Localized Trap States Research Articles

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.

Interfacial Proton-Coupled Electron Transfer via Localized Trap States on Metal Oxide Surfaces

Interfacial Proton-Coupled Electron Transfer via Localized Trap States on Metal Oxide Surfaces

Unveiling heterogeneity of hysteresis in perovskite thin films

The phenomenon of current–voltage hysteresis observed in perovskite-based optoelectronic devices is a critical issue that complicates the accurate assessment of device parameters, thereby impacting performance and applicability. Despite extensive research efforts aimed at deciphering the origins of hysteresis, its underlying causes remain a subject of considerable debate. By employing nanoscale investigations to elucidate the relationship between hysteresis and morphological characteristics, this study offers a detailed exploration of photocurrent–voltage hysteresis at the nanoscale within perovskite optoelectronic devices. Through the meticulous analysis of localized I–V curve arrays, our research identifies two principal hysteresis descriptors, uncovering a predominantly inverted hysteresis pattern in 87% of the locations examined. This pattern is primarily attributed to the energetic barrier encountered at the interface between the probe and the perovskite material. Our findings underscore the pronounced heterogeneity and grain-dependent variability inherent in hysteresis behavior, evidenced by an average Hysteresis Index value of 0.24. The investigation suggests that the localized hysteresis phenomena cannot be exclusively attributed to either photocharge collection processes or organic cation migration at grain boundaries. Instead, it appears significantly influenced by localized surface trap states, which play a pivotal role in modulating electron and hole current dynamics. By identifying the key factors contributing to hysteresis, such as localized surface trap states and their influence on electron and hole current dynamics, our findings pave the way for targeted strategies to mitigate these effects. This includes the development of novel materials and device architectures designed to minimize energy barriers and enhance charge carrier mobility, thereby improving device performance and longevity. This breakthrough in understanding the microscale mechanisms of hysteresis underscores the critical importance of surface/interface defect trap passivation in mitigating hysteretic effects, offering new pathways for enhancing the performance of perovskite solar cells.

Investigations of charge extraction and trap-assisted recombination in polymer solar cells via hole transport layer doped with NiO nanoparticles

Polymer solar cells (PSCs) are limited in their power conversion efficiency (PCE) by trap-assisted recombination caused by localized deep trap states. Therefore, developing efficient hole transport layers (HTLs) for PSCs is essential for facilitating the hole extraction stage from the photoactive layers. As part of this study, an organic/inorganic hybrid HTL was prepared by mixing synthesized nickel oxide (NiO) nanoparticles (NPs) of average size ranging from 16 to 21 nm with poly(3-hexylthiophene) (P3HT). Based on this hybrid HTL, multilayer PSCs with an ITO/ZnO/P3HT:PC61BM/P3HT:NiO NPs/MoO3/Ag structure were fabricated and characterized. As a result of the incorporation of the optimum ratio of 5 wt% NiO NPs in the P3HT layer, the PCE obtained 3.73% compared to pristine devices (2.70%) fabricated without the P3HT layer. A charge extraction by increasing voltage (CELIV) measurement confirmed enhanced hole extraction at the active layer/HTL interface. The low fluorescence emission of the P3HT/NiO NPs film enabled efficient hole extraction at the active layer/hybrid HTL interface. Adding NiO NPs beyond 5 wt% to the P3HT layer causes deep trap states to form, which facilitated trap-assisted recombination and slowed down the mobility of the holes. Consequently, we can gain insight into how to improve PSC performance with hybrid HTL.

Subthreshold Conduction of Disordered ZnO-Based Thin-Film Transistors.

