Use of porous silicon antireflection coating in multicrystallinesilicon solar cell processing

The latest results on the use of porous silicon (PS) as an antireflection coating (ARC) in simplified processing for multicrystalline silicon solar cells are presented. The optimization of a PS selective emitter formation results in a 14.1% efficiency multicrystalline (5×5 cm²) Si cell with evaporated contacts processed without texturization, surface passivation, or additional ARC deposition. Specific attention is given to the implementation of a PS ARC into an industrially compatible screen-printed solar cell process. Both the chemical and electrochemical PS ARC formation method are used in different solar cell processes, as well as on different multicrystalline silicon materials. Efficiencies between 12.1 and 13.2% are achieved on large-area (up to 164 cm² ) commercial Si solar cells     

Efficient light trapping in silicon solar cells by ultrafast‐laser‐induced self‐assembled micro/nano structures

A novel ultrafast laser processing technique is used to create self‐assembled micro/nano structures on a silicon surface for efficient light trapping. Under appropriate experimental conditions, light reflection (including scattering) of the Si surface has been reduced to less than 3% for the entire solar spectrum and the material appears completely black to the naked eye. A post‐chemical cleaning is applied to remove laser‐redeposited material and induced defects. Optical, morphological, and structural characterizations have been carried out on as‐laser‐treated and post‐chemically cleaned surfaces. Finally, we report for the first time the total efficiency of over 14% and high external quantum efficiency (EQE) results on photovoltaic devices fabricated on the ultrafast‐laser‐induced micro/nano structured silicon wafer, which can be further improved upon process optimization. Copyright © 2011 John Wiley & Sons, Ltd.     

Investigating the influence of the counter Si-cell on the optoelectronic performance of high-efficiency monolithically perovskites/silicon tandem cells under various optical sources

Nowadays, tandem structures have become a valuable competitor to conventional silicon solar cells, especially for perovskite over silicon, as metal halides surpassed Si with tunable bandgaps, high absorption coefficient, low deposition, and preparation costs. This led to a remarkable enhancement in the overall efficiency of the whole cell and its characteristics. Consequently, this expands the usage of photovoltaic technology in various fields of applications not only under conventional light source spectrum in outdoor areas, i.e., AM1.5G, but also under artificial light sources found indoors with broadband intensity values, such as Internet of things (IoTs) applications to name a few. We introduce a numerical model to analyze perovskite/Si tandem cells (PSSTCs) using both crystalline silicon (c-Si) and hydrogenated amorphous silicon (a-Si:H) experimentally validated as base cells. All proposed layers have been studied with J-V characteristics and energy band diagrams under AM1.5G by using SCAPS-1D software version 3.7.7. Thereupon, the proposed architectures were tested under various artificial lighting spectra. The proposed structures of Li4Ti5O12/CsPbCl3/MAPbBr3/CH3NH3PbI3/Si recorded a maximum power conversion efficiency (PCE) of 25.25% for c-Si and 17.02% for a-Si:H, with nearly 7% enhancement concerning the Si bare cell in both cases.     

Optimised antireflection coatings for planar silicon solar cells using remote PECVD silicon nitride and porous silicon dioxide

