Large Crystal Grains Research Articles

Strain effects on polycrystalline germanium thin films

Polycrystalline Ge thin films have attracted increasing attention because their hole mobilities exceed those of single-crystal Si wafers, while the process temperature is low. In this study, we investigate the strain effects on the crystal and electrical properties of polycrystalline Ge layers formed by solid-phase crystallization at 375 °C by modulating the substrate material. The strain of the Ge layers is in the range of approximately 0.5% (tensile) to -0.5% (compressive), which reflects both thermal expansion difference between Ge and substrate and phase transition of Ge from amorphous to crystalline. For both tensile and compressive strains, a large strain provides large crystal grains with sizes of approximately 10 μm owing to growth promotion. The potential barrier height of the grain boundary strongly depends on the strain and its direction. It is increased by tensile strain and decreased by compressive strain. These findings will be useful for the design of Ge-based thin-film devices on various materials for Internet-of-things technologies.

Crystallization behavior of electron-beam-evaporated amorphous silicon films on textured glass substrates by flash lamp annealing

Flash lamp annealing (FLA) is one of the short-duration annealing methods suitable for the crystallization of micrometer-order-thick amorphous silicon (a-Si) films for thin-film polycrystalline silicon (poly-Si) solar cells. The crystallization of electron-beam- (EB-) evaporated a-Si films formed on flat glass substrates by FLA is known to take place through explosive crystallization (EC), by which laterally stretched large crystal grains are formed. In this study, we experimentally investigate the behavior of the FLA-induced crystallization of EB-evaporated a-Si films on glass substrates with intentionally roughed surfaces. When a-Si films on the textured glass substrates receive a FL pulse light consisting of sub-pulses with variable emission frequency, macroscopic stripe patterns with a constant width are observed on the poly-Si surface. This indicates the emergence of EC in EB-evaporated a-Si films also on textured glass substrates, similar to the case of a-Si films on flat substrates. Poly-Si films formed on textured glass substrates have smaller Si crystal grains in the vicinity of the Si/glass interface than the grains near the surface. Moreover, the lateral crystallization velocity of a-Si films on textured glass substrates is smaller compared to that on flat glass substrates. These indicate that the behavior of the EC of EB-evaporated a-Si films is affected by the surface morphology of glass substrates.

A Coadditive Strategy for Blocking Ionic Mobility in Methylammonium‐Free Perovskite Solar Cells and High‐Stability Achievement

Herein, the synthesis of methylammonium‐free and bromide‐free CsxFA1−xPbI3 (FA for formamidinium) perovskite (PVK) photovoltaic layers with intrinsic outstanding properties in terms of crystallinity, defect waiving/passivation, and ionic mobility blocking by using a coadditives approach is described. It consists in mixing two chloride additives in the PVK precursor solution: potassium chloride (KCl) and ammonium chloride (NH4Cl). KCl favors the lead iodide (PbI2) precursor solubilization and results in a purer PVK phase. NH4Cl allows the control of the crystallization growth speed, leading to the formation of large crystal grains and well‐crystallized layers. By the glow‐discharge optical emission spectroscopy (GD‐OES) technique, it is directly visualized that potassium incorporated in the whole film blocks the iodide mobility by defect passivation. Also, the reduction (or suppression) of iodide mobility and the reduction (or suppression) of the J–V curve hysteresis is clearly correlated. It is found that blocking the ionic mobility is insufficient to fully stabilize the halide perovskite material which also requires the crystallization monitoring carried out in parallel by the second additive. It resulted in Cs0.1FA0.9PbI3 cells with a stabilized power conversion efficiency (PCE) of 20.02% and with superior stability.

Improved Electrochemical Performance of Li-Rich Layered Oxide Cathodes Enabled by a Two-Step Heat Treatment.