This study presents the disorderedness effects on the subthreshold characteristics of atomically deposited ZnO thin-film transistors (TFTs). Bottom-gate ZnO TFTs show n-type enhancement-mode transfer characteristics but a gate-voltage-dependent, degradable subthreshold swing. The charge-transport characteristics of the disordered semiconductor TFTs are severely affected by the localized trap states. Thus, we posit that the disorderedness factors, which are the interface trap capacitance and the diffusion coefficient of electrons, would result in the degradation. Considering the factors as gate-dependent power laws, we derive the subthreshold current-voltage relationship for disordered semiconductors. Notably, the gate-dependent disorderedness parameters are successfully deduced and consistent with those obtained by the gm/Ids method, which was for the FinFETs. In addition, temperature-dependent current-voltage analyses reveal that the gate-dependent interface traps limit the subthreshold conduction, leading to the diffusion current. Thus, we conclude that the disorderedness factors of the ZnO films lead to the indefinable subthreshold swing of the ZnO TFTs.

Boosting the Photoluminescence Efficiency of InAs Nanocrystals Synthesized with Aminoarsine via a ZnSe Thick-Shell Overgrowth.

InAs-based nanocrystals can enable restriction of hazardous substances (RoHS) compliant optoelectronic devices, but their photoluminescence efficiency needs improvement.Wereport an optimized synthesis of InAs@ZnSe core@shell nanocrystals allowing to tune the ZnSe shell thickness up to seven mono-layers (ML) and to boost the emission, reaching a quantum yield of ≈70% at ≈900nm. It is demonstrated that a high quantum yield can be attained when the shell thickness is at least ≈3ML. Notably, the photoluminescence lifetimeshows only a minor variation as a function of shell thickness, whereas the Auger recombination time (a limiting aspect in technological applications when fast) slows down from 11 to 38ps when increasing the shell thickness from 1.5 to 7MLs. Chemical and structural analyses evidence that InAs@ZnSe nanocrystals do not exhibit any strain at the core-shell interface, likely due to the formation of an InZnSe interlayer. This is supported by atomistic modeling, which indicates the interlayer as being composed of In, Zn, Se and cation vacancies, alike to the In2 ZnSe4 crystal structure. The simulations reveal an electronic structure consistent with that of type-I heterostructures, in which localized trap states can be passivated by a thick shell (>3ML) and excitons are confined in the core.

Threshold-Voltage Extraction Methods for Atomically Deposited Disordered ZnO Thin-Film Transistors.

In this paper, we present a threshold-voltage extraction method for zinc oxide (ZnO) thin-film transistors (TFTs). Bottom-gate atomic-layer-deposited ZnO TFTs exhibit typical n-type enhancement-mode transfer characteristics but a gate-voltage-dependent, unreliable threshold voltage. We posit that this obscure threshold voltage is attributed to the localized trap states of ZnO TFTs, of which the field-effect mobility can be expressed as a gate-bias-dependent power law. Hence, we derived the current-voltage relationship by dividing the drain current with the transconductance to rule out the gate-bias-dependent factors and successfully extract the reliable threshold voltage. Furthermore, we investigated the temperature-dependent characteristics of the ZnO TFTs to validate that the observed threshold voltage was genuine. Notably, the required activation energies from the low-temperature measurements displayed an abrupt decrease at the threshold voltage, which was attributed to the conduction route change from diffusion to drift. Thus, we conclude that the reliable threshold voltage of accumulation-mode ZnO TFTs can be determined using a gate-bias-dependent factor-removed current-voltage relationship with a low-temperature analysis.

Probing Intrinsic Defect-Induced Trap States and Hopping Transport in Two-Dimensional PdSe2 Semiconductor Devices.

