Grain Boundaries In Films Research Articles

Optimization of the photoelectric performances of Mg and Al co-doped ZnO films by a two-step heat treatment process

Mg and Al co-doped ZnO (AZMO) films were prepared by magnetron co-sputtering to improve the optical and electrical performances of the ZnO-based films simultaneously. A two-step heat treatment process was introduced to obtain high-quality AZMO films. AZMO films were first deposited with substrates heating of 300 °C and then annealed at 500 °C in vacuum. The individual and coupling effects of substrate heating during sputtering deposition and post-annealing treatment on AZMO films were investigated. The results show that substrate heating during sputtering deposition is mainly on increasing the grain size and crystallinity of AZMO films. However, the effects of post-annealing treatment are mainly on making Mg and Al atoms move to the lattice points and ionize to from MgZn and AlZn. Besides, post-annealing treatment can effectively desorb adsorbed oxygen from the surface or grain boundaries of AZMO films and also make lattice oxygen break away from the lattice and change into oxygen vacancy. Compared with one-step heat treatment alone, it is with the two-step heat treatment process that the coupling effects can be exerted by combining the different effects of the two heat treatments, so as to achieve more performances optimization of AZMO films.

Stabilization of FAPbI3 with multifunctional alkali-functionalized polymer.

The defects located at the interfaces and grain boundaries (GBs) of perovskite films are detrimental to the photovoltaic performance and stability of perovskite solar cells. Manipulating the perovskite crystallization process and tailoring the interfaces with molecular passivators are mainly effective strategies to mitigate performance loss and instability. Herein we report a new strategy to manipulate the crystallization process of FAPbI3 -rich perovskite by incorporating a small amount of alkali functionalized polymers into the antisolvent solution. The synergic effects of alkali cations and polyacrylic acid anion effectively passivate the defects on the surface and GBs of perovskite films. As a result, the rubidium (Rb)-functionalized polyacrylic acid significantly improves the power conversion efficiency of FAPbI3 perovskite solar cells to approaching 25% and reduces the risk of lead ion (Pb2+ ) leakage via the strong interaction between C = O bond and Pb2+ . In addition, the unencapsulated device shows enhanced operational stability, retaining 80% of its initial efficiency after 500h operation at maximum power point under one-sun illumination. This article is protected by copyright. All rights reserved.

Nano-capillary induced assemble of quantum dots on perovskite grain boundaries for efficient and stable perovskite solar cells

In recent years, perovskite solar cells (PSCs) have propelled into the limelight owing to rapid development of efficiency; however, the abundant defects at the perovskite grain boundaries result in unwanted energy loss and structural degradation. Here, the grain boundaries of perovskite polycrystalline films have been found to act as nanocapillaries for capturing perovskite quantum dots (PQDs), which enable the conformal assemble of PQDs at the top interspace between perovskite grains. The existence of PQDs passivated the surface defects, optimized the interfacial band alignments, and ultimately improved the power conversion efficiency from 19.27% to 22.47% in inverted PSCs. Our findings open up the possibility of selective assembly and structural modulation of the perovskite nanostructures towards efficient and stable PSCs.

Defects boost graphitization for highly conductive graphene films.

