Multicrystalline silicon solar cells with porous silicon emitter

A review of the application of porous silicon (PS) in multicrystalline silicon solar cell processes is given. The different PS formation processes, structural and optical properties of PS are discussed from the viewpoint of photovoltaics. Special attention is given to the use of PS as an antireflection coating in simplified processing schemes and for simple selective emitter processes as well as to its light trapping and surface passivating capabilities. The optimization of a PS selective emitter formation results in a 14.1% efficiency mc-Si cell processed without texturization, surface passivation or additional ARC deposition. The implementation of a PS selective emitter into an industrially compatible screenprinted solar cell process is made by both the chemical and electrochemical method of PS formation. Different kinds of multicrystalline silicon materials and solar cell processes are used. An efficiency of 13.2% is achieved on a 25 cm² mc-Si solar cell using the electrochemical technique while the efficiencies in between 12% and 13% are reached for very large (100–164 cm²) commercial mc-Si cells with a PS emitter formed by chemical method.     

Comparison of multicrystalline silicon surfaces after wet chemical etching and hydrogen plasma treatment: application to heterojunction solar cells

The modifications of the surface and subsurface properties of p-type multicrystalline silicon (mc-Si) after wet chemical etching and hydrogen plasma treatment were investigated. A simple heterojunction (HJ) solar cell structure consisting of front grids/ITO/(n)a-Si:H/(p)mc-Si/Al was used for investigating the conversion efficiency. It is found that the optimized wet chemical etching and cleaning processes as a last technological step before the deposition of the a-Si:H emitter are more favorable to HJ solar cells fabrication than the hydrogenation. Solar cells on p-type mc-Si were prepared without high-efficiency features (point contacts, back surface field). They exhibited efficiencies up to 13% for a cell area of 1 cm² and 12% for a cell area of 39 cm².     

Double porous silicon layer on multi-crystalline Si for photovoltaic application

Double porous silicon (d-PS) layers formed by acid chemical etching on a top surface of n⁺/p multi-crystalline silicon solar cells were investigated with the aim to improve the performance of standard screen-printed silicon solar cells. First a macro-porous layer is formed on mc-Si. The role of this layer is texturization of surface. Next, the cells have been manufactured using standard technology based on screen-printing metallization. Finally, a second mezo-porous layer in n⁺ emitter of cell has been produced. The role of this PS layer is to serve as an antireflection coating. In this way, we have obtained d-PS layers on these solar cells. The paper present observation of d-PS microstructure with SEM as well as measurements of its effective reflectance at the level of 2.5% in the 400–1000 nm length wave range. The efficiency of the solar cells with this structure is about 12%.     

Efficiency improved by acid texturization for multi-crystalline silicon solar cells

In this paper, we will show that efficiency of multi-crystalline silicon (mc-Si) solar cells may be improved by acid texturization. In order to enhance overall efficiency of mc-Si for solar-cell applications, the surface treatment of texturization with wet etching using appropriate solutions can improve incident light into the cell. Alkali etchant cannot produce uniformly textured surface to generate enough open circuit voltage (V_OC) and high efficiency of the mc-Si due to the unavoidable grain randomly oriented with higher steps formed during etching process. Optimized acid etching conditions can be obtained by decreasing the reflectance (R) for mc-Si substrate below levels generated by alkali etching. Short-circuit current (I_SC) measurements on acid textured cells reveal that current gain can be significantly enhanced by reducing reflection. The optimal acid etching ratio HF:HNO₃:H₂O = 15:1:2.5 with etching time of 60 s and lowering 42.7% of the R value can improve 112.4% of the conversion efficiency (η) of the developed solar cell. In order to obtain more detailed information of different defect region, high-resolution light beam induced current (LBIC) is applied to measure the internal quantum efficiency (IQE) and the lifetime of minority carriers. Thus, the acid texturing approach is instrumental to achieve high efficiency in mass production using relatively low-cost mc-Si as starting material with proper optimization of the fabrication steps.     

