Black Silicon Solar Cells Research Articles

Multi-Carrier Generation in Organic-Passivated Black Silicon Solar Cells with Industrially Feasible Processes.

The innate inverse Auger effect within bulk silicon can result in multiple carrier generation. Observation of this effect is reliant upon low high-energy photon reflectance and high-quality surface passivation. In the photovoltaics industry, metal-assisted chemical etching (MACE) to afford black silicon (b-Si) can provide a low high-energy photon reflectance. However, an industrially feasible and cheaper technology to conformally passivate the outer-shell defects of these nanowires is currently lacking. Here, a technology is introduced to infiltrate black silicon nanopores with a simple and vacuum-free organic passivation layer that affords millisecond-level minority carrier lifetimes and matches perfectly with existing solution-based processing of the MACE black silicon. Advancements such as the demonstration of an excellent passivation effect whilst also being low reflectance provide a new technological route for inverse Auger multiple carrier generation and an industrially feasible technical scheme for the development of the MACE b-Si solar cells.

Emitter Quality Optimization Using Lightly Doped Phosphorus Diffusion and Thermal Oxide Anneal for Cell Efficiency Improvement in Multi-Crystalline Black Silicon Solar Cells

Improving solar cell performance by increasing solar cell efficiency by various process optimization had always been a simple straight-forward methodology followed in a R&D or in a solar cell manufacturing company. This is also the most cost-effective practice to improve a product performance using the same technology without the need to procure alternative or expensive raw materials or by adopting advanced solar cell processing techniques. Aluminium Back Surface Field (Al-BSF) technology using multi-crystalline wafers (mc-Si) had been a well-established and a dominant product in the solar industry for more than two decades. However, as the industry progresses, the demand for high efficiency solar cells and modules started going up and full area Aluminium BSF based cells suffers from a lot of inherent limitations on cell efficiency. This is primarily due to the intrinsic high density of crystal lattice defects or otherwise called as grain boundary defects present dominantly only in mc-Si wafers. These grain boundaries tends to accumulate several defects and become trap centres which cause high recombination for minority carriers thereby exhibiting lower conversion efficiency and higher dispersion in electrical parameters in batches of tested cells. Years of research using this material have helped to derive the maximum benefits using this mc-Si wafer in producing industrial full area BSF cells and we can say with certainty that the efficiency potential has reached the saturation point with this technology. An interesting development that happened in the area of improving the final product performance using mc-Si wafers at both cell and module level, is by replacing the conventional acid texturing process with an introduction of a nano-texturing process called Metal Catalysed Chemical Etching (MCCE) using specialized chemicals which improves the light trapping capabilities by creation of inverted pyramid texture on the silicon wafer surface and thereby enabling the wafers to absorb sunlight over a broader range of wavelength and incident angle. With this development done in mc-Si wafers in recent past, it is still a daunting task to surpass cell efficiencies beyond 19.0% using this wafer source. Hence for cell manufacturing lines which use mc-Si wafers, there is always a constant need to improve the cell manufacturing processes to reduce the impact of poor intrinsic quality of mc-Si wafers and improve the final product performance without adding any significant cost factor.

Black Silicon Solar Cell Passivated by Sio2 Thin Film Using Liquid Phase Deposition Process

Black Silicon Solar Cell Passivated by Sio2 Thin Film Using Liquid Phase Deposition Process

Light scattering from black silicon surfaces and its benefits for encapsulated solar cells