Fabricating highly crystalline macroscopic films with extraordinary electrical and thermal conductivities from graphene sheets is essential for applications in electronics, telecommunications and thermal management. High-temperature graphitization is the only method known to date for the crystallization of all types of carbon materials, where defects are gradually removed with increasing temperature. However, when using graphene materials as precursors, including graphene oxide, reduced graphene oxide and pristine graphene, even lengthy graphitization at 3000°C can only produce graphene films with small grain sizes and abundant structural disorders, which limit their conductivities. Here, we show that high-temperature defects substantially accelerate the grain growth and ordering of graphene films during graphitization, enabling ideal AB stacking as well as a 100-fold, 64-fold and 28-fold improvement in grain size, electrical conductivity and thermal conductivity, respectively, between 2000°C and 3000°C. This process is realized by nitrogen doping, which retards the lattice restoration of defective graphene, retaining abundant defects such as vacancies, dislocations and grain boundaries in graphene films at a high temperature. With this approach, a highly ordered crystalline graphene film similar to highly oriented pyrolytic graphite is fabricated, with electrical and thermal conductivities (∼2.0× 104 S cm-1; ∼1.7 × 103W m-1 K-1) that are improved by about 6- and 2-fold, respectively, compared to those of the graphene films fabricated by graphene oxide. Such graphene film also exhibits a superhigh electromagnetic interference shielding effectiveness of ∼90dB at a thickness of 10μm, outperforming all the synthetic materials of comparable thickness including MXene films. This work not only paves the way for the technological application of highly conductive graphene films but also provides a general strategy to efficiently improve the synthesis and properties of other carbon materials such as graphene fibers, carbon nanotube fibers, carbon fibers, polymer-derived graphite and highly oriented pyrolytic graphite.

Interfacial modification to improve all-inorganic perovskite solar cells by a multifunctional 4-aminodiphenylamine layer

All-inorganic carbon-based perovskite solar cells (PSCs) offer low costs and a favorable balance between stability and power conversion efficiency (PCE). Although bulk perovskites are defect tolerant, the defect-mediated carrier nonradiative recombination at the interface between perovskite and carbon layers limits the device performance and stability. In this study, a new 4-aminodiphenylamine (4-ADPA) interlayer was applied to modify the surface and grain boundaries of CsPbI2Br perovskite films. The 4-ADPA layer passivated defects by binding with undercoordinated lead and halide ions at the surface, resulting in the improved thin film quality, reduced defects, and suppressed carrier recombination. All-inorganic CsPbI2Br PSCs by 4-ADPA passivation achieved a PCE of 11.16% with reduced hysteresis and open-circuit voltage loss. In addition, the 4-ADPA passivation inhibited the degradation of the perovskite films in ambient conditions. After storing in air at 25 °C and 25% relative humidity for 700 h, the unpackaged PSC retains 80% of the original PCE. The all-inorganic PSCs with greatly improved device performance and stability would have great prospects in practical applications.

A Review of Nanocrystalline Film Thermoelectrics on Lead Chalcogenide Semiconductors: Progress and Application

Submicron-structured films of thermoelectric materials, exhibiting an improved thermoelectric figure of merit, are reviewed, including methods of fabrication and characterization. The review emphasizes the beneficial role of the grain boundaries in polycrystalline films. The enhanced Seebeck coefficient of lead chalcogenide films is attributed to a potential relief that is built along the grain boundaries. It scatters charge carriers with low energy and does not affect carriers with higher energy. The model that accounts for the thermoelectric properties of the films is described and assessed experimentally. The application of a flexible thermoelectric device (module) based on the nanocrystalline film thermoelectric semiconductors as high sensitivity radiation detectors is suggested.

Grain Boundary Engineering with Cl-Terminated Ti2C Quantum Dots for Enhancing Perovskite Solar Cell Performance

The power conversion efficiency and stability of polycrystalline perovskite solar cells are compromised by grain-boundary-dominated ion migration. Herein, the grain boundaries of CH3NH3PbI3 films were engineered using Ti2C quantum dots. Ti2C quantum dots had a strong interaction with Pb2+ and I– ions, which retarded crystal growth, resulting in larger grain size and less grain boundary in perovskite films. Additionally, the Ti2C quantum dots at the grain boundary could anchor ions and passivate defects, increasing activation energy for ion hopping and reducing trap density. In consequence, perovskite films were used to fabricate solar cells with significantly improved efficiency and stability. These findings may allow the development of versatile grain boundary modifiers for perovskite optoelectronic devices that combine stability and efficiency.

