Microstructural Studies on Fire-Through Front

Contact Metallization of Si Solar Cells

Suhaila Sepeai, M.Y.Sulaiman, Saleem H.Zaidi, Kamaruzzaman Sopian Solar Energy Research Institute (SERI)

Universiti Kebangsaan Malaysia (UKM)

43600 UKM Bangi, Selangor, Malaysia

Email: suhailas@ukm.my/ suhaila_sepeai@yahoo.com

Abstract- Screen-printed Silver/Argentum (Ag) metallization is

used in the photovoltaic industry for the front-side emitter

contacts of crystalline silicon solar cells owing to its cost-

effectiveness and high throughput. In order to obtain a better

understanding for the formation of Ag paste and silicon (Si)

interface, the firing treatment was studied. The microstructure

properties of the fire-through Ag metal contacts formed on

pyramid textured silicon wafers are investigated. The best firing

temperature is 800!"#$%&'#()#*+,)#-%.-/%&#0*1&(2,#(&#345367#()8#

efficiency of 13.67%. The Scanning Electron Microscopy (SEM)

characterization of Ag shows that as the temperature further

increased, the particles of Ag fuse together and the decrease of the

Ag thin film thickness indicates that n+ layer has been formed.

I. INTRODUCTION

Front contact formation is critical for achieving high

efficiency solar cells. Requirements for a good front contact are

uniform, low resistance and have an ohmic contact with high

shunt resistance. Such a contact will produce a high open-

circuit voltage (Voc) and high fill factor (FF) [1]. Fire-through

metallization is the most common method for forming front

contact on screen-printed silicon solar cells. Screen printing

technique typically used in industrial metallization due to its

cost-effectiveness, rapid and simple production, compared to

the photolithography and buried-contact techniques which is

time-consuming and expensive [2]. The applied metallization

technique determines the shadowing and series resistance

losses, determines the emitter diffusion profile and surface

doping concentration[3].

An aspect of considerable importance is the issue of metal

contacts on the n-side of the silicon solar cell. Argentum (Ag)

and aluminium (Al) pastes typically use for the front and back

contacts, respectively. Requirements for a good paste are low

resistivity, good printability and good adhesion. It allows a

good contact formation on shallow homogeneous emitters [4].

Low resistivity ohmic contact also desirable to prevent current

losses and contribute in the increase of efficiency [5]. In solar

cell manufacturing, the contact firing is usually done in

infrared (IR) heated conveyor belt furnaces. Rapid firing at a

high temperature is done to avoid the degradation of the

electrical quality of the Ag metal contact [6].

In this research, we have investigated the microstructure of

the front-side contact/solar cell emitter interface fired at

temperatures of 650 to 800!C. It shows that the metal contacts

could be fired at a higher temperature in air ambient which

facilitates the formation of low resistance ohmic contacts

between the silver metal and n+ silicon surface. This process is

very simple and has advantage of making fire-through ohmic

contacts on shallower emitters.

II. EXPERIMENTAL

Solar cell devices with a basic structure which is consist of

p-type silicon (Si) wafer, n-type semiconductor, sandwiches by

Argentum (Ag) and Aluminium (Al) as a front and back

contact was fabricated. The structure is shown in Figure 1. The

<100> Si wafer substrate was used. The Si wafer was initially

cleaned by dipping into solution of hydrofluoric acid (HF) and

nitric acid (HNO3) in a ratio of 1:100 for 10 minutes. After

rinsing with deionized water, it was then dipped into HF and

water (H2O) in a ratio of 1:50 for 1 minute. The wafer was then

immersed in 10% potassium hydroxide (KOH) at a temperature

!"#$%&'#"!(#)#*+,-./01#2-30/4-/,.567#.8/#9:"/(#9:0#(/;/:./<56#

cleaned in HF:H2O. The wafer should be hydrophobic when

tested with water to indicate that it was finally cleaned.

