In order to minimize these cost and weight penalties, GaAs cells are being developed that are deposited on large area lower cost germanium substrates. These substrates can be made very thin to reduce the weight penalty. This type of cell design can be made in a dual tandem junction structure thus resulting in about 22% AMO efficiencies. Pilot production facilities are being installed now so these cells and the associated performance gains should be available within the next few years. Another advanced solar cell type that is being studied extensively is the indium phosphide (InP) cell. This material has the advantage that radiation degradation can be minimized because the crystal damage sites can be annealed out at room temperatures or at slightly elevated temperatures. Also, the theoretical efficiencies are high (similar to GaAs) with laboratory cells demonstrating about 19% AMO efficiency at this time. Material costs are very high and in limiting supply at this time so cost reduction developments, similar to those being worked on for GaAs cells, must be achieved before this cell type will receive extensive application. Further increases in solar cell efficiency are still possible by utilizing multiple band-gap devices. For instance, research is being conducted to develop a triple junction multi band-gap cell utilizing an AlGaAs, GaAs, InGaAs structure which theoretically has an AMO efficiency of 30%. As in the case above, there will need to be a great deal of cost reduction development done before this type cell becomes a competitive alternative. An alternative approach toward developing an advanced solar cell with specific mission benefits is to utilize an ultra thin film design. Amorphous silicon (ctSi) and copper indium diselenide (CuInSe2) solar cells are examples that have been extensively researched and have been manufactured in quantity for terrestrial markets. These devices have the advantages of being very low cost and can be extremely lightweight since the active device need be only a few microns thick. A major disadvantage today is that large area production type cells are only about 5% AMO efficiency. Laboratory devices can be made in the 10% efficiency range and multiple junction designs theoretically could reach greater than 15% efficiency. There are many unanswered concerns about the radiation resistance and space-worthiness of these devices, but the ultra lightweight and simple compact storage capability provide potential solutions to many space mission needs of the future. The projected power-to- weight ratios for a 10% efficiency thin film cell blanket is expected to reach about 350 watts per kilogram. Conceptually values as high as 5000 watts per kilogram are projected. A summary of the effect of some of the above solar cell advances on the complete solar array properties can be seen in Table IV. Today's typical rigid panel using thick silicon solar cells is used as a baseline for comparison of weight and area parameters. Advanced cell designs combined with advanced lightweight fold-up wing designs (typical of the space station design), provide a comparison of the advances in array technology expected in the near future.
RkJQdWJsaXNoZXIy MTU5NjU0Mg==