Handbook of Photovoltaic Silicon

The purity of polysilicon is usually between 6 N (99.9999%) and 9 N (99.9999999%). This chapter describes the test methods for measuring physical characteristics as well as quantification of elemental impurities in polysilicon materials. Float zone (FZ) process is an important method for converting granular polysilicon and polycrystalline chunk materials to monocrystalline silicon. A monocrystalline silicon rod is used to test the resistivity (n-type or p-type), minority carrier lifetime, carbon, oxygen, donors, and acceptor impurities in the polysilicon materials. Due to technological advancement, the analytical instrument detection limit (IDL) has improved in recent years allowing parts per billion atomic (ppba) to parts per trillion atomic (ppta) impurity detection in polysilicon. Donors (P, As, Sb), acceptors (B, Al), carbon, and oxygen can be measured by low-temperature FT-IR. The concentration of bulk and surface metal impurities (iron, chromium, nickel, copper, zinc, etc.) can be measured using ICP-MS.

Polysilicon is the elementary raw material for integrated circuits and photovoltaic products. In recent years, the technology of polysilicon made great progress, its cost dramatically reduced, and it provided ideal conditions for photovoltaic energy power to meet the state-set price. This chapter summarizes the achievements of Siemens process, including the technology process, the technical principle, main equipment, technological operations, polysilicon quality control, energy-saving feature, and cost reduction. This could be used as a reference for the production of polysilicon.

Silane-based fluidized-bed technology is used to produce polysilicon for solar cells by decomposing silane onto silicon particles suspended in a heated stream of silane and hydrogen. Silane-based fluidized-bed reactors potentially provide a lower cost method to produce polysilicon than the current Siemens reactors that dominate the silicon market. Production of silicon in a fluidized bed requires 80–90% less electrical energy than the currently favored Siemens process and converts a batch process into a more economical continuous process. The spherical granular silicon product from fluidized-bed reactors is preferred to the polysilicon rods produced by the Siemens process for downstream processing. Production of silicon by fluidized beds has been carried on for over 20 years, but the simpler Siemens process has dominated polysilicon production because of the high purity of its polysilicon product and the availability of low-cost electricity. The economics of the silane-based fluidized-bed technology has improved significantly due to advances in reactor design, process modeling, and operational experience. Fluidized-bed technology is the leading candidate to eventually provide less expensive polysilicon for solar cells.

Producing upgraded metallurgical-grade silicon may be the most promising method to replace the modified Siemens process to produce solar-grade silicon, which has many advantages such as no pollution and low cost in the production process. The main process is described in detail, including smelting and secondary refining of metallurgical-grade silicon and acid leaching treatment. Other technologies also are introduced and discussed: solvent refining, vacuum treatment, plasma refining, and electron beam treatment; these technologies are used to refine metallurgical-grade silicon. Purification is also very important in the production process; a solidification process is used in purification to obtain highly pure silicon.

Czochralski (CZ) silicon is widely used in the fabrication of high-efficiency solar cells in photovoltaic industry. It requires strict control of defects and impurities, which are harmful for the performances of solar cells. Therefore, the CZ silicon crystal growth aims at achieving defect-free single crystals for advanced solar cell wafers. Meanwhile, attention must be paid to the low cost of CZ silicon crystal growth. Therefore, it is necessary to develop novel crystal growth techniques suitable for practical application of photovoltaics. This chapter will review the fundamentals of CZ silicon and recent developments. The oxygen-related defects and control technologies are emphasized. Meanwhile, the novel crystal growth methods are introduced. The Ge doping in CZ silicon can not only improve the material’s mechanical strength, but also suppress the generation of boron–oxygen complexes. This will enable thinner solar cells at reduced cost and benefit the fabrication of high efficiency solar cells with low light-induced degradation effects.

