MANUFACTURING I THERMAL
Uniform thermal control
Continual improvement is the usual mantra for technology manufacturing and PV is no different. Dr. Hans Bell , Milo Gaggioli and Manuel Schwarzenbolz of Rehm Thermal Systems, along with Dr. Kay Reuter, Mathias Ribke, Dr. Norbert Müller, Christian Arndt and Dr. Armin Froitzheim of Bosch Solar Energy come together to reveal recent results for machine capability of firing systems.
FIRING IS A PRODUCTION PROCESS during which the contacts of a crystalline solar cell are conclusively formed. It’s a thermal process that takes place in a tunnel oven, in which the solar wafers are exposed to a defined temperature profile. The tolerance width and the reproducibility of this thermal process are essential criteria for assuring uniform quality of the manufactured solar cells. The temperature profile (see figure 1) is subdivided into several zones.
The solar wafers, which are printed with a thick-layer paste, are heated up to a temperature of approximately 500 to 600° C in the burnout zone. The goal of this process step is to drive out the nonvolatile organic constituents which remain in the paste after the upstream drying process. The solar pastes are sintered in the firing zone (approx. 900° C), where the electrical contact is generated. The firing process is especially decisive for the quality of the solar cells. The purpose of the cooling zone is to cool the solar cells down to room temperature.
Figure 2: Influence of conveyor speed in the peak zone
A complex relationship exists between the utilized solar pastes, the temperature gradients, the absolute temperatures, the wafer material and, in particular, the quality of the solar cell coating system. A great deal of experience and process knowledge is necessary in order to define the temperature profile that results in the greatest degree of efficiency for the manufactured solar cells. Measurement of temperature profiles presents a number of challenges in this respect. Amongst other factors, this has to do with the relatively high temperatures in the firing zone, as well as the very steep temperature gradients that occur from segment to segment within the temperature profile.
Figure 1: Temperature profile of a firing oven 22
www.solar-international.net I Issue IV 2014
A further problem results from the relatively minimal thermal mass of a solar cell as compared with the measuring equipment. Even a slightly altered thermal mass at the measuring point (caused for example by unsuitable mounting of a thermocouple) can influence the measured temperature. Limit deviation,
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