Mixed Thin Films Research Articles

Passivation engineering for hysteresis-free mixed perovskite solar cells

This work demonstrates the effectiveness of interfacial engineering and defect passivation in the bulk of mixed perovskite absorber to achieve high performance perovskite solar cells (PSCs). It is found that the extent of I–V hysteresis is most significant in PSCs with a single-layer electron transport layer (ETL) composed of SnO2 quantum dots (QD-SnO2) or SnO2 nanoparticles (NP–SnO2). The hysteresis of the PSCs can be effectively suppressed by adopting a multilayer structure for the ETL to optimize the ETL/perovskite interface. Furthermore, the performance of the PSCs can be enhanced by incorporating a controlled amount of an organic cross linker, 2,2′-(ethylenedioxy)bis (ethylammonium iodide) (EDAI), in the mixed perovskite absorber layers. The experimental results consistently show that incorporation of an optimal amount of EDAI is effective in passivating defect states in the bulk of mixed perovskite as well as the grain boundaries and film surface, while excessive EDAI significantly degrades the photovoltaic (PV) performance of PSCs due to generation of defects and electrically insulating properties of EDAI itself. Owing to the synergistic effect contributed from optimized ETL/perovskite interface and the mixed perovskite thin film, the PSC with an active area of 0.06 cm2 exhibits a champion power conversion efficiency (PCE) of ~19.0% with negligible hysteresis and improved stability. The proposed strategies were also applied to fabricate larger area devices from 0.15 cm2 to 0.85 cm2, exhibiting the PCEs of ~12%–18% with negligible hysteresis.

Halide (Cl/Br)-Incorporated Organic–Inorganic Metal Trihalide Perovskite Films: Study and Investigation of Dielectric Properties and Mechanical Energy Harvesting Performance

Compositional tuning of organic–inorganic metal trihalide perovskites (OTPs) via halide (Cl/Br) incorporation has attracted increased attention due to the rapid advancements in the performance of optoelectronic devices based on mixed halide perovskites. It is expected that the fine-tuning of OTPs can significantly enhance the dielectric and piezoelectric properties that are beneficial for constructing a high-performance piezoelectric nanogenerator (PENG). However, few studies on these aspects have so far been reported. In the present study, mixed halide perovskite thin films with various halide (Cl or Br) contents were fabricated by a simple two-step spin-coating method. The effects of fine-tuning the halide composition of methylammonium lead iodide perovskite (MAPbI3) on the morphological, electrical, dielectric, and piezoelectric properties of the perovskite thin films were systematically investigated. In addition, the feasibility of using these mixed halide-MAPbI3 thin films in mechanical energy-harvesting systems was also thoroughly examined by constructing a series of flexible PENGs. The energy-harvesting performance of the PENG was found to be significantly influenced by the amount of incorporated Cl or Br in the MAPbI3 film. Among all the fabricated PENGs, the 4Cl-MAPbI3 PENG poled at 50 kV/cm exhibited a much higher piezoelectric output performance, with an output voltage of ∼5.9 V and an output current density of ∼0.61 μA/cm2. Thus, the present study demonstrates a simple and reliable method for tuning the dielectric and piezoelectric properties of the OTP materials toward developing flexible, high-performance devices which may offer a wide range of utility in wearable and portable electronic devices.

Rational Design of Oxygen Deficiency-Controlled Tungsten Oxide Electrochromic Films with an Exceptional Memory Effect.

Owing to their nonemissive characteristics, electrochromic materials promise distinct advantages in developing next-generation eye-friendly information displays. Yet, it remains a challenge to manipulate the structure of the materials to achieve a strong memory effect with high optical contrast, which is of importance for displaying images with essentially zero energy consumption. Here, we design a mixed crystalline WOx thin film implanted with massive oxygen deficiencies based on a conventional reactive magnetron sputtering process. The obtained WOx film exhibits high dual-band optical modulation in both visible (VIS, 99.0% in 633 nm) and near-infrared (NIR, 94.2% in 1300 nm) regions as well as an exceptional memory effect (the colored transmittance increases only by 0.04% at 633 nm after 50 days). The enhanced electrochromic performance can be attributed to dense Li+-ion binding sites as well as the trapping effect provided by the massive internal oxygen deficiencies. The strategy in this work bestows the WOx thin film a promising candidate for developing electrochromic information displays and other energy-efficient devices as well.