Black silicon (b-Si) has been widely investigated as a potential replacement for more traditional antireflective schemes for silicon solar cells, such as random pyramids, due to its reduced broadband reflectance and improved light-trapping properties. Wavelength and angle resolved scattering (WARS) reflectance measurements provide the means of analysing the amount of light scattered from a textured surface, which can be of interest when considering the amount of light trapped through total internal reflectance (TIR) at various interfaces in an encapsulated photovoltaic module. Here we present and analyse results from WARS measurements on b-Si surfaces fabricated using metal assisted chemical etching (MACE). Large angle scattering is observed for the entire spectrum, increasingly so for shorter incident wavelengths and increasing height of texture features. This is predicted to result in 35–40% of the reflected light being trapped by TIR at the glass-air interface and redirected back onto the sample, when the sample is encapsulated in standard PV module materials. This leads to a calculated additional boost of up to 0.45% in the photogenerated current of an encapsulated black silicon solar cell. This exceeds the calculated 0.21% boost due to TIR predicted for an encapsulated solar cell employing the industry-standard random pyramid texture with a thin film antireflective coating. • Black silicon nanostructures are fabricated for antireflection and light-trapping. • Nanostructures exhibit broadband reduced hemispherical reflectance below 3%. • Custom setup is used to measure wavelength and angular position of reflections. • Portion of reflected light trapped at various solar cell interfaces exceeds 40%. • Photocurrent relative gain of 0.45% is calculated for multiple nanostructure heights.

Fabrication of Black Silicon via Metal-Assisted Chemical Etching—A Review

The metal-assisted chemical etching (MACE) technique is commonly employed for texturing the wafer surfaces when fabricating black silicon (BSi) solar cells and is considered to be a potential technique to improve the efficiency of traditional Si-based solar cells. This article aims to review the MACE technique along with its mechanism for Ag-, Cu- and Ni-assisted etching. Primarily, several essential aspects of the fabrication of BSi are discussed, including chemical reaction, etching direction, mass transfer, and the overall etching process of the MACE method. Thereafter, three metal catalysts (Ag, Cu, and Ni) are critically analyzed to identify their roles in producing cost-effective and sustainable BSi solar cells with higher quality and efficiency. The conducted study revealed that Ag-etched BSi wafers are more suitable for the growth of higher quality and efficiency Si solar cells compared to Cu- and Ni-etched BSi wafers. However, both Cu and Ni seem to be more cost-effective and more appropriate for the mass production of BSi solar cells than Ag-etched wafers. Meanwhile, the Ni-assisted chemical etching process takes a longer time than Cu but the Ni-etched BSi solar cells possess enhanced light absorption capacity and lower activity in terms of the dissolution and oxidation process than Cu-etched BSi solar cells.

Novel technique for large area n-type black silicon solar cell by formation of silicon nanograss after diffusion process

Metal-assisted chemical etching (MACE) method is the most convenient and cost-effective nanowire fabrication method compared to other nanowire fabrication processes although a major problem arises in silicon nanowire, formed by MACE solution during n-type c-Si solar cell fabrication steps. High-temperature boron diffusion in conventional open tube furnace breaks down the nanowire resulting in a non-uniform surface pattern which is responsible to decrease overall conversion efficiency of the finished cell. In this work, this drawback is resolved by considering silicon nanowire formation after diffusion step. A slow etchant is considered for nanostructure on diffused silicon wafer to protect the diffused junction. The generated nanowire size is very less and has forage-like structure, and so termed as nanograss. Surface morphology and the characterization of the silicon nanograss structure after diffusion process on large area (156 mm × 156 mm) c-Si solar cells using MACE method have been investigated elaborately. Further, the complete solar cell has been fabricated with an efficiency of 17.20%.

Metal‐Assisted Chemical Etching of Silicon in Oxidizing HF Solutions: Origin, Mechanism, Development, and Black Silicon Solar Cell Application

AbstractMetal‐assisted chemical etching (MacEtch) of silicon in oxidizing hydrofluoric acid (HF) solutions has emerged as a prominent top‐down micro/nanofabrication approach for a wide variety of silicon micro/nanostructures. The popularity of the process is due to its simplicity, rapidity, versatility, and scalability. In recent years, there has been a surge of interest in developing MacEtch silicon micro/nanostructures for advanced energy conversion and storage applications, such as photovoltaic devices, thermoelectric devices, lithium‐ion rechargeable batteries, and supercapacitors. Particularly, MacEtch has emerged as a powerful surface micro/nanostructuring method for low‐cost and scalable production of commercial black silicon (b‐Si) with excellent light trapping properties. This review on MacEtch processing of silicon in oxidizing HF solutions provides a critical description of its history and origin including how it evolved into what it is today, the understanding of its mechanism and important technical advances in the field. As regards MacEtch‐fabricated b‐Si, its initial discovery and further improvements to its large‐scale deployment in silicon photovoltaic industry are traced. Some fundamental challenges and perspectives in this exciting field are also discussed.

