Trap States Research Articles

Simultaneous Structural and Electronic Engineering on Bi‐ and Rh‐co‐doped SrTiO3 for Promoting Photocatalytic Water Splitting

AbstractActivating metal ion‐doped oxides as visible‐light‐responsive photocatalysts requires intricate structural and electronic engineering, a task with inherent challenges. In this study, we employed a solid (template)‐molten (dopants) reaction to synthesize Bi‐ and Rh‐codoped SrTiO3 (SrTiO3 : Bi,Rh) particles. Our investigation reveals that SrTiO3 : Bi,Rh manifests as single‐crystalline particles in a core (undoped)/shell (doped) structure. Furthermore, it exhibits a well‐stabilized Rh3+ energy state for visible‐light response without introducing undesirable trapping states. This precisely engineered structure and electronic configuration promoted the generation of high‐concentration and long‐lived free electrons, as well as facilitated their transfer to cocatalysts for H2 evolution. Impressively, SrTiO3 : Bi,Rh achieved an exceptional apparent quantum yield (AQY) of 18.9 % at 420 nm, setting a new benchmark among Rh‐doped‐based SrTiO3 materials. Furthermore, when integrated into an all‐solid‐state Z‐Scheme system with Mo‐doped BiVO4 and reduced graphene oxide, SrTiO3 : Bi,Rh enabled water splitting with an AQY of 7.1 % at 420 nm. This work underscores the significance of simultaneous structural and electronic engineering and introduces the solid‐molten reaction as a viable approach for this purpose.

Reduction of recombination at the interface of perovskite and electron transport layer with graded pt quantum dot doping in ambient air-processed perovskite solar cell

The study of charge transfer in thin film solar cells made of several layers is of high importance since they may lose their energy via the recombination process at the interfaces, specifically at the interface of the electron transport layer (ETL) and perovskite. Titanium dioxide (TiO2) is mostly used as an ETL in perovskite solar cells due to its many advantages. However, TiO2 has some disadvantages, such as low electron mobility compared to the perovskite layer and electron trap states on its top at the interface. These effects cause the accumulation of carriers at the ETL/perovskite interface then the non-radiative recombination will be enhanced, which is considered as one of the significant losses in the Perovskite Solar Cells (PSCs). In this work, a new technique is taken for more optimal ETL doping. We fabricated the ETL layers with graded doping of platinum quantum dots (Pt QDs), in which Pt QDs concentration is high at the ETL/Perovskite interface and zero at the FTO/ETL interface. This strategy not only suppresses the recombination at the ETL/perovskite interface and subsequently enhances the device efficiency from 12.92 to 14.36% but also improves the stability of the PSCs.

High‐Gain and Tunable Linear Photodetection in 2D Tunneling Heterostructures Through Potential Engineering

Abstract2D materials are extensively employed in the fabrication of high‐performance photodetectors owing to their exceptional physical properties. However, most 2D material photodetectors fail to sustain high gain under intense illumination due to the limited intrinsic trap states. Here, an n‐n type Bi2O2Se/SnSe2 van der Waals tunneling heterojunction photodetector with a detection range from visible to near‐infrared (VIS‐NIR) is presented. Under reverse bias, the heterojunction induces a significant electron barrier and hole potential well, ensuring low leakage current and ample hole defect states. Therefore, the photodetector demonstrated a responsivity of 1636.3 AW−1 and a detectivity of 1.39 × 1014 Jones under 660 nm illumination, maintaining a tunable linear dynamic range (LDR) of ≈74.7 dB. This performance is attributed to the hole potential well‐suppressing the recombination of photogenerated carriers, thereby enhancing the device's gain. Furthermore, the tunneling of photogenerated electrons within the heterojunction's space charge region under bias enables rapid response (75.1 and 15.6 µs). In summary, the study introduces a novel strategy to overcome the limited detection capabilities of 2D devices under intense illumination, characterized by outstanding linearity for rapid detection and high‐resolution imaging.

Quasi-Planar Core Based Spiro-Type Hole-Transporting Material for Dopant-Free Perovskite Solar Cells.

