Guy Ting

Surface Doping of Conjugated Polymers by Graphene Oxide and Its Application for Organic Electronic Devices

Conjugated polymers are a novel class of solution-processable semiconducting materials with intriguing optoelectronic properties. [ 1 ] They have received great attention as active components in organic electronic devices such as organic photovoltaic cells (OPVs), organic light-emitting diodes (OLEDs), and organic fi eld-effect transistors (OFETs) due to their light weight, facile tuning of electronic properties through molecular engineering, and ease of processing. The performance and lifetime of conjugated polymer-based electronic devices are critically dependent on the bulk properties of the active materials and the interfacial properties of electrode/polymer contacts. [ 2–4 ] In these devices, the electrode(s) either inject charge into or extract charges from the organic semiconductor layer(s). Mismatch of the work functions between metal or metal oxide electrodes and molecular orbital energy levels of organic semiconductors can lead to high contact resistance, which decreases the charge injection and extraction effi ciency. Therefore, it is essential to minimize contact resistance at the electrode/organic semiconductor interface. To improve charge injection/extraction across the electrode/ organic semiconductor interface, several strategies have been developed. One is to tune the interfacial dipole across the electrode/semiconductor interface to reduce the injection/collection energy barrier. This can be achieved by modifying the electrode surface with self-assembled dipolar molecules to tune the energy level alignment at the semiconductor/electrode interface. [ 5–7 ] Alternatively, the introduction of a thin layer of polymer surfactant that contains polar side chains between the conjugate polymer/electrode interface can also be used to improve the interfacial properties. The polar side chains can provide not

Low-voltage organic thin-film transistors with π-σ-phosphonic acid molecular dielectric monolayers

Pentacene-based organic thin-film transistors (OTFTs) have been fabricated using π-σ-phosphonic acid self-assembled monolayers (SAMs) on top of aluminum oxide as the gate dielectrics. With ultrathin dielectrics, high capacitances up to 760nF∕cm2 and low leakage current densities of 10−8A∕cm2 at 2V could be obtained, allowing operation of OTFTs within −3V. Vast improvements in the gate leakage current (∼2 orders), on/off current ratio (1 order), and subthreshold slope down to 85mV∕decade are achieved compared to control devices without SAMs. The OTFTs with pentacene vapor deposited at room temperature on SAM dielectrics-modified substrates exhibit mobilities of 0.14–0.30cm2∕Vs, on/off current ratios of 105, and threshold voltages of −(1.3–1.5)V.

Dielectric surface-controlled low-voltage organic transistors via n-alkyl phosphonic acid self-assembled monolayers on high-k metal oxide.

In this paper, we report on n-alkyl phosphonic acid (PA) self-assembled monolayer (SAM)/hafnium oxide (HfO(2)) hybrid dielectrics utilizing the advantages of SAMs for control over the dielectric/semiconductor interface with those of high-k metal oxides for low-voltage organic thin film transistors (OTFTs). By systematically varying the number of carbon atoms of the n-alkyl PA SAM from six to eighteen on HfO(2) with stable and low leakage current density, we observe how the structural nature of the SAM affects the thin-film crystal structure and morphology, and subsequent device performance of low-voltage pentacene based OTFTs. We find that two primary structural factors of the SAM play a critical role in optimizing the device electrical characteristics, namely, the order/disorder of the SAM and its physical thickness. High saturation-field-effect mobilities result at a balance between disordered SAMs to promote large pentacene grains and thick SAMs to aid in physically buffering the charge carriers in pentacene from the adverse effects of the underlying high-k oxide. Employing the appropriate n-alkyl PA SAM/HfO(2) hybrid dielectrics, pentacene-based OTFTs operate under -2.0 V with low hysteresis, on-off current ratios above 1 x 10(6), threshold voltages below -0.6 V, subthreshold slopes as low as 100 mV dec(-1), and field-effect mobilities as high as 1.8 cm(2) V(-1) s(-1).

Spin-cast and patterned organophosphonate self-assembled monolayer dielectrics on metal-oxide-activated Si.

An efficient process is developed for modifying Si with self-assembled monolayers (SAMs) through in situ metal oxide surface activation and microcontact printing or spin-coating of phosphonic-acid-based molecules. The utility of this process is demonstrated by fabricating self-organized and solution-processed low-voltage organic thin-film transistors enabled by patterned and spin-cast phosphonate SAM/metal oxide hybrid dielectrics.