The emergence of high-performance multicrystalline silicon (HP mc-Si) in 2011 has made a significant impact to photovoltaic (PV) industry. In addition to the much better ingot uniformity and production yield, HP mc-Si also has better material quality for solar cells. As a result, the average efficiency of solar cells made from HP mc-Si in production increased from 16.6% in 2011 to 18.5% or beyond in 2016. With an advanced cell structure, an average efficiency of more than 20% has also been reported. More importantly, the efficiency distribution became much narrower; the difference even from various wafer producers became smaller as well. Unlike the conventional way of having large grains and electrically inactive twin boundaries, the crystal growth of HP mc-Si by directional solidification is initiated from uniform small grains having a high fraction of random grain boundaries (GBs). The grains developed from such grain structures significantly relax thermal stress and suppress the massive generation and propagation of dislocation clusters. The gettering efficacy of HP mc-Si is also superior to the conventional one, which also increases solar cell efficiency. Nowadays, most of commercial mc-Si is grown by this approach, which could be implemented by either seeded with silicon particles or controlled nucleation, e.g., through nucleation agent coating. The future improvement of this technology is also discussed in this chapter.

This chapter introduces the dendritic cast method which allows us to obtain multicrystalline silicon ingot containing large-size crystal grains with specific orientations. The growth of dendrite crystals along the bottom wall of crucible in the initial stage of casting is crucial in this method. First, the features of Si dendrite crystals including the conditions for initiating dendrite growth will be explained following the concept of the dendritic cast method. The parallel twin formation and undercooling, those are prerequisites for the growth of the dendrite crystal, will be considered fundamentally. Next, experimental results of the growth of multicrystalline silicon ingots by the dendritic cast method will be summarized. The idea to control the dendrite growth will be described. Finally, problems and the future development of this method will be considered. A nonwetting dendritic cast method, which is a growth concept for the reduction of generation of dislocation and impurity during casting, will be introduced.

The mono-like method, also known as the mono cast, seed cast, and quasi-mono methods, is a candidate next-generation method of casting Si ingots for solar cell applications, replacing conventional casting methods. The mono-like method provides single crystalline Si ingots with the use of almost the same facilities as those used for growth of multicrystalline Si ingots. Hence, the mono-like method has potential to achieve Si ingots with both high quality and low cost. However, the mono-like method faces challenges owing to its crystal growth processes, such as multicrystallization, dislocation generation, and impurity contamination. To address these problems, advanced mono-like methods have been developed. In this chapter, advanced mono-like methods are reviewed from the viewpoint of crystal growth and the fundamentals of the mono-like method.

The noncontact crucible (NOC) method has the potential to be an advanced cast method. It is effective in obtaining Si single ingots with large diameter and volume using cast furnace, and solar cells manufactured with Si obtained this way have high yield and high conversion efficiency. Several novel characteristics of this method are explained based on the existence of a large low-temperature region in a Si melt, which is key to realize its enclosing potential as follows. The largest diameter ratio of 0.9 was obtained by expanding the low-temperature region in the Si melt. For p-type solar cells, the highest of 19.14% and the average conversion efficiencies of 19.0% were obtained for the NOC wafers, using the same solar cell structure and process to obtain the conversion efficiency of 19.1% for a p-type Czochralski (CZ) wafers. The present method realized solar cells with conversion efficiency and yield as high as those of CZ solar cells using cast furnace for the first time. The latest information about the growth of Si ingots using the NOC method is explained.

The fabrication of silicon wafers for solar cells and modules is an expensive step in the processing chain. The technological development is therefore primarily driven by the need to reduce cost. The dominant wafering method is multi-wire sawing with a straight steel wire and an abrasive slurry consisting of polyethylene glycols (PEG) and SiC powders (loose abrasive sawing). Substantial cost reductions are possible with structured steel wires or wires coated with diamond particles (fixed abrasive sawing) and the replacement of PEG by water-based fluids. Apart from the cost, the wafer qualities such as thickness variations, roughness, subsurface saw damage, and fracture stability play an important role and have to be improved as well. These factors depend on many sawing parameters, which makes optimization a difficult task. The chapter describes the requirements on the sawing machines, the wires, the slurries, the wafer quality, and the experimental methods, which have been developed to characterize wafers and the consumables. The fundamental micromechanical sawing processes and models are also described. Their knowledge is helpful to improve the sawing process in a controlled way. Alternative wafering methods and their perspectives are presented briefly.

