Aloysius F. Hepp and Sheila G. Bailey

Glenn Research Center, Cleveland, Ohio

Ryne P. Raffaelle

Rochester Institute of Technology, Rochester, New York

Inorganic Photovoltaics Materials and Devices:

Past, Present, and Future

NASA/TM—2005-213341

August 2005
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Aloysius F. Hepp and Sheila G. Bailey

Glenn Research Center, Cleveland, Ohio

Ryne P. Raffaelle

Rochester Institute of Technology, Rochester, New York

Inorganic Photovoltaics Materials and Devices:

Past, Present, and Future

NASA/TM—2005-213341

August 2005

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NASA/TM—2005-213341 1

Inorganic Photovoltaics Materials and Devices: 

Past, Present, and Future 

Aloysius F. Hepp and Sheila G. Bailey 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Ryne P. Raffaelle 

Rochester Institute of Technology 

Department of Physics and Microsystems Engineering 

Rochester, New York 14623 

Abstract 

This report describes recent aspects of advanced inorganic materials for photovoltaics or solar cell 

applications. Specific materials examined will be high-efficiency silicon, gallium arsenide and related 

materials, and thin-film materials, particularly amorphous silicon and (polycrystalline) copper indium 

selenide. Some of the advanced concepts discussed include multijunction III-V (and thin-film) devices, 

utilization of nanotechnology, specifically quantum dots, low-temperature chemical processing, polymer 

substrates for lightweight and low-cost solar arrays, concentrator cells, and integrated power devices. 

While many of these technologies will eventually be used for utility and consumer applications, their 

genesis can be traced back to challenging problems related to power generation for aerospace and 

defense. Because this overview of inorganic materials is included in a monogram focused on organic 

photovoltaics, fundamental issues and metrics common to all solar cell devices (and arrays) will be 

addressed. 

1. Introduction 

1.1 Recent Aspects of Advanced Inorganic Materials 

A variety of different cell types are currently under development for future utility, consumer, military, 

and space solar power applications. The vast majority of these needs are still currently being met by the 

use of single crystal silicon (ref. 1). However, a variety of other materials and even solar cell types are 

vying for the opportunity to replace Si, if not for all applications at least in certain specific ones. Close 

relatives to single crystal Si such as polycrystalline Si and amorphous Si are receiving much attention 

(ref. 2). More exotic approaches such as polycrystalline thin film CuInGaSe2 (ref. 3) or CdTe (ref. 4), 

dye-sensitized or titania solar cells (ref. 5), and even nanomaterial enhanced conjugated polymer cells 

(ref. 6) are also garnering much interest. However when it comes to highest efficiency cells, multijunction 

III-V devices are currently the leader. Of course, it is expected current triple junction GaAs cell 

technology will give way to quadruple junctions or possible nanostructured approaches. However, the use 

of ordinary Ge substrates may well give way to Ge on Si substrates. These have the advantages of lower 

cost, higher strength, and lighter weight. In addition to the traditional lattice-matched multijunction GaAs 

cells, we may also see the emergence of lattice mismatched or polymorphic cells (ref. 7). 
NASA/TM—2005-213341 2

1.2 Focus: Advanced Materials and Processing 

There is currently a tremendous amount of research being directed towards a thin film alternative to 

traditional crystalline cells. Thin films cells are quite attractive due to the fact that many of the proposed 

fabrication methods are relatively inexpensive and lend themselves well to mass production (ref. 8). 

These cells can be made to be extremely lightweight and flexible, especially if produced on polymeric 

substrates. Thin film cells have been investigated for quite some time now. Cu2S-CdS cells were 

developed as far back as the mid 1970’s. However, even the best results to date have been plagued by low 

efficiencies and poor stability. The inorganic materials that have received the most attention are 

amorphous Si, CdTe, and CuInGaSe2 (refs. 2 to 4). Recently thin film polymeric cells that incorporate 

inorganic components such as CdSe or CuInS2 quantum dots have garnered much attention (refs. 6 and 9). 

