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First demonstration of photovoltaic diodes on lunar regolith-based substrate
Acta Astronautica 56 (2005) 537 – 545 www.elsevier.com/locate/actaastro
First demonstration of photovoltaic diodes on lunar regolith-based substrate C. Horton∗ , C. Gramajo, A. Alemu, L. Williams, A. Ignatiev, A. Freundlich Texas Center for Superconductivity and Advanced Materials, 724 Science and Research Building 1, University of Houston, Houston, TX, 77204-5004, USA Received 11 December 2003; received in revised form 18 September 2004; accepted 13 October 2004
Abstract Within the framework of utilizing in situ lunar resources for the in-space fabrication of solar cells directly on the lunar surface, we report on the results of the fabrication of substrates from the lunar regolith and the first silicon and cadmium sulfide/cadmium telluride thin film photovoltaic diodes deposited on these lunar regolith substrates. © 2004 Elsevier Ltd. All rights reserved.
1. Introduction A crucial component of infrastructure for utilization of in-space resources is power. Any operations on the Moon will first and foremost require power. The source of power must be easily accessible, suitable for the lunar environment, expandable to meet needs, repairable and replaceable and, ideally, very cost effective. There are different power generation schemes available ranging from nuclear to fuel cells [1–3]. Solar energy is attractive because it can generate power without refueling or having to deal with waste products. Solar arrays are expandable as more solar panels are added. Solar arrays are used with an energy
∗ Corresponding author. Tel.: +1 713 743 3621;
fax: +1 713 747 7724. E-mail address: [email protected] (C. Horton). 0094-5765/$ - see front matter © 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.actaastro.2004.10.004
storage device (e.g. batteries or fuel cells) to provide continuous power. The important task to solve is the delivery of enough solar panels to the Moon in a cost effective manner. Current advanced space solar arrays typically achieve about 100–200 w/kg power density [4]. A 100 kW power source would require about 1 metric ton of the solar arrays. The costs of transporting solar arrays of significant power from Earth to the Moon would be prohibitively expensive. We are developing a concept where solar power is fabricated on the Moon using in situ lunar resources [5]. Rovers would travel across the surface of the Moon fabricating the solar cells. Whole power arrays would be built up over time that would generate significant amounts of power. The fabrication of solar cells will require substrates on which to deposit and a supply of material to deposit. Lunar regolith can be purified for the solar cell source material [6] or brought from Earth. The substrate
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Fig. 1. Schematic of in situ thin film solar cell fabrication on the Moon.
can be made by melting the lunar regolith to form a smooth glass. Fig. 1 shows a schematic of a solar array being fabricated by this method. The rover travels to the right, first melting the regolith to form the glass substrate and then depositing the thin film solar cell. The bottom contact is deposited first followed by the thin film photovoltaic structure. The top contact and interconnects are then deposited. In this paper, we report the results of our work on the lunar regolith substrate fabrication and the first photovoltaic diodes deposited on lunar regolith.
2. Lunar regolith substrate 2.1. Fabrication A key component in the concept of in situ power manufacturing is the development of an electrically isolating and mechanically compatible support structure for the solar cells using lunar resources. By fabricating the support structure on the Moon, the extra mass and complexity that must be added to a terrestrial support structure for it to survive the launch and transport is eliminated. The lunar-made substrate (support structure) can be much simpler in that it only needs to support the thin film solar cell. The solar cells would be deposited on this substrate. The substrate needs to be electrically insulating so that individual solar cells can be made and then connected in series or parallel circuits to provide an array with useable voltage and current. The surface will need to have features less than the dimensions of the thin film solar cell thickness, otherwise a continuous film will not be deposited. A substrate used terrestrially is glass
[7,8]. Glass is a good choice due to the low cost and the ability to isolate individual cells. The composition of the lunar regolith consists of a variety of oxides that when melted form a glassy solid. We have investigated the melting of the lunar regolith simulant JSC-1 [9] to determine if the glass produced would be suitable as a substrate for lunar solar cells. JSC-1 is similar to typical lunar mare soil with a composition of almost half silicon oxide with the aluminum oxide, calcium oxide, iron oxide and magnesium oxide comprising the majority of the balance [9]. Boats made of tungsten were resistively heated to melt the regolith samples. The boats were fitted with a type C thermocouple and the temperatures recorded as the regolith was heated. With a uniform fill of regolith in the boat, the material next to the boat at the bottom would melt but the material on the surface away from the edges would not. The regolith is a fine gray powder with low thermal conductivity and a high emissivity. The temperature of the surface regolith depends on the balance of heat flow from the material at the bottom through thermal conductivity and heat loss radiated by the surface. Under specific conditions, more energy is radiated out than what can be transported in by conduction and so the surface temperature does not reach the melting point. In one instance, the boat was over 2000 ◦ C and the regolith powder in the center had not melted. To counteract this combination of material properties, we filled the boat along the edges and left a trough in the middle with a thin layer. This way, all regolith had a short thermal path to the heat energy of the boat. This procedure allowed us to melt all the regolith in the boat and achieve a glassy substrate with a smooth surface. As the regolith is heated, it starts to soften around 1300 ◦ C. The melt is very viscous and
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Fig. 2. Melted regolith glass substrate.
