David J. Hoffman, Thomas W. Kerslake, and Aloysius F. Hepp

Glenn Research Center, Cleveland, Ohio

Mark K. Jacobs

Science Applications International Corporation, Schaumburg, Illinois

Deva Ponnusamy

Spectrum Astro, Gilbert, Arizona

Thin-Film Photovoltaic Solar

Array Parametric Assessment

NASA/TM—2000-210342

July 2000

AIAA–2000–2919
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National Aeronautics and

Space Administration

Glenn Research Center

Prepared for the

35th Intersociety Energy Conversion Engineering Conference

sponsored by the American Institute of Aeronautics and Astronautics

Las Vegas, Nevada, July 24–28, 2000

David J. Hoffman, Thomas W. Kerslake, and Aloysius F. Hepp

Glenn Research Center, Cleveland, Ohio

Mark K. Jacobs

Science Applications International Corporation, Schaumburg, Illinois

Deva Ponnusamy

Spectrum Astro, Gilbert, Arizona

Thin-Film Photovoltaic Solar

Array Parametric Assessment

NASA/TM—2000-210342 AIAA–2000–2919
Acknowledgments

Available from

NASA Center for Aerospace Information

7121 Standard Drive

Hanover, MD 21076

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Trade names or manufacturers’ names are used in this report for

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endorsement, either expressed or implied, by the National

Aeronautics and Space Administration.

This report contains preliminary

findings, subject to revision as

analysis proceeds.

The authors would like to acknowledge the support of the following individuals in the development of the

analytical model under task order contract NAS3–26565: Dennis Pellacio and Mike Stancati of SAIC;

Ken Rachocki and Kerry Wesley of Spectrum Astro; and Eli Kawam (formerly of Spectrum Astro).
NASA/TM—2000-210342 

American Institute of Aeronautics and Astronautics 

1 

AIAA–2000–2919 

THIN-FILM PHOTOVOLTAIC SOLAR ARRAY PARAMETRIC ASSESSMENT 

David J. Hoffman, Thomas W. Kerslake, and Aloysius F. Hepp 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, OH 

Mark K. Jacobs 

SAIC 

Schaumburg, IL 

Deva Ponnusamy 

Spectrum Astro 

Gilbert, AZ 

ABSTRACT

This paper summarizes a study that had the objective to 

develop a model and parametrically determine the 

circumstances for which lightweight thin-film 

photovoltaic solar arrays would be more beneficial, in 

terms of mass and cost, than arrays using highefficiency crystalline solar cells. Previous studies 

considering arrays with near-term thin-film technology 

for Earth orbiting applications are briefly reviewed. 

The present study uses a parametric approach that 

evaluated the performance of lightweight thin-film 

arrays with cell efficiencies ranging from 5% to 20%. 

The model developed for this study is described in 

some detail. Similar mass and cost trends for each 

array option were found across eight missions of 

various power levels in locations ranging from Venus to 

Jupiter. 

The results for one specific mission, a main belt 

asteroid tour, indicate that only moderate thin-film cell 

efficiency (~12%) is necessary to match the mass of 

arrays using crystalline cells with much greater 

efficiency (35% multi-junction GaAs based and 20% 

thin-silicon). Regarding cost, a 12% efficient thin-film 

array is projected to cost about half as much as a 

4-junction GaAs array. While efficiency improvements 

beyond 12% did not significantly further improve the 

mass and cost benefits for thin-film arrays, higher 

efficiency will be needed to mitigate the spacecraftlevel impacts associated with large deployed array 

areas. A low-temperature approach to depositing thinfilm cells on lightweight, flexible plastic substrates is 

Copyright © 2000 by the American Institute of Aeronautics and 

Astronautics, Inc. No copyright is asserted in the United States under 

Title 17, U.S. Code. The U.S. Government has a royalty-free license to 

exercise all rights under the copyright claimed herein for Government 

Purposes. All other rights are reserved by the copyright owner.

briefly described. The paper concludes with the 

observation that with the characteristics assumed for this 

study, ultra-lightweight arrays using efficient, thin-film 

cells on flexible substrates may become a leading 

alternative for a wide variety of space missions. 

INTRODUCTION

Very lightweight and low cost photovoltaic (PV) solar 

arrays based on thin-film PV array technology have 

held much promise for future space missions. While 

sample thin-film cells and panels have flown in space 

(LIPS-III in 1987, PASP-Plus in 1994, the Mir space 

station in 1998) and are planned to fly (Earth 

Observing-1 in 2000), a complete solar array consisting 

of thin-film cells has yet to be built. Also, the projected 

array-level efficiency of thin-film PV is currently much 

less than that of arrays based on advanced thin-crystal 

silicon (Si) and multi-junction gallium arsenide (GaAs) 

based cells. Consequently, at the spacecraft level, the 

large deployed array area required for thin-film arrays 

offsets or even negates its lower array mass and cost 

benefits. Until thin-film PV efficiency improves and 

manufacturing methods to deposit the thin-films on 

lightweight substrates over large areas are refined, 

future space missions will most likely keep using high 

efficiency silicon or multi-junction PV planar and/or 

concentrator arrays. As thin-film PV technology for 

use in space improves, more applications will consider 

its advantages, namely low cost, low mass, improved 

radiation tolerance1

, and high specific power (W/kg). 