Many believe it is these materials that will hold the key to inexpensive, easily deployed, large area, high 

specific power arrays. This is due in part to the possibility of roll-to-roll processing using low-cost spray 

chemical deposition or direct-write approaches to producing thin film solar cells on inexpensive 

lightweight substrates with these materials (refs. 3, 8, and 10). 

1.3 Increased Efficiency 

Multijunction technology has provided the most dramatic increase in inorganic solar cell technology. 

Many groups have reported laboratory efficiencies well in excess of 30 percent AM1.5. Industrial lot 

averages are already above a 28 percent threshold (ref. 11). The move towards a quadruple junction solar 

cell has been problematic due to lattice constant constraints. However, there are several new programs 

looking at lattice mismatch approaches to multijunction III-V which can give a better match to the solar 

spectrum than what is available in an ordinary lattice matched triple junction cell. In addition, there is a 

considerable amount of attention being paid to the use of nanostructures (i.e., quantum dots, wires, and 

wells) to improve the efficiencies of III-V devices. Recent theoretical results and experimental advances 

have shown that the use of such structures may afford dramatic improvement in cell efficiency (refs. 12 

and 13). 

1.4 Increased Specific Power 

When considering specific power, or the power per mass of a solar cell or array, it is clear that 

crystalline technology will be severely limited by the mass of the substrate on which they are made. In 

order to achieve the types of specific power that are required to meet portable power needs of consumer 

or military markets, as well as many large scale space power needs, will require the development of 

efficient thin film photovoltaics on polymeric substrates. When considering array specific power it is 

important to note that a cell specific power considerably higher than the array specific power goal will be 

necessary. The difference in the cell to array specific power must be enough to make up for the 

interconnects, diodes and wiring harnesses. Roughly speaking the cell mass usual accounts for 

approximately half the mass of the total balance of systems (refs. 14 and 15). 

In addition to improving the cell efficiency or making the cells lighter, gains in array specific power 

may be made by an increase in the operating voltage. Higher array operating voltages can be used to 

reduce the conductor mass. A typical array designed for 28 volt operation at several kilowatts output, with 

the wiring harness comprising ~ 10 percent of the total array mass, yields a specific mass of ~ 0.7 kg/kW. 

If this array was designed at a 300 V operation it could easily allow a reduction of the harness specific 

mass by at least 50 percent. This alone would increase the specific power by 5 percent or more without 

any other modification. 

The extremely high specific power arrays that need to be developed for several proposed space 

programs like solar electric propulsion (SEP) or space solar power (SSP) will require lightweight solar 
NASA/TM—2005-213341 3

arrays that are capable of high voltage operation in the space plasma environment. SEP missions alone 

will require 1000 to 1500 V to direct drive electric propulsion spacecraft (i.e., no voltage step up is 

required to operate the thrusters). NASA has bench marked a thin film stand-alone array specific power 

that is 15 times the state-of-the-art (SOA) III-V arrays, area power density that is 1.5 times that of the 

SOA III-V arrays, and specific costs that are 15 times lower than the SOA III-V arrays (refs. 15 and 16).

2. Overview of Specific Materials 

2.1 High-Efficiency Silicon 

Silicon solar cells are the most mature of all solar cell technologies and have been used on the vast 

majority of consumer, military, and space applications. In the early 1960s, silicon solar cells were 

~11 percent efficient, relatively inexpensive, and well suited for low power and limited lifetime 

application. The conversion efficiency of current “standard-technology” silicon ranges from around 

12 to 15 percent under standard test conditions. The lower efficiency cells are generally more resistant to 

radiation (refs. 7, 14, and 17). 

There have been many enhancements to silicon cells over the years to improve their efficiency. 