Hole-transporting material (HTMs) are crucial for obtaining the stability and high efficiency of perovskite solar cells (PSCs). However, the current state-of-the-art n-i-p PSCs relied on the use of 2,2',7,7'-tetrakis(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene (spiro-OMeTAD) exhibit inferior intrinsic and ambient stability due to the p-dopant and hydrophilic Li-TFSI additive. In this study, a new spiro-type HTM with a critical quasi-planar core (Z-W-03) is developed to improve both the thermal and ambient stability of PSCs. The results suggest that the planar carbazole structure effectively passivates the trap states compared to the triphenylamine with a propeller-like conformation in spiro-OMeTAD. This passivation effect leads to the shallower trap states when the quasi-planar HTMs interact with the Pb-dimer. Consequently, the device using Z-W-03 achieves a higher Voc of 1.178 V compared to the spiro-OMeTAD's 1.155 V, resulting in an enhanced efficiency of 24.02 %. In addition, the double-column π-π stacking of Z-W-03 results in high hole mobility (~10-4 cm2 V-1 s-1) even without p-dopant. Moreover, when the surface interface is modified, the undoped Z-W-03 device can achieve an efficiency of nearly 23 %. Compared to the PSCs using spiro-OMeTAD, those with Z-W-03 exhibit enhanced stability under N2 and ambient conditions. This superior performance is attributed to the quasi-planar core structure and the presence of multiple CH/π and π-π intermolecular stacking in Z-W-03. The multiple CH/π and π-π intermolecular contacts of HTMs can improve the hole hopping transport. Therefore, it is imperative to focus on further molecular structure design and optimization of spiro-type HTMs incorporating quasi-planar cores and carbazole moieties for the commercialization of PSCs.

Quasi‐Planar Core Based Spiro‐Type Hole‐Transporting Material for Dopant‐Free Perovskite Solar Cells

AbstractHole‐transporting material (HTMs) are crucial for obtaining the stability and high efficiency of perovskite solar cells (PSCs). However, the current state‐of‐the‐art n‐i‐p PSCs relied on the use of 2,2′,7,7′‐tetrakis(N,N‐di‐p‐methoxyphenylamine)‐9,9′‐spirobifluorene (spiro‐OMeTAD) exhibit inferior intrinsic and ambient stability due to the p‐dopant and hydrophilic Li‐TFSI additive. In this study, a new spiro‐type HTM with a critical quasi‐planar core (Z‐W‐03) is developed to improve both the thermal and ambient stability of PSCs. The results suggest that the planar carbazole structure effectively passivates the trap states compared to the triphenylamine with a propeller‐like conformation in spiro‐OMeTAD. This passivation effect leads to the shallower trap states when the quasi‐planar HTMs interact with the Pb‐dimer. Consequently, the device using Z‐W‐03 achieves a higher Voc of 1.178 V compared to the spiro‐OMeTAD's 1.155 V, resulting in an enhanced efficiency of 24.02 %. In addition, the double‐column π–π stacking of Z‐W‐03 results in high hole mobility (~10−4 cm2 V−1 s−1) even without p‐dopant. Moreover, when the surface interface is modified, the undoped Z‐W‐03 device can achieve an efficiency of nearly 23 %. Compared to the PSCs using spiro‐OMeTAD, those with Z‐W‐03 exhibit enhanced stability under N2 and ambient conditions. This superior performance is attributed to the quasi‐planar core structure and the presence of multiple CH/π and π–π intermolecular stacking in Z‐W‐03. The multiple CH/π and π–π intermolecular contacts of HTMs can improve the hole hopping transport. Therefore, it is imperative to focus on further molecular structure design and optimization of spiro‐type HTMs incorporating quasi‐planar cores and carbazole moieties for the commercialization of PSCs.

