Defects In Films Research Articles

Tailoring the Density of State of n-Type Conjugated Polymers through Solvent Engineering for Organic Electrochemical Transistors.

Conjugated polymers with ethylene glycol-type side chains are commonly used as channel materials in organic electrochemical transistors (OECTs). To improve the performance of these materials, new chemical structures are often created through synthetic routines. Herein, we demonstrate that the OECT performance of these polymers can also be improved by changing their density-of-state (DOS) profile through solvent engineering. Depending on the solvent polarity, it solvates the backbone and side chains of the conjugated polymer differently, leading to differences in molecule orientation, π-stacking paracrystallinity, and film defects, such as grain boundaries and pinholes. This then results in a change in the DOS profile of the polymer. A more intense and narrow-width DOS distribution is usually observed in organic films with an "edge on" orientation and fewer film defects, while films with a "face on" orientation and apparent defects show a broadened DOS profile. The OECT devices that use the polymer film with a more intense and narrow-width DOS profile exhibit a better-normalized transconductance and figure-of-merit μC* than those with a broadened DOS profile (0.74 to 4.29 S cm-1 and 3.5 to 14.3 F cm-1 V-1 s-1). This study provides useful insights into how the DOS profile affects the mixed ionic-electronic conduction performance and presents a new avenue for improving n-type OECT materials.

Unveiling the Effect of Cooling Rate on Grown-in Defects Concentration in Polycrystalline Perovskite Films for Solar Cells with Improved Stability.

Numerous efforts are devoted to reducing the defects at perovskite surface and/or grain boundary; however, the grown-in defects inside grain is rarely studied. Here, the influence of cooling rate on the point defects concentration in polycrystalline perovskite film during heat treatment processing is investigated. With the combination of theoretical and experimental studies, this work reveals that the supersaturated point defects in perovskite films generate during the cooling process and its concentration improves as the cooling rate increases. The supersaturated point defects can be minimized through slowing the cooling rate. As a result, the optimized FAPbI3 polycrystalline films achieve a superior carrier lifetime of up to 12.6 µs and improved stability. The champion device delivers a 25.47% PCE (certified 24.7%) and retain 90% of their initial value after >1100 h of operation at the maximum power point. These results provide a fundamental understanding of the mechanisms of grown-in defects formation in polycrystalline perovskite film.

Improving FAPbBr3 Perovskite Crystal Quality via Additive Engineering for High Voltage Solar Cell over 1.5 V.

Lead bromide-based perovskites are promising materials as the top cells of tandem solar cells and for application in various fields requiring high voltages owing to their wide band gaps and excellent environmental resistances. However, several factors, such as the formation of bulk and surface defects, impede the performances of corresponding devices, thereby limiting the efficiencies of these devices as single-junction devices. To reduce the number of defect sites, urea is added to the formamidinium lead bromide (FAPbBr3) perovskite material to increase its grain size. Nevertheless, urea undesirably reacts with lead(II) bromide (PbBr2) in the perovskite structure, creating unfavorable impurities in the device. To solve this problem, herein, in addition to urea, we introduced formamidinium chloride (FACl) into FAPbBr3. Owing to the synergistic effect of urea and FACl, the FAPbBr3 film quality effectively improved due to suppression of the generation of impurities and stabilization of film crystallinity. Consequently, the FAPbBr3 single-junction solar cell constructed using FACl and urea as additives demonstrated a power conversion efficiency of 9.6% and an open-circuit voltage of 1.516 V with negligible hysteresis. This study provides new insights into the use of additive engineering for overcoming the energy losses caused by defects in perovskite films.

