SCREEN PRINTED METALLIZATION OF SILICON SOLAR CELLS

This paper presents a screen printing process for the metallization of silicon solar cells. The physics and construction of a classical solar cell are reviewed. The results obtained with a screen printing process are comparable with other, more expensive technologies. This technology does not introduce an additional contact resistance on silicon. The process optimization and the influence of different parameters are discussed.


INTRODUCTION cell is:
The research and development of solar cells has grown considerably in the past few years, spurred on mainly by the potential use of these cells for large scale terrestrial solar energy applications.The United States Department of Energy predicts a cost reductiSn from $20.00 per peak Watt in 1976 to $0.50 per peak Watt in 1986 (Figure 1).With this cost projection they predict an annual market of 10 GW to 20 GW per year in 1990.
The silicon solar cell is one of the simpler semi- conductor device structures and consists of a single n-p junction (Figure 2).
If solar radiation falls on the upper side of the solar cell the light will be absorbed'by the material and will generate electron-hole pairs.The internal electric field at the n-p junction is of such a polarity that the minority carriers are driven towards the junction.This process builds up excess charge, causing a potential difference of about 500 to 600 mV between the n-type and p-type silicon.
When the cell is connected to a load a current will flow.To make an electrical contact on the upper side of the solar cell a finger pattern of conductor material is deposited.Connection to the bottom side is made by a metal film covering the whole surface.All the contacts have to be ohmic.Figure 3 gives the I-V characteristic in darkness and under illumination.Voc is the open circuit voltage, Isc is the short circuit current and Pmax is the maximum power point.The fill factor of the solar Ph.Lauwers is supported by the I.W.O.N.L $ R. Mertens is supported by the N.F.W.O.

Voc*Isc
The equivalent circuit of a solar cell (Figure 4) consists of a diode in parallel with a current source, delivering a current proportional to the incident light level.The parallel resistor represents the leakage while the series resistor represents the series resistance of the solar cell.

FABRICATION
The fabrication (Figure 5) of a silicon solar cell starts with a Czochralski grown p-silicon wafer as the base material.A phosphorus diffusion generates a n / layer with a thickness of about 0.4 pm.A back etch removes the parasitic junction.Then the metallization is deposited on front and back side with either an evaporation or a screen printing technology.An anti-reflective coating (TiOx) is also spun on.The screen printing process is applicable to two process steps of the silicon solar cell fabrication" the junction formation and the metallization.For the junction formation a phosphorus doped paste is screened on the front side and diffusion occurs at a temperature of about 900 C.
This paper emphasizes the optimization of screened ohmic contacts.The screen printing process for the metallization of silicon solar cells uses thick film technology and lends itself much more to automation than the conventional vacuum evaporation.The evaporation is a hand operated batch and vacuum process and it has a high facility cost.The screen printing technology avoids all these disadvantages with almost the final same efficiency.

P max
It is easy to fully automate, so it gives a cost reduction of the processing of 60% to 80%.

PROCESS OPTIMIZATION
Cu-layer is deposited, using an electroplating process, followed by a solder dip.This process yields also a lower sheet resistance of the finger pattern and decreases the series resistance of the solar cell.
The results, reported here, are obtained on different sizes of cells.The best results for 1/2 2 in.wafers are given in Table I.As can be seen, excellent open circuit voltages and fill factors are obtained, while the short-circuit currents are somewhat lower than for evaporated cells.This is due to the slightly deeper junction necessary to avoid leakage during the firing of the silver paste at this high temperature.Similar results are obtained with 3 in.wafers.
Average values under AM1 illumination of Jsc, Voc, FF and efficiency calculated out of different batches of 50 cells are given in Table II.The first great difference with an evaporation technology is that the front metallization pattern is screened on the anti-reflective coating (TiOx) instead of evaporation directly on the silicon.This yields a good electrical contact and at the same time the anti-reflective coating forms a barrier against the diffusion of impurities into the solar cell.A commercially available silver paste (ESL 5964 or similar) to which a few percent of Ti powder is added, is screen printed on the front side, using a grid contact configuration.To minimize the contact resistance, the paste has to be fired at a temperature of 860 C. Afterwards an aluminium paste (Engelhard T2497 or similar) is screen printed on the back side to form a p / back surface due to aluminium diffusion; at the same time it produces an ohmic contact.The aluminium paste is air fired at a temperature of 660C to 720C.To obtain solderable contacts a The variations in solar cell performance with the junction depth has been measured (Table III).From a junction depth of 0.36 #m (sheet resistance of diffused layer is 35 2/m), the fill factor decreases due to an increase in leakage current.
For evaporation technology, where the optimum junction depth is much smaller (+0.2/am), the silver paste used gives an optimum around 0.45 #m.Ti improves the quality of the electrical contact of Ag on Si. ,4 The fill factor as a function of Ti percentage (Figure 6) has been measured while the screen printing is done on the TiOx coating or on the silicon.There is a slight maximum between 4% and 7% Ti for the screen printing on the TiOx coating, whereas the maximum is at much higher Ti-concentrations when screen printing is done directly on the silicon.From this we can conclude that the TiOx coating has partially the same function as the Ti in the paste.

The Firing Profile
The optimum firing temperature for the modified silver paste on the front side seems to be around 860C (Figure 7).A slightly lower temperature gives an important increase in series resistance of the solar cell.A slightly higher temperature causes a decrease in output current due to degradation of the lifetime of the minority carriers in the solar cell.
For the Al-paste on the back side an optimum exists for a firing temperature around 660C to 720 C. The fill factor and with it, the efficiency, improves when the time at peak temperature increases.This is probably due to a diffusion of A1, causing a p / back surface.
4. 4. Screen Printing Parameters Snap-off distance and squeegee velocity are not very critical; they have to be chosen to give a good screen printing resolution.Somewhat more critical Influence of firing temperature on series resistance and the short-circuit current for 1/2 2 in.wafers.
is the squeegee pressure.Here one has to take care because a pressure that is too high can easily damage the very shallow junction.
4.5.Limitation in Line-Width Unlike the photolithographical and evaporation technology one has a line-width limited to 150/am while screen printing this particular paste.This has some consequences especially with regard to concentrated sunlight.Operation under highly concentrated sunlight requires very low series resistance corresponding with a very dense metallization grid such that finger widths of 50/am are necessary.However, cells fabricated with a screen printing metallization could still be used for concentration factors up to 30.
Output parameters of some of our cells are listed in Table IV.It is a 2 x 6 cm rectangular cell, optimized for a concentration factor of 15.The calculated series resistance was 15.4 m2, the measured one was 23.5 m. 5. CONCLUSIONS 1) A vacuum free process, using screen printing has been described.It gives an important processing cost reduction with almost the same efficiency.
2) Screen printing of solar cells can be controlled in such a way that it does not introduce any significant contact resistance.
3) Screen printed solar cells can be optimized to be used at concentrations as high as 30.

I
FIGURE 5 Fabrication steps of a silicon solar cell.

TABLE
Best results on cells of 1/2 2 in.wafer.Cell size: 10 cm Metal coverage: 8%.All values at AM1 illumination: 100 mW/cm and temperature of 25 C.

TABLE IV Output
parameters of a 2 x 6 cm cell.Substrate resistivity: S2.cm.Metal coverage is 12%