evolving gas bubbles out in a thick, lava like fashion. By 1600 ◦ C the regolith has completely melted and the viscosity is low. The temperature of the boat is raised to about 1800 ◦ C to ensure the volatiles are gone and the liquid regolith is uniform. The sample is then cooled. The temperature falls quickly (1–2 min) to around 300 ◦ C and then falls slowly after that. Fig. 2 shows a∼ 1 × 1 boat with a regolith glass substrate. The surface of the solidified melt is very smooth. The use of a boat to melt the regolith allows us to explore another feature of the glass substrate. Terrestrially, the backside of solar cells is textured to scatter light back through the solar cell for increased absorption [7]. If the regolith melt is stamped with a textured plate as it cools, a surface texture can be embossed into the substrate. The part of the regolith melt in contact with the boat takes on the surface texture of the boat simulating an embossing step. Thin film solar cells were deposited on both the smooth and textured surfaces to test the quality of the regolith glass as a substrate. 2.2. Lunar regolith material properties The surfaces of the smooth side and the textured side of the melted lunar regolith glass are shown in Fig. 3. The free melt surface (smooth side) shows no features larger than 50 nm (see Fig. 3a). Fig. 3b shows the textured surface on which a Si photovoltaic diode structure was deposited. The dark area is the textured regolith surface and the light area is the Si photovoltaic
Fig. 3. Surface of melted lunar regolith: (a) SEM of shiny side, (b) Optical microscope (×60) of textured side.
diode. The textured surface shows features reproducing the metal surface of the boat with the silicon photovoltaic diode following the surface texture. Reflectivity measurements of melted lunar regolith glass substrates were taken at 514.5 nm wavelength and a 45◦ angle of incidence. The reflectivity of the shiny side was 16% and for the textured side it was less than 1%. This decrease in the specular reflectivity is a result of an increase in diffuse scattering by the textured surface. Current–voltage (I–V) measurements were taken on the melted regolith substrates. The probes were spaced at 1 mm and a voltage sweep of 24 V was used. The current was below that capable of being measured by our I–V curve tracer (< 100 nA). This puts a lower limit on the resistivity of the melted
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regolith glass substrates to be 104 m. The actual resistance is likely to be much larger than this. The regolith glass charged quickly under electron bombardment during the scanning electron microscope (SEM) pictures. During the melting of the regolith, we also found that the regolith could be deposited onto substrates to form thin films. The initial deposition of regolith of ˚ film resulted in a regolith film that was so a 5000 A transparent it was difficult to see on the glass slide. Transmission spectra were taken and the index of refraction was calculated. The index of refraction varied from 1.4 to 1.8 for different regolith thin films. Auger electron spectroscopy (AES) was taken to determine the chemical composition of the regolith thin film. The film was mostly silicon oxide with a small amount of iron oxide [10]. The optical and electrical properties of the regolith thin film opens up possible uses as anti-reflection coatings and in situ fabrication of electronic circuits [6].
3. Solar cell materials and devices 3.1. Silicon solar cells Source material will be needed to deposit the solar cells. Reduction of lunar regolith can supply some of the basic ingredients (silicon, aluminum) for the solar cells. Optimal thin film silicon solar cells are made using amorphous silicon [11,12]. Terrestrially, the optimal method for silicon thin film deposition is plasma deposition using silicon hydride diluted in hydrogen as the source material. Conditions on the Moon will make plasma deposition more difficult to do. Plasma deposition will require higher power and the manufacture and storage of silane and hydrogen gas. For the deposition of thin films on the Moon, the technique most easily applicable is thermal evaporation. Silicon can be deposited by thermal evaporation but the deposition parameters will need to be optimized to provide the best nanostructure for thin film solar cells. Amorphous silicon has a direct-like bandgap and so it more efficiently absorbs photons resulting in thinner structures to achieve the same amount of absorption. In addition, the solar cells will need n-type dopants (As, P or Sb) and p-type dopants (Al or B) to form the photovoltaic diode structure. The doping
material (except for the Al) is not present in the lunar regolith, however the amount of material is small (1–10 ppm) and it can be brought from Earth for relatively little cost. The incorporation of the dopants can be achieved either by diffusion or co-evaporation. A calculation using the diffusion constant of As in Si indicated that to obtain significant diffusion of As into the Si in a short time (< 1 h) the temperature must be above 1000 ◦ C. The power and operational complexity of this approach makes it undesirable. Therefore, the dopants should be co-evaporated with the source material. In previous work, we had deposited Si that had been purified from lunar regolith [13]. We found that the evaporation of the Si resulted in a decrease in the level of impurities in the film. For deposition of nand p-doped Si on lunar regolith substrates, we started with Si source material that was already doped at levels higher than desired in the thin films. Using electron beam deposition we deposited n- and p-type silicon films on glass and silicon substrates using source material doped to 20–2000 ppm of Sb (n type) and B (p type), respectively. The films were deposited at 100, 200 and 400 ◦ C substrate temperatures. SEM cross section pictures of the films showed uniform films with no pinholes or voids visible in SEM analysis. No visible grain structure was observed down to 100 nm. An Al back contact was deposited on the smooth and the textured sides of lunar regolith substrates and then Si photovoltaic n–p diode structures were deposited (Fig. 4a). Preliminary I–V measurements were taken of the Si photovoltaic devices before top contacts were deposited. A small piece of In was used to make contact between the probe and the Si diode. The devices exhibited diode behavior but no response to light. An I–V curve was taken at different spots on the sample under no light conditions. Fig. 4b shows a dark I–V from one spot on the Si diode on the textured regolith substrate. The I–V curve shows a behavior typical of a diode. The slope of the I–V data on a semi-log plot is an indication of the source of current through the diode. The shallow slope indicates that there is a large amount of recombination current. A large recombination current would lead to a lack of light response in the photovoltaic diode. We performed electrochemical capacitance–voltage (C–V ) measurements on the individual n-Si and p-Si films to obtain the doping versus depth profile. The doping concentration in the n-Si film was
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3.2. Alternative solar cell material
(a) Dark I-V of Si PV diode on lunar regolith 0.0012
Current (mA)
0.0010 0.0008 0.0006 0.0004 0.0002
-0.5
(b)
0.0
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Fig. 4. Si photovoltaic diodes on lunar regolith substrates: (a) Picture, (b) I–V curves.
