space colonies-lunar settlements:chapter 11. solar cell fabrication on the moon from lunar resources

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11Solar Cell Fabrication on the Moon from Lunar Resources

Alex Ignatiev and Alexandre FreundlichCenter for Advanced Materials, University of HoustonHouston, Texas

Klaus Heiss and Christopher VizasHigh FrontierAlexandria, Virginia

Alex Ignatiev, PhD, is distinguished university professor of physics, chem-istry, and electrical and computer engineering. Dr. Ignatiev is a graduate of the University of Wisconsin and Cornell University where he received his PhD in materials science in 1972. He is a former Fulbright Fellow, associ-ate editor for Vacuum, and is the director of the Texas Center for Advanced Materials. He has been elected to the International Academy of Astronautics for his work in advanced materials development in space, and has been the recipient of the NSM Alumni Achievement Award, the Texas State Senate Recognition Award, and the City of Houston Science Recognition Award. He has developed the Wake Shield Facility space science payload that has flown three times on the Space Shuttle for the study of thin film growth in the vacuum of space. His research interests are focused on advanced thin film materials and device development and surface chemical interactions that form the basis for thin film growth. Recent efforts have been in the research of optical micro-detectors for artificial retina, thin film solar cells, thin film solid oxide fuel cells, thin film oxide resistive random access memory, and the fabrication of thin film solar cells on the Moon from lunar resources. He is the author of over 300 published research papers, holds 15 patents, and has been instrumental in the spinoff of five companies taking UH advanced materials technologies into the private sector.

AbstrACt The use of the indigenous resources of the Moon can result in the development of a power system on the Moon based on the fabrication of solar cells by thin film growth technology in the vacuum environment of the Moon. This can be accomplished by the deployment of a moderately-sized (~200 kg) crawler/rover on the surface of the Moon with the capabilities of preparation of the lunar regolith for use as a substrate, evaporation of the

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© 2010 Taylor and Francis Group, LLC

144 Lunar Settlements

appropriate semiconductor material for the solar cell structure, and deposition of metallic contacts and interconnects. This unique process will allow for the emplacement of a lunar electric power system that can reach a 1 MW capacity level in several years of crawler operation. This approach for the emplacement of an electric power system on the Moon would require the transportation of a much smaller mass of equipment to the Moon than would otherwise be required to install a complete electric power system brought to the Moon and emplaced there. It would also result in an electric power system that was repairable/replaceable through the simple fabrication of more solar cells.

Introduction

Energy is fundamental to nearly everything that humans would like to do in space, whether it is science, commercial development, or human exploration. If indigenous energy sources can be developed, a wide range of possibilities emerges for subsequent development. Some of these will lower the cost of future exploration, others will expand the range of activities that can be car-ried out, and some will reduce the risks of further exploration and develop-ment. This picture is particularly true for the Moon where significant electrical energy will be required for a number of lunar development scenarios, includ-ing science stations, lunar resource processing, and tourism. We present an approach to generate electrical energy on the Moon through the in-situ fabrica-tion of thin film solar cells on the surface of the Moon. In supplying this electri-cal energy by in-situ fabricated solar cells, the costly transport and installation of an immense number of solar cells to support the energy need will not be required. The fabrication of solar cells on the surface of the Moon can be accom-plished by the deployment on the Moon of a mobile solar cell fabricator, which will utilize the resources of the Moon to fabricate solar cells on location.

Lunar solar Cells

The generation of electrical energy on the Moon can be undertaken through the fabrication of thin film solar cell directly on the surface of the Moon. The Moon has an ultra-high vacuum environment at it surface (~10E-10 Torr) and hence there is no requirement for vacuum chambers to undertake vacuum deposition of thin film materials and devices. The Moon’s surface also con-tains all of the natural resources from which to fabricate thin film silicon solar cells: silicon, iron, magnesium, calcium, rutile, aluminum, etc., can be made available for thin film silicon solar cell production. The ultra-high vacuum

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Solar Cell Fabrication on the Moon from Lunar Resources 145

environment at the surface of the Moon allows for the vacuum deposition of thin film silicon solar cells directly on the surface of the Moon without the need for vacuum chambers. As a result, thin film solar cells can be directly fabricated on the surface of the Moon through the integration of both a rego-lith processing step that is robotically undertaken to extract the needed raw materials for solar cell growth, and a solar cell vacuum deposition process undertaken by an autonomous robotic rover that lays down continuous rib-bons of solar cells directly on the lunar regolith surface.