Facet orientation control of tin-lead perovskite for efficient all-perovskite tandem solar cells

Developing high-performance narrow-bandgap (NBG) perovskite solar cells based on tin-lead (Sn-Pb) perovskite is critical for the advancement of all-perovskite tandem solar cells. However, the limitations of the device current density and efficiency are magnified by the issues concerning poor carrier transport caused by a substantial number of defects in thick NBG films. This problem is further exacerbated by the quality of film crystallization, which is associated with the rapid and uncontrolled crystallization of Sn-rich perovskite chemistry using the antisolvent approach. We regulate the crystallization of Sn-contained perovskite with a mild gas-quench approach to fabricate a highly crystal-oriented and well-arranged NBG perovskite absorber. This strategy effectively boosts electron transport and light absorption of the NBG perovskite. Consequently, the average power conversion efficiency (PCE) of the NBG perovskite solar cells increases from 19.50 % to 21.18 %, with the best device achieving an excellent PCE of 21.84 %. Furthermore, when combined with a wide-bandgap perovskite subcell to form an all-perovskite tandem solar cell, a PCE of 25.23 % is achieved. After being stored in the glovebox for 1000 h, the unencapsulated device maintains over 90 % of its initial PCE, demonstrating long-term stability and durability. This work presents a promising approach for developing high-efficiency NBG perovskite solar cells.

Step-by-step in-situ preparation of magnesium-aluminum layered double hydroxide film to improve the corrosion resistance of AZ31 magnesium alloy

In this study, a hydrothermal method was employed to cultivate Mg(OH)2 precursor layers on the surface of AZ31 magnesium alloy samples. Subsequently, Mg-Al LDHs films were successfully synthesized through an in-situ growth approach. The phase composition of the films were analyzed using X-ray diffraction and infrared spectroscopy. The growth process and surface morphology of the films at the microstructure level were examined and interpreted using scanning electron microscopy and EDS. The corrosion resistance of the films was assessed through potentiodynamic polarization curves, electrochemical impedance spectroscopy, and hydrogen evolution tests. The findings indicate that the Mg(OH)2 precursor effectively mitigates defects in the Mg-Al LDHs films and improves the LDHs microstructure. Moreover, the precursor synthesized at a pH range of 10–12 better enhances the corrosion resistance of the Mg-Al LDHs film.

Gas Molecule Assisted All-Inorganic Dual-Interface Passivation Strategy for High-Performance Perovskite Solar Cells.

The trap states at both the upper and bottom interfaces of perovskite layers significantly impact non-radiative carrier recombination. The widely used solvent-based passivation methods result in the disordered distribution of surface components, posing challenges for the commercial application of large-area perovskite solar cells (PSCs). To address this issue, a novel NH3 gas-assisted all-inorganic dual-interfaces passivation strategy is proposed. Through the gas treatment of the perovskite surface, NH3 molecules significantly enhanced the iodine vacancy formation energy (1.54eV) and bonded with uncoordinated Pb2+ to achieve non-destructive passivation. Meanwhile, the reduction of the film defect states is accompanied by a decrease in the work function, which promotes carrier transport between the interface. Further, a stable passivation layer is constructed to manage the bottom interfacial defects using inorganic potassium tripolyphosphate (PT), whose ─P═O group effectively mitigated the charged defects and lowered the carrier transport barriers and nucleation barriers of PVK, while the gradient distribution of K+ improved the crystalline quality of PVK film. Based on the dual-interface synergistic effect, the optimal MA-contained PSCs with an effective area of 0.1 cm2 achieved an efficiency of 24.51% and can maintain 90% of the initial value after aging (10-20% RH and 20°C) for 2000h.

Influence of Grain Size and Film Formation Potential on the Diffusivity of Point Defects in the Passive Film of Pure Aluminum in NaCl Solution

The influence of grain size on the corrosion behavior of pure aluminum and the defect density and diffusion coefficient of surface passive films were investigated using electron backscatter diffraction (EBSD) and electrochemical testing techniques, based on the point defect model (PDM). Samples with three different grain sizes (23 ± 11, 134 ± 52, and 462 ± 203 μm) were obtained by annealing at different temperatures and times. The polarization curves and electrochemical impedance spectroscopy results for the pure aluminum in the 3.5% NaCl solution showed that with decreasing grain size, the corrosion current (icorr) decreased monotonously, giving rise to a noble corrosion potential and a large polarization resistance. The Motte–Schottky results showed that the passive films that formed on pure aluminum with fine grains of 23 ± 11 μm had a low density (3.82 × 1020 cm−3) of point defects, such as oxygen vacancies and/or metal interstitials, and a small diffusion coefficient (1.94 × 10−17 cm2/s). The influence of grain size on corrosion resistance was discussed. This work demonstrated that grain refinement could be an effective approach to achieving high corrosion resistance of passive metals.