Study on the Formation of Self-Assembled Monolayers on Sol-Gel Processed Hafnium Oxide as Dielectric Layers

High dielectric constant (k) metal oxides such as hafnium oxide (HfO2) have gained significant interest due to their applications in microelectronics. In order to study and control the surface properties of hafnium oxide, self-assembled monolayers (SAMs) of four different long aliphatic molecules with binding groups of phosphonic acid, carboxylic acid, and catechol were formed and characterized. Surface modification was performed to improve the interface between metal oxide and top deposited materials as well as to create suitable dielectric properties, that is, leakage current and capacitance densities, which are important in organic thin film transistors. Attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy, contact angle goniometry, atomic force microscopy (AFM), and simple metal-HfO2-SAM-metal devices were used to characterize the surfaces before and after SAM modification on sol-gel processed hafnium oxide. The alkylphosphonic acid provided the best monolayer formation on sol-gel processed hafnium oxide to generate a well-packed, ultrathin dielectric exhibiting a low leakage current density of 2x10(-8) A/cm2 at an applied voltage of -2.0 V and high capacitance density of 0.55 microF/cm2 at 10 kHz. Dialkylcatechol showed similar characteristics and the potential for using the catechol SAMs to modify HfO2 surfaces. In addition, the integration of this alkylphosphonic acid SAM/hafnium oxide hybrid dielectric into pentacene-based thin film transistors yields low-voltage operation within 1.5 V and improved performance over bare hafnium oxide.

Phosphonic acid self-assembled monolayer and amorphous hafnium oxide hybrid dielectric for high performance polymer thin film transistors on plastic substrates

A vacuum-free solution processed hybrid dielectric composed of an n-octadecyl-phosphonic acid self-assembled monolayer on amorphous sol-gel processed hafnium oxide (HfOx) is demonstrated for low-voltage polymer semiconductor-based thin film transistors (TFTs). The phosphonic acid/HfOx hybrid dielectric provides high capacitance (0.41 μF/cm2), low leakage current (5×10−8 A/cm2), and is compatible with plastic substrates. The utility of this dielectric is demonstrated by fabricating high performance polymer TFTs based on a spin coated thiophene-thiazolothiazole copolymer with operating voltages under −2 V, negligible hysteresis, subthreshold slopes as low as 100 mV/dec, and hole mobilities up to 0.11 cm2 V s.

Low-voltage high-performance C60 thin film transistors via low-surface-energy phosphonic acid monolayer/hafnium oxide hybrid dielectric

C60-based organic thin film transistors (OTFTs) have been fabricated using a n-octadecylphosphonic acid self-assembled monolayer/sol-gel processed hafnium oxide hybrid dielectric. With the combination of high capacitance (580 nF/cm2) and low leakage current density (8×10−9 A/cm2), this hybrid dielectric yields C60 OTFTs operating under 1.5 V with an average n-channel saturation field-effect mobility of 0.28 cm2/V s, high on-off current ratio of 105, and low subthreshold slope of 100 mV/decade. The low surface energy of the n-octadecylphosphonic acid allows C60 to form a thin film with large grains that provide an efficient charge carrier pathway for the low-voltage OTFTs.

π-σ-Phosphonic acid organic monolayer–amorphous sol–gel hafnium oxide hybrid dielectric for low-voltage organic transistors on plastic

A vacuum-free solution processed hybrid dielectric composed of an anthryl-alkyl-phosphonic acid (π-σ-PA) self-assembled monolayer on an amorphous sol–gel processed hafnium oxide (HfOx) is demonstrated for low-voltage organic thin film transistors (OTFTs) on plastic substrates. The π-σ-PA/HfOx hybrid dielectric provides high capacitance (0.54 µF cm−2) and low leakage current (2 × 10−8 A cm−2), and has a chemically and electrically compatible dielectric interface for evaporated and solution processed acene semiconductors. The utility of this dielectric is demonstrated by fabricating pentacene and 6,13-bis(triisopropyl-silylethynyl) pentacene (TIPS-PEN) based OTFTs with operating voltages under 2 V, subthreshold slopes as low as 100 mV dec−1, and average mobilities of 0.32 cm2 V−1 s−1 and 0.38 cm2 V−1 s−1, for pentacene and TIPS-PEN, respectively.

Low-voltage high-performance organic thin film transistors with a thermally annealed polystyrene/hafnium oxide dielectric

Low-voltage pentacene-based organic thin film transistors (OTFTs) are demonstrated with polystyrene (PS)/hafnium oxide (HfOx) hybrid dielectrics. Thermal annealing of PS films on HfOx at 120 °C (PS-120) induces a flatter orientation of the phenyl groups (tilt angle 65°) at the surface compared to PS films without annealing (PS-RT) (tilt angle 31°). The flatter phenyl group orientation leads to better matching of surface energy between pentacene and PS. Pentacene deposited on PS-120 display higher quality thin films with larger grain sizes and higher crystallinity. Pentacene OTFTs with PS-120/HfOx hybrid dielectrics can operate at low-voltage (<3 V) with high field-effect mobilities (1 cm2/V s), high on/off current ratios (106), and low subthreshold slopes (100 mV/dec).