IFLUECE OF THE AL-SI ALLOY FORMATIO I ARROW DIELECTRIC BARRIER OPEIGS O THE SPECIFIC COTACT RESISTACE
Elias Urrejola1, Kristian Peter1, Joachim Glatz-Reichenbach1, Eckard Wefringhaus1, Heiko Plagwitz2, and Gunnar Schubert2
1 International Solar Energy Research Center - ISC - Konstanz, Rudolf-Diesel-Str. 15, D-78467 Konstanz, Germany
2 Sunways AG, Macairestrasse 3-5, D-78467 Konstanz, Germany
ABSTRACT: in order to further improve the efficiency of multicrystalline solar cells, both a dielectric passivation and local contact formation at the rear are important design contributions. The dielectric passivation presents many advantages compared to the standard fully covered Al back contact. Our work is centred in the analysis of the contact formation between Al and Si for the passivated emitter and rear cell device structure, PERC. We give an experimental explanation for the observed dependence of the contact resistivity of Al fingers on the Si-contact area. Our observation is based on the analysis of the geometry of the Al-Si alloy formation below the contacts, giving important conclusions for PERC solar cells. Scanning electron microscopy (SEM) and energy dispersive X-ray spectrometry (EDS/EDX) supports the analysis of the Al-Si alloy geometry, giving an understanding of its formation, and effect on the contact resistivity.
Keywords: PERC, dielectric passivation, contact formation, contact resistivity, Al-Si alloy
1. INTRODUCTION
Many works have been already made in the area of Al-Si contact formation and solar cells with passivation on both sides. We will start with a brief state of art of them.
Al-Si alloy formation. Many publications are centred in the Al contact formations of solar cells. A well explained analysis of local Al contacts has been made by IMEC. It has been found that after the opening of the dielectric barrier by laser, and after the alloying pyramids of Al-Si alloy are formed in the bulk with a thickness of 60 µm [1]. More specifically is the analysis of the Al-Si alloy formations done by ISFH [2], giving an understanding of the high positive concentration normally found at aluminium-doped regions. A nice model explanation for the Al-Si alloying process, including the composition of the microstructure, has been given by Huster [3] and Popovich [4], based in the equilibrium phase-diagram of Murray [5]. Some of these results are also demonstrated within our work. Recently Lauermann [6] has shown an analysis of the cross- sectional junction of Al-Si alloying, and solar cell results.
We recommend also literature recently published about characterization and local Al formation [7, 8].
Passivated emitter and rear cell (PERC).The PERC solar cell [Figure 1(a)] was first presented by Blakers [9].
A reduction of the Si-material thickness and an improvement of the rear contact are for the PERC a design consideration. Including a rear passivation layer at the rear together with local back contacts formation, a further increasing of the solar cell efficiency has been already demonstrated [9-14]. The rear surface passivation improves the internal charge carrier reflection in the Si- bulk, compared to a standard fully covered Al-area.
Another impact is the minimizing of the rear surface recombination velocity, by a reduced Al metallization.
Contact resistance. The electrical contact resistance has been defined by Windred [15] as the resistance offered to the flow of current during its passage across the interface between two conducting materials which are in contact with each other. It is strong influenced by the state of the contact surface and its shape. If the two bodies are electrical conductors it is then possible for a current to pass from one body to the other. The absolute
contact resistance R is defined as the voltage between two points on either side of the interface and separated by a considerable distance divided by the current which flows from one body through a surface into the other body.
Berger [16] (see also Schroder [17]) has given a nice definition and discussions of many methods in order to determinate the contact resistivity. He has shown measurements for contact resistivity of Al-Si contacts depending on surface doping concentration. A variation of the contact resistivity was found already in this paper, giving the explanation on the inhomogeneity of the contacts. Different contact resistivities were found from the middle to the edges of the contact area. In other words, the contact resistivity (many times also called specific contact resistance), is defined as the reciprocal of the derivative of current density with respect to voltage.
It is the contact resistance normalized by the area.
Contact resistance measurement. One method to determinate the contact resistivity from the geometry of planar contacts, the sheet resistance of the semiconductor beneath the contact, and the contact resistivity is called the transmission line method (TLM, [16-18]).
