Air-stable Organic Semiconductor Research Articles

An organic, threshold voltage based, all PMOS, voltage reference generator for flexible sensor tags

The developments and advances achieved in organic semiconductors have promised lower costs for integrated circuit production and also fabrication of electronic circuits using printed technology on unconventional substrates such as plastic, clothing, and even skin. An important building block essential to most electronic circuits is a voltage, process, and temperature independent potential generator which can be used to bias amplifiers and produce a fixed reference for sensor devices. The generation of a voltage reference is also important for voltage regulators. Currently, most reported organic integrated circuits use only p-type OFETs in their circuits due to simpler fabrication procedures. Furthermore, air stable p-type organic semiconductors such as pentacene and CuPc are well characterized. In this paper, a low power two stage all PMOS voltage reference generator is proposed. Since properties such as threshold voltage value and device aging are dependent on the OFET structure, the type of device chosen for this purpose will have a direct impact on the circuit performance. Three different types of OFETs with silver, copper, and gold drain/source electrodes are studied in this work. Performance factors such as line sensitivity (LS), temperature coefficient (TC), power consumption, time constant, and output drifts of the fabricated integrated circuits are measured and reported to verify the characteristics of the proposed circuits. It is shown that the drain/source metal choice affects the threshold voltage dependent output potential of the reference generators.

Air and temperature sensitivity of n-type polymer materials to meet and exceed the standard of N2200

N-type organic semiconductors are notoriously unstable in air, requiring the design of new materials that focuses on lowering their LUMO energy levels and enhancing their air stability in organic electronic devices such as organic thin-film transistors (OTFTs). Since the discovery of the notably air stable and high electron mobility polymer poly{[N,N′-bis (2-octyldodecyl)- naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,29-bisthiophene)} (N2200), it has become a popular n-type semiconductor, with numerous materials being designed to mimic its structure. Although N2200 itself is well-studied, many of these comparable materials have not been sufficiently characterized to compare their air stability to N2200. To further the development of air stable and high mobility n-type organic semiconductors, N2200 was studied in organic thin film transistors alongside three N2200-based analogues as well as a recently developed polymer based on a (3E,7E)-3,7-bis(2-oxoindolin-3-ylidene)benzo[1,2-b:4,5-b′]difuran-2,6(3 H,7 H)-dione (IBDF) core. This IBDF polymer has demonstrated promising field-effect mobility and air stability in drop-cast OTFTs. While N2200 outperformed its analogues, the IBDF-based polymer displayed superior air and temperature stability compared to N2200. Overall, polymers with more heteroatoms displayed greater air stability. These findings will support the development of new air-stable materials, and further demonstrate the persistent need for the development of novel n-type semiconductors.

Investigation of External QE of P3DDT Photocathode for use in Gaseous UV Photon Detectors

The quantum efficiency results of a new air stable organic semiconductor UV photocathode P3DDT are presented in this paper.Measurement of QE was carried out as a function of wavelength in spectral region 190-250nm. It shows cut off near 230nm. However, QE is too low to be of interest as compared to CsI but being air stabile and easyto prepare photocathodeswith large area it can find applications in barium flouride scintilation based calorimetry and plasma diagnositic where photon flux is very high.

Inverted perovskite solar cells with air stable diketopyrrolopyrrole-based electron transport layer

One of the possible causes of degradation of perovskite solar cells is the instability of the electron transporting layer. In this regard, design of air stable electron transport organic semiconductors, compatible with perovskite energy levels presents challenges due to inherent vulnerability to traps, presumably originating due to water and/or oxygen. In this work, we demonstrate air stability of diketopyrrolopyrrole-based molecule (TDPP-CN4) at ambient conditions and its application as electron transporting layer (ETL) in perovskite solar cells. We investigated electron mobility and air stability of TDPP-CN4 by fabricating top-gate bottom-contact (TG-BC) thin film transistors and compared with PCBM at ambient conditions. Both TDPP-CN4 and PCBM exhibit electron transport properties with mobility of 0.13 cm2 V−1 s−1 and 0.03 cm2 V−1 s−1 respectively. However, we found remarkable air stability of the TDPP-CN4 in the OFET measurements under ambient conditions. These excellent properties of TDPP-CN4 render them as potential ETL layer in inverted planar heterojunction perovskite solar cells. Our preliminary device studies show remarkable short-circuit current (Jsc) ∼ 17.4 mA/cm2 with moderate open-circuit voltage (Voc) of 0.50 V. These results suggest that the electron mobility and air-stability of diketopyrrolopyrrole-based molecule hold a promise as ETL in perovskite solar cells at ambient conditions.

