Liquid Phase Crystallization Research Articles

Multicrystalline Silicon Thin‐Film Solar Cells Based on Vanadium Oxide Heterojunction and Laser‐Doped Contacts

Liquid phase crystallized (LPC) silicon thin films on glass substrates are a feasible alternative to conventional crystalline silicon (c‐Si) wafers for solar cells. Due to substrate limitation, a low‐temperature technology is needed for solar cell fabrication. While silicon heterojunction is typically used, herein, the combination of vanadium oxide/c‐Si heterojunction as emitter and base contacts defined by IR laser processing of phosphorus‐doped amorphous silicon carbide stacks is explored. LPC solar cells are fabricated using such technologies to identify their issues and advantages with a promising performance of an active‐area efficiency of 5.6%. Apart from the absence of light‐trapping techniques, the relatively low efficiency obtained is attributed to a low lifetime in the LPC silicon bulk. These poor material properties imply a short diffusion length that makes it that only photogenerated carriers in the emitter regions can be collected. Consequently, future devices should show narrower base contact regions, suggesting a shorter‐wavelength laser, combined with longer LPC substrate lifetimes.

Multistep Phase Transitions in Sea Surface Microlayer Droplets and Aerosol Mimics using Microfluidic Wells

Oceanic sea spray is one of the largest contributors of atmospheric aerosol particles worldwide. The phase of aerosol particles is known to impact radiative forcing and cloud nucleation. However, as chemically complex aqueous systems that include mixtures of biological, organic, and salt constituents, it is a challenge to predict the phase of sea spray aerosol especially as they age in the atmosphere. In this study, phase behavior (liquid–liquid phase separation, LLPS, and crystallization) of sea surface microlayer (SSML) sample and chemical mimics are investigated using a microfluidic pervaporation approach. Internally mixed aqueous droplets with varying compositions and concentrations are trapped in microfluidic wells, and phase transitions of the droplets are optically determined in a slow dehydration process. A system containing SSML sample combined with an organic acid 3-methyl glutaric acid (3-MGA) undergoes multiple phase changes, including two crystallization and two LLPS events. The added organic acid increases the sample’s organic-to-inorganic ratio and moves the system into the range typical of aged SSA. To better understand the contributing constituents to the observed phase changes, control experiments with inorganic salt components NaCl, MgCl2, and Na2SO4 are performed with and without 3-MGA, at varying organic to inorganic ratios. 3-MGA leads to LLPS, and the presence of Mg2+ more readily facilitates LLPS than Na+. With the systems studied, LLPS is more prevalent for the chemical mixtures in an intermediate OIR range. This study provides new insight into sea spray aerosol phase as a function of composition and relative humidity and demonstrates multistep phase transitions for these complex systems.

Demonstration of mm long nearly intrinsic GeSn single-crystalline wires on quartz substrate fabricated by nucleation-controlled liquid-phase crystallization

Millimeter long GeSn wires were fabricated on quartz substrates by using an advanced rapid melt growth method, named “nucleation-controlled liquid-phase crystallization” (NCLPC). Local melting of amorphous GeSn wires surrounded by SiO2 capping layers led to the propagation of single crystal growth from the solid/liquid interface without any crystal seeds and catalysts. Slow cooling just after rapid melting was beneficial in NCLPC for prolonged and stable lateral growth, and 1.7 mm long single-crystalline GeSn wires were successfully demonstrated. Physical characterizations revealed that, since excessive Sn atoms were swept out toward the growth direction, a mostly uniform GeSn single crystal, in terms of Sn content and crystallinity, was formed along the wires. Electrical measurements of top-gate MOSFETs with NCLPC-GeSn channels revealed the formation of nearly intrinsic GeSn single crystal, indicating a significant reduction in acceptor-like point defects achieved with the slow-cooling procedure.

Silicon Powder-Based Wafers for Low-Cost Photovoltaics: Laser Treatments and Nanowire Etching

In this study, laser-treated polycrystalline Si (pc-Si) wafers, fabricated by wire sawing of hot-pressed ingots sintered from Si powder, have been investigated. As-cut wafers and those with high-quality thin Si layers deposited on top of them by e-beam have been subjected to laser irradiation to clarify typical trends of structural modifications caused by laser treatments. Moreover, possibility to use laser-treated Si powder-based substrates for fabrication of advanced Si structures has been analysed. It is established that (i) Si powder-based wafers with thicknesses ~180 μm can be fully (from the front to back side) or partly (subsurface region) remelted by a diode laser and grain sizes in laser-treated regions can be increased; (ii) a high-quality top layer can be fabricated by crystallization of an additional a-Si layer deposited by e-beam evaporation on top of the pc-Si; and (iii) silicon nanowires can be formed by metal-assisted wet chemical etching (MAWCE) of polished Si powder-based wafers and as-cut wafers irradiated with medium laser power, while a surface texturing on the as-cut pc-Si wafers occur, and no nanowires can form in the region subject to a liquid phase crystallization (LPC) caused by high-power laser treatments.