Improvement of saw damage removal to fabricate uniform black silicon nanostructure on large-area multi-crystalline silicon wafers

The first step of preparing black silicon solar cells by Ag metal-assisted chemical etching method (Ag-MACE) is to remove the saw damage layer on the surface of diamond wire saw multi-crystalline silicon wafers. The usual way to do this is with a hot solution of KOH. However, the surface of silicon wafers processed in large quantities in this way will have many white spots and stains. These white spots and stains are the saw damage layer that have not been removed cleanly. They affect not only the appearance of the solar cells, but, most importantly, the subsequent uniform deposition of silver nanoparticles on the silicon wafers. An additive containing cyclodextrin and lignosulfonate has been developed in this work. This can remove effectively the saw damage layer and eliminate the white spots and stains on the surface of the silicon wafer. So as to improve the appearance of silicon wafer and make silver nanoparticles uniformly deposited on the surface of the whole silicon wafer. This additive has been proved to be very useful in improving solar cell efficiency, raising parallel resistance and reducing reverse leakage current and so on. Eventually, mass-produced solar cells made by Ag-MACE with this additive have a maximum efficiency of 19.49%, which is 0.46 percent absolutely higher than that without the additive.

Nanostructured pyramidal black silicon with ultra-low reflectance and high passivation

In this study, nanostructured pyramidal black silicon is prepared by metal assisted chemical etching method, in which the silver nitrate (AgNO3) is used as the metal catalyst. Effects of the concentration of AgNO3 on passivation and optical properties of the black silicon are investigated. The experimental results show that at the AgNO3 concentration of 0.03 M, the nanostructure length is about 300 nm, and the reflectance of the black silicon with a stack of silicon nitride (SiNx) and aluminum oxide (Al2O3) is 0.8%, which is comparable to that of the conventional black silicon with micrometer-long nanowires. In addition, an acceptably low surface recombination rate of 42 cm/s can be obtained. Plasma chemical vapor deposited SiNx is deposited well on the top of nanostructures of black silicon, but shows poor coverage at the bottom region. Spatial atomic layer deposited Al2O3 can conformally cover the nanostructures with high passivation quality. Simulation result indicates an improvement of 5.5% of conversion efficiency for the nanostructured pyramidal black silicon solar cell compared to industrial silicon solar cell. The short nanostructured pyramidal surface with low reflectance and high passivation is expected to be helpful for black silicon technology applied to photovoltaic applications.

High-efficiency multi-crystalline black silicon solar cells achieved by additive assisted Ag-MACE

It is important for uniform texture on crystallites with different orientation of diamond wire sawn multi-crystalline silicon solar cells by Ag metal-assisted chemical etching method (Ag-MACE). In this paper, an additive was adopted to make the nanostructures on crystallites with different orientation of multi-crystalline silicon wafers with roughly the same depth and diameter. This additive contains biological enzymes, which are macromolecular proteins with specific catalytic functions. The uniform texture produced by the additive is demonstrated to be helpful in improving the cell performance in terms of surface morphology, reflectivity, effective minority carrier lifetime, auger recombination rate, internal quantum efficiency and external quantum efficiency measurements as well as electroluminescence spectra characterizations. The chemical mechanism of additive assisted etching reaction during the texturing process was discussed. Finally, mass-produced multi-crystalline silicon solar cells made by Ag-MACE with the additive achieve a maximum efficiency and average efficiency of 19.56%, and 19.24%, respectively, which were 0.65% and 0.64% absolutely higher than that without the additive. Moreover, the effective minority carrier lifetime and auger recombination rate of the former was nearly twice and a third of that of the latter respectively.

