Efficient and bright polymer light emitting field effect transistors

Light emitting field effect transistors (LEFETs) are emerging as a multi-functional class of optoelectronic devices. LEFETs can simultaneously execute light emission and the standard logic functions of a transistor in a single architecture. However, current LEFET architectures deliver either high brightness or high efficiency but not both concurrently, thus limiting their use in technological applications. Here we show an LEFET device strategy that simultaneously improves brightness and efficiency. The key step change in LEFET performance arises from the bottom gate top-contact device architecture in which the source/drain electrodes are semitransparent and the active channel contains a bi-layer comprising of a high mobility charge-transporting polymer, and a yellow–green emissive polymer. A record external quantum efficiency (EQE) of 2.1% at 1000 cd/m² is demonstrated for polymer based bilayer LEFETs.     

Integration of a Rib Waveguide Distributed Feedback Structure into a Light‐Emitting Polymer Field‐Effect Transistor

Ambipolar light‐emitting organic field‐effect transistors (LEFETs) possess the ability to efficiently emit light due to charge recombination in the channel. Since the emission can be made to occur far from the metal electrodes, the LEFET structure has been proposed as a potential architecture for electrically pumped organic lasers. Here, a rib waveguide distributed feedback structure consisting of tantalum pentoxide (Ta₂O₅) integrated within the channel of a top gate/bottom contact LEFET based on poly(9,9‐dioctylfluorene‐alt‐benzothiadiazole) (F8BT) is demonstrated. The emitted light is coupled efficiently into the resonant mode of the DFB waveguide when the recombination zone of the LEFET is placed directly above the waveguide ridge. This architecture provides strong mode confinement in two dimensions. Mode simulations are used to optimize the dielectric thickness and gate electrode material. It is shown that electrode absorption losses within the device can be eliminated and that the lasing threshold for optical pumping of the LEFET structure with all electrodes (4.5 µJ cm^?2) is as low as that of reference devices without electrodes. These results enable quantitative judgement of the prospects for realizing an electrically pumped organic laser based on ambipolar LEFETs. The proposed device provides a powerful, low‐loss architecture for integrating high‐performance ambipolar organic semiconductor materials into electrically pumped lasing structures.     

Charge transport and recombination in heterostructure organic light emitting transistors

Light-emitting field effect transistors (LEFETs) are a class of organic optoelectronic device capable of simultaneously delivering the electrical switching characteristics of a transistor and the light emission of a diode. We report on the temperature dependence of the charge transport and emissive properties in a model organic heterostructure LEFET system from 300 K to 135 K. We study parameters such as carrier mobility, brightness, and external quantum efficiency (EQE), and observe clear thermally activated behaviour for transport and injection. Overall, the EQE increases with decreasing temperature and conversely the brightness decreases. These contrary effects can be explained by a higher recombination efficiency occurring at lower temperatures, and this insight delivers new knowledge concerning the optimisation of both the transport and emissive properties in LEFETs.     

Device Physics of White Polymer Light‐Emitting Diodes

The charge transport and recombination in white‐emitting polymer light‐ emitting diodes (PLEDs) are studied. The PLED investigated has a single emissive layer consisting of a copolymer in which a green and red dye are incorporated in a blue backbone. From single‐carrier devices the effect of the green‐ and red‐emitting dyes on the hole and electron transport is determined. The red dye acts as a deep electron trap thereby strongly reducing the electron transport. By incorporating trap‐assisted recombination for the red emission and bimolecular Langevin recombination for the blue emission, the current and light output of the white PLED can be consistently described. The color shift of single‐layer white‐emitting PLEDs can be explained by the different voltage dependencies of trap‐assisted and bimolecular recombination.     

Unlocking the full potential of light emitting field-effect transistors by engineering charge injection layers

Light emitting field-effect transistors (LEFETs) are a class of next generation devices which combine the switching properties of field-effect transistors (FETs) with light emitting capabilities of organic light-emitting diodes (OLEDs) in a single device architecture. Current LEFET architectures suffer from inefficient charge injection of electrons and holes from the source and drain electrodes, leading to unbalanced charge transport and hence poor device performance. Here we report a simple fabrication method for LEFETs that delivers asymmetric source and drain electrodes comprised of low and high work function materials. The interdigitated low and high work function source–drain electrodes consist of combinations of organic materials, salts, metal oxides and metals. Using this method we were able to obtain a maximum EQE of up to 1.2% in a single layer device with Super Yellow as the active material.     

