Failure analysis of thin film solar modules using lock-in thermography

Lock-in thermography (LIT) is an imaging method that depicts radiated heat and its diffusion in manifold samples. LIT offers versatile possibilities for the characterization of solar cells and modules since the radiated heat is proportional to the dissipation of electrical power. Up to now, the quantitative correlation of detected heat and dissi-pated electrical power has been known for silicon solar cells only. For many other types of solar cells and modules – especially thin film solar cells – LIT has been used as a qualitative measurement tool for depicting the location of defects, for example. Thus, the potential of LIT in terms of the calculation of power generation and dissipation in thin film solar cells has not been exploited. This visualization and calculation of power flows leads to a better understanding of the influences of defects on the efficiency of solar modules. Furthermore, it enables the evaluation of potential improvements, which results in solar modules with higher efficiencies, produced to lower costs.In order to interpret LIT signals accurately, the lock-in algorithm and particularly its limits have to be understood. The present thesis shows the evaluation of the lock-in algorithm and its algebraic complex result with simulations. It is found that the weak points of the lock-in algorithm lie in the sampling of the acquired heat signal. Sampling moments that are not uniformly distributed in a lock-in period produce unreliable results. A low sampling at high measurement frequencies shows significant deviations distorting the LIT result. The findings allow for the development of user-friendly LIT systems that automatically avoid sampling errors and produce reliable LIT results. The comprehension of LIT measurements of thin film solar cells needs a theoretical thermal model for the solar cells that can be used to solve the differential heat diffusion equation. The solution describes the surface temperature distribution that is acquired in LIT measurements. By the evaluation of the frequency response of a point heat source in a thin film solar cell, a simple thermal model representing a solid body is found to adequately reproduce LIT measurements. LIT investigations in the scale of the thermal diffusion length are hampered by the diffusion of heat that leads to a blurring of heat sources. With the description of the thermal model and a Fourier transform technique, it is possible to successfully deconvolute the heat generating sources from the heat diffusion, meaning the removal of the thermal blurring. This leads to the unimpeded visualization of the dissipated power of small heat sources such as shunts or the series interconnection of cells in a thin film solar module.To deconvolute LIT measurements of samples, which thermal model is not known, an advanced LIT system is developed. It uses a small laser beam, which is directed at the solar cell and is measured by the LIT setup, in order to obtain the heat dif-fusion in the sample. The deconvolution adequately reproduces the size of a shunt. Combined with an evaluation algorithm, such an LIT system is carrying out a “self-calibration” for the heat diffusion in any kind of solar cell sample. This enables the system to autonomously detect the dissipated power in manifold devices.Up to now, the estimation of power losses in solar cells commonly has been carried out with LIT measurements without illumination. This is justified for crystalline silicon solar cells, since their behavior in illuminated conditions can be calculated from unilluminated conditions with a simple model. In thin film solar modules, the electrical behavior in illuminated and unilluminated conditions differs significantly. Thus, the access to power losses lies in LIT measurements of solar modules in operation conditions: with illumination and at a voltage enabling the maximal extraction of power. The quantification of such measurements is impeded by several overlapping heat dissipation mechanisms occurring in thin film solar modules. With new developed LIT methods using a bias voltage modulation under constant illumination, the overlapping heat dissipation mechanisms are resolved and a power scaling for LIT images is achieved. These novel LIT methods thus propose an approach to realistic estimations of power losses in thin film solar modules. Furthermore, the novel LIT methods allow the investigation of single cells and their interaction in the compound of a module. In industrial solar modules, cells are capsuled and cannot be contacted to determine their electrical behavior in the compound. A specially constructed laboratory scale solar mini-module allowed the contacting of the cells and the acquisition of their electrical characteristics. A good representation of the electrical behavior of the cells in the module compound in LIT measurements with the new methods was found. In future, these methods can be used for the comparison with simulations of the electrical behavior of solar modules. Findings derived from such a comparison allow for finding critical efficiency-limiting features in solar modules