Wafer preparation for silicon PV includes wet chemical cleaning, etching, and texturization steps. Aqueous solutions containing either acids or strong bases result in very different etch rates. Underlying chemistry is used for all three applications. Typical cleaning mixtures such as RCA-SC1 and RCA-SC2, SPM, and dHF are introduced with their respective properties as well as acidic etching systems like hydrofluoric acid/nitric acid (HF/HNO₃) and alkaline mixtures such as potassium hydroxide/isopropanol (KOH/IPA). While the latter is used for texturing monocrystalline wafers due to the anisotropic etching behavior, the former HF-containing systems are generally used for isotropically texturing multicrystalline silicon wafers. Fundamental chemical, physical, thermodynamic, and kinetic aspects of these systems are presented and discussed. In all cases, it has to be pointed out that a complete understanding of the reaction mechanisms causing the observed properties is still missing to a large extent. Therefore, many aspects of silicon cleaning, etching, and texturization have only been optimized empirically. Further studies are necessary to provide a basis for future improvements, which are not only focused on scientific aspects but also on environmental and economic issues.

The technological development of silicon wafer processing for solar cells by multiwire sawing is mainly driven by the need to reduce cost but under the condition to maintain or even improve the wafer quality. This additional requirement becomes even more important because wafer and wire thickness will decrease in the future and the standard loose abrasive sawing technique will be replaced by the fixed abrasive sawing technique.

The essential quality parameters for wafers are total thickness variations (TTV), roughness and grooves, subsurface damage, and fracture strength stability. These factors depend on the properties of wires, consumables, and machine sawing parameters. Their investigation and determination require adequate characterization methods. The chapter describes standard and new methods, which have been developed to characterize wafers, wires, and consumables. Today’s optimization processes also require a basic understanding of the interaction processes between wires, sawing fluid, and the silicon material. Special experimental methods, which have been developed to investigate the fundamental micromechanical processes and their ramifications on the wafer quality parameters, are presented.

Oxygen belongs to the most important impurities in many types of solar silicon. Interstitial oxygen is already incorporated in a supersaturated state during crystal growth. Subsequent thermal treatment during solar cell manufacturing leads to its precipitation which degrades the lifetime of minority carriers in solar cell material and also the solar cell efficiency. Oxide precipitate nuclei are formed already during crystal cooling. A special form of oxygen precipitation is the generation of thermal donors which enhance the free carrier concentration and in this way degrade the lifetime of minority carriers. Controlling of oxygen precipitation for optimization of solar cell efficiency involves controlling of all important factors affecting the nucleation and growth of oxygen-related defects. Therefore, this chapter deals with the basic understanding of oxygen precipitation and thermal donor formation, its characterization and measurement, and its impact on solar cell material and solar cell performance. The interaction of intrinsic point defects, light elements, and dopants with oxygen and its impact on precipitation is also discussed.

Carbon impurity contamination during growth of crystalline silicon has been systematically studied in a representative unidirectional furnace. Mechanism of carbon incorporation into silicon has been illuminated. To better understand the carbon contamination process, a global simulation in a unidirectional solidification furnace was implemented. The effects of flow rate and pressure on impurities were examined.

To reduce carbon contamination, an improved unidirectional solidification furnace with a crucible cover was designed. Results show that this improvement enables the production of a high-purity multicrystalline silicon crystal in a unidirectional solidification furnace. In addition, the material of crucible cover has a great influence on carbon contamination. Another possible contamination mechanism due to the reaction between silica crucible and the graphite susceptor has also been given. Results show that the crucible reaction with graphite susceptor has a marked effect on carbon impurity in the crystal.

This chapter starts with the basic features of nitrogen including the existence of N-related defects, detection and measurements of N content, the solubility, and diffusion of N impurities in silicon materials. From the perspective of photovoltaic application, the nitrogen doping method for Czochralski silicon is then introduced, and the results about the influence of nitrogen impurity on N-O complexes, O-related defects, and mechanical properties are presented. A second focus of this chapter is toward N-related defects in directionally solidified photovoltaic multicrystalline silicon (mc-Si) materials. The existence and distribution of N-related defects, the formation, and influence of silicon nitride precipitates in mc-Si are comprehensively described. Then the results about mc-Si growth in ambient nitrogen are presented as an application to further understand the properties of nitrogen in mc-Si.