Innovative surface passivation of CZTSSe thin films: Ammonium sulfide treatment with plasma etching

For CZTS(e) solar cells, the passivation of interfacial defects is crucial for effectively reducing carrier recombination at the interface. This study examined the combined impact of plasma etching and surface sulfuration on the passivation of interface defects in CZTSSe thin films. The results indicated that plasma etching could improve the surface quality of the thin films and passivate surface defects. The (NH4)2S vapor had a significant sulfuration effect on the surface of the CZTSSe thin films post-etching. The surface defects of the CZTSSe film were passivated by the sulfur element entering the film by sulfuration. Moreover, the bandgap of the CZTSSe thin films was improved. The efficiency of the solar cell device increased by 1.72 % with a plasma etching duration of 5 min and (NH4)2S vaporization treatment of 15 min. This study proposes a novel approach that employs surface passivation techniques to enhance the efficiency of CZTSSe thin film solar cells.

Investigation of defects in BaSi2 thin film on Si prepared by co-sputtering technique

Defects in undoped BaSi2 thin films prepared by co-sputtering technique were investigated. The growth rate ratio between the sputtering of Ba and BaSi2 targets (GRBa/GRBaSi2) was varied between 0 and 0.5. Sample i (GRBa/GRBaSi2 = 0) exhibited a large root-mean-square (rms) roughness of 102 nm due to the presence of high density of 3D features which is attributed to Si precipitates. The rms roughness was significantly reduced to 9–26 nm for samples grown at GRBa/GRBaSi2 > 0.18. Raman data reveal the presence of Si vacancy (VSi) and Ba antisite (BaSi) defects. For GRBa/GRBaSi2 > 0.18, the Ag mode becomes blueshifted while the linewidth becomes narrower, suggesting a reduction in VSi. A similar trend was also observed for BaSi. At 10 K, defect-assisted emissions were observed from sample i with photoluminescence (PL) peak energy of 0.92 and 1.27 eV. Power dependent PL analyses suggested that the emissions were due to VSi-to-BaSi and VSi-to-free hole transitions. However, samples grown at GRBa/GRBaSi2 ≥ 0.18 showed negligible PL emission, possibly due to low density of VSi. Based on the Tauc and Urbach tail analyses, the band gap of all samples is ∼1.31 eV, regardless of the GRBa/GRBaSi2. However, samples with less structural disorder showed smaller characteristic energy of the absorption edge (Eo).

An EPR study of point defects in zinc oxide thin films

We studied ZnO thin films deposited on glass slides by thermal evaporation in vacuum followed by heat treatment in open air or Ar flow. The samples were characterized using x-ray diffraction, Raman scattering, photoluminescence (PL), and electron paramagnetic resonance (EPR) spectroscopy. We observed a broad PL band in the visible region at spectral positions of 504 and 560 nm and a low-intensity band in the UV region at 390 nm. The EPR spectra display a clear first derivative structure at g=1.96 at temperatures below 200 K. The PL spectrum shows red-shifted valence to conduction band emission due to electron hole recombination through shallow surface states. We found an activation energy of E a = 4.2 meV using the Arrhenius plot and estimated concentrations of paramagnetic defects and spin–spin relaxation time constants of 1016 m−3 and 10–14 s, respectively.