Palladium diselenide (PdSe2), as an emerging two-dimensional (2D) layered material, is gaining growing attention in nanoelectronics and optoelectronics due to its thickness-dependent band gap, high carrier mobility, and good air stability. However, its asymmetric pentagon structure is inclined to breed defects. Herein, the intrinsic Se vacancy-induced trap states and their influence on the hopping transport in PdSe2 are systematically investigated. We provide direct evidence that Se vacancies exist in the fresh PdSe2 samples, which results in the localized trapping states inside the band gap. For the few-layer PdSe2, at 77 K, the trap density (Dit) near the midgap is about 2.2 × 1013 cm-2 eV-1, whereas at 295 K, the Dit value increases to ∼7.1 × 1013 cm-2 eV-1. By comparison, the multilayer PdSe2 shows nonobvious temperature-dependent trap behaviors with almost unchanged Dit values of ∼8.1 × 1012 cm-2 eV-1 at midgap in the temperature range between 77 and 295 K. Thus, trap states in the few-layer PdSe2 are more vulnerable to temperature effect. Transport measurements demonstrated that both few-layer and multilayer PdSe2 field-effect transistor (FET) devices show n-type dominant ambipolar behaviors. The electron mobility in the multilayer PdSe2 FET is nearly 15-fold higher than that in the few-layer PdSe2 FET at 315 K, probably owing to the decreased effective mass and suppression of charge impurity scattering in the thicker channel material. However, both FET devices exhibit variable-range hopping over a temperature range from 77 to 240 K and thermally activated hopping at temperatures above 240 K. The hopping transport mechanism is strongly associated with the Se vacancy-induced localized states with poor screening and strong potential fluctuations. This study reveals the important role of structural defects in tailoring and improving the charge transport properties of PdSe2.

Vacancy-Mediated Anomalous Emission Characteristics of Size-Confined Semiconducting CoTe2.

Transition-metal tellurides (TMTs) are promising materials for "post-graphene age" nanoelectronics and energy storage applications owing to their industry-standard compatibility, high electron mobility, large spin-orbit coupling (SOC), etc. However, tellurium (Te) having a larger ionic radius (Z = 52) and broader d-bands endows TMTs with semimetallic nature, restricting their application in photonic and optoelectronic domains. In this work, we report the optical properties of the quantum-confined semiconducting phase of cobalt ditelluride (CoTe2) for the first time, exhibiting excellent two-color band photoabsorption attributes covering the UV-visible and near-infrared regions. Furthermore, novel excitonic resonances (X) of size-varying CoTe2 nanocrystals and quantum dots (QDs) are indicated by their temperature-dependent emission characteristics, which are attributed to the splitting of band edge states via confinement. On the other hand, the sudden rupture of the large-area CoTe2 nanosheets via ultrasonication incorporates Co vacancy-mediated localized trap states within the band gap, which is attributed to the superior room-temperature photoluminescence (PL) quantum yield of QDs and further corroborated using Raman analysis and atomistic density functional theory (DFT) simulations. Most interestingly, the excitonic peak of CoTe2 QDs reveals a unique positive-to-negative thermal quenching transition phenomenon, owing to the thermal activation of nonradiative surface trap states. These results introduce an exciting approach for the defect-mediated color-saturated light emission that paves the way for solution-processed telluride-based QD light-emitting diodes.

Double exponential density of states and modified charge carrier transport in organic semiconductors

This paper discusses the use of an approximated charge carrier density to model an organic thin-film transistor (OTFT) using a double exponential density of states. Traditionally, published work employs a single exponential density of states and Gaussian density of states. On the contrary, this paper employs a double exponential density of states in the Fermi integral to evaluate the charge carrier density for the OTFT. We consider two exponential density of states, one rateled to the tail region and one to a deep region, in addition to various associated parameters. The distribution of localized trap states between the highest and lowest orbital is expressed as a density of states, one for the tail states and one for the deep states. Tail states are better described by the Gaussian function, while deep states are better described by the exponential density of states. Therefore, if we require that the two regions be defined by a single function, then the function should be a sum of the two, the exponential and the Gaussian, to more accurately describe the complete region. The double exponential density of states is employed to evaluate and approximate the Fermi integral using various mathematical methods, so that the error is lower for various parameters.