Large-area multicrystalline silicon solar cell fabrication using reactive ion etching (RIE)

Surface texturing of crystalline silicon wafer improves the conversion efficiency of solar cells by the enhancement in antireflection property and light trapping. Compared to antireflection coating, it is a more permanent and effective scheme. Wet texturing with the chemicals such as alkali (NaOH, KOH) or acid (HF, HNO₃, CH₃COOH) is too difficult for thinner wafer to apply due to a large amount of silicon loss. However, Plasma surface texturing using Reactive Ion Etching (RIE) can be effective in reducing the surface reflectance with low silicon loss. In this study, we have fabricated a large-area (156×156 mm) multicrystalline silicon (mc-Si) solar cell by mask less surface texturing using a SF₆/O₂ reactive ion etching. We have accomplished texturing with RIE by reducing silicon loss by almost half of that in wet texturing process. By optimizing the processing steps, we achieved conversion efficiency, open circuit voltage, short circuit current density, and fill factor as high as 16.1%, 619 mV, 33.5 mA/cm², and 77.7%, respectively. This study establishes that it is possible to fabricate the thin multicrystalline silicon solar cells of low cost and high efficiency using surface texturing by RIE.     

Ray-trace simulation of light trapping in silicon solar cell with texture structures

We investigated the light trapping effect in a solar cell. We performed ray-tracing simulation for a light trapping structure in a silicon crystalline solar cell. By comparing theoretical and experimental values, the reliability of a simulation technique was evaluated. Using this simulation technique, we evaluated the light trapping effect in the silicon crystalline solar cell and glass with a V-shaped texture. Furthermore, we investigated the light trapping effect in a silicon thin film solar cell. In a silicon thin film with a thickness of 20 μm deposited on V-shaped glass, reflectivity which is comparable to that in a pyramidal texture structure was obtained. We concluded that the simulation technique used in this work is very effective for optimization of the structure in the enhancement of the light trapping effect.     

High-efficient operation of large-area (100 cm) thin film polycrystalline silicon solar cell based on SOI structure

High-efficient operation of a large-area thin film polycrystalline Si solar cell with a novel structure based on a silicon on insulator (SOI) structure prepared by zone-melting recrystallization (ZMR) is reported. The (100) crystal orientation area over 90% has successfully been obtained by controlling the ZMR conditions, which allowed to form a uniform random pyramidal structure at the cell surface. The effect of hydrogen passivation has also been investigated for further improvement of the cell characteristics. By employing a light trapping structure (textured surface) and hydrogen passivation, an efficiency of 14.22% was obtained for a practical 100 cm² size.     

Large area multicrystalline silicon solar cells in high volume production environment—history, status, new processes, technology transfer issues

Cast multicrystalline silicon (mc-Si) solar cell technology, accounted for nearly 41% of all the PV modules manufactured worldwide in 2000. Since 1995 the use of cast mc-Si as a substrate has increased every year and that increase is expected to accelerate in the coming years as the PV market grows further. This impressive growth has been enabled by several factors—wafer suppliers, improvements in casting technology, sawing technology and cell process technology. In this paper the enabling factors will be discussed. The new processes used to enhance the efficiency of the cast multicrystalline silicon solar cells and the criteria for technology transfer will also be discussed.     

Large-size multi-crystalline silicon solar cells with honeycomb textured surface and point-contacted rear toward industrial production

In this paper, we present a multi-crystalline solar cell with hexagonally aligned hemispherical concaves, which is known as honeycomb textured structure, for an anti-reflecting structure. The emitter and the rear surface were passivated by silicon nitride, which is known as passivated emitter and rear (PERC) structure. The texture was fabricated by laser-patterning of silicon nitride film on a wafer and wet chemical etching of the wafer beneath the silicon nitride film through the patterned holes. This process succeeded in substituting the lithographic process usually used for fabricating honeycomb textured structure in small area. After the texturing process, solar cells were fabricated by utilizing conventional fabrication techniques, i.e. phosphorus diffusion in tube furnace, deposition of anti-reflection film and rear passivation film by chemical vapor deposition, front and rear electrodes formation by screen printing, and contact formation by furnace. By adding relatively small complicating process to conventional production process, conversion efficiency of 19.1% was achieved with mc-Si solar cells of over 200 cm² in size. The efficiency was independently confirmed by National Institute of Advanced Industrial Science and Technology (AIST).     

Large area 15.8% n-type mc-Si screen-printed solar cell with screen printed Al-alloyed emitter

A record efficiency of 15.8% (independently confirmed at Fraunhofer ISE calibration laboratory) is reported on large area (120 cm²) n-type mc-Si rear junction Si solar cell. Minor modifications to the industrial process for p-type, such as optimization of Al-alloyed screen-printed emitter and phosphorus front surface field, led to an improvement in cell properties. Large improvement in short-circuit current of the cell was possible by decreasing the cell thickness to 130 μm.     

