Interfacial charges and electric field distribution in organic hetero-layer light-emitting devices

The electric field distribution in organic hetero-layer light-emitting devices based on N,N^′-diphenyl-N,N^′-bis(1-naphtyl)-1,1^′-biphenyl-4,4^′-diamine (NPB) and 8-tris-hydroxyquinoline aluminium (Alq₃) has been investigated under different bias conditions using capacitance–voltage measurements. Although this method yields primarily information on the differential capacitance, the data give clear evidence for the presence of negative interfacial charges with a density of 6.8×10¹¹e cm⁻² at the NPB/Alq₃ interface at large reverse bias. This leads to a jump of the electric field at the interface and a non-uniform field distribution in the hetero-layer device.     

Gate charge behaviors in N-channel power VDMOSFETs during HEF and PBT stresses

This paper reports the effects of high electric field stress (HEFS) and positive bias temperature instability (PBTI) in threshold voltage, input and Miller capacitances of N⁻channel power VDMOSFETs. The procedure used for this study is based on the analysis of the gate charge characteristics, the two-dimensional simulation of the structure, and the physical properties of the device. The gate charge characteristics investigated during and up to 500 h of HEFS and PBTI show some degradation of physical device properties. The results are analysed and parameters responsible of these degradations are extracted. It is shown that the main degradation issues in the Si power VDMOSFETs are the charge trapping and the trap creation at the interface of the gate dielectric, induced by energetic free carriers which have sufficient energy to cross the SiO₂/Si barrier.     

Dependence of carrier mobility on an electric field in gallium selenide crystals

The dependence of the mobility of charge carriers on voltage has been studied in undoped GaSe single crystals and crystals doped with gadolinium; the latter crystals have exhibited various values of dark resistivity (??_d.r ?? 10⁴?10⁸ ?? cm at 77 K) and of the doping level (N = 10^?5, 10^?4, 10^?3, 10^?2, and 10^?1 at %). It is established that the dependence of the charge-carrier mobility on the electric field applied to the sample E ?? 10² V/cm is observed in undoped high-resistivity GaSe crystals (??_d.r ?? 10⁴ ?? cm) and in lightly doped GaSe crystals (N ?? 10^?2 at %) in the region of T ?? 150 K. It is found that this dependence is not related to heating of the charge carriers by an electric field; rather, it is caused by elimination of drift barriers as a result of injection.     

Origin of high fill factor in polymer solar cells from semiconducting polymer with moderate charge carrier mobility

The fill factor of polymer bulk heterojunction solar cells (PSCs), which is mainly governed by the processes of charge carrier generation, recombination, transport and extraction, and the competition between them in the device, is one of the most important parameters that determine the power conversion efficiency of the device. We show that the fill factor of PSCs based on thieno[3,4-b]-thiophene/benzodithiophene (PTB7):[6,6]-phenyl C₇₁-butyric acid methylester (PC₇₁BM) blend that only have moderate carrier mobilities for hole and electron transport, can be enhanced to 76% by reducing the thickness of the photoactive layer. A drift–diffusion simulation study showed that reduced charge recombination loss is mainly responsible for the improvement of FF, as a result of manipulating spatial distribution of charge carrier in the photoactive layer. Furthermore, the reduction of the active layer thickness also leads to enhanced built-in electric field across the active layer, therefore can facilitate efficient charge carrier transport and extraction. Finally, the dependence of FF on charge carrier mobility and transport balance is also investigated theoretically, revealing that an ultrahigh FF of 80–82% is feasible if the charge mobility is high enough (∼10⁻³–10⁻¹ cm²/V s).     

Reliable extraction of organic solar cell parameters by combining steady-state and transient techniques

A method to extract reliable material and device parameters of organic solar cells is presented. We employ a comprehensive numerical device model to simulate the solar cell operation in transient and steady-state condition. Parameter extraction with numerical simulation is error-prone because model parameters are often correlated, their unique determination is very difficult and extracted parameters are likely to be inaccurate. We combine the current–voltage characteristics, the photo-CELIV currents (charge extraction with linearly increasing voltage) and the photocurrent response to a light pulse to reduce parameter correlation and increase accuracy and reliability of the extracted parameters. With a correlation matrix analysis it is shown that parameter correlation is significantly reduced when combining several experimental data sets compared with the analysis of current–voltage curves only. We find a set of parameters to reproduce the complete series of measurements with the numerical simulation. The full electrical behavior can be described using a basic drift–diffusion model with constant mobilities and direct photon-to-charge conversion. With this model we extract charge carrier mobilities in the order of 10⁻⁴ cm²/V s, a Langevin recombination prefactor of 0.08, charge injection barriers equal at both sides in the range of 0.25 eV and further device parameters for a BHJ cell with PT5DPP as donor and PCBM (C70) as acceptor. The solar cell is simulated with the extracted parameters and internal distribution of electrons, holes and the electric field are visualized.     