A study on the luminescence properties of high-performance benzothieno[3,2-b][1]benzothiophene based organic semiconductors

In terms of molecular structures, benzothieno[3,2-b][1]benzothiophene (BTBT) derivatives qualify as the best-performing organic semiconductors for hole transport, but it exhibits poor luminescence properties. For instance, a classical molecule of 2,7-Dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) only shows photoluminescence quantum yields (PLQYs) of 5.0% in chlorobenzene. Herein, we present three kinds of organic semiconductors based on phenyl-BTBT molecule core, namely, 2,7-diphenyl[1]benzothieno[3,2-b][1]benzothiophene (DPh-BTBT), 2,7-bis(4-ethylphenyl) benzothieno[3,2-b][1] benzothiophene (DBEP-BTBT), and 2,7-bis(4-methoxyphenyl)benzothieno[3,2-b][1]benzothiophene (DBOP-BTBT). Notably, blue fluorescence with relatively high PLQYs of 27%, 36%, and 48% in chlorobenzene was shown. Due to the surface defects and grain boundaries in the thin films, the PLQYs decrease to 3.4% for DPh-BTBT and 7.4% for DBOP-BTBT, respectively. Nevertheless, the DBEP-BTBT film still exhibits an absolute PLQY as high as 12%, which is a rare and precious characteristic in BTBT derivatives. Meanwhile, the differences in photophysical characteristics, crystal structures, and charge transport properties of the three BTBT derivatives were systematically studied. Similar to DPh-BTBT and DBOP-BTBT, the DBEP-BTBT crystals display typical layered herringbone packing structure with multi- and strong intermolecular interactions, which are conductive to form a two-dimensional charge transport network in layered thin films for high mobility. The mobility of DBEP-BTBT in polycrystalline thin film transistors is up to 3.22 cm2V−1s−1 in ambient air, which is close to DPh-BTBT (3.64 cm2V−1s−1) and much higher than that of DBOP-BTBT (0.5 cm2V−1s−1). Moreover, the calculations show that the intermolecular transfer integrals of HOMOs are almost isotropic for six nearest neighbour contacts in DBEP-BTBT and DPh-BTBT crystals, implying their possibility of isotropic hole transport property in single crystals.

Pseudohalogen Resurfaced CsPbBr3 Nanocrystals for Bright, Efficient, and Stable Green-Light-Emitting Diodes

Lead halide perovskite nanocrystals (LHP NCs) are regarded as promising emitters for next-generation ultrahigh-definition displays due to their high color purity and wide color gamut. Recently, the external quantum efficiency (EQE) of LHP NC based light-emitting diodes (PNC LEDs) has been rapidly improved to a level required by practical applications. However, the poor operational stability of the device, caused by halide ion migration at the grain boundary of LHP NC thin films, remains a great challenge. Herein, we report a resurfacing strategy via pseudohalogen ions to mitigate detrimental halide ion migration, aiming to stabilize PNC LEDs. We employ a thiocyanate solution processed post-treatment method to efficiently resurface CsPbBr3 NCs and demonstrate that the thiocyanate ions can effectively inhibit bromide ion migration in LHP NC thin films. Owing to thiocyanate resurfacing, we fabricated LEDs with a high EQE of 17.3%, a maximum brightness of 48000 cd m-2, and an excellent operation half-life time.

Spontaneous Internal Encapsulation via Dual Interfacial Perovskite Heterojunction Enables Highly Efficient and Stable Perovskite Solar Cells

Deep traps stemming from the defects formed at the surfaces and grain boundaries of the perovskite films are the main reasons of nonradiative recombination and material degradation, which significantly affect efficiency and stability of perovskite solar cells (PSCs). Here, a spontaneous internal encapsulation strategy was developed by constructing a dual interfacial perovskite heterojunction at the top and buried interface of the three-dimensional (3D) perovskite film. The spacer cations of the two-dimensional (2D) perovskite structure interacted strongly with the 3D perovskite to passivate the defects and optimize the energy level alignment. Meanwhile, the interfacial perovskite heterojunction underearth delayed the crystallization speed and improved the crystallization quality of the upper 3D perovskite. Thanks to these positive effects, the PSC exhibited a power conversion efficiency of 22.92% with good reproducibility. Significantly, the unencapsulated device with the dual interfacial perovskite heterojunction maintained 88% of its initial efficiency after 2900 h under 65 ± 5% RH in air.

