Applicability of thermal transient testing for the investigation of solar modules

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Supervisor:
Dr. Plesz Balázs
Department of Electron Devices

Nowadays electric energy became an indispensable a part of our life. We take it for granted and recognize the significance of electric energy only when for some reasons -either for a shorter or longer time- it is not available. Therefore it is very important that renewable energies spread more and more, to serve as a widely accessible alternative to fossil energy sources. Solar energy has the largest potential, because of its unlimited quantity, the sun is practically an inexhaustible energy source, and the installation of a photovoltaic system requires a one-time expense, than the maintenance costs are minimal. Last but not the least, the solar energy is a clean and environmentally friendly energy source and with using solar energy we do not burden the environment.

Photovoltaic devices, e. g. the devices that convert light into energy, have been readily available for decades. The structure, the physics of operation, the factors influencing the efficiency (such as the type of the silicon, the structure, the technology used) are discussed extensively in the literature. From these it is known that solar cell's temperature increase lowers its voltage resulting in less power, so the power and the efficiency of solar cells highly depend on the temperature. Despite the fact that the solar module's thermal behavior strongly influences the temperature of the solar cells in the module – and thus the delivered power, the thermal behavior of the modules hasn't been investigated extensively.

There is a commonly used and well-established measurement method, called thermal transient testing, which was originally developed for determining the thermal parameters of packaged semiconductor devices. With this measurement we can establish the thermal parameters of the device in a non-destructive way. The thermal transient testing is able to measure on any device that has a temperature dependent characteristics, for example a p-n junction. Since a solar cell is practically a large p-n junction, it can be considered as a very large area diode, so in theory it can be tested with this measurement method. From the results of the measurement we can calculate the thermal resistances and heat capacities of the components of the solar cell. From these the structure function can be derived, thus we can characterize each layer of the structure. Therefore we can create a model and it is able to test the device without destruction.

Based on these theoretical considerations, I did thermal transient tests in different solar cell structures and I measured the junction to case thermal resistance according to JEDEC JESD 51-14 standard.

Amorphous silicon and crystalline silicon solar cell structures were studied and I found phenomena which didn't appear during testing usual semiconductor devices, for example diodes.

During testing amorphous silicon solar cells, I observed changes in the structure functions, when the device was driven with the same amount of current, but at different times. The change is likely to occur in the silicon layer. The phenomenon can be explained with a degradation caused by the Staebler-Wronski effect which increases the recombination in the material. This changes the characteristics of the solar cell and the thermal sensitive parameter. In my work I tried to passivate the dangling bonds with heat treatment to confirm our theory, that the observed phenomenon is due to the Staebler-Wronski effect.

I also observed a phenomenon during thermal transient measurement when I changed the heating currents. By increasing the heating currents the thermal resistance decreased and converged to a value. This phenomenon was observed on both amorphous and crystalline silicon solar cells. This phenomenon can be explained by the extension of the area of the electric and thus the thermal excitation when the solar panel is driven with higher currents. An increase in the thermal excitation area results in a higher volume of the heat path which reduces the thermal resistance. If the current is high enough, the heat path incorporates the entire solar cell volume, thus after a certain current there is no change in the heat path volume and the thermal resistance.

In my work I carried out thermal transient measurements on different solar cell structures, and found new phenomena, which we have not experienced with other electronic devices, and in my thesis I explain these phenomena.

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