Regolith processing on the Moon to extract metallic and semiconducting elements needed for solar cell fabrications can be accomplished by a num-ber of processes.1–4 For silicon extraction, carbothermal reduction has been proposed for several abundant silicates, including anorthite (CaAl2Si2O8) and pyroxene (Ca,Mg,Fe)SiO3. Anorthite is abundant in both maria and highlands rocks, pyroxene is most abundant in the maria. Most of these pro-cesses required a closed cycle process on the Moon to reduce the resupply of reagents from Earth. Electrolytic processing is also a possible approach to extraction of elements from regolith. Preliminary studies in the development of silicon solar cells from silicon extracted from simulated lunar regolith by electrolysis have been undertaken,5 and show that such silicon can be used to fabricate thin film silicon solar cells through vacuum deposition.6 It is well to note that although the electrolysis-processed regolith silicon was of mod-erate quality, i.e., not semiconductor grade, the vacuum deposition step for the thin film growth pre-purified the silicon through vacuum purification to yield moderate quality solar cells.5

Regolith reduction and silicon production however, are very energy inten-sive processes, and there is a number of competing reduction processes that can be applied to lunar environment. The major constraints for processes on the Moon are the need to encapsulate processes that use volatiles due to the Moon’s vacuum environment, and minimization or elimination of materials/make-up materials to be brought from Earth (especially so for carbon contain-ing materials that are not found on the Moon). With this in mind, magma elec-trolysis comes to the forefront as a viable method for application on the Moon. It requires no volatiles, and can utilize directly the regolith. Of principal con-cern for magma electrolysis is the need for stable electrodes and higher level of electrical energy. The former requires some R&D to optimize electrode performance, and the latter can be mitigated by the fabrication of lunar solar cells prior to deployment of the regolith processor. In such a “bootstrapping” approach the initial amount of material to be used for solar cell production on the Moon will need to be brought from Earth. This would then allow for the fabrication of an initial amount of thin film solar cells on the surface of the Moon (nominally ~50 kW capacity) the energy from which could then be utilized by follow-on missions which will focus on processing the regolith for materials extraction to continue to “feed” the thin film solar cell fabricators.

In a case where all of the source material may be brought from Earth, then a variety of semiconductor materials systems are available from which to

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fabricate thin film solar cells. In such a case, it may be best to choose a system where the thin film solar cell fabrication power budget is as low as possible. However, for the purpose of fabricating thin film solar cell from in-situ materi-als focus needs to be maintained on the use of silicon, which is prevalent on the Moon, and thus the silicon solar cell structure becomes the structure of choice.

On Earth, silicon solar cells are not typically vacuum deposited on glass substrates but are principally fabricated from single crystal wafers not unlike those used for semiconductor device fabrication. However, as noted, the Moon possesses an ultra-high vacuum surface environment; hence vac-uum deposition of silicon can be well used on the Moon. Terrestrially, thin film silicon solar cells, when vacuum deposited, are typically deposited in crystalline form on single crystal substrates. Vacuum deposition on glass is problematic due to atomic disorder in the grown films, and when cells are achieved, they typically have low efficiencies (~3-5%). This, however, may be acceptable for the lunar environment since large areas of low efficiency solar cells can be fabricated on the Moon to give the required TOTAL power needed for lunar use, i.e., quality can be traded off for quantity.

Figure 11.1SEM micrograph of a thermally melted JSC-1 regolith simulant.

Regolith glass substrate

Back contact

p silicon

n siliconTop contact

Figure 11.2Schematic cross-section of lunar solar cell.

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For a thin film solar cell, the substrate presents the most massive part of the structure. A common terrestrial substrate for thin film cells is glass. Nominal SiO2 glass is not readily available on the surface of the Moon; however the lunar regolith can be melted to form a glass that is quite suitable as a solar cell substrate. The melted regolith simulant (JSC-1) exhibits a resistance of greater than 1011Ω, and shows a smooth surface morphology consistent with good substrate material (Figure 11.1).