Enhanced channel modulation in Aluminum- and Hydrogen-Doped Zinc-Oxide-Based transistors by complementary Dual-Gate operation

This study elucidates the enhanced channel modulation by the dual-gate operation of the ultrathin ZnO-based field-effect transistors (FETs). Bottom-gate atomic-layer-deposited zinc oxide (ZnO) transistors exhibit typical n-type enhancement-mode transfer characteristics. However, when equipped with the top Al2O3 layer, the FETs exhibit conductive transfer characteristics. These electrical property changes are attributed to Al2O3-induced hydrogen and aluminum doping effects, which increase carrier concentration. Although Al2O3-induced doping increased field-effect mobility by a factor of 2, a high off current and degraded transconductance swing were observed in FETs. However, operation in dual-gate mode after the deposition of the top-gate electrode resolved these degradations and led to achieving high-performance ZnO FETs: the off current significantly decreased, and noteworthy, field-effect mobility increased by a factor of 5. Temperature-dependent charge transport analysis revealed that the complementary dual-gate operation efficiently filled the localized trap states in the ZnO films, resulting in band-like transport. Thus, it is deduced that the complementary dual-gate operation enhanced channel modulation in the Al2O3-induced hydrogen- and aluminum-doped ZnO channel, resulting in high-performance ZnO FETs.

Element doping-induced effects in Zn-doped CdTe quantum-dot system: Insights from an ultrafast dynamics perspective.

Element doping can have a profound impact on the photoelectrochemical properties of quantum dots (QDs); nevertheless, the hitherto known information in this regard is mainly from the steady-state characterizations and remains lacking input from the dynamics perspective. Herein, we present a systematic scrutiny of the element doping-induced effects in Zn-doped CdTe QDs. By means of steady-state/time-resolved/temperature-dependent photoluminescence spectroscopy and ultrafast transient absorption spectroscopy, we reveal that the slight Zn-doping in CdTe QDs can greatly affect the involved carrier relaxation dynamics through a density-of-state modification for both near-band-edge and localized surface trap states. Furthermore, such slight doping is found to be quite significant in modulating the photoreduction efficiency (of particular relation to the localized surface trap states) as well as altering the involved relaxation/reaction activation energy and phonon effect in this QDs system. This work enriches our fundamental understanding of the element doping-induced surface/interface effects, from the dynamics perspective in particular, and, hence, offers helpful guidance for QDs-based photoelectrochemical design and optimization.

Defect Tolerance Mechanism Revealed! Influence of Polaron Occupied Surface Trap States on CsPbBr3 Nanocrystal Photoluminescence: Ab Initio Excited-State Dynamics.

Lead halide perovskite (LHP) nanocrystals (NCs) show exceptional defect tolerance which has been attributed to their unique electronic structure, where defect energy levels are not introduced inside the fundamental bandgap, and the role of polarons in screening charge carriers from defects. Here, we use ab initio atomistic simulations to explore the interplay between various surface chemistries (A = Cs+, R'NH3+; X = Br-, RCOO-) used to passivate a CsPbBr3 NC surface and their impact on the ground-state (GS) and excited-state (ES) photophysical properties. We investigate pristine fully passivated surfaces and A-X vacancy defects that reflect chemical reactions A+ + X- → AX on the surface, which result in ligand desorption. For each surface configuration, calculations are performed in the GS and lowest ES (L-ES) electronic configurations, approximating polaron formation after photoexcitation. For models with A-X surface vacancies, we find that localized electron surface trap (ST) states emerge ∼100-400 meV below the pristine Se band in the L-ES configuration due to polaronic nuclear reorganization. Surprisingly, these trap states contribute relatively bright Sh → ST spectral features. To test if these surface trap states remain bright in a dynamic (thermal) situation we implement excited-state molecular dynamics simulations. It is found that the surface defected model shows an enhanced nonradiative recombination rate which reduces the photoluminescence quantum yield (PLQY) from 95% for the pristine surface to 75%. This is accompanied by an order of magnitude reduction in PL intensity and a red shift of the transition energy. This study provides more evidence of the defect tolerance of LHP NCs along with evidence of surface trap states contributing to efficient photoluminescence. The observation of relatively bright surface trap states could provide insight into photophysical phenomena, such as size-dependent stretched-exponential photoluminescence decay and Stokes shifts.