Development of Cramics Nano-structures with Liquid Phase Crystal Growth

Development of Cramics Nano-structures with Liquid Phase Crystal Growth

Hydrothermal synthesis of MCM-49 zeolite with cyclohexylamine template

Abstract Hexamethyleneimine (HMI) is used as the structure-directing agent (SDA) in the conventional synthesis method of MCM-49 zeolite. Cyclohexanine (CA) is less toxic and cheaper than HMI. Hence, in this paper, MCM-49 zeolite was successfully synthesized by using CA as a sole SDA with the aid of seeds. Both the seed and template CA are indispensable for the synthesis of MCM-49 zeolite. The optimum synthesis conditions were obtained. The range of the SiO2/Al2O3 ratio of the initial gel was broadened to 70. The synthesis of MCM-49 followed the liquid-phase crystallization mechanism. The MCM-49 zeolites synthesized by using CA and conventional template hexamethyleneimine (HMI) exhibit similar intrinsic properties, such as morphology, acidity, BET surface area, SiO2/Al2O3, and so on. The MCM-49(CA) exhibite good catalytic activity and product distribution in alkylation of benzene with methanol.

Integration Challenges of Flash Lamp Annealed LTPS for High Performance CMOS TFTs

The development of low-temperature polycrystalline silicon (LTPS) based on excimer laser annealing (ELA) has realized CMOS TFTs with notable electrical performance. The flat-panel display industry is searching for alternative LTPS strategies which are cost-effective and easily scalable to large glass panel production, which has led to recent interest in the application of flash-lamp annealing (FLA) for the LTPS crystallization process. The FLA-LTPS process exposes large areas of amorphous silicon with high irradiance from xenon flashlamps for pulse durations that are in microsecond timescale. Amorphous silicon absorbs a sufficient portion of the xenon emission spectrum to melt and crystallize into polysilicon while staying within the thermal constraints of the underlying glass substrate. Multi-lamp exposure systems with high repetition pulse rates would potentially offer significant advantages in manufacturing throughput and cost. The application of FLA-LTPS for CMOS TFTs has been recently reported, with best-case performance comparable to ELA-LTPS. Investigations to improve upon this technology are in progress to address integration challenges to reduce variation in device operation and enable device scaling. The focus areas of this work are material uniformity, source/drain dopant activation, and defect passivation; all of which are fundamental issues facing FLA-LTPS TFTs. The importance of patterning the amorphous silicon film into separate super-mesa elements prior to FLA crystallization was previously established. The FLA system for this work was a NovaCentrix PulseForge 3300 system, used to crystallize polygons of a-Si with a 100 nm SiO2 capping layer. The FLA response of densely packed mesa arrays exhibits a proximity effect which impedes the melting of interior mesas, whereas exterior mesas demonstrate a crystallization response consistent with isolated mesas. This comparison is highlighted in Fig. 1 by the pronounced gradient in crystallization morphology. These observations have led to a theory of competing rates between solid-phase and liquid-phase crystallization, hypothesized due to the large difference between the melting points of amorphous and crystalline silicon. The challenge of dopant activation is exacerbated by the need to limit lateral diffusion. Dopants within silicon that has entered a liquid phase will experience dramatically enhanced diffusion, even within the ultra-short pulse duration of the FLA process. The result is a compromised intrinsic channel which limits device scaling. Thus, dopants must be introduced into post-FLA LTPS and activated at glass-compatible temperatures, which is also a requirement for self-aligned devices. Solid-phase dopant activation in FLA-LTPS films has been investigated using furnace annealing and multiple shot FLA exposures at lower intensity. Pre-amorphizing the source/drain regions with electrically inactive species has also been explored. Hydrogen passivation treatments using both plasma and sintering processes have been investigated in efforts to reduce the influence of defect states. Pre-amorphization and passivation processes have yielded promising device characteristics with low series resistance, low leakage current, and high carrier mobility, as shown in Fig. 2. Figure 1

Percolated Si:SiO₂ Nanocomposites: Oven- vs. Millisecond Laser-Induced Crystallization of SiOx Thin Films.

