Microcrystalline Silicon Research Articles

Using phosphorus doped hydrogenated silicon oxycarbide film as a window layer on the light entrance side of silicon heterojunction solar cells: The role of phase separation on electron transport

The phosphorus doped hydrogenated nanocrystalline silicon oxycarbide (n-nc-SiCO:H) layer can be used as a window layer on the light incident side of silicon heterojunction solar cells (HJT). The chemical composition, structural organization and properties of the n-nc-SiCO:H layer deposited from plasma enhanced chemical vapor deposition (PECVD) method can be easily manipulated via radio frequency power density tuned. Phase separation is observed in the n-nc-SiCO:H layer in which nanoscale silicon crystallites were embedded in amorphous silicon oxycarbide matrix. The optical properties of the n-nc-SiCO:H layer are depended on oxygen and carbon atoms incorporation ratio. The conductivity of the n-nc-SiCO:H layer is dominated by activated phosphorus concentration and the phase separation. The activated phosphorus atoms which play an important role on electron transportation are distributed in both crystalline silicon phase and amorphous silicon phase. Both activated phosphorus concentration and band gap value for oxygen rich amorphous silicon oxycarbide layer are determined by incorporation ratio of O atoms. The interplay effects of optical and electrical properties of the n-nc-SiCO:H layers on HJT cells electric performance variation are revealed out in this report. An average efficiency (across ∼120 cells) of 26.3 % was achieved in cells with optimized power density for the n-nc-SiCO:H layer. The results demonstrate that phase separation in the n-nc-SiCO:H layer plays a critical role on electron transportation.

Flexible Triboelectric Sensor based on Catalyst‐Diffusion Self‐Encapsulated Conductive Liquid‐Metal‐Silicone Ink for Somatosensory Soft Robotic System

AbstractThe combination of fluidity and metallic conductivity has attracted considerable attention to liquid metal (LM), but its development remains challenging due to enormous surface tension. Here, vinyl‐terminated silicone oil and platinum catalyst are added to LM to reduce its surface tension, which develops a special type of liquid‐metal‐silicone (LMS) ink with a catalyst diffusion effect. Combined with an embedded three‐dimentional (3D) printing method, the LMS ink is printed on the support matrix, and the catalyst diffuses outward along the print path to cure the silicone around it, directly constructing self‐encapsulated conductive composites with excellent conductivity and self‐encapsulated flexible tactile sensors based on triboelectric nanogenerator (TENG). The sensor exhibits excellent sensitivity (0.308 V kPa−1), high linearity (≈0.99), and good durability (over 10 000 cycles). Furthermore, when used in flexible wearable electronics, the sensor demonstrates a good performance with an accuracy of ≈96% in classifying different human postures using a convolutional neural network. Finally, through embedded 3D printing with LMS ink and silicone ink, a somatosensory soft robotic gripper with complex cavity structures is designed and manufactured in one step, achieving the all‐in‐one integration of sensors and actuators. This study shows great application potential in flexible electronics and soft robotic systems.

Non-thermal atmospheric jet plasma effects on the morphological, electrical and optical properties of nanocrystalline silicon

In this study, a non-thermal atmospheric argon plasma jet (NTAPJ) was designed and used to treat the porous silicon layer, prepared by photoelectrochemical etching (PECE). The porous silicon layers were distinguished before and after the cold plasma treatment utilized by X-ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM), photoluminescence (PL), and dark and photo current-voltage characteristics. The results confirmed the nanostructure of the formed porous silicon. After the treatment with argon plasma, the intensity of the XRD peak increased, and the porosity decreased from 93% to 62%. Additionally, porous silicon's average roughness (Sa) and root mean square (Sq) were reduced from 60.10 to 12.31nm and 73.70 to 16.53nm, respectively. The current-voltage characteristic indicated the increase in photocurrent and the ideality factor, as well as a decrease in dynamic resistance. The photoluminescence results showed a change in PL peaks, represented by a blue shift, an increase in intensity, and a vanishing of another peak.

