Grain Boundaries In Films Research Articles

Grain-Boundary Elimination via Liquid Medium Annealing toward High-Efficiency Sb2Se3 Solar Cells.

Suppression of charge recombination caused by unfavorable grain boundaries (GBs) in polycrystalline thin films is essential for improving the optoelectronic performance of semiconductor devices. For emerging antimony selenide (Sb2Se3) materials, the unique quasi-1D structure intensifies the dependence of GB properties on the grain size and orientation, which also increases the impact of defects related to grain structure on device performance. However, these characteristics pose significant challenges in the preparation of thin films. In this study, a novel annealing approach using ammonia-thiourea is developed mixed solution as the liquid medium (LM) to finely regulate the crystallization of Sb2Se3 films, resulting in micron-sized large grains with enhanced [hk1] orientation and fewer defects. Mechanistic studies indicate that the intermediate phase formed at the GBs promotes the growth of large grains. Moreover, LM creates a closed and uniform environment for thin-film annealing, suppressing the volatilization of Se and reducing the types of deep-level defects. Consequently, the film delivers a device efficiency of 9.28%, the highest efficiency achieved for Sb2Se3 solar cells fabricated via thermal evaporation. Hence, this study provides a facile and effective annealing method for controlling the crystallization of low-dimensional materials and offers valuable guidance for the development of chalcogenide materials.

Advances in Self-Healing Perovskite Solar Cells Enabled by Dynamic Polymer Bonds.

This comprehensive review addresses the self-healing phenomenon in perovskite solar cells (PSCs), emphasizing the reversible reactions of dynamic bonds as the pivotal mechanism. The crucial role of polymers in both enhancing the inherent properties of perovskite and inducing self-healing phenomena in grain boundaries of perovskite films are exhibited. The review initiates with an exploration of the various stability problems that PSCs encounter, underscoring the imperative to develop PSCs with extended lifespans capable of self-heal following damage from moisture and mechanical stress. Owing to the strong compatibility brought by polymer characteristics, many additive strategies can be employed in self-healing PSCs through artful molecular design. These strategies aim to limit ion migration, prevent moisture ingress, alleviate mechanical stress, and enhance charge carrier transport. By scrutinizing the conditions, efficiency, and types of self-healing behavior, the review encapsulates the principles of dynamic bonds in the polymers of self-healing PSCs. The meticulously designed polymers not only improve the lifespan of PSCs through the action of dynamic bonds but also enhance their environmental stability through functional groups. In addition, an outlook on self-healing PSCs is provided, offering strategic guidance for future research directions in this specialized area.

Pyridalthiadiazole-Based Molecular Chromophores for Defect Passivation Enables High-Performance Perovskite Solar Cells.

Passivation of defects at the surface and grain boundaries of perovskite films has become one of the most important strategies to suppress nonradiative recombination and improve optoelectronic performance of perovskite solar cells (PSCs). In this work, two conjugated molecules, abbreviated as CPT and SiPT, are designed and synthesized as the passivator to enhance both efficiency and stability of PSCs. The CPT and SiPT contain pyridalthiadiazole (PT) units, which can coordinate with undercoordinated Pb2+ at the surface and grain boundaries to passivate the defects in perovskite films. In addition, with the incorporation of CPT, the crystallized perovskite films exhibit more uniform grain size and smoother surface morphology relative to the control ones. The efficient passivation by CPT also results in better charge extraction and less carrier recombination in PSCs. Consequently, the CPT-passivated PSCs yield the highest power conversion efficiency (PCE) of 23.14 % together with better storage stability under ambient conditions, which is enhanced relative to the control devices with a PCE of 22.14 %. Meanwhile, the SiPT-passivated PSCs also show a slightly enhanced performance with a PCE of 22.43 %. Our findings provide a new idea for the future design of functional passivating molecules towards high-performance PSCs.

Mapping domain structures near a grain boundary in a lead zirconate titanate ferroelectric film using X-ray nanodiffraction.

