Vacuum Co-deposition Research Articles

Evolution of Defects, Morphology, and Strain during FAMAPbI3 Perovskite Vacuum Deposition: Insights from In Situ Photoluminescence and X-ray Scattering.

At present, the power conversion efficiency of single-junction perovskite-based solar cells reaches over 26%. The further efficiency increase of perovskite-based optoelectronic devices is limited mainly by defects, causing the nonradiative recombination of charge carriers. To improve efficiency and ensure reproducible fabrication of high-quality layers, it is crucial to understand the perovskite nucleation and growth mechanism along with associated process control to reduce the defect density. In this study, we investigate the growth kinetics of a promising narrow bandgap perovskite, formamidinium methylammonium lead iodide (FAMAPbI3), for high-performance single-junction solar cells. The temporal evolution of structural and optoelectronic properties during FAMAPbI3 vacuum codeposition was inspected in real time by grazing-incidence wide-angle X-ray scattering and photoluminescence. Such a combination of analytical techniques unravels the evolution of intrinsic defect density and layer morphology correlated with lattice strain from the early stages of the perovskite deposition.

Hydrogenation of CO2 on NiGa thin films studied by ambient pressure x-ray photoelectron spectroscopy

Successful catalytic conversion of carbon dioxide into high-value chemicals, as a means of abating global CO2 emissions is a pressing challenge. Herein, we report on an ambient pressure x-ray photoelectron spectroscopy study of CO2 hydrogenation reaction using a mixed gas of CO2 and H2 at a pressure up to 0.1 mbar on polycrystalline nickel-gallium thin films prepared by vacuum co-deposition that are compositionally equivalent to Ni, Ni3Ga, Ni5Ga3 and NiGa2, which are some noted low pressure methanol synthesis catalysts. Ni film is very reactive at 300 K, with the reaction sequence initiated by facile CO2 dissociation, yielding C2O4 −2, CO3 −2, CO2 − and CO, but little HCOO− and HCO3 −. The product species observed on NiGa alloy films are not generally different from those observed from using Ni film but are now of different proportions. The NiGa2 film, with the highest Ga content, is the least reactive, as judged by a dominant molecular adsorption feature of CO2. Interestingly, the NiGa alloys are found to undergo composition change and surface restructuring induced by the reactant gas and heating during the reaction. A surface depletion of Ni and a surface enrichment of oxidic Ga are found to occur and continue unabated toward high temperature. For example, for the alloy film with a composition similar to that of Ni5Ga3 when exposed to the mixed gas at 500 K for the duration of measurements (a few h), the near surface region delimited by an XPS probing depth (3λ) of 36 Å is composed of 29% Ni, 24% Ga, and 47% oxidic Ga vs. 62% Ni, 34% Ga, and 4% oxidic Ga for the freshly prepared film at 300 K. Our study demonstrates that a complex interplay exists between the surface reactivity and the evolving chemical makeup as the alloy films are exposed to the reactive gas at elevated pressure and temperature.

Gallium-Enhanced Aluminum and Copper Electromigration Performance for Flexible Electronics.

Wide range binary and ternary thin film combinatorial libraries mixing Al, Cu, and Ga were screened for identifying alloys with enhanced ability to withstand electromigration. Bidimensional test wires were obtained by lithographically patterning the substrates before simultaneous vacuum co-deposition from independent sources. Current–voltage measurement automation allowed for high throughput experimentation, revealing the maximum current density and voltage at the electrical failure threshold for each alloy. The grain boundary dynamic during electromigration is attributed to the resultant between the force corresponding to the electron flux density and the one corresponding to the atomic concentration gradient perpendicular to the current flow direction. The screening identifies Al-8 at. % Ga and Cu-5 at. % Ga for replacing pure Al or Cu connecting lines in high current/power electronics. Both alloys were deposited on polyethylene naphthalate (PEN) flexible substrates. The film adhesion to PEN is enhanced by alloying Al or Cu with Ga. Electrical testing demonstrated that Al-8 at. % Ga is more suitable for conducting lines in flexible electronics, showing an almost 50% increase in electromigration suppression when compared to pure Al. Moreover, Cu-5 at. % Ga showed superior properties as compared to pure Cu on both SiO2 and PEN substrates, where more than 100% increase in maximum current density was identified.

