Conference Paper

NREL/CP-520-45001

February 2009

Studies on Backside Al-Contact 

Formation in Si Solar Cells: 

Fundamental Mechanisms 

Preprint

B. Sopori, V. Mehta, P. Rupnowski, and 

H. Moutinho

National Renewable Energy Laboratory

A. Shaikh and C. Khadilkar

Ferro Electronic Materials

M. Bennett and D. Carlson

BP Solar

Presented at the Materials Research Society (MRS) 

Fall Meeting 2008

Boston, Massachusetts

December 1-5, 2008
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1 

Studies on Backside Al-Contact Formation in Si Solar Cells: Fundamental Mechanisms

Bhushan Sopori,1 Vishal Mehta,

1 Przemyslaw Rupnowski,1

 Helio Moutinho,

1 Aziz Shaikh,2

Chandra Khadilkar,2 Murray Bennett,3 and Dave Carlson

3 

1

National Renewable Energy Laboratory, Golden, CO 80401, USA

2

Ferro Electronic Materials, Vista, CA 92083, USA 

3

BP Solar, Frederick, MD 21703, USA

ABSTRACT

We have studied mechanisms of back-contact formation in screen-printed Si solar cells by a 

fire-through process. An optimum firing temperature profile leads to the formation of a P-Si/P+-

Si/ Si-Al eutectic/agglomerated Al at the back contact of a Si solar cell. Variations in the 

interface properties were found to arise from Al-Si melt instabilities. Experiments were 

performed to study melt formation. We show that this process is strongly controlled by diffusion 

of Si into Al. During the ramp-up, a melt is initiated at the Si-Al interface, which subsequently 

expands into Al and Si. During the ramp-down, the melt freezes, which causes the doped region 

to grow epitaxially on Si, followed by solidification of the Si-Al eutectic. Any agglomerated (or 

sintered) Al particles are dispersed with Si. Implications on the performance of the cell are 

described. 

INTRODUCTION

The contact formation of most commercial Si solar cells is done by co-firing the cells with 

screen-printed front and back contacts. Typically, a gridded front-contact pattern of Ag-based

ink is directly applied on the antireflection coating of SiN:H, whereas the backside has a blanket 

Al-based contact. The firing process, also called fire-through contact metallization, performs the 

following functions: (i) On the front contact, the glass frit dissolves the antireflection coating of 

SiN:H and allows Ag to react with Si. This reaction involves participation of a solvent metal that 

leaches from the glass frit and helps to lower the eutectic point of Si-Ag alloy; (ii) Diffuses 

hydrogen from the SiN:H-Si interface into the bulk of the cell and passivates impurities and 

defects; and (iii) Produces an interaction of Si and Al to form a deep back-surface field (BSF)

and sinters unreacted Al to form a low-resistance contact. It is important to understand how each 

of these functions of this complex process can be optimized. 

This paper describes our investigations on the formation of the back contact, and how most 

of the requirements of a good back contact can be met by a suitable process. We will first 

describe requirements of a back contact and then discuss our experiments and results. 

These requirements may be divided into electronic and optical categories. The electronic 

properties include: (i) a BSF for minority-carrier reflection, (ii) a uniform low-resistance ohmic 

contact, and (iii) a smooth, dimple-free surface of Al. Optically, the back contact must be 

reflecting with very little absorption so that it contributes effectively to light trapping. In 

addition to these, the process of making the back contact should be compatible with efficient 

gettering of impurities. In commercial solar cell fabrication, the primary consideration is given 

to the electronic properties, while optical reflection and gettering are assumed as byproducts, 

Requirements of a Good Back Contact
2 

because there is some concern whether all the requirements can be met simultaneously. Indeed, 

our detailed analyses indicate that the performance of the back contact of most commercial cells 

is below optimum. 

The back-contact formation relies heavily on alloying of Si and Al to produce a controlled 

stratified structure. To obtain the desired electronic properties of the back contact, the kinetics of 

Si-Al melting and re-solidification of various phases during the firing cycle must be controlled. 

The next section briefly discusses requirements and issues with each region. 

Back-surface field

The purpose of the BSF is to provide a minority-carrier reflector that can lower the loss of 

photo-generated carriers at the back surface [1–6]. As is well known, the BSF arises from the 

band bending at the P+

–P region. Figure 1(a) illustrates the geometry of the back contact and 

 (a)

 

 

 

 

 

 (b) 

shows a band diagram of the backside of a typical boron-doped solar cell. One approach to 

determine the effectiveness of the BSF is to determine the surface recombination velocity (SRV) 

of the P+

–P interface, which depends on many factors including the thickness of the P+ region. 