Lithium-rich layered oxide cathodes with high specific energy have become one of the most popular cathode materials for high-performance lithium-ion batteries. However, spinel phase formation due to the migration of transition metals and the release of lattice oxygen leads to the degradation of electrochemical performance. Here, we develop a synthesis approach for Li-rich layered oxide cathodes by a two-step heat-treatment process, which includes precursor calcination and pellet sintering. Compared with the sample prepared by the traditional one-step calcination, the oxide particles prepared by the two-step heat treatment show increased grain size from 217 to 425 nm. The Li-rich layered oxide cathodes with larger crystal grains indicate a mitigated formation of spinel phase and reduced voltage decay, which result in improved specific capacity, cycle stability, and rate capability. In addition, the thermal stability of the oxides is also improved. The improved electrochemical performance is because of the large single grains having a reduced contact area with a liquid electrolyte and the stable crystal lattice during cycling. Our strategy not only provides a simple and effective way to enhance the stability of the Li-rich layered oxide cathodes but also extends to the preparation of oxide powders with large grains.

Comparison of failure analysis on reheater and waterwall tube power plant base on outer surface

Visual observation on waterwall tube failure showed crack at the outer surface and fish mouth on reheater tube. An analysis of this research is to compare the causes of failure of waterwall and reheater tube boiler. Method of this used descriptive qualitative with macro and micro visual observation. Crack appeared from the outside waterwall tube occurred at a place attached to the oxide layer, it indicated that failure on this tube achieved in long period. The small crack as a place for new oxide layer and corrosive media. Larger crystal grains at the edge of the oxide layer and tip of crack because the corrosion reaction between oxides and metal, it produces local heat on the area. Failure on reheater tube was presence of scale attached on the outer caused differences in the distribution of temperature between inner and outer surface tube.The dominant factor of this failure occurred intergranular creep fracture. The results shows that main mechanism on waterwall tube failure is physical and mechanical factor on reheater tube.

Raman spectroscopy real-time detection for the performance of perovskite solar cell

In this work, Raman signals are used to characterize the crystallinity of perovskite films and the photoelectric conversion efficiency (PCE) of perovskite solar cells. By controlling the ratios of EtOH and CHCl3 during the perovskite crystallization process, the CH3NH3PbI3 film with larger crystal grain size, higher crystallinity and no pinholes can be obtained. Good crystallinity of perovskite films promotes charge transfer (CT), and Raman spectroscopy is a practical method for the real time detection of CT in perovskite films. The results manifest that SERS intensities are conforming to X-ray diffraction (XRD) intensities and the PCE of perovskite solar cells. Meanwhile, the Raman signals are tested in the nitrogen environment, which can avoid the decomposition of perovskite films caused by H2O and O2. These results indicate that this method provides an early, real-time and harmless method to study the performance of perovskite photovoltaic devices.

Microstructure and lattice strain control towards high-performance ambient green-printed perovskite solar cells

High-quality methylammonium lead iodide-based perovskite layer with large crystal grains and relaxed lattice strain is demonstrated via simple ambient blade-coating from green solvents, with an efficiency of 20.21% in solar cells.

Morphology control of perovskite film for efficient CsPbIBr2 based inorganic perovskite solar cells

Inorganic perovskite solar cell (PSC) has been emerging as one of the most promising photovoltaic technologies for future application due to its excellent thermal stability and high power conversion efficiency (PCE). However, the efficiency of state-of-the-art CsPbIBr2–based PSCs is not satisfactory due to the poor quality of CsPbIBr2 perovskite film. Herein, by combining both substrate preheat treatment (SPT) and NH4PF6 precursor additive engineering, we prepared the high-quality CsPbIBr2 perovskite film featuring full coverage, high crystallinity and large crystal grain size, resulting in improved light harvest and reduced nonradiative charge recombination. With these morphology optimizaitons, the PSCs demonstrate a champion efficiency of 10.1% under 100 mW cm−2 illumination. Moreover, CsPbIBr2-based PSC without encapsulation exhibits excellent moisture stability by showing negligible drop in PCE under 40% relative humidity for 30 days.

