Crystal Growth of Si for Solar Cells

This chapter will give a short introduction to the well-described processes for the production of metallurgical silicon and solar grade silicon by the Siemens process. Among the new methods for the production of solar grade silicon, the upgraded metallurgical silicon is a good alternative to replace the feedstock produced by the Siemens process. Many of the new methods consist of several steps. The most common refining processes are described in this chapter. The intention of this chapter is to give an overview and understanding of the new processes that are emerging and to give some tools to optimize and develop new methods for production of solar grade silicon.

The fast growing photovoltaic market is mainly based on crystalline silicon. The strong demand on silicon requires wafer manufacturers to produce high-quality material through high productivity processes with low-cost. Due to the higher energy conversion efficiency of single crystalline silicon (sc-i), the Czochralski (Cz) pulling remains the key technology in photovoltaics. However, when compared with the multicrystalline silicon (mc-Si) production by the directional solidification, the current Cz technology is still more costly, due to the lower throughput and more energy consumption. Therefore, to retain the competition of sc-Si in the PV market, high efficient Cz ingot pulling is needed. In this chapter, we discuss some important issues in the Cz sc-Si production. Special focuses will be on the hot-zone design and multiple charges. The implementation of these concepts has led to significant cost reduction and yield improvement for both 6 in. and 8 in.-diameter solar-grade silicon in production. Some comments for the future development are also given.

This chapter outlines one of the two practically important bulk crystal growth methods for silicon, the crucible-less floating zone (FZ) technique, which cannot be evaluated without comparing it to the other one, the Czochralski (CZ) method. The main advantage of FZ silicon is the high purity and the resulting high electrical and structural material quality. Although, till now, FZ silicon for solar cells is mainly a matter of R&D and not of cell production for terrestrial utilization, it has a big potential for future applications, because efficiency and long-term stability of FZ silicon cells are considerably higher than that of CZ silicon cells.

Presently, the main problem with FZ solar cells is not only the high price of the special feed material for FZ, boosted by the currently unbalanced situation of the feed stock market, but also the limited crystal cross section of FZ crystals, which does not fit the standard cell formats. New concepts to overcome these difficulties are to grow crystals directly with the desired square cross section of the solar wafer, the use of cheaper feed material like granular silicon, or pulling feed rods after a downgraded CZ technique from low-price raw silicon like upgraded metallurgical grade (UMG) silicon with the benefit of further purification by segregation.

This chapter introduces crystallization process of multicrystalline silicon by using a directional solidification method. Numerical analysis, which includes convective, conductive, and radiative heat transfers in the furnace is also introduced. Moreover, a model of impurity segregation is included in this chapter. A new model for three-dimensional (3D) global simulation of heat transfer in a unidirectional solidification furnace with square crucibles was also introduced.

Fundamental understanding of crystal growth behaviors from Si melt is significant for the researchers, who are involved in the development of crystal growth technologies. In the light of Si crystals for solar cells, it is imperative to improve the crystal-quality of Si-multicrystal ingot grown by casting, because it is widely used for solar cell substrates in the present and future. Faceted dendrite has unique structural features and has a potential to be used for controlling the crystal structure in Si-multicrystal ingots. In addition, basically, its growth behavior is fascinating. In this chapter, the growth mechanism of Si faceted dendrite will be described with recent experimental results.

Generally, Si multicrystals, which are grown by a casting method using a crucible, contain many grain boundaries (GBs) and crystal grains with various orientations. Since the grain size has increased as a result of improving in the growth technique, instead of GBs, subgrain boundaries (sub-GBs) have become major defects acting as recombination centers for photogenerated carriers. In this chapter, the study of sub-GBs in Si multicrystals is comprehensively reviewed with the authors’ current results.

The objective of this chapter is to review, for photovoltaic application, the current status of crystalline silicon ribbon technologies as an alternative to wafers originating from ingots. Increased wafer demand, the current silicon feedstock shortage and the need of a substantial module cost reduction are the main issues that must be faced in the booming photovoltaic market. Ribbon technologies make excellent use of the silicon, as wafers are crystallised directly from the melt in the desired thickness and no kerf losses occur. Therefore, they offer a high potential to significantly reduce photovoltaic electricity costs when compared to wafers cut from ingots. Nevertheless, the defect structure present in the ribbon silicon wafers can limit material quality and cell efficiency.