Influence of bowl-like nanostructures on the efficiency and module power of black silicon solar cells

In this work, black multi-crystal silicon (Mc-Si) solar cells with bowl-like nanotextured surfaces were successfully fabricated by a metal-assisted chemical etching (MACE) method. Defect removal etching processes of various durations were used to form bowl-like nanostructures of three sizes on the wafer surface. Overall, a low depth and large diameter of bowl-like structure in nanotextured surfaces is demonstrated to be helpful in reducing surface recombination and improving the cell and module performance. The average cell module power of the bowl-like nanotextured surfaces with an average bowl diameter 680 nm is clearly higher by 1.51 W, 1.46 W, and 1.26 W than for nanotextured surfaces with bowl diameter 460 nm in the 18.8%, 18.9%, and 19.0% efficiency bins. A maximum cell efficiency of 19.17% and module power of 279.74 W were obtained using our MACE process in an industrial mass production line. The techniques presented in this paper can be used for the mass production of diamond wire sawing Mc-Si solar cells and meet the requirements of high efficiency and low cost in the photovoltaic industry.

A new passivation scheme for the performance enhancement of black silicon solar cells

It is well known that, in c-Si solar cells, the surface treatments are very important and have great relevance in the final efficiency of the devices. Black silicon solar cells are the terminology commonly used for solar cells with different grades of nanometric surface texturizations. In this work, we prepared silicon solar cells with a diverse grade of chemical surface treatment (alkaline etch and MACE treatment), plasma annealing (ammonia and hydrogen) with silicon-rich silicon nitride as passivation and antireflection coating. Spectral response, surface reflectance, and I–V measurements were carried out to obtain the final performance of the cell fabricated in the present work. From all the devices fabricated in the present work, the best results were obtained for the structure where the double stack layer (pm-Si/SiNx) was used as the passivation scheme, and plasma treatment was carried out in ammonia atmosphere as in comparison to existing conventional passivation schemes.

Design of experiment approach to the optimization of diffusion process on nanoscopic silicon solar cell

The temperature and duration of diffusion process were optimized for fabricating nanoscopic silicon based solar cells using design of experiments (DoE) technique. The optimum nano-structure silicon substrate had been etched for 180s and had porosity of 25%, depth of 0.34 μm and diameter of 99.8 nm. Modeling and experimental design were implemented by D-optimal design of experiment with 12 random experiments. The temperature and time parameters were varied at ranges of 810–910 °C and 15–60 min, respectively. The result indicated that the diffused substrate at temperature of 840 °C and time of 60 min had the highest voltage and current-density which were 0.607 V and 24.32 mA/cm2, respectively. The reflectance of fabricated optimum solar cell was gaged. The result presented that the reflectance of black silicon solar cell had been 600 times less than the non-porous solar cells using SiNx coated polished surface as antireflection layer. In addition, considerable gains of 13.64% on average efficiency, 0.53 on fill factor, 0.611 V on open circuit voltage and 43.43 mA/cm2 on current-density were achieved for optimum black silicon solar cell.

An 18.9% efficient black silicon solar cell achieved through control of pretreatment of Ag/Cu MACE

With diamond wire sawn (DWS) technique becoming mainstream of multicrystalline silicon (mc-Si) solar cells, the corresponding texturing technology for light harvesting is more prominent. In order to further reduce production costs of mature Ag-based metal assisted chemical etching (MACE), an Ag/Cu MACE method was proposed. In this paper, the influence of different pretreatment method which has few studies reported before was investigated. The experimental results indicated that appropriate pretreatment could contribute to achieve uniform nanostructure, low reflectivity and recombination velocities of the silicon wafers. The light trapping mechanism of different texturing method was analyzed. The impact of nanostructure on surface passivation was also studied. Industrial large area solar cells have been fabricated by applying different texturing method. The results showed that the pretreatment using hot alkaline solution with texturing additive was more beneficial to achieve high conversion efficiency. Finally, an efficiency of 18.91% was obtained on DWS mc-Si wafer, which is 0.4% absolutely higher than the cells with traditional acid texturing.