Impact ionization and light emission in AlGaAs/GaAs HEMT's

Impact ionization and light emission phenomena have been studied in AlGaAs/GaAs HEMTs biased at high drain voltages by measuring the gate excess current due to holes generated by impact ionization and by analyzing the energy distribution of the light emitted from devices in the 1.1-3.1 eV energy range. The emitted spectra in this energy range can be divided into three energy regions: (i) around 1.4 eV light emission is dominated by band-to-band recombination between cold electrons and holes in GaAs; (ii) in the energy range from 1.5 to 2.6 eV energy distribution of the emitted photons is approximately Maxwellian; and (iii) beyond 2.6 eV the spectra are markedly distorted due to light absorption in the n⁺ GaAs cap layer. The integrated intensity of photons with energies larger than 1.7 eV is proportional to the product of the drain and gate currents. This suggests recombination of channel electrons with holes generated by impact ionization as the dominant emission mechanism of visible light     

Electron–Hole Recombination in Uniaxially Aligned Semiconducting Polymers

A promising, general strategy for improving performance of optoelectronic devices based on conjugated polymer semiconductors is to make better use of the fast intrachain transport along the covalently bonded polymer backbone. Little is known, however, about how the recombination rate between electrons and holes would be affected in device structures in which current flow is primarily along the polymer chain. Here a light‐emitting field effect transistor (LFET) structure with a uniaxially aligned semiconducting polymer is used to show that the width and shape of the recombination zone depend strongly on polymer alignment. For alignment of the polymer parallel to the current the emission zone is 5–10 times wider than for perpendicular alignment. 2D drift‐diffusion modeling is used to show that such significant widening of the recombination zone in the case of parallel alignment implies that the recombination rate constant is more than 100 times lower than expected for standard Langevin recombination. On the basis of Monte Carlo modeling it is proposed that such unexpected weak recombination is a result of the significant mobility anisotropy of the aligned polymer. These results provide new fundamental insight into the recombination physics of polymer semiconductors.     

Effects of substrate topography on current injection and light emission properties of organic light emitting devices

Substrate topography plays a critical role in the function of nano-scale materials and devices. We study small molecular organic light emitting devices (OLEDs) deposited onto non-planar substrates, where the substrate’s radius of curvature in some regions approaches the thickness of the active device layers. As a result, the electric field profile inside the organic charge transport layers is modified, influencing carrier injection, transport, and light emission properties. Experiments and numerical modeling suggest that charge balance and electroluminescence efficiency potentially can be improved in electron injection-limited OLED architectures via substrate geometry. These findings elucidate the optoelectronic behavior (and degradation) of OLEDs on imperfect substrates, and suggest a strategy based on substrate topography for controlling device behavior.     

Ambipolar Light‐Emitting Transistors of a Tetracene Single Crystal

The first ambipolar light‐emitting transistor of an organic molecular semiconductor single crystal, tetracene, is demonstrated. In the device configuration, electrons and holes injected from separate magnesium and gold electrodes recombined radiatively within the channel. By varying the applied voltages, the position of the recombination/emission zone could be moved to any position along the channel. Because of the changes made to the device structure, including the use of single crystals and polymer dielectric layers and the adoption of an inert‐atmosphere fabrication process, the set of materials that can be used for light‐emitting transistors has been expanded to include monomeric molecular semiconductors.     

Simultaneous Tenfold Brightness Enhancement and Emitted‐Light Spectral Tunability in Transparent Ambipolar Organic Light‐Emitting Transistor by Integration of High‐k Photonic Crystal

In organic light‐emitting transistors, the structural properties such as the in‐plane geometry and the lateral charge injection are the key elements that enable the monolithic integration of multiple electronic, optoelectronic, and photonic functions within the same device. Here, the realization of highly integrated multifunctional optoelectronic organic device is reported by introducing a high‐capacitance photonic crystal as a gate dielectric into a transparent single‐layer ambipolar organic light‐emitting transistor (OLET). By engineering the photonic crystal multistack and bandgap, it is showed that the integration of the photonic structure has a twofold effect on the optoelectronic performance of the device, i.e., i) to modulate the spectral profile and outcoupling of the emitted light and ii) to enhance the transistor source–drain current by a 25‐fold factor. Consequently, the photonic‐crystal‐integrated OLET shows an order of magnitude higher emitted power and brightness with respect to the corresponding polymer‐dielectric device, while presenting as‐designed electroluminescence spectral and spatial distribution. The results validate the efficacy of the proposed approach that is expected to unravel the technological potential for the realization of highly integrated optoelectronic smart systems based on organic light‐emitting transistors.     

