Neue Infrarotmeßtechniken für die Photovoltaik

Abstract

The significant increase in solar cell production over the last few years has triggered an increased demand for characterization tools suitable for in-line and / or off-line process-control. Simultaneously, from a scientific perspective, there is a demand for characterization tools with high spatially resolution that may significantly contribute to a better understanding of the loss mechanisms in laterally inhomogeneous solar cells. This thesis contributes to the development of such characterization tools. Measurement instruments were developed that are capable to characterize performance limiting parameters of finished solar cells and solar cell precursor with sufficient lateral resolution. At the moment, the two- or three-dimensional modeling of solar cells with inhomogeneous parameters is only attainable in complex semiconductor simulation tools. Therefore, modeling tools were developed, that allow for a quick and simple evaluation of the influence of measured inhomogeneities on solar cell parameters. Among others, the characterization and modeling tools described in the following were developed in the framework of this thesis: The class model is a possibility to incorporate the effect of the frequency distribution of a laterally inhomogeneous parameter, as e.g. carrier lifetime, in a one dimensional solar cell simulation. The basic idea is to calculate an appropriately weighted mean of the lifetime distribution and use this weighted mean in the 1D simulation. It was shown, that especially for lifetime and diffusion length distributions, the class model is capable to incorporate the complete effect of inhomogeneous material quality in a 1D simulation. A measurement station for Dark Lock-In Thermography (DLT) after the principle proposed by Breitenstein and Langenkamp was built. A disadvantage of Dark Lock-In Thermography is that the measurements are performed on an un-illuminated solar cell with electrical excitation. It is well known, that illuminated and un-illuminated current paths in a solar cell may differ considerably. Hence it is not amazing, that in a number of cases DLT-measurements may not give a realistic image of the relative impact of different loss mechanisms in the solar cell. For this reason Dark Lock-In Thermography was further developed to the principle of Illuminated Lock-In Thermography (ILT) within this thesis. In Illuminated Lock-In Thermography the electrical excitation source is replaced by a modulated light source. This method allows for a direct and spatially resolved measurement of the power losses in a solar cell at the operation point (MPP). This enables a realistic evaluation of the influence of the spatial distribution of power losses and different loss mechanisms on solar cell performance. The method of Carrier Density Imaging (CDI) enables for the first time the measurement of lifetime topographies of essentially every silicon wafer relevant for photovoltaics without the need for scanning the sample. Thus CDI decreases measurement times for spatially resolved lifetime images by at least a factor of 100 compared to other state-of-the-art lifetime measurement equipment. An additional advantage of CDI is the contactless measurement of absolute lifetime values under low-level injection conditions. CDI can be used in either an absorption or an emission mode. In Emission-CDI a slightly increased sample temperature results in a significant increase in signal strength and thus leads to a further reduction of measurement time. A second possibility for the further reduction of measurement time is to use a flash instead of the semiconductor laser for illumination purposes. Measurement times below 100 ms were realized with this Flash-CDI within this work. The setup that is used for CDI-measurements may as well be used for the investigation of emitter sheet resistance RSh if the measurement conditions are appropriately altered. An according measurement method (Sheet Resistance Imaging SRI) was developed. In contrary to four-point-probing, which is the standard method for measurements of RSh at the moment, SRI is a contactless measurement with measurement times of a few seconds. As in CDI measurements a good spatial resolution of e.g. 350 µm on a 100x100 mm² sample may be achieved without the need for time-consuming scanning of the sample. The class model was used to simulate the dependence of solar cell performance on the distance from the bottom of a multicrystalline ingot. With this method important information about the size of the useable part of a silicon ingot was attained. In addition, the influence of different conditions in the phosphor diffusion process on bulk lifetime of multicrystalline samples was investigated. In this investigation, the class model helped to determine the optimal diffusion conditions from the controversial demands for high and low lifetime areas.