Solar-grade Si usually contains a considerable amount of metal impurities and extended defects. The detrimental effect of transition metals on solar cell performance is well documented. This necessitates a study of transition metal properties in Si and their interaction with point and extended defects. Such studies will form the basis for the development of procedures on suppressing the detrimental effects of transition metals. This chapter briefly describes the solubilities, diffusivities, and electrical properties of metal impurities that are most frequently detected in multicrystalline silicon. The diffusion length values necessary for highly efficient crystalline silicon solar cells, which set a limit on the electrically active metal concentration, are estimated. The effects of dislocations and grain boundaries on the effective diffusion length are evaluated. Finally, the efficiency of gettering and hydrogen passivation procedures used in silicon solar cell technology is considered.

Current understanding of various properties of dislocations in Si is given comprehensively for photovoltaic applications. Dislocations cause spatial variations in the electrical and optical properties of semiconductor materials and also the degradation of various kinds of semiconductor devices. Thus, establishing knowledge on mechanical properties of dislocations and also interactions between dislocations and impurities is important from both the fundamental and practical viewpoints for development of semiconductor technology. Indeed, the knowledge is widely applied as the basis for dislocation-free crystal growth and device fabrication process in Si.

Directionally solidified (DS) silicon is typically multicrystalline (mc), i.e., it contains per definition grain boundaries. Even so-called quasi-mono silicon is not free of grain boundaries. The crystallographic arrangement of neighboring grains is used for a definition of the certain types of grain boundaries by the so-called coincidence site lattice parameter Σ. It turns out that the predominant types of grain boundaries are twin (Σ = 3), small angle (Σ ~ 1), and large angle (“random”) grain boundaries. For the solar cell application, it is of great relevance that the nontwin boundaries are often accompanied by dislocation defects. These dislocations, especially their clusters, are well known to reduce the minority charge carrier lifetime and hence the efficiency of solar cells. Therefore, the corresponding characterization methods for the types of grain boundaries, their length, spatial distribution, and grain size will be presented in this chapter.

The main part of the chapter presents a detailed treatment of the occurrence of the various types of grain boundaries and the related dislocations structures for different variants of the DS method. The most important DS variants differ from each other mainly by the seeding and nucleation processes which result in different sizes of the grains and also different prevailing grain boundaries. The so-called classic mc, dendritic mc, and quasi-mono Si material have relatively large average grain sizes ranging from mm up to cm. The solar cell performance of this material is mainly limited by the occurrence of dislocation structures which can easily spread in the relatively large grains. This problem seems to be decreased in a recently developed fine grained material (micro-meter up to mm scale). The variety of nucleation concepts to achieve a fine grained structure reaches from seeding with small Si feed or non-Si particles to specially structured profiles of the crucible bottom. The resulting higher performance of solar cells is promising for the future and gave reason to call the material high performance mc Si (HPM).

The whole chapter includes results of recent worldwide research and development activities but provides also its proving under production-like conditions. All results are illustrated by corresponding figures and allocated to important references.

