Deep Trap States Research Articles

First-Principles Modeling of Bismuth Doping in the MAPbI3 Perovskite

Heterovalent doping in lead halide perovskites was only marginally explored. Particular attention was focused on Bi3+ dopant, which was found to increase the α-phase stability for CsPbI3, leading to high efficiency of fully inorganic perovskite solar cells. It was recently demonstrated that the absorption onset red-shift of the Bi-doped perovskite is due to the increased number of defect states and a significant increase in the sub-band-gap density of states. Here we computationally simulated the electronic properties of the Bi-doped MAPbI3 (MA = CH3NH3+) perovskite to gain insight into the electronic structure modifications occurring upon heterovalent doping. Our results confirm the presence of deep trap states induced by the Bi dopant, with the Bi3+ acting as deep electron trap. The absorption onset red-shift observed upon Bi-doping of MAPbI3 is mainly related to transitions to the Bi defect states, while the perovskite band gap is essentially unaltered.

The photoluminescence, ultrafast nonlinear optical properties and carrier dynamics of 1D In-doped ZnO nanostructures: Experiment and mechanism

1D In-doped ZnO (IZO) nanostructures with different morphologies, namely nanowires, nanoseries(serial), nanorods and nanoprisms, have been successfully synthesized by chemical vapour deposition method. The structural, morphology and optical properties of IZO nanowires have been characterized. The ultrafast nonlinear optical and carrier dynamics characteristics of IZO were studied using femtosecond Z-scan, time-resolved photoluminescence (TRPL) and two-color pump-probe (TCPP) technique, respectively. The TRPL showed that the luminescence decay lifetimes of about several 10 ps and nanoseconds time response scale can be attributed to the shallow trap (ST) state, and deep trap (DT) state emissions, respectively. The mechanisms of the TRPL behavior in the different emission band were analyzed. The result of TCPP experiment showed that the bleach recovery with lifetimes on the fast (100–200 ps), and slow (>1 ns) time scale, caused by the shallow donor mixed with donor bound recombination, and donor−acceptor pair recombination (DAP), respectively. The optical transition process in the IZO nanostructures has demonstrated that the energy transfer process from the host electron-hole pairs can efficiently excite the metal ions. Comparing with the results obtained from the TRPL experiment, we found the fast decay process was resulted from the exciton migration and the slow decay to the recombination process of the relaxed excitons. Our results show that IZO nanostructures are a promising candidate in further optoelectronic device applications.

Complexity at simple interfaces: Dynamically generated deep trap states at the noble-metal/alkali-halide interface

The trapping of delocalized electrons at ultrathin layers of NaCl on Ag(100) is investigated in real time using two-photon photoemission. Above 79 K, electrons localize to deep trap states assigned to low-coordinated sites on the NaCl surface. Below 79 K, these low-coordinated trap states serve as precursors for a new deep trap state at increased binding energies. The observation that the emerging trap state requires a localized precursor state suggests that a dynamic polarization of the NaCl lattice may be required for its formation. This behavior is consistent with the expectations for the formation of a interfacial (electron) small polaron which is theoretically unstable in bulk NaCl. The alkali-halide/noble-metal interface shows the sequential population of multiple metastable deep trap states in two dimensions in real time.

Highly transparent, flexible and hydrophilic ZnS thin films prepared by a facile and environmentally friendly chemical bath deposition method

Growth of highly transparent and hydrophilic ZnS thin films on flexible polymeric and glass substrates has been performed using an environmentally friendly chemical bath deposition method with an ammonia and thioacetamide free reaction solution. Deposition time effect on surface morphology, structure and optical properties of the ZnS thin films has been investigated by field emission scanning electron microscopy (FE-SEM), atomic force microscopy (AFM), transmission electron microscopy (TEM), X-ray diffractometer (XRD), ultraviolet-visible light (UV–vis) spectroscopy, and photoluminescence (PL) spectroscopy. Also, the chemical composition of the prepared films has been studied in detail by energy-dispersive X-ray spectrometer (EDX), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. TEM images show that the ZnS films consisted of small nanocrystals with diameters ≤5 nm. Moreover, the water contact angles, 38.5° and 25.4° for ZnS films deposited on PS and on glass substrates, respectively, indicate a highly hydrophilic nature. Undoubtedly, the chemical analysis shows the formation of the films with a chemical composition of ZnS0.89, lacking impurities such as ZnO and/or Zn (OH)2. Additionally, FE-SEM images show a homogenous deposition over a very large surface area substrates. Ultimately, the obtained photoluminescence spectrum indicates a strong and broad emission peak ranging from 400 to 600 nm, which are due to deep levels or trap state defects in the crystalline structure of the ZnS films.