Textured front surfaces for better light absorption, extremely thin cells with back surface reflectors for 

internal light trapping, and passivated cell surfaces to reduce losses due to recombination effects are just a 

few examples. The highest measured efficiency for a large area (i.e., 5 in2

) crystalline silicon solar cell 

stands at 21.5 percent. Currently, high efficiency Si cells approaching 17 percent AM0 efficiency in 

production lots are available from Japanese and German producers. Tecstar and Spectrolab, the two large 

U.S. companies that have produced virtually all of the domestic silicon space solar cells, only offer the 

conventional type of silicon cells with efficiencies around 14.8 percent because they have concentrated 

their development efforts on GaAs based multijunction cells. The advantage of a high efficiency silicon 

cell to that of a III-V lies in their relatively lower cost, lower material density, and higher strength (refs. 1, 

7, and 17). 

2.2 Polycrystalline Silicon 

A lower cost alternative, although less efficient, to standard crystalline silicon technology lies in its 

polycrystalline alternative (ref. 18). This material is manufactured by pouring liquid silicon into a mold. 

Upon solidification, multicrystallites form with associated grain boundaries. The resulting blocks of 

material are sliced into suitable wafers. Polycrystalline silicon (p-Si) has been extensively studied over 

the past decade due to its applications in optoelectronics. The light absorption properties along with its 

simple manufacture and broadly tunable morphology have made it an attractive photovoltaic material for 

some time now. Due to the defects associated with the grain boundaries the best polycrystalline Si solar 

cell efficiencies stand at 19.8 percent, less than its monocrystalline Si counterpart (ref. 19). 

2.3 Amorphous Silicon 

Silicon based solar cells can also be produced using amorphous thin films that are evaporated onto 

glass or even polymeric substrates (ref. 2). This approach requires very little active material and thus can 

relate to tremendous savings in production costs. These types of cells are well suited to large area roll-toroll processing (ref. 8). Unfortunately this method of manufacture results in high defect densities and thus 

is even less efficiency than p-Si solar cells. However, this approach does lend itself to multilayer 

processing and thus multijunction solar cells are possible. This approach has resulted in dramatic 
NASA/TM—2005-213341 4

increases in overall amorphous Si solar cell efficiencies. Multijunction amorphous Si solar cell 

efficiencies as high as 13 percent AM1.5 have been reported (ref. 2). 

2.4 Gallium Arsenide and Related III-V Materials 

Gallium Arsenide (GaAs) has been a material of interest to the solar cell community for many years 

(ref. 20). In 1955, an RCA group was funded by the U.S. Army Signal Corps, and later by the Air Force 

to work on the development of GaAs based cells. GaAs has a nearly ideal direct bandgap of 1.42 eV for 

operation in our solar spectrum. It also has favorable thermal stability and radiation resistance as 

compared to silicon. However, it took nearly 30 years of efficiency improvements until the use of GaAs 

based cells could be argued because of its much higher costs. The arena in which this premium could 

most easily be justified was in space utilization. In fact, now the vast majority of cells being launched for 

space solar power are multijunction III-V cells (refs. 14 and 15). 

The lattice matched heterojunction cells developed in the early 1970’s led to the acceptance of GaAs 

as a viable photovoltaic material. The Air Force launched the Manufacturing Technology for GaAs Solar 

Cells or (MANTECH) program in 1982. This program was designed to develop metal organic chemical 

vapor deposition techniques (MOCVD) necessary for the large-scale production quantities of GaAs solar 

cells (ref. 21). 

The other significant development in GaAs technology was in the use of alternative substrates. In 

1986, the Air Force supported work by the Applied Solar Energy Corp. (ASEC) in which they developed 

GaAs cells grown on Ge substrates. This was possible due to the similarity in the lattice constants and 

thermal expansion coefficients of the two materials. This resulted in the improvement in the mechanical 

stability of the cells and a lowering of the production costs. 