多晶PERC太阳电池的背面与正面结构性能优化

多晶硅太阳电池以其价格低廉的优势成为低成本太阳电池的首选,但其光电转换效率提升空间有限。钝化发射极和背面电池（PERC）技术是当前晶硅太阳电池提效的主要途径。多晶PERC电池结合了多晶硅电池的低成本和PERC电池的高效,是当前多晶硅电池的研究热点。本文研究了多晶PERC电池的背面和正面结构优化与设计,提出了提高多晶PERC电池效率的产业化技术方法。通过在硅片背面用三层SiN_x:H薄膜来代替常规双层SiN_x:H薄膜,在保证优良的背面钝化的同时,使电池长波响应得到改善,电池光电转换效率由20.19% 提升至20.26%。优化多晶PERC电池的背面激光开窗工艺,使多晶电池效率较常规工艺提升0.11%。而在多晶PERC电池的正面叠加选择性发射极技术,可较常规工艺提升电池效率0.10%。综合运用多种提效手段有利于保持多晶PERC电池的竞争力。     

Maximum light trapping for the random back-reflective silicon film solar cell with omnidirectional light transmission characteristics

The light trapping characteristics in the wavelength range of 0.5−1.2 μm for the random back-reflective silicon film with omnidirectional top anti-reflection are numerically analyzed based on the simplified probability method. The spectrum averaged maximum external quantum efficiency (EQE) for the 5 μm thick silicon film is evaluated with an increase of 10.6% compared with the best bulk planar silicon solar cell—suggesting that an efficiency higher than those of the best bulk planar cell can be obtained for thin film silicon solar cells several microns thick. The light absorption curves drop slowly with increased back absorption, exhibiting that the performance of the thin film silicon solar cell with light trapping is tolerant of back absorption.     

A high efficiency thin film silicon solar cell and module

A photoelectric conversion efficiency of over 10% has been achieved in thin-film microcrystalline silicon solar cells which consist of a 2 μm thick layer of polycrystalline silicon. It was found that an adequate current can be extracted even from a thin film due to the very effective light trapping effect of silicon with a low absorption coefficient. As a result, this technology may eventually lead to the development of low-cost solar cells. Also, an initial aperture efficiency as high as 13.5% has been achieved with a large area (91 cm × 45 cm) tandem solar cell module of microcrystalline silicon and amorphous silicon (thin film Si hybrid solar cell). An even greater initial efficiency of 14.7% has been achieved in devices with a small size (area of 1 cm²), and further increases of efficiency can be expected.     

Enhancement of photovoltaic properties of multicrystalline silicon solar cells by combination of buried metallic contacts and thin porous silicon

Photovoltaic properties of buried metallic contacts (BMCs) with and without application of a front porous silicon (PS) layer on multicrystalline silicon (mc-Si) solar cells were investigated. A Chemical Vapor Etching (CVE) method was used to perform front PS layer and BMCs of mc-Si solar cells. Good electrical performance for the mc-Si solar cells was observed after combination of BMCs and thin PS films. As a result the current-voltage (I-V) characteristics and the internal quantum efficiency (IQE) were improved, and the effective minority carrier diffusion length (Ln) increases from 75 to 110 μm after BMCs achievement. The reflectivity was reduced to 8% in the 450-950 nm wavelength range. This simple and low cost technology induces a 12% conversion efficiency (surface area = 3.2 cm²). The obtained results indicate that the BMCs improve charge carrier collection while the PS layer passivates the front surface.     

Hybrid tandem solar cell for concurrently converting light and heat energy with utilization of full solar spectrum

A two-compartment hybrid tandem cell comprising a dye-sensitized solar cell as top cell and a thermoelectric cell as bottom cell has been developed to increase the overall photovoltaic conversion efficiency by utilization of full solar spectrum. The photovoltaic properties of the four-wire and two-wire hybrid tandem cells have been characterized and the working principle has been demonstrated using the electron energy band diagram. For two-wire hybrid tandem cells, the overall conversion efficiency can be improved by optimal designing DSC module in order to match the output current of the selected thermoelectric cell. Comparing with the individual dye-sensitized solar cell, an efficiency increase of 10% has been obtained for the hybrid tandem cell. The incident light intensity has no influence on the matching of the two compartments of the two-wire hybrid tandem cell.     

A hybrid amorphous silicon photovoltaic and thermal solar collector

A hybrid amorphous silicon (a-Si) photovoltaic and thermal solar collector was developed and its performance tested. The solar cells, deposited on glass panels and having an average efficiency of 4% and a total area of 0.9 m², were bonded to the fin and tube aluminum heat-exchange plate using simple technology. This hybrid unit performed well as a thermal solar collector, heating water up to 65°C, while the electric characteristics of the photovoltaic modules showed little change. In addition to saving space this integral unit substantially reduces the balance-of-system cost of the photovoltaic generator. The transmission of light through various layers of an a-Si cell was measured and, in order to improve the thermal efficiency, a novel transparent type of a-Si cell was developed and tested in the hybrid unit. The results obtained show that it is possible to construct simple and cheap hybrid systems having good photovoltaic as well as thermal efficiencies.     

