Black silicon (BSi) represents a very active research area in renewable energy materials. The rise of BSi as a focus of study for its fundamental properties and potentially lucrative practical applications is shown by several recent results ranging from solar cells and light-emitting devices to antibacterial coatings and gas-sensors. In this paper, the common BSi fabrication techniques are first reviewed, including electrochemical HF etching, stain etching, metal-assisted chemical etching, reactive ion etching, laser irradiation and the molten salt Fray-Farthing-Chen-Cambridge (FFC-Cambridge) process. The utilization of BSi as an anti-reflection coating in solar cells is then critically examined and appraised, based upon strategies towards higher efficiency renewable solar energy modules. Methods of incorporating BSi in advanced solar cell architectures and the production of ultra-thin and flexible BSi wafers are also surveyed. Particular attention is given to routes leading to passivated BSi surfaces, which are essential for improving the electrical properties of any devices incorporating BSi, with a special focus on atomic layer deposition of Al2O3. Finally, three potential research directions worth exploring for practical solar cell applications are highlighted, namely, encapsulation effects, the development of micro-nano dual-scale BSi, and the incorporation of BSi into thin solar cells. It is intended that this paper will serve as a useful introduction to this novel material and its properties, and provide a general overview of recent progress in research currently being undertaken for renewable energy applications.

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Black silicon: fabrication methods, properties and solar energy applications

We fabricate and measure graded-index “black silicon” surfaces and find the underlying scaling law governing reflectance. Wet etching (100) silicon in HAuCl4, HF, and H2O2 produces Au nanoparticles that catalyze formation of a network of [100]-oriented nanopores. This network grades the near-surface optical constants and reduces reflectance to below 2% at wavelengths from 300 to 1000 nm. As the density-grade depth increases, reflectance decreases exponentially with a characteristic grade depth of about 1/8 the vacuum wavelength or half the wavelength in Si. Observation of Au nanoparticles at the ends of cylindrical nanopores confirms local catalytic action of moving Au nanoparticles.

/pdf/nanostructured-black-silicon-and-the-optical-reflectance-of-1qqog9or3j.pdf

Nanostructured black silicon and the optical reflectance of graded-density surfaces

Nonthermal plasmas have emerged as a viable synthesis technique for nanocrystal materials. Inherently solvent and ligand-free, nonthermal plasmas offer the ability to synthesize high purity nanocrystals of materials that require high synthesis temperatures. The nonequilibrium environment in nonthermal plasmas has a number of attractive attributes: energetic surface reactions selectively heat the nanoparticles to temperatures that can strongly exceed the gas temperature; charging of nanoparticles through plasma electrons reduces or eliminates nanoparticle agglomeration; and the large difference between the chemical potentials of the gaseous growth species and the species bound to the nanoparticle surfaces facilitates nanocrystal doping. This paper reviews the state of the art in nonthermal plasma synthesis of nanocrystals. It discusses the fundamentals of nanocrystal formation in plasmas, reviews practical implementations of plasma reactors, surveys the materials that have been produced with nonthermal pla...

Nonthermal Plasma Synthesis of Nanocrystals: Fundamental Principles, Materials, and Applications

Potential-induced degradation (PID) has received considerable attention in recent years due to its detrimental impact on photovoltaic (PV) module performance under field conditions. Both crystalline silicon (c-Si) and thin-film PV modules are susceptible to PID. While extensive studies have already been conducted in this area, the understanding of the PID phenomena is still incomplete and it remains a major problem in the PV industry. Herein, a critical review of the available literature is given to serve as a one-stop source for understanding the current status of PID research. This paper also aims to provide an overview of future research paths to address PID-related issues. This paper consists of three parts. In the first part, the modelling of leakage current paths in the module package is discussed. The PID mechanisms in both c-Si and thin-film PV modules are also comprehensively reviewed. The second part summarizes various test methods to evaluate PV modules for PID. The last part focuses on studies related to PID in the omnipresent p-type c-Si PV modules. The dependence of temperature, humidity and voltage on the progression of PID is examined. Preventive measures against PID at the cell, module and system levels are illustrated. Moreover, PID recovery in standard p-type c-Si PV modules is also studied. Most of the findings from p-type c-Si PV modules are also applicable to other PV module technologies.

https://pubs.rsc.org/en/content/articlepdf/2017/ee/c6ee02271e

Potential-induced degradation in photovoltaic modules: a critical review

https://www.cell.com/joule/pdfExtended/S2542-4351(20)30035-0

Blade-Coated Perovskites on Textured Silicon for 26%-Efficient Monolithic Perovskite/Silicon Tandem Solar Cells

We investigate the influence of the quality of precursor amorphous silicon (a-Si) films on the quality of flash-lamp-crystallized (FLC) polycrystalline Si (poly-Si) films by tuning the conditions of a-Si deposition by catalytic chemical vapor deposition. Electron spin resonance measurement reveals that the defect density of FLC poly-Si films is affected by the defect density of a-Si films, and FLC poly-Si films with lower defect density can be formed by using precursor a-Si films with lower defect density. The same tendency is also confirmed through μ-PCD measurement. Improvement in the characteristics of thin-film crystalline Si (c-Si) solar cells can be expected by using high-quality FLC poly-Si films formed from a-Si films with low defect density.

