Two-step Sequential Deposition Research Articles

3D Mesoporous van der Waals Heterostructures for Trifunctional Energy Electrocatalysis.

The emergence of van der Waals (vdW) heterostructures of 2D materials has opened new avenues for fundamental scientific research and technological applications. However, the current concepts and strategies of material engineering lack feasibilities to comprehensively regulate the as-obtained extrinsic physicochemical characters together with intrinsic properties and activities for optimal performances. A 3D mesoporous vdW heterostructure of graphene and nitrogen-doped MoS2 via a two-step sequential chemical vapor deposition method is constructed. Such strategy is demonstrated to offer an all-round engineering of 2D materials including the morphology, edge, defect, interface, and electronic structure, thereby leading to robustly modified properties and greatly enhanced electrochemical activities. The hydrogen evolution is substantially accelerated on MoS2 , while the oxygen reduction and evolution are significantly improved on graphene. This work provides a powerful overall engineering strategy of 2D materials for electrocatalysis, which is also enlightening for other nanomaterials and energy-related applications.

A strategy toward air-stable and high-performance ZnO-based perovskite solar cells fabricated under ambient conditions

Despite the recently reported efficiency over 20%, the long-term stability of perovskite solar cells (PSCs) under ambient conditions remains a great challenge. For most of the highly efficient PSCs, fabrication processes are usually carried out in glovebox to guarantee their performance and stability. Herein, a new manufacturing strategy is introduced to optimize the device performance under ambient conditions. We skillfully merge both of ultrasonic assisted method and a two-step sequential spin-coating deposition technique to prepare a more compact and pinhole-free ZnO and perovskite films in air. As a result, the ZnO-based PSC with a conventional structure of glass/FTO/ZnO/CH3NH3PbI3/Spiro-OMeTAD/Au demonstrate an efficiency of up to 14.25%. Notably, the efficiency can be maintained at 86.25% of its initial value even though it is exposed to air for 45 days. Most importantly, all processes including fabrication, measurement as well as storage are performed in ambient air. We believe the proposed synergetic strategy can open up a new pathway for boosting the commercial applications of PSCs.

Uniform, high crystalline, (100) crystal orientated perovskite films without PbI2 residue by controlling the nanostructure of PbI2

The photovoltaic performance and stability of perovskite solar cells (PVSCs) is deeply dependent on the morphology of the perovskite film and the amount of lead iodide (PbI2) residue. However, the morphology of perovskite film fabricated through two-step sequential deposition method usually suffers from poor film coverage and low crystallinity due to the ungovernable crystallization kinetics. Moreover, owing to the compact stacking of PbI2 crystals, the conversion of PbI2 is incomplete as well. In this work, we propose for the first time to control the nanostructure of PbI2 to get high crystallinity, identical (100) crystal orientation and PbI2 residue-free film. Porous PbI2 with plate structure was obtained by selected solvent annealing (SSA). The reaction rate between PbI2 and methylammonium iodide (MAI) was accelerated because the contact area between PbI2 and MAI was enlarged due to the porous nanostructure of PbI2. Thus, uniform, high crystalline perovskite film without PbI2 residue was obtained. Furthermore, (100) crystal orientation of perovskite was obtained due to the PbI2 shape changing from grain to plate, which originated from the changing growth frontier of PbI2 crystals. As a result, a promising power conversion efficiency of 16.61% is achieved in planar-heterojunction PVSCs due to improved photoabsorption and carrier extraction efficiency. This work for the first time establishes the relationship between PbI2 structure and the corresponding structure of perovskite in detail, and shows a reasonable design of PbI2 nanostructure is important to improve the perovskite morphology.

Improved performance of CH3NH3PbBr3 perovskite solar cells utilizing PbI2 precursors

High-quality CH3NH3PbBr3 films have been made utilizing the PbI2 precursor via a two-step sequential deposition method. The use of the PbI2 precursor has significantly improved the film quality. UV–Vis results show that the PbI2 precursor leads to moderately enhanced absorptions. XRD and SEM characterizations suggest that compact and uniform films with large grain sizes and high crystallinity can be obtained utilizing PbI2. The power conversion efficiency of the FTO/compact-TiO2/MAPbBr3/Spiro-OMeTAD/Ag device increased from 5.3% to 7.6% after changing the PbBr2 precursor to PbI2. The reaction between PbI2 and excess MABr generated MAPbBr3 through halide anion exchange.

