Shallow Trap States Research Articles

Charge Transport in and Electroluminescence from sp3-Functionalized Carbon Nanotube Networks.

The controlled covalent functionalization of semiconducting single-walled carbon nanotubes (SWCNTs) with luminescent sp3 defects leads to additional narrow and tunable photoluminescence features in the near-infrared and even enables single-photon emission at room temperature, thus strongly expanding their application potential. However, the successful integration of sp3-functionalized SWCNTs in optoelectronic devices with efficient defect state electroluminescence not only requires control over their emission properties but also a detailed understanding of the impact of functionalization on their electrical performance, especially in dense networks. Here, we demonstrate ambipolar, light-emitting field-effect transistors based on networks of pristine and functionalized polymer-sorted (6,5) SWCNTs. We investigate the influence of sp3 defects on charge transport by employing electroluminescence and (charge-modulated) photoluminescence spectroscopy combined with temperature-dependent current–voltage measurements. We find that sp3-functionalized SWCNTs actively participate in charge transport within the network as mobile carriers efficiently sample the sp3 defects, which act as shallow trap states. While both hole and electron mobilities decrease with increasing degree of functionalization, the transistors remain fully operational, showing electroluminescence from the defect states that can be tuned by the defect density.

Morphology Control in 2D Carbon Nitrides: Impact of Particle Size on Optoelectronic Properties and Photocatalysis

AbstractThe carbon nitride poly(heptazine imide), PHI, has recently emerged as a powerful 2D carbon nitride photocatalyst with intriguing charge storing ability. Yet, insights into how morphology, particle size, and defects influence its photophysical properties are virtually absent. Here, ultrasonication is used to systematically tune the particle size as well as concentration of surface functional groups and study their impact. Enhanced photocatalytic activity correlates with an optimal amount of those defects that create shallow trap states in the optical band gap, promoting charge percolation, as evidenced by time‐resolved photoluminescence spectroscopy, charge transport studies, and quantum‐chemical calculations. Excessive amounts of terminal defects can act as recombination centers and hence, decrease the photocatalytic activity for hydrogen evolution. Re‐agglomeration of small particles can, however, partially restore the photocatalytic activity. The type and amount of trap states at the surface can also influence the deposition of the co‐catalyst Pt, which is used in hydrogen evolution experiments. Optimized conditions entail improved Pt distribution, as well as enhanced wettability and colloidal stability. A description of the interplay between these effects is provided to obtain a holistic picture of the size–property–activity relationship in nanoparticulate PHI‐type carbon nitrides that can likely be generalized to related photocatalytic systems.

In-situ observation of trapped carriers in organic metal halide perovskite films with ultra-fast temporal and ultra-high energetic resolutions

We in-situ observe the ultrafast dynamics of trapped carriers in organic methyl ammonium lead halide perovskite thin films by ultrafast photocurrent spectroscopy with a sub-25 picosecond time resolution. Upon ultrafast laser excitation, trapped carriers follow a phonon assisted tunneling mechanism and a hopping transport mechanism along ultra-shallow to shallow trap states ranging from 1.72–11.51 millielectronvolts and is demonstrated by time-dependent and independent activation energies. Using temperature as an energetic ruler, we map trap states with ultra-high energy resolution down to < 0.01 millielectronvolt. In addition to carrier mobility of ~4 cm2V−1s−1 and lifetime of ~1 nanosecond, we validate the above transport mechanisms by highlighting trap state dynamics, including trapping rates, de-trapping rates and trap properties, such as trap density, trap levels, and capture-cross sections. In this work we establish a foundation for trap dynamics in high defect-tolerant perovskites with ultra-fast temporal and ultra-high energetic resolution.

Decoding the charge carrier dynamics in triple cation-based perovskite solar cells

By deciphering the charge carrier dynamics in perovskite solar cells (PSCs) we noted deep (254 meV) and shallow (91 meV) trap states observed at low and high frequencies respectively, corresponds to ionic migration/electronic trapping.

