Bulk Trap States Research Articles

Organic Chain Length Effect on Trap States of Lead Halide Perovskite Scintillators

Scintillators fabricated from organic–inorganic layered perovskites have attracted wide attention due to their excellent properties, including fast decay times, superior light yield, and high exciton binding energy. In relation to their optoelectronic properties, hybrid organic–inorganic perovskites are known for their tunability, which could be manipulated by modifying the organic cations. In this study, we investigate the optical and scintillation properties of lead halide perovskites A2PbBr4, where A vary from amylammonium (AA), hexylammonium (HA), octylammonium (OA), and benzylammonium (BZA) organic ligands. Photoluminescence (PL) spectra display dual peaks due to surface and bulk trap states contributions, while fast average decay times from time-resolved photoluminescence (TRPL) for all samples are within the range of 0.69 ± 0.11–0.99 ± 0.13 ns. The optical band gap of these hybrid perovskites is within ∼3 eV range, which fulfill the criteria of promising scintillators. Radioluminescence (RL) spectra show negative thermal quenching behavior (NTQ) in all samples, with the AA2PbBr4 peak intensity appearing at relatively lower temperature compared to other samples. Thermoluminescence (TL) measurement reveals trap-free states in AA2PbBr4, while other samples possess shallow traps (<40 meV) as well as low trap density, which is beneficial for fast-decay scintillators, X-ray detection and energy conversion for solar cells. Overall, our results demonstrate that the extension of linear organic chains in lead-based perovskite is a deterministic strategy for a fast response hybrid-based scintillator to date.

IZO Thin Film Transistor-Oriented Trap State UV Analysis Using Random Matrix

With the popularization of electronic equipment, indium zinc oxide (IZO) thin film transistor (TFT) research has become a hot spot. Due to the excellent characteristics of solution method, solution-processed IZO TFTs have seen wide applications in various fields. However, the study on the trapping state distribution of solution-processed IZO TFTs is not thorough enough, and the current research methods have limitations. Therefore, the ultraviolet (UV) analysis method is introduced for analyzing the trap state distributions. Then, the UV analysis method is optimized using the random matrix. The interface and bulk trap states are extracted, and the feature of trap states is analyzed. Experimental results show that under the proposed method, the bulk trap concentration of IZO TFTs is 1018∼1019 cm−3 eV−1, and the interface state density is 1012∼1013 cm−2V−1. They all change with wavelength. The proposed method is effective, feasible, and easy to implement. The research will make an important contribution to the preparation of high-performance IZO TFTs.

Impact of process-dependent SiNx passivation on proton-induced degradation in GaN MIS-HEMTs

In this study, AlGaN/GaN heterojunction-based metal–insulator-semiconductor high-electron-mobility transistors (MIS-HEMTs) were fabricated using silicon nitride (SiN) passivation layer. This layer was deposited under the different gas ratio conditions. The effects of proton irradiation on the current characteristics of the devices were investigated according to the deposition conditions of the SiN film. The drain current (ID) characteristics of two devices (with Si-rich or N-rich SiN passivation layers) were reduced by proton irradiation regardless of the deposition condition of SiN film because of the proton-induced displacement damage. In terms of the threshold voltage (Vth), off-state current (Ioff) and gate leakage current (IG), the device with Si-rich SiN exhibited less variation after proton irradiation compared with the N-rich SiN device. This is because the Si-rich SiN was less affected by the proton irradiation. Furthermore, the device with Si-rich SiN showed better dynamic performances before and after proton irradiation than the device with N-rich SiN. This was achieved suppressing the drain-lag phenomenon because the high SiH4 rate for Si-rich SiN deposition considerably reduces the bulk trap states. The capacitance characteristics of the Si-rich SiN device were also improved by the high SiH4 rate. Therefore, the Si-rich SiN-passivated MIS-HEMTs achieved enhanced performance and higher reliability for applications involving space radiation.

Excellent passivation with implied open-circuit voltage of 710 mV for p-type multi-crystalline black silicon using PECVD grown a-Si:H passivation layer

Hydrogenated amorphous silicon (a-Si:H) deposited using plasma-enhanced chemical vapor deposition (PECVD) is used as the passivation layer for p-type multi-crystalline black silicon (p-mc-b-Si) wafers. The effects of deposition conditions on passivation quality are investigated. The optimized a-Si:H passivation layer enables the best surface passivation with an implied open-circuit voltage (iVoc) of 710 mV, which is better than other conventional passivation materials. The excellent passivation quality possibly resulted from that the activated hydrogen from a-Si:H at ~200 °C was able to passivate both surface defect states and bulk trap states of p-mc-b-Si effectively. Moreover, an additional SiNx capping layer helps to reduce reflectance significantly and to keep passivation quality. This work suggests that b-Si passivated by a-Si:H/SiNx stack film shows potential for various optoelectronic devices that requires both excellent optical anti-reflectance and surface passivation.