Shape, Electronic Structure, and Trap States in Indium Phosphide Quantum Dots

Localized states on undercoordinated atoms at the surface of a quantum dot (QD) can become electron or hole traps when the atomic orbitals involved fall within the highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) gap of the material. Therefore, QDs of more covalent materials are predicted to be more sensitive to trap-state formation. Here, we analyze the relation between the presence of undercoordinated surface atoms and the formation of localized trap states for InP QDs by means of density functional theory. We argue that the requirement of charge neutrality limits the possible shapes cation-rich InP QDs can take, among which small cuboctahedrons and tetrahedrons of any size. We thus select both structures as InP model QDs and show that a complete InCl3 surface passivation results in an electronic structure with a clean, delocalized HOMO–LUMO gap in either case. Upon removal of InCl3 formula units from the InP cuboctahedron, hole traps linked to 2-coordinated surface P can form, a behavior similar to that of CdSe QDs. On the other hand, InCl3 displacement from the InP tetrahedron can lead to hole and electron traps, related to 2- or 3-coordinated surface P and 2-coordinated surface In, respectively. Since we obtain a very similar electronic structure of pristine InP and CdSe cuboctahedrons, we argue that the concept of ionicity provides little guidance to understand the enhanced sensitivity of InP to trap-state formation. Therefore, more apt descriptors are needed to predict the energetic position of atomic orbitals of uncoordinated atoms relative to the delocalized band-edge states.

Wavelength conversion through plasmon-coupled surface states

Surface states generally degrade semiconductor device performance by raising the charge injection barrier height, introducing localized trap states, inducing surface leakage current, and altering the electric potential. We show that the giant built-in electric field created by the surface states can be harnessed to enable passive wavelength conversion without utilizing any nonlinear optical phenomena. Photo-excited surface plasmons are coupled to the surface states to generate an electron gas, which is routed to a nanoantenna array through the giant electric field created by the surface states. The induced current on the nanoantennas, which contains mixing product of different optical frequency components, generates radiation at the beat frequencies of the incident photons. We utilize the functionalities of plasmon-coupled surface states to demonstrate passive wavelength conversion of nanojoule optical pulses at a 1550 nm center wavelength to terahertz regime with efficiencies that exceed nonlinear optical methods by 4-orders of magnitude.

Photodriven Charge Accumulation and Carrier Dynamics in a Water-Soluble Carbon Nitride Photocatalyst.

Charge accumulation in photoactive molecules and materials holds great promise in solar energy conversion as it allows for decoupling solar‐driven charging from (dark) redox reactions. In this contribution, light‐driven charge accumulation was investigated for a recently reported novel water‐soluble carbon nitride [K,Na‐poly(heptazine imide); K,Na‐PHI] photocatalyst, which exhibits excellent activity and stability in highly selective photocatalytic oxidation of alcohols and concurrent reduction of dioxygen to H2O2 under quasi‐homogeneous conditions. An excellent charge storage ability of the K,Na‐PHI material was demonstrated, showing an optimal density of accumulated electrons (32.2 μmol of electrons per gram) in the presence of 10 vol % MeOH as a sacrificial electron donor. The long‐lived electrons accumulated under anaerobic conditions as K,Na‐PHI.− radical ions were utilized in interfacial electron transfer to O2 or methyl viologen in a subsequent dark reaction. Ultrafast time‐resolved spectroscopy was employed to reveal the kinetics of charge‐carrier recombination and methanol oxidation. Geminate recombination of electrons and holes within approximately 100 ps was followed by trap‐assisted recombination. The presence of methanol as a sacrificial electron donor accelerated the decay of the transient absorption signal when a static sample was used. This behavior was ascribed to the faster charge recombination in the presence of the radical anions generated after hole extraction. The work suggests that photodriven electron storage in the water‐soluble carbon nitride is enabled by localized trap states, and highlights the importance of the effective electron donor for creating long‐lived photo‐generated carbon nitride radicals.