The development of GaAs-based cells continued throughout the late 1980's with the primary focus 

being on the development of multijunction approaches to photovoltaic conversion. The initial work was 

mechanical stacked cells, however this quickly transitioned into epitaxially grown dual-junctions with 

tunnel junctions in between. Overall cell efficiencies increase dramatically during this time (see fig. 1) 

(ref. 22). 

2.5 Thin-Film Materials 

Thin film photovoltaics is an intriguing technology due to flexible lightweight construction, 

permitting arrays to be “molded” onto non-rigid or uniform structures for innovative power systems. 

Photovoltaic modules based on ternary chalcopyrite absorber materials, (i.e. I-III-VI2 - Cu(In:Ga)(S:Se)2) 

have been the focus of intense investigation for over two decades (ref. 3). The use of chalcopyrite 

absorbers is attractive since their bandgaps correlate well to the maximum photon power density in the 

solar spectrum for both terrestrial (AM 1.5) (ref. 8), and space applications (AM0) (fig. 2) (refs. 14 to 16). 

Additionally, by adjusting the percent atomic composition of either Ga for In and/or S for Se, the bandgap 

can be tuned from 1.0 eV to 2.4 eV, thus permitting fabrication of high, or graded bandgaps (refs. 3 

and 8). 

One of the first thin film cells, Cu2S/CdS, was developed for space applications. Reliability issues 

eliminated work on this particular cell type for both space and terrestrial considerations even though 

AM1.5 efficiencies in excess of 10 percent were achieved. Thin film cells require substantially less 

material and thus lower mass and promise the advantage of large area, low cost manufacturing (refs. 2 to 

4, and 8). 

Development of other wide bandgap thin film materials that can be used in conjunction with CIS to 

produce a dual-junction device is underway. As has already been demonstrated in III-V cells for space 

use, a substantial increase over single-junction device efficiency is possible with a dual-junction device.

NASA and NREL have both initiated dual-junction CIS-based thin film device programs (refs. 3 and 16). 
NASA/TM—2005-213341 5

The use of Ga to widen the bangap of CIS and thus improve the efficiency is already well known. The 

substitution of S for Se also appears to be an attractive top cell material. The majority of thin film devices 

developed thus far have been on heavy substrates such as glass; progress is being made in reducing 

substrate mass through the use of thin metal foils and lightweight flexible polyimide or plastic substrates.

A major problem with the use of plastic substrates is the incompatibility with many of the deposition 

processes. The most efficient thin film cells to date are made by some combination of co-evaporation of 

the elements and subsequent annealing. AM0 cell efficiencies as high as 7 percent have been measured 

for CuIn0.7Ga0.3S2 (Eg 1.55 eV) thin film devices on flexible substrates (R. Birkmire, personal 

communication, 2003). The use of such plastic substrates as poly(p-phenylene-2,6-benzobisoxazole) 

(PBO), UpilexTM or KaptonTM puts an unacceptable restriction on the processing temperatures. The 

current world records for thin film CdTe and CuInGaSe2 solar cells AM1.5 efficiencies stand at 16.5 and 

19.2 percent, respectively (refs. 3, 4, and 7). 

3. Advanced Concepts 

3.1 Multijunction III-V Devices 

Investigations into further efficiency improvement toward the end of the 20th century turned toward 

the development of multiple junction cells and concentrator cells. Much of the development of 

multijunction GaAs-based photovoltaics was supported by a cooperative program funded by the Air Force 

(ManTech program, Space Vehicles Directorate, and the Space Missile Center) and NASA. This work 

resulted in the development of a “dual-junction” cell which incorporates a high-bandgap GaInP cell 

grown on a GaAs low-bandgap cell. The 1.85 eV GaInP converts higher energy photons and GaAs 

converts lower energy photons. Commercially available dual-junction GaAs/GaInP cells have an AM0 

efficiency of 22 percent with a Voc of 2.06 V (refs. 20 to 22). 