TiI4-doping induced bulk defects passivation in halide perovskites for high efficient photovoltaic devices

Abstract Power conversion efficiency (PCE) and stability are two important properties of perovskite solar cells (PSCs). Particularly, defects in the perovskite films could cause the generation of trap states, thereby increasing the nonradiative recombination. To address this issue, suitable dopants can be incorporated to react with non-bonded atoms or surface dangling bonds to passivate the defects. Herein, we introduced TiI4 into CH3NH3PbI3 (MAPbI3) film and obtained a dense and uniform morphology with large crystal grains and low defect density. The champion cell based on 0.5% TiI4-doped MAPbI3 achieved a PCE as high as 20.55%, which is superior to those based on pristine MAPbI3 (17.64%). Moreover, the optimal solar cell showed remarkable stability without encapsulation. It retained 88.03% of its initial PCE after 300 h of storage in ambient. This work demonstrates TiI4 as a new and effective passivator for MAPbI3 film.

Realization of Moisture-Resistive Perovskite Films for Highly Efficient Solar Cells Using Molecule Incorporation.

The development of highly crystalline perovskite films with large crystal grains and few surface defects is attractive to obtain high-performance perovskite solar cells (PSCs) with good device stability. Herein, we simultaneously improve the power conversion efficiency (PCE) and humid stability of the CH3NH3PbI3 (CH3NH3 = MA) device by incorporating small organic molecule IT-4F into the perovskite film and using a buffer layer of PFN-Br. The presence of IT-4F in the perovskite film can successfully improve crystallinity and enhance the grain size, leading to reduced trap states and longer lifetime of the charge carrier, and make the perovskite film hydrophobic. Meanwhile, as a buffer layer, PFN-Br can accelerate the separation of excitons and promote the transfer process of electrons from the active layer to the cathode. As a consequence, the PSCs exhibit a remarkably improved PCE of 20.55% with reduced device hysteresis. Moreover, the moisture-resistive film-based devices retain about 80% of their initial efficiency after 30 days of storage in relative humidity of 10-30% without encapsulation.

PEAI-Based Interfacial Layer for High-Efficiency and Stable Solar Cells Based on a MACl-Mediated Grown FA0.94MA0.06PbI3 Perovskite.

Among the three-dimensional (3D) organic-inorganic hybrid perovskites (OIHPs), mixed formamidinium and methylammonium cation lead iodide is one of the most promising for solar cell application. After optimizing the use of a methylammonium chloride (MACl) additive for the preparation of compact, high-quality, and large crystal grain layers made of a pure α-phase perovskite with the FA0.94MA0.06PbI3 composition, the treatment of the perovskite surface by a 2-phenylethylammonium iodide (PEAI) solution has been performed. This treatment, without any thermal annealing, leads notably to the spontaneous formation of a crystallized (PEA)2PbI4 two-dimensional (2D) perovskite nanolayer at the film surface due to partial organic cation dissolution. This buffer layer is shown to favor a fast transfer of the holes toward the hole transporting layer (HTL) and to reduce the recombinations at and near the perovskite/HTL interface in perovskite solar cells (PSCs). It is shown to boost their maximum power conversion efficiency (PCE) from 20.37 to 22.18%, while the hysteresis becomes negligible. A comprehensive study of the electrical response of the device has been performed. The electrical impedance spectroscopy (EIS) measurements have been fitted with ad hoc equivalent electrical circuits. The electrical responses due to interface stabilization, the intrinsic dielectric relaxation of the perovskite, and the charge depletion and charge recombinations have been distinguished. The low-frequency capacitance is analyzed as a charge recombination capacitance. The perovskite surface buffer layer is notably shown to suppress charge recombinations from the boosting of the high- and low-frequency recombination resistances as well as from the marked decrease of the low-frequency recombination capacitance. The prepared devices are proven to be especially resistant to electrical stresses, light irradiation, and moisture.

Sputtered and selenized Sb2Se3 thin-film solar cells with open-circuit voltage exceeding 500 mV

