EPD Congress 2012 TMS (The Minerals, Metals & Materials Society), 2012

MONO-LIKE INGOT/WAFERS M A D E O F S O L A R - G R A D E SILICON FOR SOLAR CELLS APPLICATION

Sergey Beringov', Timur Vlasenko1, Sergiy Yatsuk', Oleksandr Liaskovskiy1, Iryna Buchovska1

'Pillar Ltd, member of Pillar Group, Severo-Syretskaya Str. 3, 04136, POB 42, Kiev, Ukraine Phone: +38 044 490 99 80; Fax: +38 044 490 99 84

Keywords: Mono-like, Efficiency, Multicrystalline, Monocrystalline, Texturing, Seeding

Abstract

Working with solar-grade silicon for solar cells application usually meets some difficulties in comparison to polysilicon produced by Siemens method: higher impurities content leads to lower yield, worse electro-physical parameters and poor performance in solar cells application. However some adjustments in ingot production can show significant improvements in quality of final product: crystalline wafers and cells. Recent investigations have shown that solar-grade silicon is suitable for high effective production process such as mono-like multicrystalline wafer production. Production-scale lots made at Pillar show promising results for solar-grade silicon application: high-quality wafers with significantly big crystalline grains of <100> orientation.

Introduction

The standard technology of producing multicrystalline silicon ingots by directional solidification method in quartz crucibles has a number of disadvantages resulting in limited efficiency of solar cells made of multicrystalline material: crystallites of multicrystalline ingot are relatively small (down to 5 mm size), grain borders of crystallites contain parasitic impurities such as metals, silicon nitride etc., crystallographic orientation of crystallites is random. The listed deficiencies are an integral characteristic of the technology which affects the efficiency of solar cells, such as presence of solar cells with low efficiency even at a high level of average efficiency, problems with shunted cells, silicon nitride inclusions, which adversely affect cell efficiency and are centres for silicon carbide formation, impossibility of application of high effective alkaline texturing etc. Also with regard to the targets set by photovoltaic to reduce the cost per Wp in the nearest future, standard technology of multicrystalline silicon ingots production by directional solidification faces certain challenges some of which are hard to overcome due to rather low efficiency of solar cells made of multicrystalline wafers. Therefore alternative methods of multicrystalline silicon production for photovoltaic application having a number of advantages in comparison to traditional technology are developing rapidly. To produce multicrystalline silicon ingots for solar cells application, which have relatively big crystallites with specified crystallographic orientation, the authors developed new technology for growing multicrystalline silicon ingots with improved crystalline structure resulting in higher efficiency obtained for solar cells using both polysilicon and solar-grade silicon.

Growing technology

The authors used the modified method of directional solidification which allows growing multicrystalline ingots with structure of crystallites close to monocrystalline. The method is accomplished by using monocrystalline seeds which are placed on the quartz crucible's bottom

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and almost completely cover it. Monocrystalline seeds allow getting the ingot with crystallographic structure close to monocrystalline with specified crystallographic orientation. Control of temperature gradient during the process of seeding of crystals is performed during the growing process. The authors have designed a special system which allows to prevent monocrystalline seeds from melting as" the seeding happens resulting in growing ingots with crystallographic structure close to monocrystalline. Designed mono-like system is appropriate to be installed on the standard directional solidification furnace without any special redesign. The quartz crucible with monocrystalline seeds positioned on the bottom and loaded with solar-grade silicon chunks on the top of seed was set into the directional solidification furnace. Mono-like system was started at pre-final steps of the melting stage when all silicon chunks were completely melted. Installed on the growing furnace mono-like system allows to control the temperature and prevents monocrystalline seeds from melting. The solidification process of the melt starts from the bottom and ends at the top of the quartz crucible. Consequently growing of the mono-like structure with relatively big crystallites starts from partially melted monocrystalline seeds. Therefore it is highly important to control the time when monocrystalline seed will be partially melted with the help of specially designed mono-like system. The initial stage of the growing process is shown on the Figure 1.

Figure 1. The initial stage of the growing process.

By the time when the seeds are partially melted at the pre-defined height the furnace shifts to the growing stage at the same time ensures growing of big crystallites with defined crystallographic orientation from the monocrystalline seeds. The crystallographic structure of the ingot strictly depends on crystallographic orientation of monocrystalline seed [1], Therefore for the growing process monocrystalline seeds with monocrystalline structure and defined crystallographic orientation <100> were used. At the same time it is very important to adjust growing regimes to achieve optimal conditions for seeding big crystallites at the initial step of growing. Mono-like system controls a few centimeters of ingot's growing then it is switched off. In the grown multicrystalline ingot the predominant crystal orientation is the same as in monocrystalline seeds namely <100>. The grown ingot with the rest of the seeds is shown on the Figure 2. The final height of the rest of the seeds hf is less than initial height hi of seeds.