Evaluation of the Role of Deep Trap State Using Analytical Model in the Program/Erase Cycling of NAND Flash Memory and Its Process Dependence

A method to analyze the kinetics of the charge accumulation in the tunnel oxide by the nand flash memory program and erase (P/E) cycling is proposed. Both electron trapping and detrapping processes are required to be considered owing to the oxide high electric field during P/E cycles. Consequently, the electron trapping in the deep trap state is concluded, whose trap energy ( $\text{E}_{\text {trap}}$ ) is more than 3.5 eV. Furthermore, the as-grown trap state density ( ${\text{N}}_{e}$ ), the trapping capture cross section ( $\sigma $ ), and the number of trapped positive charges can also be extracted to explain the tunneling current modulation and the $\text{V}_{\text {T}}$ shift by oxide-trapped charges under the P/E stress. The trapped electrons are mainly distributed in the center of tunnel oxide, and the distributed area extends as the P/E bias increases. In addition, the dependence of oxidation process is also shown. Both thermal dry and plasma oxidation have almost the same value of $\sigma $ ( $\sim {4} \times {10}^{ {-17}}$ cm2). However, 30% reduction of ${\text{N}}_{e}$ is shown in plasma oxidation ( $\sim {1.25} \times {10}^{ {19}}$ cm−3) when compared with thermal dry oxidation ( $\sim {1.88} \times {10}^{ {19}}$ cm−3).

Mechanistic Investigation of Upconversion Photoluminescence in All-Inorganic Perovskite CsPbBrI2 Nanocrystals

Perovskite quantum dots emit upconversion photoluminescence (UCPL) excited by long wavelength photons. However, UCPL with different wavelengths and widths are irradiated in perovskite colloidal excited by different energy photons. This study investigated efficient UCPL in colloidal CsPbBrI2 nanocrystals and proposed a simple energy level model to illustrate the UC mechanism. Red-shifted UCPL relative to normal PL was attributed to recombination of carriers from the surface to the valence band and involved a single-photon process, whereas unshifted UCPL was attributed to recombination of carriers from the conduction band to the valence band and involved a two-photon process. Trap states within the valence band played an important role in the UC process. Single photon UC emission occurred when the absorbed photons possessed sufficient energy to excite electrons in deep trap states in CsPbBrI2 nanocrystals. However, when the photons could only excite electrons in shallow trap states, some excited photons wer...

How Methylammonium Cations and Chlorine Dopants Heal Defects in Lead Iodide Perovskites

AbstractLead tri‐iodide methylammonium (MAPbI3) perovskite polycrystalline materials show complex optoelectronic behavior, largely because their 3D semiconducting inorganic framework is strongly perturbed by the organic cations and ubiquitous structural or chemical inhomogeneities. Here, a newly developed time‐dependent density functional theory‐based theoretical formalism is taken advantage of. It treats electron–hole and electron–nuclei interactions on the same footing to assess the many‐body excited states of MAPbI3 perovskites in their pristine state and in the presence of point chemical defects. It is shown that lead and iodine vacancies yield deep trap states that can be healed by dynamic effects, namely rotation of the methylammonium cations in response to point charges, or through slight changes in chemical composition, namely by introducing a tiny amount of chlorine dopants in the defective MAPbI3. The theoretical results are supported by photoluminescence experiments on MAPbI3−mClm and pave the way toward the design of defect‐free perovskite materials with optoelectronic performance approaching the theoretical limits.

Interface engineering of compact-TiOx in planar perovskite solar cells using low-temperature processable high-mobility fullerene derivative

Interface engineering plays a promising strategy to produce highly efficient planar heterojunction (PHJ) perovskite solar cells. The deep trap states on the compact-TiOx surface leading to a large leakage current and recombination of charge carriers. To solve the problems, interfacial engineering of electron collecting layer (ECL) compact-TiOx by a thin-layer of one-step solution-processed and low-cost organic material is applied. In contrast, commonly used PCBM is still expensive material. Herein, a new, low-temperature processable higher potential of [60]fulleropyrrolidine derivatives named as N-phenyl[60]fulleropyrrolidines (PNP) was introduced as an interfacial modification of ECL compact-TiOx with the varying thickness of 10, 20, and 30nm to replace the commonly used PCBM in PHJ perovskite solar cells. The modified surface morphology was achieved by introducing PNP interfacial layers that enhanced the surface-energy properties of the cells in terms of enhanced photocurrent. Compared with PCBM, PNP features a higher electronic mobility and stronger hydrophobic nature. The enhancement of power conversion efficiency was obtained from 5.12% to 8.23%, with an increase in short-circuit current density (Jsc) from 11.90 to 21.44mAcm−2 and fill factor (FF) from 0.49 to 0.56 owing to insertion of optimum 10-nm-thickness PNP that led to more efficient electron transport and charge extraction in the solar cell performances. The present work provides an important sign in the aspects to the low-cost mass production of perovskite solar cells.

