Scholarly article on topic 'The use of laser technology to shape properties of the contacts of silicon solar cells and their structure'

The use of laser technology to shape properties of the contacts of silicon solar cells and their structure Academic research paper on "Materials engineering"

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Academic research paper on topic "The use of laser technology to shape properties of the contacts of silicon solar cells and their structure"

Cent. Eur. J. Phys. • 12(12) • 2014 • 836-842 DOI: 10.2478/s11534-014-0512-5

VERS ITA

Central European Journal of Physics

The use of laser technology to shape properties of the contacts of silicon solar cells and their structure

Research Article

Matgorzata Musztyfaga-Staszuk1*, LeszekA. Dobrzanski2t

1 Welding Departament.Silesian University of Technology Konarskiego St. 18a, 44-100 Gliwice, Poland

2 Institute of Engineering Materials and Biomaterials. Silesian University of Technology Konarskiego St. 18a, 44-100 Gliwice, Poland

Received 16 March 2014; accepted 17 June 2014

Abstract: The paper presents the results of the investigation of the front contact manufactured using silver pastes

(based on experimentally prepared silver powder) on monocrystalline silicon solar cells in order to reduce contact resistance. Various deposition and fabrication techniques were applied to improve the electrical properties of contacts. The aim of the paper was to apply an unconventional method (selective laser sintering) to improve the quality of forming contacts of silicon solar cells. The topography of both the melted/sintered contact and textured silicon was investigated using atomic force microscopy. Resistance of front electrodes was measured using the Transmission Line Model (TLM). Both surface topography and cross section of front contacts were researched using the scanning electron microscopy.

PACS C200B): 68.37.Hk, 68.37.Ps, 81.05.-t, 81.65.-b

Keywords: electrical properties • crystalline solar cell • selective laser sintering

© Versita Sp. zo.o.

1. Introduction

In view of civilization and technological advances, the world's demand for energy to be utilized in everyday life of modern society is constantly increasing. Furthermore, it is an indispensable condition to maintain social and economic development of all countries to ensure free access to energy. Pollution of the environment, depletion of conventional raw materials for energy generation and instability involving their acquisition is enforcing the search

*E-mail: malgorzata.musztyfaga-staszuk@polsl.pl (Corresponding author) ^E-mail: leszek.dobrzanski@polsl.pl

for alternative energy sources worldwide, principally such sources which would not contribute to further degradation of the natural environment. One of the renewable sources of energy is the sun, which satisfies all the above criteria and which allows energy security for generations to come [1,2].

One of the methods to acquire solar energy are photovoltaic cells which directly convert solar radiation energy into electrical energy. Photovoltaic systems can provide electrical energy with the application of tools and/or power generation networks. The characteristic features which make the difference between this type of energy acquisition and conventional methods include principally high reliability, low noise emission and little negative

Springer

Impact on the natural environment. The governments of many countries (USA, EU, Japan) put a great emphasis on the development of this technology and on the promotion of "green energy". In the countries of the European Union the share of electrical energy production from non-conventional sources has already reached 14 % on average (in Poland around 2 - 4 %), and therefore new EU directives enforce respective regulations to be introduced in particular member countries to increase the share of electric energy produced from these sources. Poland will be obliged to increase this share to 15 % by the year 20201 [3]. Therefore, it is necessary to take the issues related to improving the functional properties of materials for photovoltaic applications. Due to the introduction of new technologies the costs of photovoltaic systems can drop whereby they will be more viable. It is generally expected that a considerable decrease of energy production costs will contribute to their wider application both in the developed and also developing countries. It is also predicted that this technology will be developed into the production of modules, cells and photovoltaic systems of higher efficiency, higher durability and reliability and the application of new materials [4-8].

At present, the fabrication techniques of silicon plates and photovoltaic cells have dominated a considerable part of commercial photovoltaic market through their huge surplus. It is predicted that the current state of domination will be maintained until the year 2020. A bulk part of solar cells is made from silicon, and one of the essential stages of their fabrication is the production of the front electrode. It is predicted that laser technology will considerably simplify the fabrication of silicon photovoltaic cells, including also the fabrication process of front metallization, improving the efficiency of solar energy transformation into electrical energy. In view of the increasing demand for materials of appropriate properties we can deem it scientifically grounded and up-to-date to utilize laser techniques [8-27] to shape the properties of materials and their structure for example for selective sintering/melting front electrodes on silicon surface. The electrode coating should meet specific requirements to ensure low resistance of the electrode interface zone with the substrate, and in effect to improve the efficiency of the photocell. The essential requirements are as follows: material (of the electrode and substrate) [1, 2, 4, 17, 25] and the conditions of its fabrication, shape and size of the electrode, adhesion of the electrode to the

1 Statement EP and Council no 2009I28IWE in case of promote application energy from different renewable sources very and in sequence revoke statements

2001I77IWE as well 2003I30IWE

substrate and substrate morphology and the appropriate level of emitter doping [17].