The effect of an electric field on local domain structure near a 24° tilt grain boundary in a 200 nm-thick Pb(Zr0.2Ti0.8)O3 bi-crystal ferroelectric film was probed using synchrotron nanodiffraction. The bi-crystal film was grown epitaxially on SrRuO3-coated (001) SrTiO3 24° tilt bi-crystal substrates. From the nanodiffraction data, real-space maps of the ferroelectric domain structure around the grain boundary prior to and during application of a 200 kV cm-1 electric field were reconstructed. In the vicinity of the tilt grain boundary, the distributions of densities of c-type tetragonal domains with the c axis aligned with the film normal were calculated on the basis of diffracted intensity ratios of c- and a-type domains and reference powder diffraction data. Diffracted intensity was averaged along the grain boundary, and it was shown that the density of c-type tetragonal domains dropped to ∼50% of that of the bulk of the film over a range ±150 nm from the grain boundary. This work complements previous results acquired by band excitation piezoresponse force microscopy, suggesting that reduced nonlinear piezoelectric response around grain boundaries may be related to the change in domain structure, as well as to the possibility of increased pinning of domain wall motion. The implications of the results and analysis in terms of understanding the role of grain boundaries in affecting the nonlinear piezoelectric and dielectric responses of ferroelectric materials are discussed.

Grain rotation mechanisms in nanocrystalline materials: Multiscale observations in Pt thin films.

Near-rigid-body grain rotation is commonly observed during grain growth, recrystallization, and plastic deformation in nanocrystalline materials. Despite decades of research, the dominant mechanisms underlying grain rotation remain enigmatic. We present direct evidence that grain rotation occurs through the motion of disconnections (line defects with step and dislocation character) along grain boundaries in platinum thin films. State-of-the-art in situ four-dimensional scanning transmission electron microscopy (4D-STEM) observations reveal the statistical correlation between grain rotation and grain growth or shrinkage. This correlation arises from shear-coupled grain boundary migration, which occurs through the motion of disconnections, as demonstrated by in situ high-angle annular dark-field STEM observations and the atomistic simulation-aided analysis. These findings provide quantitative insights into the structural dynamics of nanocrystalline materials.

Optical and microstructural studies of erbium-doped TiO2 thin films on silicon, SrTiO3, and sapphire

Rare-earth ion doped oxide thin films integrated on silicon substrates provide a route toward scalable, chip-scale platforms for quantum coherent devices. Erbium-doped TiO2 is an attractive candidate: the Er3+ optical transition is compatible with C-band optical fiber communications, while TiO2 is an insulating dielectric compatible with silicon process technology. Through structural and optical studies of Er-doped TiO2 thin films grown via molecular beam deposition on silicon, SrTiO3, and sapphire substrates, we have explored the impact of polycrystallinity and microstructure on the optical properties of the Er emission. Comparing polycrystalline TiO2(rutile)/Si with single-crystalline TiO2(rutile)/r-sapphire and polycrystalline TiO2(anatase)/Si with single-crystalline TiO2(anatase)/SrTiO3, we observe that the inhomogeneous linewidth (Γinh) of the most prominent peak in the Er spectrum (the Y1–Z1 transition, 1520 and 1533 nm in rutile and anatase TiO2) is significantly narrower in the polycrystalline case. This implies a relative insensitivity to extended structural defects and grain boundaries in such films (as opposed to, e.g., point defects). We show that the growth of an undoped, underlying TiO2 buffer on Si can reduce Γinh by a factor of 4–5. Expectedly, Γinh also reduces with decreasing Er concentrations: we observe a ∼2 order of magnitude reduction from ∼1000 ppm Er to ∼10 ppm Er. Γinh then gets limited to a residual value of ∼5 GHz that is insensitive to further reduction in the Er concentration. Based upon the above results, we argue that the optical properties in these thin films are limited by the presence of high “grown-in” point defect concentrations.

Thermal-triggered healing strategy for efficient and stable perovskite solar cells with efficiency over 25 %

The power conversion efficiency (PCE) of organic-inorganic hybrid perovskite solar cells (PSCs) is hindered by the abundant defects within the grain boundaries (GBs) of perovskite films. Herein, a directed thermal-triggered healing strategy based on phthalide-3-acetic acid (P3AC) is proposed, aiming to mitigate GBs defects and optimize energy level arrangement by utilizing the strong polarity effect of P3AC and its thermal-triggered interaction with perovskite. The introduced dual-sites molecular (P3AC) anchors at the GBs of perovskite, which can passivate the uncoordinated Pb2+ and form hydrogen bonds with FA+, yielding dense, uniform and low-defects and large-grain perovskite film. Compared to the pristine devices, the P3AC-modified devices achieved a champion PCE of 25.28 % (certified PCE of 24.46 %) and impressive operational stability. This work provides an effective and direct solution for enhancing the performance of PSCs.