Trapped‐Electron‐Induced Hole Injection in Perovskite Photodetector with Controllable Gain

AbstractPerovskite is an excellent photosensitive material but it exhibits a shortcoming in providing photoconductive gain for layered photodetectors due to lacking of trap states. Here, the perovskite photodetectors are fabricated with controllable photoconductive gain by designing a trapped‐electron‐induced hole injection structure of [6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM):2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane (F4‐TCNQ)/Bathocuproine/Au. The deep trap states provided by F4‐TCNQ can capture the photogenerated electrons from perovskite, which makes the hole injection barrier thin enough to be tunneled through, allowing the hole injection and forming a gain. Meanwhile, the gain is controllable by adjusting the electron trapping and hole transport capacities of PCBM:F4‐TCNQ dual‐functional layer, concomitantly realizing the transition of device from photovoltaic to photoconductive. Thus fabricated device achieves a higher external quantum efficiency of 6 × 104% at a lower bias of −1 V, and simultaneously maintains the rectifying behavior in dark, providing the detectivity of about 1 × 1015 Jones.

Degradation of encapsulated perovskite solar cells driven by deep trap states and interfacial deterioration

The degradation of perovskite device performance is found to be driven by interfacial deterioration in the device and induced deep trap assisted recombination in the perovskite with aging.

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.

Leakage current transport mechanism under reverse bias in Au/Ni/GaN Schottky barrier diode

The leakage current transport mechanism under reverse bias of Au/Ni/GaN Schottky diode is studied using temperature dependent current-voltage (I-V-T) and capacitance-voltage (C-V) characteristics. I-V measurement in this study is in the range of 140 K–420 K in steps of 10 K. A reduction in voltage dependent barrier height and a strong internal electric field in depletion region under reverse bias suggested electric field enhanced thermionic emission in carrier transport via defect states in Au/Ni/GaN SBD. A detailed analysis of reverse leakage current revealed two different predominant transport mechanisms namely variable-range hopping (VRH) and Poole-Frenkel (PF) emission conduction at low (<260 K) and high (>260 K) temperatures respectively. The estimated thermal activation energies (0.20–0.39 eV) from Arrhenius plot indicates a trap assisted tunneling of thermally activated electrons from a deep trap state into a continuum of states associated with each conductive threading dislocation.

Interface and transport properties of gamma irradiated Au/n-GaP Schottky diode

The effect of 10Mrad γ-ray exposure on the interface and transport properties of Au/n-GaP Schottky diode is studied in the 220–400K temperature range. There is significant alteration in the interface and defect state density after irradiation. The Arrhenius plot of σac shows the presence of both shallow and deep trap states corresponding to activation energies 0.073eV and 0.21eV respectively. The influence of additional defects is reflected in the charge transport mechanism at low temperature regimes, where tunneling mechanisms are observed. The perceived transport and capacitance/conductance properties are ascribed due to further interfacial and deep defect state formation upon gamma irradiation.

Thermally activated electrical conductivity of thin films of bis(phthalocyaninato)terbium(III) double decker complex

The temperature and field dependence of the electrical conductance of neutral radical bis(phthalocyaninato)terbium(III) (TbPc2) thin films was measured in situ under ultrahigh vacuum. The films behave electrically as narrow gap intrinsic semiconductors, having high conductivity and weakly thermally activated conductance. Taking advantage of the exceptionally high electrical stability of the material and the fast-settling response in the low-field region, precise measurements of the temperature dependence could resolve a linear temperature dependence in the pre-exponential factor of the conduction equation. The activation energy of conductance of the annealed TbPc2 film was determined to be 0.158eV after adjusting the fitting procedure to take into account the temperature dependent pre-exponential. This new conductance equation, which differs only slightly from the usual Arrhenius expression, arises as the natural consequence of thermal excitation of carriers from a continuous density of deep trap states, or a similar activated process where either the states or the barrier heights distribute over a finite width.