Non-thermal atmospheric jet plasma effects on the morphological, electrical and optical properties of nanocrystalline silicon

In this study, a non-thermal atmospheric argon plasma jet (NTAPJ) was designed and used to treat the porous silicon layer, prepared by photoelectrochemical etching (PECE). The porous silicon layers were distinguished before and after the cold plasma treatment utilized by X-ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM), photoluminescence (PL), and dark and photo current-voltage characteristics. The results confirmed the nanostructure of the formed porous silicon. After the treatment with argon plasma, the intensity of the XRD peak increased, and the porosity decreased from 93% to 62%. Additionally, porous silicon's average roughness (Sa) and root mean square (Sq) were reduced from 60.10 to 12.31nm and 73.70 to 16.53nm, respectively. The current-voltage characteristic indicated the increase in photocurrent and the ideality factor, as well as a decrease in dynamic resistance. The photoluminescence results showed a change in PL peaks, represented by a blue shift, an increase in intensity, and a vanishing of another peak.

Improved electrical contact properties in Indium-free silicon heterojunction solar cells with amorphous SnO2 TCO layers

Silicon heterojunction (SHJ) solar cells are recognized as one of the most efficient architectures in silicon-based photovoltaic devices. However, the reliance on indium (In)-based transparent conductive oxides (TCO) is anticipated to constrain their production capacity due to the critical and economically volatile nature of In. Recently, low-temperature-grown amorphous SnO2 (a-SnO2) films have been explored as an earth-abundant alternative TCO material. In this study, we examine the electrical contact properties of a-SnO2 layers employed as TCO layers in SHJ cells, focusing on their interaction with the underlying carrier selective contact layers. Our findings indicate that a stack of doped amorphous silicon (a-Si:H) and a-SnO2 exhibits relatively high specific contact resistivity, leading to a significant reduction in the device's fill factor. To address this issue, we propose two approaches: the insertion of a thin ZnO-based TCO layer between a-Si:H and a-SnO2, and the use of nanocrystalline silicon layers in place of a-Si:H. Both approaches effectively reduce the contact resistivity, resulting in improvements in fill factor and conversion efficiency comparable to those of benchmark device with In-based TCOs. Based on these findings, we demonstrate a high-efficiency, In-free, SnO2-based SHJ cell.

Spatially Resolved Oxidation and Enhanced Photooxidation in Nanocrystalline Silicon

Spatially Resolved Oxidation and Enhanced Photooxidation in Nanocrystalline Silicon

Tantalum doped tin oxide enabled indium-free silicon heterojunction solar cells with efficiency over 25 %

Reducing indium consumption has received increasing attention in contact schemes of high efficiency silicon heterojunction (SHJ) solar cells. It is imperative to discover suitable, low-cost, and resource-abundant transparent electrodes to replace the conventional, resource-scarce indium-based transparent electrodes. Herein, tantalum doped tin oxide (TTO), prepared through low-temperature sputtering, is selected as an alternative material. Notably, the Anti-Burstein-Moss effect of TTO is observed, which is ascribed to variations of stress in films. Based on this finding, TTO was first applied to SHJ solar cells, and the photoelectric tradeoff in front and rear contacts is gained by matching nanocrystalline silicon layers and adjusting TTO thickness. Ultimately, an indium-free SHJ solar cell with an efficiency of 25.15 % is obtained (certified efficiency 25.10 %, 274.30 cm2).

A Review: Application of Doped Hydrogenated Nanocrystalline Silicon Oxide in High Efficiency Solar Cell Devices.

Due to the unique microstructure of hydrogenated nanocrystalline silicon oxide (nc-SiOx:H), the optoelectronic properties of this material can be tuned over a wide range, which makes it adaptable to different solar cell applications. In this work, the authors review the material properties of nc-SiOx:H and the versatility of its applications in different types of solar cells. The review starts by introducing the growth principle of doped nc-SiOx:H layers, the effect of oxygen content on the material properties, and the relationship between optoelectronic properties and its microstructure. A theoretical analysis of charge carrier transport mechanisms in silicon heterojunction (SHJ) solar cells with wide band gap layers is then presented. Afterwards, the authors focus on the recent developments in the implementation of nc-SiOx:H and hydrogenated amorphous silicon oxide (a-SiOx:H) films for SHJ, passivating contacts, and perovskite/silicon tandem devices.