Effect of solution‐processed cesium carbonate on Cu(In,Ga)Se2 thin‐film solar cells

AbstractHeavy alkali‐metal treatments have been the most recent breakthrough in improving the efficiency of Cu(In,Ga)Se2 (CIGS) solar cells. Alkali halides are generally evaporated onto the surface of the CIGS thin film by a vacuum process. Here, we report an alternative, low‐cost solution process for the surface treatment of CIGS thin films using cesium carbonate (CsCO3) as a new route to incorporate cesium (Cs) for improving solar cell performance. CIGS thin films were fabricated using pulsed hybrid reactive magnetron sputtering and the surface treatment was performed by spin‐coating CsCO3 solution on the surface of CIGS at room temperature, followed by vacuum annealing at 400°C. The surface chemistry of the CIGS thin film changed after the treatment and the efficiency of respective solar cells improved by more than 30%, mostly driven by an enhancement in open‐circuit voltage. X‐ray photoelectron spectroscopy revealed the depletion of copper and the presence of Cs on the surface of the CIGS thin film. Ultraviolet photoelectron spectroscopy showed the lowering of the valence band maximum by around 0.25 eV after the treatment, which plays a positive role in reducing interfacial recombination. High‐resolution transmission electron microscopy indicates the presence of Cs and depletion of Cu at the grain boundaries of the CIGS thin film. These findings open a low‐cost route for improving the performance of CIGS solar cells by surface modification using a solution process.

Judicious Fluorination of Perovskite Quantum Wells Enables Over 25% Efficiency in Inverted Solar Cells

AbstractThe photovoltaic performance of inverted perovskite solar cells (PSCs) is often hindered by trap‐induced non‐radiative recombination and photochemical degradation occurring at the upper interfaces and the grain boundaries of perovskite films. Herein, ortho‐, meta‐, and para‐isomers of fluorophenylethylammonium iodine (F‐PEAI) organic spacer molecules are evaluated for the construction of perovskite quantum wells (2D or quasi‐2D, PQWs) to encapsulate 3D perovskites. Among the three variants, p‐F‐PEAI leads to the most symmetric charge distribution and the weakest steric hindrance which resulting in reinforced interactions with PbI2 and perovskite, the enhanced out‐of‐plane orientation is confirmed by Grazing incidence wide‐angle X‐ray scattering (GIWAXS) results. Density functional theory and crystal orbital Hamilton population (COHP) calculations further confirm that p‐F‐PEAI engages most strongly with the perovskite structure. Moreover, transmission electron microscopy (TEM) characterization is used to illustrate that p‐F‐PEAI‐assisted 2D PQWs effectively passivate both the grain boundaries and surfaces of perovskites. This configuration facilitates effective surface passivation, improves charge carrier transport, and significantly suppresses non‐radiative recombination. The resultant inverted PSCs achieve an excellent power conversion efficiency (PCE) of 25.03% with a fill factor (FF) of 85.11%. The unencapsulated devices exhibit enhanced long‐term stability under ambient environments and continuous light illumination.

Stabilizing Lead Halide Perovskites via an Organometallic Chemical Bridge for Efficient and Stable Photovoltaics.

Defects around the surface and grain boundaries of perovskite films normally cause severe nonradiative recombination and imbalanced charge carrier transport, further limiting both the efficiency and stability of perovskite solar cells (PSCs). To tackle this critical issue, we propose a chemical bridge strategy to reconstruct the interface using organometallic molecules. The commercially available molecule bis(diphenylphosphino)ferrocene (FcP2), with a unique bridge molecular structure, anchors and chelates Pb atoms by forming strong Pb-P bonds and further passivates both surfaces and grain boundaries. Detailed characterization revealed that bridge molecule FcP2 reconstruction can effectively suppress nonradiative recombination, and the electron delocalization properties of the ferrocene core can further achieve more balanced interfacial carrier transport. The resultant N-i-P PSC device outputs close to 25% efficiency together with one of the best reported operational stabilities, maintaining over 95% of the initial efficiency after 1000 h of continuous operation at the maximum power point under 1-sun illumination.