Figure 1 depicts two ways to obtain very high specific 

power using photovoltaic arrays. Flexible planar arrays 

of moderate area density (1-2 kg/m2

) using either 

relatively heavy but very efficient multi-junction solar 

cells, or relatively lighter but less efficient thin silicon 

cells, could obtain an array-level specific power 

approaching 300 W/kg. To get to this level, new solar 
NASA/TM—2000-210342 

American Institute of Aeronautics and Astronautics 

2 

array substrates, support structures and deployment 

concepts may be needed in conjunction with improved 

cell technology.2

 Ultra-lightweight arrays (0.25 to 

0.75 kg/m2

) using lightweight thin-film solar cells of 

moderate efficiency may enable the attainment of even 

greater array-level specific power. As the plot in figure 

1 implies, ultra-lightweight thin-film arrays may be the 

most feasible means of approaching the very high 

specific power necessary to enable missions with very 

high power requirements, such as space solar power 

satellites, manned Mars or lunar missions and some 

solar electric propulsion concepts.3, 4

The objective of the present assessment is to develop a 

model and parametrically determine the circumstances, 

both in terms of solar array technology and mission 

scenarios, for which thin-film PV solar arrays would be 

more beneficial than alternatives. NASA Glenn 

research Center’s (GRC) approach to depositing thinfilm cells on lightweight substrates with the aim of 

ultimately achieving higher efficiencies is also 

described. 

BACKGROUND

A number of past studies have compared solar cell and 

array technologies for Earth orbiting missions. Ralph 

performed system trades for presently available and 

near-term crystalline and thin-film cells on rigid, 

flexible and concentrator arrays in Low Earth Orbit 

(LEO) and Geostationary Earth Orbit (GEO).5, 6 Ralph’s 

results in reference 5 indicate that GEO arrays using 

high efficiency multi-junction GaAs cells have mass 

and cost advantages over alternatives, especially when 

the area penalty (increased attitude control fuel) of 

arrays using the less efficient thin-film cells is included. 

With Ralph’s assumptions, thin-film cell efficiency 

needs to be at least 12.6% to be competitive in GEO. 

For LEO, Ralph concluded that while the most efficient 

multi-junction cell array has the lowest mass, arrays 

with 9% to 12.6% efficient thin-film cells have 

competitive area-adjusted costs. 

In a similar study, Gaddy looked at the cost 

performance of multi-junction GaAs and advanced Si 

arrays on small, medium and large LEO spacecraft.7

 

This study included the cost of the spacecraft support to 

the payload and concludes that the most efficient multijunction arrays result in the greatest spacecraft-level 

mass and cost benefits. 

The paper by Bell outlines a model developed by the 

Aerospace Corporation to “determine optimal power 

subsystem suites as a function of spacecraft design and 

total system cost.”

8

 Example model results are reported 

for a 100 satellite high-power (15kW) LEO 

constellation and a small, single-mission 1 kW LEO 

satellite. Study results for both cases favored high 

efficiency cell solar arrays. Because satellites in the 

LEO constellation were delivered to a low parking orbit 

and then transferred to the final 1852 km orbit, the large 

area of the 8% to 10% efficient thin-film arrays led to 

significant attitude control system impacts, and 

ultimately higher mission costs. For the single-mission 

low power LEO case, the Aerospace model favored 

mature, low nonrecurring cost array technologies using 

16% efficient Si and 21.5% efficient GaAs cells. 

Each of the studies reviewed above looked at near-term 

thin-film cell technology on flexible, but not necessarily 

lightweight arrays for Earth orbiting applications. Only 

when the cell efficiency of a thin-film array was greater 

than 10% did they compare favorably with crystalline 

cell arrays for some of the missions studied. In the 

present study, the performance of ultra-lightweight 

thin-film arrays with assumed cell efficiencies ranging 

from 5% to 20% are evaluated for missions in Earth 

orbit and beyond. 

ANALYTICAL APPROACH

One objective of this study is to estimate the 

improvement in cell efficiency required for thin-film 

arrays to be more competitive with higher-efficiency 

crystalline cells from a mass and area perspective. 