From solar cell application point of view, this chapter reviews the aspects of hydrogenated amorphous silicon (a-Si:H) based materials. Spear and LeComer made the first a-Si:H films with glow discharge by decomposing hydrogen containing gases such as SiH₄, in which hydrogen atoms terminate the Si dangling bonds and reduce the defect density significantly. The reduction of defect density lead to the possibility of doping a-Si:H to form N-type and P-type films and made it possible to make p-n junction, which is the foundation of thin film silicon-based electronic devices such as solar cells and thin film transistors. Since then several methods have been developed to deposit a-Si:H materials and devices. From solar cells point of view, a-Si:H is one of the useful materials with several unique properties: (1) high absorption coefficients in the visible range allowing to use a thin absorber layer, (2) large area uniform deposition for low-cost mass productions, and (3) deposition on various foreign substrates for rigid and flexible solar modules. The main disadvantage of a-Si:H is the lower carriers mobility and lifetime than the crystal copartners, which result in the lower energy conversation efficiency in a-Si:H solar cells than c-Si solar cells. In addition, the so-called Staebler-Wronski effect causes a light-induced degradation in a-Si:H solar cell efficiency. Over the years, a-Si:H science and technology have been very well developed driving by the applications in thin film silicon photovoltaic (PV) solar energy and large area display industries. In the PV applications, a-Si:H solar cell has been one of the most important technologies for the so-called second-generation PV solar energy. However, with the significant cost reduction of poly-Si and c-Si solar panels in the last few years, a-Si:H PV industry has been shrunk to the sideline and can only provide products in the niche market. Fortunately, new application as the surface passivation layer on c-Si to form a-Si:H/c-Si heterojunction solar cells gives a-Si:H a new life in the PV industry.

We review μc-Si:H material’s properties, deposition techniques, and applications in solar cells. The focus is on the issues that limit μc-Si:H solar cell performance. Because of unintentional impurity incorporation, μc-Si:H shows an N-type conductivity, especially in the early days, μc-Si:H was used only as the doped layers. Meier et al. used VHF-PECVD technique with reduced impurity levels and made first μc-Si:H solar cell in 1994. Since then the research and development of μc-Si:H solar cells have attracted a great attention, and a significant progress has been made for using μc-Si:H as the bottom cell in multijunction structures. Compared to a-Si:H, μc-Si:H structure is much complicated with nanometer sized grains, grain boundaries, amorphous tissues, and microvoids. The complexity in the structure leads to complicated material properties, such as electronic structure, optical absorption, carrier transport, and stability. The material’s property affects the solar cell performance significantly. The material structure changes with the film thickness under a constant deposition condition, which influences the solar cell performance. For solar cell applications, it has been found that the best μc-Si:H material should be compact with a low defect density and a crystalline volume fraction around 50% in the whole absorber layer. In order to obtain a high photocurrent density, an effective light trapping is achieved using textured substrates. However, a high textured substrate causes a degradation of material quality. A thick μc-Si:H up to 5 μm requires a high rate deposition. Here, we will discuss the techniques for resolving the μc-Si:H issues and the approaches for achieving the high quality materials and high efficiency μc-Si:H solar cells.

By eliminating the costly steps of Si wafer, polycrystalline silicon (poly-Si) thin film solar cells become the very promising candidates for cost-effective photovoltaics in the future. In order to maintain the high efficiency character of crystalline silicon (c-Si) wafer-based solar cells, competitive material qualities and appropriate device structures are required for poly-Si thin film solar cells on inexpensive substrates. Low cost fabrication processes are also demanded from the point of view of industrial production.

In the past few decades, a wide variety of poly-Si thin film solar cell approaches have been investigated to improve device performance and to identify suitable technology to boost poly-Si thin film solar cells towards competitive photovoltaic devices. The efficiencies of poly-Si thin film solar cells increase gradually. However, they are still much lower than that of c-Si solar cells or other compound semiconductor thin film solar cells. More efforts are needed in the future.

This chapter reviews the technological and scientific developments in the field of poly-Si thin films and solar cells. After an introduction, basic knowledge involved in the fabrication of poly-Si thin films is presented in the first part. In the second part, seed layer and transfer techniques for poly-Si thin film solar cells are described. In the third part, suitable light trapping technology is discussed. In the fourth part, material characterization techniques and properties of poly-Si thin films are shown. In the final part, the developing status of poly-Si thin film solar cells is summarized.