Optimized SnO2 preparation and coherent interlayers for enhanced charge dynamics and efficient perovskite photovoltaics

The dense, uniform and conformal electron transport layers (ETLs) will largely promote charge separation and extraction. Here, the mixed acid (hydrochloric acid and nitric acid) was used to regulate preparation process and enhance utilization of materials, and the colloids of tin oxide (SnO2) nanocrystals were prepared through hydrothermal process. The complete dissolution of Sn source can increase purity, produce homogeneous precursor, reduce grain sizes and improve film-coverage. As confirmed, a coherent interlayer at the SnO2 ETLs/perovskite interfaces will be achieved by coupling a Cl-bonded SnO2 film with a Cl-containing perovskite precursor. This thin coherent interlayer will largely reduce interface traps, enhance rapid carrier extraction, and impede charge recombination. The uniform polymer phase of (PEO)120-(PPO)30 will be used to passivate traps at the grain boundaries of perovskite films and further improve the photovoltaic performance. The maximum energy conversion efficiency (23.17%, a VOC of 1.153 V, a JSC of 24.75 mA cm−2 and a FF of 0.812) of perovskite solar cells was achieved. The charge separation, extraction, and recombination kinetics (charge dynamic process) was determined by the related characterization techniques. The functionalized SnO2-ETLs and formed coherent interlayer will provide a simple strategy to effectively decrease interface traps, enhance charge extraction, and facilitate development of perovskite solar cells.

Elevating Charge Transport Layer for Stable Perovskite Light-Emitting Diodes.

Ion migration is a major factor affecting the long term stability of perovskite light-emitting diodes (LEDs), which limits their commercialization potential. The accumulation of excess halide ions at the grain boundaries of perovskite films is a primary cause of ion migration in these devices. Here, it is demonstrated that the channels of ion migrations can be effectively impeded by elevating the hole transport layer between the perovskite grain boundaries, resulting in highly stable perovskite LEDs. The unique structure is achieved by reducing the wettability of the perovskites, which prevents infiltration of the upper hole-transporting layer into the spaces of perovskite grain boundaries. Consequently, nanosized gaps are formed between the excess halide ions and the hole transport layer, effectively suppressing ion migration. With this structure, perovskite LEDs with operational half-lifetimes of 256 and 1774 h under current densities of 100 and 20mAcm-2 respectively are achieved. These lifetimes surpass those of organic LEDs at high brightness. It is further found that this approach can be extended to various perovskite LEDs, showing great promise for promoting perovskite LEDs toward commercial applications.

Cathodic Deposition‐Assisted Synthesis of Thin Glass MOF Films for High‐Performance Gas Separations

AbstractGlass metal–organic framework (MOF) films can be fabricated from their crystalline counterparts through a melt‐quenching process and are prospective candidates for gas separation because of the elimination of the grain boundaries in crystalline MOF films. However, current techniques are limited to producing glass MOF films with a thickness of tens of micrometers, which leads to ultralow gas permeances. Here, we report a novel cathodic deposition‐assisted synthesis of glass ZIF‐62 films with a thickness as low as ~1 μm. Electrochemical analyses and deposition experiments suggest that the cathodic deposition can lead to pure crystalline ZIF‐62 films with a controllable thickness of ~2 μm to ~15 μm. Accordingly, glass ZIF‐62 films with a thickness of ~1 μm to ~10 μm can be obtained after a thermal treatment. The fabricated defect‐free glass ZIF‐62 film measuring 2 μm in thickness shows a remarkable CO2/N2 and CO2/CH4 selectivity of 31.4 and 33.4, respectively, with a CO2 permeance which is over 30 times higher than the best‐performing glass ZIF‐62 films in literature.

Cathodic Deposition-Assisted Synthesis of Thin Glass MOF Films for High-Performance Gas Separations.

