Bias-dependent generation and quenching of defects in pentacene

Effects of Charge Traps on Hysteresis in Organic Field-Effect Transistors and Their Charge Trap Cause Analysis through Causal Inference Techniques

Hysteresis in organic field-effect transistors is attributed to the well-known bias stress effects. This is a phenomenon in which the measured drain-source current varies when sweeping the gate voltage from on to off or from off to on. Hysteresis is caused by various factors, and one of the most common is charge trapping. A charge trap is a defect that occurs in an interface state or part of a semiconductor, and it refers to an electronic state that appears distributed in the semiconductor's energy band gap. Extensive research has been conducted recently on obtaining a better understanding of charge traps for hysteresis. However, it is still difficult to accurately measure or characterize them, and their effects on the hysteresis of organic transistors remain largely unknown. In this study, we conduct a literature survey on the hysteresis caused by charge traps from various perspectives. We first analyze the driving principle of organic transistors and introduce various types of hysteresis. Subsequently, we analyze charge traps and determine their influence on hysteresis. In particular, we analyze various estimation models for the traps and the dynamics of the hysteresis generated through these traps. Lastly, we conclude this study by explaining the causal inference approach, which is a machine learning technique typically used for current data analysis, and its implementation for the quantitative analysis of the causal relationship between the hysteresis and the traps.

Suppressing bias stress degradation in high performance solution processed organic transistors operating in air

Solution processed organic field effect transistors can become ubiquitous in flexible optoelectronics. While progress in material and device design has been astonishing, low environmental and operational stabilities remain longstanding problems obstructing their immediate deployment in real world applications. Here, we introduce a strategy to identify the most probable and severe degradation pathways in organic transistors and then implement a method to eliminate the main sources of instabilities. Real time monitoring of the energetic distribution and transformation of electronic trap states during device operation, in conjunction with simulations, revealed the nature of traps responsible for performance degradation. With this information, we designed the most efficient encapsulation strategy for each device type, which resulted in fabrication of high performance, environmentally and operationally stable small molecule and polymeric transistors with consistent mobility and unparalleled threshold voltage shifts as low as 0.1 V under the application of high bias stress in air.

Electrical instability of organic field-effect transistors (OFETs) during operation remains a challenge that limits the device’s real-world technological viability. Here, the authors report a method for diagnosing and suppressing bias stress in solution-processed OFETs operated in air.

Fabrication of ring oscillators using organic molecules of phenacene and perylenedicarboximide

Organic field-effect transistors (FETs) can be applied to radio-frequency identification tags (RFIDs) and active-matrix flat-panel displays. For RFID application, a cardinal functional block is a ring oscillator using an odd number of inverters to convert DC voltage to AC. Herein, we report the properties of two ring oscillators, one formed with [6]phenacene for a p-channel FET and N,N′-dioctyl-3,4,9,10-perylenedicarboximide (PTCDI-C8) for an n-channel FET, and one formed with 3,10-ditetradecylpicene ((C₁₄H₂₉)₂-picene) for a p-channel FET and PTCDI-C8 for an n-channel FET. The former ring oscillator provided a maximum oscillation frequency, f_osc of 26 Hz, and the latter a maximum f_osc of 21 Hz. The drain–drain voltage, V_DD, applied to these ring oscillators was 100 V. This may be the first step towards a future practical ring oscillator using phenacene molecules. The values of field-effect mobility, μ in the p-channel [6]phenacene FET and n-channel PTCDI-C8 FET, which form the building blocks in the ring oscillator with an f_osc value of 26 Hz, are 1.19 and 1.50 × 10⁻¹ cm² V⁻¹ s⁻¹, respectively, while the values in the p-channel (C₁₄H₂₉)₂-picene FET and n-channel PTCDI-C8 FET, which form the ring oscillator with an f_osc of 21 Hz, are 1.85 and 1.54 × 10⁻¹ cm² V⁻¹ s⁻¹, respectively. The μ values in the p-channel FETs are higher by one order of magnitude than those of the n-channel FET, which must be addressed to increase the value of f_osc. Finally, we fabricated a ring oscillator with ZrO₂ instead of parylene for the gate dielectric, which provided the low-voltage operation of the ring oscillator, in which [6]phenacene and PTCDI-C8 thin-film FETs were employed. The value of f_osc obtained in the ring oscillator was 24 Hz. In this ring oscillator, the V_DD value applied was limited to 20 V. The durability of the ring oscillators was also investigated, and the bias stress effect on the f_osc and the amplitude of the output voltage, V_out are discussed. This successful operation of ring oscillators represents an important step towards the realization of future practical integrated logic gate circuits using phenacene molecules.