Investigation structural heterogeneities in hydrogenated nanocrystalline silicon thin films from argon-diluted silane dusty plasma PECVD

This study presents a novel approach for achieving a high deposition rate of nanocrystalline hydrogenated silicon (nc-Si:H) thin films using dusty plasma, eliminating the need for hydrogen dilution. Contrary to conventional beliefs, the research demonstrates that crystalline seeds can nucleate directly from the plasma environment itself. The investigation explores the relationship between plasma parameters and the degree of crystallinity in the deposited films, along with the characterization of structural inhomogeneities throughout the film's thickness. Raman spectroscopy, X-ray diffraction, and Transmission Electron Microscopy (TEM) are utilized for detailed analysis. The results reveal intriguing variations in crystallinity along the film's thickness, with higher crystallinity observed near the surface compared to regions closer to the substrate interface. In-depth analysis of grain size variation reveals that the average crystallite size decreases near the surface and increases in the bulk volume of the material. Additionally, deposition parameters, specifically power and pressure, are found to influence grain sizes, with higher values promoting larger grain growth. Notably, the observation of conical growth in layers deposited from dusty plasma presents a promising avenue for achieving controlled and desirable microstructures. This study sheds light on the critical influence of deposition parameters on film properties and the potential of dusty plasma for achieving desirable microstructures in nc-Si:H thin films. The findings offer valuable insights for tailoring thin film properties in diverse technological applications.

Phosphorus doped hydrogenated silicon oxycarbide: Film formation, properties and its application on silicon heterojunction solar cell

An ultrathin phosphorus doped hydrogenated microcrystalline silicon oxycarbide (n-μc-SiCO:H) layer deposited by plasma enhanced chemical vapor deposition (PECVD) method can be used as a window layer for silicon heterojunction solar cells. The optical and electrical properties of n-μc-SiCO:H films were controlled by PECVD deposition conditions. The interplay effects of H2, CO2 and PH3 to SiH4 gas ratio on the chemical composition, material structure and properties of n-μc-SiCO:H film were systematically determined. O, H, C and P atoms were observed to incorporate into the silicon network of n-μc-SiCO:H layer, while most of the atoms bond directly to Si atoms. Incorporation of oxygen and carbon atoms into n-μc-SiCO:H film, confirmed by XPS as formation of Si-O and Si-C bonds, enlarged the optical absorption band gap (Eg) value. Both film crystallinity and phosphorus concentration in n-μc-SiCO:H layer were observed to be controlled by the H2, CO2 and PH3 to SiH4 gas ratio. The electric performance of HJT solar cell was depended on the optical and electrical properties of n-μc-SiCO:H layer. An average efficiency (across ∼ 120 cells) of 26.32 % was achieved in cells with optimized deposition parameters for the n-μc-SiCO:H layer. The reaction gas ratio was also the dominant factor in determining the number and size of nanoscale Si crystallites embedded in the n-μc-SiCO:H layer. The conductivity of n-μc-SiCO:H layer was determined by the activated phosphorus concentration but independent of the total phosphorus concentration. It was also found that large concentrations of small Si crystallites of size less than 3 nm improved film conductivity.

Heavy Boron-Doped Silicon Tunneling Inter-layer Enables Efficient Silicon Heterojunction Solar Cells.

P-type hydrogenated nanocrystalline silicon (nc-Si:H) has been used as a hole-selective layer for efficient n-type crystalline silicon heterojunction (SHJ) solar cells. However, the presence of an additional valence band offset at the interface between intrinsic amorphous hydrogenated silicon and p-type nc-Si:H films will limit the hole carrier transportation. In this work, it has been found that when a heavily boron-doped silicon oxide layer deposited with high hydrogen dilution to silane (pB) was inserted into their interface, the fill factor of SHJ solar cells increases 3% absolutely because of the reduced valence band offset and the increased opportunity to provide a hopping tunnel assisted by the doping energy level and valence band tail states. Furthermore, the additional boron incorporation in intrinsic amorphous silicon adjacent to pB helps to enhance the built-in electric field, thus increasing the hole selectivity. By these means, the power conversion efficiency was improved from 23.9% to approximately 25%.