Modeling of the electrical characteristics of an organic field effect transistor in presence of the bending effects

Abstract An analytical model incorporating the density of trap states for a bendable organic field effect transistor (OFET) is presented in this paper. The aim of this work is to propose a novel modeling framework to quantitatively characterize the bending effects on the electrical properties of an OFET in the linear and saturation regimes. In this model, the exponentially distributed shallow trap states are introduced into the Poisson equation to describe the carrier transports in the channel. The carrier mobility takes into account the low field mobility enhancement under gradual channel approximation and high field degradation. As a result, the generalized current-voltage transistor equations are derived for the first time to reflect the transconductance relationships of the OFET with trap states. In addition, an electro-mechanical coupling relationship is established per the metaphorical analogy between inorganic and organic semiconductor energy band models to quantify the stress-induced variations of the carrier mobility, and the threshold voltage. It is revealed that the before- and after-bending transconductances, predicted from the derived analytical model, are in good agreement with the experimental data measured from DNTT-based OFET bending tests.

Edge Influence on Charge Carrier Localization and Lifetime in CH3NH3PbBr3 Perovskite: Ab Initio Quantum Dynamics Simulation.

The distribution of charge carriers in metal halide perovskites draws strong interest from the solar cell community, with experiments demonstrating that edges of various microstructures can improve material performance. This is rather surprising because edges and grain boundaries are often viewed as the main source of charge traps. We demonstrate by ab initio quantum dynamics simulations that edges of the CH3NH3PbBr3 perovskite create shallow trap states that mix well with the valence and conduction bands of the bulk and therefore support mobile charge carriers. Charges are steered to the edges energetically, facilitating dissociation of photo-generated excitons into free carriers. The edge-driven charge separation extends carrier lifetimes because of decreased overlap of the electron and hole wave functions, which leads to reduction of the nonadiabatic coupling responsible for nonradiative electron-hole recombination. Reduction of spatial symmetry near the edges activates additional vibrational modes that accelerate coherence loss within the electronic subsystem, further extending carrier lifetimes. Enhanced atomic motions at edges increase fluctuations of edge energy levels, enhancing mixing with band states and improving charge mobility. The simulations contribute to the atomistic understanding of the unusual properties of metal halide perovskites, generating the fundamental knowledge needed to design high-performance optoelectronic devices.

Shallow Iodine Defects Accelerate the Degradation of α-Phase Formamidinium Perovskite

Summary Shallow defects are mostly benign in covalent semiconductors, such as silicon, given that they do not constitute non-radiative recombination sites. In contrast, the existence of shallow defects in ionic perovskite crystals might have significant repercussions on the long-term stability of perovskite solar cells (PSCs) because of the metastability of the ubiquitous formamidinium lead triiodide (FAPbI3) perovskite and the migration of charged point defects. Here, we show that shallow iodine interstitial defects ( I i ) can be generated unintentionally during commonly used post-fabrication treatments, which can lower the cubic-to-hexagonal transformation barrier of FAPbI3-based perovskites to accelerate its phase degradation. We demonstrate that concurrently avoiding the generation of I i and the more effective passivation of iodine vacancies ( V I ) can improve the thermodynamic stability of the films and operational stability of the PSCs. Our most stable PSC retained 92.1 % of its initial performance after nearly 1,000 h of continuous illumination operational stability testing.

Formation of TiO2 nanostructures modified Eumelanin films with enhanced properties for biopolymer implementations

Thin films of hybrid melanin-TiO2 nanoparticles (eumelanin: TiO2) deposited from solution by electro-spray were accurately inspected to unveil modified structural and electronic properties for device implementations. Based on the reorganization of the melanin electronic valence orbital, among other interesting behaviors, we observed up to a two orders increase in the absorption coefficient in the visible range. Furthermore the red-shifted absorption features, disclose that the extended π- stacking of oligomers is mainly responsible for the tunability of the optical gap as confirmed by photoluminescence. TiO2 nanostructures via oxidative polymerization, further improve the oligomeric character observed by means of Raman scattering. Finally, the photocatalytic activity of TiO2 nanoparticles helps the saturation of shallow trap states in melanin structure and consequently enhances the charge carrier transport.