A ring oscillator consisting of p-channel and n-channel organic FETs.

Molecular Semiconductors for Logic Operations: Dead-End or Bright Future?

The field of organic electronics has been prolific in the last couple of years, leading to the design and synthesis of several molecular semiconductors presenting a mobility in excess of 10 cm² V^-1 s^-1 . However, it is also started to recently falter, as a result of doubtful mobility extractions and reduced industrial interest. This critical review addresses the community of chemists and materials scientists to share with it a critical analysis of the best performing molecular semiconductors and of the inherent charge transport physics that takes place in them. The goal is to inspire chemists and materials scientists and to give them hope that the field of molecular semiconductors for logic operations is not engaged into a dead end. To the contrary, it offers plenty of research opportunities in materials chemistry.

Organic semiconductor crystals

A comprehensive overview of organic semiconductor crystals is provided, including the physicochemical features, the control of crystallization and the device physics.

Dynamic Charge Carrier Trapping in Quantum Dot Field Effect Transistors

Noncrystalline semiconductor materials often exhibit hysteresis in charge transport measurements whose mechanism is largely unknown. Here we study the dynamics of charge injection and transport in PbS quantum dot (QD) monolayers in a field effect transistor (FET). Using Kelvin probe force microscopy, we measured the temporal response of the QDs as the channel material in a FET following step function changes of gate bias. The measurements reveal an exponential decay of mobile carrier density with time constants of 3-5 s for holes and ∼10 s for electrons. An Ohmic behavior, with uniform carrier density, was observed along the channel during the injection and transport processes. These slow, uniform carrier trapping processes are reversible, with time constants that depend critically on the gas environment. We propose that the underlying mechanism is some reversible electrochemical process involving dissociation and diffusion of water and/or oxygen related species. These trapping processes are dynamically activated by the injected charges, in contrast with static electronic traps whose presence is independent of the charge state. Understanding and controlling these processes is important for improving the performance of electronic, optoelectronic, and memory devices based on disordered semiconductors.

25th anniversary article: microstructure dependent bias stability of organic transistors

Recent studies of the bias-stress-driven electrical instability of organic field-effect transistors (OFETs) are reviewed. OFETs are operated under continuous gate and source/drain biases and these bias stresses degrade device performance. The principles underlying this bias instability are discussed, particularly the mechanisms of charge trapping. There are three main charge-trapping sites: the semiconductor, the dielectric, and the semiconductor-dielectric interface. The charge-trapping phenomena in these three regions are analyzed with special attention to the microstructural dependence of bias instability. Finally, possibilities for future research in this field are presented. This critical review aims to enhance our insight into bias-stress-induced charge trapping in OFETs with the aim of minimizing operational instability.

Mapping of trap densities and hotspots in pentacene thin-film transistors by frequency-resolved scanning photoresponse microscopy

Frequency-resolved scanning photoresponse microscopy of pentacene thin-film transistors is reported. The photoresponse pattern maps the in-plane distribution of trap states which is superimposed by the level of trap filling adjusted by the gate voltage of the transistor. Local hotspots in the photoresponse map thus indicate areas of high trap densities within the pentacene thin film.

Spectroscopic characterization of charged defects in polycrystalline pentacene by time- and wavelength-resolved electric force microscopy

Spatial maps of topography and trapped charge are acquired for polycrystalline pentacene thin-film transistors using electric and atomic force microscopy. In regions of trapped charge, the rate of trap clearing is studied as a function of the wavelength of incident radiation.

Light-assisted deep-trapping of holes in conjugated polymers

The injection of positive charge carriers (holes) into a single conjugated polymer chain was observed to be light-assisted. This effect may underlie critical, poorly understood organic electronic device phenomena such as the build-up of functional deeply trapped charge layers in polymer light emitting diodes. The charging/discharging dynamics were investigated indirectly by a variety of single molecule electro-optical spectroscopic techniques, including an "image-capture" approach.