Exploring Deep and Shallow Trap States in a Non-Fullerene Acceptor ITIC-Based Organic Bulk Heterojunction Photovoltaic System

Nonfullerene organic molecules that usually act as acceptors have exhibited prominent optoelectronic properties in organic bulk heterojunction (BHJ) solar cells. Although the highly efficient exciton dissociation can happen even with a relatively smaller driving force, the underlying energy loss including exciton dissociation and electron–hole recombination in the nonfullerene acceptor (NFA) system remains unclear. In principle, the energetic disorder characteristic is a common feature in organic semiconductors as well as their organic blends due to the molecular random distribution and the donor–acceptor interpenetration network. Owing to their completely different molecular structures in contrast to the bulky-ball shaped fullerene-derivative counterpart such as PCBM, a comprehensive understanding for the trap-associated energetic disorder characteristic and distribution are merits to further eliminate the open-circuit voltage (Voc) loss. In this work, a highly efficient planar organic BHJ solar cell comprising ITO(glass)/ZnO/PBDB-T:ITIC/MoO3/Al was fabricated. Both the polymeric donor PBDB-T and NFA ITIC form an interpenetrating network. With an assist from the nondestructive impedance spectroscopic technique, surface and bulk trap states can be fully revealed. More importantly, both of them respectively contain deep and shallow traps with different energetic locations in an energy gap. The results help to explain the appearance of double and triple capacitance–voltage (C–V) bands. Furthermore, with pump–probe laser spectroscopic measurements, the trap states may locate far below the photoabsorption band-edge. This work serves an insightful view on the trap states in the NFA-based organic BHJ system, and it is valuable for minimizing the energy loss.

Effect of rapid thermal annealing of copper indium aluminium gallium diselenide solar cell devices and its deposition challenges

Thin-film photovoltaic research based on ternary or quaternary absorber materials has mainly concentrated on copper (indium/gallium) diselenide, CuInxGa1-xSe2 (CIGS). This material has demonstrated exceptional energy conversion efficiencies. By altering the In/Ga ratio the band gap can be varied from 1.02 eV (for CuInSe2) to 1.68 eV (for CuGaSe2). However, research from leading groups showed that cells have maximum efficiency at or below 1.35 eV. This paper reports the challenges of using aluminium alloyed CIGS deposited with a single step co-evaporation method. Adding aluminium is found to reduce the bulk trap state density for wide gap devices. However, it created significant safety issues when compared to conventional CIGS co-evaporation deposition systems. The release of H2Se when moisture comes in contact with aluminium selenide was resolved by placing exhaust lines at various places of the deposition chamber. A single phase CIAGS device with a bandgap of 1.30 eV was prepared using a co-evaporation method. The fabricated solar cell devices with CIAGS absorber layers and resulted in a photoconversion efficiency of 10.3%. A progressive rapid thermal annealing at various temperature resulted in a 10% increase in the overall efficiency at 300 °C. The efficiencies were reduced when the RTA temperature increased above 300 °C.

Fast-Switching Mixed A-Cation Organic-Inorganic Hybrid Perovskite TFTs

We use a popular organic-inorganic hybrid perovskite, methyl-ammonium lead iodide (MAPbI3), for thin-film transistors (TFTs). Given the sensitivity of organic-based perovskites to wet processes, we employed an inverted coplanar TFT structure with a UV-treated, high- ${k}$ $\text{AlO}_{\textit {x}}$ gate insulator and deposited the perovskite from solution into a photoresist bank as the final process step. We also explored the impact of substitutional doping of the MAPbI3 with a smaller inorganic cation, cesium (Cs+), or the larger organic cation, formamidinium (FA), to form the mixed-A cation perovskites, $\text{FA}_{\textit {x}}$ MA1- x PbI3 and Cs x MA1- x PbI3, respectively. We demonstrated a significant improvement in the TFT switching speed and ON/ OFF ratio with Cs x MA1- x PbI3 but failed to achieve any transistor operation with FA x MA1- x PbI3. Incorporation of Cs into MAPbI3 reduced bulk and interfacial trap states and improved film density and morphology, whereas incorporation of FA resulted in increased surface roughness and thicker grain boundaries.