A typical lunar solar cell structure is shown in Figure 11.2, where melted lunar regolith is the substrate, a metallic back contact layer is evaporated onto the regolith substrate, p- and n- doped silicon is then evaporated onto the contact layer, a top contact is deposited onto the silicon through a contact mask and an antireflection coating can then be deposited on top of the whole cell. It has been shown that in addition to being a very favorable substrate layer for silicon solar cells, evaporated regolith has excellent optical trans-mission properties and can be used as an antireflection coating for enhance-ment of fabricated solar cell efficiency. In this manner, all the components of a thin film solar cell are available for fabrication of thin film silicon solar cells on the surface of the Moon.

These lunar solar cells would be fabricated by a facility deployed on the surface of the Moon. A movable “crawler”—a Cell Paver, of ~150 kg mass would traverse the lunar surface depositing solar cells7 as part of the traverse (Figure 11.3). The Cell Paver wheeled vehicle could clear larger rocks and boulders from the terrain directly in front of it, thus preparing a bed for the fabrication of the lunar glass substrate onto which the solar cells would be directly fabricated by vacuum deposition.

Figure 11.3Artist’s drawing of Cell Paver fabricating thin film solar cells on the surface of the Moon.

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The thermal energy required for each set of the evaporations in the above process would be obtained from direct solar energy collected by an array of small parabolic concentrators used to focus the solar energy and couple it into fiber optic bundles routed over the Cell Paver to whatever location it is needed. A large array of concentrators can be made so that the source mate-rial evaporation and regolith melting can be effectively undertaken. The fiber optic bundles would first focus energy to regolith melting to form the regolith glass substrate. They would then be moved to a metal evaporator for bottom electrode deposition. This would be followed by silicon (p- and n-doped) evaporation, top electrode evaporation, antireflection coating evaporation, and finally metallic interconnect evaporation to form the solar cell depicted in Figure 11.2.

The in-situ solar cells would be fabricated while the Cell Paver is moving. Individual cells would be connected in alternating series/parallel fashion by the deposition of thin film metallic strips (wires), and thus the intercon-nected cells would form arrays. In this manner, the crawler could continue to migrate over the lunar surface (maneuvering around large obstacles) and continuously lay down solar cells on an undulating landscape.

It is projected that in the initial version of the Cell Paver, silicon solar cells of ~1 m width and ~5% efficiency would be deposited on the lunar surface at a rate of approximately 1 m2 per hour giving the Cell Paver a motive speed of about 1m/hr. The cells would be integrated into a power system with both built in by-pass transistors also grown by thin film methods as part of the solar cell growth, and periodic array-grouping junctions which would be connected to a power management and distribution system to be brought from the Earth.

As noted, the initial set of lunar solar cells could be fabricated from raw materials brought from the Earth. Approximately 20 kg of raw materials

Figure 11.4Artist’s drawing of the Regolith Processor which would extract raw materials from the regolith by magma electrolysis.

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would be required for the fabrication of ~50 kW of thin film solar cell elec-tric power capacity. Beyond this amount, additional raw materials would need to be extracted to feed the Cell Paver. A Regolith Processor (Figure 11.4) would be required for this purpose. Noting that the initial run of the Cell Paver resulted in the fabrication of ~50 kW of solar cells, this power capacity could be used for regolith processing as well as follow-on development of the lunar site. The Regolith Processor would then be a second robotic vehicle8 deployed on the Moon at the lunar solar cell production site. The Regolith Processor of ~150 kg mass would use power from the fabricated lunar solar arrays to process the lunar regolith to yield up to 200 kg of raw materials per year including silicon, iron-silicide, and aluminum among others. These raw materials would then be supplied to the solar cell crawler to continu-ously fabricate large numbers of silicon-based solar cells. These two vehicles would comprise the initial facility for the development of a lunar solar power system. The projected yield for this two-facility system is ~1MW capacity of solar cells fabricated over a 5-year period. It is clear that a series of such facilities deployed on the Moon could result in the generation of a significant amount of electrical energy on the surface of the Moon.