Passivating detrimental grain boundaries in perovskite films with strongly interacting polymer for achieving high-efficiency and stable perovskite solar cells

The grain boundaries in polycrystalline metal-halide perovskite films contain numerous defects, which contribute to performance loss and degradation in perovskite solar cells. In this study, we report a cyano-containing conjugated polymer, composed of benzodithiophene and bicyanobenzothiadiazole (PBDT2CNBT), that functionalizes perovskite crystalline domains for defect passivation and charge carrier extraction. We demonstrate that cyano groups can effectively passivate these defects through strong interactions between the conjugated polymer and perovskite components. The PBDT2CNBT polymer not only modifies the perovskite film surface but also infiltrates the grain boundaries during perovskite crystallization. Notably, these conjugated polymers create favorable energy band bending at the boundary interface between the perovskite and conjugated polymer, enhancing charge separation and collection through grain boundaries by forming a cascade energy level structure. Consequently, we show that PBDT2CNBT transforms the intrinsically detrimental grain boundary into a beneficial feature for defect-free properties and improved charge collection. These advantages directly contribute to achieving high power conversion efficiencies of up to 22.9% and enhanced light, humidity, and thermal stability in perovskite solar cells. This study offers a novel strategy for developing multifunctional conjugated polymers for next-generation perovskite optoelectronics.

Surface potential variation across (hk1) and non-(hk1) grain boundaries of antimony triselenide

Electronic materials that can tolerate defects or create self-passivated interfaces expand the horizons to more sustainable electronic devices. Two heuristic models suggest that (i) antimony triselenide is a defect tolerant semiconductor (to certain point defects) and that (ii) the boundaries of grains with a specific orientation are self-passivated. Yet, experimental support or scrutiny of these models is lacking. Partly because measuring a passive quality of a material is perplexing. A possible way forward is to measure defect activity in correlation with other experimental evidence for defect presence, for example, by relating the grain boundaries’ (GBs) nanostructure and electronic quality. One measure of the electronic quality is the magnitude of the surface potential variation across grain boundaries—a high (charged) defect concentration and low defect tolerance typically leads to a significant surface potential variation. On the other hand, direct observation of defects (e.g., by electron microscopy) and a small surface potential variation indicates that the material is defect tolerant. In parallel, a comparison of the surface potential variation across grain boundaries in films with grains of different orientations provides a qualitative measure of the self-passivation of material because only certain crystallographic surfaces are expected to be self-passivated. Herein we report that regardless of the grains’ mutual respective orientation, the grain boundaries of antimony triselenide have a relatively benign electronic quality. This is evident in a shallow surface potential variation across grain boundaries (10–50 mV; 195 GBs were investigated). Yet, a high-resolution TEM investigation reveals that a variety of defects is prevalent at and near the GBs. Additionally, we found that GBs in films with distinct (hk1) orientation indeed have a lower surface potential variation than grains with (hk0) orientation. These findings support both heuristic models and provide a practical way to scrutinize defect tolerance in other polycrystalline semiconductors.

Decoupling the electrical resistivity contribution of grain boundaries in dilute Fe-alloyed Cu thin films

To study the role of chemical composition on the resistivities of grains and grain boundaries (GB) for dilute Fe-alloyed Cu thin films, Cu films with grain sizes varying over three orders of magnitude and compositions of 0.025 and 0.25 at.% Fe were prepared by magnetron co-sputtering. Character, morphology and compositions of bulk and GBs were studied using electron backscatter diffraction, transmission electron microscopy and atom probe tomography, respectively. The specific resistivities of both individual GBs and within grains were obtained through local electrical measurements assisted by micromanipulation in situ within a scanning electron microscope. In addition, global resistivity characterisation of the thin films allowed for calculation of the GB reflection coefficient. A decoupling of GB and grain interior resistivities is found with alloying, where the GB resistivity increases by an order of magnitude while the grain interior is affected only to a minor extent in comparison.