The highest efficiency solar cells currently available are triple-junction cells consisting of GaInP, 

GaAs, and Ge (see fig. 3). They are grown in series connected layers and have been produced with a 

28.3 percent efficient with a Voc of 2.66 V in production lots, with laboratory cells of 29 percent. Emcore, 

Inc., and Spectrolab, Inc., currently produce cells that are commercially available in the 25 to 27 percent 

range. Their high efficiencies are due to their ability to effectively convert a larger portion of the available 

sunlight. In fact, it is expected as better materials (for triple junction cells) are developed and eventually 

as quadruple junction cells are developed, cell efficiencies will surpass the 40 percent efficiency barrier 

(see fig. 4). Hughes Space and Communications Company’s HS601 and HS702 spacecraft currently use 

multijunction technology as do most other contractors for their high-performance spacecraft (refs. 14, 15, 

21, and 22). 

Spectrolab, supplier of more than half of the world’s spacecraft solar cells, has reached a milestone 

of 1400 kW of multijunction cells delivered, with an average ((BOL + EOL)/2) conversion efficiency of 

26.3 percent. They are currently producing 1 MW of cells for commercial sales to a variety of array 

manufacturers (e.g., Hughes Space and Communication Company, Ball Aerospace & Technologies 

Group, Lockheed Martin, and Boeing). Their high efficiency cells retain 86 percent of their original 

power after 15 years of operation. There is currently more than 375 kW of Spectrolab multijunction solar 

cells in operation (ref. 14). 

NREL recently announced a new world record conversion efficiency for a multijunction GaAs solar 

cell at 32.5 percent (ref. 23). The new multijunction III-V cells have allowed a reduction in solar array 

size and mass over the previously used Si cells to achieve comparable power levels. This is especially 

important in space applications. Scientists expect the majority of the 800 commercial and military 

spacecraft launched in the next 5 years to use multijunction technology. This should result in the lower 

costs for telecommunications, internet, television, and other wireless services. The AFRL recently 

initiated a 35 percent efficient four-junction solar cell program. 
NASA/TM—2005-213341 6

Multijunction III-V cells are relatively expensive to produce. The development of large area arrays 

using these cells can become cost prohibitive (see section 3.4). One option to reduce the overall cost is to 

use the cells in solar concentrators, where a lens or mirror is used to decrease the required cell area 

(ref. 24). 

3.2 Nanotechnology—Specifically Quantum Dots 

A recent approach to increasing the efficiency of thin-film photovoltaic solar cells involves the 

incorporation of quantum dots (refs. 6 and 9). Semiconductor quantum dots are currently a subject of 

great interest mainly due to their size-dependent electronic structures, in particular the increased band gap 

and therefore tunable optoelectronic properties. To date these nanostructures have been primarily limited 

to sensors, lasers, LEDs, and other optoelectronic devices. However the unique properties of the size 

dependent increase in oscillator strength due to the strong confinement exhibited in quantum dots and the 

blue shift in the band gap energy of quantum dots are properties that can be exploited for developing 

photovoltaic devices that offer advantages over conventional photovoltaics. Theoretical studies predict a 

potential efficiency of 63.2 percent for a single size quantum dot cell, which is approximately a factor of 2 

better than any SOA device available today. For the most general case, a system with an infinite number 

of sizes of quantum dots has the same theoretical efficiency as an infinite number of bandgaps or 86.5 

percent (refs. 12 and 13). 

A collection of different size quantum dots can be regarded as an array of semiconductors that are 

individually size-tuned for optimal absorption at their band gaps throughout the solar energy emission 

spectrum. This is in contrast with a bulk material where photons are absorbed at the band gap; energies 

above the band gap result in less efficient photogeneration of carriers. In addition, bulk materials used in 

photovoltaic cells suffer from reflective losses for convertible photons near the band gap, whereas for 

individual quantum dots, reflective losses are minimized. Some recent work has shown that quantum dots 

may also offer some additional radiation resistance and favorable temperature coefficients. 