Improved quality of flash-lamp-crystallized polycrystalline silicon films by using low defect density Cat-CVD a-Si films

Ultrathin Al-doped Si oxide (SiO x ) layers were formed by a simple wet chemical treatment, and their hole-selective passivating contact and electrical properties were investigated. From the evaluated contact resistivity (ρ c) and saturation current density (J 0), carrier selectivity (S 10) was estimated to be 13.3. Moreover, in Si nitride (SiN y )/Al-doped SiO x stacks, negative values of fixed charge density (Q f) were obtained, despite a high positive Q f existing in the single SiN y layer. This result implies that Al-doped SiO x has high negative fixed charges and overcompensates the charge polarity in the stacks, which forms an inversion layer and accumulates holes on the Si surface. Furthermore, the negative fixed charges realize excellent carrier separation by the induced upward band bending. In addition, we proposed a novel device architecture named Al-induced charged oxide inversion layer solar cells and confirmed device operation in a simple device configuration.

Ultrathin Al-doped SiO x passivating hole-selective contacts formed by a simple wet process

We have investigated carrier transport properties of μm-order-thick polycrystalline silicon (poly-Si) films formed by flash lamp annealing (FLA) of precursor amorphous Si (a-Si) films on glass substrates. The Hall mobility of flash-lamp-crystallized (FLC) poly-Si films decreases as doping concentration increases, and then reversely increase with further increase in doping concentration, both in the cases of p- and n-type poly-Si films. The tendency observed is characteristic of poly-Si materials having a number of grain boundaries at which carriers are trapped. N 2 -atmosphere furnace annealing of FLC poly-Si films with low doping concentration significantly recovers their carrier concentration and improve their carrier mobility up to more than 10 cm2/Vs, which is probably because of the termination of the trapping states by hydrogen (H) atoms that exists in FLC poly-Si films on the order of 1021 /cm3. The realized mobility of FLC poly-Si films exceeds those of conventional chemical-vapor-deposited (CVD) microcrystalline Si (μc-Si) films, and the remarkable carrier transport properties would lead to effective carrier collection and resulting high quantum efficiency of FLC poly-Si solar cells.

Carrier transport properties of flash-lamp-crystallized poly-Si films

We investigated the long-term durability of our newly developed encapsulant-less p-type crystalline silicon (c-Si) photovoltaic (PV) modules, with a base made of polycarbonate (PC), against potential-induced degradation (PID) in dry and damp-heat (DH) environments. Encapsulant-less modules were found to have high PID resistance compared to conventionally encapsulated c-Si PV modules in both PID conditions. We observed a slight PID for the encapsulant-less modules in which the cover glass was in contact with the solar cell. The slight PID can be suppressed by using a base with a deeper groove so that a sufficient gap between the cover glass and the cell is prepared. Yellow precipitates were formed in the encapsulant-less modules in the DH environment. This is probably due to the hydrolysis of the PC, and proper measures to prevent the precipitate formation should be applied for the industrialization of the encapsulant-less modules.

Potential-induced degradation of encapsulant-less p-type crystalline Si photovoltaic modules

This is to study on the feasibility of photo-carrier generation at amorphous-silicon (a-Si) insertion layer in silicon-nitride (SiNx)/a-Si stacked passivation system, which realizes extremely low surface recombination velocity (SRV). When the thickness of crystalline silicon (c-Si) becomes thinner than 100 ixm for reducing cell cost, the method collecting 100% sun light is to be very important. If a-Si or a-Si-compounds on c-Si in hetero-structure can absorb sun light enough and if the photo-carriers generated inside such a-Si or a-Si compounds are effectively transferred to c-Si and used as currents of c-Si solar cells, the formation of hetero-structure has an advantage for thinning c-Si substrates. In the present research, the ratio of photo-generated carriers in a-Si to the carriers transferred to c-Si is experimentally studied in SiNx/a-Si stacked passivation system, which is expected to be used as surface passivation of back-contact c-Si solar cells. We compared the SiNx/a-Si/c-Si system prepared by the plasma-enhanced chemical vapor deposition (PECVD) with that by catalytic chemical vapor deposition (Cat-CVD), often called “Hot-Wire CVD”. It is found that the a-Si/c-Si interface prepared by Cat-CVD is much smoother than that by PECVD, indicating better quality of Cat-CVD a-Si/c-Si interface and that almost 100% photo-carriers generated in Cat-CVD a-Si can be transferred into c-Si through the interface when the a-Si thickness is less than 15 nm. This demonstrates the feasibility as power generator for Cat-CVD a-Si in a-Si/c-Si hetero-structure.

Keisuke Ohdaira

Papers

Improved quality of flash-lamp-crystallized polycrystalline silicon films by using low defect density Cat-CVD a-Si films

Ultrathin Al-doped SiO x passivating hole-selective contacts formed by a simple wet process

Carrier transport properties of flash-lamp-crystallized poly-Si films

Potential-induced degradation of encapsulant-less p-type crystalline Si photovoltaic modules

Photo-carrier generation at a-Si layer in SiNx/a-Si stacked passivation with extremely low surface recombination velocity