field-effect charge transport with average hole mobilities of 3.87 × 10−7 to 3.32 × 10−5 cm2/(V s). Bulk heterojunction solar cells based on blends of the polymers with [6,6]phenyl-C71-butyric acid methyl ester (PC71BM) gave power conversion efficiencies as high as 0.68% for HPP-BTPt and 2.41% for BT-BTPt. These results demonstrate the molecular engineering of the electronic band structures and the optical, charge transport, and photovoltaic properties of organometallic donor−acceptor conjugated polymer semiconductors.