3.3 Advanced Processing for Low-Temperature Substrates 

A key technical issue outlined in the 2001 U.S. Photovoltaic roadmap (ref. 25) is the need to develop 

low cost, high throughput manufacturing for high-efficiency thin film solar cells. Thus, a key step for 

device fabrication for thin film solar cells is the deposition onto flexible, lightweight substrates such as 

polyimides. Current methods for depositing ternary crystallite compounds often include high-temperature 

processes, often followed by toxic sulphurization/selenization steps (refs. 3, 4, and 8). The high 

temperature requirements make this protocol incompatible with all presently known flexible polyimides, 

or other polymer substrates. In addition, the use of toxic reagents is a limiting factor. The use of 

multisource inorganic/organometallic precursors in a chemical vapor deposition (CVD) process is more 

appealing due to milder process parameters. However, stoichiometric control of deposited films can be 

difficult to achieve and film contamination has been reported (ref. 26). A novel alternative approach is the 

use of ternary single source precursors, (SSP’s), which have the I-III-VI2 stoichiometry “built in” and are 

suitable for low temperature deposition (fig. 5). Although, a rich and diverse array of binary SSP’s are 

known, characterized, reviewed and tested, the number of known ternary SSP’s is limited, as is their use 

in deposition processes (ref. 27). We briefly summarize the highly promising technique for thin film 

growth: molecular design of single source precursors for use in a chemical (vapor or spray) deposition 

process. 

Spray CVD has become an often-studied deposition technique; a precursor solution is ultrasonically 

nebulized, and is swept into a two-zone, hot-wall reactor (fig. 6). The carrier-solvent is evaporated in the 

warm zone, and the gaseous precursors are decomposed in the hot zone, where film growth occurs as in 

conventional CVD. Spray CVD maintains the most desirable features of MOCVD and spray pyrolysis, 
NASA/TM—2005-213341 7

such as film growth in inert atmospheres, large area deposition, laminar flow over the substrate, and low 

temperature solution reservoir, while avoiding the major difficulties of each (ref. 28). It minimizes the 

high volatility and temperature requirements for the precursor, which are essential in MOCVD, by 

delivering the precursor to the furnace as an aerosol propelled by a fast-flowing carrier gas from a lowtemperature precursor reservoir analogous to that employed in spray pyrolysis. The latter feature is an 

important benefit that can prevent premature precursor decomposition when using thermally labile 

precursors. 

In 1990 Kanatzidis et al., reported the preparation of hetro binuclear complexes consisting of 

tetrahedrally arranged Cu and In centers, with two bridging thiolato and selenolato groups (see fig. 5 for 

similar structure) (ref. 29). Pyrolysis studies undertaken revealed that the Se derivative could be converted 

into CuInSe2 at 400 to 450 °C, but none of the precursors had been evaluated in a thin-film deposition 

study. Buhro and Hepp were able to demonstrate that [{PPh3}2Cu(SEt)2In(SEt)2] could be utilized in a 

spray CVD process, for depositing thin-film CuInS2 below 400 °C (refs. 10 and 30). Thin films where 

deposited using a dual solvent system of toluene and dichloromethane, (CH2Cl2) as the carrier solvent. 

Single-phase 112 orientated CuInS2 thin films were successfully deposited at a range of temperatures 

from 300 to 400 °C; at elevated temperatures (>500 °C), CuIn5S8 phase thin films were deposited. 

Materials analysis showed that the films were free from any detectable impurities and highly crystalline, 

thus concluding the precursor decomposes cleanly. 

In the course of our investigations for improved single source precursors for the spray CVD of 

chalcopyrite thin-films, for the general semiconductor stoichiometry Cu(Ga:In)(Se:S)2, we have continued 

to expand the molecular design of SSP’s based on the [{LR3}2Cu(ER’)2M(ER’)2] architecture (ref. 10). 

Furthermore, the number of “tunable” sites within the complex allows for their utility in preparing a 

number of ternary chalcopyrites of varying composition, in addition to engineer the SSP’s to match a 

given spray CVD process. 