Silicon nitride (SiN) films fabricated by remote plasma‐enhanced chemical vapour deposition (RPECVD) have recently been shown to provide an excellent electronic passivation of silicon surfaces. This property, in combination with its large refractive index, makes RPECVD SiN an ideal candidate for a surface‐passivating antireflection coating on silicon solar cells. A major problem of these films, however, is the fact that the extinction coefficient increases with increasing refractive index. Hence, a careful optimisation of RPECVD SiN based antireflection coatings on silicon solar cells must consider the light absorption within the films. Optimal optical performance of silicon solar cells in air is obtained if the RPECVD SiN films are combined with a medium with a refractive index below 1·46, such as porous SiO₂. In this study, the dispersion of the refractive indices and the extinction coefficients of RPECVD SiN, porous SiO₂, and several other relevant materials (MgF₂, TiO_x, ZnS, B270 crown glass, soda lime glass, ethylene vinyl acetate and resin as used in commercial photovoltaic modules) are experimentally determined. Based on these data, the short‐circuit currents of planar silicon solar cells covered by RPECVD SiN and/or porous SiO₂ single‐ and multi‐layer antireflection coatings are numerically maximised for glass‐encapsulated as well as non‐encapsulated operating conditions. The porous SiO₂/RPECVD SiN‐based antireflection coatings optimised for these applications are shown to be universally suited for silicon solar cells, regardless of the internal blue or red response of the cells. Copyright © 1999 John Wiley & Sons, Ltd.     

Experimental testing of a random medium optical model of porous silicon for photovoltaic applications

We have developed a model for light propagation in porous silicon (PS) based on the theory of wave propagation in random media. The low porosity case is considered, with silicon being the host material assuming randomly distributed spherical voids as scattering particles. The specular and the diffuse part of the light could be determined and treated separately. The model is applied to the case in which porous silicon would be used as a diffuse back reflector in a thin‐film crystalline silicon solar cell realized in an ultrathin (1–3 μm) epitaxially grown Si layer on PS. Three layer structures (epi/PS/Si) have been fabricated by atmospheric pressure chemical vapor deposition (APCVD) of 150–1000 nm epitaxial silicon layers on silicon wafers of which 150–450 nm of the surface has been electrochemically etched. An excellent agreement is found between the experimentally measured reflection data in the 400–1000 nm wavelength range and those calculated using the proposed model. The values of the layer thickness agree, within a reasonable experimental error, with those obtained independently by cross sectional transmission electron microscopy (XTEM) analysis. This provides an experimental verification of the random medium approach to porous silicon in the low porosity case. The analysis shows that the epitaxial growth process has led to appreciable porosity decrease of an initially high porosity layer from about 60% to 20–30%. Copyright © 2001 John Wiley & Sons, Ltd.     

A simple analytical model of thin films crystalline silicon solar cell with quasi-monocrystalline porous silicon at the backside

An analytical model that simulates the performance of an elementary thin silicon solar cell with a thin film quasi-monocrystalline porous silicon (QMPS) at the backside reflector is developed. A complete set of equations for the photocurrent generated under the effect of the reflected light is solved analytically in each region. The collection of the light absorbed by the QMPS layer has been discussed and the analytical solution of the light-generated current in this layer is derived. The maximum of the photocurrent density calculated in the present study is in accordance with the numerical values established by Bergmann et al. Furthermore, the influence that the layer's number of double porosities and high porosity have on the photovoltaic parameters is studied. It is demonstrated that the photovoltaic parameters increase with the number of double porosities that the layer might have in a given structure. When the QMPS layer is formed by three double-porosity layers 20%/80% and for a 5-μm-thick film c-Si, the backside reflector gives a total improvement of about 6 mA/cm² for the photocurrent density and 3.2% for the cell efficiency.     

Photosensitive heterostructures based on porous nanocrystalline silicon

The features of the manufacturing process, as well as the results of studies on the morphology, electrophysical, and photoelectric properties of photosensitive structures based on silicon containing siliconcarbide and porous silicon layers, are considered. A porous layer is created on the surface of a monocrystalline silicon substrate via electrolytic etching in fluorine-containing solutions. Wafers with a different surface microrelief such as a ground, polished, and textured one, has been used. The carbonization of the samples resulting in the formation of SiC/Si heterostructures has been carried out via gas transport endotaxy in a hydrogen flow using a vertical reactor with cold walls and a graphite container. The structure and composition of the manufactured SiC/Si heterostructures formed on different types of structured surfaces on polycrystalline and monocrystalline silicon, including the surface porous silicon layer, are investigated. It is shown that the process of endotaxy on all the types of surfaces leads to the formation of a single-crystal phase of silicon carbide by cubic modification. Using scanning and transmission electron microscopy, the morphology of the produced structures is investigated. Filiform entities with a different structure have been revealed on nonporous surfaces identified as silicon carbide, whereas the cylindrical or conical structures, whose nature is uncertain, have been observed on porous surfaces. The current-voltage and current-power curves are plotted for all types of manufactured structures, the general form of which indicates the presence of several potential barriers there. The photoelectric properties of the structures and the prospects of their use in photoelectric converters of solar cells are analyzed.     