An isopropanol-assisted fabrication strategy of pinhole-free perovskite films in air for efficient and stable planar perovskite solar cells

As a promising photovoltaic device, perovskite solar cells have attracted numerous attention in recent years, where forming a compact and pinhole-free perovskite film in air is of great importance. Herein, we evaluate highly efficient and air stable planar perovskite solar cells in air (relative humidity over 50%) with the modified two-step sequential deposition method by adjusting the CH3NH3I (MAI) concentrations and regulating the crystallization process of the perovskite film. The optimum MAI concentration is 60 mg mL−1 in isopropanol. With a planar structure of FTO/TiO2/MAPbI3/spiro-OMeTAD/Au, the efficient devices composed of compact and pinhole-free perovskite films are constructed in air, achieving a high efficiency of up to 15.10% and maintaining over 80% after 20 days storing without any encapsulation in air. With a facile fabrication process and high photovoltaic performance, this work represents a promising method for fabricating low-cost, highly efficient and stable photovoltaic device.

A 200-nm length TiO2 nanorod array with a diameter of 13 nm and areal density of 1100 µm−2 for efficient perovskite solar cells

Abstract Rutile TiO 2 nanorod arrays with lengths of 200 nm, 490 nm and 800 nm were prepared by a hydrothermal method using an aqueous grown solution of 38 mM titanium isopropoxide and 6 M hydrochloric acid at 170 °C for 75 min, 105 min and 115 min, respectively. CH 3 NH 3 PbI 3−x Br x thin films were obtained by two-step sequential deposition using a 1.7 M PbI 2 ·DMSO complex precursor solution and a 0.465 M isopropanol solution of a methylammonium halide mixture with a molar ratio of CH 3 NH 3 I/CH 3 NH 3 Br = 85/15. The influence of the microstructure of the TiO 2 nanorod array on the charge separation at the perovskite/TiO 2 interface and the photovoltaic performance of the corresponding solar cells were investigated. Perovskite solar cells based on a 200-nm length TiO 2 nanorod array with a diameter of 13 nm and areal density of 1100 µm −2 exhibited the best photoelectric conversion efficiency (PCE) of 16.23%, with an open-circuit voltage ( V oc ) of 1.02 V, short-circuit photocurrent density ( J sc ) of 21.98 mA cm −2 and fill factor (FF) of 0.72, as well as an average PCE of 15.63 ± 0.60%, with a V oc of 1.00 ± 0.03 V, J sc of 21.80 ± 0.81 mA cm −2 and FF of 0.71 ± 0.02 under illumination of simulated AM 1.5 sunlight (100 mA cm −2 ).

Enhanced performance in perovskite solar cells via bromide ion substitution and ethanol treatment

Mixed lead halide (Pb(I1-xBrx)2) as the seed layer was employed to prepare mixed lead-halide perovskite (MAPbI3-2xBr2x, MA=CH3NH3) films through two-step sequential deposition method. Ethanol treatment process was also introduced for the control of morphology and microstructure of Pb(I1-xBrx)2 films. The ethanol treatment accelerates the crystallization of Pb(I1-xBrx)2 and the resulted Pb(I1-xBrx)2 films exhibit a porous structure which facilitates more complete conversion of PbI2 at the same time. As a result, high purity and highly crystallized MAPbI3-2xBr2x films are obtained. The photovoltaic performance of assembled perovskite solar cells based on MAPbI3-2xBr2x films are improved upon ethanol treatment and a champion power conversion efficiency (PCE) of 15.53% is obtained with x=0.2. After exposed in air condition for 14days, a 86% of initial PCE remains in the champion device.

Optimization of spray coating for the fabrication of sequentially deposited planar perovskite solar cells

We use facile coating techniques including spray coating and drop casting to fabricate methylammonium lead iodide perovskite solar cells through a two-step sequential deposition approach. In the first step, for the deposition of the lead iodide, spray coating substitutes for the commonly used lab-scale spin coating, while the operating parameters of the former process are optimized to achieve a fully covered and uniform film of lead iodide. In the second step, to deposit methylammonium iodide atop the lead iodide layer to form methylammonium lead iodide perovskite, dip-coating process is replaced by the touch-free drop casting and scalable pulsed-spray coating. It is found that the performance of the perovskite films and devices made by pulsed-spray coating and drop casting is similar to those prepared by dip coating, while the large-scale production capabilities of such methods beside the low material consumptions of drop casting prove their potential to replace dip coating in large-scale manufacturing of perovskite solar cells. The champion devices fabricated by spray-drop and spin-drop techniques demonstrated power conversion efficiencies of 6.92% and 9.48%, respectively. It is expected that device fabrication in a low-humidity environment using the optimized parameters and optimization of other layers will result in higher efficiencies.

