Crystalline Si Research Articles

A New Concept of Molten Salt Systems for the Electrodeposition of Si, Ti, and W.

ConspectusThis Account describes the results of the electrodeposition of film-like Si, Ti, and W by utilizing molten salts selected based on a new concept. The proposed molten salt systems, KF-KCl and CsF-CsCl, have high fluoride ion concentrations, relatively low operating temperatures, and high solubility in water.First, KF-KCl molten salt was used for the electrodeposition of crystalline Si films to establish a new fabrication method for Si solar cell substrates. The electrodeposition of Si films from the molten salt at 923 and 1023 K was successfully achieved using K2SiF6 or SiCl4 as the Si ion source. The crystal grain size of Si was larger at higher temperatures, indicating that higher temperatures are advantageous for the application of Si solar cell substrates. The resulting Si films underwent photoelectrochemical reactions. Second, the electrodeposition of Ti films using the KF-KCl molten salt was investigated to easily impart the properties of Ti, such as high corrosion resistance and biocompatibility, to various substrates. Ti films with a smooth surface were obtained from the molten salt containing Ti(III) ions at 923 K. Electrochemical tests in artificial seawater revealed that the electrodeposited Ti films had no voids and cracks and that the obtained Ti-coated Ni plate had a high corrosion resistance against seawater. Finally, the molten salts were used for the electrodeposition of W films, which are expected to be used as diverter materials for nuclear fusion. Although the electrodeposition of W films was successful in the KF-KCl-WO3 molten salt at 923 K, the surface of the films was rough. Therefore, we used the CsF-CsCl-WO3 molten salt, which can be employed at lower temperatures than KF-KCl-WO3. We then successfully electrodeposited W films with a mirror-like surface at 773 K. Such a mirror-like metal film deposition has not been reported before using high-temperature molten salts. Further, the temperature dependence of the crystal phase of W was revealed by the electrodeposition of W films at 773-923 K. β-W was obtained at 773 and 823 K, α-W was obtained at 923 K, and a mixed phase of α- and β-W was obtained at 873 K. In addition, single-phase β-W films with a thickness of approximately 30 μm were electrodeposited, which has not been reported before.The results show that our proposed molten salt systems are advantageous for electroplating Si, Ti, and W. Our approach is also expected to be applicable for the electrodeposition of other metals such as Zr, Nb, Mo, Hf, and Ta.

Electrodeposition of Crystalline Si Using a Liquid Zn Electrode in Molten KF–KCl–K2SiF6

In this study, we propose a novel Si electrodeposition method using a liquid Zn electrode in molten KF–KCl. Electrochemical measurements and electrolysis were conducted in a KF–KCl–K2SiF6 melt at 923 K. Cyclic voltammograms at a liquid Zn electrode revealed that the reduction currents at 0.75–1.0 V vs K+/K were attributed to the formation of Si–Zn liquid alloy. Additionally, Si was deposited through potentiostatic electrolysis at 0.75 V using liquid Zn in a boron nitride (BN) crucible as an electrode. Cross-sectional scanning electron microscopy and energy-dispersive X-ray spectroscopy showed that deposited Si was located at the bottom and side of the interface between Zn and the BN crucible instead of at the interface between Zn and the molten salt, indicating the electrodeposition of Si attributed to Si–Zn liquid alloy formation. The obtained Si was confirmed to be the crystalline form by X-ray diffractometry, and the maximum grain size was approximately 2 mm. Galvanostatic electrolysis at –20 mA cm–2 with varying electrical charges showed that the Si grain size increased with increasing charge, confirming the growth of crystalline Si. Finally, the mechanism of Si electrodeposition on a Zn electrode through Si–Zn alloying was discussed.

Tolerance to hygrothermal and high-voltage stresses for Si-related photovoltaic modules with newly-developed polyvinyl butyral encapsulants

Newly-developed polyvinyl butyral (PVB) encapsulants were employed for Si-related photovoltaic modules. Modules of amorphous Si with a glass–glass structure and those of crystalline Si with glass and a back-sheet structure were subjected to hygrothermal stresses by a damp-heat (DH) test and high-voltage stress by a potential-induced degradation (PID) test, respectively. Even after a quite long DH test at 85 °C and 85% relative humidity for 49 000 h, retention of output power of about 85% was obtained without edge sealant. No decrease in output power was also observed after a PID test with −1000 V at 60 °C and 85% relative humidity for 168 h. These facts suggest that newly-developed PVB is well applicable to encapsulant materials for highly reliable photovoltaic modules.