Deposition and properties of mixed molybdenum sulfide (MoS2) and copper sulfide (CuxS) films on glass surface using elemental sulfur as a precursor

Mixed molybdenum sulfide (MoS2) and copper sulfide (CuxS) materials were synthesized on glass slides as a model substrate by successive ionic layer adsorption and reaction method using elemental sulfur as a sulfur source and solutions of copper (II/I) and molybdenum (IV) salts. X-ray diffraction data showed the presence of copper sulfides (anilite Cu7S4 and djurleite Cu31S16 phases), molybdenum disulfide and molybdenum oxide with adjustable crystalline phase composition dependent on the precursor exposure time and reaction temperature. Scanning electron microscopy images showed that the surface had a homogenous layer distribution at lower temperatures/short exposures and comprised of ~1 μm particles. X-ray photoelectron spectroscopy showed no oxidized sulfate (SO42−) compounds present on the sample surface while a thin MoO3 layer was found to cover the sample surface. Optical ultraviolet-visible spectroscopy showed mixed layers formed, containing optical properties of both MoS2 and CuxS. After annealing in an N2 atmosphere at 100 °C, the sample was composed primarily of MoS2 and copper sulfides in the crystalline phase with minimal residual S8. Usability of this mild mixed metal sulfide thin film preparation method on supports for sour natural gas catalyst synthesis is also discussed.

Co-Solvent Controllable Engineering of MA0.5FA0.5Pb0.8Sn0.2I3 Lead–Tin Mixed Perovskites for Inverted Perovskite Solar Cells with Improved Stability

Use of a lead–tin mixed perovskite is generally considered an effective method to broaden the absorption wavelength of perovskite thin films. However, the preparation of lead–tin mixed perovskites is a major challenge due to the multivalent state of tin and stability in the atmosphere. This study attempted to replace the organic cation and metal elements of perovskites with a relatively thermal stable formamidinium (FA+) and a more environmentally friendly tin element. MA0.5FA0.5Pb0.8Sn0.2I3 lead–tin mixed perovskite thin films were prepared with the one-step spin-coating method. By adjusting the dimethylformamide (DMF):dimethyl sulfoxide (DMSO) concentration ratio of the lead–tin mixed perovskite precursor solution, the surface morphologies, crystallinity, and light-absorbing properties of the films were changed during synthesis to optimize the lead–tin mixed perovskite films as a light-absorbing layer of the inverted perovskite solar cells. The quality of the prepared lead–tin mixed perovskite film was the highest when the ratio of DMF:DMSO = 1:4. The power-conversion efficiency of the perovskite solar cell prepared with the film was 8.05%.

Pulsed Laser Deposition Deposited Layers of Pseudo-Binary Zinc Oxides and Zinc-Porphyrin for Steel Corrosion Inhibition

Obtaining of ZnTa2O6 and ZnV2O6 pseudo-binary oxide materials by hydrothermal and coprecipitation methods and of Zn(II) 5-pyridyl-10,15,20-tris-(3,4-di-methoxy-phenyl) porphyrin (ZnPydiMeOPP) are presented. Mixed thin films of pseudo-binary oxide materials and Zn-porphyrin were realized by pulsed laser deposition (PLD) technique on steel disks and corrosion tests in 0.1 M HCl solution were performed. The inhibition efficiency (IE) in all cases containing porphyrin was always higher than 80%. The best IE (84.50%) was obtained for the sandwich structure using the hydrothermally obtained ZnV2O6 oxide as the first layer and the ZnPydiMeOPP porphyrin as the second layer. The mechanism of corrosion protection can be explained by the physical/mechanical barrier effect.