1–2 × 1018 /cm3 , which is close to the desired doping level for a solar cell. The p-Si film doping concentration was 1–2 × 1020 /cm3 which is 100 times higher than desired. With the p-doping concentration this high, the Si has almost a metallic behavior with the fermi level well below the valence band. With such a concentration one almost expects a tunnel diode. The vapor pressure of Sb is significantly higher than Si leading to the reduction in the n-type films but B has a lower vapor pressure than Si and the p type doping level remained high. The starting source material will need to be carefully manipulated in order to obtain the appropriately doped Si layers for solar cells. The alternative would be to use a separate dopant source and calibrate the deposition to achieve the correct doping levels.
Another alternative would be to bring specific source material from Earth. The amount of material in a 5 m thick film with 1 m2 area is on the order of 15–30 g (material dependent). For a 10% efficient cell, this is 30–60 kg for more than 250 kW. If the source material is brought from Earth, then other solar cell material systems besides silicon can be considered. The requirements for consideration would be material systems where the power budget for deposition is low and the normal terrestrial deposition techniques are more in line with what can be easily utilized on the Moon. We are investigating the cadmium sulfide/cadmium telluride (CdS/CdTe) solar cell material system as one possibility. On Earth, CdS/CdTe solar cells are typically deposited on glass superstrates using various forms of thermal evaporation [14,15]. Based on vapor pressure curves, CdS and CdTe source material temperatures are around 600 ◦ C for evaporation, which is a substantial reduction in power required over the ∼ 1400 ◦ C necessary for Si. In addition, the CdS/CdTe material system is more tolerant of defects. The defects are essentially the source of doping for the material [16,17]. CdTe typically deposits as p type and the CdS typically deposits as n type. The band gap of this material system is direct and so it is a more efficient absorber of photons than silicon. In addition, the CdTe band gap of 1.51 eV is well suited for the solar spectrum. Many deposition techniques have been used to fabricate the solar cells with all delivering 10–15% efficiencies [15,16]. We deposited CdTe and CdS on glass slides and GaAs wafers to determine the quality of the thin film material. The films were deposited at ambient substrate temperature. The temperature of the substrate typically rose to about 35 ◦ C due to radiation heating from the source boats. The CdTe and CdS films were then annealed at 100–300 ◦ C for 0.5 and 1.5 h using a cover sample to prevent loss of Te or S during the anneal. Transmission spectra were taken of the samples deposited on glass. The transmission spectra showed sharper band edge spectra for the samples annealed at 200 and 300 ◦ C and for 1.5 h. Photoluminescence (PL) spectroscopy was taken on the CdTe and CdS films deposited on GaAs substrates. The spectra were dominated by defect peaks. The main feature of the CdTe
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spectrum was a peak at 1.399 eV. This peak is due to Cd vacancies [14]. This is the defect that causes the CdTe film to be p type [17]. The main feature of the CdS spectrum is the 2.04 eV peak. This peak is identified with Cd interstitials [18]. Both CdTe and CdS are n-type when Cd rich. Two back contact materials were chosen for the test solar cell. Aluminum is present in the lunar regolith and gold is a common contact material for terrestrial cells. Aluminum and Aluminum/gold were deposited on glass and regolith glass substrates. A CdTe layer 2 m thick was deposited on the back contact and then ˚ of CdS was deposited on top of the CdTe. In 1500 A order to simulate lunar fabrication conditions, shadow masks were used to deposit the CdS/CdTe solar cell material onto the regolith substrates. Shadow masks eliminate the need for wet chemical processing of the solar cells and so the technique is more representative of what could be used in the lunar environment. After depositing the CdS/CdTe solar cell, the cells were tested for light response before depositing the top contact. This test was performed using small balls of In to make contact between the I–V probe and the CdS top layer. Indium is an appropriate material for making n-type contact with CdS and the physical contact should be enough to determine the light responsiveness of the solar cell. The CdS/CdTe solar cells deposited on the smooth side and the textured side of the lunar regolith substrates showed response to light at a level of 1–3%. The I–V data showed a definite ohmic leakage current in addition to the light response. Fig. 5 shows a spectral quantum efficiency spectrum from the solar cell deposited on the textured side of the lunar regolith melt. The spectral quantum efficiency is a measure of the photo-response of the cell as a function of incident wavelength. The photo-response has a small signal starting around the CdTe band gap (1.51 eV, 821 nm) and a larger signal starting at the CdS band gap (2.37 eV, 524 nm). This is an indication that most of the carriers participating in the device photocurrent response are photoconverted in the CdS layer rather than the CdTe layer. The weak photocurrent response associated with below CdS band gap photons indicates a problem with the CdTe layer. However, considering the strong absorption coefficient of CdTe and the thicknesses of the CdTe layer, one would expect a significant absorption of photons in the CdTe layer and
CdS/CdTe solar cell on lunar regolith substrate 1.0 Spectral Response (arb units)
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300K 0.8
CdS (bandgap)
0.6 0.4
CdTe (bandgap)
0.2 0.0 500
600 700 800 Wavelength (nm)
900
Fig. 5. Spectral response of CdS/CdTe solar cell on textured side of regolith melt.
Fig. 6. CdS/CdTe with top contacts.
thus a more significant current response. Thus recombination (radiative and non radiative) losses must be the origin of the poor CdTe photoresponse. The preliminary I–V curves taken before the top contacts were deposited have also indicated a problem with the solar cell. While the I–V curve was light responsive, there was an ohmic leak evident in the I–V curve. Combined with the spectral response information, the ohmic loss path is probably in the CdTe layer. Aluminum was then deposited for top contacts (see Fig. 6).