Figure 2(b) shows the calculated SRV at the back contact as a function of the thickness of the P+ 

region [7]. These results are given for two values of S1. Here, S1 is the recombination velocity 

at the alloy-P+ interface and S2 is the effective recombination velocity due to the entire contact. 

It is clear that the width of the P+ region should be about 10 μm for a good BSF. Thus, it is 

important to determine how the cell should be processed to create a uniform, 10-μm-wide P+ 

region in conjunction with a good front contact. 

Low-resistance contact formation necessitates that a well-formed, ohmic contact between Si 

and Al be produced beyond the P+ region. An ideal contact would consist of Al/P+

/Si (as shown 

in Fig. 1(a). However, as shown in the actual cross-section of a cell later in the paper, a eutectic 

composition accompanies the formation of the P+

 region. Because the eutectic composition has a 

higher sheet resistivity compared to Al, this can lead to increased series resistance of the cell. 

An obvious way to mitigate this issue is to increase the thickness of the Si-Al eutectic. We also 

Low-resistance contact

Figure 1. (a) Illustration of band 

bending within the P+-P region, (b) 

calculated effective surface 

recombination velocity at a BSF. 

Alloy 

/metal
3 

show that the back contact typically leaves a region of disconnected sintered Al particles in a 

glass matrix. These regions can also contribute to the series resistance of the finished cell.

In addition to being a minority-carrier reflector, a back contact must also be a good optical 

reflector to enhance light trapping within the cell [8]. The co-firing process should also 

effectively getter the impurities in the cell. The details of these will be discussed elsewhere. 

Other properties

EXPERIMENT

Experiments were done on single wafers and multicrystalline (mc) Si solar cells. These cells

were fabricated on 124 mm x 124 mm, textured wafers using commercial screen-printed front 

and back contacts. The wafers were single crystal, single-side polished with Al deposited by 

electron-beam deposition either on the polished side or rough side. The firing was done in an 

optical furnace in which the cell is illuminated with a tailored optical flux profile, and the 

temperature of the wafer/cell is monitored to determine local temperatures at various critical 

areas such as under the metallization and away from it. All single-crystal samples were 

illuminated from the Si side.

0

100

200

300

400

500

600

700

800

0 100 200 300 400 500 600

Time (Sec)

Temperature (0C)

0

1

2

3

4

5

6

7

8

Light Intensity Signal (V)

Thermocouple 

Profile

Figure 2. A typical firing profile along with measured cell temperature for Si solar cell.

A typical firing profile for a solar cell is shown in Fig. 2. The ramp-up segment accomplishes 

the following: bake the cell to drive out organics, produce a molten glass to etch the SiN:H 

antireflection coating, melt Si-Al on the back side, and sinter Si-Ag on the front side. The rampup is followed by a short constant temperature. This maximum temperature determines the 

maximum concentration of Al in the Si melt, which controls the maximum doping of the P+ 

region (see the discussion). In the ramp-down part, the molten regions of both the front and back 

contact solidify. Because the single-crystal samples had evaporated Al, the ramp-up portion of 

the profile was eliminated.

The processed samples were characterized by a variety of techniques. Scanning electron 

microscopy (SEM) was used for high-resolution imaging of the processed contacts and for 

Characterization Techniques
4 

dopant profiling to study the BSF formation. Scanning Kelvin probe microscopy (SKPM) was 

used to generate potential profiles to evaluate the formation of the BSF and to determine local 

conductivity. Secondary-ion mass spectrometry (SIMS) profiles were used to measure Si 

diffusion in Al and profiles of Al after firing. Cross-sectioning was performed by a new 

technique [9]. This technique produces a highly flat cross-section over a large length of the cell 

that allows one to perform statistically meaningful analysis. Solar cells were characterized by 

dark and illuminated current-voltage (I-V) analyses. 

RESULTS AND DISCUSSION

Optical microscopy of our cross-sectioned samples provided valuable information on melting 

of Al and formation of Si-Al alloy. Figure 3 shows an optical micrograph of the back side of an 

optimally fired cross-sectioned Si solar cell with screen-printed contacts. This figure shows that 

the back contact consists of: (i) Al-doped region that has grown epitaxially on the Si surface.

The thickness of this region is about 15 μm. The interface waviness is due to texturing of the 

cell. (ii) Alloyed region (Si-Al eutectic) shows typical lamellar growth consisting of Si-rich and 

Al-rich phases. (iii) A granular region consisting of sintered Al particles in a matrix of glass.