Density of bulk trap states of hybrid lead halide perovskite single crystals: temperature modulated space-charge-limited-currents

Temperature-modulated space-charge-limited-current spectroscopy (TMSCLC) is applied to quantitatively evaluate the density of trap states in the band-gap with high energy resolution of semiconducting hybrid lead halide perovskite single crystals. Interestingly multicomponent deep trap states were observed in the pure perovskite crystals, which assumingly caused by the formation of nanodomains due to the presence of the mobile species in the perovskites.

Effects of trifluoromethyl substituents on interfacial and bulk polarization of polystyrene gate dielectrics

The ability to control the bulk and interfacial polarization of dielectric polymers is important to their application in organic electronics. We examine the effect of the trifluoromethyl substituent on poly(3-trifluoromethylstyrene) (P3TFMS) as compared to unsubstituted polystyrene (PS) on the I-V relationships of pentacene-based organic field-effect transistors (OFETs). Single- and double-layered films of these polymers were used, with lower layers crosslinked through vinylbenzocyclobutene comonomers before deposition of upper layers. Control experiments verified that the electronic effect of the crosslinking was negligible. We found that the TFM substituent markedly and independently affected both the initial threshold voltage Vth and the nonvolatile, shifted Vth observed after the application of static gate voltage, depending on its position adjacent or apart from the pentacene. The trifluoromethyl-bearing polymers exhibited significantly lower magnitude initial threshold voltages (Vth,i of ca. −17 V for P3TFMS compared to −35 V for PS), large threshold voltage shifts after charging by the application of static electric fields (ΔVth of ca. 32 V for P3TFMS and 17 V for PS), and greater stability of the ΔVth under repeated charge/discharge cycles. These results are consistent with P3TFMS having fewer interfacial trap states but more stable bulk trap states. The results are applicable to organo-electronic systems such as piezoelectrics for energy harvesting and nonvolatile OFETs such as memory, sensing, and logic elements.

Perovskite-Polymer Blends Influencing Microstructures, Nonradiative Recombination Pathways, and Photovoltaic Performance of Perovskite Solar Cells.

Solar cells based on organic-inorganic halide perovskites are now leading the photovoltaic technologies because of their high power conversion efficiency. Recently, there have been debates on the microstructure-related defects in metal halide perovskites (grain size, grain boundaries, etc.) and a widespread view is that large grains are a prerequisite to suppress nonradiative recombination and improve photovoltaic performance, although opinions against it also exist. Herein, we employ blends of methylammonium lead iodide perovskites with an insulating polymer (polyvinylpyrrolidone) that offer the possibility to tune the grain size in order to obtain a fundamental understanding of the photoresponse at the microscopic level. We provide, for the first time, spatially resolved details of the microstructures in such blend systems via Raman mapping, light beam-induced current imaging, and conductive atomic force microscopy. Although the polymer blend systems systematically alter the morphology by creating small grains (more grain boundaries), they reduce nonradiative recombination within the film and enhance its spatial homogeneity of radiative recombination. We attribute this to a reduction in the density of bulk trap states, as evidenced by an order of magnitude higher photoluminescence intensity and a significantly higher open-circuit voltage when the polymer is incorporated into the perovskite films. The solar cells employing blend systems also show nearly hysteresis-free power conversion efficiency ∼17.5%, as well as a remarkable shelf-life stability over 100 days.

First Experimental Demonstration and Mechanism of Abnormal Palladium Diffusion Induced by Excess Interstitial Ge

This letter represents the first direct experimental demonstrations and mechanism proposal regarding abnormal palladium diffusion into germanium (Ge). Our experiments indicated that excess Ge atoms among palladium germanide alloy formation indirectly induce the abnormal out-diffusion of mass palladium atoms into Ge. Consequently, palladium germanide alloy on both n-type and p-type Ge form ohmic-like Schottky junctions. To identify this phenomenon, first-principle calculations and technology computer-aided design simulation were used to evaluate the electrical influence of palladium atoms in Ge. We discovered that the activated palladium atoms in Ge induce large midgap bulk-trap states, which contribute to a severe increment of trap-assisted tunneling current at the palladium germanide/Ge junction.

