This protocol describes how to install an infrared camera into a conveyor belt furnace, conduct a customer correction of a factory calibrated IR camera, and evaluate the spatial surface temperature distribution of an object of interest. The example objects are industrial silicon solar cells.
Measuring the surface temperature of objects that are processed in conveyor belt furnaces is an important tool in process control and quality assurance. Currently, the surface temperature of objects processed in conveyor belt furnaces is typically measured via thermocouples. However, infrared (IR) thermography presents multiple advantages compared to thermocouple measurements, as it is a contactless, real-time, and spatially resolved method. Here, as a representative proof-of-concept example, an inline thermography system is successfully installed into an IR lamp powered solar firing furnace, which is used for the contact firing process of industrial Si solar cells. This protocol describes how to install an IR camera into a conveyor belt furnace, conduct a customer correction of a factory calibrated IR camera, and perform the evaluation of spatial surface temperature distribution on a target object.
Process control and quality assurance of objects processed in conveyor belt furnaces1 is important and accomplished by measuring the surface temperature of the object. Currently, the temperature is typically measured by a thermocouple1. As thermocouple measurements require contact with the object, thermocouples inevitably damage the object. Therefore, it is common to choose representative samples of a batch for temperature measurements, which are not further processed since they become damaged. The measured temperatures of these damaged objects are then generalized to the remaining samples from the batch, which are further processed. Accordingly, production must be interrupted for thermocouple measurements. Furthermore, the contact is local, needs to be readjusted after each measurement, and influences the local temperature.
Infrared (IR) thermography2 has a number of advantages over classic thermocouple measurements and represents a contactless, in-situ, real-time, time-saving, and spatially resolved temperature measurement method. Using this method, each sample of the batch, including those that are further processed, can be measured without interrupting production. In addition, the surface temperature distribution can be measured, which provides insight into temperature homogeneity during the process. The real-time feature allows correction of temperature settings on-the-fly. So far, the possible reasons for not using IR thermography in conveyor belt furnaces are 1) unknown optical parameters of hot objects (especially for nonmetals3) and 2) parasitic environmental radiation in the furnace (i.e., reflected radiation detected by the IR camera in addition to the emitted radiation from the object), which leads to false temperature output2.
Here, as a representative proof-of-concept example of IR thermography in a conveyor belt furnace, we successfully installed an inline thermography system into an IR lamp powered solar firing furnace (Figure 1), which is used during the contact firing process of industrial Si solar cells (Figure 2A,B)4,5. The firing process is a crucial step at the end of industrial solar cell production6. During this step, the contacts of the cell are formed7,8, and surface passivation is activated9. To successfully achieve the latter, the time-temperature profile during the firing process (Figure 2C) must be accurately realized. Therefore, sufficient and efficient temperature control is required. This protocol describes how to install an IR camera into a conveyor belt furnace, conduct a customer correction of a factory calibrated IR camera, and evaluate the spatial surface temperature distribution of a target object.
1. Installation of IR camera into a conveyor belt furnace
2. Global customer temperature correction of a fabrication calibrated IR camera
CAUTION: The fabrication of the IR camera is assumed to include a radiometric calibration.
3. Evaluation of spatial surface temperature distribution via IR thermography
NOTE: The firing conditions are assumed to be identical for this section.
As shown in Figure 3B−D, the example object (here, a silicon solar cell; strictly speaking, a passivated emitter and rear cell [PERC]12; Figure 2A,B) can be clearly detected by the IR camera in different configurations4. The different configurations are monofacially metallized (Figure 3B), bifacially metallized13 (Figure 3C) and nonmetalized PERC samples (Figure 3D). The difference between the monofacial and bifacial configuration is that the former has a full area aluminum layer, whereas the latter has an H-pattern grid (similar to the silver front side) on the rear side. Here, the IR camera was positioned in a way that the camera FOV captured the peak temperature of the firing process. The peak phase is the most crucial phase during the firing process, since the contacts are actually formed during this phase14. Here, the temperature range of interest resembled the typical peak temperature range of the firing process (i.e., ca. 700–900 °C1).
For the latter temperature range, the spectral emissivity is quite high and homogenous in the short, middle, and long wavelength infrared spectra3. A double sapphire layer was used as a transmissive window, allowing for good transmission in the short and middle IR wavelength spectra. In order to minimize detection of light from the IR lamps of the furnace (peak wavelength in short wavelength infrared range), an IR camera type with InSb as detector material was chosen, with a detection range of 3.7−4.1 µm (including filters). Only one-third of the wafer in the throughput direction can be detected at the same time. However, it was sufficient for this work, since the wafer passes the existing field of view entirely. Naturally, temperature corrected thermography images are shown here. Strictly speaking, the image is temperature-corrected with respect to the solar cells.
As can be seen in Figure 3A, the contacting thermocouple on the opposite side of the optical path caused a temperature drop around itself (with a temperature drop of 10 K), most likely due to heat dissipation and shading. The latter drop is important to estimate the cell temperature during firing without thermocouples, compared to the temperature measured by the thermocouple. Here, the cell was positioned onto a frame when contacted by a thermocouple (Figure 3E). The heat dissipation by the frame caused a temperature drop of around 10 K. Together with the additional heat drop by the thermocouple, the latter measured a 20 K lower temperature than what the cells displayed during standard processing (without the thermocouple equipment). It is important to estimate the latter offset for the used thermocouple system, which is performed with the help of thermography, as shown. The IR camera allows observation of the local heat dissipation of the cells by the conveyor belt if placed directly on the belt (Figure 3F). This is the reason why cells are usually placed on belt elevations to minimize contact between them and the belt.