Single step deposition of Cs2SnIxCl6-x thin films with uniform morphology, composition and high air stability

The present work reports single step spray deposition of cesium tin mixed halide perovskite thin films using a precursor solution containing cesium chloride (CsCl) tin chloride (SnCl2) and iodine powder. At the optimized spray conditions, the role of solvents, substrate temperature and iodine concentration on the structure and properties of the perovskite films were investigated. X-ray diffraction patterns and Raman spectra showed that the thin films were Cs2SnIxCl6-x, a mixed halide double perovskite having cubic crystal structure. The elemental composition and their chemical states were determined using X-ray photoelectron spectroscopy. The Cs2SnIxCl6-x perovskite thin films showed bandgap values ranging from 3.1 to 1.5 eV as x varied from 0 to 0.25. Further, the films showed uniform and compact morphology with good ambient air stability. Our results demonstrated that spray deposition is a reproducible technique to synthesize stable inorganic lead-free perovskite thin films.

Thin Film of TiO2–ZnO Binary Mixed Nanoparticles as Electron Transport Layers in Low-Temperature Processed Perovskite Solar Cells

Organic–inorganic hybrid perovskite solar cells have become one of the highly promising candidates for photovoltaic technologies because of their low processing cost, rapid-growing power conversion efficiency and easy preparation process. Electron transfer layer (ETL) plays an important role in exciton separation and charge transport for perovskite devices. A TiO2–ZnO binary mixed nanoparticle (NP) ETL, which can be prepared in low-temperature hydrothermal method, was proposed. By analyzing the XRD and SEM, the incorporation of mixed NPs thin film improved the interfacial stability of ZnO/perovskite and prevented the perovskite layer from being decomposed as compared to the pure ZnO NPs thin film. Furthermore, UV spectrum and EIS results show that TiO2–ZnO mixed NP ETL has high transmittance and maintains good electrical properties of pure ZnO NPs basically. Finally, the efficiency of perovskite device based on TiO2–ZnO mixed NP ETL was improved to 15%. Our research provides a simple way for the application of ZnO in PCSs.

Coexistence of cubic and tetragonal phase with colored excitonic emissions in mixed halide Perovskite CH3NH3PbI3-xClx thin film

SynopsisCH3NH3PbI3-xClx crystalline film composed of cuboids and polyhedral structure was prepared by dipping and withdrawing method at ambient. X-ray diffraction shows the presence of tetragonal and cubic phases having high absorption coefficient of 106 m-1. Photoluminescence study confirms shallow and deep level emissions. Temperature dependent-cathodoluminescence shows a green luminescence at 2.51 eV accompanied by weak red and ultraviolet luminescence at 80 and 300 K.

Crystallization kinetics of rapid spray plasma processed multiple cation perovskites in open air

Wide angle X-ray scattering of perovskite conversion in ∼100 ms using rapid spray plasma process.

Visualizing the impact of chloride addition on the microscopic carrier dynamics of MAPbI3 thin films using femtosecond transient absorption microscopy.

The spatial heterogeneity of carrier dynamics in mixed halide perovskite CH3NH3PbI3-xClx thin films with a range of different chloride additions is mapped using femtosecond transient absorption microscopy (TAM). The comparison of TAM images of fibrous and granular polycrystalline CH3NH3PbI3-xClx films indicates that the impact of chloride addition on the local heterogeneity of carrier dynamics is highly dependent on the film preparation method and the resulting morphology. In addition to signals of pristine CH3NH3PbI3, CH3NH3PbI3-xClx films with a fibrous structure show long-lived excited state absorption (ESA) signals in localized, microscopic regions. The ESA signal exhibits transient absorption with a rise time of about 5 ps after the excitation pulse, indicating that these distinct micrograins have preferential carrier trapping properties. The chemical composition of these micrograins does not differ detectably from their surroundings. In contrast, in CH3NH3PbI3-xClx films with a granular structure, Cl addition does not seem to affect the charge carrier dynamics. These results provide insight into the localized effects of halide mixing and on the resulting photophysical properties of mixed halide perovskite materials on the micrometer length scale.