Quantum Efficiency of P3HT a new Air-Stable Photocathode for use in Gaseous UV Photon Detectors

We present the QE results of a new air stable organic semiconductor UV photocathode P3HT. QE was measured as a function of wavelength in spectral range 190-250nm. It shows nonnegligible QE near 230nm. Though QE is low as compared to CsI but due to ease of preparation of large area photocathodes and air stability it may replace CsI in some experiments where photon flux is high.

Intrinsic charge carrier mobility in single-crystal OFET by “fast trapping vs. slow detrapping” model

Abstract This study investigates the gate stress-induced mobility discrepancy in p-type single-crystal organic field-effect transistors (OFETs) on an octyltrichlorosilane (OTS)-modified SiO2/Si substrate. During measurements in atmosphere, anti-clockwise hysteresis was observed in the transfer curve, and the mobility calculated from the forward sweep was smaller than that calculated from the reverse sweep. Hysteresis has often been observed for OFETs but the mobility discrepancy has not been clearly understood. We formulated a “fast trapping vs. slow detrapping” model and suggested that the mobility values calculated from the reverse sweep represent the intrinsic property of the material. To verify the validity of this model, we investigated mobility anisotropy of an air-stable organic semiconductor, 2,7-bis(4-methoxyphenyl)benzo[b]benzo[4,5]thieno[2,3-d]thiophene (DBOP-BTBT). By measuring the single-crystal OFET characteristics of many crystals with different orientations, we observed anisotropic hole mobility calculated from the reverse sweep. The mobility along the b-axis, which corresponds to the π-π stacking direction, was 13.9 cm2 V−1 s−1, and that along the a-axis was 6.2 cm2 V−1 s−1. However, we did not see clear anisotropy when mobility was calculated from the forward sweep due to a variation in the data. The threshold voltage from the reverse and the forward sweeps showed isotropic characteristics within the range from −60 to −70 V and from −50 to −60 V, respectively. These results indicate that the numbers of filled traps were different between the reverse and the forward sweeps at the interface, and confirm the validity of our model.

Printed Nonvolatile Resistive Memories Based on a Hybrid Organic/Inorganic Functional Ink

Organic memories are increasingly being considered as promising candidates for a number of novel consumer applications, such as smart labels and smart packaging devices. Indeed, organic memories can be fabricated on highly flexible substrates at low temperatures from liquid phase, employing for instance printing techniques. In this work, a nonvolatile resistive memory element conceived for large‐area processing and operation in ambient conditions is presented. In particular, a functional ink made out of an air stable organic semiconductor, namely ActivInk N1400, and gold nanoparticles (NPs) is developed and optimized for the fabrication of high performance memories through inkjet printing. The ink formulation is varied in order to explore the influence of Au NPs concentration on the switching behavior. Devices are operated in ambient conditions with reproducible memory behavior, high ON/OFF current ratios, and low programming voltages. In depth material analysis with time‐of‐flight secondary ion mass spectrometry and X‐ray photoelectron spectroscopy depth profiles are carried out on operated devices to shine a light on resistive switching mechanism and to determine the average gold content and its 3D distribution in stable memories.

Novel Air Stable Organic Radical Semiconductor of Dimers of Dithienothiophene, Single Crystals, and Field-Effect Transistors.