2. Experimental procedure

The investigations were done on monocristalline silicon (produced by Deutsche Solar AG) of type p doped with boron in the form of inserts of the thickness ~ 300^m and area ~ 50x50cm2, resistivity ~ 1Q • cm2. The silver powder about 40 nm granulation was applied during preliminary investigations. It was applied in investigations based on metallographic observations of front electrode manufactured by a selective laser sintering method. Table 1 presents the selection of elements in the contact layer of the solar cell. The election of chemical composition of the front contact was carried out by experiment and the mixture was prepared using a mechanical mixer. The technology used to produce the solar cell was developed in the Institute of Metallurgy and Materials Science in Krakow, Poland. Figure 1 presents the process sequence for producing solar cells. The emitter of the solar cell was produced on the basis of Si with surface resistance of approximately 60-70 ohm/square with the modified diffusion process. The alteration of the process consisted of primitive production on the wafer Si of the layer of silicon-phosphoric enamel from the POCl3 source at temperatures below 800 °C, at which the appropriate diffusion process did not occur. The rediffusion of atoms of phosphorus to the Si was then achieved using an infrared belt furnace at temperatures above 850 °C. The lower level of additive and the changed profile of the emitter was of key importance for the quantum yield of the link in the period of the wavelength 400-600 nm of the radiation generating the photoelectricity that directly affects the increase in the efficiency of the photoconversion.

One of the aspects of the paper was to manufacture experimental contact systems (fig. 2) to improve the quality by minimizing the resistance of a joint between the electrode and the substrate. A special experimental contact system was prepared, which was composed of a series of parallel paths with different distances between them. Firstly, a few initial series of solar wafers were prepared for testing by laser micro-treatment (Tab. 2). Secondly, laser micro-treatment conditions were selected for a final series of experimental of solar cells contact systems (Tab. 3) based on the results of metallographic observations. In order to perform a selective laser sintering method the Eosint M 250 Xtended device was applied during investigations. This device was equipped with a CO2 laser. The other technical parameters of it were the

Chemical etching and texturization

P - N junction formation

Diffusion of phosphorous impurity and creation of p-n junction

Deposition of passivation layer and antireflection layer

Figure 3. Transmission line method (TLM) a) the measuring position, and b) test structure for determining the contact resistance (where Rp - the sheet resistance, k - front contact length, L - width of contact, d - distance between contacts).

Screen printing deposition offrant side metallsation

Formation of front side metallisation in a selective laser sintering process

Figure 1. Scheme of the technological process of silicon solar cell production.[3, 28].

Figure 2. Overview of experimental front contact system.

following: the feed rate of passage through the laser beam - max. 3.0 m/s, the diameter of laser beam - 300 jm, wavelength - 10640 nm, shielding gas - nitrogen.

3. Methodology

The investigations of electrical parameters such as: the contact resistance Rc, specific contact resistance pc (for currents: 10, 30 and 50 mA), transfer length (LT) of front contact solar cell were performed using the Transmission

Line Model (TLM) method onto measuring position worked out in the Institute of Engineering Materials and Biomaterials (fig. 3). The Transmission Line Model (TLM) method was applied to determine the electrical parameters mentioned above. TLM consists of a direct current (I) measurement and a voltage (U) measurement between any two separate contacts. The contact resistance is characterized by the specific contact resistance Rc and the sheet resistance Rp. The quality of the ohmic contact to semiconductor can be studied by measuring the value of specific contact resistance. The specific contact resistance defines not only the real joint zone of contact with the Si substrate, but also the regions directly under and below surface of phase separation. The specific contact resistance of the front contact was calculated from the formula in the literature [29-32], but other parameters like Rc and LT were calculated using linear regression. The sheet resistance was measured with a four - point probe and calculated according to formula from in the literature [33], The topography of both surface and cross section of the front contacts was observed using a Zeiss Supra 35 scanning electron microscope and a Zeiss confocal laser scanning microscope. The profile of contact thickness was determined on the basis of eight medium measurements. Microchemical analysis of the chosen front contacts was performed using a scanning electron microscope equipped with an energy dispersive X-ray (EDS) spectrometer. Phase composition analysis of the chosen front contacts was performed using the XRD method. Observations of the textured surface of the silicon wafer and measurement of the average height of the pyramids was performed using the atomic force microscope with uncontacted trybe.