Modeling of the effectiveness of novel edge seal designs for fast, low‐Cap‐Ex manufacturing

AbstractIn the manufacturing of photovoltaic (PV) modules, the lamination process can take up to 20 min to complete. In this work, new lamination processes are being developed, and have been prototyped, which hope to be able to cut this time down to as little as 30 s. This could provide significant savings in the cost of lamination equipment, floor space, and energy. PV modules are expected to have a lifespan exceeding 20 to 30 years. For moisture‐sensitive PV technologies, the edge seal between the two layers of glass can be the weakest point of its reliability. There is an inherent challenge when evaluating edge seal materials due to their low permeation rates. As part of Colorado State University's Photovoltaic Research and Development 2, work at the National Renewable Energy Laboratory has developed models to evaluate edge seal configurations in glass‐glass PV modules. Here, this new manufacturing process is evaluated for long‐term moisture durability. Different edge seal design options within glass–glass PV modules are explored. Most of these designs are targeting a superstrate on glass configuration, e.g CdTe, but some designs could be used on conventional crystalline Si cells. Using COMSOL finite element simulation software, we investigated the edge seal and interlayer design configurations containing silicone perimeter edge adhesive, desiccated polyisobutylene‐based edge seal, air, and polyolefin while integrating climate conditions equivalent to a hot and humid environment such as Miami, Florida. We found optimized configurations that will allow the module to prevent moisture ingress over 50 years minimizing the amount of time and material used while utilizing polymers that are easily dispensed.

Self-Oscillated Growth Formation of Standing Ultrathin Nanosheets out of Uniform Ge/Si Superlattice Nanowires

Self-oscillation is an intriguing and omnipresent phenomenon that governs a broad range of growth dynamics and formation of nanoscale periodic and delicate heterostructures. A self-oscillating growth phenomenon of catalyst droplets, consuming surface-coating a-Si/a-Ge bilayer, is exploited to accomplish a high-frequency alternating growth of ultrathin crystalline Si and Ge (c-Si/c-Ge) nano-slates, with Ge-rich layer thickness of 14–19 nm, embedded within a superlattice nanowire structure, with pre-known position and uniform channel diameter. A subsequent selective etching of the Ge-rich segments leaves a chain of ultrafine standing c-Si nanosheets down to ∼ 6 nm thick, without the use of any expensive high-resolution lithography and growth modulation control. A ternary-phase-competition model has been established to explain the underlying formation mechanism of this nanoscale self-oscillating growth dynamics. It is also suggested that these ultrathin nanosheets could help to produce ultrathin fin-channels for advanced electronics, or provide size-specified trapping sites to capture and position hetero nanoparticle for high-precision labelling or light emission.

Wire-electrical discharge machining on single crystalline Si brick as a new wafering process for photovoltaic applications

Wire-electrical discharge machining on single crystalline Si brick as a new wafering process for photovoltaic applications

Acceleration of potential-induced degradation in crystalline Si photovoltaic modules after a lightning impulse strike

The potential-induced degradation (PID) is one of the significant issues in realizing low-cost electricity from photovoltaic (PV) power generation plants. In this paper, we have investigated PID in crystalline Si (c-Si) PV modules with conventional p-type multicrystalline Si solar cells after the application of lightning impulse strikes. Lightning impulses with a voltage of −40 kV were applied to the module between the shorted electrodes of the c-Si cell and the mimic aluminum frame. It is confirmed that no degradation in the electrical characteristics of the c-Si cell occurs by applying the impulse only. We have found that the PID of c-Si PV modules was accelerated by applying the impulses between a c-Si cell and a metal frame. The acceleration of PID in the module applied with a lightning impulse might be caused by the migration of Na+ ions easily toward the c-Si cell owing to damage to the ethylene-vinyl acetate encapsulant by impulses.

Two-Dimensional Siloxene Nanosheets: Impact of Morphology and Purity on Electrochemistry.

Two-dimensional (2D) siloxene (Si6O3H6) has shown promise as a negative electrode material for Li-ion batteries due to its high gravimetric capacity and superior mechanical properties under (de)lithiation compared to bulk Si. In this work, we prepare purified siloxene nanosheets through the removal of bulk Si contaminants, use ultrasonication to control the lateral size and thickness of the nanosheets, and probe the effects of the resulting morphology and purity on the electrochemistry. The thin siloxene nanosheets formed after 4 h of ultrasonication deliver an average capacity of 810 mA h/g under a 1000 mA/g rate over 200 cycles with a capacity retention of 76%. Interestingly, the purified siloxene shows lower initial capacity but superior capacity retention over extended cycling. The 2D morphology benefit is illustrated where the parent siloxene nanosheet morphology and structure were largely maintained based on operando optoelectrochemistry, in situ Raman, ex situ scanning electron microscopy, and ex situ transmission electron microscopy. Furthermore, the purified siloxene-based electrode free from crystalline Si impurity experiences the least expansion upon (de)lithiation as visualized by cross-section electron microscopy of samples recovered post-cycling.