Impact of Bulk Trapping Phenomena on the Maximum Attainable Photovoltage of Semiconductor–Liquid Interfaces

In this work, we present a generic semiclassical approach for theoretically evaluating the impact of bulk trap states on the maximum attainable photovoltage in solar-assisted water-splitting reactions. Our method explicitly considers the charge contribution due to the capture and emission of conduction band electrons and valence band holes by trap states situated in the band gap, integrated within a self-consistent solution procedure for calculating electrostatic and charge transport properties. Utilizing the hematite photoanode as our model system, our approach reveals that bulk trap states may significantly degrade both the maximum attainable photovoltage and the concentration of mobile electrons (majority carriers) in the conduction band. These results suggest that bulk trap states can have two primary consequences in photoanodes. First, they may limit the degree to which a favorable cathodic shift in the photocurrent can be achieved (by lowering the maximum attainable photovoltage). Second, they may i...

Quantitative analysis of charge trapping and classification of sub-gap states in MoS2 TFT by pulse I–V method

The threshold voltage instabilities and huge hysteresis of MoS2 thin film transistors (TFTs) have raised concerns about their practical applicability in next-generation switching devices. These behaviors are associated with charge trapping, which stems from tunneling to the adjacent trap site, interfacial redox reaction and interface and/or bulk trap states. In this report, we present quantitative analysis on the electron charge trapping mechanism of MoS2 TFT by fast pulse I–V method and the space charge limited current (SCLC) measurement. By adopting the fast pulse I–V method, we were able to obtain effective mobility. In addition, the origin of the trap states was identified by disassembling the sub-gap states into interface trap and bulk trap states by simple extraction analysis. These measurement methods and analyses enable not only quantitative extraction of various traps but also an understanding of the charge transport mechanism in MoS2 TFTs. The fast I–V data and SCLC data obtained under various measurement temperatures and ambient show that electron transport to neighboring trap sites by tunneling is the main charge trapping mechanism in thin-MoS2 TFTs. This implies that interfacial traps account for most of the total sub-gap states while the bulk trap contribution is negligible, at approximately 0.40% and 0.26% in air and vacuum ambient, respectively. Thus, control of the interface trap states is crucial to further improve the performance of devices with thin channels.

Extraction method of trap densities for indium zinc oxide thin-film transistors processed by solution method

In accordance with the positive bias stress (PBS) instability, generalized equations are derived to extract distributions of trap states in indium zinc oxide (IZO) thin film transistors (TFTs) processed by solution method. In this extraction method, densities of both interface trap states and bulk trap states can be obtained simultaneously. It demonstrates that in the double-active-layer IZO TFT, the front-channel (the bottom layer) annealed by rapid thermal annealing (RTA) in oxygen (O2) atmosphere with high flow rate has more traps at the interface between insulator and IZO than that annealed in nitrogen (N2). Meanwhile, the density of bulk trap states in the bilayer stack IZO TFTs is lower than that in the single-active-layer IZO TFTs. In addition, the specific association of electric characteristics with trap states density also has been explored. Results imply that the subthreshold swing of TFT relies more heavily on interface states. Furthermore, contrary to IZO TFT with single active layer, the positive shift of transfer curves per second increased for stacked TFT in the range from 100 to 200 s, at this point one should take into account the interfacial region between two active layers. Nevertheless, on the whole, double-layered active structure can optimize the performance of devices to some extent.

Study of trapping and recombination processes in thin films of MAPbI3, MAPbI2Br and MAPbI2Cl through photoconductivity measurements

Trapping and recombination processes in thin films of MAPbX3 (X is I, Br or Cl) were studied by means of transient photoconductivity measurements and theoretical simulation of the photocurrent response; in particular the influence of temperature, intensity of illumination and pressure inside the measurement system on the transient photocurrent were studied. The study revealed that the photocurrent of thin films of MAPbI3, MAPbI2Br and MAPbI2Cl measured at atmospheric pressures is mainly affected by surface transient photocurrent, whereas the photocurrent resulting from measurements performed in vacuum is mainly governed by bulk trap states. It was also found that, in general the surface processes are delayed whereas the bulk processes gives raise to fast recombination. The results allowed to establish also, that the transport in MAPbI3, MAPbI2Br and MAPbI2Cl films is affected by both recombination attributed to deep trap states and trapping processes attributed to shallow trap states.