It may be noticed that some of the Al particles have melted and combined with neighboring 

particles. The size of the Al particle is between 5 and 20 μm. Because our technique allows 

large samples to be cross-sectioned, we have observed that large variations exist in the thickness 

of alloyed regions and in the size of the sintered Al particles. 

Figure 4 shows SEM images of the back contact of a fired mc-Si solar, taken in dopant 

contrast. It shows a P+

 region, as well as the alloyed eutectic and sintered Al regions. The 

thickness of this region is only 3 μm and varies along the length of the interface. Figure 4b is an 

image of another section of the same cell showing a significant variation in the thickness of the 

alloyed layer and of P+ region and that some correlation exists between the thicknesses of these 

regions. 

Figure 5a shows an SEM image of a back contact and its potential profile (Fig 5b) under 

different bias conditions obtained by SKPM. A voltage of 110 mV appears across this P+-P 

region. Figure 6 is an optical micrograph showing a very important feature of the alloy thickness 

variation; the thickness of the alloy is largest at the valley of the texture and a minimum at the 

peak of the texture. This indicates that during the formation of Si-Al alloy, the melt initially fills 

up the texture valley. Any excess melt spills over the adjacent valleys and then builds up. The 

depth of the P+

 region is generally larger at thicker alloyed regions. 

Interface roughness 

due to texturing of 

the wafer. Note: 

The texture of the 

interface is 

retained. 

20 μm

P+-Si

Si-Al eutectic

Sintered Al particles

Unmelted 

Si

20 μm

P+ region

Figure 3. A cross-section of an 

optimally fired solar cell showing backcontact region.

 (a) (b)

Figure 4 (a) and (b). SEM images of different 

areas of same optimally fired multi-Si cell.
5 

Our results show that the melting and solidification of the back Al contact agrees well with 

the binary phase diagram of Al and Si [10]. It is rather remarkable that the general (qualitative) 

features of the phase diagram, which is applicable to a thermal equilibrium process, are indicated 

in our results (which may not be an equilibrium process). Indeed, there are variations imposed 

by the departures from a phase diagram. These departures arise from the following: 

1. We have observed that the firing process involves a very rapid diffusion of Si into Al. This 

diffusion controls the initial composition and location of the initial melt near the Si-Al 

interface. Figure 7 shows a SIMS profile of Si diffused into Al caused by a short firing 

profile; the thickness of Al was kept at only 1 micron to show that the diffused Si can 

accumulate at the Al-air interface. In a screen-printed cell, the diffused Si can disperse 

between melted Al particles. Figure 8 shows an SEM (Fig. 8a) and EDX images of Si (Fig 

8b) and Al (Fig 8c). In Fig. 8b, one can see that Si has diffused deep into Al beyond the 

alloyed region. The presence of Si around Al particles will cause an increase in the series 

resistance of the cell.

2. Melting of Al on a Si surface produces a very nonuniform melt because of high surface 

tension (large contact angle) and balling up into Al spheres. This phenomenon will lead to 

pockets of melt of different sizes, concomitantly producing P+

 and Si-Al eutectic regions of 

different thicknesses (as seen in Figs. 4 and 6). We have found that Si diffusion into Al can 

be used to increase the adhesion between the Al melt and Si, thereby producing a uniform 

melt, and hence, a uniform contact. 

3. During ramp-down, the melt cools and tries to regrow epitaxially over Si. This doped layer 

of Si follows the topology of the Si surface. On further cooling, an outside layer of Al 

freezes, which traps a liquid layer between solid Al (particles) and Si (with regions of soft 

glass). As the melt reaches the eutectic point, the entire melt is expected to freeze. It may be 

recognized that beyond the eutectic point Si will continue to diffuse into Al-Si and will pile 

up at interstitial spaces. Another important significance of diffusion of Si is that it competes 

with Al for oxygen that may be available at the interface, and it prevents a strong oxidation 

of melted Al particles to allow them to join with each other.

 

Silicon

Al-Si alloy

p+ 3.6 



m

p

Silicon

Al-Si alloy

p+ 3.6 



m

p

 

 -50

0

50

100

150

200

250

300

350

400

0 1 2 3 4 5 6 7 8 9 10

Di ( )

Vbias = -1.0 V

Vbias = 0.0 V

Vbias = 1.0 V Figure 6. Optical 

micrograph of a c/s cell, 

showing thickness 

variations of the alloyed 

region corresponding to the 

textured profile of the Si. 