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After the deposition of the top contacts, the CdS/CdTe solar cells were no longer responsive to light. The samples were shorted with a resistance of about 12 . The predeposition I–V traces indicated leakage current in the cell. This leakage could have been a result of pinholes in the films that allowed short circuits. When the Al was deposited, it would have filled in these holes to create a short similar to what was measured. To investigate improving the film quality, we deposited CdTe and CdS on glass substrates at a deposition temperature ∼ 75 ◦ C. The adhesion of the films to the glass was greatly enhanced even though the temperature increase of the substrate was relatively small. The surface of the CdTe film showed no pinholes under 500X on an optical microscope. We are in the process of determining optimal deposition temperatures and film quality prior to depositing more solar cells.
4. Discussion The production of solar power on the Moon will require substrate (support structure) fabrication and thin film solar cell deposition. The fabricated solar array is like a macroscopic version of an electronic device on a chip. The chip (substrate) must be insulating so that the different elements (solar cells) can be made and then connected together in appropriate circuits. Fabricating the substrate on the Moon is advantageous, in that it eliminates the need for the support structure to be strong enough to survive launch and transport stresses and using lunar resources to do fabrication eliminates the transport of material. The substrate must be easily fabricated in order that the process be reliable and cost effective. Melting of the lunar regolith produces a smooth, insulating glass. Individual solar cells can be isolated on the insulating lunar regolith glass. The surface of the regolith glass is very smooth but can also be textured to increase the scattering of light at the back of the solar cell. This feature causes any light not absorbed on the first pass through the solar cell to be scattered back into the solar cell for another chance. Reflectivity measurements showed a decrease in the specular reflection as a result of the textured surface. In the lunar environment, the regolith will be melted by heating from the surface. The problems encountered in melting the regolith in the boats
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will actually be helpful in melting the regolith on the lunar surface. In heating from the surface, the low thermal conductivity will help trap the heat at the surface. The regolith does not start to soften until ∼ 1300 ◦ C. As the regolith starts to melt, it is very viscous and so will support the melt at the surface. We are preparing to melt regolith by heating the surface with concentrated light to determine the suitability for forming the regolith glass substrate. Photovoltaic diodes were deposited on both smooth and textured lunar regolith substrates. The deposition of silicon photovoltaic film structures on lunar regolith substrates produced diodes that were not light responsive. Analysis of the n- and p-doped layers indicated that the p-layer was too highly doped for the device to work as a solar cell. The high doping reduces the diffusion length of the charge carriers and provides recombination centers. The photo-generated electrons and holes recombine before they can be collected. The slope of the dark I–V curve indicated a large recombination current. For silicon, the composition of the initial source material will have to be determined. If the source silicon is already doped n and p, then the amount of dopant will have to be determined so that after deposition, the correct doping in the films is achieved. If the dopants are transported separately, then both the Si and dopant deposition rates will need to be carefully calibrated so that the correctly doped films are deposited. This would increase the complexity of operation and so is less desirable. More optimization work is needed before a silicon solar cell can be made on the lunar regolith substrate. The choice of solar cell depends on the source of material to be used. Refining lunar regolith can produce silicon, aluminum and other potentially useful elements but the cost depends on the amount needed. The amount of material needed in a thin film solar cell is small. For example, a silicon solar cell 20 m thick would need less than 50 g to cover 1 m2 . If the solar cells were 10% efficient, then 100 kg of Si would be enough for more than 250 kW of solar cell fabrication. By reducing the solar cell film structure to 2 m, the amount of silicon could be reduce to 10 kg to supply the same amount of power. This amount of material is small compared to the mass of a solar cell fabricating lunar rover. The deposition of thin film solar cells will require careful optimization. For alternate solar cell material systems, the deposition of the solar cells will
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need to be tested. While solar cells have been optimized for terrestrial fabrication techniques, the cells will need to be optimized for lunar thermal deposition. An ideal material system would be one that is normally deposited by thermal deposition techniques and that with simple processing reach efficiencies on the order of 5–10%. While this efficiency is much lower than terrestrial, there is plenty of lunar real estate to make up for the lower efficiency. The CdS/CdTe solar cell system is compatible with these requirements. The material is deposited by variations of thermal evaporation at lower temperatures than silicon. The cadmium telluride naturally dopes p type under thermal evaporation with no intentional doping. The doping is related to Cd vacancy defects. The cadmium sulfide naturally dopes n type due to Cd interstitials. All these attributes combine to make the CdS/CdTe system viable for the lunar solar cell. CdS/CdTe photovoltaic diodes were deposited on smooth and textured lunar regolith substrates. The deposition of the thin film structures was not optimized. Before the top contacts were deposited, the photovoltaic diodes had a response to light on the order of 1–3%. The I–V curve indicated that the diode had an ohmic leakage current in the device. The spectral quantum efficiency indicated that the CdTe layer was not providing adequate photoconversion. When the top contacts were deposited, the photovoltaic diode became unresponsive to light. The failure is most likely in the CdTe layer. Both the CdS and the CdTe films were deposited with nonoptimal deposition parameters. Optimizing the deposition parameters should improve the quality of the films.
5. Summary We have demonstrated the first photovoltaic diodes fabricated on lunar regolith substrates. We have fabricated the solar cell structures using techniques similar to those that would be available on the Moon. Lunar regolith was melted to produce glass substrates that are suitable for thin film deposition. For the thin film photovoltaic structures, both smooth surface and textured surface lunar regolith substrates were utilized. The textured surface increases the diffuse scattering at the back of the solar cell. The Si diode deposited on lunar regolith substrates displayed diode behavior in
I–V measurements but had incorrect doping levels and a high recombination current leading to a lack of light response. The CdS/CdTe photovoltaic diodes also displayed diode behavior in I–V measurements and had a light response on the order of 1–3%. The spectral quantum efficiency of the CdS/CdTe indicated that most of the photoconverted carriers were emanating from the CdS layer with only a little in the CdTe. The CdS/CdTe photovoltaic diode lost the light response when the top contact was deposited due to poor film quality. Further work is needed to optimize the Si, CdS and CdTe film deposition parameters to optimize the film quality and improve the light response.