Thickness-derived optical-electrical management in Sn-based perovskite solar cells

Tin-based perovskite solar cells (TPSCs) have attracted great attention due to their promising photovoltaic performance and environmental friendliness. Adequate photon trapping and efficient carrier utilization are known to be the keys to achieving high-performance devices, which are closely related to the thickness of the light-absorbing layer and the quality of film formation, respectively. Due to the ultra-fast crystallization characteristics and low defect formation energy, thick tin-based perovskite films are faced with poor electrical performance due to poor quality. Thin tin-based perovskite films are easy to achieve low defects and high crystallization quality, but insufficient thickness leads to the problem of insufficient light trapping. To address these issues, we have thoroughly investigated the various aspects of the photoelectric conversion process in TPSC devices and, for the first time, explored the behavior of front-end optical field management. It is found that the self-constructed microcavity effect can effectively improve the light trapping efficiency under the premise that a thin light-absorbing layer is used, so that sufficient photon trapping and excellent carrier transport characteristics can be ensured simultaneously to realize a high-performance device. Different from the traditional film formation and defect modulation strategy, the present work provides a feasible idea for the performance enhancement of TPSC devices, and is also of great significance for cost control and environmental protection issues in commercial applications.

The influence mechanism of ink viscosity on ink transfer rates and film defects of the roll-to-roll printed organic photoactive layers

The influence mechanism of ink viscosity on ink transfer rates and film defects of the roll-to-roll printed organic photoactive layers

Wide-Range and Multistate Work Functions of Organometallic Halide Perovskite Films Regulated by Ferroelectric Substrates.

Work function of organometallic halide perovskite (OHP) films is one of the most crucial photoelectric properties, which dominates the carrier dynamics in OHP-based devices. Despite surface treatments by additives being widely used to promote crystallization and passivate defects in OHP films, these chemical strategies for modulation of work functions face two trade-offs: homogeneity on the surface versus along the thickness; the range versus the accuracy of modulation. Herein, by using ferroelectric substrates of uniform polarization and subnanometer roughness, homogeneous CH3NH3PbI3 films are fabricated with five states of work functions with large spanning (∼0.8 eV) and high precision (sd ∼ 0.01 eV). We reveal that the ferroelectric polarizations and the smooth surfaces regulate CH3NH3+ orientations and suppress distortions of PbI6 octahedrons. The wide-range and multistate work functions originate from the ordered CH3NH3+ orientations and PbI6 octahedrons, which result in intensity enhancements and wavelength shifts in photoluminescence with a 30-fold increase of photoexcited carrier lifetime.

Ni/Graphene Coating for Enhanced Corrosion Resistance of Metal Foam Flow Field in Simulated PEMFC Cathode Environment.

Metal foam flow field suffers serious corrosion issues in proton exchange membrane fuel cells due to its large surface area. Ni and Ni/graphene coatings are prepared under constant and gradient current modes, respectively, to improve the corrosion resistance. The effect of the electrodeposition current mode and the deposition mechanism is studied. Compared with Ni coating, Ni/graphene coating brings low corrosion current density and high coating resistance, effectively enhancing the stability of Ni foam in an acidic environment. Different from Ni coating with a single layer, Ni/graphene deposits have core-shell structure, with graphene coated on the surface of Ni nanoparticles. It is shown that graphene deposits cover the Ni particles during the electrodeposition, which protects nickel particles from agglomeration and forms an inert film on the surface of the porous structure. After an 8 h constant potential test, no significant pitting is observed on the surface of Ni/graphene coating, showing excellent anticorrosion performance. As to the effect of the deposition current mode, it is shown that more composite particles deposit on the upper layer under the gradient current mode, which brings denser protective film and fewer surface defects on the surface. Ni/graphene coating electrodeposited under a gradient current mode between 0 and 10 mA·cm-2 exhibits the lowest corrosion current densities. The values at 50 and 80 °C are only 62.9 and 26.0% of those of uncoated Ni foam, respectively.

Eliminating Resistance-Capacitance Coupling Shielding for Depicting the Defect Landscape in Perovskite Solar Cells by Capacitance Spectroscopy.