Impact of Dual Functionalities of Mo‐Doped BiFeO3 Decorated with Bi4MoO9 Heteroatoms for Piezo‐Photocatalytic Activity

AbstractPiezocatalysts have shown great promise for effcient hydrogen generation. However, their application is limited by the rapid charge recombination and sluggish charge transfer rate of piezo‐induced charge carriers. This work aims to address the preceding limitations by controlled synthesis of a representative piezocatalyst, BiFeO3 doped with Mo atoms and decorated with Bi4MoO9 (BMO) heteroatoms using sol‐gel method. The substitutional doping of Mo on BFO (BFMO) was shown to induce a shallow energy level near the conduction band that acted as a trapping state, to suppress recombination of charge carriers, enhance charge mobility for transport to the surface, modulate band alignment, and lower the energy barrier for surface reaction. The formation of BFMO/BMO heterostructures induced electrical band bending, leading to the electric potential gradient, hence promoting charge carrier separation and transfer. The BFMO/BMO with 0.5 mol % Mo (Mo‐5) demonstrated four times faster piezocatalytic Rhodamine B (RhB) dye degradation rate (k of 0.032 min−1) compared to pristine BFO (0.009 min−1). Further, the Mo‐5 exhibited a 45% higher piezocatalytic H2 production rate (14.4 µmol/g/h) compared to pristine BFO (9.8 µmol/g/h) while being cocatalyst‐free. Coupling piezocatalysis with photocatalysis was found to reduceperformance relative to the individual processes, which is attributed to Auger recombination which impedes any potential synergistic effect.

Identifying Rare Events in Quantum Molecular Dynamics of Nanomaterials with Outlier Detection Indices.

Nanoscale and condensed matter systems evolve on multiple length- and time-scales, and rare events such as local phase transformation, ion segregation, defect migration, interface reconstruction, and grain boundary sliding can have a profound influence on material properties. We demonstrate how outlier detection indices can be used to identify rare events in machine-learning based, high-dimensional molecular dynamics (MD) simulations. Designed to order data-points from typical to untypical, the indices enable one to capture atomic events that are hard to detect otherwise. We demonstrate the approach with a nanosecond MD simulation of a grain boundary in a metal halide perovskite that is extensively studied for solar energy and optoelectronic applications. The method captures the initial grain boundary sliding and a spontaneous fluctuation half a nanosecond later, both events giving rise to persistent deep electronic trap states that impact charge carrier lifetime and transport and material performance. The approach offers a generalizable and simple method for identifying outlier events in complex condensed matter, molecular, and nanoscale systems.

Controlling the luminescence of CdSe quantum dots in the fluorinephosphate glass

A series of fluorophosphate glasses with CdSe and CdSe+ZnSe are successfully synthesized. After heat treatment above the glass transition temperature, CdSe quantum dots with sizes 1.5–5.5 nm are formed. The luminescence spectra of CdSe quantum dots are analyzed at various temperatures, excitation energies and chemical composition of glass. The influence of selenium content on the growth features of quantum dots is shown. The size dependence of luminescence quantum yield on the QDs size is found. The 2.0–2.5 nm-sized QDs demonstrate trap emission in the region of 1.7–1.85 eV with a maximum quantum yield of 60 %. It is found that only a broad band due to the transition from the low state of the conduction band and the shallow donor trap states to the deep accepter trap levels is observed. This broad emission band is observed at any selenium cadmium ratio and zinc introduction.

Distributions and evolution of trap states in non-fullerene organic solar cells

Distributions and evolution of trap states in non-fullerene organic solar cells

Deep Traps in AlN Hydride Vapor Phase Epitaxy Film on Low-Temperature AlN/Sapphire

Deep trap states were examined in c-plane, Al-polar AlN epilayers, deposited via metal organic chemical vapor deposition on 270 nm thick AlN buffer layers on sapphire substrates. Contacts were created using e-beam deposited Ni through a shadow mask, with I-V characteristics revealing a trapped limited current (TLC) regime and voltage-dependent hysteresis upon light-emitting diode illumination. Thermally stimulated current and photothermal ionization current spectroscopy measurements demonstrated a prominent trap activation energy of approximately 0.75 eV and additional trap energies of 0.6, 0.4, 0.25, 1.05, and 1.1 eV. The observed differences in photocurrent responses between forward and reverse biases suggest that forward bias induces electron trapping at deeper levels, influencing the TLC behavior. Comparisons with bulk n-type AlN crystals from previous studies show similarities in deep trap spectra, suggesting commonality in trap characteristics across different AlN samples.