Acknowledgement The authors would like to thank NASA for support through the Cross Enterprise Technology Development Program (CETDP).
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First demonstration of photovoltaic diodes on lunar regolith-based substrate C. Horton∗ , C. Gramajo, A. Alemu, L. Williams, A. Ignatiev, A. Freundlich Texas Center for Superconductivity and Advanced Materials, 724 Science and Research Building 1, University of Houston, Houston, TX, 77204-5004, USA Received 11 December 2003; received in revised form 18 September 2004; accepted 13 October 2004
Abstract Within the framework of utilizing in situ lunar resources for the in-space fabrication of solar cells directly on the lunar surface, we report on the results of the fabrication of substrates from the lunar regolith and the first silicon and cadmium sulfide/cadmium telluride thin film photovoltaic diodes deposited on these lunar regolith substrates. © 2004 Elsevier Ltd. All rights reserved.
1. Introduction A crucial component of infrastructure for utilization of in-space resources is power. Any operations on the Moon will first and foremost require power. The source of power must be easily accessible, suitable for the lunar environment, expandable to meet needs, repairable and replaceable and, ideally, very cost effective. There are different power generation schemes available ranging from nuclear to fuel cells [1–3]. Solar energy is attractive because it can generate power without refueling or having to deal with waste products. Solar arrays are expandable as more solar panels are added. Solar arrays are used with an energy
∗ Corresponding author. Tel.: +1 713 743 3621;
fax: +1 713 747 7724. E-mail address: [email protected] (C. Horton). 0094-5765/$ - see front matter © 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.actaastro.2004.10.004
storage device (e.g. batteries or fuel cells) to provide continuous power. The important task to solve is the delivery of enough solar panels to the Moon in a cost effective manner. Current advanced space solar arrays typically achieve about 100–200 w/kg power density [4]. A 100 kW power source would require about 1 metric ton of the solar arrays. The costs of transporting solar arrays of significant power from Earth to the Moon would be prohibitively expensive. We are developing a concept where solar power is fabricated on the Moon using in situ lunar resources [5]. Rovers would travel across the surface of the Moon fabricating the solar cells. Whole power arrays would be built up over time that would generate significant amounts of power. The fabrication of solar cells will require substrates on which to deposit and a supply of material to deposit. Lunar regolith can be purified for the solar cell source material [6] or brought from Earth. The substrate
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Fig. 1. Schematic of in situ thin film solar cell fabrication on the Moon.
can be made by melting the lunar regolith to form a smooth glass. Fig. 1 shows a schematic of a solar array being fabricated by this method. The rover travels to the right, first melting the regolith to form the glass substrate and then depositing the thin film solar cell. The bottom contact is deposited first followed by the thin film photovoltaic structure. The top contact and interconnects are then deposited. In this paper, we report the results of our work on the lunar regolith substrate fabrication and the first photovoltaic diodes deposited on lunar regolith.
2. Lunar regolith substrate 2.1. Fabrication A key component in the concept of in situ power manufacturing is the development of an electrically isolating and mechanically compatible support structure for the solar cells using lunar resources. By fabricating the support structure on the Moon, the extra mass and complexity that must be added to a terrestrial support structure for it to survive the launch and transport is eliminated. The lunar-made substrate (support structure) can be much simpler in that it only needs to support the thin film solar cell. The solar cells would be deposited on this substrate. The substrate needs to be electrically insulating so that individual solar cells can be made and then connected in series or parallel circuits to provide an array with useable voltage and current. The surface will need to have features less than the dimensions of the thin film solar cell thickness, otherwise a continuous film will not be deposited. A substrate used terrestrially is glass
[7,8]. Glass is a good choice due to the low cost and the ability to isolate individual cells. The composition of the lunar regolith consists of a variety of oxides that when melted form a glassy solid. We have investigated the melting of the lunar regolith simulant JSC-1 [9] to determine if the glass produced would be suitable as a substrate for lunar solar cells. JSC-1 is similar to typical lunar mare soil with a composition of almost half silicon oxide with the aluminum oxide, calcium oxide, iron oxide and magnesium oxide comprising the majority of the balance [9]. Boats made of tungsten were resistively heated to melt the regolith samples. The boats were fitted with a type C thermocouple and the temperatures recorded as the regolith was heated. With a uniform fill of regolith in the boat, the material next to the boat at the bottom would melt but the material on the surface away from the edges would not. The regolith is a fine gray powder with low thermal conductivity and a high emissivity. The temperature of the surface regolith depends on the balance of heat flow from the material at the bottom through thermal conductivity and heat loss radiated by the surface. Under specific conditions, more energy is radiated out than what can be transported in by conduction and so the surface temperature does not reach the melting point. In one instance, the boat was over 2000 ◦ C and the regolith powder in the center had not melted. To counteract this combination of material properties, we filled the boat along the edges and left a trough in the middle with a thin layer. This way, all regolith had a short thermal path to the heat energy of the boat. This procedure allowed us to melt all the regolith in the boat and achieve a glassy substrate with a smooth surface. As the regolith is heated, it starts to soften around 1300 ◦ C. The melt is very viscous and
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Fig. 2. Melted regolith glass substrate.