Multiscale molecular simulations of the morphological evolution of small-molecule organic solar cells during the vacuum codeposition process

The mesoscale morphologies of organic small molecular films fabricated via vacuum deposition processes are critical to the performance of small-molecule solar cells and organic light-emitting diodes. In the present study, the morphological evolution of the active layer of DPDCPB:${\mathrm{C}}_{70}$ small-molecule solar cells during vacuum codeposition processes was revealed by a series of GPU-accelerated coarse-grained molecular-dynamics simulations. The ${\mathrm{C}}_{70}$ and DPDCPB molecules were coarsened into ellipsoids and bonded ellipsoids, respectively. The interactions between ellipsoids were described by the Gay-Berne formulation and were parametrized to reproduce potential energy surfaces from all-atom atomistic simulations using a genetic algorithm. Due to the significantly reduced overall degrees of freedom, this coarse-grained scheme allowed us to simulate the vacuum codeposition processes and monitor the morphological evolution of systems with system length scales compatible with those of the experiments (\ensuremath{\sim}30 nm). Our simulations indicate that the film morphologies are closely correlated with the DPDCPB:${\mathrm{C}}_{70}$ blending ratio. High ${\mathrm{C}}_{70}$ concentration leads to a rough film surface, which is in accordance with experimental observations and can be attributed to the strong self-aggregation behavior of ${\mathrm{C}}_{70}$ molecules. The morphological property analysis indicates that the rough film surface has an almost negligible impact on the DPDCPB/${\mathrm{C}}_{70}$ domain percolations, and the device with the optimal deposition ratio should give the most balanced hole/electron transfer in respective DPDCPB/${\mathrm{C}}_{70}$ domains. The present study demonstrates that by using the ellipsoid-based coarse-grained model, it is possible to study the morphological evolution of small-molecule organic thin film during vacuum deposition processes with molecular scale details, which can provide valuable insights for experimental teams to further optimize device fabrication protocols for the next generation of organic optoelectronic devices.

Vacuum-Deposited Multication Tin–Lead Perovskite Solar Cells

The use of a combination of tin and lead is the most promising approach to fabricate narrow bandgap metal halide perovskites. This work presents the development of reproducible tin and lead perovskites by vacuum co-deposition of the precursors, a solvent-free technique which can be easily implemented to form complex stacks. Crystallographic and optical characterization reveal the optimal film composition based on cesium and methylammonium monovalent cations. Device optimization makes use of the intrinsically additive nature of vacuum deposition, resulting in solar cells with 8.89% photovoltaic efficiency. The study of the devices by impedance spectroscopy identifies bulk recombination as one of the performance limiting factors.

Control over Crystal Size in Vapor Deposited Metal-Halide Perovskite Films.

Understanding and controlling grain growth in metal halide perovskite polycrystalline thin films is an important step in improving the performance of perovskite solar cells. We demonstrate accurate control of crystallite size in CH3NH3PbI3 thin films by regulating substrate temperature during vacuum co-deposition of inorganic (PbI2) and organic (CH3NH3I) precursors. Films co-deposited onto a cold (−2 °C) substrate exhibited large, micrometer-sized crystal grains, while films that formed at room temperature (23 °C) only produced grains of 100 nm extent. We isolated the effects of substrate temperature on crystal growth by developing a new method to control sublimation of the organic precursor, and CH3NH3PbI3 solar cells deposited in this way yielded a power conversion efficiency of up to 18.2%. Furthermore, we found substrate temperature directly affects the adsorption rate of CH3NH3I, thus impacting crystal formation and hence solar cell device performance via changes to the conversion rate of PbI2 to CH3NH3PbI3 and stoichiometry. These findings offer new routes to developing efficient solar cells through reproducible control of crystal morphology and composition.