Melczarasky [19] have found a variation in the contact resistance of screen-printed Ag fingers, explaining it as an inefficiency of this method.
2. EXPERIMENTAL PART
The material used in this study is formed by p-type multicrystalline silicon wafers of 156x156mm2 and resistivity of 1.5Ωcm. The samples are cleaned in a sodium hydroxide (NaOH) bath, in order to remove as cut damage or texture on the surface. Later on, a dielectric layer is deposited as a barrier against the Al- paste on the back of the p-type Si wafers. The barrier is selectively opened by screen printing of a phosphorus containing etching paste. The activation of the etching properties is done by drying the wafers slowly on a belt furnace. The cleaning of the etching paste is completed in an ultrasonic bath with deionised water and 0.2%
potassium hydroxide (KOH). The samples are fired under a lamp-heated conveyor belt furnace.
Our PERC cells [see figure 1(a)] present at the rear fine screen printed openings through the rear passivation First publ. in: EU PVSEC proceedings : 25th European Photovoltaic Solar Energy Conference and Exhibition ; 5th World
Conference on Photovoltaic Energy Conversion, 6-10 September 2010, Valencia, Spain / Europäische Kommission Gemeinsame Forschungsstelle. - München : WIP-Renewable Energies, 2010. - pp. 2176-2179. - ISBN 3-936338-26-4
Konstanzer Online-Publikations-System (KOPS) URL: http://nbn-resolving.de/urn:nbn:de:bsz:352-194909
layer, covering about 9% of the rear surface. The contact of the Al paste is made to the Si-substrate via these fine opening lines. The contact formation (Al-Si alloy) takes place only in the fine dielectric barrier openings. A width limitation of 200 µm for the geometry of the opening lines is presented, in order to do not have too much loss.
Parallel to that, dielectric openings less than 50 µm are hard to realize, by using common screen printed technology. Therefore, a range between the above limits is chosen for the screen printing openings on p-type Si material (50, 75, 100 and 125 µm).
The fully covered Al area is replaced by variable screen printed Al finger widths on these dielectric openings, in order to analyze the effect of the contact geometry and the Al-Si alloy formation on the contact resistance.
We will give an observation and an understanding of the Al-Si alloy formation under local contacts for PERC solar cells and explains its influence on the minimization of the contact resistivity. The SEM-identification of the structure and geometry of the samples is obtained after special laser cutting and cleaning.
Figure 1. (a) PERC structure (texture not showed).
(b) Al-Si alloy junction formed at the rear of the PERC.
Figure 2. (a) Al finger alloyed on the same p-Si width.
(b) Al finger alloyed on a narrow dielectric open area.
Figure 3. Al-Si alloy phase diagram [5] .
2.1. The Al-Si alloy junction
Figure 1(b) shows our cross-section model of the alloyed junction structure at the rear of the PERC cell, used in order to analyze the contact resistance and contact formation. d1 represents the dielectric barrier opening width (the contact surface), and d2 represents the screen printed Al finger width (Al mass). Due to the spreading of 30 to 45 µm of the etching paste during the drying, the real values for d1, after optical microscopy analysis, are: 80, 110, 135 and 170 µm. The screen printing of the Al-fingers on the openings is achieved by optical alignment. Four different Al finger widths are printed. The values for d2 are: 600, 700, 800 and 900 µm.
These values have demonstrated, in previous experiments, deeply formed Al-Si alloys, and a reduction in the absolute contact resistance to 1.1Ω. In figure 1(a) a darker gray region is showed in the Al-matrix, which represents the high concentration of Si found after the alloying, and which indicates also that a minimum quantity of Al mass is needed for the Al-Si alloy process.
It is important to remark that a high overlapping of Al material is present on each side of the dielectric openings, since the Al fingers are wider than the dielectric opening widths (d2 > d1).
2.2. Contact formation
After the drying of the Al paste most of the organic binders are burnt out. Increasing the temperature the alloying process starts with the melting of the aluminum at 660°C (see Fig. 3). At the peak temperature of 840°C almost 30% of the liquid phase consists of silicon. On cooling down, the silicon is rejected from the melt building up the high positive doped layer called the back surface field (BSF [20]). Under the temperature of 577°C [21] the remaining liquid phase solidifies, forming the eutectic layer. Figure 2(a) shows a 700µm Al finger alloyed on a p-type silicon surface. The eutectic formed (10µm) all over the p-Si surface is normally found at the rear of standard solar cells. By reducing the contact area a strong Al-Si alloy formation is found in the bulk (20µm) below the contacts, compared to the standard cells [Fig.