From a mass perspective, array specific mass is the 

figure of merit. Array specific mass can be obtained by 

dividing the specific area (W/m2

) by the array’s area 

density (kg/m2

). Specific area is a function of the cell 

efficiency and array packing factor. Area density is a 

function of the cell material density and thickness and 

the array substrate, wiring, support structure and 

mechanisms. To a first order, the cell efficiency 

required to match the specific power of an array of a 

given type but using different cells (i.e. the array area 

density not including cells is assumed to be constant) 

can be estimated with the following equation, 

ηTF = η2

PF2

PFTF





 



 Array + CellTF

Array + Cell2





 



 (1) 

where η is the cell efficiency, PF is the array packing 

factor, Array is the area density (kg/m2

) of the array, 

including its wiring, substrate, support structure and 

mechanisms, and Cell is the cell area density. While 

the array area density is held constant in this first order 

approximation, in actuality, it should decrease with the 

use of lighter cell technology. The more detailed array 

model discussed later accounts for this effect. 

Figure 2 shows the approximate thin-film cell 

efficiency required to match the specific power of a 

high efficiency cell array using equation 1. Cell 
NASA/TM—2000-210342 

American Institute of Aeronautics and Astronautics 

3 

material densities, including the coverglass, of 

0.50 kg/m2

 for the Si cell, 1.0 kg/m2 for the multijunction GaAs cells, and 0.16 kg/m2

 for the thin-film 

cells are assumed in figure 2. In practice, the actual cell 

efficiency required to match array specific mass will 

also depend on the cell operating temperature and 

degradation of the cell efficiency from environmental 

effects over the mission life. Nevertheless, figure 2 can 

be used to discern trends. For example, the figure 

shows that for ultra-light arrays (area densities from 

0.25 to 0.75 kg/m2

), only moderate thin-film cell 

efficiencies are required to match the specific power of 

arrays using much higher efficiency, but heavier cells. 

Improvements in thin-film cell efficiencies may still be 

necessary in order to reduce the size of thin-film arrays 

in order to minimize attitude control system impacts 

and to reduce array stowed volume and deployment 

complexity for missions with these concerns. 

To perform the main analysis of this study, a 

spreadsheet model was developed that calculates the 

size and estimates the cost of PV arrays based on 

different cell and array technologies for a given set of 

mission requirements. Comparative metrics (e.g. W/kg, 

W/m2

, kg/m2, etc.) are calculated for various array 

components, at the array level itself, and then at the 

power subsystem and spacecraft level. 

Representative mission information was gathered for 

eight missions at various locations in the solar system 

with various end-of-life (EOL) power requirements. 

The model was applied to each mission in a parametric 

fashion in an effort to determine meaningful trends. 

MODEL DESCRIPTION

The Array Design Assessment Model (ADAM) was 

developed to support evaluation of array design 

alternatives. ADAM includes several integrated array 

design modules, five databases to manage input set 

alternatives for running the design modules, and a user 

interface with input forms and model outputs. Outputs 

include nearly 100 items representing array 

performance, including PV array, other power 

subsystem elements, and spacecraft development. 

ADAM elements and estimating methodology flow are 

shown in figure 3. 

Mission candidates in the ADAM database cover array 

sizes from several hundred watts to around 

20 kilowatts. Size and costing relationships have not 

been tested for very small (<100 W) or very large 

(>25 kW) arrays. 

PV Array Sizing

For PV array sizing, ADAM separates the array into 

several elements, as shown in figure 4. The model first 

estimates cell area requirements based on cell 

performance characteristics in the selected operating 

environment, including the effects of operating 

temperature, cell mismatch, interconnects, radiation, 

thermal cycling, contamination deposition, meteoroid 

and orbital debris, ultraviolet degradation, shadowing, 

offpointing and the array packing factor. Additional 

blanket layers are built up based on material selections 

and layer thicknesses. Many advanced features are 

incorporated to address scaling issues. For example, as 

required rigid array wing areas grow, less dense and 

thicker substrate core materials are used to maintain 

reasonable structural characteristics. 

After all blanket requirements are estimated, structure 

and mechanical elements are added based on blanket 

properties and required structural characteristics. 

ADAM handles structural design differently for rigid 

and flexible arrays. For rigid arrays, a yoke is used to 

reduce losses from shadowing and stiffness is based on 

properties of the blanket panels and hinges between 

panels. For flexible arrays, a deployable boom is sized 

to support the panel and meet first fundamental 

frequency requirements. 

For rigid panels, the model uses a sandwich structure, 

which includes a honeycomb core and aluminum or 

composite face sheets. A parametric curve, correlating 

mass to the substrate area has been developed based on 

past data, and the mass is initially estimated using this 

curve. The masses of other mechanical elements are 

computed as a fraction of the substrate mass. 