4. Results and discussion

As a result of investigations of a few initial series of solar wafers (fig. 4) the authors decided to give up further work on the selective laser sintering of test electrode system 1,11 silicon solar cells with medium thickness 50 jm from silver paste. The reason was that the authors observed a partial

Figure 4. Topography of the surface layer of silver paste on the surface of a silicon deposited by screen printing and SLS with the laser beam feed rate 50 mm/s and various laser beam power, where z - the number of passes of the laser beam, r- a laser beam, A1 -A9 - the symbol of the sample, h -the thickness of electrode before the process, z=3 for A1 -A9, r= 37.8 W for A1,A4,A7 and r= 39.15 W for A2,A5,A8, r= 40.5W for A3,A6,A9, h=15 pm for A1 -A3, h=35 pm for A4 -A6, h=50 цm for A7 -A9 (SEM - magnification in the range from 20 000 to 80 000 x).

evaporation of electrode, melting of elements, as well the areas of fully exposed silicon substrate in the electrode ((fig. 4) - samples from A7 to A9), as a result, abandoned the use of samples with the amount of further research. However, continued research on samples from A1 to A6. Figure 5a presents an example of the series of experimental solar cells, where front side metallization was deposited from X paste on the surface of the silicon solar cell with texture and ARC layer and SLS with the laser beam feed rate 50 mm/s and various laser beam powers. Based on electrical properties investigations using the TLM method in the first series it was found that the smallest specific contact resistance values of experimental contacts system I,II were respectively 1.03 -.—3.21 fi • cm2, 1.12 .—1.88 fi • cm2 for solar cells with texture and with or without ARC coating. The smallest value of experimental contacts system I, II (1.03 fi • cm2; 1.12 fi • cm2) were observed for medium thickness 15 pm onto substrate with

Jj/ ^ jj 0.5 1 1.5 2

Laser beam [mm/,]

a \ s*

L Lr a d [cm]*

Figure 5. a) An example plot of resistance versus contact distance for the determination of contact parameters (LT -a track of current impact and Rc - contact resistance, slope=Rp/z, Rp - sheet resistance, Z - contact length), b) typical graphic method from the literature used to determine factors [9]

Figure 6. Image of two- and three- dimensional surface topography (CLSM) of front electrode experimental contact system I from paste X onto Si substrate with texture and ARC layer by laser microtreatemn

texture and ARC coating with applied respectively laser beam 37.8 W and 40.5 W and the feed rate of passage the laser beam 50 mm/s.

Based on electrical properties it was found that the smallest specific contact resistance values of experimental contacts system I,II were respectively 0.36 .—0.51 fi • cm2, 1.29 .—1.63 fi • cm2 solar cells with texture and with or without ARC coating. The minimum value of experimental contacts system I, II (0.36 fi • cm2; 1.29 fi • cm2) about

Table 1. Properties of paste.

Paste symbol Mass concentration of elements Basic powder Organic carrier Ceramic glaze Solar cells with different morphology*

X 80 18 2 1,2

Where: 1.textured surface with deposited TiOx coating, 2. textured surface without deposited TiOx coating

Table 2. Initial conditions of laser micro-treatment for experimental contacts systems of silicon wafers.

Series Silicon wafer surface Paste symbol Laser beam passage feed rate (v), Laser beam, (P)W The thickness of printed front

mm/s electrode by screen printing

method,^m

1 " . , , I 3 60 -140 27 -46 15,35,50

2 Chemical cleaned X 50 27 -41 1535,50

medium thickness 35 ^m onto substrate with texture and without ARC coating with respectively applied laser beam 37.8 W and 40.5 W and the feed rate of passage the laser beam 50 mm/s.