Liquid phase epitaxy (LPE) is a growth technique that can be suitable for photovoltaic applications regarding its simplicity and its capacity to produce high-quality thin film. The growth of Silicon proceeds from a molten solution (metal + Si), which is slowly cooled. Temperature range is typically 700–1,000°C and growth rate can be as high as 1 μm min^-1. In this chapter, we first introduce the fundamental principles of LPE and then we discuss on the influence of the metallic solvent (In, Sn, Cu, Al …), the temperature range on the growth rate and on the quality of the epitaxial layers. We also review epitaxial growth on polycrystalline silicon or foreign substrates. We finally discuss the development of high throughput LPE deposition equipment.

The main advantages of the vapor phase epitaxy (VPE) are the ability to grow very good quality layers, with high growth rate (higher than μm min ^–1). Its principle is relatively simple and allows great flexibility (change in doping level or type of doping …). In addition, the VPE can handle several large wafers, which is particularly desirable for photovoltaic applications. In this chapter, we introduce the principle of this method before discussing the theories and modeling for understanding the mechanisms governing the kinetics of crystal growth. It is followed by a detailed description of SiH₂Cl₂/H₂ system, well adapted to the growth of films for photovoltaic applications.

Flash lamp annealing (FLA) has attracted attentions as a technique of rapidly crystallizing precursor a-Si films to form poly-Si films with high crystallinity on low-cost glass substrates. In this chapter, a brief explanation on fundamental physics typically seen in nonthermal equilibrium annealing and in utilization of metastable a-Si as precursor films have been given. Recent findings concerning FLA-triggered crystallization of micrometer-order-thick a-Si films and microstructures of the poly-Si films are also introduced.

Thin, large-grained polycrystalline Si (poly-Si) films can be formed on foreign substrates (e.g., glass) by the aluminum-induced layer exchange (ALILE) process, which is based on the aluminum-induced crystallization (AIC) of amorphous Si (a-Si). During an annealing step, below the eutectic temperature of the Al/Si system (577°C), the initial substrate/Al/a-Si stack is transformed into a substrate/poly-Si/Al(+Si) stack. In this chapter, the ALILE process itself and the properties of the resulting poly-Si films are discussed in detail from the scientific as well as technological point of view.

The fabrication of solar cell grade silicon (SOG-Si) feedstock involves processes that require direct contact between solid and a fluid phase at near equilibrium conditions. Knowledge of the phase diagram and thermochemical properties of the Si-based system is, hence, important for providing boundary conditions in the analysis of processes. A self-consistent thermodynamic description of the Si-Ag-Al-As-Au-B-Bi-C-Ca-Co-Cr-Cu-Fe-Ga-Ge-In-Li-Mg-Mn-Mo-N-Na-Ni-O-P-Pb-S-Sb-Sn-Te-Ti-V-W-Zn-Zr system has recently been developed by SINTEF Materials and Chemistry. The assessed database has been designed for use within the composition space associated with the SoG-Si materials. An assessed kinetic database covers the same system as in the thermochemical database. The impurity diffusivities of Ag, Al, As, Au, B, Bi, C, Co, Cu, Fe, Ga, Ge, In, Li, Mg, Mn, N, Na, Ni, O, P, Sb, Te, Ti, Zn and the self diffusivity of Si in both solid and liquid silicon have extensively been investigated. The databases can be regarded as the state-of-art equilibrium relations in the Si-based multicomponent system. The thermochemical database has further been extended to simulate the surface tensions of liquid Si-based melts. Many surface-related properties, e.g., temperature and composition gradients, surface excess quantity, and even the driving force due to the surface segregation are possible to obtain directly from the database. By coupling the Langmuir-McLean segregation model, the grain boundary segregations of the nondoping elements in polycrystalline silicon are also possible to estimate from the assessed thermochemical properties.