Singly linked and vinyl-linked dimers of dithienothiophenes exhibit different electronic behaviors. Single crystals of the singly linked dimer show a high conductivity of 0.265 S cm(-1) , five orders of magnitude higher than that of the vinyl-linked dimer. The huge increase in the hole density of singly linked dimers results from the formation of radicals, which can be reversibly tuned by facile thermal de-doping.

Towards solution processable air stable p-type organic semiconductors: synthesis and evaluation of mono and di-fluorinated pentacene derivatives

A series of mono and di-fluorinated soluble pentacene derivative organic semiconductors have been synthesised and subjected to a study of the relationship between molecular structure and thermal/photochemical stability.

Thiophene-Diketopyrrolopyrrole-Based Quinoidal Small Molecules as Solution-Processable and Air-Stable Organic Semiconductors: Tuning of the Length and Branching Position of the Alkyl Side Chain toward a High-Performance n-Channel Organic Field-Effect Transistor.

A series of thiophene-diketopyrrolopyrrole-based quinoidal small molecules (TDPPQ-2-TDPPQ-5) bearing branched alkyl chains with different side-chain lengths and varied branching positions are synthesized. Field-effect transistor (FET) measurement combined with thin-film characterization is utilized to systematically probe the influence of the side-chain length and branching position on the film microstructure, molecular packing, and, hence, charge-transport property. All of these TDPPQ derivatives show air-stable n-channel transporting behavior in spin-coated FET devices, which exhibit no significant decrease in mobility even after being stored in air for 2 months. Most notably, TDPPQ-3 exhibits an outstanding n-channel semiconducting property with electron mobilities up to 0.72 cm(2) V(-1) s(-1), which is an unprecedented value for spin-coated DPP-based n-type semiconducting small molecules. A balance of high crystallinity, satisfactory thickness uniformity and continuity, and strong intermolecular interaction accounts for the superior charge-transport characteristics of TDPPQ-3 films. Our study demonstrates that tuning the length and branching position of alkyl side chains of semiconducting molecules is a powerful strategy for achieving high FET performance.

Hybrid dielectric layer for low operating voltages of transparent and flexible organic complementary inverter

Although flexibility and transparency are considered advantages of organic electronic devices along with low processing cost and the possibility of large-area production, high operating voltages and metallic contacts are obstacles to their application in real electronic products. In this work, flexible and transparent organic complementary inverters that can be operated with low voltage were fabricated on a plastic substrate. Two different air-stable organic semiconductors, fluorinated copper phthalocyanine and pentacene, are used for n-type and p-type transistors, respectively. An ITO gate electrode was deposited by sputtering, and a hybrid dielectric layer with a thin Al2O3 layer and self-assembled monolayers (SAMs) was fabricated to reduce the operation voltage. To confirm the properties of the hybrid dielectric layer, the capacitance and gate leakage current were measured. Then, source and drain electrodes were formed from gold or ITO specifically for fully transparent devices. For the ITO electrodes, a MoO3 interlayer was incorporated between the pentacene and ITO to reduce the contact resistance caused by mismatch of workfunction. Finally, we evaluated the low-voltage operation of the flexible organic inverters and the fully transparent device through transmittance measurement.

The International Polymer World and Industry: Advances and Recent Developments

The International Polymer World and Industry: Advances and Recent Developments

Systematic Reliability Study of Top-Gate p- and n-Channel Organic Field-Effect Transistors

We report on a systematic investigation on the performance and stability of p-channel and n-channel top-gate OFETs, with a CYTOP/Al2O3 bilayer gate dielectric, exposed to controlled dry oxygen and humid atmospheres. Despite the severe conditions of environmental exposure, p-channel and n-channel top-gate OFETs show only minor changes of their performance parameters without undergoing irreversible damage. When correlated with the conditions of environmental exposure, these changes provide new insight into the possible physical mechanisms in the presence of oxygen and water. Photoexcited charge collection spectroscopy experiments provided further evidence of oxygen and water effects on OFETs. Top-gate OFETs also display outstanding durability, even when exposed to oxygen plasma and subsequent immersion in water or operated under aqueous media. These remarkable properties arise as a consequence of the use of relatively air stable organic semiconductors and proper engineering of the OFET structure.