Investigation on hot-carrier-induced degradation of SOI NLIGBT

The hot-carrier-induced (HCI) degradations of silicon-on-insulator (SOI) lateral insulated gate N-type bipolar transistor (NLIGBT) are investigated in detail by DC voltage stress experiment, TCAD simulation and charge pumping test. The substrate current I_sub and on-state resistance Ron at different voltage stress conditions are measured to assess the HCI effect on device performance. The electric field and impact ionization rate are simulated to assist in providing better physical insights. And charge pumping current is measured to determinate the front-gate interface states density directly. The degradation mechanisms under different gate voltage stress conditions are then presented and summarized.     

Construction of Self‐Healing Internal Electric Field for Sustainably Enhanced Photocatalysis

The construction of internal electric field is generally considered an effective strategy to enhance photocatalytic performance due to its significant role in charge separation. However, static internal electric field is prone to be saturated either by inner or outer shield effect, and thus its effect on the improvement of photocatalysis can easily vanish. Here, the self‐healing internal electric field is proposed and successfully endowed to a designed helical structural composite microfiber polyvinylidene fluoride/g‐C₃N₄ (PVDF/g‐C₃N₄) based on the bioinspired simple harmonic vibration. Importantly, the saturation and recovery of internal electric field are characterized by transient photovoltage and photoluminescence. The results indicate that the internal electric field could be saturated within about 10 min and refreshed with the assistance of rebuilt piezoelectric potential. The lifetime of photogenerated carriers is about 10^?4 s and the number of effective carriers is greatly increased in the presence of self‐healing internal electric field. The results provide direct experimental evidence on the role of self‐healing internal electric field in charge transfer behavior. This work represents a new design strategy of photocatalysts, and it may open up new horizons for solving energy shortage and environmental issues.     

Silicon nitride for the improvement of silicon inversion layer solar cells

It is demonstrated that CVD Si-nitride films as transparent dielectric satisfy all requirements for achieving MIS/IL solar cells with high efficiency and long term stability. Deposited on silicon by the SiH₄/NH₃ reaction at temperatures lower than usual (between 600° and 650°C) fixed positive interface charge densities Q_N/q up to 7 × 10¹² cm^?2 with excellent stability have been obtained. Utilizing the Si-nitride charge storage effect, the highest known Q_N/q values (> 10¹³cm^?2) combined with low values of N_it have been achieved. The charge distribution is discussed and an energy band diagram modified according to new analytical results is presented. MIS/IL solar cells with AM1 efficiencies of 15% (active area) and high UV sensitivity have been obtained.     

Photorefractive device using self-assembled monolayer coated indium-tin-oxide electrodes

Photorefractive (PR) performances of methyl-substituted poly(triarylamine) (PTAA)-based PR device were demonstrated using chemically modified electrodes (CMEs) of self-assembled monolayer (SAM) coated indium-tin-oxide (ITO) electrodes. The SAM-ITO electrodes successfully suppressed dark current which blocks the formation of space-charge field and also causes the dielectric breakdown. The PR device consisted of composite of PTAA, 4-azacycloheptylbenzylidenemalononitrile (7-DCST), N-ethylcarbazole (ECz), and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) sandwiched between the SAM-ITO electrodes. The PR devices showed the PR performances: optical gain Γ, refractive index Δn, diffraction efficiency η, response time τ, sensitivity S, phase shift Φ, trap-limited field E_q, number density of traps N_T, and space-charge field E_SC. The remarkable response time of 11.3 ms was achieved at the low electric field of 20 V μm⁻¹, which was comparable to the response time of high-definition television (HDTV) quality of 16 ms. Our approach will widen the usage of higher mobility materials to photorefractive field and give us more favorable materials to achieve the best performance of photorefractivity in the future.     

