The present invention is directed to a tandem solar module as well as to a method for the manufacture thereof.
High-efficiency, large-area, stable and, cost-effective solar cells or modules are needed for photo-voltaic energy generation in the power range.
Known solar modules of crystalline silicon (c-Si) currently achieve effective efficiencies between 10% and 14% for the conversion of sunlight into electrical energy. However, the crystal quality of the silicon must be extremely pure and results in high costs for silicon material. As a result such solar modules cannot economically compete with traditional energy sources.
Solar modules of amorphous, hydrogenated silicon (a-Si:H) are in fact relatively inexpensive to manufacture for this purpose. However, current solar modules of a-Si:H only achieve efficiencies of about 5% through 6%. Further, these modules tend toward long-term instabilities when exposed to light. A noticeable decrease in power of up to about 20% can be observed after a modest operating time.
Further, the solar sprectrum is not optimally exploited by standard solar modules. As a result of the higher band spacing of a-Si:H in comparison to c-Si, modules of amorphous silicon predominantly exhibit absorption in the blue range of the visible spectrum, whereas the crystalline cells absorb more in the red, longer-wave length range. This disadvantage can in fact be avoided by using high-grade, single-crystal silicon wafers and complex cell structures. However, the costs of such solar cells are extremely high.
Even the use of gallium arsenide, which is more receptive to the natural solar spectrum, for photo-voltaic elements has not produced the desired results and is not capable of supplying price-beneficial and high-efficiency solar cells.
Tandem solar cells offer another solution for better adaption of solar cells to the solar spectrum. These are thin-film solar cells of amorphous, hydrogenated silicon and other materials that are deposited or formed directly on top of one another, as is disclosed for example, in a report by J. Yang, R. Mohr and R. Ross in Proceeding of the First International Photovoltaic Science and Engineering Conference, Kobe, Japan, 1984, A-IIa-L6. These cells, whose active zones can have p-n or p-i-n structures, utilize the transparency of the a-Si:H material in order to also utilize those photons not absorbed in the upper cell for generating pairs of charge carriers in the lower cell. On the basis of this structure, tandem cells are "series-connected" and are treated like a single cell as a result of the electrical connection. Additives to the a-Si:H layers make it possible to modify the band spacing of the individual cells, so that the tandem cell operates in a greater spectral range than that of an individual cell.
However, the disadvantages such as long-term instability and poor efficiency also occurs in these tandem solar cells. Moreover, the layer thicknesses of the individual cells must be exactly matched to one another, so that the demand for equal currents is met by both individual cells and so that the double cell can operate satisfactorily.