Controllable extrinsic ion transport in two-dimensional perovskite films for reproducible, low-voltage resistive switching

Organic-inorganic halide perovskite has attracted significant interest in being switching medium for resistive random access memory (RRAM), yet the in-depth understanding of ion spatial distribution and transport kinetics—which is responsible for the filament formation and normally suffers from a paradox between stochasticity and dynamics—remains limited. Herein, we show evidence of space-confined, fast extrinsic ion transport within grain boundaries (GBs) of two-dimensional perovskite films through systematic ex-situ and in-situ studies. The filament growth can be dominated by the geometrical feature of GBs that act as the channel for cation transport in the dielectric film. By tailoring the structure of perovskite GBs, electroforming-free RRAM devices are fabricated with an ultralow set voltage of 0.09 V (1.8kVcm−1) and small temporal/spatial variations (<10%). The devices can also be integrated with flexible substrates for multifunctional applications, including multilevel writing and light erasing. Our work may open new perspectives for regulating filament formation kinetics in RRAMs and provides a reliable building block for future applications in electronic and photonic circuits.

The improvement of endurance characteristics in a superlattice-like material-based phase change device

Improvement of endurance characteristics has been a hot area of phase-change memoryresearch. The properties of a phase-change material are believed to play an important role in device endurance. Repeated SET–RESET operation always leads to material failure problems, such as composition deviation and phase separation. Moreover, the quality of the electrode and the electrode contact also determine the endurance characteristics. In this study, C nanolayers were periodically inserted into the phase-change material Ge2Sb2Te5 (GST) to fabricate a superlattice-like (SLL) structure. Although some of C bonded with some of the Ge, Sb and Te atoms, more C atoms prefer nanometer-scale clusters at the grain boundary in the SLL film. The typical local configuration of GST was unchanged when artificial C nanolayers were inserted. Transmission electron microscopy experiments revealed that the bonded C atoms and nanometer-scale C clusters may occupy the spontaneously created holes and defects, preventing composition deviation of the phase-change material and prolonging the electrode service life. The contact surface between the phase-change material and the electrode is also improved. As a result, we found that the endurance cycle could be improved by up to 106 for a GST/C SLL film-based device.

Treating CsPbI3 Perovskite with Pyrrolidinium Iodide to Improve the Performance of Perovskite Solar Cells

All-inorganic CsPbI3 perovskite has attracted wide attention due to its desirable optical bandgap (Eg: ∼1.7 eV) as well as high chemical stability. Nevertheless, the photovoltaic performance of CsPbI3 perovskite solar cells (PSCs) was limited by severe nonradiative charge recombination due to high defect density at the grain boundary and surface of perovskite films. To address this issue, a pyrrolidinium iodide (PyI) molecule was introduced to modify the surface and grain boundary of CsPbI3 perovskite films to passivate defects, which improves the quality of CsPbI3 perovskite films as well as induces the generation of a quasi-2D Py2CsPb2I7 capping layer between perovskite layer and hole transport layer. Such quasi-2D Py2CsPb2I7 capping layer optimizes interface contact between CsPbI3 perovskite layer and hole transport layer and blocks the electron transfer from CsPbI3 perovskite photoactive layer to the hole transport layer. As a result, the performance of CsPbI3 PSCs is well improved to 17.87% for power conversion efficiency (PCE) with an ultra-high fill factor (FF) of 0.84. In addition, the PyI molecule modified CsPbI3 perovskite devices exhibit excellent stability, which remains its initial PCE almost unchanged after aging for 35 days under the dry air atmosphere (temperature: 20°C–30°C, control relative humidity (RH):

Oxygen distribution in the structure of YBa-=SUB=-2-=/SUB=-Cu-=SUB=-3-=/SUB=-O-=SUB=-7-delta-=/SUB=- thin films after vacuum exposure at 300 K