Antimony selenide (Sb 2 Se 3 ) is a potential absorber material for environment-friendly and cost-efficient photovoltaics and has achieved considerable progress in recent years. However, the severe open-circuit voltage ( V oc ) deficit ascribed to the interface and/or bulk defect states has become the main obstacle for further efficiency improvement. In this work, Sb 2 Se 3 absorber layer was prepared by an effective combination reaction involving sputtered and selenized Sb precursor thin films. The self-assembled growth of Sb 2 Se 3 thin films with large crystal grains, benign preferential orientation, and accurate chemical composition were successfully fulfilled under an appropriate thickness of Sb precursor and an optimized selenization scenario. Substrate structured Sb 2 Se 3 thin-film solar cells, a champion device with a power-conversion efficiency of 6.84%, were fabricated. This device is comparable to state-of-the-art ones and represents the highest efficiency of sputtered Sb 2 Se 3 solar cells. Importantly, the high V oc of 504 mV is closely related to the reduced deep level defect density for the Sb 2 Se 3 absorber layer, the passivated interfacial defects for Sb 2 Se 3 /CdS heterojunction interface, and the additional heterojunction heat treatment-induced Cd and S inter-diffusion. This significantly improved V oc demonstrates remarkable potential to broaden its scope of applications for Sb 2 Se 3 solar cells. • An effective combination reaction of sputtered and selenized Sb precursors for Sb 2 Se 3 thin film solar cells. • The self-assembled growth of Sb 2 Se 3 thin films with large crystal grains, benign preferential orientation and accurate chemical composition. • A highly interesting V oc record of 504 mV obtained due to the reduced deep level defect density, the passivated interfacial defects and heterojunction heat treatment-induced Cd and S inter-diffusion. • PCE of 6.84% represents the highest efficiency of sputtered Sb 2 Se 3 solar cells.

Spark plasma sintering of Sb2Se3 sputtering target towards highly efficient thin film solar cells

Antimony selenide (Sb2Se3) is a potential absorber material for environment-friendly and cost-efficiently photovoltaics due to its material advantages and superior optoelectronic properties. In this work, we proposed a facile and versatile method of ball milling followed with spark plasma sintering (SPS) to prepare high-quality Sb2Se3 sputtering target. Then the highly crystalline Sb2Se3 thin film consisted of large crystal grains can be prepared by using radio frequency (RF) magnetron sputtering with an additional post-selenization heat treatment. An efficient substrate structured Sb2Se3 thin film solar cell with configuration of Mo/Sb2Se3/CdS/ITO/Ag was fabricated and a champion device with highly interesting power conversion efficiency (PCE) of 5.08% has been achieved. Superior device performances are closely related to the Sb2Se3 absorber layer with benign growth orientation and the Sb2Se3/CdS heterojunction interface with smooth contact, which induced less recombination loss. The combined features of homemade sputtering target and advantageous thin film preparation technology further demonstrated its attractive application potential in thin film photovoltaic scenarios.

Mechanism of size effects of a filler on the wear behavior of ultrahigh molecular weight polyethylene

Abstract Although the size effects of a filler are closely related to the complex multi-level structures of their polymer composites; unfortunately, such relationships remain poorly understood. In this study, we investigated the effects of various sizes (40–600 nm) of silicon carbide (SiC) fillers on the wear behavior of ultrahigh molecular weight polyethylene (UHMWPE) in the presence of the silane coupling agent KH-560. All of these SiC fillers improved the wear resistance of UHMWPE significantly, with a medium size (150 nm) being optimal. To examine the reasons for this behavior, we analyzed the multi-level structures of the samples in terms of their matrix structures (crystalline; amorphous; interphase), matrix–filler interactions (physical adsorption; chemical crosslinking; hybrid network) and the external effects of SiC fillers (bearing loads; transferring frictional heat). The high rigidity and thermal conductivity of SiC fillers and, more importantly, the intrinsic characteristics of the matrix structures (larger crystal grains; higher interphase; stronger amorphous entangled networks) were the key parameters affecting the enhancement in the wear-resistance of the UHMWPE. Herein, we also provide interpretations of the corresponding physical effects. Our results should improve our understanding of the structure–property relationships and, thus, should guide the formula design of UHMWPE composites.