Full coverage of perovskite layer onto ZnO nanorods via a modified sequential two-step deposition method for efficiency enhancement in perovskite solar cells

Full coverage of perovskite layer onto ZnO nanorod substrates with less pinholes is crucial for achieving high-efficiency perovskite solar cells. In this work, a two-step sequential deposition method is modified to achieve an appropriate property of perovskite (MAPbI3) film. Surface treatment of perovskite layer and its precursor have been systematically performed and their morphologies have been investigated. By pre-wetting of lead iodide (PbI2) and letting it dry before reacting with methylammonium iodide (MAI) provide better coverage of perovskite film onto ZnO nanorod substrate than one without any treatment. An additional MAI deposition followed with toluene drop-casting technique on the perovskite film is also found to increase the coverage and enhance the transformation of PbI2 to MAPbI3. These lead to longer charge carrier lifetime, resulting in an enhanced power conversion efficiency (PCE) from 1.21% to 3.05%. The modified method could been applied to a complex ZnO nanorods/TiO2 nanoparticles substrate. The enhancement in PCE to 3.41% is observed. These imply that our introduced method provides a simple way to obtain the full coverage and better transformation to MAPbI3 phase for enhancement in performances of perovskite solar cells.

TiO2 colloid-based compact layers for hybrid lead halide perovskite solar cells

In this report, we present a simple method of TiO2 colloidal preparation and its application to form compact conformal TiO2 (c-TiO2) layers on transparent conducting substrates. The synthesized colloid has ∼5nm particle size distribution and is stable over a period exceeding one year. The blocking nature of c-TiO2 is evaluated by the method of cyclic voltammetry using Spiro-OMeTAD as the redox probe. This method of c-TiO2 preparation is successfully employed in the fabrication of hybrid perovskite solar cells. The critical requirement of c-TiO2 colloid-based compact layer as hole-blocking material is justified with the realization of photo-conversion efficiencies of 8.08% for small active areas of 0.07cm2 and 7.52% with relatively large active areas of 0.23cm2 in one-step method. In two-step sequential deposition method, a photo-conversion efficiency of 4.61% is achieved over relatively large active area of 0.64cm2. Our observations suggest that c-TiO2 formation by TiO2 colloid can be an alternative potential route to approach for perovskite solar cell fabrication. All device processing is carried out in ambient air of relative humidity 40±5% at room temperature.

The Influence of PbI<sub>2</sub> on Characteristic of Organic-Inorganic Hybrid Perovskite Thin Films

Organic-inorganic hybrid perovskite materials have attracted significant research efforts because of their outstanding properties. Meanwhile the crystallization of organic-inorganic hybrid perovskite materials can significantly influence the films quality. Here, we research the influence of the characteristics of PbI2 thin film on final perovskite films and the mechanisms of film formation based on the two-step sequential deposition method. We found that the characteristics of PbI2 thin film, such as the grain size, the grain shape, the surface roughness and the film densification, have significant effects on the final perovskite films due to different film crystallization process. According to the analysis on the characteristics of the perovskite films obtained from different PbI2precursor, we suggested that the formation of perovskite film begins from the PbI2 crystals expanding when they are converted to MAPbI3 perovskite by migration of MA+ cations from the grain boundaries.

Enhanced efficiency of planar perovskite solar cells via a two-step deposition using DMF as an additive to optimize the crystal growth behavior

We present a facile way towards the use of the polar solvent additive in the inter-diffusion two-step sequential deposition method for a high quality perovskite film.

