The efficiency of energy conversion of solar energy into electric current can be increased significantly by application of nanomaterials. Beyond that, anti-reflecting coatings for solar cells and collectors can increase the light conversion efficiency. For applications in space, however, clearly higher demands on solar cells must be fulfilled rather than for terrestrial applications. While, due to the mass restrictions in space transportation, a maximum efficiency is aimed at, even if expensive manufacturing processes and materials are to be accepted, an appropriate durability of the collectors under space conditions must also be ensured (radiation and corrosion resistance).
The Most Efficient Solar Cells are Based on III/V Semiconductors
At present the most efficient solar cells for space applications are based on III/V-semiconductors such as GaAs (Gallium Arsenide) and InP (Indium Phosphide). These cells are manufactured by heteroepitactical deposition on semiconductor substrates. By vertical alignment of two or more compounds (junctions) with different gaps the energy output can be optimized (binary or multi junction cells). By means of optical concentrators the energy output can be increased additionally. At present, the most efficient solar cells for space applications have a conversion efficiency of approximately 30% and are manufactured, for example, by the US-American company Spectrolab.
Conversion Efficiency Rates Using Compound Semiconductor Solar Cells - Practice versus Theory
In principle, conversion efficiencies of over 50% appear possible with such compound semiconductor solar cells. In case of an optimal use of the solar spectrum, even conversion efficiencies up to 66% are theoretically conceivable, without using optical concentrators. Practically, however, numerous obstacles thwart the realization of the theoretically possible conversion efficiencies, like, for instance, different lattice constants of the semiconductor materials, which lead to mechanical stress and defects in the crystal structure.
Methods and Projects to Improve III/V Semiconductor Solar Cells
At present, however, there is still a substantial potential for further improvements of III/V semiconductor solar cells. For example, the use of indium gallium nitride seems promising for solar cells. This material system has, as scientists of the Lawrence Berkeley National Laboratory recently discovered, an optimal gap range for the conversion of almost the entire solar spectrum, and is very tolerant with regard to lattice mismatches.
Producing Multi-Junction Solar Cells Using the (Metal Organic Chemical Vapor Deposition) MOCVD and Molecular Beam Epitaxy (MBE) Techniques
In Germany, the development of III/V-semiconductor solar cells for space applications is promoted by the DLR, and accomplished in a joint project with the participation of the Fraunhofer Institute for Solar Energy Systems and the RWE Solar AG. The production of multi-junction solar cells with MOCVD and MBE procedures requires process control on a nanoscale level. Disadvantages of III/V semiconductor solar cells are the relatively high material costs and a complex process technology.
Improving the Efficiency of Solar Cells by Using Semiconductor Quantum Dots (QD)
Another starting point for the increase of the conversion efficiency of solar cells is the use of semiconductor quantum dots (QD). By means of quantum dots, the band gaps can be adjusted specifically to convert also longer- wave light and thus increase the efficiency of the solar cells. These so called quantum dot solar cells are, at present still subject, to basic research.
Material Systems for Quantum Dot (QD) Solar Cells and Benefits of Using Silicon/Germanium (Si/Ge)
As material systems for QD solar cells, III/V-semiconductors and other material combinations such as Si/Ge or Si/Be Te/Se are considered. In a BMBF joint project with the participation of DaimlerChrysler, QD solar cells, on the basis of self-structured Ge-islands on Si substrates, are investigated at present. Potential advantages of these Si/Ge QD solar cells are:
• Higher light absorption in particular in the infrared spectral region,
• Compatibility with standard silicon solar cell production (in contrast to III/V semiconductors),
• Increase of the photo current at higher temperatures,
• Improved radiation hardness compared with conventional solar cells.
Increasing Efficiency via an Improved Layer Structure and Parallel Contacting of the Quantum Dot (QD) Solar Cells
The present results show that the improved photo response within the IR range is overshadowed by a worse response in the visible and UV spectral region, so that, altogether, smaller efficiencies than with pure Si cells are obtained. However, a potential still exists for an improvement of the efficiency by an improved layer structure and parallel contacting of the QD solar cells. Figure 1 shows the schematic structure of a Si/Ge QD solar cell.
Figure 1. Schematic structure of a Si/Ge QD solar cell with layers of Ge quantum dots in the active layer of the Si solar cell substrate.
Thin Film Solar Cells in Space Applications
Further approaches for nanotechnology applications within the range of space solar cells are thin film solar cells, which have already been used for solar cell panels of satellites. Thin film solar cells for space applications are based, for example, on amorphous silicon or on Cu(In)(Se,S)2- layers, which are attached to thin metal or polymer foils. For space applications in particular, thin film cells on polymer substrates are interesting due to their small weight and their flexibility. The US-American company United Solar develops, for example, amorphous silicon thin film cells on thin Kapton foils, which reach conversion efficiencies of 12% under space conditions and also demonstrate a good radiation hardness. The company Solarion in Leipzig, Germany, has recently developed an ion beam process, which allows a cheap production of large area CIS thin film solar cells on Kapton foils. Such flexible large area thin film cells are interesting not only for satellites, but particularly for new visionary spacecrafts such as solar sails, Gossamer-Spacecrafts or solar power plants in space.
Using Organic Solar Cells for Space Travel
In the future, organic solar cells could also play a role in space travel, which, in principle, can be manufactured substantially more economically than inorganic cells. At present, however, they still exhibit relatively small conversion efficiencies. Organic solar cells use dyes, conjugated polymers or also fullerene derivatives, for the conversion of sunlight.
Graetzel Organic Solar Cells
A special type of organic solar cell, the Graetzel cell, uses a nanoporous titanium dioxide layer coated with organic dyes, in order to achieve a higher conversion efficiency by surface enlargement, and a better electron transfer from the light absorber to the electrode. Graetzel cells at present reach conversion efficiencies of approx. 10% under diffuse illumination. Organic solar cells are investigated intensively and possess a high development potential for the future.
Another approach for the conversion of solar light into electric energy is based on thermoelectrics. Thermoelectric converters produce electricity from solar energy through a two-step thermoelectric process, in which electromagnetic radiation is first converted to heat and then into electricity. Thermoelectric converters harness the whole spectrum of solar light, and therefore have high theoretical conversion efficiencies of up to 70%.
Thermoelectric Converters Based on Polycrystalline Diamond Films
Particularly interesting for space applications are thermoelectric converters based on thin polycrystalline diamond films, consisting of myriads of nanoscale diamond tips. When heated, these diamond nanotips act as a field emitter cathode, that emits electrons, flowing across an intervening vacuum to the anode and creating an electric current. For this temperatures of 1000°C to 1500°C have to be achieved by means of a solar absorber plate.
Benefits of Using Thermoelectric Converters
Advantages of thermoelectric converters, in comparison to photovoltaic cells, are an increased conversion efficiency as well as a high radiation resistance. Such diamond thin film solar cells can be manufactured by CVD processes. R&D activities aiming at utilization of this technology for space applications are accomplished at present by the Vanderbilt University School of Engineering.