Capacitance spectroscopy techniques have been widely utilized to evaluate the defect properties in perovskites, which contribute to the efficiency and operation stability development for perovskite solar cells (PSCs). Yet the interplay between the charge transporting layer (CTL) and the perovskite on the capacitance spectroscopy results is still unclear. Here, they show that a pseudo-trap-state capacitance signal is generated in thermal admittance spectroscopy (TAS) due to the enhanced resistance capacitance (RC) coupling caused by the carrier freeze-out of the CTL in PSCs, which could be discerned from the actual defect-induced trap state capacitance signal by tuning the series resistance of PSCs. By eliminating the RC coupling shielding effect on the defect-induced capacitance spectroscopy, it is obtain the actual defect density which is 4-folds lower than the pseudo-trap density, and the spatial distribution of defects in PSCs which reveals that the commonly adopted interface passivators can passivate the defects about 60 nm away from the decorated surface. It is further revealed that phenethylammonium ions (PEA+) possess a better passivation capability over octylammonium ions (OA+) due to the deeper passivation depth for PEA+ on the surface defects in perovskite films.

Additive engineering by dicyandiamide for high-performance carbon-based inorganic perovskite solar cells

Hole transport layer (HTL)-free, all-inorganic CsPbIxBr3-x carbon-based perovskite solar cells (C-PSCs) have attracted much attention due to their low cost and excellent stability. The poor device efficiency is a barrier to constrain its commercialization, mainly due to the large amount of interfacial and bulk defects existed in inorganic perovskite films. In this study, an organic small molecule dicyandiamide (DCD) is added to the perovskite precursor as an additive to adjust the crystallization kinetics and passivate defects of inorganic perovskite films, simultaneously. It is demonstrated that the introduction of DCD can not only accelerate the transition process from intermediate-phase DMAPbI3 to inorganic perovskite, but also passivate defects through the Lewis acid-base interaction between cyano (CN), imine (CN) groups, and uncoordinated Pb2+. Meanwhile, the energy level alignment was optimized, which effectively improves the charge transport efficiency of CsPbIxBr3-x C-PSCs. As a result, optimized device shows an enhanced efficiency from 14.07% to 15.84%, accompanied by improved long-term stability.

The synergistic anti‐corrosion performance and mechanism of meso‐tetra(4‐carboxyphenyl)porphine on steel bars in alkaline environments

AbstractCorrosion protection of steel bars in alkaline concrete environments poses a common challenge in marine engineering. One approach to mitigate steel bar corrosion is the addition of corrosion inhibitors to the concrete. In alkaline environments, the passivation of rebars occurs through anodic passivation coupled with the cathodic oxygen reduction reaction (ORR). The catalysis of ORR can expedite anode passivation. To investigate the corrosion inhibition of steel bars in alkaline environments, meso‐tetra(4‐carboxyphenyl)porphine (TCPP), known for its ORR catalytic properties, is selected. TCPP forms adsorption films on the surface of steel bars, facilitating the formation of passivation films. TCPP primarily adsorbs onto active sites on the surface of the passivation film, where lattice iron ions have leached. The adsorbed TCPP accelerates the formation of the passivation film through ORR catalysis, inhibiting the development of passivation film defects and enhancing the integrity and protection of the passivation film. The most significant effect is observed when the concentration of TCPP is 0.5 mmol/L. The physical adsorption of TCPP is primarily determined by the negative charge centers, namely the carboxyl group O and the pyrrole N. However, due to steric hindrance caused by the unrestricted rotation of the carboxyl benzene, the pyrrole N does not play a dominant role in chemical adsorption. Instead, the active site for chemical adsorption is the carboxyl group O. The adsorption process significantly reduces the diffusion coefficient of TCPP molecules, providing a robust and stable adsorption binding. Phthalocyanine molecules without carboxyl benzene groups adopt a planar structure, allowing them to form stable adsorption configurations on the iron surface through flat adsorption. This observation provides guidance for the design of novel metal phthalocyanine molecules. Specifically, the development of metal phthalocyanine molecules with modifying groups that are coplanar with the phthalocyanine ring and possess restricted rotation can achieve flat adsorption, improve coverage rate, and enhance adsorption configuration stability.