The deployed fundamental frequency is one of the basic 

requirements of the array, and it is calculated to further 

validate the sizing and configuration. The natural 

frequency is calculated using the Jones’ equation, 

2 max

1.2769

π δ

g f n = (2) 

where, fn is the natural frequency in first bending 

mode, g is the acceleration due to gravity, and δmax is 

the maximum deflection of array. This is a close 

approximation of the fundamental frequency of a 

uniform thin plate of arbitrary shape, having any 

combination of fixed, partially fixed or simply 

supported boundaries. 
NASA/TM—2000-210342 

American Institute of Aeronautics and Astronautics 

4 

Substrate materials are selected and each layer’s 

thickness is calculated to match the substrate mass 

estimated earlier. For the purpose of this calculation, 

the solar array is assumed to be a uniform thin plate and 

the total deflection under 1 g due to the bending of the 

substrates and the compliance of the hinge lines is 

calculated. Hinge stiffness is assumed to be 105

-106

Nm/rad. Details like the aspect ratio of the array are 

chosen to achieve a fundamental frequency of about 0.5 

Hz as the model default, although the user can specify 

other fundamental frequency values. 

In the case of the flexible panel, the total mass of the 

blanket, cells and all other add-ons is estimated by 

ADAM’s Blanket Design Module. Given the total 

mass and the aspect ratio of the array, the uniformly 

distributed mass on the boom is calculated. The boom 

used in this study is a coilable lattice boom. The 

diameter of the boom, which is limited to a minimum of 

10cm, is chosen to provide the equivalent stiffness, 

necessary to achieve a user-defined fundamental 

frequency (typically 0.5 Hz) for the given load.9

 This 

approach results in boom dimensions and masses that 

are realistic, even though the strength of the boom in 

bending or buckling is not taken into account. The mass 

of the canister is assumed to be 1.5 times the mass of 

the boom. The mass of the array stowage and 

tensioning systems is calculated as 25% of the sum of 

the boom, canister, blanket and wiring masses. 

For both rigid and flexible arrays, the mass of a single 

axis drive actuator (SADA) is accounted for and is 

assumed to scale linearly with the beginning-of-life 

(BOL) power level (1.5 kg/kW). Wiring mass for 

either type of array is assumed to be 1.2 kg/kW. 

Array sizing accounts for energy storage to support 

eclipse operations or other mission requirements. 

ADAM includes nine PMAD and energy storage inputs 

to estimate other power subsystem element 

requirements and additional array output required for 

charging the storage system. 

Cost Assumptions/Methodology

ADAM includes parametrics to estimate spacecraft 

hardware development costs in fixed year dollars (fiscal 

year 2000). This covers activities typically performed in 

Phases B/C/D. Cost estimating relationships (CERs) 

were developed for each ADAM Reference Mission 

Candidate using proven methods. For Earth orbiting 

missions, CERs were derived from the NASA GSFC 

Space Systems Quick Estimating Guide (Version 2.0, 

August 1997). For the other planetary missions, SAIC's 

Planetary Development Model was used. Heritage 

credits were applied to approximately 75% of each 

subsystem and the other 25% is assumed to be new 

development with available technology. Advanced 

technology development costs are not included. 

Parametrics are based on costs per kg for all spacecraft 

subsystems except power, and are only intended to be 

accurate for concepts reasonably similar to the selected 

Reference Mission Candidate. Spacecraft system-level 

assembly/integration/test costs are estimated to be 15% 

of the subsystem total. Cost results should be 

interpreted as relatively representative, not absolute 

values. 

Power system costs are built up from several elements. 

Hardware costs are estimated at the component-level 

(e.g. cells, substrate, structures, etc.). Non-recurring 

costs are assumed to be 50% of the hardware costs, and 

assembly/integration/test labor is added at a rate of 

$500 per Watt. Because ADAM does not estimate 

advanced technology development, each array design 

concept is assumed to be at an equivalent technology 

readiness level. Savings from advanced array concepts 

need to offset costs to demonstrate flight readiness. 

Model Inputs/Outputs

Table 1 shows a summary of ADAM’s databases, 

inputs, and outputs. ADAM generates almost 100 

output items from over 50 inputs to compare 

performance of different array design concepts. Four 

high-level inputs – Mission Type, Operating 

Environment, End-of-Life (EOL) Power Required, and 

Array Design Lifetime – interface with the ADAM 

databases to determine initial default values for 24 

Level 1 and 30 Level 2 inputs. Level 1 inputs interface 

with the model databases to determine Level 2 input 

defaults. ADAM users can choose to operate at the 

high-level or modify any Level 1 or 2 input to better 

represent their array/mission design concept. As the 

ADAM databases are expanded, model capabilities are 

enhanced. Future versions of ADAM may incorporate 

more database candidates and additional/enhanced 

databases, inputs/outputs, and design modules. 