Toxicity of Perovskite Solar Cells

Over the past decade, there has been significant and rapid developments in organic-inorganic hybrid perovskite solar cells (PVSCs). Despite the fact that the power conversion efficiency (PCE) of PVSCs has increased from 3.8% to 25.8%, approaching that of commercial single crystalline Si solar cells, the market is still dominated by Si-based photovoltaic (PV) technology. This can be attributed to the challenges associated with upscaling PVSCs, improving device stability, and reducing the toxicity of PVSCs, which are hurdles in commercializing perovskite PV technologies. In particular, the toxicity due to lead leakage of PVSCs makes it difficult for them to enter the market. Hence, in this article, the structure and working principle of PVSCs are first summarized. Then, the toxicity of PVSCs is discussed, including the impacts of organic solvents and perovskite precursor materials on the health and environment. In this section, examples of advanced strategies for reducing the toxicity of PVSCs are also provided. Finally, challenges and a perspective for developing nontoxic PVSCs are given.

Thermoelectric properties of Mg2Si thin films prepared by thermal evaporation of Mg and face-to-face annealing

Mg2Si thin films were prepared by thermal evaporation of Mg and face-to-face annealing (FTFA) on a crystalline Si substrate and on a P-doped hydrogenated amorphous Si thin films deposited on a quartz substrate. FTFA suppressed the evaporation of Mg, the oxidation of Mg2Si, and the decomposition of Mg2Si. The polycrystalline Mg2Si thin film with a large grain size was obtained. The effects of annealing temperature (TA) during FTFA on crystal structure and thermoelectric properties of Mg2Si thin films were investigated. Raman scattering spectra revealed that Mg2Si was formed by FTFA in the range of TA = 623–923 K, and lower TA led to lower strain and higher crystallinity. From the thermoelectric properties of the sample prepared at TA = 623 K, a relatively high ZT of 0.68 was obtained at 712 K. This ZT was achieved by a drastic increase in electrical conductivity, a relatively high Seebeck coefficient of −235 μV･K−1, and a very low thermal conductivity of 1.4–1.7 W･m−1･K−1.

High efficiency perovskite/heterojunction crystalline silicon tandem solar cells: towards industrial-sized cell and module

29.2%-conversion efficiency of a two-terminal (2T) perovskite/crystalline Si heterojunction tandem solar cell using 145 μm thick industrial Czochralski (CZ) Si wafer is obtained. The structural optimization, such as surface passivation of the perovskite layer and better light management techniques, improved power conversion efficiency (PCE). To our knowledge, this PCE is the best in 2T-tandem solar cells using CZ wafers. Towards industrialization, crucial issues with the 2T tandem solar cells with crystalline Si bottom cell are discussed. Four-terminal (4T) tandem solar cells are evaluated as an approach to avoid the crucial issues. Examining our base technologies which realize 22.2%-conversion efficiency perovskite single junction solar cell module and 26%-heterojunction back-contact solar cells, we clarified that the based technologies were ready to realize 30%-conversion efficiency 4T perovskite/heterojunction crystalline Si tandem solar cells with approximately quarter size of an industrial crystalline Si solar cell (∼64 cm2).

Quantitative evaluation of implied open-circuit voltage after metal electrode deposition on TiO x /Si heterostructures by photoluminescence imaging: impact of metallization on passivation performance

The degradation of surface passivation performance by metallization is a challenge in realizing highly efficient crystalline Si solar cells that use novel carrier-selective contacts. Here, we report on a simple method to study the effect of metallization on passivation of titanium oxide (TiO x )/Si heterostructures. We investigated the relationship between the implied open-circuit voltage (iV OC) and the photoluminescence (PL) intensity imaging of solar cell precursors before metallization. Based on the relationship obtained, the change of the iV OC before and after metallization on the TiO x was evaluated quantitatively. The results showed that the iV OC predicted by the PL measurement decreases by 23–104 mV after metal deposition and shows a good agreement with the measured V OC in the finished solar cells. These results demonstrate that the iV OC evaluation by PL measurement provides a good prediction of the V OC after metallization, which is useful in analyzing the passivation degradation induced by metallization.