We have found out that vacuum exposure of YBa2Cu3O7-delta thin epitaxial films at 300 K for 24 hours decreases oxygen content and induces changes in their crystal structure depending on the films synthesis conditions. YBa2Cu3O7-delta films on SrTiO3 (100) substrates were synthesized by pulsed laser deposition. The crystal structure was studied using XRD technique. Additional maxima of diffraction peak (005) before and after vacuum exposure indicate the presence of regions with different oxygen content; and the change in the shape of the rocking curves indicate a change in the crystallites orientation. After vacuum exposure, the oxygen deficiency delta in the films increased from ~0.01 to ~0.6 for films with a crystallites size of 2-10 nm and from ~0.25-0.35 to ~0.6 for crystallites size of 500-1000 nm. The appearance of a residual resistance after the superconducting transition in the dependence R(T) may be explained by the fact that oxygen loss mainly in the boundary regions of the crystallites takes place, so that their boundaries become dielectric, while the crystallites themselves remain superconductive. Based on the data of X-ray diffraction analysis, as well as resistive measurements, we have come to the conclusion about oxygen loss through grain boundaries in films with nanocrystallites of 2-10 nm in size, and also through structural defects in films with larger crystallites. Keywords: pulsed laser deposition, surface relief, transport characteristics of films, film evolution, SrTiO3.

Распределение кислорода в структуре тонких пленок YBa-=SUB=-2-=/SUB=-Cu-=SUB=-3-=/SUB=-O-=SUB=-7-delta-=/SUB=- после хранения в вакууме при 300 K

Abstract We have found out that vacuum exposure of YBa2Cu3O7‒δ thin epitaxial films at 300 K for 24 hours decreases oxygen content and induces changes in their crystal structure depending on the films synthesis conditions. YBa2Cu3O7‒δ films on SrTiO3 (100) substrates were synthesized by pulsed laser deposition. The crystal structure was studied using XRD technique. Additional maxima of diffraction peak (005) before and after vacuum exposure indicate the presence of regions with different oxygen content; and the change in the shape of the rocking curves indicate a change in the crystallites orientation. After vacuum exposure, the oxygen deficiency δ in the films increased from ~0.01 to 0.6 for films with a crystallites size of 2-10 nm and from ~0.25−0.35 to 0.6 for crystallites size of 500−1000 nm. The appearance of a residual resistance after the superconducting transition in the dependence R(T) may be explained by the fact that oxygen loss mainly in the boundary regions of the crystallites takes place, so that their boundaries become dielectric, while the crystallites themselves remain superconductive. Based on the data of X—ray diffraction analysis, as well as resistive measurements, we have come to the conclusion about oxygen loss through grain boundaries in films with nanocrystallites of 2-10 nm in size, and also through structural defects in films with larger crystallites.

An effective modulation of bulk perovskite by V2CTx nanosheets for efficient planar perovskite solar cells

Crystallization modulation and defect passivation are key for high performance perovskite solar cells (PSCs) through suppressing defects in the surface and/or near the grain boundaries (GBs) of solution-processed perovskite films.

Orientation-controlled mesoporous PbI2 scaffold for 22.7% perovskite solar cells

Sequential deposition is a promising tactic for perovskite film fabrication. However, the highly crystalline and compact morphology of PbI2 film will hinder the quality and reproducibility of perovskite films. Here, nicotinamide (NTM) derivatives were introduced as simple and effective multifunctional ligands into the PbI2 precursor solution. Orientation-controlled and mesoporous PbI2 scaffolds formed due to strong chemical interaction and accelerated solvent volatilization. Nanobelt PbI2 crystals were also generated at the grain boundaries of perovskite films with NTM derivatives, which can passivate the defects and enhance the carrier lifetime. As a result, the perovskite solar cells based on 6-(trifluoromethyl) nicotinamide-modulated films showed an enhanced power conversion efficiency of 22.7%, of which 98.8% was maintained even after 2000 h of aging. Our findings offer a distinctive understanding of regulating the crystallization of PbI2 films and manipulating excess PbI2 structure for efficient perovskite solar cells.