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In the previous chapters, we have focused on the study of single organic solar cells. To power an actual electrical tool with such a device, one might think of simply enlarging the active device area, thereby nominally generating a higher output current and a higher amount of electrical energy. Unfortunately, from a technological and practical point of view, this is not always the best solution. Therefore, integration of solar cells into electrical applications creates the need for interconnecting a number of single cells with each other into a larger structure.
We have seen that a standard single solar cell is characterized by an open-circuit voltage below 1 V. The voltage at the maximal power point is still somewhat lower than this, even at high illumination levels. The actual output current varies, of course, strongly with the light intensity, but the size of the active area of the device is also crucial. However, due to many technological issues, the dimensions of single solar cells will be limited. Small inhomogeneities in the active layer may deteriorate the performance, especially for the very thin films that are used in organic photovoltaic cells. These effects scale with the size of the device. Furthermore, the collection of the photogenerated current for large solar cells can be substantially influenced by the limited conductivity of the electrode material. Thus, the output of a single solar cell is too small to power most practical applications. Therefore, interconnecting separate devices into a module to enlarge the overall output has to be considered. This can be achieved by externally connecting solar cells that have been fabricated separately. This is what is commonly done in the wafer-based Si solar cell industry. Besides this, an integrated approach in which monolithic production of photovoltaic modules is achieved can also be considered. There, the different layers are processed on one substrate in such a way that the different cells are directly interconnected. This is, of course, a very attractive production process that is pursued for many thin-film solar cell technologies and also in the field of organic solar cells. Accurate structuring and patterning of the subsequent layers is absolutely necessary here.
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