Brief Outlook on Top Cell Absorber of Silicon‐Based Tandem Solar Cells

Silicon (Si) is a low‐cost, stable photovoltaic market contributor which is approaching its single‐junction theoretical efficiency limit. To further elevate the efficiency of crystalline Si (c‐Si) afterward, Si solar cells (SCs) need a proper top cell absorber in Si tandem SCs. Herein, the top cell absorber is studied by evaluating their current state of the art, the existing challenges, and possible future solutions which can enhance their efficiency. The progress of perovskite/Si tandem SC both for monolithic and four terminals is exceptional but still many challenges limit their commercialization. In view of these challenges, the relative's solutions are prescribed in detail to make possible their realization. III–V semiconductors being mature candidates still face a growth/fabrication problem, which makes them a costly option for III–V/Si tandem SCs. Effective growth strategies along with technical solutions are discussed in detail. Emerging chalcogenides are considered the future of tandem technology. Recent developments in the chalcogenides/Si tandem SC are highlighted and the possible options are presented. In the end, a short note is given on the near future tandem materials for Si tandem SCs.

Synthesis of passively prelithiated SiOx nanoparticles for Li‐ion battery anode

AbstractA new concept of passive prelithiation to SiOx nanoparticles is introduced and evaluated by investigating their nanostructures and electrochemical properties. Specifically, Li is incorporated into SiOx nanoparticles during the nanoparticle synthesis. We obtain Si‐Li‐O‐based nanoparticles, which we call SILIO; these are much larger than SiOx particles and have totally different nanostructures. Due to nanostructures with various phase distributions, SILIO nanoparticles show enhanced electrochemical properties. The initial reversible capacity (IRC) and initial columbic efficiency (ICE) of SILIO nanoparticles are 946 to 1107 mAh/g and 72% to 77%, respectively, while SiOx exhibits 1,064 mAh/g of IRC only with 41.5% of ICE. In addition, the stability of SILIO in the air is evaluated to guarantee no unstable phases such as Li2Ox (x = 0–2) are present in SILIO. Through our findings, we suggest a new nanostructure model composed of crystalline Si, amorphous SiOx, and lithium silicate.

Self-formation of SiGe oxide, Ge, and void multilayers via thermal oxidation of hydrogenated epitaxial SiGe films

In this work, hydrogenated SiGe thin films epitaxially grown on crystalline Si (c-Si) at 175 °C are annealed at 800 °C in a low vacuum system. The influence of H from the thin film and residual O atoms in the vacuum system on the structural changes of SiGe is presented. Multilayers consisting of c-Si/SiGe/void/Ge/SiOx/SiGeOx are obtained by taking advantage of the hydrogen present in the epitaxial SiGe layer. Various annealing conditions are applied to study the evolution of SiGe layers. The surface morphology, crystallinity, composition and micro structure of the multilayer stack formed during the annealing process are investigated in detail. A discussion based on the diffusion of H and O atoms is presented to account for this extraordinary process.

The improved thermoelectric properties of Mg2Sn/Mg multilayer films with nano-sized period by layer interface

The Mg2Sn/Mg multilayer films with different nano-sized modulation period were successfully prepared on single crystalline Si substrate with 500 ​nm thickness SiO2 layer by alternately sputtering using high pure metal Mg target and Mg–Sn alloy target. The chemical composition, surface and cross-sectional morphology, phase composition, carrier transport process and thermoelectric properties of deposited films were deeply investigated. The results show that the deposited films possess obvious layered structure, which is composed of metal Mg layer and antifluorite cubic Mg2Sn layer. The atom ratio [Mg]/[Sn]<2.0 in Mg2Sn layer between metal Mg layers and deposited films are p-typed semiconductor due to the existence of Mg vacancy in Mg2Sn lattice. The carrier concentration, mobility and thermal conductivity at room temperature decrease with reducing the modulation period of the multilayer films due to scattering of layer interface, at which depletion layer and built-in electric field form, impeding motion of the carriers. The electrical conductivity slightly decrease, but the Seebeck coefficient, power factor and figure of merit (ZT) greatly increase with reducing modulation period due to the scattering of the carriers. The ZT value of the film with the smallest modulation period of 171 ​nm reaches 0.51. The introduction of the layer interface greatly strengthens the carrier scattering process and improves the thermoelectric properties of the deposited films. This work demonstrates that introduction of heterogeneous interfaces composed of the same elements into films can effectively improve its thermoelectric properties.