The thickness of the test electrodes was determined by checking the height profile of the three-dimensional surface topography measured using a confocal laser scanning microscope (CLSM) (fig. 6). CLSM provides a 3D image by using a computer program to process recorded images from scanned reflections. Confocal microscopy improves the resolving power compared to the situation where the laser beam focuses on the sample surface - in which case there is a lot of noise. The narrow beam of laser light provides a spot light source with a higher resolution and is characterized by improved contrast compared to an optical microscope. Using the aperture ring allows the creation of optical and three-dimensional topographical cross-sectional images. As a result of SEM investigations, the investigated laser-sintered front electrodes show a wide range of cross-section thicknesses in the range from 580 nm to 2 ^m.

Based on metallographic observations, it was found that the front contacts obtained from X paste and selective laser sintering show uniformly melted structure, often with irregular shapes, which show a grid of micro cracks in some places (fig. 7). The investigations confirm that the antireflection coating creates a barrier into connection zone, which has an influence on the increase of resistance between the electrode layer and the silicon substrate. The thickness of the deposited layer has an influence on the structure of the electrode layer and the resistance value of the resistance electrode. Electrical properties of the electrode are closely dependent on participation of the particular components of pastes from which they are formed. The conduction of the investigated silver paste depends

Figure 7. SEM images of front contact layer obtained from X paste and selective laser sintered with the laser beam power 40.5 W and the laser beam feed rate 50 mm/s on Si substrate with texture and ARC layer

on granulation, particle shape and quantity of powder in its composition.

Based on fractographic investigations, it was found that

Table 3. Conditions of laser micro-treatment of experimental silicon solar cell contact systems.

Series Solar cells with different morphology* Paste symbol Laser beam passage feed rate (v), mm/s Laser beam,(P) W The thickness of printed front electrode by screen printing method, pm

1 1,2 X 50 27 -40.5 15

2 1,2 X 50 27 -40.5 35

* Where: 1.textured surface with deposited TiOx coating, 2. textured surface without deposited TiOx coating

Figure 8. Images of front experimental electrode system layer from paste X onto Si substrate with texture and ARC layer by laser micro-machinining: a) topography image (SEM), b,c) fracture image (SEM), d) EDS spectra from X

manufacturing contacts yield layers well concentrated without pores and discontinuities, also adhering well to the silicon substrate (fig. 8).

Figure 9. X-ray diffraction pattern of front metallization performed from X paste onto silicon surface with texture selective laser sintered with the laser beam 37.8 W for sample B1 and 40.5 for sample B2 and the laser beam feed rate of 50 mm/s

The qualitative analysis of phase composition carried out with the X-ray diffraction method confirms that the layers of the contact system contain the phase Ag, which was generated in congruence with the assumptions (fig. 9). On the X-ray diffractograms obtained with the use of Bragg-Brentano technique the presence of the reflexes from phase Si present in the substrate materials was demonstrated. In the atomic force microscope, topographies of silicon wafers with texture were observed. A medium thickness of pyramids was determined by atomic force microscopy, equal to 3 pm (fig. 10). It was found that the silicon substrate morphology has a significant influence on the minimum resistance value that can be obtained for electrodes made by selective laser sintering. The value is larger for the substrate with texture than for the one without texture. This is probably connected with occurrences of empty areas under contacts, because the medium thickness of pyramids can be different.

Figure 10. Topography of the textured surface of monocrystalline solar cell (AFM) (an example)

5. Summary

This investigation studies the elaboration of fabrication conditions of front contacts with the use of silver pastes (including nanopastes) with the application of laser technique, which are currently an indispensable element of modern photovoltaic technology. It will have an innovative contribution to the solution of present photovoltaic problems which will become a challenge since the problem is relatively new in the country. Hence the results of this work may provide guidelines for other research workers for the focus of future studies.

Based on investigation results, it was shown that laser

micro-treatment of the silicon elements of solar cells made from monocrystalline silicon, including selective laser sintering of the front electrode to its surface using a CO2 laser, improves the quality by minimizing the connection resistance of the front electrode with surface. It was found that among analyzed front electrodes the best electrical and structural properties are characteristic of the selective laser sintered test systems (0.36 Q • cm2; 1.29 Q • cm2) obtained from nanopaste and medium thickness 35 jm onto substrate with texture and without ARC coating with respectively applied laser beam 37.8 W and 40.5 W and the feed rate of passage the laser beam 50 mm/s. The results of new research will be supporting and complementing the research undertaken in the present project. In effect they can develop the existing knowledge and experience in this paper.

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