Three-dimensional nanocomposite networks consisting of percolated Si nanowires in a SiO matrix, Si:SiO, were studied. The structures were obtained by reactive ion beam sputter deposition of (x ≈ 0.6) thin films at 450 C and subsequent crystallization using conventional oven, as well as millisecond line focus laser treatment. Rutherford backscattering spectrometry, Raman spectroscopy, X-ray diffraction, cross-sectional and energy-filtered transmission electron microscopy were applied for sample characterization. While oven treatment resulted in a mean Si wire diameter of 10 nm and a crystallinity of 72% within the Si volume, almost single-domain Si structures of 30 nm in diameter and almost free of amorphous Si were obtained by millisecond laser application. The structural differences are attributed to the different crystallization processes: conventional oven tempering proceeds via solid state and millisecond laser application via liquid phase crystallization of Si. The five orders of magnitude larger diffusion constant in the liquid phase is responsible for the three-times larger Si nanostructure diameter. In conclusion, laser treatment offers not only significantly shorter process times, but moreover, a superior structural order of nano-Si compared to conventional heating.

Wafer-scale epitaxial germanium (100), (111), (110) films on silicon using liquid phase crystallization

A wafer-scale method to obtain epitaxial germanium (Ge) on crystalline silicon (Si) using liquid-phase-crystallization (LPC) is presented. The technique provides a simple yet versatile method to grow epitaxial germanium on silicon with any crystallographic orientation: (100), (110) or (111). The process starts with amorphous Ge, which is melted and cooled in a controlled manner to form epitaxial germanium. LPC Ge films are continuous with an average grain-size of 2-5 μm. Rocking scan confirms that the LPC Ge is oriented with a threading dislocation density of ∼109 cm-2. The phi-scan confirms that LPC germanium is epitaxial with Ge (100), Ge (110) and Ge (111) showing four-fold, two-fold, and three-fold symmetry, respectively. The epitaxial quality of the Ge is influenced by the cleanliness of the Ge/Si interface; rate of cooling and ambient gas during LPC; and Ge layer thickness. Best films are obtained for 1 μm thick LPC Ge(100), cooled at ∼3-4 C/min in hydrogen ambient. Electron Hall mobility in these LPC Ge films is 736cm2/Vs, a high value that confirms the electronic quality of LPC Ge film.

LiFePO4/C cathode material prepared with sphere mesoporous-FePO4 as precursors for lithium-ion batteries

The spherical mesoporous iron phosphate (P-FP) were prepared by liquid phase crystallization method, and then Carbon-coated lithium iron phosphate was synthesized by high temperature solid phase method using the spherical mesoporous iron phosphate as precursors (P-LFP/C) or using the non-mesoporous iron phosphate as precursors (LFP/C). The P-LFP/C were characterized by X-Ray-Diffraction, Scanning-Electron-Microscope, Fourier-Transform Infrared Spectrometer and electrochemical tests. The results show that the initial discharge capacity of the P-LFP/C at 0.1 C increase by 27.10%, reaching 159.29 mAh g−1, and the electrode charge transfer resistance reduce by 88.16%, just 48.32 Ω. compare with the LFP/C. And the P-LFP/C exhibited better discharge capacity at initial 151.78 and 112.11 mAh g−1 at 0.2 C and 5 C rate, respectively. It is beneficial to the lithium ion escaped and embed in the process of charging and discharging and enhanced the electronic conductivity and rate performance of the cathode materials.

Honeycomb micro-textures for light trapping in multi-crystalline silicon thin-film solar cells.

The liquid phase crystallization (LPC) of silicon is an emerging technology for fabricating 10 - 20 µm thin multi-crystalline silicon layers on glass. LPC silicon solar cells exhibit similar electronic performance to multi-crystalline wafer-based devices. Due to the reduced absorber thickness, however, effective measures for light trapping have to be taken. We present tailor-made micro-structures for light trapping at the LPC silicon back-side, whereby a nano-imprinted resist layer serves as a three-dimensional etching mask in subsequent reactive ion etching. Contrary to state-of-the-art random pyramid textures produced by wet-chemical etching, this method allows to produce tailor-made textures independent of grain orientation. Differently shaped micro-textures were replicated in LPC silicon. Absorptance and external quantum efficiency of periodic honeycomb patterns and random pyramids were found to be equivalent. Thus, the method enables the potential to further optimize light trapping in LPC silicon solar cells.