Air-stable organic semiconductors based on 6,6′-dithienylindigo and polymers thereof

We report on the synthesis and properties of 6,6′-dithienylindigo (DTI) and poly(DTI). Redox chemistry and ambipolar charge transport with μh = 0.11 cm2 V−1 s−1 and μe = 0.08 cm2 V−1 s−1 and excellent air stability are shown.

Solution-processable ambipolar organic field-effect transistor based on Co-planar bisphthalocyaninato copper

A soluble binuclear phthalocyaninato copper (II) complex, Cu2[Pc(COOC8H17)6]2 (1), with planar molecular structure and extended conjugation system, has been designed and synthesized. By fusing two phthalocyanine rings side by side and introducing electron withdrawing groups at periphery positions, the energy levels of HOMO and LUMO have been tuned successfully into the range of an air-stable ambipolar organic semiconductor required as revealed by the electrochemical studies. With the help of a solution-based quasi-Langmuir–Shäfer (QLS) method, thin solid films of this compound were fabricated and organic field effect transistors (OFETs) based on these QLS thin solid films were constructed. Because of the promising electrochemical properties as well as the high ordered packing structure of the molecules in the thin solid films, the OFETs performed excellent ambipolar operating properties, with the electron and hole mobility in air as high as 1.7×10−1 and 2.3×10−4cm2V−1s−1, respectively. For comparison purpose, mononuclear compound Cu[Pc(COOC8H17)8] (2) was comparatively studied. The QLS thin solid films of this compound possess similar ordered structure with that of Cu2[Pc(COOC8H17)6]2 (1), but the OFETs based on the thin solid films of this compound can only show n-type properties under nitrogen atmosphere with an extremely small electron mobility of 1.6×10−4cm2V−1s−1. This result suggests that extension on the conjugation system of an aromatic compound with multiple electron withdrawing groups can tune the molecule into an air stable ambipolar organic semiconductor.

AC characterization of organic thin-film transistors with asymmetric gate-to-source and gate-to-drain overlaps

This paper presents S-parameter characterization and a corresponding physics-based small-signal equivalent circuit for organic thin-film transistors (OTFTs). Furthermore, the impact of misalignment between the source/drain contacts and the patterned gate on the dynamic TFT performance is explored and a simple method to estimate the misalignment from the measured S-parameters is proposed. An excellent fit between theoretical and experimental S-parameters is demonstrated. For this study, OTFTs based on the air-stable organic semiconductor dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT) having a channel length of 1μm and a gate-to-contact overlap of 5 or 20μm and being operated at a supply voltage of 3V are utilized. The intentional asymmetry between gate-to-source and gate-to-drain overlaps is precisely controlled by the use of high-resolution silicon stencil masks.

Conjugate acene fused buckybowls: evaluating their suitability for p-type, ambipolar and n-type air stable organic semiconductors

Elaborate and exhaustive first principles calculations were carried out to screen the novel properties of a series of acene fused buckybowls. The acene fused compounds exhibit hole transport property due to their higher electron injection and lower hole transport barrier relative to the work function potential of Au electrodes. The higher HOMO and lower LUMO energy levels suggest lower hole and electron injection barriers of F and CN substituted and boron doped bowls which indicates ambipolar property of these bowls. The dicyano substituted pentacene fused bowls show only electron transport property with lower LUMO (-4.26 eV to -4.27 eV) and higher HOMO (-5.56 eV to -5.90 eV) energy levels. High electron affinity (>2.80 eV) and low LUMO energy (< -4.00 eV) attributes air stability to these bowls. Curvature decreased frontier orbital energies and increased ionization energy and electron affinity of bowls. This study reveals substitution of electron withdrawing groups and boron doped acene fused bowls can be a prominent materials for ambipolar and electron transport organic semiconductors.