A near-infrared organic light-emitting diode based on an Yb(iii) complex synthesized by vacuum co-deposition.

Yb(DBM)3(DPEPO), an emitter, was directly synthesized on a substrate by the vacuum co-deposition of the precursor Yb(DBM)3(H2O)2 and the ligand DPEPO. OLEDs incorporating the co-deposited emitting layer (Yb(DBM)3(H2O)2 : DPEPO molar ratio = 1 : 1) achieved an ηEQE value of 0.15% at 1.0 mA cm-2.

Ionic liquid-assisted growth of DBTTF–TCNQ complex organic crystals by vacuum co-deposition

High-crystalline DBTTF–TCNQ charge-transfer complex crystals with larger grains visible even by an optical microscope have been successfully grown, assisted by ionic liquid (IL) in vacuum co-deposition. Although the charge transfer reaction between the DBTTF and TCNQ molecules was ready to occur to form the complex regardless of the presence or absence of the IL even at room temperature, the subsequent crystal growth of the DBTTF–TCNQ complexes was enhanced by the IL, especially much more at temperatures higher than room temperature, leading to a significant improvement in the crystallinity of the complexes. The crystal growth mechanism of the DBTTF–TCNQ complexes in the IL was discussed based on the results of in situ optical microscope observation during the deposition of the DBTTF and TCNQ molecules into the IL.

Composition-controlled organometal halide perovskite via CH3NH3I pressure in a vacuum co-deposition process

We report an efficient method to adjust the compositions of CH3NH3PbI3 perovskite by controlling the working pressure of CH3NH3I and its effects on perovskite films in a vacuum co-deposition process.

Structure and mechanical properties of Ag–Cu films prepared by vacuum codeposition of Au and Cu

We have studied in detail the structure formation process in Ag–Cu films in the course of vacuum deposition of the metals, followed by thermal annealing, and compared the hardness values of nanocrystalline Ag, Cu, and Ag–Cu films. Under equivalent deposition conditions, the hardness of the Ag–Cu films produced by codeposition of the metals exceeds that of the Ag and Cu films. The high hardness of the mixedphase Ag–Cu films is due to their amorphous–nanocrystalline structure. We have determined the limiting grain size above which plastic deformation follows a dislocation mechanism.

Visible-Light-Active NiV2O6 Films for Photoelectrochemical Water Oxidation

NiV2O6 films were successfully fabricated and characterized as photoanodes for photoelectrochemical (PEC) water oxidation. The films were synthesized by vacuum codeposition of Ni and V followed by annealing in air. The resulting triclinic NiV2O6 films were n-type semiconductors with optical transitions at ∼2.1 eV (indirect) and 2.4 eV (direct). Photoelectrochemical testing in 1 M KOH showed a photoelectrochemical band gap of ∼2.4 eV and that ∼45% of the photocurrent came from light with λ > 420 nm. NiV2O6 electrodes showed quite stable photocurrent under bias in basic electrolyte, indicative of chemical stability against photocorrosion despite its relatively small band gap. The flat band potential of NiV2O6 was ∼0.6 V vs RHE, thus allowing photogenerated holes to oxidize water thermodynamically, and confirmed by oxygen evolution measurements (Faradaic efficiency ∼80%). Suitable oxygen evolution catalysts and n-type doping are suggested to improve the PEC performance of this material further. Regarding the...