Deeper Insight into the Mechanisms Behind Sputter Damage in Silicon Solar Cells Based on the Example of Nanocrystalline Silicon Carbide

AbstractTransparent conducting oxides, like indium tin oxide, enable lateral charge carrier transport in silicon heterojunction solar cells. However, their deposition can damage the passivation quality in the solar cell. This damage during the sputter deposition is a complex issue that has not been fully understood, particularly in various silicon‐based materials like amorphous silicon, polycrystalline silicon, or nanocrystalline silicon carbide. The degradation in passivation quality observed in, for example, amorphous silicon is not only explainable by UV light degradation. This study explores the origin of this degradation based on the example of hydrogenated nanocrystalline silicon carbide by combining simulations with experimental analyses. It delves into potential sources of damage during the sputtering process and determines that neither primary nor secondary effects from plasma luminescence or electron bombardment are likely contributors to the damage. Similarly, the implantation of ions, as well as the creation of vacancies and ionization of lattice atoms, are also considered improbable causes. It is, however, proposed that the transfer of energy to the crystalline silicon interface via phonons can factor into the degradation of the passivation quality. This transfer might be a plausible explanation for the damage observed in the passivation layers during the sputtering process.

Fully 3D printed dielectric elastomer actuators based on silicone and its composites

AbstractDielectric elastomer actuator (DEA) is one of the most promising types of soft actuation technology, which has great potential in the fields of wearable devices and soft robotics. It consists of a dielectric elastomer layer, which is an electroactive polymer that can produce large deformation, and compliant electrodes to bring charges to certain locations. In this article, direct ink writing (DIW) technology, an emerging 3D printing method, was used to realize the preparation of the electrode‐elastomer‐electrode stack of the DEA. The dielectric and electrode materials were designed with suitable rheological properties to fulfill the need for the extrusion process. The formulated silicone material not only presented excellent dielectric and mechanical properties, but also good printability. Extrudable electrodes were prepared based on silicone composites with the characteristics of mechanical compliance and high conductivity. The fully printed DEA achieved a maximum actuation strain of 11.11%, a fast response time of 0.76 s and excellent electromechanical repeatability. DEA arrays were also achieved, possessing the ability to carry out on‐demand actuation, allowing each actuator to be activated singly or work in groups. Thanks to the design freedom of the DIW technology, this strategy is able to manufacture fine and complex structures with precise active zones, paving a way for the fabrication of next‐generation smart devices.Highlights Printable silicone ink was formulated with good dielectric property and softness. Carbon black/silicone composites were obtained with high conductivity and compliant nature. The silicone composites were printed into thin films to act as electrodes. Fully 3D printed dielectric elastomer actuators (DEA) were achieved by direct ink writing. DEA arrays with on‐demand actuation were realized by well‐defined printing.

Comparative simulation analysis of μc Si:H and SnS BSF layers in CIGS solar cells

In this paper, we presented Cu(In1-xGax)Se2 (CIGS) based solar cells in which hydrogenated microcrystalline silicon (μc Si:H) and tin sulfide (SnS) are used as back surface field (BSF) layers. In CIGS thin film solar cell technology, thickness serves a crucial factor. Optimizing absorber thickness reduces the usage of scarce and expensive materials such as indium (In) and gallium (Ga) in CIGS cells. Integrating a heavily doped layer between the absorber and the back surface, known as the back surface field (BSF) layer, in solar cell design is a successful strategy for minimizing the thickness of the absorber layer. The suggested configuration utilizes μc-Si:H and SnS as the back surface field (BSF) layer. The buffer layer in this configuration employs less harmful InP instead of the typically utilized CdS layer. All the simulation work is performed using SCAP-1D simulation tool. The utilization of μc Si:H and SnS BSF layers yields optimized efficiencies of 31.51% and 31.01%, respectively. This simulation work demonstrates that use of a proper BSF layer is very effective not only in performance enhancement but also in reducing absorber material thickness.