Synthesis and structure of α-substituted pentathienoacenes

Recent progress in thienoacene development has produced numerous high performance, air-stable organic semiconductor materials. Pentathienoacene (T5) thin film devices have shown poor performance despite promising computational studies, likely due to high mobility anisotropy and poor film morphology. 2,6-Bis-alkyl-pentathienoacene has been synthesized to enable solution-processing routes to better microstructures of T5-based devices. Soluble side groups are introduced to thieno[3,2-b]thiophene precursors through deprotonation at α-positions. Introduction of the sulfur bridge was achieved by Pd-catalyzed coupling reaction with bis(tri-n-butyltin)sulfide (Bu3SnSSnBu3), followed by final ring closure through oxidative coupling with CuCl2. This method achieves higher purity and higher yield than sulfide-quenched Li–Br exchange. UV-vis and fluorescence emission spectra show a bathochromic shift of ≈10 nm, indicating the introduction of alkyl chains decreases the HOMO–LUMO gap. X-ray analysis yields unit cells for 2,6-bis-octyl and 2,6-bis-dodecyl substituted T5s (C8-T5 and C12-T5, respectively). C8-T5 grows orthorhombic crystals with lattice parameters a = 1.15 nm, b = 0.43 nm and c = 3.05 nm; C12-T5 grows monoclinic crystals (c-unique) with unit cell with parameters a = 1.10 nm, b = 0.42 nm, c = 3.89 nm and γ = 92.9°. Both materials grow large (>50 μm), faceted, single crystals that are microscopically composed of alternating layers of semiconducting cores and insulating substitutions.

Flexible Active-Matrix Organic Light-Emitting Diode Display Using Air-Stable Organic Semiconductor of $\hbox{Dinaphtho}[\hbox{2}, \hbox{3}\hbox{-b}: \hbox{2}^{ \prime}, \hbox{3}^{\prime} \hbox{-f}]\hbox{thieno}[\hbox{3}, \hbox{2} \hbox{-b}] \hbox{-} \hbox{thiophene}$

We developed a flexible active-matrix (AM) organic light-emitting diode (OLED) display driven by an organic thin-film transistor (TFT) (OTFT) using an air-stable organic semiconductor (OSC) of dinaphtho[2, 3 -b:2', 3' -f]thieno[ 3, 2 -b]-thiophene (DNTT) for the first time. We employed a low-temperature cross-linkable olefin-type polymer as the gate insulator (GI) and investigated the properties of the interface between DNTT and polymer GI. The top-contact TFT demonstrated a high mobility of up to 0.8 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /V·s and a near-zero turn-on voltage. DNTT has a deeper highest occupied molecular orbital level than other OSC materials, and this leads to large contact resistance between the source/drain (S/D) contact and OSC. Surface modifications to the S/D contact were investigated to enable efficient carrier injection to fabricate high-performance bottom-contact TFTs. The short-channel TFT we fabricated exhibited a high hole mobility of 0.5 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /V·s, a low subthreshold slope of 0.31, and excellent environmental and operational stability. The DNTT-based TFT also demonstrated good applications to processes with less deterioration. Finally, a 5-in flexible OLED display was successively fabricated by integrating the AM backplane with a phosphorescent OLED device. A high luminescence over 300 cd/ m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> was achieved by driving the DNTT-based OTFTs.

Band-like transport in solution-crystallized organic transistors

Patternable solution-crystallized organic transistors are developed with high carrier mobility that exceeds 10 cm2/V. We have investigated techniques of crystallization during the fabrication of organic semiconductor thin films from solution. Regulating direction of the crystal growth in the process, we successfully formed crystalline films with the mobility as high as 10–12 cm2/V from newly developed air-stable organic semiconductor compounds of 2,7-dialkyl[1]benzothieno[3,2-b]benzothiophene (Cn-BTBT) and 2,9-dialkyl-dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (Cn-DNTT). Furthermore, comprehensive measurements of Hall-effect and temperature dependent mobility for solution-crystallized single-crystal transistors tell us that their fundamental charge-transport mechanism is band transport.