More details describing ADAM can be found in the 

final review presentation for the task order contract 

under which the model development was performed.10

ANALYSIS CASES

As previously mentioned, this study assessed eight 

representative missions throughout the solar system: a 

Venus orbiter, LEO and GEO missions, a lunar lander, 

a Mars communication orbiter and a Mars lander, a 

Main Belt Asteroid Tour, and a Jupiter orbiter. Given 

the lightweight substrate and parametric thin-film cell 

efficiency assumptions used in this study, the same 

overall trends were found for all missions. 
NASA/TM—2000-210342 

American Institute of Aeronautics and Astronautics 

5 

The results for the Main Belt Asteroid Tour (MBAT) 

are used to illustrate the trends from the parametric 

analysis. The MBAT mission was chosen because it is 

a relatively high power mission using solar electric 

propulsion (SEP). MBAT mission characteristics are as 

follows: 

• Location 1.5 AU 

• Design Life 6 years 

• EOL Power Required 7.5 kW (1.5 AU) 

• Spacecraft Dry Mass 560 kg 

• Spacecraft Wet Mass 956 kg 

The specific thin-film technology considered in the 

trade study is a 0.2-mil (5 micron) copper indium 

disulfide (CuInS2, CIS2 or CIS2) cell on 0.08-mil of 

molybdenum and 2 mils of a polyimide, resulting in an 

area density of 0.16 kg/m2. The CIS2 cell also contains 

ZnO and CdS layers and is estimated to cost $60/W. 

The BOL, 28 degree C, AM0 cell efficiency for CIS2 is 

parametrically varied from a low of 6% up to 20%. 

CIS2 performance metrics are compared with a 

presumed 35% efficient four-junction (4-j) cell based 

on single-crystal GaAs/Ge technology (1.1 kg/m2 and 

$400/W) and a 20% efficient single-crystal thin-Si cell 

(0.55 kg/m2

 and $220/W). For reference, present stateof-the-art AM0, 1-sun efficiency is about 25% for 

multijunction GaAs based cells and 17% for thin Si. 

Both crystalline cells have a 4-mil coverglass. All cells 

are mounted on a 5-mil composite flexible substrate 

with a coilable deployment boom sized for a 0.5 Hz 

minimum first fundamental frequency. 

RESULTS

Figures 5 through 8 show the model results in graphical 

form. Figure 5 plots the PV blanket and total array 

specific power for each array. Figure 6 depicts the total 

array area for each array on a relative basis, normalized 

to the 4-j GaAs case (50 m2

 total). Figures 7 and 8 show 

the array mass and cost breakdowns on a relative basis, 

again normalized to the 4-j GaAs case (121 kg, $14.1M 

total array mass and cost). 

DISCUSSION

Pertaining to specific power, figure 5 shows the arrays 

with 4-j GaAs and Si crystalline cells have comparable 

values for this key metric at both the PV blanket and 

total array levels. For the thin-film array, progressively 

higher specific power at the PV blanket level results as 

a linear function of cell efficiency. However, at the 

total array level, which includes array wiring, 

structures, mechanisms and a single-axis drive actuator 

for pointing, the increase is not linear and is much less 

rapid than at the blanket level. This illustrates the 

difficulty in attaining very high total-array-level 

specific power when accounting for all typical array 

“ancillaries”. 

With respect to array total deployed area, figure 6 

confirms what is expected – area scales linearly with 

cell efficiency (assuming similar packing factors and 

mission cell efficiency knockdown factors). For the 

MBAT mission, unbalanced drag torques would not be 

a problem for the much larger array sizes with the 

lowest thin-film cell efficiencies. However, other 

disturbance torques and or spacecraft/array slewing to 

maintain SEP thrust vectors may be an issue. 

Figure 7 indicates that for the assumptions underlying 

the present study, a moderate thin-film cell efficiency 

of 12% is necessary to match the total mass of arrays 

using crystalline cells with much greater efficiency. 

The array component mass breakdowns reveal the 

leading contributors to each array’s total mass. 

Mechanical components, which include the array 

stowage and tension mechanisms and SADA contribute 

a significant portion to all arrays. The cell and 

coverglass mass dominate the crystalline PV blanket 

mass, while the substrate mass dominates the thin-film 

blanket. This highlights the point that in order to take 

full advantage of the mass benefits of thin-film cell 

technology, very lightweight substrates and support 

structures are necessary. 

The cost breakpoint for the thin-film arrays occurs at 

thin-film efficiencies greater than 12% according to 

figure 8, resulting in an array that costs about half as 

much as the 4-j GaAs array. Improving the thin-film 

cell efficiency beyond 12% did not significantly further 

improve the cost benefit. 

THIN-FILM CELL DEVELOPMENT AT GRC

Among the desirable attributes in any space-bound 

component, subsystem or system are high specific 

power, radiation tolerance and high reliability, without 

sacrificing performance. NASA GRC is currently 

developing space-bound technologies in thin film 

chalcopyrite solar cells and thin-film lithium polymer 

batteries. The thin-film solar cell efforts at GRC are 

summarized below. 