It is well to note here that this energy scenario for the Moon recognizes the economics of energy in space in that taking only the tools to the Moon to fabricate solar cells can be much more cost effective in the long run than just bringing the solar cells themselves and erecting them on the Moon. An initial cost analysis of the two scenarios indicates that at the current price of ~$1,000/W for space solar cells, and current projected lunar launch costs of ~$150,000/kg, the economic break-even point in bringing or fabricating solar cells on the Moon is approximately 125–150 kW. It is well to know, how-ever, that this assumes only one-half year of operation for the Cell Paver/Regolith Processor. The projected lifetime for the Cell Paver and the Regolith Processor (based on past performance of deployable planetary robotic vehi-cles) is ~5 years. Hence, the deployment of the Cell Paver system for a near-term need for ~125 kW of electrical power capacity will result in a total of ~1 MW capacity after the five year operating period—a major benefit for timed expansion of a lunar base.

The longer-term development of lunar solar cell power systems would be driven by several factors including expanding needs for lunar electri-cal energy, energy needs for sis-lunar space, and electrical energy needs on Earth. The latter electrical energy need—energy for Earth—is one that will require a significant amount of lunar solar cell fabrication. To support this, a second generation of Cell Pavers and Regolith Processors would need to be developed that would benefit both from improvement of processes gained from first generation facilities operation on the Moon, and from advance-ment of the science and technology of thin film silicon solar cells.

Preliminary studies have indicated that thin film silicon cell efficiencies could reach up to 10% through the optimization and control of grain size in the thin film silicon to consistently yield grain sizes larger than 2 μm.9

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The integration of higher efficiency thin film silicon solar cells (> 8%) with the ability to fabricate them at an increased rate (> 6 m2/hr) would yield ~2 MW/yr growth capacity per Cell Paver II. One hundred of such sec-ond generation Cell Pavers deployed on the Moon could therefore result in 200 MW capacity of lunar solar cells fabricated in one year, and 1 GW of lunar solar cells fabricated in a 5-year period. The existence of such a large solar energy capacity on the Moon can now significantly impact the electrical energy environment on the Earth. Transporting 1 GW of electrical energy to the Earth from the Moon can be accomplished by energy beam-ing technologies. Both microwave and laser energy beaming have been discussed and proposed previously, especially in light of recent comments on space solar power generation and power beaming from artificial satel-lites containing massive solar cell arrays.10 Such artificial solar cell satel-lite concepts are more complex and costly as compared to using the Moon and its resources as a solar power satellite, not only because it is an excep-tionally stable platform on which to collect and transmit energy, but also because it is a platform on which the solar cell arrays can be built using in-situ resources. Even though a good deal has been said about microwave and laser power beaming, much is still needed in the technology develop-ment to realize high efficiency power beaming over large distances and at high power levels. The communications industry has done much to move the power beaming technology forward. However, their interests are to principally transmit a microwave signal to a widely dispersed area on the surface of the earth. For power beaming this concept needs to be inverted to address the transmission of a significant amount of power to a localized spot on the surface of the Earth. Progress in this arena is expected, with the promise of near-term realization of efficient and effective power beaming over large distances. This coupled with the ability to fabricate immense solar cell capacity on the Moon will enable a new lunar electrical energy source for Earth consumption.

Conclusion

The ability to “live off the land” brings a new paradigm to space exploration and utilization for the space programs of the world. Electrical energy will be required everywhere man or robots go in space. The development of space-fabricated solar power as described here can significantly lower the costs of operating in space, and will provide an energy-rich environment where rapid expansion of space activities can be undertaken. Furthermore, expan-sion of the scale of lunar solar cell array development to realize GWs of cell capacity can enable energy beaming to the Earth to help alleviate a portion of Earth’s energy problems in the near future.

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Acknowledgments

This work was supported in part by the NASA Cooperative Agreements NCC8-239, NAG9-1287, CAM through the State of Texas, and the R.A. Welch Foundation. The contributions of Drs. Michael Duke, Laurent Sibille, Peter Curreri, Sanders Rosenberg, and Charlie Horton are gratefully acknowledged.

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10. See as Example: Special Issue of Ad Astra, “Space-Based Solar Power,” (Spring 2008).

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