18.78% hierarchical black silicon solar cells achieved with the balance of light-trapping and interfacial contact

18.78% efficiency of crystalline silicon (Si) solar cells was achieved through introducing the combined nanopore/pyramid textures. These hierarchical structures possessed the ultra-low reflectivity with values <6% under the various illumination angles from 0 to 60°, evidencing their remarkable omnidirectional light-trapping capability. Although the greatly improved light-trapping effect of the hierarchical structures was demonstrated, we found that the increased formation durations of Si nanopores led the substantial increase of photoluminescent characteristics that could limit the efficient separation of photogenerated carriers through charge recombination. Moreover, the positive correlation in surface roughness of nanopore arrays with respect to the elongated etching durations was evidenced, and these issues could increase the series resistance of solar cells owing to poor interfacial contact between nanopores and front electrodes. Thus, there existed the optimal combination between two-scale textures critically for both photonic and electrical management of cell design. By optimizing the etching durations for the controlled nanopore formation, the improved light-trapping characteristics and fairly unchanged contact resistance with front electrode was achieved, possessing the improved conversion efficiency with >0.58% of increased value beyond the typical microtexture-based solar cells.

Multi-physics analysis: The coupling effects of nanostructures on the low concentrated black silicon photovoltaic system performances

Black silicon (nanostructured front surface) can significantly improve the efficiency of the silicon solar cell by absorbing more solar irradiance. However, due to the band gap of the semiconductor material, most part of the absorbed solar irradiance is converted into heat to increase the temperature of solar cell. Especially under the concentrating condition, the waste heat is more. Besides, multi-physics coupling problem makes it difficult to study the effects of nanostructures on the performances of the low concentrated black silicon photovoltaic system. In this study, a multi-physics model of a low concentration photovoltaics (LCPV) that employs an all-back-contact black silicon solar cell is built. As for the multi-physics problem in LCPV, a multi-physics coupling mathematical model is developed. Its advantage is to couple near-flied optics, photoelectric conversion, and heat transfer. Furthermore, through parameter sensitivity analysis, it is proved that the coupling mathematical model is more accurate, because it takes full account of the nonlinear correlation between the output power and the temperature, and the effects of series resistance under concentrating condition. Hence, the mathematical model is used to investigate the coupling effects of nanostructures, and it is found that the nanostructure with lower reflectance may not be beneficial for the low concentrated black silicon photovoltaic due to the increased temperature. According to different circumstances, the maximum output power with the lower cost could be achieved by choosing the nanostructure with appropriate reflectance. At last, the dynamic analysis proves that the coupling effects of nanostructure lead to the result that the nanostructure with lower reflectance reduces the annual power generation per square meter of the low concentrated black silicon photovoltaic system by about 116 kW h/m2 when the concentration ratio is 10.

Hybrid black silicon solar cells textured with the interplay of copper-induced galvanic displacement

Metal-assisted chemical etching (MaCE) has been widely employed for the fabrication of regular silicon (Si) nanowire arrays. These features were originated from the directional etching of Si preferentially along <100> orientations through the catalytic assistance of metals, which could be gold, silver, platinum or palladium. In this study, the dramatic modulation of etching profiles toward pyramidal architectures was undertaken by utilizing copper as catalysts through a facile one-step etching process, which paved the exceptional way on the texturization of Si for advanced photovoltaic applications. Detailed examinations of morphological evolutions, etching kinetics and formation mechanism were performed, validating the distinct etching model on Si contributed from cycling reactions of copper deposition and dissolution under a quasi-stable balance. In addition, impacts of surface texturization on the photovoltaic performance of organic/inorganic hybrid solar cells were revealed through the spatial characterizations of voltage fluctuations upon light mapping analysis. It was found that the pyramidal textures made by copper-induced cycling reactions exhibited the sound antireflection characteristics, and further achieved the leading conversion efficiency of 10.7%, approximately 1.8 times and beyond 1.2 times greater than that of untexturized and nanowire-based solar cells, respectively.