PDMS microspheres as rheological additives for PDMS-based DIW inks

Direct Ink Writing holds vast potential for additive manufacturing with broad material compatibility as long as appropriate rheological properties are exhibited by the material of choice. Additives are often included to attain the desired rheological properties for printing, but these same additives can yield products with undesirable mechanical properties. For example, silica fillers are used to create silicone inks appropriate for printing but yield cured structures that are too stiff. In this work, we investigate the applicability of PDMS microspheres as a rheological and thixotropic additive for PDMS based DIW inks. We utilize a facile oil-in-water emulsion method to reproducibly obtain small (∼5 μm) PDMS microspheres, which are then incorporated into PDMS-based inks. More traditional inks with fumed silica and thixotropic additive were compared with inks containing PDMS microspheres at equal volume loadings to determine whether the PDMS microspheres could impart the desired rheological properties for DIW. Inks including PDMS microspheres exhibited surprising thixotropic effects, which enabled prints with fidelity analogous to traditional ink employing silica filler, while producing mechanically softer prints.

Development of high conducting phosphorous doped nanocrystalline thin silicon films for silicon heterojunction solar cells application

We have investigated the plasma-enhanced chemical vapor deposition growth of the phosphorus-doped hydrogenated nanocrystalline silicon (n-nc-Si:H) film as an electron-selective layer in silicon heterojunction (SHJ) solar cells. The effect of power densities on the precursor gas dissociation are investigated using optical emission spectra and the crystalline fraction in n-nc-Si:H films are correlated with the dark conductivity. With the P d of 122 mW cm−2 and ∼2% phosphorus doping, we observed Raman crystallinity of 53%, high dark conductivity of 43 S cm−1, and activation energy of ∼23 meV from the ∼30 nm n-nc-Si:H film. The n-nc-Si:H layer improves the textured c-Si surface passivation by two-fold to ∼2 ms compared to the phosphorus-doped hydrogenated amorphous silicon (n-a-Si:H) layers. An enhancement in the open-circuit voltage and external quantum efficiency (from >650 nm) due to the better passivation at the rear side of the cell after integrating the n-nc-Si:H layer compared to its n-a-Si:H counterpart. An improvement in the charge carrier transport is also observed with an increase in fill factor from ∼71% to ∼75%, mainly due to a reduction in electron-selective contact resistivity from ∼271 to ∼61 mΩ-cm2. Finally, with the relatively better c-Si surface passivation and carrier selectivity, a power conversion efficiency of ∼19.90% and pseudo-efficiency of ∼21.90% have been realized from the SHJ cells.

4D printing of cellular silicones with negative stiffness effect for enhanced energy absorption and impact protection

Cellular silicone foams are renowned for their exceptional flexibility, ultra-elasticity, and durability, which have gained significant attention in diverse applications. However, manufacturing porous silicone foams featuring substantial bending curvatures and customized mechanical properties remains a challenge. Herein, this work presents a new strategy for manufacturing curved porous silicone foams with both 4D printed bending curvature and tailored meta-mechanical properties. The foam is achieved by direct ink writing of composite silicone inks embedded with thermal expandable microspheres as foaming agents. The work studied in detail the thermal expansion of microspheres, foaming of composite silicone inks, as well as properties of the expanded foam. Utilizing the strain mismatch under thermal stimuli, the printed bilayer structures achieved shape-shifting. Additionally, silicones composed of stacked bilayer filaments exhibited negative stiffness properties under compression, leading to enhanced energy absorption capacity, which can be fine-tuned through different printing structural designs, demonstrating its potential in fields such as energy dissipation and shock absorption while protecting objects with curved shapes.

