Tuesday, May 24, 2016
VOLUME -26 NUMBER 7
Publication Date: 07/1/2011
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ARCHIVE >  July 2011 Issue >  Special Features: SMT and Production > 

Maximizing Efficiency of Silicon Solar Cells
Firing furnace available from Torrey Hills.

Silicon solar cells are one of the most widely used and highly efficient photovoltaics. With growing competition from second and third generation photovoltaics and decreasing module prices, there is an increasing demand for improving efficiency and decreasing the cost of silicon solar cells.

Screen printed metallization, which is one of the most widely used contact formation methods, has been reported as having a significant impact in determining the efficiency of the solar cell. Certain firing process parameters are extremely important, and it is crucial that the appropriate furnace be used for achieving the best results.

Contact Formation
The metallization contact formation is an important step in determining the efficiency of the solar cell. The screen-printing method is a very common contact formation method. In this process, the printed thick film paste is dried at about 150°C to remove a major part of the solvents. The dried substrates are then fired inside a firing furnace. The first step in firing is the initial temperature ramp-up where the paste solvents are volatilized. The second step is the burn-out where all organic binders are removed. The burn-out phase is carried out at 300-400°C. The third step is the sintering, or the firing process, which is done between 700-800°C. During this process, the Ag metal forms a bond with the underlying silicon substrate to form metal contact. The final step in the firing process is the wafer cool-down phase. The mechanism behind contact formation in a fire-through contact is very complex and not fully understood. The glass frit melts at 400-600°C and the sintering of the Ag particles takes place. Furthermore, from 600-800°C, molten glass with some amount of dissolved Ag etches the silicon nitride anti-reflection coating and then reaches the Si surface. Here, it reacts and etches a very thin layer of Si. Ag in the glass will then precipitate onto the Si surface in the form of crystallites.

Peak Temperature
The peak temperature of the sintering process is observed as having a significant impact in determining the efficiency of the silicon solar cell. The ideal peak temperature range for metallization firing has been observed to be 780-800°C. When the silicon solar cells are under-fired, one half of the cells display high contact resistance while the other half displays low contact resistance. At the same time, when the solar cells are fired within the optimum range of 780-800°C, the contact resistance has been reported as being uniform throughout the entire cell. However, when the solar cells were over fired, there was an overall decrease in efficiency due to shunting.

Soak Time
The soak time determines the diffusion of silicon and silver and is a function of time and temperature. From the results in the published literature, the highest efficiencies have always been reported for shorter soak times. Also, soak time increases the contact resistance due to increase in the thickness of the interfacial glass layer.

The design of the belt furnace has a significant effect on firing results as well. Solar cells that are fired in cleaner furnaces have been reported to have higher efficiencies. In addition, the thickness of the oxide layer has also been reported as having a significant impact on cell efficiency. The oxide layer thickness can be controlled by controlling the atmosphere inside of the muffle. Hilali has reported an increase in the conductivity of the silicon solar cell fired within a reduced atmosphere with a small percentage of Hydrogen. [Hilali, M, M., Al-Jasim, M, M., "Understanding the Formation and Temperature Dependence of Thick-Film Ag Contacts on High-Sheet-Resistance Si Emitters for Solar Cells", The Journal of the Electrochemical Society, Vol. 152, No.10, 2005, pp. 742-749.]

Furnace for Firing
The HSH series furnace is a specially designed infrared furnace that caters to the needs of photovoltaic metallization firing requirements. The furnace is rated at 1000°C. The belt width can be customized to meet wafer size requirements. Water cooling is available to yield superior cooling. This furnace comes with a computer control system that not only allows controlling the furnace parameters — the belt speed and the zone temperatures — but also allows storing and retrieving profiles. The furnace is equipped with a redundant overheat safety protection system which incorporates an additional type "K" thermocouple in the center of each controlled zone and the multi-loop alarm.

The firing process is a crucial step in determining the efficiency of the silicon solar cell. The peak temperature of the firing, the soak time and the furnace design itself has a significant influence on the efficiency of the silicon solar cell. The HSH series is an IR furnace that matches well with the requirements for the firing operation.

Contact: Torrey Hills Technologies, LLC, 6370 Lusk Blvd., F-111 San Diego, CA 92121 E-mail: sales@torreyhillstech.com Web:
http://www.torreyhillstech.com

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