Charge trapping in organic transistor memories: On the role of electrons and holes

In this work, we study charge trapping in organic transistor memories with a polymeric insulator as gate dielectric. We found that the mechanism of charge trapping is tunneling from the semiconductor channel into the gate dielectric. Depending on the semiconductor and its processing, charge trapping can result in large bi-directional threshold voltage shifts, in case the semiconductor is ambipolar, or in shifts in only one direction (unipolar semiconductor). These results indicate that optimal memory performance requires charge carriers of both polarities, because the most efficient method to lower the programming field is by overwriting a trapped charge by an injected charge of opposite polarity.     

TFT-LCD高温光照漏电流改善研究

造成TFT不稳定的问题点一般认为有两种:一是沟道内半导体材料内部的缺陷,另一个是栅极绝缘层内的或是绝缘层与沟道层界面的电荷陷阱。TFT-LCD在长期运行时由于高温及光照的影响会导致漏电流增加,进而对TFT造成破坏。分析研究表明,TFT沟道在刻蚀完成后,沟道内部存在一定的缺陷以及绝缘层与沟道层界面存在电荷陷阱,平面电场宽视角核心技术-高级超维场转换技术型产品由于设计的原因面临着如果进行氢处理会导致与其与氧化铟锡中的铟发生置换反应,导致铟的析出,所以无法采用氢处理。理论分析表明Si-O键稳定,本文主要介绍通过氯气/氧气和六氟化硫/氧气对TFT沟道进行处理改善高温光照漏电流。结果表明,通过氯气/氧气和六氟化硫/氧气处理TFT沟道后,高温光照漏电流从18.19pA下降到5.1pA,可见氯气/氧气和六氟化硫/氧气对沟道处理可有效改善高温光照漏电流。     

Organic Light‐Emitting Diodes Based on Poly(9,9‐dioctylfluorene‐co‐bithiophene) (F8T2)

A study of the optical properties of poly(9,9‐dioctylfluorene‐co‐bithiophene) (F8T2) is reported, identifying this polymer as one that possesses a desirable combination of charge transport and light emission properties. The optical and morphological properties of a series of polymer blends with F8T2 dispersed in poly(9,9‐dioctylfluorene) (PFO) are described and almost pure‐green emission from light emitting diodes (LEDs) based thereon is demonstrated. High luminance green electroluminescence from LEDs using only a thin film of F8T2 for emission is also reported. The latter demonstration for a polymer previously primarily of interest for effective charge transport constitutes an important step in the development of emissive materials for applications where a union of efficient light emission and effective charge transport is required.     

Stabilized Blue Emission from Polymer–Dielectric Nanolayer Nanocomposites

Blue‐light‐emitting polymer (polyfluorene)/dielectric nanolayer nanocomposites were prepared by the solution intercalation method and employed in an electroluminescent (EL) device. Their photoluminescence (PL) and electroluminescence characteristics demonstrates that the interruption of interchain interaction in intercalated organic/inorganic hybrid systems reduces the low‐energy emission that results from keto‐defects. By reducing the probability that the excitons initially generated on the polyfluorenes will find keto‐defects, both the color purity and the luminescence stability were improved. Furthermore, the dielectric nanolayers have an aspect ratio of about five hundred, and therefore act as efficient exciton blocking layers and barriers to oxygen diffusion, producing a dramatic increase in the device stability. A nanocomposite device with a Li:Al alloy cathode gave a quantum efficiency of 1.0 %(ph/el), which corresponds to an approximate five times enhancement compared to the neat polymer device. The nanocomposite emitting layer is considered to have a pseudo‐multiple quantum well structure.     

Enabling High-Energy-Density High-Efficiency Ferroelectric Polymer Nanocomposites with Rationally Designed Nanofillers

Ferroelectric polymers have been regarded as the preferred matrix for high-energy-density dielectric polymer nanocomposites because of their highest dielectric constants among the known polymers. Despite a library of ferroelectric polymer-based composites having been demonstrated as highly efficient in enhancing the energy density, the charge–discharge efficiency remains moderate because of the high intrinsic loss of ferroelectric polymers. Herein, a systematic study of the oxide nanofillers is presented with varied dielectric constants and the vital role of the dielectric match between the filler and the polymer matrix on the capacitive performance of the ferroelectric polymer composites is revealed. A combined experimental and simulation study is further performed to specifically investigate the effect of the nanofiller morphology on the electrica properties of the polymer nanocomposites. The solution-processed ferroelectric polymer nanocomposite embedded with Al₂O₃ nanoplates exhibits markedly improved breakdown strength and discharged energy density along with an exceptional charge–discharge efficiency of 83.4% at 700 MV m⁻¹, which outperforms the ferroelectric polymers and nanocomposites reported to date. This work establishes a facile approach to high-performance ferroelectric polymer composites through capitalizing on the synergistic effect of the dielectric properties and morphology of the oxide fillers.     

