1. Field of the Invention
The present invention relates to the field of photoelectric solar cell arrays, and to fabrication processes utilizing, for example, multijunction solar cells based on III-V semiconductor compounds fabricated into interconnected solar cell strings including discrete bypass diodes.
2. Description of the Related Art
Solar power from photovoltaic cells, also called solar cells, has been predominantly provided by silicon semiconductor technology. In the past several years, however, high-volume manufacturing of III-V compound semiconductor multijunction solar cells for space applications has accelerated the development of such technology not only for use in space but also for terrestrial solar power applications. Compared to silicon, III-V compound semiconductor multijunction devices have greater energy conversion efficiencies and generally more radiation resistance, although they tend to be more complex to manufacture. Typical commercial III-V compound semiconductor multijunction solar cells have energy efficiencies that exceed 27% under one sun, air mass 0 (AM0), illumination, whereas even the most efficient silicon technologies generally reach only about 18% efficiency under comparable conditions. Under high solar concentration (e.g., 500×), commercially available III-V compound semiconductor multijunction solar cells in terrestrial applications (at AM1.5D) have energy efficiencies that exceed 37%. The higher conversion efficiency of III-V compound semiconductor solar cells compared to silicon solar cells is in part based on the ability to achieve spectral splitting of the incident radiation through the use of a plurality of photovoltaic regions with different band gap energies, and accumulating the current from each of the regions.
In satellite and other space related applications, the size, mass and cost of a satellite power system are dependent on the power and energy conversion efficiency of the solar cells used. Putting it another way, the size of the payload and the availability of on-board services are proportional to the amount of power provided. Thus, as payloads become more sophisticated, the power-to-weight ratio of a solar cell becomes increasingly more important, and there is increasing interest in lighter weight, “thin film” type solar cells having both high efficiency and low mass.
Typical III-V compound semiconductor solar cells are fabricated on a semiconductor wafer in vertical, multijunction structures. The individual solar cells or wafers are then disposed in horizontal arrays, with the individual solar cells connected together in an electrical series circuit. The shape and structure of an array, as well as the number of cells it contains, are determined in part by the desired output voltage and current.
Sometimes, the individual solar cells are rectangular, and often square. Photovoltaic modules, arrays and devices including one or more solar cells may also be substantially rectangular, for example, based on an array of individual solar cells. Arrays of substantially circular solar cells are known to involve the drawback of inefficient use of the surface on which the solar cells are mounted, due to space that is not covered by the circular solar cells due to the space that is left between adjacent solar cells due to their circular configuration.
However, solar cells are often produced from circular or substantially circular wafers. For example, solar cells for space applications are typically multi junction solar cells grown on substantially circular wafers. These circular wafers are sometimes 100 mm or 150 mm diameter wafers. However, as explained above, for assembly into a solar array (henceforth, also referred to as a solar cell panel), the circular wafers are often divided into other form factors to make solar cells. One preferable form factor for a solar cell for space is a rectangle, such as a square, which allows for the area of a rectangular panel consisting of an array of solar cells to be filled 100%, assuming that there is no space between the adjacent rectangular solar cells.
Space applications frequently use high efficiency solar cells, including multijunction solar cells based on III-V compound semiconductors. High efficiency solar cell wafers are often costly to produce. Thus, the waste has conventionally been accepted in the art as the price to pay for a high fill factor, that is, the waste that is the result of cutting the rectangular solar cell out of the substantially circular solar cell wafer can incur a considerable cost.
One method of reducing waste is by using solar cells having oblique cut corners, also referred to as cropped corners. Solar cells with cropped corners can be obtained from a substantially circular solar cell wafer, which allows a substantial part of the wafer to be used for the production of a substantially octagonal solar cell. As the four oblique sides at the corners are shorter than the other four sides, the general layout of the solar cell is substantially rectangular or square, and a high coverage factor is obtained when the solar cells are placed in an array to provide a substantially rectangular solar cell array. Some space is wasted at the corners of the solar cells, as the space where the solar cells meet at the cropped corners will not be used for the conversion of solar energy into electrical energy. However, this wasted space amounts to a relatively small portion of the entire space occupied by the solar cell array and typically can be used to house other components of the solar cell assembly, such as bypass diodes.
Bypass diodes are frequently used for each solar cell in solar cell arrays comprising a plurality of series connected solar cells or groups of solar cells. One reason for this is that if one of the solar cells or groups of solar cells is shaded or damaged, current produced by other solar cells, such as by unshaded or undamaged solar cells or groups of solar cells, can flow through the bypass diode, and thus avoid the high resistance of the shaded or damaged solar cell or group of solar cells. Placing the bypass diodes at the cropped corners of the solar cells can lead to increased efficiency, because the bypass diodes make use of a space that is not used for converting solar energy into electrical energy. As a solar cell array or solar panel often includes a large number of solar cells, and often a correspondingly large number of bypass diodes, the efficient use of the area at the cropped corners of individual solar cells can represent an important enhancement of the efficient use of space in the overall solar cell assembly.