Characteristics of large‐scale nanohole arrays for thin‐silicon photovoltaics

Nanostructured crystalline silicon is promising for thin‐silicon photovoltaic devices because of reduced material usage and wafer quality constraint. This paper presents the optical and photovoltaic characteristics of silicon nanohole (SiNH) arrays fabricated using polystyrene nanosphere lithography and reactive‐ion etching (RIE) techniques for large‐area processes. A post‐RIE damage removal etching is subsequently introduced to mitigate the surface recombination issues and also suppress the surface reflection due to modifications in the nanohole sidewall profile, resulting in a 19% increase in the power conversion efficiency. We show that the damage removal etching treatment can effectively recover the carrier lifetime and dark current–voltage characteristics of SiNH solar cells to resemble the planar counterpart without RIE damages. Furthermore, the reflectance spectra exhibit broadband and omnidirectional anti‐reflective properties, where an AM1.5 G spectrum‐weighted reflectance achieves 4.7% for SiNH arrays. Finally, a three‐dimensional optical modeling has also been established to investigate the dimension and wafer thickness dependence of light absorption. We conclude that the SiNH arrays reveal great potential for efficient light harvesting in thin‐silicon photovoltaics with a 95% material reduction compared to a typical cell thickness of 200 µm. Copyright © 2012 John Wiley & Sons, Ltd.     

Simple analytical model and efficiency improvement of polysilicon solar cells with porous silicon at the backside

The present study developed a simple analytical model to simulate the performance of polysilicon solar cells with porous silicon (PS) layer at the backside. It analytically solved the complete set of equations necessary for the determination of the photocurrent generated under the effect of the reflected light. It also investigated the contribution of the light absorbed by the PS layer and explored the effect that the latter’s number of double porosities and high porosity have had on photovoltaic parameters. The findings suggest that the photovoltaic parameters increase with the number of double porosities that the layer might have in a given structure. When the PS layer is formed by three-double porosity layers 20%/80% and for a 5 μm-thick film c-Si, the backside reflector gives a total improvement of about 2.65 mA/cm² in photocurrent density and 1.4% in cell efficiency. This improvement can even be of much more important for well passivated grain boundaries and back contact of solar cells.     

Preparation and degradation of the optical properties of nano-, meso-, and macroporous silicon

In the work some peculiarities in formation of the porous silicon, its morphology and composition were investigated, as well as some optical properties of this material. Porous silicon was obtained on the substrates of single-crystalline Si as well as on the silicon structures with p–n junctions. In order to obtain nano-, meso- and macroporous silicon as well as multi-layered porous structures several technological parameters (were controlled) varied—orientation of the substrates, conductivity type and composition of the etchant. Correlation between photoluminescence intensity of the obtained samples and intensity of the absorption band in their IR spectra (the band at 616 cm⁻¹) that is due to the presence of Si–Si bonds in porous layer was observed. The influence of the porous silicon storage in the air on the degradation of its photoluminescence parameters was also estimated.     