Room temperature humidity sensors based on co-evaporated TeO2 and Sn

Thin layers prepared by vacuum co-deposition of TeO2 and Sn are studied as humidity sensors working at room temperature. They are amorphous and, due to chemical interaction between the two substances, are built up of a mixed Sn- and Te-oxide matrix and a dispersed phase that at RSn/Te < 2.0 consists of elemental Te. The optimal composition of the layers is obtained by varying the atomic ratio RSn/Te. Auger analysis is used to follow the in-depth distribution of the components.It is shown that the layers have a homogeneous composition over their entire depth. For RSn/Te = 0.8 – 0.9, they exhibit very high sensitivity to humidity – the electrical resistance changes by more than 4 decades in the range of 20 - 90% RH. The sensor response is very fast and the recovery period, very short. A very high long-term stability is observed as well.

As-grown superconducting MgB2 thin films prepared at extreme deposition conditions

Superconducting (SC) MgB2 thin films were prepared in situ in one step by vacuum co-deposition at extremely low growth temperature(Ts ∼ 110–150 °C) and at very low Mg deposition rate, far off from theMgB2 stability regime of the thermodynamic phase diagram. As-grown films are nanocrystallineand exhibit (002) texture, moderate surface roughness and SC onset up toTcon = 18 K. This is the highest critical temperature ever observed inlow-Ts as-grownMgB2 films. Our preparationmethod at very low Ts is favorable for the fabrication of layered SC hybrid electronic devices, wheredeposition and post-annealing at high temperature are detrimental to devicefunctioning. Simple post-annealing of our films in ultrahigh vacuum enhancesTcon up to 22 K due to the improvement in structural properties.

Effect of main ligands on organic photovoltaic performance of Ir(iii) complexes

The photovoltaic performance of devices fabricated using three iridium complexes (1, 2, and 3) containing different main ligands (1-phenylisoquinoline, (4-isoquinolin-1-yl-phenyl)diphenylamine, and 1-pyren-1-yl-isoquinoline for 1, 2, and 3, respectively) was investigated. Two different devices, one fabricated by spin coating and one produced by vacuum deposition, were tested. Among the bulk heterojunction solar cells (BHJCs) fabricated by spin coating, the cell fabricated using 2 had the highest power conversion efficiency (PCE, 0.50%). The PCEs of 1 and 3 were 0.43% and 0.34%, respectively. These results suggested that the superior hole-transport ability of the triphenylamine moiety in 2 was responsible for the high photovoltaic performance of the device fabricated using this complex. This assumption was confirmed by fabricating electron-only devices using the three Ir complexes and comparing the turn-on voltage of each device. The photovoltaic performance of device C fabricated by the vacuum co-deposition of 2 and C60 in a 50 nm-thick active layer was 50% higher than that of device A (bilayer heterojunction solar cell) and device B (fabricated by the co-deposition of 2 and C60 with a 30 nm-thick active layer).

Photochromic films prepared by vacuum co-deposition of polymer and spiropyrans

Photochromic polymer thin films have been prepared by vacuum co-deposition of PTFE and several spirocompounds. Their photochromic properties were studied with the use of the spectral-kinetic method. A number of positive properties for application of these films were found.

A New Method and Equipment to Manufacture the Composite Powder Materials by Vacuum Co-Deposition of Double Sources

In the process of manufacturing the composite materials, some of the chemical components are difficult to combine with each other. Aiming at this problem, a new method and supporting equipment of manufacturing the composite powder materials is reported which can make any two (or more) kinds of components mixing in nano-scale. This method is derived from the vacuum co-depositing technology by two (or more) vaporization sources for vacuum film coating, and some of the structures in vacuum film coil coating machine are used for reference. The principle, structure, process and characteristics of this method are introduced in detail. The models for depositing rate calculation are built respectively for both the electric resistance evaporation sources and the magnetron sputtering targets. The mixing component uniformity in the depositing powder material is analyzed by calculation.