单晶硅太阳电池发射极的模拟优化

针对传统p型衬底晶硅太阳电池,通过PCID数值计算,模拟了发射极扩散峰值浓度、方块电阻、结深等对电池性能的影响规律以及该规律与硅衬底电阻率之间的依赖关系,分析了其中所蕴含的作用机理.对于磷原子浓度梯度符合余误差分布的发射极,得到扩散制结的标准为:扩散峰值浓度介于1×10~(19)～5×10~(19)cm~(-3)之间,方块电阻在100Ω/□以上.尽管电池效率在衬底电阻率为1Ω·cm时最高,并随衬底电阻率的增大而明显下降,但上述发射极扩散标准基本保持不变.     

TCO and light trapping in silicon thin film solar cells

For thin film silicon solar cells and modules incorporating amorphous (a-Si:H) or microcrystalline (μc-Si:H) silicon as absorber materials, light trapping, i.e. increasing the path length of incoming light, plays a decisive role for device performance. This paper discusses ways to realize efficient light trapping schemes by using textured transparent conductive oxides (TCOs) as light scattering, highly conductive and transparent front contact in silicon p–i–n (superstrate) solar cells. Focus is on the concept of applying aluminum-doped zinc oxide (ZnO:Al) films, which are prepared by magnetron sputtering and subsequently textured by a wet-chemical etching step. The influence of electrical, optical and light scattering properties of the ZnO:Al front contact and the role of the back reflector are studied in experimentally prepared a-Si:H and μc-Si:H solar cells. Furthermore, a model is presented which allows to analyze optical losses in the individual layers of a solar cell structure. The model is applied to develop a roadmap for achieving a stable cell efficiency up to 15% in an amorphous/microcrystalline tandem cell. To realize this, necessary prerequisites are the incorporation of an efficient intermediate reflector between a-Si:H top and μc-Si:H bottom cell, the use of a front TCO with very low absorbance and ideal light scattering properties and a low-loss highly reflective back contact. Finally, the mid-frequency reactive sputtering technique is presented as a promising and potentially cost-effective way to up-scale the ZnO front contact preparation to industrial size substrate areas.     

Reactive ion etching (RIE) technique for application in crystalline silicon solar cells

Saw damage removal (SDR) and texturing by conventional wet chemical processes with alkali solution etch about 20 micron of silicon wafer on both sides, resulting in thin wafers with which solar cell processing is difficult. Reactive ion etching (RIE) for silicon surface texturing is very effective in reducing surface reflectance of thin crystalline silicon wafers by trapping the light of longer wavelength. High efficiency solar cells were fabricated during this study using optimized RIE. Saw damage removal (SDR) with acidic mixture followed by RIE-texturing showed the decrease in silicon loss by ∼67% and ∼70% compared to conventional SDR and texturing by alkaline solution. Also, the crystalline silicon solar cells fabricated by using RIE-texturing showed conversion efficiency as high as 16.7% and 16.1% compared with 16.2%, which was obtained in the case of the cell fabricated with SDR and texturing with NaOH solution.     

High-efficiency drift-field thin-film silicon solar cells grown on electronically inactive substrates

A drift-field in the base region of a solar cell can enhance the effective minority-carrier diffusion length, thus increasing the long-wavelength spectral response and energy-conversion efficiency. Silicon thin-films of 20–32 μm thickness as a cell base layer were grown by liquid-phase epitaxy (LPE) on electronically inactive heavily doped p⁺⁺-type CZ silicon substrates. Growth was performed from In/Ga solutions, and in a purified Ar/4%H₂ forming gas ambient, rather than pure H₂. The Ga dopant concentration was tailored throughout the p-type film to create a drift-field in the base layer of the solar cell. An independently confirmed efficiency of 16.4% was achieved on such an LPE drift-field thin-film silicon solar cell with a total cell area of 4.11 cm². Substrate thinning, combined with light trapping which is encouraged by the textured front surface and a highly reflective aluminium rear surface, is demonstrated to improve the long-wavelength response and thus, increase cell efficiency by a factor of up to 23.7% when thinned to a total cell thickness of 30 μm.