Mechanistic understanding of the phase behavior of supersaturated solutions of poorly water-soluble drugs

Amorphous solid dispersions (ASDs) are a promising formulation strategy to increase both the apparent aqueous solubility and bioavailability of poorly water-soluble drugs. Upon dissolution under nonsink conditions, ASDs can generate highly supersaturated drug solutions which can undergo liquid–liquid phase separation (LLPS) and/or crystallization. In this study, the phase behavior of supersaturated solutions generated by antisolvent addition and upon the dissolution of ASDs was evaluated using fluorescence lifetime measurements and several other orthogonal techniques, including steady-state fluorescence spectroscopy, ultraviolet (UV) extinction and concentration profiles, ultracentrifuge measurements and nanoparticle tracking analysis. Ritonavir and lopinavir were chosen as poorly water-soluble model drugs, and the polymer, Kollidon VA64, was selected to form the dispersions. The fluorescence lifetime of the environment-sensitive fluoroprobe, PRODAN, was monitored to determine the occurrence of LLPS and crystallization. It was found that only the 10% w/w drug loading ASDs dissolved to a concentration in solution higher than the LLPS concentration and this led to an increase in the lifetime of PRODAN due to partitioning of the fluoroprobe into the drug-rich phase. In contrast, the 50% w/w drug loading ASDs did not reach the amorphous solubility, pointing to a dissolution behavior controlled by the low water solubility and high hydrophobicity of the drug. Fluorescence lifetime measurements were demonstrated to be extremely useful for the characterization of the phase behavior of supersaturated solutions of poorly water-soluble drugs.

Progress in and potential of liquid phase crystallized silicon solar cells

Liquid phase crystallization of silicon (LPC-Si) offers great potential for high-quality Si films and a cost effective fabrication technique for thin crystalline silicon solar cells on glass. In this work, we report on the progress on LPC-silicon at HZB in the past years. Beginning with a brief description of the fabrication process, we summarize the work on the different contact systems developed for these absorbers before focusing on the interdigitated back contact architecture on which the highest efficiencies were reported. State-of-the art cells form the basis for a detailed discussion of the status of this technology. We investigate the current loss mechanisms and explore the potential for further improvement. Finally, based on this comprehensive quality assessment, we develop a roadmap to increase the cell efficiencies to wafer-equivalent values.

Impact of Dielectric Layers on Liquid-Phase Crystallized Silicon Solar Cells

Liquid-phase crystallization (LPC) of thin silicon layers on glass substrates is a technique to fabricate solar cells with low energy and material consumption and open-circuit voltages comparable to multicrystalline silicon wafer cells. We studied the impact of different passivation layers deposited with plasma-enhanced chemical vapor deposition (PECVD) on the cell quality. Silicon nitride (SiNx) and ultrathin silicon oxide (SiO x ) layers with varying thicknesses were used. In addition, we plasma-treated the SiN x surface to form a thin silicon oxynitride (SiO x N y ) passivation layer. Plasma oxidation is an attractive alternative to PECVD, since thicknesses of ultrathin layers can be controlled easier as compared with PECVD. We found that the cell performance is influenced by the passivation layer. Particularly, cells on the 9 nm PECVD SiO x passivation layer are worse than cells on the 11 nm SiO x N y layer. We modeled cell parameters employing ASPIN3 to estimate effective diffusion lengths. Using transmission electron microscopy as well as electron energy loss spectroscopy, we studied the effect of the plasma treatment on the morphology and elemental distribution at the interface between passivation layer and LPC-Si absorber. Our best interdigitated back contacted solar cell with an efficiency >14% is based on the SiO x N y layer.

Confined and Directed Polymer Crystallization at Curved Liquid/Liquid Interface

AbstractSpherical crystals are ubiquitous in nature and the necessary breaks in translational symmetry not seen in flat crystals render them structurally unique. Polymer crystals have been shown to exhibit nonflat morphologies, but control over their formation is difficult to achieve. One strategy is directing the crystallization by spatially and/or temporally tuning chain segmental mobility. This has been studied early on using polymer blends or polymer/solvent systems where coupling liquid–liquid phase separation with crystallization could provide morphological control. In this Trend article, a recent trend in using miniemulsion systems to act as nanoscale confinement on chain segmental mobility is reviewed. The confinement at this length scale causes unique features to arise in ordering processes such as liquid–liquid phase separation and crystallization that are not observed at the macroscale. The generality of this approach makes it a good candidate to direct the formation of new and unique hierarchical polymer nanostructures that could be utilized in numerous applications.