Chlorine gas sensors using hybrid organic semiconductors of PANI/ZnPcCl16

PANI/ZnPcCl16 (polyaniline doped with sulfosalicylic acid/hexadecachloro zinc phthalocyanine) powders were vacuum co-deposited onto Si substrates, where Pt interdigitated electrodes were made by micromachining. The PANI/ZnPcCl16 films were characterized and analyzed by SEM, and the influencing factors on its intrinsic performance were analyzed and sensitivities of the sensors were investigated by exposure to chlorine (Cl2) gas. The results showed that powders prepared with a stoichiometric ratio of (ZnPcCl16)0.6(PANI)0.4 had a preferential sensitivity to Cl2 gas, superior to those prepared otherwise; the optimal vacuum co-deposition conditions for the films are a substrate temperature of 160 °C, an evaporation temperature of 425 °C and a film thickness of 75 nm; elevating the operation temperature (above 100 °C) or increasing the gas concentration (over 100 ppm) would improve the response characteristics, but there should be upper levels for each. Finally, the gas sensing mechanism of PANI/ZnPcCl16 films was also discussed.

Bulk heterojunction photovoltaic cells based on vacuum evaporated cadmium sulfide–phthalocyanine hybrid structures

Bulk heterojunction (BHJ) photovoltaic (PV) cells have been prepared by vacuum co-deposition of inorganic cadmium sulfide (CdS) and organic zinc phthalocyanine (PcZn) materials. Characteristics of the BHJ cells were compared with the cells based on vacuum evaporated bilayer CdS/PcZn heterojunctions, and it was found that the BHJ structure possesses higher PV performance due to higher interface area at which excitons can dissociate. Variation of the component ratio in the BHJ devices and comparison of their photocurrent spectra allowed us to distinguish the contribution of CdS, CdS/PcZn interface and PcZn to the PV response. The mechanism of charge transport was attributed to the model of space-charge-limited current with traps; however, the concentration of traps was different in the bilayer and BHJ structures.

A highly efficient OLED based on terbium complexes

A neutral ligand 9-(4-tert-butylphenyl)-3,6-bis(diphenylphosphineoxide)-carbazole (DPPOC) and its complex Tb(PMIP) 3DPPOC ( A, where PMIP stands for 1-phenyl-3-methyl-4-isobutyryl-5-pyrazolone) were synthesized. DPPOC has a suitable lowest triplet energy level (24,691 cm −1) for the sensitization of Tb(III) ( 5D 4: 20,400 cm −1) and a significantly higher thermal stability (glass transition temperature 137 °C) compared with the familiar ligand triphenylphosphine oxide (TPPO). Experiments revealed that the emission layer of the Tb(PMIP) 3DPPOC film could be prepared by vacuum co-deposition of the complex Tb(PMIP) 3(H 2O) 2 ( B) and DPPOC (molar ratio = 1:1). The electroluminescent (EL) device ITO/ N, N′-diphenyl- N, N′-bis(1-naphthyl)-1,1′-diphenyl-4,4′-diamine (NPB; 10 nm)/Tb(PMIP) 3 (20 nm)/co-deposited Tb(PMIP) 3DPPOC (30 nm)/2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP; 10 nm)/tris(8-hydroxyquinoline) (AlQ; 20 nm)/Mg 0.9Ag 0.1 (200 nm)/Ag (80 nm) exhibited pure emission from terbium ions, even at the highest current density. The highest efficiency obtained was 16.1 lm W −1, 36.0 cd A −1 at 6 V. At a practical brightness of 119 cd m −2 (11 V) the efficiency remained above 4.5 lm W −1, 15.7 cd A −1. These values are a significant improvement over the previously reported Tb(PMIP) 3(TPPO) 2 ( C).

ORGANIC PHOTOSENSING COMPOSITE STRUCTURE ON THE BASE OF PENTACENE AND POLYANILINE

Organic photosensing composite polyaniline:pentacene structure with a wide spectrum of photosensitivity (1.7-2.7 eV) by means of vacuum co-deposition of polyaniline and pentacene on flexible substrate (PET) with ITO layer was fabricated. As rectifying contact the deposited aluminum film on composite structure was used. The bulk and barrier properties of composite structure were evaluated by means of current-voltage measurements.