Halide Perovskite-Lead Chalcohalide Nanocrystal Heterostructures.

We report the synthesis of colloidal CsPbX3–Pb4S3Br2 (X = Cl, Br, I) nanocrystal heterostructures, providing an example of a sharp and atomically resolved epitaxial interface between a metal halide perovskite and a non-perovskite lattice. The CsPbBr3–Pb4S3Br2 nanocrystals are prepared by a two-step direct synthesis using preformed subnanometer CsPbBr3 clusters. Density functional theory calculations indicate the creation of a quasi-type II alignment at the heterointerface as well as the formation of localized trap states, promoting ultrafast separation of photogenerated excitons and carrier trapping, as confirmed by spectroscopic experiments. Postsynthesis reaction with either Cl– or I– ions delivers the corresponding CsPbCl3–Pb4S3Br2 and CsPbI3–Pb4S3Br2 heterostructures, thus enabling anion exchange only in the perovskite domain. An increased structural rigidity is conferred to the perovskite lattice when it is interfaced with the chalcohalide lattice. This is attested by the improved stability of the metastable γ phase (or “black” phase) of CsPbI3 in the CsPbI3–Pb4S3Br2 heterostructure.

Photogating-driven enhanced responsivity in a few-layered ReSe2 phototransistor

The paper presents a thorough investigation of photoconductive properties of few layers of rhenium diselenide (ReSe2). A correlation between responsivity (R) and power exponent (γ) indicates localized trap states plays a crucial role in photocurrent generation, commonly known as photogating. These trap states can be modulated by external factors such as temperature and gate voltage.

(Invited) Phase Stable and Less-Defect Perovskite Quantum Dots: Optical Property, Photoexcited Carrier Dynamics, and Application to Solar Cells