Probing defects and electronic structure of Eu doped t-Mg2B2O5 nanocrystals using X-ray absorption near edge spectroscopy and luminescence techniques

We report here the defects and electronic structure study of Eu (1%,3%,5%) doped t-Mg2B2O5 nanocrystals synthesized using low temperature combustion method, and probed using x-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray absorption near edge spectra (XANES), photoluminescence (PL) and thermoluminescence (TL). The XRD analysis of all samples shows single phase triclinic crystal structure and average crystallite size decreases with Eu doping in Mg2B2O5. The experimental XANES spectra of each sample acquired at Eu M5,4-edges were compared with simulated absorption edges using atomic-multiplet calculations, which clearly shows the presence of Eu3+ in Mg2B2O5. The O K-edge spectra of Eu doped Mg2B2O5 predicts the formation of O and Mg defects upon increasing Eu concentrations. The photoluminescence of Mg2-xEuxB2O5 nanocrystals at an excitation of 325 nm comprises of a group of sharp peaks owing to intra-4f transitions in Eu3+ and confirms the lowering of local symmetry with Eu concentration in Mg2B2O5. The effect of Eu doping on the trapping states was probed using TL after irradiating Mg2B2O5 with ultra-violet radiations (365 nm), which shows the presence of shallow and deep level trapping states. The corresponding activation energies of trapping states were determined using glow curve deconvolution method based on Kitti's general order equation.

Synthesis and Localized Photoluminescence Blinking of Lead‐Free 2D Nanostructures of Cs3Bi2I6Cl3 Perovskite

AbstractTwo‐dimensional (2D) lead‐free halide perovskites have generated enormous perception in the field of optoelectronics due to their fascinating optical properties. However, an in‐depth understanding on their shape‐controlled charge‐carrier recombination dynamics is still lacking, which could be resolved by exploring the photoluminescence (PL) blinking behaviour at the single‐particle level. Herein, we demonstrate, for the first time, the synthesis of nanocrystals (NCs) and 2D nanosheets (NSs) of layered mixed halide, Cs3Bi2I6Cl3, by solution‐based method. We applied fluorescence microscopy and super‐resolution optical imaging at single‐particle level to investigate their morphology‐dependent PL properties. Narrow emission line widths and passivation of non‐radiative defects were evidenced for 2D layered nanostructures, whereas the activation of shallow trap states was recognized at 77 K. Interestingly, individual NCs were found to display temporal intermittency (blinking) in PL emission. On the other hand, NS showed temporal PL intensity fluctuations within localized domains of the crystal. In addition, super‐resolution optical image of the NS from localization‐based method showed spatial inhomogeneity of the PL intensity within perovskite crystal.

Synthesis and Localized Photoluminescence Blinking of Lead-Free 2D Nanostructures of Cs3 Bi2 I6 Cl3 Perovskite.

Two-dimensional (2D) lead-free halide perovskites have generated enormous perception in the field of optoelectronics due to their fascinating optical properties. However, an in-depth understanding on their shape-controlled charge-carrier recombination dynamics is still lacking, which could be resolved by exploring the photoluminescence (PL) blinking behaviour at the single-particle level. Herein, we demonstrate, for the first time, the synthesis of nanocrystals (NCs) and 2D nanosheets (NSs) of layered mixed halide, Cs3 Bi2 I6 Cl3 , by solution-based method. We applied fluorescence microscopy and super-resolution optical imaging at single-particle level to investigate their morphology-dependent PL properties. Narrow emission line widths and passivation of non-radiative defects were evidenced for 2D layered nanostructures, whereas the activation of shallow trap states was recognized at 77 K. Interestingly, individual NCs were found to display temporal intermittency (blinking) in PL emission. On the other hand, NS showed temporal PL intensity fluctuations within localized domains of the crystal. In addition, super-resolution optical image of the NS from localization-based method showed spatial inhomogeneity of the PL intensity within perovskite crystal.