Computer model and charge transport studies in short gate charge-coupled devices

A computer model has been developed that simulates charge transport of carriers in a surface channel charge-coupled device. This model is based on the charge continuity and current transport equations with a time dependent surface field. The device structure of the model includes a source diffusion an input gate and transfer gate. The present model is the first real simulation of the input scheme of the surface-channel CCDs. The scooping and spilling techniques associated with the charge injection process are simulated by the input diffusion which is included in the model.As an application to a CCD practical problem the present model has been used to study the linearity of the electrical charge injection into surface channel charge-coupled devices. The generated harmonic components of a sinusoidal input are calculated using the transfer characteristics of the input stage obtained from the computer simulation.Using this model the spatial variations of the self-induced fringing field and total currents under the storage and transfer gates were computed. The charge transfer mechanisms for short-gate (L ≤ 8 μm) CCDs was investigated. It was found that for short gates the charge transfer efficiency is governed mainly by the fringing field and self-induced current mechanisms. The results of this study help to clarify the mechanism by which the signal-charge level and gate length affect the charge transfer efficiency.     

Charge transport properties in liquid carbazole

The hole mobility in 9-(2-ethylhexyl)carbazole so-called liquid carbazole, in poly(N-vinylcarbazole) (PVK) and in their blends is determined by time-of-flight experiment using a phthalocyanine charge generation layer. With an applied electric field of 2.5 × 10⁵ V/cm, mobilities of 4 × 10⁻⁶ cm²/Vs and 6 × 10⁻⁷ cm²/Vs are measured in liquid carbazole and PVK, respectively. The enhancement of the charge carrier mobility in liquid carbazole is attributed to both a larger transfer integral and changes in the distribution of the excimer trapping sites. The results show the potential interest of liquid carbazole for electroactive applications in optoelectronics.     

Conductivity of Se95As5 chalcogenide glassy semiconductor layers containing the EuF3 rare-earth impurity in high electric fields

By investigating the I?CV characteristics of an Al-Se₉₅As₅:EuF₃-Te structure, it is established that, upon the application of a positive potential to Te, the current flows in it by the mechanism of space-charge-limited currents (SCLCs) of unipolar-injection, and the N-type I-V characteristic is observed for the opposite polarity. It is shown that these are processes of the thermal-field ionization of neutral and negatively charged U ^? centers, which play a dominant role in the current-flow mechanism in the investigated structures upon the application of an electric field with intensities exceeding 10⁵ V/cm. These are also processes of the electron-hole recombination and capture of charge carriers at U ^? centers. The energy position and concentration of the local states, which correspond to the indicated centers and characterize the effect of the electric field??activation length are determined. It is established that it is the EuF₃ impurities that predominantly affect the local-state concentration.     

Effect of a d.c. electric field parallel to the depletion layer on the microwave oscillations in a p-i-n IMPATT diode

High frequency properties of a p-i-n IMPATT diode have been analysed when an additional d.c. electric field parallel to the area of the junction and perpendicular to the high electric field caused by reverse biasing the device is established in the space charge layer. The transit time required by the carriers to cross the depletion layer is increased due to the increase in the path length caused by the change in the direction of the resultant field while the velocity of the carriers remain saturated. It is found that the frequency for maximum negative resistance decreases with the increase of the additional d.c. electric field.     

Analysis of contact effects in fully printed p-channel organic thin film transistors

Contact effects have been analyzed in fully printed p-channel OTFTs based on a pentacene derivative as organic semiconductor and with Au source–drain contacts. In these devices, contact effects lead to an apparent decrease of the field effect mobility with decreasing L and to a failure of the gradual channel approximation (GCA) in reproducing the output characteristics. Experimental data have been reproduced by two-dimensional numerical simulations that included a Schottky barrier (Φ_b = 0.46 eV) at both source and drain contacts and the effects of field-induced barrier lowering. The barrier lowering was found to be controlled by the Schottky effect for an electric field E < 10⁵ V/cm, while for higher electric fields we found a stronger barrier lowering presumably due to other field-enhanced mechanisms. The analysis of numerical simulation results showed that three different operating regimes of the device can be identified: (1) low |V_ds|, where the channel and the Schottky diodes at both source and drain behave as gate voltage dependent resistors and the partition between channel resistance and contact resistance depends upon the gate bias; (2) intermediate V_ds, where the device characteristics are dominated by the reverse biased diode at the source contact, and (3) high |V_ds|, where pinch-off of the channel occurs at the drain end and the transistor takes control of the current. We show that these three regimes are a general feature of the device characteristics when Schottky source and drain contacts are present, and therefore the same analysis could be extended to TFTs with different semiconductor active layers.     