1(b)].
Figure 4. SEM Al-Si alloyed junction with different layers.
Figure 5. Cavity bellow the Al-matrix, with thin BSF formed.
20µm BSF(Si-1%Al)
Al-12.6%Si
Al matrix (Al-17%Si)
During the alloying process of Al on Si [3, 5], the solid Al particles change to liquid state above the eutectic temperature, starting to alloy locally on the Si surface (also probed by our pre-experiments, and by [1]). The reduction of the dielectric barrier opening width for thick printed Al fingers causes a higher flow of Al material in to the Si wafer, [fig. 2(b) compare to fig. 2(a)]. Since the Al-paste used does not fire through the dielectric barrier, the offered material overlap of Al alongside the dielectric opening is strongly attracted into the Si substrate (also observed by [1]). Consequently, the liquid Al penetrates in to the openings, alloying deeply with the Si substrate [fig. 4]. This explains why, after the cooling down of the material, a strong Al-Si alloy formation is found in the bulk and below the contacts, compared to a fully covered Al back-surface for standard solar cells processing. This strong Al-Si alloy formation is present below narrow dielectric opening areas for Al finger widths wider than 500 µm and 50 µm thicknesses.
3. RESULTS AND DISCUSSION
The different layers (measured by SEM, EDS/EDX) formed after the alloying process are showed in figure 4 [layer a, b, c in fig 1(a), respectively], and they are: the Al-matrix layer in porous state (a: Al-17%Si) formed by Al-spherical particles, Si, Al2O3, and other lower concentrated defects; the strong Al-Si alloy formation (b:
eutectic layer Al-12.6%Si [21]); and the BSF (c:
composition Si-1%Al). Sometimes a cavity beneath the contacts is found (see figure 5) probably because of the high firing temperatures, and fast cooling down that does not allow the re-crystallization of the Si (remaining on the top of the Al-matrix) in the eutectic. Therefore, the concentration of Si is higher in the matrix than in the eutectic-layer (17% compared to 12.6% of the eutectic).
The importance to keep the contact resistivity low for solar cells lies on the FF losses that can be minimized.
The measurements results for the contact resistivity ρc are shown in figure 6, for different firing conditions.
Simplifying, only the results for a 700 µm Al finger width (d2) alloyed on four different dielectric barrier opening widths (d1: 80 to 170 µm) are shown. Similar results were found for different Al finger widths (300 to 1000 µm) on mc-Si and CZ-Si material. The total contact resistance R is also plotted which is not increasing for broader openings, as expected. There is a dependence of the contact resistivity on the dielectric barrier openings, but less on the firing temperature.
Figure 6. y-axis left: contact resistivity of a 700µm Al finger width versus four different dielectric barrier opening widths, for different firing conditions (T1-T4). y-axis right: absolute constant resistance.
A decrease of the fill factor losses in 1% is achieved by reducing the contact resistivity from 16 to 8 mΩcm2 for 9% contacted area. Although the TLM method seems not to be suited for measuring the specific contact resistance of such Al screen-printed contacts, we will give our explanation to its variation. Berger [16] found a variation of the resistivity for Al-Si contacts, changing from the edges to the middle. The contact resistivity seams to depend on the homogeneity of the surface.
Theoretically the dependence of the contact resistivity should be only on the doping and temperature [16, 20], therefore our expectation was to observe a non dependence for the contact resistivity on the increased contact area. Since the same Si material, the same Al- paste, the same screen printed mass, and firing conditions were used for all the experiment, the minimization of the contact resistivity is due to the geometry of the Al-Si alloy formation.