The key to achieving high specific power solar arrays is 

the development of a high-efficiency, thin-film solar 

cell that can be fabricated directly on a flexible, 

lightweight, space-qualified durable substrate. Such 

substrates include KaptonTM (DuPont) or other 

polyimides or suitable polymer films. While the results 

of the present study indicate that lightweight thin-film 
NASA/TM—2000-210342 

American Institute of Aeronautics and Astronautics 

6 

cells with moderate efficiency on lightweight substrates 

can compete on a mass basis, higher cell efficiencies 

will be required to mitigate impacts associated with 

large array area. Current thin-film cell fabrication 

approaches are limited by either (1) the ultimate 

efficiency that can be achieved with the device material 

and structure, or (2) the requirement for hightemperature deposition processes that are incompatible 

with all presently known flexible polymides, or other 

polymer substrate materials. 

At GRC, a chemically based approach is enabling the 

development of a process that will produce highefficiency cells at temperatures below 300 °C. Such 

low temperatures minimize the problems associated 

with the difference between the coefficients of thermal 

expansion of the substrate and thin-film solar cell 

and/or decomposition of the substrate. 

Polymer substrates can be used in low temperatures 

processes. As such, thin-film solar cell materials can be 

deposited onto molybdenum-coated Kapton, or other 

suitable substrates, via a chemical spray process using 

advanced single-source precursors, or by direct 

electrochemical deposition. A single-source precursor 

containing all the required chemically-coordinated 

atoms such as copper, indium, sulfur and others, will 

enable the use of low deposition temperatures that are 

compatible with the substrate of choice.11

A combination of low-temperature electrochemical 

deposition and chemical bath deposition has been used 

to produce ZnO/CdS/CuInSe2 thin-film photovoltaic 

solar cells on lightweight flexible plastic substrates, 

depicted in figure 9.12

CONCLUSION

Once available and space qualified, moderate to 

relatively high efficiency thin-film cells on lightweight 

flexible substrates will offer significant mass and cost 

benefits. This approach may even enable ultralightweight solar arrays to attain the very high specific 

mass required for future high-power missions and 

applications. Further, as thin-film cell efficiency 

improves, the packaging, deployment and 

attitude/control impacts of the larger array area will 

diminish. With these characteristics, ultra-lightweight 

arrays using efficient, thin-film cells on flexible 

substrates may become a leading alternative for a wide 

variety of space missions. 

REFERENCES

1. Woodyard, J., Landis, G., “Radiation Resistance of 

Thin Film Solar Cells”, Solar Cells, Vol. 31, No. 4, 

pp. 297–329, 1991 (also NASA Technical 

Memorandum TM–103715). 

2. Jones, P.A., White, Stephen F., Harvey, T.J., “A 

High Specific Power Solar Array for Low to MidPower Spacecraft”, Proceedings of the Space 

Photovoltaic Research and Technology Conference 

(SPRAT XII), 1992, NASA CP–3210, pp. 177–187. 

3. Landis, G., Hepp, A., “Applications of Thin-Film 

PV for Space”, Proceedings of the 26th Intersociety 

Energy Conversion Engineering Conference, 

Vol. 2, pp. 256–261, Aug. 1991. 

4. Kerslake, Thomas W., Gefert, Leon P., “Solar 

Power System Analyses for Electric Propulsion 

Missions”, NASA/TM–1999–209289, SAE 99–01–

2449, 34th IECEC, August 1999. 

5. Ralph, E.L., Woike, T.W., “Solar Cell Array 

System Trades – Present and Future”, AIAA, 1999. 

6. Ralph, E.L., “High Efficiency Solar Cell Arrays 

System Trade-Offs”, IEEE, 1994. 

7. Gaddy, Edward M., “Cost Performance of MultiJunction, Gallium Arsenide, and Silicon Solar 

Cells on Spacecraft, 25th PVSC, May 1996. 

8. Bell, Kevin D., Marvin, Dean C., “Power 

Generation and Storage Technology Selection for 

an Optimal Spacecraft System Design”, IECEC 

1999–01–2531. 

9. Conley, Peter, “Space Vehicle Mechanisms: 

Elements of Successful Design”, John Wiley and 

Sons Inc., 1998.

10. “Thin-Film PV Solar Array Assessment for 

Satellites, Final Review Report,” contract number 

NAS3–26565, Task Order 31.

11. Hollingsworth, J.A., Hepp, A.F., and Buhro, W.E., 

Chemical Vapor Deposition, Vol.3, pp. 105–108, 

1999.

