In Situ Surface Temperature Measurement in a Conveyor Belt Furnace via Inline Infrared Thermography

As it is crucial to measure the temperature of the objects processed in inline furnaces, here we present inline thermography as a promising alternative to classic temperature measurements by thermocouples. Thermocouples damage the object, measure the temperature locally, and require a production interruption. Our inline thermography camera however, measures the object temperature in a contactless, real-time, spatially-resolved manner.

We use inline furnaces for contact firing of silicon solar cells. Therefore, we installed a thermography camera inline into our furnace to investigate these advantages. Select a camera with a detection wavelength range that matches the wavelength range of the highest emission of the object of interest in the temperature range of interest as much as possible.

To install the camera outside of the furnace chamber, remove the furnace wall and isolation at the location where the optical path should be located, avoiding disturbing objects, such as infrared lamps, in the optical path. Close the hole with a window that isolates the furnace chamber thermally while being as transparent as possible for the detection wavelength range of the camera. Then, place the camera above the windows so that the camera has a visual on the moving belt.

Avoid as much parasitic radiation detection by the camera as possible by avoiding nearby objects that emit or reflect radiation in the detection wavelength range of the camera. Then, examine the thermography image via the infrared camera software to check the resulting field of view of the camera. For a customer temperature correction for Silicon solar cells, first check the solar cell for local optical artifacts.

As the temperature correction is based on thermocouples, to check the thermocouple validity, mount the thermocouple on the rear aluminum side of the wafer, and measure the time temperature profile for a standard firing process. If the time temperature profile shows a disruption in the form of a flatter curve at the aluminum silicon eutectic temperature of 577 degrees Celsius, the thermocouple is most likely correctly calibrated. Conduct thermocouple measurements with the validated thermocouple mounted on the rear side of the solar cell, and record the wafer with the infrared camera.

Conduct multiple thermocouple measurements in the temperature range of interest at the same object spot and at spatially various random object spots to obtain statistically significant time temperature profiles. To determine the local uncorrected thermography solar cell temperature under the thermocouple, extract the local temperature at the position of the thermocouple. Log the measured temperatures via thermocouples against the determined temperatures via uncorrected infrared thermography, and obtain a curve fit as a general uniform global correction formula for the uncorrected thermography image.

Then use this curve fit data to correct the uncorrected thermography image globally. To create a two-dimensional peak temperature distribution map, write a script in an appropriate programming language to track the surface temperature for each object’s surface spot along the entire camera field of view to act as a virtual thermocouple placed at all of the wafer spots simultaneously. Then, extract the peak temperature value for each spot, and plot these temperatures in a corresponding 2D distribution map.

To perform an average temperature distribution in the throughput direction, average the 2D temperature distribution in the dimension perpendicular to the throughput direction. To perform an average temperature distribution perpendicular to the throughput direction, average the 2D temperature distribution in the dimension in the throughput direction. As demonstrated in this figure, the corrected temperature of this Silicon solar cell can be clearly detected by the infrared camera in different configurations.

Monofacially metalized, bifacially metalized, and non-metalized perk samples. In these analyses, the temperature range of interest resembled the typical peak temperature range of the firing process. As observed in this image, the contacting thermocouple on the rear side of the solar cell causes a temperature drop around itself, most likely due to heat dissipation and shading.

The latter drop is important for estimating the cell temperature during firing without thermocouples, compared to the temperature measured by the thermocouple, as for this cell positioned onto a frame when contacted by a thermocouple. If placed directly on the belt, the infrared camera allows observation of the local heat dissipation of the cells by the conveyor belt. This image shows a representative two-dimensional spatial solar cell peak temperature distribution, and the deduced average distribution in and perpendicular to the transport direction.

As we use inline furnaces for contact firing of silicon solar cells, we installed an infrared camera into our furnace to create an innovative thermography application. Obtaining spatially resolved peak temperature distributions during the firing process allows the investigation of temperature distribution correlations to the spatially resolved solar cell parameters that are significantly effected by the firing.