Deciphering the Mechanisms of Bacterial Inactivation on HiPIMS Sputtered CuxO-FeOx-PET Surfaces: From Light Absorption to Catalytic Bacterial Death.

The production of nontoxic, affordable, and efficient antibacterial surfaces is key to the well-being of our societies. In this aim, antibacterial thin films have been prepared using earth-abundant metals deposited using high-power impulse magnetron sputtering (HiPIMS). The sputtered FeOx, CuxO, and mixed CuxO-FeOx films exhibited fast bacterial inactivation properties under exposure to indoor light (340-720 nm) showing total bacterial inactivation within 180, 120, and 60 min, respectively. The photocatalytic mechanisms of these films were investigated, from the absorption of photons up to the bacteria's fate, by means of ultrafast transient spectroscopy, flow cytometry, and malondialdehyde (MDA) quantification justifying the cell wall disruption. The primary driving force leading to bacterial inactivation was found to be the oxidative stress at the interface between the sputtered thin films and the microorganism. This was justified by using engineered porinless bacteria disabling the possible ion diffusion leading to internal bacterial inactivation. Such stress is a direct consequence of the photogenerated electron-hole pairs at the interface of the sputtered layers. By diffuse reflectance spectroscopy, we found that both FeOx and CuxO present a band gap of ∼2.9 eV (>425 nm), while the mixed CuxO-FeOx thin film has a band gap below 2.3 eV (>540 nm). The structure and atomic composition of the films were characterized by energy-dispersive X-ray, X-ray photoelectron, and optical spectroscopy. While the composition and metal oxidation states are distinct in all three films, the difference in photocatalytic efficiency can, at first sight, be explained as the direct consequence of their absorbance and the unique interaction between Fe and Cu oxides in the composite film.

Rapid fabrication of perovskite solar cells through intense pulse light annealing of SnO2 and triple cation perovskite thin films

Rapid evolution of perovskite solar cells (PSCs) performance and stability has inclined the research focus towards scalable bulk fabrication through high speed and cost-effective automated methods. For the first time, intense pulsed light (IPL) is utilized to rapidly fabricate efficient PSCs through swift annealing of both the SnO2 electron transport layer (ETL) and mixed triple cation perovskite thin films. The addition of di-iodomethane (CH2I2) alkyl-halide could enhance the PSC efficiency by retarding the crystallization and improving the surface morphology of the perovskite photoactive film through supplying iodine cleaved by ultraviolet energy during IPL process. The maximum efficiency and fill factor of the PSCs fabricated by IPL annealing were 12.56% and 78.3% for the rigid glass-FTO slides, and 7.6% and 64.75% for flexible PET-ITO substrates when processed in the ambient with relative humidity of 60%, respectively. The annealed materials were characterized through Scanning electron microscopy (SEM), UV–Vis, photoluminescence (PL), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) techniques. In addition, impedance spectroscopy (IS) and current-voltage measurements were conducted to study the functionality of fabricated cells. Our results delineated the feasibility of sequential step IPL annealing on rapid fabrication of efficient PSCs which is directly applicable for scalable roll-to-roll manufacturing.

Crystal growth of (Bi, Pb)2Sr2Ca2Cu3O thin films fabricated via post annealing

Post-annealing of superconducting Bi2Sr2Can-1CunOx (BSCCO) thin films deposited by sputtering is essential for obtaining a single 2223 phase. However, optimum annealing temperature is limited to a very narrow range, needing to pass liquid phase for 2223 crystallization while not exceeding re-evaporation point. In this study, we optimized annealing conditions and observed the crystal growth process. As a result, single phase 2223 thin films with critical temperature of 108 K were obtained at 850 °C, while mixed phase thin films of 2223 and 2212 were grown above 855 °C. This is due to the re-evaporation of the thin film compounds, and these results reflect the recrystallization process after melting.