The Microstructure of Underdense Hydrogenated Amorphous Silicon and its Application to Silicon Heterojunction Solar Cells

The application of thin underdense hydrogenated amorphous silicon (a‐Si:H) films for passivation of crystalline Si (c‐Si) by avoiding epitaxy in silicon heterojunction (SHJ) solar cell technology has recently been proposed and successfully applied. Herein, the microstructure of such underdense a‐Si:H films, as used in our silicon heterojunction solar cell baseline, is investigated mainly by Raman spectroscopy, effusion, and secondary ion mass spectrometry. In H effusion experiments, a low‐temperature (near 400 °C) effusion peak which has been attributed to the diffusion of molecular H2 through a void network is seen. The dependence of the H effusion peaks on film thickness is similar as observed previously for void rich, low substrate‐temperature a‐Si:H material. Solar cells using underdense a‐Si:H as i1‐layer with a maximum efficiency of 24.1% are produced. The passivation quality of the solar cells saturates with increasing i1‐layer thickness. The fact that with such underdense material combined with a following high‐quality i2‐layer, instead of only high‐quality a‐Si:H with a low defect density direct on the c‐Si substrate, good passivation of c‐Si solar cells is achieved, which demonstrates that in the passivation process, molecular hydrogen plays an important role.

Dose-dependency of contact resistance and sheet resistance of B-implanted emitters for N-type crystalline Si solar cells fabricated using screen-printed fire-through Ag/Al paste metallization process

Dose-dependency of contact resistance and sheet resistance of B-implanted emitters for N-type crystalline Si solar cells fabricated using screen-printed fire-through Ag/Al paste metallization process

Perovskite quantum dot-based tandem triboelectric-solar cell for boosting the efficiency and rain energy harvesting

Silicon (Si) solar cell efficiently convert solar energy into electricity under the sun illumination, but its inherent intermittent power generation due to diurnal and seasonal cycle has usually allowed for the requirement for alternative power sources. Owing to the abundance, diversity and availability, rain drop energy is one of the most promising green energy sources. Hence, hybrid energy harvesting could be a promising strategy of achieving preferential hybrid performances without sacrificing the advantages in individual devices. Herein, we proposed a hybrid solar and rain energy harvesting device featuring the integration of triboelectric nanogenerator (TENG) top-cell and a crystalline Si solar cell bottom-cell by laminating perovskite quantum dots-embedded polydimethylsiloxane composite film (PQDP) onto the Si solar cell, which endows enhanced electrical output performances under rainy days. To overcome the limitation of low external quantum efficiency (EQE) in the ultraviolet region of the Si solar cell, the PCE of the hybrid solar cell is significantly enhanced from 18.47% of bare Si solar cell to 20.16% owing to the anti-reflection effect of PQDP film and the down-conversion property of CsPbBr3 QDs doped with Ce3+ ions. Meanwhile, this hybrid solar cell presents excellent durability after continuous measurement over 7 days. This tandem hybrid solar cell provides a promising strategy to simultaneously scavenging the solar and rain energy and boosting the power conversion efficiency by introducing the perovskite QDs without affecting the efficiency of bare Si solar cell.

Theoretical analysis of Si3N4/TiC interfacial properties and material preparation

Computer simulation plays an instrumental role in modern tool material design. In this study, we have calculated the temperature and pressure conditions for the phase transition of Si3N4 from α to β using first principles. The simulation results show that the phase transition temperature is 1330 K at a phase transition pressure of 32 MPa. The modulus of elasticity, hardness, ductile/brittle properties (B/G), Poisson's ratio, elastic anisotropy, and phonon spectral curves at the phase transition point were also calculated for the material at temperatures ranging from 873 K to 2073 K. According to the phase transition conditions, the surface energies of β-Si3N4 and TiC interfaces were calculated, and 4-layer models of Si3N4/TiC interfaces were constructed. The total energy and adhesion work of the Si3N4(110)/TiC(100) interface were analyzed by molecular dynamics. The results show that the Si3N4(110)/TiC(100) interface exhibits the maximum total energy and adhesion work of 12,806.62 eV and 2.33 J/m2, respectively, indicating that the interface bond was the most stable. Furthermore, the mechanical properties of different interface models were calculated and compared. The effects of Si3N4(110)/TiC(100) interfacial 5-layer and 6-layer crystalline surfaces and Si and Ti vacancy defects on the mechanical properties of the interfacial model were further investigated. The Si3N4/TiC composite ceramic tool material was prepared experimentally based on the results of simulation, and the properties show reasonable agreement.