Unveiling full-dimensional distribution of trap states toward highly efficient perovskite photovoltaics

To gain a deep understanding and address key issues in perovskite photovoltaics, such as power conversion efficiency (PCE) and long-term stability, defect passivation and analysis of the device performance are required. Here, we proposed a non-contact characterization technique named scanning photocurrent measurement system (SPMS) for device surface detection, having conducted signal analysis and method adjustments based on perovskite photovoltaic devices. This technique enables the monitoring of minority carriers in the device, allowing for the investigation of carrier behavior based on photocurrent signals. By integrating SPMS with thermal conductance spectroscopy (TAS) and drive-level capacitance profiling (DLCP), we further simulated a spatial three-dimensional (3D) distribution of trap states in the device and analyzed the distribution of trap states matched with energy space. Through extensive case studies, we have validated the universality and accuracy of this method. The integration of trap state characterization techniques provides strong support for targeted defect passivation and performance evaluation of perovskite photovoltaic devices, yielding a highly efficient perovskite solar cell with PCE as high as 25.74%.

Bottom Contact Engineering for Ambient Fabrication of >25% Durable Perovskite Solar Cells.

The bottom contact in perovskite solar cells (PSCs) is easy to cause deep trap states and severe instability issues, especially under maximum power point tracking (MPPT). In this study, sodium gluconate (SG) is employed to disperse tin oxide (SnO2) nanoparticles (NPs) and regulate the interface contact at the buried interface. The SG-SnO2 electron transfer layer (ETL) enabled the deposition of pinhole-free perovskite films in ambient air and improved interface contact by bridging effect. SG-SnO2 PSCs achieved an impressive power conversion efficiency (PCE) of 25.34% (certified as 25.17%) with a high open-circuit voltage (VOC) exceeding 1.19V. The VOC loss is less than 0.34V relative to the 1.53eV bandgap, and the fill factor (FF) loss is only 2.02% due to the improved contact. The SG-SnO2 PSCs retained around 90% of their initial PCEs after 1000h operation (T90 = 1000h), higher than T80 = 1000h for the control SnO2 PSC. Microstructure analysis revealed that light-induced degradation primarily occurred at the buried holes and grain boundaries and highlighted the importance of bottom-contact engineering.

In-situ synthesis of Fe phosphate layer on the porous hematite nanocube for efficient photoelectrochemical water splitting

Hematite (α-Fe2O3) is considered as an ideal photoanode material in photoelectrochemical (PEC) water splitting system due to its suitable band gap, excellent stability and abundant reserves. However, the actual PEC water oxidation performance of α-Fe2O3 is greatly limited by its severe charge recombination and sluggish water oxidation kinetics. Here, an in-situ generated Fe phosphate (Fe-Pi) coating strategy is applied on the surface of porous hematite nanocube (Fe2O3 NC) to realize PEC water oxidation effectively. The obtained Fe-Pi/Fe2O3 NC photoanode exhibits a remarkable PEC property with a photocurrent density of 2.7mAcm-2 at 1.23V vs. RHE, which was about 5.7-fold enlarged compared to the pristine nanorod-like hematite (Fe2O3 NR) photoanode. Detailed studies have shown that the porous Fe2O3 NC possesses an enhanced ability in light absorption and charge separation than the traditional Fe2O3 NR photoanode. In addition, the in-situ loading of Fe-Pi layer as a co-catalyst can not only effectively passivate the surface trapping states of Fe2O3 NC and suppress interfacial charge recombination, but also quickly extract the holes to participate in the water oxidation reaction. Our study suggests that the further surface modification based on morphological engineering is an effective strategy to improve the PEC performance of hematite.

Advanced spectroscopic methods for probing in-gap defect states in amorphous SiNx for charge trap memory applications

Silicon nitride (SiNx) serves as the charge trap layer in current 3D NAND flash memory devices. The precise formation mechanism and electronic structure of localized defect trap states in SiNx remain elusive. Here, we present a refined experimental methodology to elucidate the in-gap defect states and the band gaps in amorphous SiNx thin films. Our approach integrates high-resolution reflection electron energy loss spectroscopy (REELS) and spectroscopic ellipsometry (SE) for comprehensive analysis. By systematical analysis, we aim to provide a robust method for determining in-gap electronic states in SiNx. We investigated two different SiNx films prepared by plasma-enhanced chemical vapor deposition and sputtering. Our analysis revealed several distinct in-gap states and determined band gap energies. This approach not only provide advanced spectroscopic methods to characterize the defect electronic states in SiNx, but also applicable to other large band gap semiconductors or dielectrics to predict device-level characteristics for future devices.