Comparison of GeSn alloy films prepared by ion implantation and remote plasma-enhanced chemical vapor deposition methods

Thin films of germanium-tin (GeSn) alloy with Sn content well above its equilibrium solubility limit in Ge are produced using both remote plasma-enhanced chemical vapor deposition (RPECVD) directly on silicon substrates and ion implantation of Sn into Ge. For RPECVD, the growth temperature of 302 °C resulted in fully relaxed GeSn alloys with high defect density, principally threading dislocations related to the large lattice mismatch between Si and GeSn. For the implantation case, pulsed laser melting was used to melt and crystallize the GeSn layer on a time scale of a few tens of nanoseconds. The resulting GeSn layers were also relaxed and defective, presumably again as a result of lattice mismatch with the underlying Ge lattice. However, the nature of the defects was quite different to the RPECVD method, whereby the line defects were not threading dislocations but stackinglike defects, which developed into arrays of these defects in the high Sn content region close to the surface. For the purpose of comparing RPECVD and ion-implantation methods, alloy films of similar thickness (400–450 nm) and Sn content (4.5–6.5 at. %) were examined. Film parameters (thickness, Sn content, Sn solubility, and segregation), as well as film quality and defect structures, were examined for both fabrication methods using several analytical techniques. This comparison provided us with a better physical understanding of our GeSn films and will help inform future growth/fabrication strategies targeted at minimizing defects formed in the GeSn films for the realization of optoelectronic devices.

Investigations on temperature dependent properties of spray deposited tin oxide thin films

The present study reported the temperature dependent properties of tin oxide (SnO2) thin films deposited by using conventional, cost effective and non-vacuum-based spray pyrolysis technique (SPT). The as deposited films were characterized by various characterization techniques such as X ray diffraction (XRD), UV-Vis spectroscopy and Scanning electron microscopy (SEM) to study their structural, optical and morphological properties respectively. XRD analysis revealed tetragonal rutile crystal structure of SnO2 with preferred orientation along (200) plane. The refinement of XRD data was also carried out by Rietveld refinement to obtain the fitting parameters, crystallite size and lattice parameters. UV-Vis study showed considerable change in optical absorbance and transmittance with change in substrate temperature. Effect of substrate temperature on film thickness, refractive index and band gap energy were determined by Swanepoel method and by Tauc’s plot respectively. The Urbach energy was also calculated and were correlated to the defects in SnO2 films. SEM micrograph showed the transformation from granules to nanospheres as a function of substrate temperature. Electrical properties were determined by Hall effect measurements which gives carrier concentration, electrical resistivity and mobility in the order of 1019-1020 /cm3, 10−2-10−3 Ω.cm and 4–23 cm2/Vs respectively.

Achieving High Efficiency and Stability by Blocking Moisture‐Oxygen Permeation Paths on Perovskite Surfaces Using the Curing Agent 4‐(2‐Carboxyvinyl) Benzoic Acid

Organic–inorganic hybrid perovskite solar cells (PSCs) are ideal candidates for the new generation of photovoltaics (PV) due to their excellent physical and chemical properties. However, the active layer of perovskite may have many defects such as vacancies, interstitials, and substitutions during the solution method preparation process. The defects in films can become fast channels for moisture‐oxygen penetration, which promotes the interaction of moisture and oxygen with the defects and accelerates the degradation of PV in air, significantly limiting the efficiency and stability of PSCs. Herein, the curing agent 4‐(2‐carboxyvinyl) benzoic acid is added to the perovskite precursor solution to enhance the device stability by forming a moisture‐oxygen‐blocking barrier at the grain boundaries, at the same time, the Lewis base property of the molecule plays a key role to realize the preferred crystal orientation of perovskite films, passivate the defects and enhance the PV performance, which can significantly improve the devices efficiency. The final optimized PSCs achieve an outstanding power conversion efficiency of 20.28% and show excellent stability, with the unencapsulated device maintaining 78% of its initial efficiency after 1000 h of under ambient environment.