Simulation of current-voltage characteristics of a ferroelectric field-effect transistor

Current-voltage (I-V) characteristics of an all-perovskite ferroelectric-semiconductor field-effect transistor (FET) were simulated. The modeling is based on an analysis of an experimental hysteresis loop of a metal-ferroelectric-metal structure. The charge in the semiconductor, electric fields in the semiconductor and ferroelectric (FE), and FE polarization at the FE-semiconductor interface are calculated at a given semiconductor surface potential. The Poisson equation is solved numerically across the FE thickness. The semiconductor surface potential, semiconductor charge, FE polarization, electric field and voltage drop in the FE are calculated as functions of the applied voltage. By using appropriate semiconductor thickness and built-in voltage between the FE and the gate, it is possible to provide a remanent polarization necessary for the opening and blocking of the FET channel in the ascending and descending portions of the hysteresis loop, respectively. The I-V characteristics and the voltage drop along the FET channel are calculated and analyzed for both polarities of the drain bias. The results make it possible to predict I-V characteristics of an all-perovskite ferroelectric FET.     

OLED器件电荷注入模型的MATLAB分析计算

介绍了电荷从OLED金属电极注入到一个随机跳跃体系（例如配对聚合物或分子掺杂聚合物）的模型。它包括电荷载流子从电极的费米能级到介质跳跃态分布尾态之间的注入,其结果可能是电荷载流子返回到电极,或者是受到镜像引力势的作用而形成扩散逃逸。后者类似于一维Onsager型激子解离。该模型将注入电流处理为电场、温度以及跳跃态分布能量宽度的函数。提出了OLED器件中相对电流密度的概念。介绍了用MATLAB软件分析计算该模型的方法和技巧。     

Dynamic Processes in Sandwich Polymer Light‐Emitting Electrochemical Cells

The operational mechanism of polymer light‐emitting electrochemical cells (LECs) in sandwich geometry is studied by admittance spectroscopy in combination with numerical modeling. At bias voltages below the bandgap of the semiconducting polymer, this allows the determination of the dielectric constant of the active layer, the conductivity of mobile ions, and the thickness of the electric double layers. At bias voltages above the bandgap, p–n junction formation gives rise to an increase in capacitance at intermediate frequencies (≈10 kHz). The time and voltage dependence of this junction are successfully studied and modeled. It is shown that impedance measurements cannot be used to determine the junction width. Instead, the capacitance at intermediate biases corresponds to a low‐conductivity region that can be significantly wider than the recombination zone. Finally, the long settling time of sandwich polymer LECs is shown to be due to a slow process of dissociation of salt molecules that continues after the light‐emitting p–n junction has formed. This implies that in order to significantly decrease the response‐time of LECs an electrolyte/salt combination with a minimal ion binding energy must be used.     

Non‐Volatile Organic Memory Elements Based on Carbon‐Nanotube‐Enabled Vertical Field‐Effect Transistors

High‐performance non‐volatile memory elements based on carbon‐nanotube‐enabled vertical field‐effect transistors (CN‐VFETs) are demonstrated. A thin crosslinking polymer layer, benzocyclobutene (BCB), on top of the gate dielectric acts as the charge storage layer. This results in a large, fully gate sweep programmable, hysteresis in the cyclic transfer curves exhibiting on/off ratios >4 orders of magnitude. The carbon nanotube random network source electrode facilitates charge injection into the charge storage layer, realizing the strong memory effect without sacrificing mobility in the vertical channel. Given their intrinsically simple fabrication and compact size CN‐VFETs could provide a path to cost‐effective, high‐density organic memory devices.     

Monitoring the Channel Formation in Organic Field‐Effect Transistors via Photoinduced Charge Transfer

Conducting channel formation in organic field‐effect transistors (OFETs) is considered to happen in the organic semiconductor layer very close to the interface with the gate dielectric. In the gradual channel approximation, the local density of accumulated charge carriers varies as a result of applied gate bias, with the majority of the charge carriers being localized in the first few semiconductor monolayers close to the dielectric interface. In this report, a new concept is employed which enables the accumulation of charge carriers in the channel by photoinduced charge transfer. An OFET employing C₆₀ as a semiconductor and divinyltetramethyldisiloxane‐bis(benzocyclobutene) as the gate dielectric is modified by a very thin noncontinuous layer of zinc‐phthalocyanine (ZnPc) at the semiconductor/dielectric interface. With this device geometry, it is possible to excite the phthalocyanine selectively and photogenerate charges directly at the semiconductor/dielectric interface via photoinduced electron transfer from ZnPc onto C₆₀. Thus the formation of a gate induced and a photoinduced channel in the same device can be correlated.