12. Raffaelle, R.P., et al, Solar Energy Materials and 

Solar Cells, Vol. 57, pp.167–178, 1999. 
NASA/TM—2000-210342 

American Institute of Aeronautics and Astronautics 

7 

Lightweight Solar Array Technology Thrusts

0.0 

0.2 

0.4 

0.6 

0.8 

1.0 

1.2 

1.4 

1.6 

1.8 

2.0 

5% 10% 15% 20% 25% 30% 35% 40%

Cell Efficiency, AM0 @ 28°C

100 W/kg 50 W/kg

SOA for Flight 

200 W/kg

1000 W/kg

Better

Better

High η Cell 

Flexible Planar Arrays

Thin Film Arrays

Area Density, kg/m2

300 W/kg

Figure 1 - Lightweight solar array technology thrusts.

Approximate Thin-Film Cell Efficiency

 Required to Match High Efficiency Cell Array Specific Power

5%

6%

7%

8%

9%

10%

11%

12%

13%

14%

15%

16%

17%

18%

19%

20%

0 0.5 1 1.5 2 2.5 3

Array Area Density not including Cells , kg/m^2

Thin Si (17%)

GaAs (18%)

GaAs (23%)

GaAs (25%)

GaAs (30%)

Note: Array packing factor

is 0.85 for all options.

Figure 2 - Approximate thin-film cell efficiency required to match high efficiency cell array

specific power.
NASA/TM—2000-210342 

American Institute of Aeronautics and Astronautics 

8 

Figure 3 - ADAM Elements and Estimating Methodology Flow

1.0 Mission

Application

Requirements

3.0 Solar

Array

Mass

4.0 Solar

Array

Size

5.0

Mechanical/

Structural

6.0

Cost

2.0

Baseline

Comparisons

STRUCTURES CELLS MATERIALS MISSIONS ENVIRONMENTS

INPUTS

DATABASES

DESIGN

MODULES

OUTPUTS

Databases provide initial input defaults/Input sets can be saved as a new option in the corresponding database

Rigid & Flexible

Structures/Mechanical Blanket PMAD & Storage

Level 1 - PV Design

Level 2 - Cells/Blanket

Level 2 - Structures/Substrate

Level 1 -

Mission

Level 1 -

PMAD &

Storage

Figure 4 – Solar array hardware elements and ADAM model nomenclature.

BLANKET LAYER DRAWING

Level 2

INPUT Sheet

Level 1 MAIN

Term Level 2 Input Term

Coverglass/Superstrate

Adhesive layer

Cell front contact

CELL

Cell back contact

Adhesive layer

Facesheet

Core

Facesheet

Cover

Cells

Substrate

Cells/

Blanket

Substrate/

Structure

Canister

Drive

Mechanism

Array Wing

Hinges Yoke

Tensioning System

Array Panels

Rigid

Flexible

= Blanket

S/C

Boom

Array elements and blanket layer thicknesses not shown to scale
NASA/TM—2000-210342 

American Institute of Aeronautics and Astronautics 

9 

Main Belt Asteroid Tour - Solar Array Specific Power 

(9 kW Array @ 1 AU, 28°C)

0 W/kg

100 W/kg

200 W/kg

300 W/kg

400 W/kg

500 W/kg

600 W/kg

700 W/kg

GaAs 4-j (35%) Thin Si (20%) CIS2 (6%) CIS2 (12%) CIS2 (16%) CIS2 (20%)

 - PV Blanket BOL W/kg - Total Array BOL W/kg

Figure 5 - PV blanket and total solar array specific power.

Main Belt Asteroid Tour - Solar Array Area 

(9 kW Array @ 1AU 28°C)

100%

150%

200%

250%

300%

350%

400%

450%

500%

550%

600%

GaAs 4-j (35%) Thin Si (20%) CIS2 (6%) CIS2 (12%) CIS2 (16%) CIS2 (20%)

Figure 6 - Relative solar array total area (GaAs 4-j = 50 m2 total).
NASA/TM—2000-210342 

American Institute of Aeronautics and Astronautics 

10 

Figure 7 - Solar array mass breakdown and relative comparison (GaAs 4-j = 121 kg).

Figure 8 - Solar array cost breakdown and relative comparison (GaAs 4-j = $14.1M).

Main Belt Asteroid Tour Solar Array (9 kW @ 1 AU 28°C) with a Flexible Substrate 

Array Mass Breakdown with Different Cell Technology

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

110%

120%

130%

140%

150%

160%

GaAs 4-j (35%) Thin Si (20%) CIS2 (6%) CIS2 (12%) CIS2 (16%) CIS2 (20%)

Cover

Cells

Substrate

Wiring

Boom/Structure

Mechanical

Main Belt Asteroid Tour Solar Array (9 kW @ 1 AU 28°C) with a Flexible Substrate

Array Cost Breakdown with Different Cell Technology

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

GaAs 4-j (35%) Thin Si (20%) CIS2 (6%) CIS2 (12%) CIS2 (16%) CIS2 (20%)

Cover

Cells

Substrate

Wiring

Boom/Structure

Mechanical
NASA/TM—2000-210342 

American Institute of Aeronautics and Astronautics 

11 

Table 1 - Summary Descriptions of ADAM Database, Inputs and Outputs. 