The texturing process was undertaken after the cleaning

procedure. The wafers were textured using a solution of KOH,

iso-propil alcohol (IPA) and H2O in the ratio of 1:5:125. After

the texturing process, the wafers were subjected to the n-type

diffusion procedure. The heavily doped n+ region was formed

on the Si wafer surface by phosphorous diffusion at 900°C for

RSM2011 Proc., 2011, Kota Kinabalu, Malaysia

364 978-1-61284-846-4/11/$26.00 ©2011 IEEE

30 minutes using phosphorous oxychloride (POCl3) as the

diffusion source. The edges of Si wafers were then etched by

Xenon Difluoride (XeF2).

The wafers were then ready for the metallization process.

The Ag and Al pastes were screen-printed on both of Si wafers

forming the front and back contacts, respectively. After oven

drying at 100°C for 15 min, the solar cells were fired in a rapid

thermal annealing (RTA) furnace. The firing temperature was

varied in the range of 650&'#.!#=%%&'1##>8/#"+,+08/<#0!5:(#?/550#

were analyzed using light Current-Voltage (LIV) Measurement

System. The cross section images of solar cells and grain size

images of Ag paste have been characterized by Scanning

Electron Microscope with a magnification of 1000.

Fig. 1 The structure of basic Si solar cell device

III. RESULT AND DISCUSSION

Figure 2 shows the current-voltage (I-V) curve of basic Si solar cell. Based on the figure, it can be seen that the voltage or

open circuit voltage (Voc) increased with the increasing of

firing temperature. In typical result, the Voc value for the device

fired at 650, 730, 750 and 800°C are 0.468, 0.468, 0.480 and

0.492 V, respectively (Table 1). It is important to note here

that, despite the increase in the Voc as the temperature

increases, it also causes an increase in the short circuit current

(Isc), fill factor (FF), shunt resistance (Rsh) and efficiency. The

efficiency for the device fired at 650, 730, 750 and 800°C are

5.25, 5.69, 8.25 and 10.49, respectively. The inconsistent value

of ISC is due to the leakage current. When shunt resistance is low, the diode acts more like a resistive element. Thus, the

behavior becomes similar to that of a photoconductor.

It can be simply understood that, to increase the Voc, the

optimization of the interface characteristic and the carrier

mobility of the device are required. We have carried out the

firing treatment on the device, to achieve enhancement in

efficiency. We found that the performance of the device can be

substantially improved, reflecting the enhancement of the

carrier mobility in the device.

Fig. 2. I-V curve of solar cell with a variation on firing temperature, x = 650 –

880 °C

TABLE 1

SUMMARIZED RESULT FROM I-V CURVE WITH A VARIATION OF FIRING

TEMPERATURE

Temperature Voc Isc FF Rsh Eff

(%)

650 0.468 0.08523 0.41458 0.75 5.25

730 0.468 0.09241 0.48171 0.78 5.69

750 0.480 0.10970 0.49360 0.80 8.25

800 0.492 0.10805 0.62195 0.83 10.49

This assumption strongly confirmed by the Ag grain size

images of un-fired cell, fired at 650°C and 742°C (Figure 3-5).

SEM has characterized these images. The grain size of Ag

before firing process is 416.44 nm. There is an increase in the

particle size because the adjoining particles fuse together. As

the temperature is increased, many Ag particles agglomerate

and fuse together into bunches. At the peak temperature, some of the agglomerates close to the surface react with Si to form

n+ layer. The n+ layer provides the tunnel for electron. The

electron can move easily to the electrode and this enhances the

open circuit voltage (Voc).

Fig. 3. The grain size of Argentum paste before firing process.

RSM2011 Proc., 2011, Kota Kinabalu, Malaysia

365

Fig. 4. The grain size of Argentum paste for a firing temperature at 650°C

Fig. 5. The grain size of Argentum paste for a firing temperature at 742°C

There are other reasons for the increasing of the Voc of the

solar cells, for instance the thickness of the thin film that

closely related to the carrier diffusivity in the device. Figure 6

shows the cross section images of solar cells before firing

process. It is clearly shown that the Ag particles have not fuse

yet and the thin film of Ag is on the pyramid textured Si wafer.

Figure 7 and 8 shows cross section images of firing

temperature at 650 and 742°C, respectively. The thickness of

cell fired at 650°C is 16.72 µm, while the thickness of cell fired

at 742°C is 14.74 µm. From these figures, it is clearly shown

that the Ag paste diffuse in the n-type Si layer after firing process. This is proved that n+ layer has been form.