多孔硅可见光发射研究

用常规电化学方法制备了发射高效可见光的多孔硅样品。对样品进行了光谱研究。用非化学方法从样品表面得到了粉末状荧光物质(非多孔硅膜研磨产物),光谱测定证实它的光致发光光谱与原多孔硅样品光致发光光谱相同,粉末进一步研磨后仍能发出同样的可见光。多孔硅样品还可以实现阴极射线激发发出同样的高效可见光,但易因电子束的轰击而发光强度较快减弱。用扫描电镜(SEM)对多孔硅样品的形貌、结构、荧光粉末的形状、尺寸、多孔硅样品阴极荧光发射区域进行了系统的研究。实验结果表明多孔硅样品一般可分为三层结构:表面层、多孔层和单晶硅衬底,样品荧光是来源于其表面层的。对样品表层组分的x射线光电子谱(XPS)分析表明,此时的多孔硅表层有大量非硅元素存在,如:C、O、F(没考虑H),硅元素的原子个数比只占30％～50％。用同样的电化学方法在单晶硅未抛光面上和多晶硅未抛光面上制得了均匀发射可见光样品。上述实验结果说明,多孔硅的高效可见光发射是来源于样品制备过程中在其表面层中形成的粉末状荧光物质。     

Textured monocrystalline thin‐film Si cells from the porous silicon (PSI) process

We have fabricated a textured monocrystalline Si solar cell with a thickness of 15·5 μm and a confirmed efficiency of 12·2% using porous silicon (PSI) for layer transfer. The PSI process avoids photolithography and high‐temperature oxidation. The cell has a surface that is textured with randomly positioned inverted pyramids for light trapping. The device does not yet fully exploit the light‐trapping capability of this film shape, owing to a small back‐surface reflectance. Copyright © 2001 John Wiley & Sons, Ltd.     

多孔硅纵向孔隙率与残余应力分布的研究

利用电化学方法在p型重掺杂单晶硅(100)基体上制备了多孔硅薄膜,通过质量计算法得到多孔硅的孔隙率,并研究了多孔硅孔隙率随腐蚀深度变化的规律。利用显微拉曼光谱技术对多孔硅纵切面上的残余应力进行了测量,结果表明,多孔硅的孔隙率随腐蚀时间/深度的增加有先增加后减少的趋势;多孔硅纵向上存在拉伸残余应力,拉伸应力的分布与纵切面上孔隙率的分布成正比,先增大,再减小;到达多孔硅与基体硅的界面处时,拉伸应力减小为零,靠近硅一侧,转变为压应力;残余应力的最大值出现在临近多孔硅表面以下的区域。这主要与多孔硅制备过程中孔内HF酸浓度的降低和硅/电解液表面的电偶层有关。     

Influence of substrate on the growth of microcrystalline silicon thin films deposited by plasma enhanced chemical vapor deposition

Microcrystalline silicon (μc-Si) thin films are widely used for silicon thin film solar cells, especially in the high performance tandem solar cells which comprise an amorphous silicon junction at the top and a μc-Si junction at the bottom. One of the major factors affecting the photovoltaic properties of μc-Si thin film solar cells of thin films is the quality of the μc-Si thin films. In this work, we investigated the effect of substrates on the crystallization characteristics and growth behaviors of μc-Si thin films grown by the plasma enhanced chemical vapor deposition method (PECVD), and found that substrates have a strong effect on the crystallization characteristics of μc-Si thin films. In addition, the growth rate of μc-Si thin films was also highly influenced by the substrates. Three types of substrates, quartz glass, single crystalline silicon and thermally oxidized single crystalline silicon, were used for growing μc-Si thin films from SiH₄/H₂ with a flow rate ratio 2:98 at different temperatures. Crystallization characteristics of these μc-Si thin films were studied by Raman scattering and X-ray diffraction techniques.     

Classification of electrical properties of porous silicon

The classification of electrical properties of porous silicon is performed on the basis of differences in the structure of this material and in the processes of formation of the regions depleted of charge carriers. It is shown that porous silicon as a class can be subclassified into four groups, each of which has a specific set of characteristic features. For each group, the most probable properties of metal/(porous silicon) and (porous silicon)/(single-crystalline silicon) junctions are described. It is shown that the diversity of electrical properties of porous silicon and its contacts with metallic electrode and silicon substrate brings about the experimentally observed wide set of characteristics of multilayer structures with porous silicon layers.     