An effective combination reaction involved with sputtered and selenized Sb precursors for efficient Sb2Se3 thin film solar cells

An effective combination reaction involved with sputtered and selenized Sb precursor was used for preparing Sb 2 Se 3 thin film solar cells. Self-assembled growth of Sb 2 Se 3 with large crystal grains, benign orientation, stoichiometric composition and high-quality heterojunction interface. A champion device with PCE of 6.15% is comparable to the state-of-the-art devices and represents the highest efficiency of sputtered Sb 2 Se 3 solar cells. • An effective combination reaction of Sb metallic precursors for efficient Sb 2 Se 3 solar cell. • Self-assembled growth of stoichiometric Sb 2 Se 3 thin film with desired orientation. • An over-thick MoSe 2 interfacial layer could be effectively suppressed. • The highest PCE of 6.15% in sputtered Sb 2 Se 3 thin film solar cell with excellent stability. Sputtering followed by post annealing is extensively used for fabrication of copper indium gallium selenide (CIGS), copper zinc tin sulfide (CZTS) and copper zinc tin sulfur selenide (CZTSSe) thin film solar cells. In this work, Sb 2 Se 3 as an emerging alternative absorber was fabricated by an effective combination reaction of annealing sputtered Sb metallic precursors under Se vapor. Self-assembled growth of Sb 2 Se 3 thin films consist of large grains that across the whole films have been successfully fulfilled via this combination reaction. Sb 2 Se 3 thin films with desired orientation, stoichiometric composition and high-quality Sb 2 Se 3 /CdS heterojunction could be achieved once a proper selenization scenario was employed. Further, by selecting Sb films as precursors, the thickness of interfacial MoSe 2 located at the back-contact region can be well controlled, leading to a significant enhancement in fill factor (FF) of the devices. This is in good accordance with our DFT simulation results which demonstrated Se vapor would be prone to react with the Sb lattice thermodynamically and thus limiting the thickness of the MoSe 2 layer. Finally, a champion Sb 2 Se 3 thin film solar cell with power conversion efficiency of 6.15% was achieved, which represents the highest efficiency of sputtered Sb 2 Se 3 solar cells.

Interface Engineering of Air‐Stable n‐Doping Fullerene‐Modified TiO2 Electron Transport Layer for Highly Efficient and Stable Perovskite Solar Cells

AbstractAs one common electron transport material for planar n‐i‐p perovskite solar cell, titanium dioxide (TiO2) compact layer has several challenging issues, such as surface hydroxyl groups, high defect density, and unmatched energy levels, causing severe energy loss and poor stability at contact. To solve these problems, the authors introduce a thin [6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM) interlayer doped with an air stable n‐type dopant, 3‐dimethyl‐2‐phenyl‐2,3‐dihydro‐1H‐benzoimidazole (DMBI) to modify the TiO2 surface. The state‐of‐the‐art characterizations demonstrate such modification significantly improves charge transfer at MAPbI3/TiO2 interface together with smaller energy level offset, leading to suppressed charge recombination. High‐quality perovskite film with larger crystal grain size grows on the n‐doped PCBM/TiO2 attributed to the better surface affinity. As a result, the average power conversion efficiency of perovskite solar cell exhibits a prominent improvement from 17.46% to 20.14%, with an enhancement in all device photovoltaic parameters. In addition, the stability of the device with n‐doped PCBM/TiO2 is much better than that of the control device with the bare TiO2 due to hydrophobicity nature of PCBM and low defect densities in the perovskite film and at the interface. This work indicates that many further device performance improvements should be conceivable by focusing on the perovskite interface.

Crystallization tailoring of cesium/formamidinium double-cation perovskite for efficient and highly stable solar cells

Achieving high-quality perovskite crystal films is a critical prerequisite in boosting solar cell efficiency and improving the device stability, but the delicate control of nucleation and growth of the perovskite film remains limited success. Herein, a facile but effective strategy has been developed to finely tailor the crystallization of thermally stable cesium/formamidinium (Cs/FA) based perovskite via partially replacing PbI2 with PbCl2 in the precursor solution. The incorporation of chlorine into the perovskite crystal lattice derived from PbCl2 changes the crystallization process and improves the crystal quality, which further results in the formation of larger crystal grains compared to the control sample. The larger crystal grains with high crystallinity lead to reduced grain boundaries, suppressed non-radiative recombination, and enhanced photoluminescence lifetime. Under the optimized conditions, the methylammonium free perovskite solar cells (PSCs) delivers a champion power conversion efficiency (PCE) of 21.30% with an open-circuit voltage as high as 1.18 V, which is one of the highest efficiencies for Cs/FA based PSCs up to now. Importantly, the unencapsulated PSC devices retain more than 95% and 81% of their original PCEs even after long-term (over one year) storage under ambient conditions or 2000 h's thermal aging at 85 °C in a nitrogen atmosphere, respectively.