Database Summary Inputs Summary Outputs Summary 

6 Cell Candidates 

1 Rigid Substrate Type 

5 Flexible Substrate Types 

Over 50 Material Candidates 

8 Mission Types 

- 2 Earth Orbiting 

- 4 Planetary 

- 2 Lunar/Mars 

Landers 

8 Operating Environments 

(specific to Mission Types) 

4 Primary Inputs: 

- Mission Type 

- Operating Environment 

- EOL Power Required 

- Array Design Lifetime 

24 Level 1 Inputs: 

- 4 Mission 

- 11 PV Array Design 

- 9 PMAD/Storage 

30 Level 2 Inputs: 

- 21 Cells/Blankets 

- 9 Structures/Substrates 

38 Level 1 Outputs: 

- 15 Option 

Comparison 

Summary Metrics 

- 8 Mass Elements 

- 4 Size 

Characteristics 

- 3 Structural 

Characteristics 

- 8 Cost Elements 

54 Level 2 Outputs: 

- 11 Power System 

Items 

- 23 Spacecraft 

Elements 

- 20 Power System 

Equipment List 

Items 

Anneal in 

argon at

400 oC

for 1 hour

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5

Energy (eV)

(α hν)

2 (a.u.)

CuInSe2 CdS ZnO

-20.0

-10.0

0.0

10.0

20.0

30.0

40.0

-0.5 -0.3 -0.1 0.1 0.3 0.5

Voltage (V)

Current Density (mA/cm 2

)

KaptonTM

- Thin-film space 

qualified flexible

polymer substrate

Molybdenum

- Electron-beam 

evaporated 200

angstrom film

CuInSe2

- Electrochemically 

deposited 1 micron 

absorber layer

CdS - Chemical 

bath deposited 

n-type window layer

ZnO - Chemical 

bath deposited

transparent contact

Thermally evaporated

aluminum contacts

Optical Optical Bandgaps Bandgaps I-V photoresponse

Figure 9 - NASA GRC thin-film cell approach. 
This publication is available from the NASA Center for AeroSpace Information, (301) 621–0390.

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17

A03

Thin-Film Photovoltaic Solar Array Parametric Assessment

Solar arrays; Electric power

Unclassified -Unlimited

Subject Categories: 18 and 20 Distribution: Nonstandard

Prepared for the 35th Intersociety Energy Conversion Engineering Conference sponsored by the American Institute of

Aeronautics and Astronautics, Las Vegas, Nevada, July 24–28, 2000. David J. Hoffman, Thomas W. Kerslake, and

Aloysius F. Hepp, NASA Glenn Research Center; Mark K. Jacobs, Science Applications International Corporation,

1501 Woodfield Road, Suite 202N, Schaumburg, Illinois 60173; and Deva Ponnusamy Spectrum Astro, 1440 North

Fiesta Blvd., Gilbert, Arizona 85233.

David J. Hoffman, Thomas W. Kerslake, Aloysius F. Hepp,

Mark K. Jacobs, and Deva Ponnusamy

This paper summarizes a study that had the objective to develop a model and parametrically determine the circumstances for which

lightweight thin-film photovoltaic solar arrays would be more beneficial, in terms of mass and cost, than arrays using high-efficiency

crystalline solar cells. Previous studies considering arrays with near-term thin-film technology for Earth orbiting applications are

briefly reviewed. The present study uses a parametric approach that evaluated the performance of lightweight thin-film arrays with cell

efficiencies ranging from 5 to 20 percent. The model developed for this study is described in some detail. Similar mass and cost

trends for each array option were found across eight missions of various power levels in locations ranging from Venus to Jupiter. The

results for one specific mission, a main belt asteroid tour, indicate that only moderate thin-film cell efficiency (~12 percent) is

necessary to match the mass of arrays using crystalline cells with much greater efficiency (35 percent multi-junction GaAs based and

20 percent thin-silicon). Regarding cost, a 12 percent efficient thin-film array is projected to cost about half as much as a 4-junction

GaAs array. While efficiency improvements beyond 12 percent did not significantly further improve the mass and cost benefits for

thin-film arrays, higher efficiency will be needed to mitigate the spacecraft-level impacts associated with large deployed array areas. A

low-temperature approach to depositing thin-film cells on lightweight, flexible plastic substrates is briefly described. The paper

concludes with the observation that with the characteristics assumed for this study, ultra-lightweight arrays using efficient, thin-film

cells on flexible substrates may become a leading alternative for a wide variety of space missions.