Effects of in-situ annealing on the electroluminescence performance of the Sn-based perovskite light-emitting diodes prepared by thermal evaporation

The Sn-based perovskite light-emitting diodes (PeLED) with the structure of (ITO)/MoO3/4,4′-cyclohexylidenebis[N,N-bis(p-tolyl)aniline](TAPC)/4,4′,4′-Tris(carbazol-9-yl)-triphenylamine (TCTA)/perovskite/1,3,5-Tri(m-pyrid-3-yl-phenyl)benzene (TmPyPB)/lithium fluoride(LiF)/Al were fabricated by thermal evaporation and in-situ annealing. The perovskite emitting layers were annealed at the various temperature of 50, 75 and 85 °C during the dual sources thermal evaporation of CsBr and SnBr2 instead of routine post-synthetic annealing for the formation of crystalline CsSnBr3 film. The effects of annealing temperature on the morphology, structure and photophysical properties of the CsSnBr3 perovskite films as well as the electroluminescence(EL) performance of the PeLED were investigated. Little temperature dependence of perovskite emitting layers' morphology was found while obviously intensified absorbance, crystallinity and steady-state photoluminesce(PL) as well as apparently increased PL decay lifetime and slight red-shifted PL spectra of the CsSnBr3 perovskite films upon increasing in-situ annealing temperature were demonstrated. The similar temperature dependent behavior of steady-state PL intensity, transient PL decay life time and photoluminescence quantum yield(PLQY) of all the CsSnBr3 films verified that the hybrid effect of suppressed deep acceptor states and enhanced emitting from shallow trap states would be responsible for the PL behavior of in-situ annealed CsSnBr3 films. In-situ annealing was also capable of extending carriers lifetime and reducing trap density, and leading to the occurrence of the suppressed “self-doping” of Sn4+ and reduction of acceptor states. Meanwhile, the improved crystal quality of the CsSnBr3 nanocrystals should be closely related to the enhanced charge carriers dynamics and improved PL performance. The PeLED with 85 °C in-situ annealed CsSnBr3 emitting layer demonstrated optimal EL performance with the current efficiency(CE) of 0.34 cd/A and external quantum efficiency(EQE) of 0.16%, which was consistent with the annealing temperature dependence of PL behavior of the CsSnBr3 perovskite emitting layer and could be attributed to its superior PL performance, low trap density and high crystallinity.

Effect of doping concentration for the properties of Fe doped TiO2 thin films applications

This paper reports the sol–gel preparation and subsequent characterization of Fe doped TiO2 thin films with different doping concentrations (1%, 3% and 5%). X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Energy Dispersive X-Ray analysis (EDX), UV–Visible analysis and Photoluminescence spectra analysis (PL) were used to characterize the prepared samples. The crystallite size of anatase phase increases from 4.76 nm to 7.61 nm when doping concentration increases from 1% to 3% and the anatase phase is transformed to brookite at the doping of 5% as the crystallite size of more than 14.82 nm. The surface morphological pictures are observing to be almost smooth and particle size dispensation of Fe doped titanium dioxide particles is non-spherical agglomeration crystallite formation. An optical band gap of 3.63 eV, 3.41 eV and 3.11 eV were obtained for the TiO2 thin films doped with 1%, 3% and 5% Fe respectively. PL analysis results the emission peaks at 347 nm for 1% Fe doped sample seems to shift slightly towards higher wavelength for 3% and 5% Fe doped samples. This may be due to reduction in bandgap because of the shallow trap state created near the absorption band edge as a result of doping.