High-quality CH3NH3PbI3 thin film fabricated via intramolecular exchange for efficient planar heterojunction perovskite solar cells

The high-quality CH3NH3PbI3 perovskite thin film with excellent coverage and uniformity was prepared using an intramolecular exchange technology via a low-temperature, two-step sequential deposition process. The PbI2(DMSO) complex was synthesized at room temperature without any additives and was deposited, then the CH3NH3I solution was deposited subsequently. The further controllable thermal annealing process resulted in the complete formation of flat and uniform CH3NH3PbI3 thin film with large-size grains and (110) preferred crystallographic orientation. The perovskite solar cells (PSCs) with a very simple inverted planar heterojunction structure of ITO/PEDOT:PSS/CH3NH3PbI3/PCBM/Al and without other buffer layers, e.g., C60, LiF, BCP, etc., were fabricated, resulting in a power conversion efficiency (PCE) as high as 14.26%. The results suggest that the low-temperature, two-step sequential deposition process with intramolecular exchange technology provides a good route to fabricate high-quality perovskite thin film and efficient PSCs, which would match with large-scale, high-output roll-to-roll (R2R) printing/coating techniques.

Antifouling Electrospun Nanofiber Mats Functionalized with Polymer Zwitterions.

In this study, we exploit the excellent fouling resistance of polymer zwitterions and present electrospun nanofiber mats surface functionalized with poly(2-methacryloyloxyethyl phosphorylcholine) (polyMPC). This zwitterionic polymer coating maximizes the accessibility of the zwitterion to effectively limit biofouling on nanofiber membranes. Two facile, scalable methods yielded a coating on cellulose nanofibers: (i) a two-step sequential deposition featuring dopamine polymerization followed by the physioadsorption of polyMPC, and (ii) a one-step codeposition of polydopamine (PDA) with polyMPC. While the sequential and codeposited nanofiber mat assemblies have an equivalent average fiber diameter, hydrophilic contact angle, surface chemistry, and stability, the topography of nanofibers prepared by codeposition were smoother. Protein and microbial antifouling performance of the zwitterion modified nanofiber mats along with two controls, cellulose (unmodified) and PDA coated nanofiber mats were evaluated by dynamic protein fouling and prolonged bacterial exposure. Following 21 days of exposure to bovine serum albumin, the sequential nanofiber mats significantly resisted protein fouling, as indicated by their 95% flux recovery ratio in a water flux experiment, a 300% improvement over the cellulose nanofiber mats. When challenged with two model microbes Escherichia coli and Staphylococcus aureus for 24 h, both zwitterion modifications demonstrated superior fouling resistance by statistically reducing microbial attachment over the two controls. This study demonstrates that, by decorating the surfaces of chemically and mechanically robust cellulose nanofiber mats with polyMPC, we can generate high performance, free-standing nanofiber mats that hold potential in applications where antifouling materials are imperative, such as tissue engineering scaffolds and water purification technologies.

From unstable CsSnI3 to air-stable Cs2SnI6: A lead-free perovskite solar cell light absorber with bandgap of 1.48 eV and high absorption coefficient

All-inorganic and lead-free cesium tin halides (CsSnX3, X=Cl, Br, I) are highly desirable for substituting the organolead halide perovskite solar cells. However, the poor stability of CsSnX3 perovskites has so far prevented the fabrication of devices that can withstand sustained operation under normal conditions. In this paper, a two-step sequential deposition method is developed to grow high-quality B-γ-CsSnI3 thin films and their unique phase change in atmosphere is explored in detail. We find the spontaneous oxidative conversion from unstable B-γ-CsSnI3 to air-stable Cs2SnI6 in air. Allowing the phase conversion of the CsSnI3 film to evolve in ambient air it gives the semiconducting perovskite Cs2SnI6 with a bandgap of 1.48eV and high absorption coefficient (over 105cm−1 from 1.7eV). More importantly, the Cs2SnI6 film, for the first time, is adopted as a light absorber layer for a lead-free perovskite solar cell and a preliminary estimate of the power conversion efficiency (PCE) about 1% with open-circuit voltage of 0.51V and short-circuit current of 5.41mA/cm2 is realized by optimizing the perovskite absorber thickness. According to the bandgap and the Shockley-Queisser limit, such inorganic perovskite solar cell with higher efficiency and pronounced stability can be expected by material quality improvement and device engineering.

Ambient Engineering for High-Performance Organic-Inorganic Perovskite Hybrid Solar Cells.