evolving gas bubbles out in a thick, lava like fashion. By 1600 ◦ C the regolith has completely melted and the viscosity is low. The temperature of the boat is raised to about 1800 ◦ C to ensure the volatiles are gone and the liquid regolith is uniform. The sample is then cooled. The temperature falls quickly (1–2 min) to around 300 ◦ C and then falls slowly after that. Fig. 2 shows a∼ 1 × 1 boat with a regolith glass substrate. The surface of the solidified melt is very smooth. The use of a boat to melt the regolith allows us to explore another feature of the glass substrate. Terrestrially, the backside of solar cells is textured to scatter light back through the solar cell for increased absorption [7]. If the regolith melt is stamped with a textured plate as it cools, a surface texture can be embossed into the substrate. The part of the regolith melt in contact with the boat takes on the surface texture of the boat simulating an embossing step. Thin film solar cells were deposited on both the smooth and textured surfaces to test the quality of the regolith glass as a substrate. 2.2. Lunar regolith material properties The surfaces of the smooth side and the textured side of the melted lunar regolith glass are shown in Fig. 3. The free melt surface (smooth side) shows no features larger than 50 nm (see Fig. 3a). Fig. 3b shows the textured surface on which a Si photovoltaic diode structure was deposited. The dark area is the textured regolith surface and the light area is the Si photovoltaic
Fig. 3. Surface of melted lunar regolith: (a) SEM of shiny side, (b) Optical microscope (×60) of textured side.
diode. The textured surface shows features reproducing the metal surface of the boat with the silicon photovoltaic diode following the surface texture. Reflectivity measurements of melted lunar regolith glass substrates were taken at 514.5 nm wavelength and a 45◦ angle of incidence. The reflectivity of the shiny side was 16% and for the textured side it was less than 1%. This decrease in the specular reflectivity is a result of an increase in diffuse scattering by the textured surface. Current–voltage (I–V) measurements were taken on the melted regolith substrates. The probes were spaced at 1 mm and a voltage sweep of 24 V was used. The current was below that capable of being measured by our I–V curve tracer (< 100 nA). This puts a lower limit on the resistivity of the melted
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regolith glass substrates to be 104 m. The actual resistance is likely to be much larger than this. The regolith glass charged quickly under electron bombardment during the scanning electron microscope (SEM) pictures. During the melting of the regolith, we also found that the regolith could be deposited onto substrates to form thin films. The initial deposition of regolith of ˚ film resulted in a regolith film that was so a 5000 A transparent it was difficult to see on the glass slide. Transmission spectra were taken and the index of refraction was calculated. The index of refraction varied from 1.4 to 1.8 for different regolith thin films. Auger electron spectroscopy (AES) was taken to determine the chemical composition of the regolith thin film. The film was mostly silicon oxide with a small amount of iron oxide [10]. The optical and electrical properties of the regolith thin film opens up possible uses as anti-reflection coatings and in situ fabrication of electronic circuits [6].
3. Solar cell materials and devices 3.1. Silicon solar cells Source material will be needed to deposit the solar cells. Reduction of lunar regolith can supply some of the basic ingredients (silicon, aluminum) for the solar cells. Optimal thin film silicon solar cells are made using amorphous silicon [11,12]. Terrestrially, the optimal method for silicon thin film deposition is plasma deposition using silicon hydride diluted in hydrogen as the source material. Conditions on the Moon will make plasma deposition more difficult to do. Plasma deposition will require higher power and the manufacture and storage of silane and hydrogen gas. For the deposition of thin films on the Moon, the technique most easily applicable is thermal evaporation. Silicon can be deposited by thermal evaporation but the deposition parameters will need to be optimized to provide the best nanostructure for thin film solar cells. Amorphous silicon has a direct-like bandgap and so it more efficiently absorbs photons resulting in thinner structures to achieve the same amount of absorption. In addition, the solar cells will need n-type dopants (As, P or Sb) and p-type dopants (Al or B) to form the photovoltaic diode structure. The doping
material (except for the Al) is not present in the lunar regolith, however the amount of material is small (1–10 ppm) and it can be brought from Earth for relatively little cost. The incorporation of the dopants can be achieved either by diffusion or co-evaporation. A calculation using the diffusion constant of As in Si indicated that to obtain significant diffusion of As into the Si in a short time (< 1 h) the temperature must be above 1000 ◦ C. The power and operational complexity of this approach makes it undesirable. Therefore, the dopants should be co-evaporated with the source material. In previous work, we had deposited Si that had been purified from lunar regolith [13]. We found that the evaporation of the Si resulted in a decrease in the level of impurities in the film. For deposition of nand p-doped Si on lunar regolith substrates, we started with Si source material that was already doped at levels higher than desired in the thin films. Using electron beam deposition we deposited n- and p-type silicon films on glass and silicon substrates using source material doped to 20–2000 ppm of Sb (n type) and B (p type), respectively. The films were deposited at 100, 200 and 400 ◦ C substrate temperatures. SEM cross section pictures of the films showed uniform films with no pinholes or voids visible in SEM analysis. No visible grain structure was observed down to 100 nm. An Al back contact was deposited on the smooth and the textured sides of lunar regolith substrates and then Si photovoltaic n–p diode structures were deposited (Fig. 4a). Preliminary I–V measurements were taken of the Si photovoltaic devices before top contacts were deposited. A small piece of In was used to make contact between the probe and the Si diode. The devices exhibited diode behavior but no response to light. An I–V curve was taken at different spots on the sample under no light conditions. Fig. 4b shows a dark I–V from one spot on the Si diode on the textured regolith substrate. The I–V curve shows a behavior typical of a diode. The slope of the I–V data on a semi-log plot is an indication of the source of current through the diode. The shallow slope indicates that there is a large amount of recombination current. A large recombination current would lead to a lack of light response in the photovoltaic diode. We performed electrochemical capacitance–voltage (C–V ) measurements on the individual n-Si and p-Si films to obtain the doping versus depth profile. The doping concentration in the n-Si film was
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3.2. Alternative solar cell material
(a) Dark I-V of Si PV diode on lunar regolith 0.0012
Current (mA)
0.0010 0.0008 0.0006 0.0004 0.0002
-0.5
(b)
0.0
0.5
1.0
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Fig. 4. Si photovoltaic diodes on lunar regolith substrates: (a) Picture, (b) I–V curves.