Electrochemical Semiconductor Analysis By Square Wave Voltammetry

Studies in the past 20 years have utilized cyclic voltammetry (CV) for determining various electronic properties (e.g. density of states, electron carrier concentrations, trap state capacitances) of semiconducting titanium oxide films in photoelectrocatalytic systems such as dye-sensitized solar cells [1-4]. Though primarily used for redox sensing, this work presents square wave voltammetry (SWV) as an alternative to determining said properties. As an electrochemical characterization technique, SWV can provide significantly less background current and greater sensitivity than CV [5]. This is beneficial for the accurate measurement and calculation of electronic properties from electrochemical semiconductor current-voltage data. By varying the applied frequency as well as step and/or pulse sizes of the square wave, one can probe electronic processes of various time scales, in contrast to CV which only allows control of the potential scanning rate across the same potential window [5, 6]. This work presents a comparative study of electronic properties as determined by SWV versus CV for various semiconducting electrodes such as silicon, germanium, titanium oxide, and doped diamond films. This technique was also extended to semiconductor heterostructures - such as noble metal-decorated titanium oxide - for observation of intraband gap states and their contribution to the overall electronic properties of the electrode as determined by SWV.[1] Liu, B. et. al., “Intrinsic intermediate gape states of TiO2 materials and their roles in charge carrier kinetics”, J. Photochem. and Photobio. C: Photochem. Rev., 39 (2019) 1-57[2] Zare, M., Mortezaali, A., and Shafiekhani, A., “Photoelectrochemical determination of shallow and deep trap states of platinum-decorated TiO2 nanotube arrays for photocatalytic applications”, J. Phys. Chem. C, 120 (2016) 9017-9027[3] Fabregat-Santiago, F. et al., “High carrier density and capacitance in TiO2 nanotube arrays induced by electrochemical doping”, J. Am. Chem. Soc., 130 (2008), 11312-11316[4] Abayev, I. et al., “Properties of the electronic density of states in TiO2 nanoparticles surrounded with aqueous electrolyte”, J. Solid State Electrochem, 11 (2007) 647-653[5] Mirceski, V. et al., ”Square-wave voltammetry: A review on the recent progress”, Electroanalysis, 25 (2013) 2411-2422[6] Bard, A. J. and Faulkner, L. R., Electrochemical Methods: Fundamentals and Applications, 2nd ed. Hoboken, NJ: John Wiley & Sons, Inc., 2001.

(Invited) Gap Nanowires with Twinning Superlattices: Structure, Optical Properties and Applications

Semiconductor nanowires (NWs) are a good candidate for future optoelectronic devices. However, the control of the essential parameters that determine the electronic and optical quality of NWs, such as crystal structure and incorporation of impurity dopants, are still challenging problems. Most III-V NWs exhibit crystal defects, which are typically randomly distributed zincblende twinning segments and stacking faults that can affect the optical and electrical properties of NW devices. The incorporation of intentional impurity dopants in NWs is important for the fabrication of p-n junctions and control of the electrical conductivity of NWs. The effect of Te and Be impurity dopant concentration on the crystal structure, surface roughness and optical properties of GaAs NWs will be presented. Four identical GaAs NW arrays were grown: an undoped sample (as a reference) and 6 samples with different Te and Be doping concentration. High resolution transmission electron microscopy (HRTEM) revealed an unusual superlattice twinning, with periodicity that became wider and more regular as the doping level increased. Twin boundaries in GaP are shown to act as an atomically narrow plane of wurtzite phase with a type-I homostructure band alignment. Twin boundaries and stacking faults (wider regions of the wurtzite phase) lead to the introduction of shallow trap states observed in photoluminescence studies. Controlling the surface roughness, the periodicity, and the width of twinning planes with doping concentration might open new possibilities for high efficiency NW-based thermoelectric devices, but also has wide implications for all NW devices.

Shallow and Deep Trap State Passivation for Low-Temperature Processed Perovskite Solar Cells

While perovskite solar cells (PSCs) have emerged as promising low-cost solar power generators, most reported high-performance PSCs employ electron transport layers (ETLs, mainly TiO2) treated at hi...

Luminescence study on Mn,Ni co-doped zinc sulfide nanocrystals.