Figure 5. (a) An 

SEM image (b) 

corresponding 

SKPM image 

(potential 

profile). 

Distance (μm)

Potential (mv) 
6 

0.0

1017

1018

1019

1020

1021

1022

1023

0.5 1.0

Si

1.5 2.0

Al-Si Alloy

p+

p 

6.9 μm 

 

Figure 7. SIMS profile of Si diffusion in Al.

CONCLUSION

An optimum firing temperature profile leads to the formation of a P-Si/P+

-Si/ Si-Al 

eutectic/unmelted Al at the back contact of a Si solar cell. Variations in the interface properties 

were found to arise from Al-Si melt instabilities. Experiments show that melt formation is 

strongly controlled by diffusion of Si into Al. This affects the series resistance of the cell.

Based on the above results, a desirable profile should encourage initial Si diffusion. This 

will initiate a melt very close to the eutectic point of 577°C. Next, the temperature should be 

raised to about 775°–800°C to ensure an adequate amount of Al concentration in the melt to 

form heavily doped P+ upon solidification. The duration of the peak temperature strongly 

controls the total thickness of P+ and the eutectic regions. However, a slow cooling from peak 

temperature to the eutectic point is desirable to ensure a large P+ region.

REFERENCES

1. J. D. Alamo, J. Eguren, and A. Luque, Solid-State Electron. 24, 415 (1981). 

2. C. Khadilkar, S. Kim, A. Shaikh, S. Sridharan, and T. Pham, Tech. Digest. of International 

PVSEC- 15, (2005). 

3. A. Kaminski, B. Vandelle, A. Fave, J.P. Boyeaux, L. Q. Nam, R. Monna, D. Sarti, and 

A. Laugier, Solar Energy Materials & Solar Cells 72,373 (2002). 

4. S. Narasimha, A. Rohatgi, and A. W. Weeber, IEEE Trans. on Electron Devices 46(7), 1363 

(1999).

5. F. Huster and G. Schubert, 20th European Photovoltaic Solar Energy Conference and 

Exhibition, Barcelona, 2DV2.48, 6–10 June (2005).

6. C.H. Lin, S. Y. Tsai, S. P. Hsu, and M. H. Hsieh, Solar Energy Materials & Solar Cells 92, 

986 (2008). 

7. B. Sopori, V. R. Mehta, P. Rupnowski, D. Domine, M. Romero, H. Moutinho, B. To, 

R. Reedy, M. Al-Jassim, A Shaikh, N. Merchant, and C. Khadilkar, Proc. 22nd European 

Photovoltaic Solar Energy Conference, Milan, 841(2007). 

8. M. Cudzinovic and B. Sopori, Proc. 25th IEEE PVSC, 501 (1996). 

9. B. Sopori, V. Mehta, N. Fast, H. Moutinho, D. Domine, B. To, and M. Al-Jassim, Proc.17th

Workshop on Crystalline Silicon Solar Cells & Modules: Materials and Processes, Vail, 

Colorado, 222 (2007). 

10. J. L. Murray and A. J. McAlister, Bull. Alloy Phase Diagrams 5, 74 (1984). 

(a) (b) (c)

Figure 8. Dopant-contrast SEM image (a) and corresponding 

EDX images of (b) Si and (c) Al.

 Depth (μm) Concentration (atoms/cc)
F1147-E(10/2008) 

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Studies on Backside Al-Contact Formation in Si Solar Cells: 

Fundamental Mechanisms; Preprint

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B. Sopori, V. Mehta, P. Rupnowski, H. Moutinho, A. Shaikh, 

C. Khadilkar, M. Bennett, and D. Carlson

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14. ABSTRACT (Maximum 200 Words)

We have studied mechanisms of back-contact formation in screen-printed Si solar cells by a fire-through process. An 

optimum firing temperature profile leads to the formation of a P-Si/P+-Si/ Si-Al eutectic/agglomerated Al at the back 

contact of a Si solar cell. Variations in the interface properties were found to arise from Al-Si melt instabilities. 

Experiments were performed to study melt formation. We show that this process is strongly controlled by diffusion of 

Si into Al. During the ramp-up, a melt is initiated at the Si-Al interface, which subsequently expands into Al and Si. 

During the ramp-down, the melt freezes, which causes the doped region to grow epitaxially on Si, followed by 

solidification of the Si-Al eutectic. Any agglomerated (or sintered) Al particles are dispersed with Si. Implications on 

the performance of the cell are described.

15. SUBJECT TERMS

PV; back-contact formation; solar cells; fundamental mechanisms; impurities; 

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