Lightly doped n-type tensile-strained single-crystalline GeSn-on-insulator structures formed by lateral liquid-phase crystallization

A tensile-strained single-crystalline n-type GeSn-on-insulator structure was demonstrated by lateral liquid-phase crystallization of an Sb-doped GeSn layer, and the diffusion and activation behaviors of Sb atoms in a liquid-phase-grown GeSn wire were investigated. A photoluminescence-based study revealed that a substantial amount of Sb was swept out during the liquid-phase growth of the GeSn wire. A thin-film transistor based on an Sb-doped GeSn wire on a quartz substrate exhibited n-channel accumulation-mode operation, and the temperature dependence of field-effect mobility revealed that the single-crystalline GeSn wire is a lightly doped n-type (∼1017 cm−3). The combination of Sb-doped n-type and undoped p-type GeSn wires would be an ideal platform for GeSn-based optoelectronic integration.

Vibrational dynamics of glass forming: 2-phenylbutan-1-ol (BEP), 2-(trifluoromethyl)phenethyl alcohol (2TFMP) and 4-(trifluoromethyl)phenethyl alcohol (4TFMP) in their thermodynamic phases

ABSTRACTThe complex polymorphism and vibrational dynamics of three glass-forming single-phenyl-ring alcohols (with and without fluorine atoms) have been studied by complementary methods. Glass of isotropic liquid phase and cold crystallization of metastable supercooled liquid state were detected. Temperature investigations of vibrational motions show important role of hydrogen bonds in interactions between molecules. Theoretical calculations for isolated molecule, as well as dimer- and tetramer-type aggregates of non-covalently bound molecules, allow for a good description of experimental spectra. Intermolecular interactions of molecules with ortho and para positions of CF3 group in phenyl ring have a similar influence on the spectra observed.

Potential of interdigitated back-contact silicon heterojunction solar cells for liquid phase crystallized silicon on glass with efficiency above 14%

Liquid phase crystallization of silicon (LPC-Si) on glass is a promising method to produce high quality multi-crystalline Si films with macroscopic grains. In this study, we report on recent improvements of our interdigitated back-contact silicon heterojunction contact system (IBC-SHJ), which enabled open circuit voltages as high as 661mV and efficiencies up to 14.2% using a 13µm thin n-type LPC-Si absorbers on glass. The influence of the BSF width on the cell performance is investigated both experimentally and numerically. We combine 1D optical simulations using GenPro4 and 2D electrical simulations using Sentaurus™ TCAD to determine the optical and electrical loss mechanisms in order to estimate the potential of our current LPC-Si absorbers. The simulations reveal an effective minority carrier diffusion length of 26µm and further demonstrate that a doping concentration of 4 × 1016cm−3 and a back surface field width of 60µm are optimum values to further increase cell efficiencies.

Dynamics of interstitial atoms and vacancies during the crystallization of amorphous Si and Ge films by flash lamp annealing

We examined the dynamics of interstitial atoms and vacancies in amorphous Si (a-Si) and a-Ge films crystallized by flash lamp annealing in consideration of the self-diffusion coefficients of Si and Ge. We found that the interstitial atoms play an important role in the liquid-phase crystallization (LPC) of a-Si films, whereas the vacancies are more important for the solid-phase crystallization (SPC) of a-Si films along with the LPC and SPC of a-Ge films. For Si, the crystal defect density of the film crystallized by LPC was higher than that of the film crystallized by SPC; the opposite result was achieved for Ge. This phenomenon is considered to be attributed to the existence of interstitial atoms introduced in Si. The thermodynamic calculated results related to the relationship between the point defect and SPC or LPC supported the crystallization mechanism.

Applicability of an economic diode laser emitting at 980 nm for preparation of polycrystalline silicon thin film solar cells on glass

Recently, polycrystalline silicon thin film solar cells on glass are fabricated by a laser induced liquid phase crystallization (LPC) process. This study compares a new economic diode laser, emitting a line focus at 980 nm, with the 808 nm laser normally used concerning its absorption during LPC. We measured the optical constants of amorphous silicon by spectral ellipsometry and UV/VIS spectroscopy. Together with the literature data for crystalline and liquid silicon combined with numerical temperature simulations, we calculated the absorption during LPC and the overall power needed for successful crystallization. Solar cells prepared with both laser types show comparable crystallographic and optoelectronic characteristics. Concerning the economic advantages, the use of such a 980 nm diode laser system would be the choice for the potential industrial production.Polycrystalline silicon thin film solar cells on glass are fabricated by laser‐induced liquid phase crystallization. A new economic diode laser emitting a line focus at 980 nm is compared to the 808 nm laser normally used. Solar cells show comparable crystallographic and optoelectronic characteristics.