Zinc sulfide (ZnS) doped with transition metal has been used as phosphor for various optoelectronic applications. In the present report, ZnS:Mn doped and ZnS:Mn,Ni co-doped were prepared using chemical co-precipitation method using polyvinyl pyrolidone as a surfactant. The structural studies were carried out using an X-ray diffraction technique; optical studies have been performed using ultraviolet-visible light absorption and photoluminescence (PL) spectroscopy. The presences of functional groups were confirmed using Fourier transform infrared spectroscopy. The X-ray diffraction study and Reitveld analysis confirms the formation of cubic phase with crystalline size 2-3 nm for undoped and doped ZnS nanoparticles. A novel and enhanced luminescence characteristic have been observed in PL spectra. The luminescence intensity of Mn,Ni co-doped ZnS in the blue region is much higher of that of ZnS. The PL results indicate that the doping of Ni creates shallow trap states or luminescence centres in the forbidden energy gap, which quenches the Mn states emission. Concentration quenching has been observed in Mn-doped ZnS nanoparticles. From CIE coordinates, it is seen that the yellow and blue light emission of ZnS:Mn,Ni co-doped nanophosphor may be a promising candidate for display devices and phosphor converted light-emitting diode applications.

Temperature dependences of photoluminescence intensities observed from AgInGaS and AgInGaS/GaSx core–shell nanoparticles

We report on the observation of similar temperature dependences of PL intensities of AgInGaS nanoparticles and AgInGaS / GaSx core–shell nanoparticles. The intensity of band-edge emission in AgInGaS / GaSx increases with temperature up to 200 K, and the intensity at room temperature is on the order of that at 5 K. Using a model that includes effects of thermal activation of carriers from trap states, we propose that a shallow trap state exists in the AgInGaS core, and the higher PL intensity of the band-edge emission at 200 K is due to the radiative recombination of carriers that have been thermally activated from the shallow state. Time-resolved PL measurements are employed to support this interpretation.

Soft Lattice and Defect Covalency Rationalize Tolerance of β-CsPbI3 Perovskite Solar Cells to Native Defects.

Although all-inorganic metal halide perovskites (MHPs) have shown tremendous improvement, they are still inferior to the hybrid organic-inorganic MHPs in efficiency. Recently, a conceptually new β-CsPbI3 perovskite reached 18.4 % efficiency combined with good thermodynamic stability at ambient conditions. We use ab initio non-adiabatic molecular dynamics to show that native point defects in β-CsPbI3 are generally benign for nonradiative charge recombination, regardless of whether they introduce shallow or deep trap states. These results indicate that MHPs do not follow the simple models used to explain defect-mediated charge recombination in the conventional semiconductors. The strong tolerance is due to the softness of the perovskite lattice, which permits separation of electrons and holes upon defect formation, and only allows carriers to couple to the low-frequency vibrations. Both factors decrease notably the non-adiabatic coupling and slow down the dissipation of energy to heat.

Soft Lattice and Defect Covalency Rationalize Tolerance of β‐CsPbI3 Perovskite Solar Cells to Native Defects

AbstractAlthough all‐inorganic metal halide perovskites (MHPs) have shown tremendous improvement, they are still inferior to the hybrid organic–inorganic MHPs in efficiency. Recently, a conceptually new β‐CsPbI3 perovskite reached 18.4 % efficiency combined with good thermodynamic stability at ambient conditions. We use ab initio non‐adiabatic molecular dynamics to show that native point defects in β‐CsPbI3 are generally benign for nonradiative charge recombination, regardless of whether they introduce shallow or deep trap states. These results indicate that MHPs do not follow the simple models used to explain defect‐mediated charge recombination in the conventional semiconductors. The strong tolerance is due to the softness of the perovskite lattice, which permits separation of electrons and holes upon defect formation, and only allows carriers to couple to the low‐frequency vibrations. Both factors decrease notably the non‐adiabatic coupling and slow down the dissipation of energy to heat.