Thin-film solar cell technology based on nanocrystalline silicon has made a significant progress since the production of the first hydrogenated nanocrystalline silicon (nc-Si:H) solar cell in 1994. Up to date, the highest conversion efficiency of single-junction nc-Si:H thin-film solar cells has reached 11.8%, and further progress is expected. In this chapter, we aim to outline the progress, trends, and major approaches to enhance the nanocrystalline silicon solar cell technology and achieve considerably higher efficiency numbers. Comprising of two parts, this chapter in its first part describes the fundamentals of nanocrystalline silicon properties, typical fabrication methods, and technologies for solar cells as well as recent progress in nc-Si:H fabrications and properties. The second part states the recent advanced technologies for efficiency improvement and provides an overview of the significant achievements, current status, and future prospects of the thin-film solar cells based on nc-Si:H. In particular, the highest reported open-circuit voltage of 608 mV has been demonstrated in a 650-nm-thick single-junction nc-Si:H solar cell by applying amorphous silicon passivation layers at the n/i interface. Besides, the multijunction solar cell technique has significantly contributed to the improvement of the conversion efficiency. In particular, the best triple-junction tandem solar cells reach an initial active-area efficiency of about 16% in the a-Si:H/a-SiGe:H/nc-Si:H structure. Such an impressive success in light management paves the way to the application of nanocrystalline silicon for many vital fields such as novel texture structures, window layers, intermediate reflectors for the improvements of photocurrent density, and conversion efficiency.

Nanocrsytalline Silicon (nc-Si) is a promising material to develop the next generation of solar cells since it can efficiently absorb the incident solar light in a wide spectral range via the size modulation. The idea of all Si-based tandem type solar cells containing the sub-cells with various bandgaps motivates the extensive studies on the synthesis, physical properties, as well as the device applications of nc-Si material. Currently, the fabrication of size-controllable nc-Si is one of the challenging issues and the utilization of nc-Si in actual photovoltaic device is still at the stage of exploration. In this chapter, we describe the preparation of size-controllable nc-Si dots in multilayer by using thermally annealing or laser crystallization technique to crystallize amorphous Si/SiO₂ or amorphous Si/SiC stacked structures. It is shown that the dot size can be well confined with the initial amorphous Si layer thickness and the size-dependent properties are observed. The chapter focuses on the utilization of prepared nc-Si-based multilayers in prototype hetero-junction solar cells. The photovoltaic properties with different dot size, surrounding insulator materials, as well as the novel grade-sized structures are present. Furthermore, the efforts to improve the device performance by combining light trapping structures with nc-Si-based multilayers are introduced.

Nanostructured silicon (Si) materials are exciting new building blocks for Si-based photovoltaics to achieve a stronger light trapping, absorption, and antireflection with the least material consumption. Constructed upon Si nanowires (NWs), a novel 3D radial junction solar cell architecture decouples the optical absorption thickness from the electric distance that photocarriers need to travel to be collected. This allows a radical reduction of the absorber layer thickness that will benefit a fast photo-carriers separation and extraction. In addition, the light incoupling and absorption distribution among the antenna-like radial junction units can be largely enhanced by the resonant modes in the nanostructured photonic cavities, which necessitates a set of new theoretical models and high-precision simulation capabilities to address and predict the photovoltaic performance of the radial junction units, as a key basis for seeking optimal structural design. Recent progress in radial junction solar cells has accomplished a device performance comparable or even superior to their planar counterparts, with still plenty of room for further improvement. This chapter starts with a presentation of hydrogenated polymorphous silicon, a nanostructured material with enhanced optoelectronic properties with respect to hydrogenated amorphous silicon, and then continues with a review on the major fabrication strategies, growth theories, and key technologies involved in developing a new generation of high performance and low cost Si solar cells, with a particular focus on the radial junction thin film solar cells fabricated upon SiNWs grown via a plasma-assisted low temperature vapor-liquid-solid procedure. Critical issues, such as the geometry, density, and doping control in Si nanowires and the radial junction deposition and optimization, will be addressed in a systematical but concise way.

Colloidal silicon quantum dots (Si QDs) that are usually no more than 10 nm large crystalline Si nanoparticles dispersed in solvent have been attracting great attention largely due to their remarkable electronic and optical properties and flexible incorporation into device structures. In this work we begin with the introduction to the preparation of colloidal Si QDs. The electronic and optical properties of both undoped and doped Si QDs are then discussed. The use of colloidal Si QDs in Si-wafer-based solar cells and all kinds of hybrid solar cells is reviewed in detail. We briefly present the outlook of the development of colloidal Si QDs for solar cells in the end.