Glass metal-organic framework (MOF) films can be fabricated from their crystalline counterparts through a melt-quenching process and are prospective candidates for gas separation because of the elimination of the grain boundaries in crystalline MOF films. However, current techniques are limited to producing glass MOF films with a thickness of tens of micrometers, which leads to ultralow gas permeances. Here, we report a novel cathodic deposition-assisted synthesis of glass ZIF-62 films with a thickness as low as ~1 μm. Electrochemical analyses and deposition experiments suggest that the cathodic deposition can lead to pure crystalline ZIF-62 films with a controllable thickness of ~2 μm to ~15 μm. Accordingly, glass ZIF-62 films with a thickness of ~1 μm to ~10 μm can be obtained after a thermal treatment. The fabricated defect-free glass ZIF-62 film measuring 2 μm in thickness shows a remarkable CO2/N2 and CO2/CH4 selectivity of 31.4 and 33.4, respectively, with a CO2 permeance which is over 30 times higher than the best-performing glass ZIF-62 films in literature.

Multifunctional Interface Modification Enables Efficient and Stable HTL-Free Carbon-Electroded CsPbI2Br Perovskite Solar Cells.

In recent years, hole transport layer-free all-inorganic CsPbI2Br carbon-electroded perovskite solar cells (C-PSCs) have garnered significant attention due to a trade-off between stability and photovoltaic performance. However, there are inevitably many defects generated at the surfaces or grain boundaries of CsPbI2Br perovskite films, which will serve as carrier non-radiative recombination centers, and CsPbI2Br perovskite films are sensitive to water molecules to degrade, together with energy level mismatch between CsPbI2Br perovskite and carbon electrodes. Herein, 1-benzyl-3-methylimidazolium hexafluorophosphate (1-B-3-MIMPF6), an imidazolium-based ionic liquid simultaneously containing benzene ring and fluorine atoms, was introduced for the modification of the perovskite/carbon interface. The results showed that it could effectively reduce defects, enhance carrier transfer, mitigate carrier non-radiative recombination, facilitate energy alignment, and block moisture intrusion. Therefore, the photovoltaic performance of the modified PSCs with ITO/SnO2/CsPbI2Br/1-B-3-MIMPF6/carbon architecture has been boosted with a champion power conversion efficiency (PCE) of 13.47 %, open circuit voltage of 1.20 V, short circuit current density of 14.69 mA/cm2, and fill factor of 76 %. Moreover, the unencapsulated modified devices exhibited an improved stability and the PCE maintained 78 % of their initial PCE after 24 h storage at room temperature in a 30 %-35 % humidity environment, whereas that of the pristine devices dropped to almost zero.

Bifunctional interfacial engineering enabled efficient and stable carbon-based CsPbIBr2 perovskite solar cells

Carbon-based inorganic CsPbIBr2 perovskite solar cells (C-IPSC) have attracted widespread attention due to their low cost and excellent thermal stability. Unfortunately, due to the soft ion crystal nature of perovskite, inherent bulk defects and energy level mismatch at the CsPbIBr2/carbon interface limit the performance of the device. In this study, we introduced aromatic benzyltrimethylammonium chloride (BTACl) as a passivation layer to passivate the surface and grain boundaries of the CsPbIBr2 film. Due to the reduction of perovskite defects and better energy level arrangement, carrier recombination is effectively suppressed and hole extraction is improved. The champion device achieves a maximum power conversion efficiency (PCE) of 11.30% with reduces hysteresis and open circuit voltage loss. In addition, unencapsulated equipment exhibits excellent stability in ambient air.

The Effect of Antioxidants on the Stability of Perovskite Solar Cells

The passivation of defects present in the bulk and surface of the perovskite absorbing layers of solar cells is currently the subject of intense research. To address this issue, dopants can be used to minimize the density of defects that occur at the surface and grain boundaries of perovskite films. Herein, we introduced two new antioxidants (Morin and Quercetin) in the precursor solutions to passivate the defects present in the Cs0.1FA00.9PbI3 perovskite absorbing layer. The positive impact of antioxidants on the efficiency of perovskite cells has been recently reported. However, more experimental data are needed to identify the most effective passivating agent. The time resolved photoluminescence measurements revealed that the added molecules increased the photocarrier lifetime by 120-194%. Our results showed that the optimum concentrations are 2.223 mM for Morin and 0.0156 mM for Quercetin. These results demonstrate that the new molecules are effective passivating agents and have the potential to improve the device performance. The variation of the cell power conversion efficiency as function of the concentration of the antioxidants will be discussed.