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Figure 2. The final stage of the growing process.

The coating of quartz crucible was standard, used for growing directionally solidified multicrystalline ingots. Growing regimes were optimized for growing ingots with the same productivity as standard directionally solidified multicrystalline ingots. There was no special preparation or treatment of the charge for mono-like process except for the use of monocrystalline seeds in the quantity sufficient to cover crucible's bottom. The furnace for growing multicrystalline ingots requires minor adjustments for installation of mono-like system. The mono-like system needs no specific maintenance or service. This system does not violate the basic modes of operation of directional solidification multicrystalline growing furnace. The mono-like system requires minimum control from the operator, which is mainly limited to monitoring the growing cycle.

Results

In order to develop suitable modes for growing mono-like multicrystalline ingots different types of silicon feedstock were used as a material for melting: polysilicon and solar-grade silicon from different producers. The properties of the resulting mono-like multicrystalline ingots were investigated by application of standard analytical methods used in photovoltaic: oxygen and carbon concentration measurement by FTIR, non-contact bulk resistivity measurement, μ-PCD minority carrier lifetime measurement, metal concentration measurements by ICP-OES etc. The results obtained were compared to the properties of multicrystalline ingots produced by directional solidification at Pillar Ltd. Minority carrier lifetime measured on the side surface of ingots (see Figure 3) is higher for mono-like ingots in comparison to ingots produces by directional solidification.

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Figure 3. Typical minority carrier lifetime maps measured on the side surface of ingots produced by the mono-like (left) and directional solidification (right) technologies.

As seen from research results mono-like ingots have a number of advantages as compared to directionally solidified ingots. Mono-like method succeeds in ensuring bigger crystallites as compared to directionally solidified ingots, which is an advantage for solar cells application. Traditional directional solidification technology provides for high quality multicrystalline silicon. However, the crystallographic orientation of crystallites in traditional directional solidification technology is random, which doesn't allow the application of high effective alkaline texturing in solar cells production. The opportunity to control the crystallographic orientation of crystallites in mono-like process by using monocrystallme seeds with <100> crystallographic orientation allows the producers of solar cell to use alkaline texturing which opens the prospect of increasing light absorption and improving the quality of solar cells. Multicrystalline ingot made from solar-grade silicon by mono-like growing technology was sliced into wafers. All wafers were divided into three groups depending on their crystallographic orientation (Figure 4).

Figure 4. Types of mono-like wafers.

This experimental lot of wafers was used for solar cells production in an industrial production line. The obtained efficiency results of these solar cells are presented in Table I.

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Table I. Efficiency results for solar cells made of mono-like wafers with using of solar-grade silicon in charge

Wafer type

Group 1

Group 2

Group 3

Crystalline structure

> 70% of wafer area <100> orientation

40%-70% of wafer area <100> orientation

< 40% of wafer area <100> orientation

Solar cells efficiency alkaline texturing

18.9%

17.7%

16.5%

Solar cells efficiency acidic texturing

17.7%

17.3%

16.7%

It can be easily seen that wafers from Group 1 and Group 2 are suitable for both acidic and alkaline texturing in solar cell production process. Alkaline texturing gives obvious benefit of 0.4% - 1.2% in efficiency. Wafers from Group 3 have similar crystallographic properties to wafers produced by traditional directional solidification process therefore are more appropriate for acidic texturing process.

Conclusions

Obtained data confirm that mono-like growing method is applicable for different types of feedstock, highly effective and promising for producing multicrystalline ingots for photovoltaic application and the resulting multicrystalline silicon has improved crystalline structure and is suitable for high effective alkaline texturing.

Acknowledgements

Authors would like to thank Q-Cells SE for producing solar cells from wafers manufactured by mono-like technology and measuring their parameters.

References

[1] K. Nakajima, N. Usami, K. Fujiwara, K. Kutsukake and S. Okamoto, "Control of microstructures and crystal defects in Si multicrystals grown by the casting method - how to improve the quality of multicrystals to the level of single crystals -", Proceedings of 24th European Photovoltaic Solar Energy Conference, (2009) 1219-1221. [2] K. Fujiwara, W. Pan, K. Sawada, M. Tokairin, N. Usami, Y. Nose, A. Nomura, T. Shishido and K. Nakajima, "Directional growth method to obtain high quality polycrystalline silicon from its melt", J. Cryst. Growth, 292 (2006) 282-285.

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