Passivation of p-n junction in crystalline silicon by amorphous silicon

Hydrogenated amorphous silicon, a-Si:H, is shown to be an excellent passivant for crystalline silicon (c-Si) p-n junctions. A two-orders-of-magnitude reduction in reverse leakage current from that of a typical thermal oxide passivated junction is obtained. This is achieved through a lowering of the interface state density by hydrogenation of the c-Si surface. Superior bias-temperature stability of the passivated junctions also is observed. There is evidence that the hydrogen in the bulk of the a-Si:H can act as a hydrogen reservoir for rehydrogenation of the interface between c-Si and a-Si:H. Thermal stability of the a-Si:H is adequate for temperatures up to 500°C for 30 min, which is sufficient for most device-processing requirements. Above 550°C, significant dehydrogenation from both the interface and the bulk a-Si:H regions and an increase in leakage are observed. The passivation properties were assessed through studies of the current-voltage and current-temperature characteristics of the p-n junctions.     

Photoelectrochemical properties of porous silicon based novel photoelectrodes

Porous silicon interfaces have been modified with nitrided TiO₂ (TiON) nanoparticles to develop highly efficient photoelectrodes. Photoelectrodes were prepared by impregnating the electrochemically prepared porous silicon microchannels with titanium oxynitride. Photocatalytic measurements were carried out on titanium oxynitride particles in water‐methanol mixture and the results showed a dependence on the nitrogen concentration. Among the photoelectrodes used for photocurrent measurements, porous silicon impregnated with TiO₂ nitrided at 600 °C showed maximum photocurrent increase after exposure to sunlight‐type radiation. The enhancement in photocurrent was one order more for the porous silicon/titanium oxynitride hetero‐structure than that of polished silicon/titanium oxynitride hetero‐structure. Photoelectrodes thus prepared were found to have stable performance for a period of six months. This observation promises the possibility of using porous silicon/titanium oxynitride hetero‐structures as efficient electrodes for photovoltaic cells. Copyright © 2010 John Wiley & Sons, Ltd.     

Low-cost solar cells based on large-area unconventional silicon

Low-cost approaches to solar cell manufacture require the use of inexpensive low-grade nonsingle crystalline silicon. Earlier experimental results indicate that conventional polysilicon, as it is used as ingot for the single crystal growing process, leads to solar cells of poor photovoltaic performance. These problems were overcome by utilizing unconventional nonsingle crystalline silicon, which is characterized by controlled size and structure of the individual grains. With modified processing, optimized in respect to the unique structure of the material, large-area solar cells could be realized under production scheme methods. Cells exhibiting dimensions up to 11 cm × 11 cm were fabricated, AM0 efficiencies of 8 percent could be achieved corresponding to AM1 values exceeding 10 percent. On test samples of 2 cm × 2 cm area AM0 efficiency Of 12.5 percent (AM1 value equivalent 14.0 percent) could be reached. The new material together with the optimized processes offer potentials for significant cost reduction by virtue of their being applicable to volume production and to automated fabrication techniques.     

High frequency capacitance-voltage and conductance-voltage characteristics of d.c. sputter deposited a-carbon/silicon MIS structures

Dark and illuminated C–V and G–V characteristics of A1/a-carbon/silicon MIS structures have been measured in the frequency range of 10 kHz to 10 MHz. The a-carbon passivating layer is prepared by d.c. sputtering of a graphite target in Ar gas. There is no evidence of Fermi level pinning, and surface carrier inversion is clearly demonstrated. Electrical instabilities are noted and briefly discussed. The high frequency electrical behavior seems to be dominated by states in the dielectric rather than states at the interface. a-carbon appears to be a promising dielectric material for use in silicon solid state electronics.