Revealing Suppressed Intermolecular Coupling Effects in Aggregated Organic Semiconductors by Diluting the Crystal: Model System Perfluoropentacene:Picene.

In order to investigate the effects of intermolecular interactions on the optical properties of organic semiconductors, we employ mixing of the organic semiconductor perfluoropentacene (PFP; C22F14) with the wide band-gap organic semiconductor picene (PIC; C22H14). The binary mixed thin films are prepared by simultaneous coevaporation of PIC and PFP in vacuum. We determine the optical properties of the blends by differential reflectance spectroscopy (absorption) and photoluminescence (emission). PFP:PIC thin films are a rare case of mixed thin films with a known molecular packing. The formation of equimolar mixed domains with a crystal structure clearly different from that of the pure compounds is, in the case of nonequimolar blends, accompanied by pure domains of the excess compound. Due to the wide band gap of PIC, the effect of reduced intermolecular interactions between PFP molecules can be studied in detail without any direct contributions of PIC to the spectra. We find a strongly enhanced emission from PFP in the mixed thin films, which can be explained by decoupling. Real-time investigations of the absorption spectra during growth provide further insight into intermolecular coupling effects on optical properties.

Efficiency Enhancement with the Ferroelectric Coupling Effect Using P(VDF-TrFE) in CH3NH3PbI3 Solar Cells.

A novel ferroelectric coupling photovoltaic effect is reported to enhance the open‐circuit voltage (V OC) and the efficiency of CH3NH3PbI3 perovskite solar cells. A theoretical analysis demonstrates that this ferroelectric coupling effect can effectively promote charge extraction as well as suppress combination loss for an increased minority carrier lifetime. In this study, a ferroelectric polymer P(VDF‐TrFE) is introduced to the absorber layer in solar cells with a proper cocrystalline process. Piezoresponse force microscopy (PFM) is used to confirm that the P(VDF‐TrFE):CH3NH3PbI3 mixed thin films possess ferroelectricity, while the pure CH3NH3PbI3 films have no obvious PFM response. Additionally, with the applied external bias voltages on the ferroelectric films, the devices begin to show tunable photovoltaic performance, as expected for the polarization in the poling process. Furthermore, it is shown that through the ferroelectric coupled effect, the efficiency of the CH3NH3PbI3‐based perovskite photovoltaic devices is enhanced by about 30%, from 13.4% to 17.3%. And the open‐circuit voltages (V OC) reach 1.17 from 1.08 V, which is reported to be among the highest V OCs for CH3NH3PbI3‐based devices. It should be noted in particular that the thickness of the layer is less than 160 nm, which can be regarded as semi‐transparent.

Detection of X‐Rays by Solution‐Processed Cesium‐Containing Mixed Triple Cation Perovskite Thin Films

AbstractMaterials and technology development for designing innovative and efficient X‐ray radiation detectors is of utmost importance for a wide range of applications ranging from security to medical imaging. Here, highly sensitive direct X‐ray detectors based on novel cesium (Cs)‐based triple cation mixed halide perovskite thin films are reported. Despite being in a thin film form, the devices exhibit a remarkably high X‐ray sensitivity of (3.7 ± 0.1) µC Gy−1 cm−2 under short‐circuit conditions. At a small reverse bias of 0.4 V, the sensitivity further increases by orders of magnitude reaching a record value of (97 ± 1) µC Gy−1 cm−2 which surpasses state‐of‐the‐art inorganic large‐area detectors (a‐Se and poly‐CZT). Based on detailed structural, electrical, and spectroscopic investigations, the exceptional sensitivity of the triple cation Cs perovskite is attributed to its high ambipolar mobility‐lifetime product as well as to the formation of a pure stable perovskite phase with a low degree of energetic disorder, due to an efficient solution‐based alloying of individual n‐ and p‐type perovskite semiconductors.