1–2 × 1018 /cm3 , which is close to the desired doping level for a solar cell. The p-Si film doping concentration was 1–2 × 1020 /cm3 which is 100 times higher than desired. With the p-doping concentration this high, the Si has almost a metallic behavior with the fermi level well below the valence band. With such a concentration one almost expects a tunnel diode. The vapor pressure of Sb is significantly higher than Si leading to the reduction in the n-type films but B has a lower vapor pressure than Si and the p type doping level remained high. The starting source material will need to be carefully manipulated in order to obtain the appropriately doped Si layers for solar cells. The alternative would be to use a separate dopant source and calibrate the deposition to achieve the correct doping levels.
Another alternative would be to bring specific source material from Earth. The amount of material in a 5 m thick film with 1 m2 area is on the order of 15–30 g (material dependent). For a 10% efficient cell, this is 30–60 kg for more than 250 kW. If the source material is brought from Earth, then other solar cell material systems besides silicon can be considered. The requirements for consideration would be material systems where the power budget for deposition is low and the normal terrestrial deposition techniques are more in line with what can be easily utilized on the Moon. We are investigating the cadmium sulfide/cadmium telluride (CdS/CdTe) solar cell material system as one possibility. On Earth, CdS/CdTe solar cells are typically deposited on glass superstrates using various forms of thermal evaporation [14,15]. Based on vapor pressure curves, CdS and CdTe source material temperatures are around 600 ◦ C for evaporation, which is a substantial reduction in power required over the ∼ 1400 ◦ C necessary for Si. In addition, the CdS/CdTe material system is more tolerant of defects. The defects are essentially the source of doping for the material [16,17]. CdTe typically deposits as p type and the CdS typically deposits as n type. The band gap of this material system is direct and so it is a more efficient absorber of photons than silicon. In addition, the CdTe band gap of 1.51 eV is well suited for the solar spectrum. Many deposition techniques have been used to fabricate the solar cells with all delivering 10–15% efficiencies [15,16]. We deposited CdTe and CdS on glass slides and GaAs wafers to determine the quality of the thin film material. The films were deposited at ambient substrate temperature. The temperature of the substrate typically rose to about 35 ◦ C due to radiation heating from the source boats. The CdTe and CdS films were then annealed at 100–300 ◦ C for 0.5 and 1.5 h using a cover sample to prevent loss of Te or S during the anneal. Transmission spectra were taken of the samples deposited on glass. The transmission spectra showed sharper band edge spectra for the samples annealed at 200 and 300 ◦ C and for 1.5 h. Photoluminescence (PL) spectroscopy was taken on the CdTe and CdS films deposited on GaAs substrates. The spectra were dominated by defect peaks. The main feature of the CdTe
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spectrum was a peak at 1.399 eV. This peak is due to Cd vacancies [14]. This is the defect that causes the CdTe film to be p type [17]. The main feature of the CdS spectrum is the 2.04 eV peak. This peak is identified with Cd interstitials [18]. Both CdTe and CdS are n-type when Cd rich. Two back contact materials were chosen for the test solar cell. Aluminum is present in the lunar regolith and gold is a common contact material for terrestrial cells. Aluminum and Aluminum/gold were deposited on glass and regolith glass substrates. A CdTe layer 2 m thick was deposited on the back contact and then ˚ of CdS was deposited on top of the CdTe. In 1500 A order to simulate lunar fabrication conditions, shadow masks were used to deposit the CdS/CdTe solar cell material onto the regolith substrates. Shadow masks eliminate the need for wet chemical processing of the solar cells and so the technique is more representative of what could be used in the lunar environment. After depositing the CdS/CdTe solar cell, the cells were tested for light response before depositing the top contact. This test was performed using small balls of In to make contact between the I–V probe and the CdS top layer. Indium is an appropriate material for making n-type contact with CdS and the physical contact should be enough to determine the light responsiveness of the solar cell. The CdS/CdTe solar cells deposited on the smooth side and the textured side of the lunar regolith substrates showed response to light at a level of 1–3%. The I–V data showed a definite ohmic leakage current in addition to the light response. Fig. 5 shows a spectral quantum efficiency spectrum from the solar cell deposited on the textured side of the lunar regolith melt. The spectral quantum efficiency is a measure of the photo-response of the cell as a function of incident wavelength. The photo-response has a small signal starting around the CdTe band gap (1.51 eV, 821 nm) and a larger signal starting at the CdS band gap (2.37 eV, 524 nm). This is an indication that most of the carriers participating in the device photocurrent response are photoconverted in the CdS layer rather than the CdTe layer. The weak photocurrent response associated with below CdS band gap photons indicates a problem with the CdTe layer. However, considering the strong absorption coefficient of CdTe and the thicknesses of the CdTe layer, one would expect a significant absorption of photons in the CdTe layer and
CdS/CdTe solar cell on lunar regolith substrate 1.0 Spectral Response (arb units)
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300K 0.8
CdS (bandgap)
0.6 0.4
CdTe (bandgap)
0.2 0.0 500
600 700 800 Wavelength (nm)
900
Fig. 5. Spectral response of CdS/CdTe solar cell on textured side of regolith melt.
Fig. 6. CdS/CdTe with top contacts.
thus a more significant current response. Thus recombination (radiative and non radiative) losses must be the origin of the poor CdTe photoresponse. The preliminary I–V curves taken before the top contacts were deposited have also indicated a problem with the solar cell. While the I–V curve was light responsive, there was an ohmic leak evident in the I–V curve. Combined with the spectral response information, the ohmic loss path is probably in the CdTe layer. Aluminum was then deposited for top contacts (see Fig. 6).
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After the deposition of the top contacts, the CdS/CdTe solar cells were no longer responsive to light. The samples were shorted with a resistance of about 12 . The predeposition I–V traces indicated leakage current in the cell. This leakage could have been a result of pinholes in the films that allowed short circuits. When the Al was deposited, it would have filled in these holes to create a short similar to what was measured. To investigate improving the film quality, we deposited CdTe and CdS on glass substrates at a deposition temperature ∼ 75 ◦ C. The adhesion of the films to the glass was greatly enhanced even though the temperature increase of the substrate was relatively small. The surface of the CdTe film showed no pinholes under 500X on an optical microscope. We are in the process of determining optimal deposition temperatures and film quality prior to depositing more solar cells.