Mitigating bulk halide defects in blue-emissive perovskite nanocrystals using benzoyl halides for efficient light-emitting diodes

While recent advances have demonstrated highly efficient and stable perovskite light-emitting diodes (LEDs), blue perovskite LEDs still show low performance compared to green, red, near IR counterparts. Despite perovskites being well-known for their defect tolerance, blue perovskites are highly vulnerable to defects, which introduce deep trap states. In this study, we mitigated bulk halide vacancies in perovskite nanocrystals (PNCs) via the diffusion-assisted halide repairing strategy. We confirmed halide vacancies inside as-synthesized PNCs, both in the core and on the surface, through HADDF-STEM. By treating PNCs with benzoyl halides, these halide vacancies were further mitigated due to halide diffusion via concentration gradients between the surface and the core, resulting in enhanced PLQY of the PNCs. In the fabrication of blue PNC LEDs, this approach yielded a comparable efficiency with an EQE of 5.37 % at 469 nm, demonstrating the robustness of our method for efficient blue perovskite LEDs. Furthermore, the peak wavelengths of benzoyl halide-treated PNC LEDs remained stable during operation, whereas those of pristine PNC LEDs exhibited peak shifts. We believe that our novel approach can be universally applied to various types of perovskites, reducing bulk vacancies. This advancement will be a significant breakthrough for the performance enhancement of blue perovskite LEDs in the future.

Enhanced carrier transfer performance on blade-coating PbS quantum dot solar cells by coordinated operation of iodine ligands

The significant breakthrough in the photovoltaic conversion efficiency of lead sulfide (PbS) quantum dot solar cells has primarily been achieved on small substrates using the spin-coating method. However, the blade-coating technique offers a scalable alternative for large-scale fabrication of absorption film, addressing the limitations of spin-coating for commercial applications. Given the inherent challenges of the blade-coating method, which often produces low-quality and defective films. Thus, an iodide ligand passivation strategy to realize the formation of high-quality films with reduced trap states was proposed. Typically, the iodide ligand is incorporated into the precursor solution with the aim of fine-tuning the nanoparticle’s functionality, facilitating the formation of systematically aligned and densely coupled film configurations during the blade coating procedure. The functional mechanisms of the aligned and compact-coupled configurations, as well as the effect of iodide introduction on defect mitigation, have been systematically investigated. The results reveal that iodide ligand introduction modulates the interparticle electrostatic potential and inter-dot space, facilitating the formation of a uniform structure with enhanced energy level alignment and reduced trap states. The highly aligned and compact structure improves interfacial integration with the ZnO electron transport layer, enabling efficient photon-electron extraction and transfer. As a result, PbS quantum dot solar cells fabricated using the blade coating technique have achieved an optimized efficiency of 4.68 %, offering significant potential for the development and production of directly-synthesized ink for quantum dot solar cells.

Suppressed nonradiative electron–hole recombination in bismuth halide perovskite Cs3Bi2Cl9: Time domain ab initio analysis

Time-domain density functional theory, coupled with non-adiabatic molecular dynamics simulations, was employed to explore the defect characteristics and the associated nonradiative recombination processes in the bismuth halide perovskite Cs3Bi2Cl9. Our findings indicate that Cs3Bi2Cl9 inherently exhibits p-type semiconductor behavior, with vacancies at the Cs and Bi sites acting as predominant shallow acceptor defects. Although Cl vacancy and interstitial Cl defects introduce trap states within the bandgap of Cs3Bi2Cl9, the by-defect electron–hole (e-h) recombination is substantially impeded, which is attributed to the remarkable local structural deformations associated with the BiCl63− octahedral compression around the defects, which further results in decoupling between the defect state and the band edge state. As a result, the enhanced delocalization of defect states leads to a notably small wave function overlap between defect states and band edge states, as well as weak nonadiabatic couplings dominated by low-frequency phonons. Our study offers crucial insights into the mechanism of defect-mediated e-h recombination in bismuth-based perovskites and provides guidelines for designing efficient optoelectronic devices based on these materials.