Novel Fast Cure Silicone Inks for Single‐Step, Support‐Free 3D Printing of Tall, Overhanging, and High Aspect Ratio Structures

AbstractSilicone elastomers have a broad variety of applications, such as soft robotics, biomedical devices, and structural metamaterials. The extrusion‐based method known as direct ink write (DIW) has enabled the production of additively manufactured silicone structures. However, this method is limited to manufacturing mostly planar or pseudo‐3D structures. Due to the low self‐supporting capabilities of extruded strands for traditional silicone‐based “inks,” obtaining tall or overhanging structures, or structures comprised by thin walls is not feasible. Here, a novel Fast Cure silicone‐based ink is demonstrated that enables manufacturing of complex 3D structures. The Fast Cure ink is a two‐part mixture and silicone structures are produced by inline mixing and coextrusion of a part containing a catalyst (part A) and a part containing a crosslinker (part B). By the virtue of crosslinking, the extruded strands rapidly rigidize, increasing their self‐supportive capacity. Hence, structures can be obtained with superior shape retention and previously unobtainable parts are realized that are tall, with aspect ratios higher than 3, and have overhanging features, achieving inclination angles smaller than 35° with respect to the printing plane. These minimal sag parts are achieved without requiring extra curing or mechanisms, support structures, or suspension baths.

Interdigitated‐back‐contacted silicon heterojunction solar cells featuring novel MoOx‐based contact stacks

AbstractThe fabrication process of interdigitated‐back‐contacted silicon heterojunction (IBC‐SHJ) solar cells has been significantly simplified with the development of the so‐called tunnel‐IBC architecture. This architecture utilizes a highly conductive (p)‐type nanocrystalline silicon (nc‐Si:H) layer deposited over the full substrate area comprising pre‐patterned (n)‐type nc‐Si:H fingers. In this context, the (p)‐type nc‐Si:H layer is referred to as blanket layer. As both electrodes are connected to the same blanket layer, the high lateral conductivity of (p)nc‐Si:H layer can potentially lead to relatively low shunt resistance in the device, thus limiting the performance of such solar cells. To overcome such limitation, we introduce a thin (<2 nm) full‐area molybdenum oxide (MoOx) layer as an alternative to the (p)nc‐Si:H blanket layer. We demonstrate that the use of such a thin MoOx minimizes the shunting losses thanks to its low lateral conductivity while preserving the simplified fabrication process. In this process, a novel (n)‐type nc‐Si:H/MoOx electron collection contact stack is implemented within the proposed solar cell architecture. We assess its transport mechanisms via electrical simulations showing that electron transport, unlike in the case of tunnel‐IBC, occurs in the conduction band fully. Moreover, the proposed contact stack is evaluated in terms of contact resistivity and integrated into a proof‐of‐concept front/back‐contacted (FBC) SHJ solar cells. Contact resistivity as low as 100 mΩcm2 is achieved, and fabricated FBC‐SHJ solar cells obtain a fill factor above 81.5% and open‐circuit voltage above 705 mV. Lastly, the IBC‐SHJ solar cells featuring the MoOx blanket layer are fabricated, exhibiting efficiencies up to 21.14% with high shunt resistances above 150 kΩcm2. Further optimizations in terms of layer properties and fabrication process are proposed to improve device performance and realize the efficiency potential of our novel IBC‐SHJ solar cell architecture.

Active mixing of two-component viscoelastic silicone ink at molecular level for spatiotemporally controlled 3D/4D printing of cellular silicones

3D printed silicones have demonstrated great potential in diverse areas such as soft robots, tissue engineering, flexible sensors and energy dissipation. However, most silicone inks face inevitable curing issue once it is prepared. During the materials extrusion based printing process, the curing-related viscosity change in silicone inks hamper the ink stability and printing controllability, leading to complex relationship among ink properties and printing parameters, as well as printing structures and material properties of final products. Herein, this work presents active mixing of two component silicone ink to realize stable printing, achieving exact and continuous match of ink properties and printing parameters. Using a specially formulated ink containing separate starting components with reactive groups, catalyst and thixotropic agents, this work studies the active mixing process thoroughly for the first time from molecular level to materials performances under various conditions regarding inlet flow rates, impeller rotation speed and component ratios. Besides, cellular silicones with controlled distribution of component ratios and tailored gradient structures are printed, leading to integrated manufacturing of compression performance and shape morphing property. This work will pave way for active mixing and printing of two-component silicones, and provide guidance for other reactive multi-material 3D printing systems.