The geometry of the alloy is depending on the ratio of the printed Al finger width to the dielectric barrier opening width. The alloying of Al into the Si material is increased (penetrates deeper) when increasing this ratio [figure 7(a)], and the contact resistivity is reduced. Then, a homogeneous Al-Si alloy and BSF are formed deeply in the barrier opening, for narrow dielectric openings (BSF of 8µm, extending underneath the dielectric layer). By increasing the dielectric openings, we found that the alloy presents an inhomogeneous geometry formed by two strong alloy formations at the edges and a planar surface in between. By further increasing its value the Al-Si alloy starts to separate into two identical forms at the open edges [see figure 7(b)]. In order to better analyse the homogeneity of the contact surface, the Al matrix and the eutectic were etched off from the alloy junction, as shown in figure 7(b). The deep alloying forms are influenced by the high offered material overlap of Al-paste alongside the contact openings compared to the Al-mass on the middle part. The shape of the alloy formation in the Si substrate is found. Similar two strong Al-Si alloy formations are deep formed at the edges of the dielectric barrier opening.
Figure 7. (a) model to calculate the real contact surface 2*l2. (b) for broader openings a planar surface appears in between the
two deep alloying forms at the edges, increasing with the dielectric opening width.
As shown in figure 7(a) a real contact surface should be take in to consideration for the TLM calculations. The real surface of contact is not two times the radio r of the circle, but two times the large l2. From the figure we get
that,
π 25 π
1
= 2 r =
l
(1).Experimentally we have found that, 3 . 1
2 1=
=l
cte l (2).
Using (2) in (1) we get,
6 .
2
2
l = π r
(3).During the TLM measurement, the surface 2r should be replaced by 2l2, for the calculation in TLM, where 2l2 is the real surface of contact for the Al-Si alloy. Now for an opening of 80µm (r=40µm, l2=48µm), we have obtained a contact resistivity of 8mΩcm2. Using now the real contact surface 2l2, we get:
2
2[ ]) 10
* 2
* ] [ 1 (
* ] [ 1 . 1
*A cm l µm m cm
R c
c= = Ω = Ω
ρ
(4)The contact resistivity of a 700µm Al finger contacted on 80µm dielectric barrier opening is 10 mΩcm2, where the real contact surface is 96µm.
Explanation for the contact resistivity dependence. The two alloy-forms found at the edges of broader openings present similar geometry. If we observe closer the two shapes of the alloy at the edges of fig. 7(b) and compare this with the alloy of fig. 7(a), we can conclude that the shape of this two forms together make the one of fig.
7(a). The real surface of contact of these two Al-Si alloy formations remains constant (2l2 ≈ 96µm) and does not change for wider dielectric openings. Only the planar surface in between showed by fig. 7(b), and appearing between these two formations, is changing and its width varies proportional with the dielectric opening width. We can said, that two Al-Si alloys appears at the edges of broader dielectric openings than 100µm, separated by a shallow alloyed surface in between.
Comparing our analysis with the contact resistance results, the planar surface is not needed in the development of solar cells because it contributes to a high contact resistivity (as an increasing value). If we take into consideration just the real contact surface 2l2, and remove the variable planar surface from each opening, the contact resistivity of screen printed Al fingers on p-Si areas should be constant. We also assume that the contact resistivity should be constant for the same Al mass alloyed on even narrower contact areas than the presented in this work.
4 CONCLUSION
We showed that the contact resistivity depends on the surface of contact, where the Al-Si alloy is formed. A minimization of the FF losses can be achieved by decreasing the contact resistivity. This is found only for thick screen printed Al fingers alloyed on narrow dielectric barrier openings. A dependence on the Al mass has been found for the Al-Si alloy. An analysis of the Al- Si alloy junction has been done. We found similar alloying structures at the edges of the openings, where the good contact is formed. Just a very narrow contact
area is required for the design of the back contacts of the PERC cell. This work can have application in Al-Si alloying processes and advanced solar cells concepts, like PERC and back-contact solar cells, where the minimization of the resistance losses and the improvement of the back-side (contacts and passivation) are a solar cell design consideration.
Further investigations have to be done using other measuring methods and pastes, in order to demonstrate the same comportment and result as observed in this paper (please refer to [23]).
ACKNOWLEDGEMENTS
The authors would like to acknowledge the financial support by the German Federal Ministry of Education and Research under contract no. 03SSF0335I, and the kindly supply of the etching paste by Merck KGaA.
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