Charging and discharging of single conjugated-polymer nanoparticles

Despite intense, long-term interest in organic semiconductors from both an applied and fundamental perspective, key aspects of the electronic properties of these materials remain poorly defined. A particularly challenging problem is the molecular nature of positive charge carriers, that is, holes or oxidized species in organics. Here, the unique ability of single-molecule spectroelectrochemistry (SMS-EC) to unravel complex electrochemical process in heterogeneous media is used to study the oxidation of nanoparticles of the conjugated polymer poly(9,9-dioctylfluorene-co-benzothiadiazole). A reversible hole-injection charging process has been observed that occurs primarily by initial injection of shallow (untrapped) holes, but soon after the injection, a small fraction of the holes becomes deeply trapped. Good agreement between experimental data and simulations strongly supports the presence of deep traps in the studied nanoparticles and highlights the ability of SMS-EC to study energetics and dynamics of deep traps in organic materials at the nanoscale.

Time-Resolved Electric Force Microscopy of Charge Trapping in Polycrystalline Pentacene

Here we introduce time-resolved electric force microscopy measurements to directly and locally probe the kinetics of charge trap formation in a polycrystalline pentacene thin-film transistor. We find that the trapping rate depends strongly on the initial concentration of free holes and that trapped charge is highly localized. The observed dependence of trapping rate on the hole chemical potential suggests that the trapping process should not be viewed as a filling of midgap energy levels, but instead as a process in which the very creation of trapped states requires the presence of free holes.

Mechanism of carrier photogeneration and carrier transport in molecular crystal tetracene

Models for the carrier photoexcitation mechanism in molecular crystals have been established initially on the bases of measurements on oligoacenes and later applied to conjugated polymers as well. These models emphasize the localized nature of photoexcitations and describe carrier generation as a secondary process involving exciton dissociation. The results of our photoconductivity studies of single crystal tetracene are at variance with these widely accepted models, and in fact indicate that the photocarrier quantum efficiency appears independent of temperature, photon energy, and light intensity, thus featuring the hallmarks of direct interband carrier photogeneration and coherent carrier transport at band states.

Transferring Self-Assembled, Nanoscale Cables into Electrical Devices

This study details a new derivative of the contorted HBCs that self-organizes into one-dimensional, single-crystalline fibers. X-ray diffraction, transmission electron microscopy, and electron diffraction studies show that they have an orthorhombic unit cell with dimensions of 5.8 nm x 4.5 nm x 0.45 nm. Each fiber is composed of a few thousands columns. A method is put forth that utilizes elastomer stamps to manipulate and position isolated fibers in organic field effect transistors.

Voltage-induced metal-insulator transition in polythiophene field-effect transistors

We have extensively studied the carrier transport in regio-regular polythiophene field-effect transistors (FETs) from room temperature to 4.2 K. At low temperatures, Zabrodskii plots (dlnsigma/dlnT) demonstrate that the gate voltage and source-drain voltage combine to induce the insulator-to-metal transition at a carrier density of 5x10(12) cm-2. The carrier transport in the insulating regime is well described by phonon assisted hopping in a disordered Fermi glass with Coulomb interaction between the hopping charge carrier and the opposite charge left behind, as described by Efros and Shklovskii.

Space-charge-limited current fluctuations in organic semiconductors

Low-frequency current fluctuations are investigated over a bias range covering Ohmic, trap-filling, and space-charge-limited current regimes in polycrystalline polyacenes. The relative current noise power spectral density S(f) is constant in the Ohmic region, steeply increases at the trap-filling transition region, and decreases in the space-charge-limited-current region. The noise peak at the trap-filling transition is accounted for within a continuum percolation model. As the quasi-Fermi level crosses the trap level, intricate insulating paths nucleate within the Ohmic matrix, determining the onset of nonequilibrium conditions at the interface between the insulating and conducting phase. The noise peak is written in terms of the free and trapped charge carrier densities.

Amorphouslike density of gap states in single-crystal pentacene

We show that optical and electrical measurements on pentacene single crystals can be used to extract the density of states in the highest occupied molecular orbital-lowest unoccupied molecular orbital band gap. It is found that these highly purified crystals possess band tails broader than those typically observed in inorganic amorphous solids. Results on field-effect transistors fabricated from similar crystals imply that the gap state density is much larger within 5-10 nm of the gate dielectric. Thus, organic thin-film transistors for such applications as flexible displays might be significantly improved by reducing these defects.