Considering the evaporation of solvents during fabrication of perovskite films, the organic ambience will present a significant influence on the morphologies and properties of perovskite films. To clarify this issue, various ambiences of N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), and chlorobenzene (CBZ) are introduced during fabrication of perovskite films by two-step sequential deposition method. The results reveal that an ambient CBZ atmosphere is favorable to control the nucleation and growth of CH3NH3PbI3 grains while the others present a negative effect. The statistical results show that the average efficiencies of perovskite solar cells processed in an ambient CBZ atmosphere can be significantly improved by a relatively average value of 35%, compared with those processed under air. The efficiency of the best perovskite solar cells can be improved from 10.65% to 14.55% by introducing this ambience engineering technology. The CH3NH3PbI3 film with large-size grains produced in an ambient CBZ atmosphere can effectively reduce the density of grain boundaries, and then the recombination centers for photoinduced carriers. Therefore, a higher short-circuit current density is achieved, which makes main contribution to the improvement in efficiency. These results provide vital progress toward understanding the role of ambience in the realization of highly efficient perovskite solar cells.

Well-Organized Mesoporous TiO2 Photoanode by Using Amphiphilic Graft Copolymer for Efficient Perovskite Solar Cells

The control of the thickness and porosity of a mesoporous TiO2 layer is important to improve the photovoltaic performance of perovskite solar cells. We produced organized mesoporous TiO2 (om-TiO2) layers using a low-cost amphiphilic graft copolymer, poly(vinyl chloride)-graft-poly(oxyethylene methacrylate) (PVC-g-POEM), as a sacrificial template. This simple but effective synthetic approach generates highly mesoporous and well-organized TiO2 nanostructures with interconnected and size-tunable features. Specifically, the average pore size increased with the amount of hydrophobic PVC main chain in the graft copolymer, which acted as the pore forming agent. Perovskite layers were prepared on top of an om-TiO2 layer according to a two-step sequential deposition: after coating the PbI2 solution in dimethylformamide (DMF) on an om-TiO2 substrate, the substrate was prewetted in isopropyl alcohol (IPA) solvent before immersing into a CH3NH3I/IPA solution. This prewetting treatment not only improves the yields of ...

Enhanced performance in hole transport material free perovskite solar cells via morphology control of PbI2 film by solvent treatment

The morphology of PbI2 film plays a critical role in determining the quality of the resultant CH3NH3PbI3 film and power conversion efficiency of CH3NH3PbI3 perovskite solar cell. Here, we propose a solvent treatment method in the two-step sequential deposition process to control the morphology of PbI2 film, which leads to enhanced power conversion efficiency. Hole transport material free perovskite solar cell is chosen as a paradigm to demonstrate this idea. Solvent (isopropanol, chlorobenzene, or ethanol) treated PbI2 films exhibit dendrite-like or flake-like morphologies, which facilitate more complete conversion of PbI2 to CH3NH3PbI3 perovskite in ambient atmosphere with a relative high humidity. Therefore, enhanced performance is obtained with the solvent treated PbI2 films. Average power conversion efficiency has been improved from 9.42% in the traditional two-step sequential deposition to 11.22% in solar cells derived from ethanol treated PbI2 films.

Investigating relation of photovoltaic factors with properties of perovskite films based on various solvents

Methylammonium lead iodide (MAPbI3) perovskite solar cells are fabricated by two-step sequential deposition processes. The influence of the properties of PbI2 precursor film on the cell performance is investigated based on different solvents. The X-ray diffraction (XRD) patterns, Urbach energy value and scan electron microscope (SEM) images reveal that solvents have considerable influence on the crystal structure, band tail and morphology of PbI2 precursor films. The addition of a small amount of dimethyl sulfoxide (DMSO) in N,N-dimethylformamide (DMF) solvent promotes the compactness and uniformity of PbI2 precursor films, but also causes defect in band tail. The relation of the properties of perovskite film with photovoltaic parameters is explored. A high conversion efficiency of 16.6% for solar cells based on mixed solvents with 10vol% DMSO is achieved. Meanwhile, light beam-induced current (LBIC) images directly evidence the improvement of uniformity for perovskite solar cells caused by DMSO addition.

Organic solvent vapor treatment of lead iodide layers in the two-step sequential deposition of CH3NH3PbI3-based perovskite solar cells

The two step sequential deposition of CH3NH3PbI3-based perovskite solar cells has been modified by applying toluene and chlorobenzene vapors during the preparation of PbI2 films leading to the growth of the PbI2 grain size.