Perovskite quantum dots (QDs) as a new type of colloidal nanocrystals have gained significant attention for both fundamental research and commercial applications owing to their appealing optoelectronic properties and excellent chemical processability.1 For their wide range of potential applications, synthesizing colloidal QDs with high crystal quality is of crucial importance. However, like most common QD systems, those reported perovskite QDs still suffer from a certain density of trapping defects, giving rise to detrimental non-radiative recombination centers and thus low photoluminescence quantum yield (PL QY). Halide vacancy in perovskites has been widely accepted to be the main factor preventing near-unity PL QY by creating uncoordinated Pb octahedra and the subsequent localized deep trap states.2 Very recently, we have succeeded in synthesis of phase-stable and less-defect perovskite QDs, including FAPbI3 QDs, CsPbI3 QDs and Sn-Pb alloyed QDs.3-6 We have demonstrated that a high room-temperature PL QY of up to 100% can be obtained in FAPbI3 and CsPbI3 perovskite QDs, signifying the achievement of almost complete elimination of the trapping defects. This is realized by the modulation of halide precursor involved in the synthesis, in which trioctylphosphine (TOP) is used to dissolve halide salts, allowing sufficient releasing of halide ions as well as improved surface passivation. Ultrafast kinetic analysis with time-resolved transient absorption spectroscopy evidences the negligible electron or hole trapping pathways in our QDs, which explains such a high quantum efficiency. For Sn-Pb alloyed QDs, although the alloyed QDs possess much improved structural stability than that of the pure Pb- and Sn-based one, they still suffer from an extremely low PL QY of ~0.3%, indicative of the severe charge trapping in these Sn-containing perovskites. Such low PL QY is believed to result from the widely known facile oxidation of Sn2+ to Sn4+. To address this issue, we have demonstrated very recently that the deep-level trap states in Sn-based perovskites can be well suppressed by Na+ doping, which gives rise to a dramatically improved PL QY of ~28% without alerting their favorable electronic structure.7 X-ray photoelectron spectroscopy (XPS) studies suggest the formation of a stronger chemical interaction between I- and Sn2+ ions upon Na doping, which is in line with the results obtained by DFT calculations. On the other hand, though the reported TOP-based route has demonstrated efficacy in producing CsSnxPb1-xI3 QDs (x = 0~0.6) with superior structural stability, such synthetic protocol can only find success in making these I-related QDs, preparing those pure Cl- and Br-based analogues remains challenging because of the insufficient solubility of PbBr2 and PbCl2 in TOP. Therefore, by taking advantage of the strong coordinating ability of trioctylphosphine oxide (TOPO), we have recently reported the first synthesis of ASnxPb1-xX3 QDs (A = Cs, FA, MA) with excellent tunability in both Sn/Pb ratio and a full range of halide covering. The ability to generate all possible, unique combinations has offered great material versatility for the perovskite QD family. Further, by employing ultrafast spectroscopic techniques, we have resolved the fundamental photoexcited charge carrier dynamics in these alloyed QDs including trapping states-involved recombination, hot carrier relaxation, and charge transfer dynamics, which shall pave the way for future study for enhancing their usefulness in optoelectronic applications.8 In addition, photoexcited hot and cold carrier dynamics as well as charge transfer at the heterojunction of QD/metal oxide were systematically investigated.9 Solar cells based on these high-quality perovskite QDs exhibit power conversion efficiency of over 12%, showing great promise for practical application. We expect the successful synthesis of the “ideal” perovskite QDs will exert profound influence on their applications to both QD-based light-harvesting and -emitting devices in the near future. References Swarnkar, A. et al., Science 2016, Vol. 354, pp. 92-95.Alivisatos et al., J. Am. Chem. Soc. 2018, Vol. 140, pp. 17760F. Liu and Q. Shen et al., ACS Nano , Vol. 11, No. 10, pp. 10373-10383, 2017.F. Liu and Q. Shen et al., J. Am. Chem. Soc. , Vol. 139, No. 46, pp. 16708-16719, 2017.F. Liu and Q. Shen et al., J. Phys. Chem. Lett. , Vol. 9, No. 2, pp. 294-297, 2018.F. Liu and Q. Shen et al., Chem. Mater. , Vol. 31, No. 3, pp. 798-807, 2019.F. Liu and Q. Shen et al., Angew. Chem. Int. Ed. , Vol. 59, pp. 1-5, 2020.F. Liu and Q. Shen et al., Chem. Mater. , Vol. 32, No. 3, pp. 1089-1100, 2020.C. Ding and Q. Shen et al., Nano Energy 2020, 67, 104267.

Effect of the Degree of the Gate‐Dielectric Surface Roughness on the Performance of Bottom‐Gate Organic Thin‐Film Transistors

AbstractIn organic thin‐film transistors (TFTs) fabricated in the inverted (bottom‐gate) device structure, the surface roughness of the gate dielectric onto which the organic‐semiconductor layer is deposited is expected to have a significant effect on the TFT characteristics. To quantitatively evaluate this effect, a method to tune the surface roughness of a gate dielectric consisting of a thin layer of aluminum oxide and an alkylphosphonic acid self‐assembled monolayer over a wide range by controlling a single process parameter, namely the substrate temperature during the deposition of the aluminum gate electrodes, is developed. All other process parameters remain constant in the experiments, so that any differences observed in the TFT performance can be confidently ascribed to effects related to the difference in the gate‐dielectric surface roughness. It is found that an increase in surface roughness leads to a significant decrease in the effective charge‐carrier mobility and an increase in the subthreshold swing. It is shown that a larger gate‐dielectric surface roughness leads to a larger density of grain boundaries in the semiconductor layer, which in turn produces a larger density of localized trap states in the semiconductor.