Figure 4 shows the surface temperature distribution. Since silicon solar cells are typically around 160 µm thick and processed in the furnace for 30 s, it is likely that the temperature distribution along the cell depth is homogenous. Therefore, the results most likely suggest a temperature distribution rather than only a surface temperature distribution. Opposite to the throughput direction, an average temperature gradient of 1 K/cm was obtained. In the throughput direction, the incoming wafer quarter was substantially colder than the trailing wafer rest. The colder incoming portion experienced a gradient of 7 K/cm, while the hotter trailing part experienced a gradient of 0.5 K/cm.
In both directions, the cell edges (the remaining 2 cm) were ignored for determination of the gradients, since the detected temperature at the edges mixed with the colder outside boundary of the cells, resulting in false temperatures. Figure 4C shows a representative 2D temperature distribution of a monofacial solar cell, which was not metallized at the front side. The abovementioned trends in the same and opposite transport directions were observed here, as well. All in all, these results reveal that the solar cells in this work experienced a certain degree of spatial temperature inhomogeneity.
Figure 1: Most important equipment used in the protocol. (A) Lateral scheme of the conveyor belt furnace. This figure panel has been modified from Ourinson et al.4. (B) Zoomed-in last firing zone, visualizing the setup of the thermography system. 1) Furnace wall and isolation, 2) IR camera, 3) IR lamps, 4) insulating window, 5) object transport direction, 6) camera FOV, 7) transportation belt, 8) object, and 9) thermography software. This figure panel has been modified from Ourinson et al.4. (C) The firing furnace used during this protocol. (D) Image illustrating the used IR camera and transmissive IR window positioned in the firing furnace. The numbers correspond to the numbers from panels A and B. Please click here to view a larger version of this figure.
Figure 2: Measured objects and their temperatures. (A) Schematic cross-section of a monofacial PERC solar cell. (B) Front (left) and rear (right) side view of an industrial PERC cell. (C) Thermocouple-measured industrial time-temperature profile of a PERC solar cell during the firing process, including segmentation into phases and section, which is covered by the camera field of view. This figure has been modified from Ourinson et al.5. (D) Demonstration of disruption around the eutectic temperature (TEUT) of aluminium and silicon in a firing profile measured by a thermocouple, when the thermocouple is placed on the aluminium rear side of the solar cell. This figure has been modified from Ourinson et al.5. Please click here to view a larger version of this figure.
Figure 3: Representative temperature-corrected thermography images of PERC solar cells for identical firing conditions. (A) Visible local temperature drop caused by contact of a thermocouple from the rear side. (B) Thermography image of the upper one-third of a monofacially metallized PERC cell, including (1) visible busbars (2) positioned on the visible conveyor belt. TAV shows the average temperature on the wafer. (C) Thermography image of a bifacially metallized PERC cell. (D) Thermography image of a nonmetallized PERC wafer. (E) Thermography image of a wafer placed on a thermocouple frame and contacted by a thermocouple. TTC shows the wafer temperature measured by the thermocouple. (F) Thermography image of a wafer placed directly on the conveyor belt. (G) Color map of the temperature range measured by the IR camera. This figure has been modified from Ourinson et al.5. Please click here to view a larger version of this figure.
Figure 4: Temperature distribution of a PERC solar cell for identical firing conditions. (A) 2D peak temperature distribution of a monofacial PERC solar cell from the front side. (B) Average peak temperature distribution in (right picture) and perpendicular (left picture) to the cell transport direction.”Please click here to view a larger version of this figure.
Commonly, thermography temperature is corrected via measuring and adapting the optical parameters of the object, transmissive window and path, and environmental temperature of the object and transmissive window2. As an alternative method, a temperature correction technique based on thermocouple measurements is described in this protocol. For the latter method, knowledge of the parameters mentioned above is not required. For the application shown here, this method is sufficient. However, it cannot be guaranteed that the thermocouple method is sufficient for all thermography applications in a conveyor belt furnace.
In the protocol, a uniform global temperature correction of the thermography image is proposed; although, it is more precise to correct the spatially resolved temperature. However, it has been found that the uniform temperature correction is more appropriate in cases of moving objects. Furthermore, it is intended to correct the temperature of the object rather than the surrounding objects (e.g., the belt and walls).
As mentioned in step 2.3.2.2, the example provided here is assumed to have a homogeneous temperature distribution along the object depth. In cases of objects with inhomogeneous temperature distribution along their depths, the temperature on one surface does not resemble the temperature on the opposite surface. Thus, the steps described in section 2.3.2.2 do not apply for these cases. A solution for inhomogeneous temperature distribution along the object depth must be further studied.
The authors have nothing to disclose.
This work is supported by the German Federal Ministry for Economic Affairs within the project “Feuerdrache” (0324205B). The authors thank the co-workers that contributed to this work and the project partners (InfraTec, Rehm Thermal Systems, Heraeus Noblelight, Trumpf Photonic Components) for co-financing and providing outstanding support.
Datalogger incl. Thermal barrier | Datapaq Ltd. | ||
IR thermography camera "Image IR 8300" | InfraTec GmbH | ||
IR thermography software "IRBIS Professional 3.1" | InfraTec GmbH | ||
Solar cells | Fraunhofer ISE | ||
Solar firing furnace "RFS 250 Plus" | Rehm Thermal Systems GmbH | ||
Sheath thermocouples type K | TMH GmbH | ||
Thermocouple quartzframe | Heraeus Noblelight GmbH |