Modulation of crystal structure and electrical properties of Hf0.6Zr0.4O2 thin films by Al-doping

The electrical properties of Hf- and Zr-based oxide thin films, which are related to their crystal structure, are sensitively dependent on their composition and process conditions. In particular, HfZrO2 mixed thin films, which have potential applicability as high-k dielectrics, can have various polarization phases, including paraelectric, ferroelectric, and anti-ferroelectric phases. Therefore, we investigated the enhancement of the dielectric properties of HfZrO2 thin films upon Al doping by confirming their polarization characteristics. Doping of the HfZrO2 thin film with a small amount of Al (∼2.5%) caused a remarkable increase in the dielectric constant to ∼47 and modified the polarization phase from ferroelectric to paraelectric, and the doped film subsequently showed an excellent leakage current of about 2 × 10−8 A/cm2 at ± 1 MV/cm. On the basis of these findings, we believe that Al-doped HfZrO2 films are promising candidates for use as paraelectric high-k dielectrics for dynamic random access memory capacitors, as gate dielectrics for high-speed transistors, and so on.

Drastic increase in thermoelectric power factor of mixed Sb2Te3-In2Te3 thin films

Thermoelectric power factor is an indicator of the performance of a thermoelectric material. Attempts have been made by various techniques, like doping, to improve the thermoelectric conversion efficiency of materials. In the present study, a layer structured thermoelectric material Sb2Te3 is alloyed to In2Te3 using vacuum deposition method at 423 K to significantly enhance the power factor of ∼118 μWm-1K−2 (at 450 K). Structurally, all films were polycrystalline in nature as clearly reflected in XRD patterns. All films were showing p-type conductivity, and electrical conductivity of In2Te3 films increased with increasing Sb2Te3 content. The seebeck coefficient is found to be higher for un-doped In2Te3 than that of Sb2Te3-In2Te3 and pure Sb2Te3 films. However, the thermoelectric power factor of 25% Sb2Te3 alloyed In2Te3 films is enhanced by 11.9 times that of In2Te3 films and 4 times that of Sb2Te3 films at 320 K. It is interesting to note that efficiency of the mixed films is higher than that of the individual films.

Expanding Multinary Selenide Based High-Efficiency Oxygen Evolution Electrocatalysts through Combinatorial Electrodeposition: Case Study with Fe–Cu–Co Selenides

Developing low-cost electrocatalysts with high efficiency for water splitting is a critical task to make this technology viable for large-scale clean energy generation. Transition metal selenides, comprising earth abundant elements, such as Fe, Co and Cu, have gained attention as superior electrocatalysts for oxygen evolution reaction (OER) in the alkaline medium. In this article, we have systematically investigated the evolution of OER catalytic activity as a function of composition for a series of Fe–Co–Cu quaternary selenides by exploring a trigonal phase diagram. The OER activity was dependent on the quantity of Cu and Fe in the Fe-Cu-Co-Se quaternary selenide electrocatalysts, while surprisingly, Fe–Cu ternary selenides exhibit reduced OER activity in comparison to their pure parent compounds FeSe and Cu₃Se₂. Quaternary selenides exhibited more efficient catalytic activity with increasing amount of Fe or Cu in the catalysts, and the quaternary mixed metal selenide thin film of composition (Fe₀.₄₈Co₀.₃₈Cu₀.₁₄)Se showed the best catalytic performance with a small overpotential of 256 mV at 10 mA cm–² and a low Tafel slope of 40.8 mV dec–¹ in N₂-saturated 1.0 M KOH solution. The outstanding catalytic performance of quaternary selenides may be explained by the possible electron cloud delocalization among the transition metal sites in the catalytic system through d-bands, leading to lower charge transport resistance at the catalyst–electrolyte interface as well better film conductivity, as has also been observed through electrochemical impedance spectroscopy. Such enhanced charge transfers eventually facilitate the rate of O₂ release from the catalyst surface, leading to enhanced activity.