Understanding the Interplay of Binary Organic Spacer in Ruddlesden–Popper Perovskites toward Efficient and Stable Solar Cells

AbstractRuddlesden–Popper perovskite (RPP) materials have attracted great attention due to their superior stability, where the organic spacer dominantly determines the stability and efficiency of RPP solar cells, but research still lacks the systematical understanding of the interplay of binary spacer in the overall mixture range of 0–100% in RPPs on the precursor chemistry, film quality, and carrier behavior. Herein, a series of novel binary spacer RPP films of (PBA1−xBAx)2MA3Pb4I13 (BA = n‐butylammonium, PBA = 4‐phenylbutan‐1‐aminium, and MA = methylammonium) is successfully fabricated to reveal the interplay of binary spacers. The incorporation of 50% BA into the (PBA)2MA3Pb4I13 precursor solution increases the colloidal size and reduces nucleation sites, and therefore forms a very smooth film with much larger crystal grains and a higher degree of crystal preferential orientation, resulting in a significant reduction of trap states. The resulting combination of fast electron transfer and efficient electron extraction facilitates to effectively suppress the trap‐assisted charge recombination and remarkably decrease charge recombination losses. Consequently, the (PBA0.5BA0.5)2MA3Pb4I13 device achieves a champion efficiency of 16.0%, among the highest reported efficiencies for RPP devices. Furthermore, this device demonstrates good ambient, illumination, and thermal stabilities, retaining 60–93% of its initial efficiency after 30 days of various ageing.

CH3NH3PbI3:MoS2 heterostructure for stable and efficient inverted perovskite solar cell

Organic-inorganic perovskite solar cells emerge as one of the most promising photovoltaic technology due to its high performances. Particularly, inverted perovskite device architecture, due to low temperature processing, have a great potential in commercialization. High-crystalline quality perovskite film and interfacial passivation are essential to yield high performance devices. In this work, we employ a simple strategy of using molybdenum disulfide (MoS2) as both the interfacial layer and the additive to prepare efficient PSCs. MoS2 as an additive in perovskite can form the CH3NH3PbI3:MoS2 heterostructure, resulting in the homogeneous perovskite film with larger crystal grains. In addition, MoS2 as the buffer layer (BL) between poly (3,4-ethylene dioxythiophene)-poly (styrene sulfonate) (PEDOT:PSS) and perovskite can prevent the decomposition of perovskite film by avoiding the direct contact with the hydrophilic PEDOT:PSS films. On tedious optimization, the champion device based on active layer of CH3NH3PbI3:MoS2 (10 v%) as well as employing MoS2 buffer layer shows a remarkable improvement in the power conversion efficiency (PCE) (from 15.29% to 18.31%) and a better stability, with 87% of the initial efficiency sustained after 20 days. Our finding herein provides a promising way to fabricate high efficiency and stable photovoltaic devices.

Single Source Thermal Evaporation of Two-dimensional Perovskite Thin Films for Photovoltaic Applications

Hybrid two-dimensional (2D) halide perovskites has been widely studied due to its potential application for high performance perovskite solar cells. Understanding the relationship between microstructural and opto-electronic properties is very important for fabricating high-performance 2D perovskite solar cell. In this work, the effect of solvent annealing on grain growth was investigated to enhance the efficiency of photovoltaic devices with 2D perovskite films based on (BA)2(MA)3Pb4I13 prepared by single-source thermal evaporation. Results show that solvent annealing with the introduction of solvent vapor can effectively enhance the crystallization of the (BA)2(MA)3Pb4I13 thin films and produce denser, larger-crystal grains. The thin films also display a favorable band gap of 1.896 eV, which benefits for increasing the charge-diffusion lengths. The solvent-annealed (BA)2(MA)3Pb4I13 thin-film solar cell prepared by single-source thermal evaporation shows an efficiency range of 2.54–4.67%. Thus, the proposed method can be used to prepare efficient large-area 2D perovskite solar cells.