Understanding the Size‐Dependent Photostability and Photoluminescence Intermittency of Blue‐Emitting Core/Graded Alloy/Shell “giant”‐Quantum Dots

AbstractRecently, giant quantum dots (g‐QDs) with a core/interface graded alloy shell/shell structure have shown promise in reducing photoluminescence (PL) intermittency and improving photostability. However, this approach has been mainly demonstrated with red and green emitting g‐QDs but the blue‐emitting graded alloy QDs has remained less explored. To tackle this challenge, a composition gradient method is employed to create three blue‐emitting CdZnS/CdxZn1–xS/ZnS core/interface graded alloy shell/shell (C/A/S) quantum dots (QDs) with different diameters. The sample with the largest diameter (gQD‐3) exhibits superior optical characteristics, with a photoluminescence quantum yield (PLQY) of approximately 62% and around 80% ON/radiative events at the single‐particle level. Conversely, the smallest diameter (gQD‐1) sample shows lower PLQY and only 30% radiative events with longer OFF/nonradiative events. Probability distribution analysis of PL trajectories, fitted with a truncated power law, reveals a significantly higher carrier de‐trapping rate in gQD‐3 compared to gQD‐1, attributed to its proximity to band edge trap states. Additionally, the largest diameter sample retains remarkable optical performance during 48 h of continuous UV irradiation in colloidal suspension and single‐particle levels. These findings show optimized core/shell structures, gradual alloy interfaces, and outer shell coatings can stabilize blue‐emitting quantum dots, advancing next‐gen optoelectronics.

Fe-N Co-Doped BiVO4 Photoanode with Record Photocurrent for Water Oxidation.

Bismuth vanadate ranks among the most promising photoanodes for photoelectrochemical water splitting. Nonetheless, slow charge separation and transportare key barriers to its photoefficiency. Here, we present a co-doping strategy that significantly improves the charge separation performance of BVO. Under standard one sun illumination, the Fe-N co-doped BVO photoanode (Fe-N-BVO) by N-coordinated Fe precursor reaches a record photocurrent density of 7.01 mA cm-2 at 1.23 V vs RHE after modified a surface co-catalyst. By contrast, much lower photocurrent density is obtained for the N-doped and Fe-doped BVO with separated N and Fe precursors. The detailed characterizations show that the high activity of the Fe-N-BVO is attributed to the enhanced photo-induced bulk charge separation and the accelerated surface water oxidation kinetics. XPS, EXAFS and DFT calculations clearly show that, instead of formation of deep trapping state in the individually doped BVO, the co-doping of Fe-N into BVO generates Fe-based electronic states just below the bottom of conduction band and N-derived states just above the top of valence band. Such modulations in electronic structure enable the efficient trap of the electrons and holes to enhance the separation of photo-induced carriers, but hinder the charge recombination originated from the deep trapping sites.

Non-Traditional Response to Intensive NBS and NBIS Treatments on IGZO TFTs

This work provides an interpretation of defect and trap state behavior following the application of intensive negative-bias stress (NBS) and negative-bias illumination stress (NBIS) on IGZO TFTs. Staggered bottom-gate TFTs were fabricated with a 50 nm IGZO film sandwiched between a 50 nm gate oxide and a 50 nm passivation oxide layer. The working metal was molybdenum for the gate electrode and source/drain contacts. A passivation anneal was done and an alumina capping layer was deposited via ALD to promote electrical stability; full process details are provided in a previous report. Devices were tested with a Keysight B1500 parameter analyzer to establish initial device characteristics, with +20 V PBS and -10 V NBS stress applications for 20 ks having a negligible response. Intensive stress testing was then done for 10 ks at either high bias (-20 V NBS) or with the addition of illumination (-10 V NBIS at 410 nm). The responses to intensive NBS and NBIS applications were both non-traditional and distinctive.