Performance investigation of black silicon solar cells with surface passivated by BiFeO3/ITO composite film

In order to prepare black silicon material with excellent optical absorption performance for solar cell application, a micro/nano bilayer-structure is formed on the surface of textured silicon wafer by a silver assisted chemical etching method. It is found that the deeper nanoholes could form as the etching time is longer, and the surface reflectivity is reduced obviously due to the increased time of photon reflection from the nanowires. The incident light reflectivity of the prepared black silicon is significantly reduced to 2.3%, showing obviously better optical reflectance characteristics than general monocrystalline silicon wafer, especially in a wavelength range of 300-830 nm. Considering the fact that a large number of carrier recombination centers is introduced into the nanostructured crystal silicon surface, BiFeO3/ITO composite film is coated on the surface of the black silicon solar cell by magnetron sputtering process to optimize the surface defect states and improve the cell performance. The experimental results show that the lengths of the nanowires are predominantly in a range of 180-320 nm for the prepared black silicon with micro/nano double-layer structure. The reflectivity of the incident light is below 5% in a wavelength range from 300 nm to 1000 nm, and reaches a maximal value at about 700 nm. The reflectance increases slightly as BiFeO3/ITO composite film is coated on the surface of black silicon solar cell, but it is still much lower than that of general monocrystalline silicon solar cell. The open circuit voltage and short circuit current density of the black silicon solar cell increase respectively from 0.61 V to 0.68 V and from 28.42 mA/cm2 to 34.57 mA/cm2 after it has been coated with BiFeO3/ITO composite film, and the photoelectric conversion efficiency of the cell increases from 13.3% to 16.8% accordingly. The improvement in performance of black silicon solar cell is mainly due to the promotion of effective separation of photogenerated carriers, thereby enhancing the spectral response of black silicon solar cell in the whole wavelength range. This indicates that the spontaneously polarized BiFeO3 film can play a better role in improving the surface properties of black silicon solar cell. On the other hand, for the BiFeO3 film deposited on the surface of black silicon, a spontaneous polarization positive electric field could be produced, pointing from the film surface to the inside of the solar cell. This polarization electric field could also act as part of built-in electric field to contribute the photoelectric transformation of the black silicon solar cell, leading to the open circuit voltage of cell increasing from 0.61 V to 0.68 V.

Research on Black Silicon Based on Surface Photo Voltage

The solar cell research has experienced more than half a century. The black silicon solar cells have the advantages of low cost, low reflectivity. Due to the very large surface state density of Black silicon solar cells, the surface passivation effect directly affects its efficiency. Since the Surface photo voltage technology is very sensitive to surface states, three kinds of black silicon solar cells: no surface passivation and SiO2 passivation were tested and analyzed by this technology. The electrical characteristics of the solar cells were tested. The passivation effects were studied using surface photo voltage technology. We found that the recombination in the bulk and on the surface dominated at shorter times and at longer times respectively from the transient photo voltage results. Surface photo voltage can reveal the generation and recombination mechanisms of light-induce charge carriers, it provides the theoretical guidance to improve the efficiency of the solar cells.

High-Performance Black Multicrystalline Silicon Solar Cells by a Highly Simplified Metal-Catalyzed Chemical Etching Method

A wet-chemical surface texturing technique, including a two-step metal-catalyzed chemical etching (MCCE) and an extra alkaline treatment, has been proven as an efficient way to fabricate high-efficiency black multicrystalline (mc) silicon solar cells, whereas it is limited by the production capacity and the cost cutting due to the complicated process. Here, we demonstrated that with careful control of the composition in etching solution, low-aspect-ratio bowl-like nanostructures with atomically smooth surfaces could be directly achieved by improved one-step MCCE and with no posttreatment, like alkali solution. The doublet surface texture of implementing this nanobowl structure upon the industrialized acidic-textured surface showed concurrent improvement in optical and electrical properties for realizing 18.23% efficiency mc-Si solar cells (156 mm × 156 mm), which is sufficiently higher than 17.7% of the solely acidic-textured cells in the same batch. The one-step MCCE method demonstrated in this study may provide a cost-effective way to manufacture high-performance mc-Si solar cells for the present photovoltaic industry.