Separate Extraction of Densities of Interface and Bulk Trap States in High-Mobility ZnON Thin-Film Transistors

Separate Extraction of Densities of Interface and Bulk Trap States in High-Mobility ZnON Thin-Film Transistors

Determination of Interface and Bulk Trap Densities in High-Mobility p-type WSe2Thin-Film Transistors

In this letter, the energy distributions of the interface and bulk trap densities were separately determined in p-type tungsten diselenide (WSe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) thin-film transistors (TFTs) by measuring and analyzing the subthreshold transfer characteristics and space-charge-limited current under the flat-band condition at high drain-to-source voltages. From the experimental results, approximately 79% of the subgap trap states are attributed to the interface states at the valence band edge, which represents that the contribution of the interface trap states is more significant than that of the structural disorder-induced bulk trap states in the subgap density of states of the WSe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> TFT. Quantitative information about the separately determined interface and bulk trap densities presented in this letter will facilitate the further development of the fabrication processes to passivate the defect states at the interface in transition metal dichalcogenide-based TFTs, including WSe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> TFTs.

Extraction of bulk and interface trap densities in amorphous InGaZnO thin-film transistors

The authors determine the density of interface and bulk trap states in the amorphous indium-gallium-zinc oxide (a-IGZO) thin-film transistors (TFTs) by using a simple extraction method. To determine the bulk trap density, the current–voltage curve is measured between the source and drain electrodes of the TFT at room temperature under the flat-band condition. In the high voltage region, the carrier transport is well described by the space charge limited current controlled by the bulk trap states that are exponentially distributed in energy with a trap density at the conduction band edge of 6.27 × 1017 cm−3 eV−1 and an inverse slope for the trap distribution of 0.12 eV. The density of traps at the a-IGZO/gate dielectric interface is calculated by subtracting the bulk trap components from the density of total subgap trap states extracted from the subthreshold slope in the transfer curve and the frequency-independent capacitance-voltage characteristics. The experimental results show that the contribution of the interface trap is more significant compared to that of the bulk trap in the subgap density of states of the fabricated a-IGZO TFTs.

Modulation of surface trap induced resistive switching by electrode annealing in individual PbS micro/nanowire-based devices for resistance random access memory.

Bipolar resistive switching (RS) devices are commonly believed as a promising candidate for next generation nonvolatile resistance random access memory (RRAM). Here, two-terminal devices based on individual PbS micro/nanowires with Ag electrodes are constructed, whose electrical transport depends strongly on the abundant surface and bulk trap states in micro/nanostructures. The surface trap states can be filled/emptied effectively at negative/positive bias voltage, respectively, and the corresponding rise/fall of the Fermi level induces a variation in a degenerate/nondegenerate state, resulting in low/high resistance. Moreover, the filling/emptying of trap states can be utilized as RRAM. After annealing, the surface trap state can almost be eliminated completely; while most of the bulk trap states can still remain. In the devices unannealed and annealed at both ends, therefore, the symmetrical back-to-back Fowler-Nordheim tunneling with large ON/OFF resistance ratio and Poole-Frenkel emission with poor hysteresis can be observed under cyclic sweep voltage, respectively. However, a typical bipolar RS behavior can be observed effectively in the devices annealed at one end. The acquirement of bipolar RS and nonvolatile RRAM by the modulation of electrode annealing demonstrates the abundant trap states in micro/nanomaterials will be advantageous to the development of new type electronic components.

Pump-probe surface photovoltage spectroscopy measurements on semiconductor epitaxial layers.

Pump-probe Surface Photovoltage Spectroscopy (SPS) measurements are performed on semiconductor epitaxial layers. Here, an additional sub-bandgap cw pump laser beam is used in a conventional chopped light geometry SPS setup under the pump-probe configuration. The main role of pump laser beam is to saturate the sub-bandgap localized states whose contribution otherwise swamp the information related to the bandgap of material. It also affects the magnitude of Dember voltage in case of semi-insulating (SI) semiconductor substrates. Pump-probe SPS technique enables an accurate determination of the bandgap of semiconductor epitaxial layers even under the strong influence of localized sub-bandgap states. The pump beam is found to be very effective in suppressing the effect of surface/interface and bulk trap states. The overall magnitude of SPV signal is decided by the dependence of charge separation mechanisms on the intensity of the pump beam. On the contrary, an above bandgap cw pump laser can be used to distinguish the signatures of sub-bandgap states by suppressing the band edge related feature. Usefulness of the pump-probe SPS technique is established by unambiguously determining the bandgap of p-GaAs epitaxial layers grown on SI-GaAs substrates, SI-InP wafers, and p-GaN epilayers grown on Sapphire substrates.