Solar Water Splitting: Enhancing Charge Carrier Lifetime in Metal Oxide Photoelectrodes through Mild Hydrogen Treatment (Adv. Energy Mater. 22/2017)

Time-resolved conductivity measurements reveal that a mild hydrogen treatment at 300 °C for 10 minutes successfully extends the lifetime of carriers by more than two-fold in bismuth vanadate (BiVO4) photoelectrodes. This enhancement is a result of passivation of deep trap states and/or reduction of their density. Consequently, the AM1.5 photocurrent and onset potential for the hydrogen-treated BiVO4 are significantly improved. This is reported by Fatwa F. Abdi and co-workers in article number 1701536.

High performance SONOS flash memory with in-situ silicon nanocrystals embedded in silicon nitride charge trapping layer

In this paper, SONOS-type flash memory device with highly improved charge-trapping efficiency is suggested by using silicon nanocrystals (Si-NCs) embedded in silicon nitride (SiNX) charge trapping layer. The Si-NCs were in-situ grown by PECVD without additional post annealing process. The fabricated device shows high program/erase speed and retention property which is suitable for multi-level cell (MLC) application. Excellent performance and reliability for MLC are demonstrated with large memory window of ∼8.5 V and superior retention characteristics of 7% charge loss for 10 years. High resolution transmission electron microscopy image confirms the Si-NC formation and the size is around 1–2 nm which can be verified again in X-ray photoelectron spectroscopy (XPS) where pure Si bonds increase. Besides, XPS analysis implies that more nitrogen atoms make stable bonds at the regular lattice point. Photoluminescence spectra results also illustrate that Si-NCs formation in SiNx is an effective method to form deep trap states.

Deep-Trap States of GaN-Based Light Emitting Diodes Analyzed by Space Charge Limited Conduction Model

Deep-Trap States of GaN-Based Light Emitting Diodes Analyzed by Space Charge Limited Conduction Model

Identification of the spatial location of deep trap states in AlGaN/GaN heterostructures by surface photovoltage spectroscopy

Spatial and spectral origin of deep level defects in molecular beam epitaxy grown AlGaN/GaN heterostructures are investigated by using surface photovoltage spectroscopy (SPS) and pump-probe SPS techniques. A deep trap center ∼1 eV above the valence band is observed in SPS measurements which is correlated with the yellow luminescence feature in GaN. Capture of electrons and holes is resolved by performing temperature dependent SPS and pump-probe SPS measurements. It is found that the deep trap states are distributed throughout the sample while their dominance in SPS spectra depends on the density, occupation probability of deep trap states and the background electron density of GaN channel layer. Dynamics of deep trap states associated with GaN channel layer is investigated by performing frequency dependent photoluminescence (PL) and SPS measurements. A time constant of few millisecond is estimated for the deep defects which might limit the dynamic performance of AlGaN/GaN based devices.

Analytical Drain Current Model for Amorphous InGaZnO Thin-Film Transistors at Different Temperatures Considering Both Deep and Tail Trap States

Surface-potential-based drain current model is presented for amorphous InGaZnO thin-film transistors considering both exponential deep and tail trap states densities in the energy gap. The trap states densities are determined by the numerical calculation on the basis of the assumption that the trapped carrier concentration is much higher than the free carrier concentration. The analytical drain current model is developed consistent with the numerical calculation, and verified by the experimental data at different temperatures.

Enhancing Charge Carrier Lifetime in Metal Oxide Photoelectrodes through Mild Hydrogen Treatment

AbstractWidespread application of solar water splitting for energy conversion is largely dependent on the progress in developing not only efficient but also cheap and scalable photoelectrodes. Metal oxides, which can be deposited with scalable techniques and are relatively cheap, are particularly interesting, but high efficiency is still hindered by the poor carrier transport properties (i.e., carrier mobility and lifetime). Here, a mild hydrogen treatment is introduced to bismuth vanadate (BiVO4), which is one of the most promising metal oxide photoelectrodes, as a method to overcome the carrier transport limitations. Time‐resolved microwave and terahertz conductivity measurements reveal more than twofold enhancement of the carrier lifetime for the hydrogen‐treated BiVO4, without significantly affecting the carrier mobility. This is in contrast to the case of tungsten‐doped BiVO4, although hydrogen is also a donor type dopant in BiVO4. The enhancement in carrier lifetime is found to be caused by significant reduction of trap‐assisted recombination, either via passivation or reduction of deep trap states related to vanadium antisite on bismuth or vanadium interstitials according to density functional theory calculations. Overall, these findings provide further insights on the interplay between defect modulation and carrier transport in metal oxides, which benefit the development of low‐cost, highly‐efficient solar energy conversion devices.