衬底浮空N 环高压器件新结构与击穿电压模型

提出带有衬底重掺杂N 环的高压器件新结构,称为FR LDMOS。在衬底中引入高掺杂N 环,漏极偏压由环结和漂移区主结分担,降低了主结电场,纵向击穿特性获得显著改善。基于柱坐标Poisson方程,建立击穿电压模型。结果表明：FR LDMOS较常规器件击穿电压提高56%。     

The longitudinal diffusion coefficient and the mobility of hot electrons in silicon

The longitudinal diffusion coefficient of electrons in silicon is (i) calculated as a function of electric field strength from noise and current measurements on a n⁺nn⁺ planar silicon device and (ii) measured vs field strength with the help of the Time-of-Flight technique at 200, 160 and 77 K.The electric field was oriented along the 〈111〉 direction. The first method yielded results for fields lower than 10⁵ Vm^?1 whereas the second method yielded results for fields ranging from 2 × 10⁴ to 2 × 10⁶ Vm^?1. Values for the mobility obtained with the first method are presented as well and compared with Time-of-Flight data.     

Modulations in line shapes of magnetoconductance curves for diodes of pentacene:fullerene charge transfer complexes

This work investigates that the applied electric field modulates the line shapes of magnetoconductance (MC) responses in pentacene:fullerene diodes under illumination. We attribute the line shape of MC curves herein is correlated with the strength of the exchange interaction in pentacene:fullerene charge transfer (CT) complex states. Applying the reversed bias increases the built-in electric field (E_built-in) of the diodes to offset the exchange interaction of CT complex states and narrows the line shapes of MC responses. The saturation field of MC curves in the pentacene:fullerene bulk heterojunction device is 752 Oe when the device is biased at 0.4 V and decreases to 212 Oe at a reversed bias of −1.0 V. The line shape of MC curves for the diode made of pristine pentacene or fullerene as the active layer does not change with the applied bias voltage. Our results indicate the correlations of MC responses with the exchange interaction of CT complexes as modulated by the applied electric field.     

Bright green organic light-emitting devices having a composite electron transport layer

A bright green organic light-emitting device employing a co-deposited Al-Alq₃ layer has been fabricated. The device structure is glass/indium tin oxide (ITO)/ N, N′-diphenyl-N, N′- (3-methylphenyl)-1, 1′-biphenyl-4, 4′-diamine (TPD)/tris(8-quinolinolato) aluminum (Alq₃)/ Al-Alq₃/Al. In this device, Al-Alq₃ is used as electron transport layer (ETL). The device shows an operation voltage of 6.1 V at 20 mA/cm². At optimal condition, the brightness of a device at 20 mA/cm² is 2195 cd/m² achieved a luminance efficiency of 5.64lm/W. The result proves that the composite Al-Alq₃ layer is suitable for the ETL of organic light-emitting devices (OLEDs).     

Famos—A new semiconductor charge storage device

A new non-volatile charge storage device is described. The floating gate avalanche injection MOS (FAMOS) structure is a p-channel silicon gate field effect transistor in which no electric contact is made to the silicon gate. It combines the floating gate concept with avalanche injection of electrons from the surface depletion region of a p-n junction to yield reproducible charging characteristics with long term storage retention.     

Influence of irradiation with neutrons on the characteristics of the voltage terminating structure in silicon radiation detectors

The distribution of potentials over a voltage terminating structure (VTS) has been studied in silicon nuclear-radiation detectors irradiated with neutrons in the range of doses from 1 × 10¹⁰ to 5 × 10¹⁵ n_eq/cm², where the VTS represents a system of floating ring p ⁺-n junctions. It is shown that variation in the profile of an electric field in the bulk of the detector as the radiation dose is increased is the determining factor in the distribution of potentials over the VTS. The mechanisms of VTS operation at irradiation doses lower than 5 × 10¹⁴ n_eq/cm² are established: the distribution of potentials between the rings is accomplished by a punch-through mechanism in the inter-ring gap, while, at higher doses, the distribution is controlled by a current-related mechanism, which is based on the density of the electron??hole generation current flowing in the bulk of the detector. The suggested mechanisms of VTS operation are confirmed experimentally and by simulation.