Fig. 6. The cross-section images of solar cell before firing process

Fig. 7. The cross-section images of solar cell for a firing temperature at 650°C

Fig. 8. The cross-section images of solar cell for a firing temperature at 742°C

Although the solar cell devices have been successfully obtained, however, their efficiency was still low, namely

10.49%. Since our assumption that the improvement of the

solar cells performance upon firing might be caused by the

change in the interface characteristic and carrier mobility, we

investigate the effect of Silicon Nitride (SiN) as anti-reflecting

coating in order to improve the efficiency. We study the optical

property, which is the reflectivity of the device. From the result

shows in Figure 9, it was found that the textured wafer still

have a high reflectivity in a range of visible light, 400 – 700

nm. It is indicates that the light is not fully absorb by the

textured wafer. Hence, fewer electrons generated and it leads to a low efficiency. The reflectivity is better after we applied SiN,

which is below 0.1. We then applied SiN to all the devices. The

result shows in Table 2, that the Voc, Isc, FF and efficiency were

improved. The highest efficiency obtained is 13.67%.

RSM2011 Proc., 2011, Kota Kinabalu, Malaysia

366

Fig. 9. Reflectance of the planar, textured and textured with a SiN thin film on

Si wafer.

TABLE 2

SUMMARIZED RESULT FROM I-V CURVE WITH A VARIATION OF FIRING

TEMPERATURE

Temperature After SiN deposition

(°C) Voc Isc FF Eff.

(%)

650 0.468 0.081395 0.49877 6.03

730 0.504 0.12664 0.58529 11.86

750 0.492 0.14837 0.53305 12.35

800 0.504 0.13577 0.62937 13.67

IV. CONCLUSION

The best firing temperature of crystalline silicon solar cell have

3//,# !3.:+,/<1# >8/# 3/0.# "+(+,@# ./*;/(:.-(/# +0# =%%&'# 9+.8# :,#

open circuit voltage at 0.5042V and efficiency of 13.67%. The

choice of this parameter will be used for future advanced solar

cell fabrication. It is hoped that it may lead to an increase in the

contact adhesion. The result of Scanning Electron Microscope

proved the formation of the n+ layer, which is, provides the

tunnel for the electron. Further research on higher firing

temperature and its effect on contact adhesion and thickness

will be reported out in the future.

ACKNOWLEDGEMENT

This work has been carried out with the support of the

Malaysia Ministry of Science, Technology and Innovation

(MOSTI) under the TechnoFund and Fundamental Research

Grant Scheme.

REFERENCES

[1] B.Sopori, V. Mehta, P.Rupnowski, J.Appel, M.Romero, H.Moutinho,

D.Domine,B.To, R.Reedy, M.Jassim, A.Shaikh, N.Merchant, C.Khadilkar,

D.Carlson and M.Bennet, “Fundamental Mechanism in the Fire-Through

Contact Metallization of Si Solar Cells: A Review”, 17th Workshop on

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

Cascade Resort, Vail, Colorado USA, August 5 - 8, 2007.

[2] T.Markvart and L.Castaner, Solar Cells Materials, Manufacture and

Operation,Oxford:Great Britain, 2005, pp.96-105.

[3] S.R.Wenham and M.A.Green, “Buried Contact Solar Cell”, U.S.Patent No.

4,726,850,1988.

[5] R.R.Bilyalov, L.Stalmans, L.Schirone and C.Levy-Clement,”Formation of

fire-through silver metal contacts on the porous silicon surface for silicon

solar cells”, IEEE Trans. on Electron Devices, vol. 46, pp.1-24,1999.

[6] J.W.Jeong, A.Rohatgi, V.Yelundur, A.Ebong and M.D.Rosenblum,

”Enhanced Silicon Solar Cell Performance by Rapid Thermal Firing of

Screen-Printed Metals”, IEEE Transaction on Electron Devices, vol. 48,

pp. 2836-2841, 2001.

[7] P.N.Vinod, “Formation of fire-through silver metal contacts on the porous

silicon surface for silicon solar cells”, Semiconductor Science and

Technol.,vol. 20, pp.966-977,2005.

RSM2011 Proc., 2011, Kota Kinabalu, Malaysia

367