Zn–Porphyrin Antisolvent Engineering‐Enhanced Grain Boundary Passivation for High‐Performance Perovskite Solar Cell

Perovskite solar cells (PSCs) represent a promising and rapidly evolving technology in the field of photovoltaics due to their easy fabrication, low‐cost materials, and remarkable efficiency improvements over a relatively short period. However, the grain boundaries in the polycrystalline films exhibit a high density of defects, resulting in not only heightened reactivity to oxygen and water but also hampered charge transport and long‐term stability. Herein, an approach involving Zn‐porphyrin (Zn‐PP)‐upgraded antisolvent treatment to enhance the grain size and meanwhile passivate grain boundary defects in FA0.95MA0.05PbI2.85Br0.15 perovskites is presented. The Zn‐PP molecules significantly improve structural and optical properties, effectively mitigating defects and promoting carrier transport at the perovskite/hole transport layer interface. The density functional theory simulation confirms that Zn‐PP forms a strong chemical bonding with the perovskite surface. With Zn‐PP passivation, the total density of state shifts to higher‐energy regions with molecular adsorption, especially near the valence and conduction band edges, indicating that there is an increase in conducting properties of the surface with molecular adsorption. The power conversion efficiency (PCE) of PSCs increases significantly as a result of this improvement, rising from 15.38% to 19.11%. Moreover, unencapsulated PSCs treated with Zn‐PP exhibit outstanding stability, retaining over 91% of their initial PCE.

Difference in the structure and morphology of CVD diamond films grown on negatively charged and grounded substrate holders: Optical study

Microcrystalline diamond films were grown by plasma-enhanced chemical vapor deposition from a CH4/H2 gas mixture on Si single-crystalline substrates placed on negatively charged and grounded substrate holders. The obtained diamond films had the (100) predominant faceting of microcrystals. The film structure and morphology were analyzed by scanning electron microscopy, photoluminescence, Raman and FTIR spectroscopies. The main physical factor causing the difference in the structure of the diamond films grown on the grounded and charged substrate holders was found to be the flow of low-energy (up to 200 eV) Si+, N2+, H, O ions in the latter holder. These ions predominantly embedded into the structure of the diamond films grown on the charged substrate holder leading to appearance of residual mechanical stress up to 2 GPa. Ion bombardment led to increase in the volume fraction of non-diamond carbon component in the film grain boundaries, decrease in sp3-bonded carbon fraction and reduction of the diamond microcrystals lateral size. Larger amount of grain boundaries in the diamond films grown on the charged substrate holder promoted diffusion of Si atoms from the substrate to the plasma and growing film surface, inducing formation of SiV centers in the diamond microcrystals even in the 150…200 μm thick films. The concentration of Si-related defects was much smaller in the films grown using the grounded substrate holder. These films had substantially smaller volume fraction of graphite-like carbon in the grain boundaries and were more homogeneous.

Enhancing Efficiency and Stability of Tin‐Based Perovskite Solar Cells Through the Addition of Nickel (II) Porphyrin Complex for Defect Passivation and Mitigation of Ion Migration

The stability and efficiency of tin perovskite solar cells (TPSCs) are often decreasing with high‐density defects occurring at the grain boundaries of the perovskite film and the heterojunction interfaces of hole‐transport material (HTM) and poly(3,4‐ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS). Hence, this study addresses these challenges using nickel (II) meso amino octaethylporphyrin (NiNH2 OEP) as an effective defect‐passivating agent for the device fabricated through a two‐step method with an inverted device architecture. The precursor solution contains NiNH2 OEP porphyrin coating over PEDOT:PSS, where NiNH2 OEP is self‐assembled uniformly at the perovskite grain boundary and interface between perovskite and HTM. NiNH2 OEP is protonated to form nickel (II) ammonium porphyrin ions (NiNH3+ OEP) to retard nonradiative charge recombination and inhibit ion migration that occurred through the high‐density defect states to improve device stability and performance. Consequently, the TPSC device fabricated with NiNH2 OEP unveils a remarkable power conversion efficiency of 9.6%. Moreover, the stability of the device shows a prominent improvement that maintains an initial efficiency of 90% for more than 6,000 h inside the glove box.