4. Discussion The production of solar power on the Moon will require substrate (support structure) fabrication and thin film solar cell deposition. The fabricated solar array is like a macroscopic version of an electronic device on a chip. The chip (substrate) must be insulating so that the different elements (solar cells) can be made and then connected together in appropriate circuits. Fabricating the substrate on the Moon is advantageous, in that it eliminates the need for the support structure to be strong enough to survive launch and transport stresses and using lunar resources to do fabrication eliminates the transport of material. The substrate must be easily fabricated in order that the process be reliable and cost effective. Melting of the lunar regolith produces a smooth, insulating glass. Individual solar cells can be isolated on the insulating lunar regolith glass. The surface of the regolith glass is very smooth but can also be textured to increase the scattering of light at the back of the solar cell. This feature causes any light not absorbed on the first pass through the solar cell to be scattered back into the solar cell for another chance. Reflectivity measurements showed a decrease in the specular reflection as a result of the textured surface. In the lunar environment, the regolith will be melted by heating from the surface. The problems encountered in melting the regolith in the boats
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will actually be helpful in melting the regolith on the lunar surface. In heating from the surface, the low thermal conductivity will help trap the heat at the surface. The regolith does not start to soften until ∼ 1300 ◦ C. As the regolith starts to melt, it is very viscous and so will support the melt at the surface. We are preparing to melt regolith by heating the surface with concentrated light to determine the suitability for forming the regolith glass substrate. Photovoltaic diodes were deposited on both smooth and textured lunar regolith substrates. The deposition of silicon photovoltaic film structures on lunar regolith substrates produced diodes that were not light responsive. Analysis of the n- and p-doped layers indicated that the p-layer was too highly doped for the device to work as a solar cell. The high doping reduces the diffusion length of the charge carriers and provides recombination centers. The photo-generated electrons and holes recombine before they can be collected. The slope of the dark I–V curve indicated a large recombination current. For silicon, the composition of the initial source material will have to be determined. If the source silicon is already doped n and p, then the amount of dopant will have to be determined so that after deposition, the correct doping in the films is achieved. If the dopants are transported separately, then both the Si and dopant deposition rates will need to be carefully calibrated so that the correctly doped films are deposited. This would increase the complexity of operation and so is less desirable. More optimization work is needed before a silicon solar cell can be made on the lunar regolith substrate. The choice of solar cell depends on the source of material to be used. Refining lunar regolith can produce silicon, aluminum and other potentially useful elements but the cost depends on the amount needed. The amount of material needed in a thin film solar cell is small. For example, a silicon solar cell 20 m thick would need less than 50 g to cover 1 m2 . If the solar cells were 10% efficient, then 100 kg of Si would be enough for more than 250 kW of solar cell fabrication. By reducing the solar cell film structure to 2 m, the amount of silicon could be reduce to 10 kg to supply the same amount of power. This amount of material is small compared to the mass of a solar cell fabricating lunar rover. The deposition of thin film solar cells will require careful optimization. For alternate solar cell material systems, the deposition of the solar cells will
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need to be tested. While solar cells have been optimized for terrestrial fabrication techniques, the cells will need to be optimized for lunar thermal deposition. An ideal material system would be one that is normally deposited by thermal deposition techniques and that with simple processing reach efficiencies on the order of 5–10%. While this efficiency is much lower than terrestrial, there is plenty of lunar real estate to make up for the lower efficiency. The CdS/CdTe solar cell system is compatible with these requirements. The material is deposited by variations of thermal evaporation at lower temperatures than silicon. The cadmium telluride naturally dopes p type under thermal evaporation with no intentional doping. The doping is related to Cd vacancy defects. The cadmium sulfide naturally dopes n type due to Cd interstitials. All these attributes combine to make the CdS/CdTe system viable for the lunar solar cell. CdS/CdTe photovoltaic diodes were deposited on smooth and textured lunar regolith substrates. The deposition of the thin film structures was not optimized. Before the top contacts were deposited, the photovoltaic diodes had a response to light on the order of 1–3%. The I–V curve indicated that the diode had an ohmic leakage current in the device. The spectral quantum efficiency indicated that the CdTe layer was not providing adequate photoconversion. When the top contacts were deposited, the photovoltaic diode became unresponsive to light. The failure is most likely in the CdTe layer. Both the CdS and the CdTe films were deposited with nonoptimal deposition parameters. Optimizing the deposition parameters should improve the quality of the films.
5. Summary We have demonstrated the first photovoltaic diodes fabricated on lunar regolith substrates. We have fabricated the solar cell structures using techniques similar to those that would be available on the Moon. Lunar regolith was melted to produce glass substrates that are suitable for thin film deposition. For the thin film photovoltaic structures, both smooth surface and textured surface lunar regolith substrates were utilized. The textured surface increases the diffuse scattering at the back of the solar cell. The Si diode deposited on lunar regolith substrates displayed diode behavior in
I–V measurements but had incorrect doping levels and a high recombination current leading to a lack of light response. The CdS/CdTe photovoltaic diodes also displayed diode behavior in I–V measurements and had a light response on the order of 1–3%. The spectral quantum efficiency of the CdS/CdTe indicated that most of the photoconverted carriers were emanating from the CdS layer with only a little in the CdTe. The CdS/CdTe photovoltaic diode lost the light response when the top contact was deposited due to poor film quality. Further work is needed to optimize the Si, CdS and CdTe film deposition parameters to optimize the film quality and improve the light response.
Acknowledgement The authors would like to thank NASA for support through the Cross Enterprise Technology Development Program (CETDP).
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