Spray CVD, in conjunction with SSP design, provides a proof-of-concept for a reproducible high 

manufacturability process. An outlook for further investigation that needs to be undertaken is: (1) 

Precursor Design: Development of more volatile/thermally labile systems. This can be achieved by the 

incorporation of fluorinated or silylated function groups. Importantly due to the well-known propensity of 

fluorine to react with silyl moieties, incorporation of both elements in the molecule can serve not only to 

increase volatility, but also as a “self-cleaning” mechanism should the precursor decompose through an 

undesired pathway (ref. 31). (2) Processing Parameters: Spray CVD has a number of tunable variables, 

such as droplet size, flow-rate, concentration, solvent polarity, which are advantageous to achieve the 

desired film characteristics. Thus an in-depth study needs to address these parameters on film 

composition (ref. 32). (3) Device fabrication: working devices from deposited films need to be tested to 

aide SSP design and spray CVD process parameters; this is the current focus of work on-going at NASA 

Glenn Research Center (ref. 33). The work summarized here on the molecular design of SSP’s for their 

use in a spray CVD process, although still in its infancy, undoubtedly shows it as a mass producible, cost 

effective method for fabricating commercial thin film PV devices. Furthermore, we have discovered that 

single-source precursors are a valuable route to semiconductor nanoparticles (refs. 9 and 31). 

3.4 Concentrator Cells 

Concentrating solar collectors use devices such as Fresnel lenses and parabolic mirrors to concentrate 

light onto solar cells (ref. 24). This reduces the area of cells needed and also raising the efficiency of the 

cells by operating them at higher light concentration. Normally these collectors are mounted onto an axis 

tracking system to follow the track of the Sun across the sky. Figure 7 shows a point-focus Stirling engine 

concentrator system. Figure 8 shows a linear concentration system in Austin, Texas. 
NASA/TM—2005-213341 8

3.5 Integrated Power Devices 

NASA has also been working to develop lightweight, integrated space power systems on small or 

flexible substrates. These systems generally consist of a high efficiency thin film solar cell, a high energy 

density solid-state lithium-ion battery, and the associated control electronics in a single monolithic 

package. These devices can be directly integrated into microelectronic or MEMS devices and are ideal for 

distributed power systems on satellites or even the main power supply on a nanosatellite. These systems 

have the ability to produce constant power output throughout a varying or intermittent illumination 

schedule as would be experienced by a rotating satellite or “spinner” and by satellites in a low earth orbit 

(LEO) by combining both generation and storage (ref. 34). 

An integrated thin film power system has the potential to provide a low mass and cost alternative to 

the current SOA power systems for small spacecraft. Integrated thin film power supplies simplify 

spacecraft bus design and reduce losses incurred through energy transfer to and from conversion and 

storage devices. It is hoped that this simplification will also result in improved reliability (see fig. 9). 

The NASA Glenn Research Center has recently developed a microelectronic power supply for a 

space flight experiment in conjunction with the Project Starshine atmospheric research satellite (see 

fig. 10) (ref. 35). This device integrates a 7 junction small-area GaAs monolithically integrated 

photovoltaic module (MIM) with an all-polymer LiNi0.8Co0.2O2 lithium-ion thin film battery. The array 

output is matched to provide the necessary 4.2 V charging voltage and minimized the associated control 

electronic components. The use of the matched MIM and thin film Li-ion battery storage maximizes the 

specific power and minimizes the necessary area and thickness of this microelectronic device. This power 

supply was designed to be surface mounted to the Starshine 3 satellite, which was be ejected into a lowearth orbit (LEO) with a fixed rotational velocity of 5 degrees per second. The supply is designed to 

provide continuous power even with the intermittent illumination due to the satellite rotation and LEO. 

4. Applications 

4.1 Terrestrial 

The use of photovoltaic power continues to increase steadily in the U.S. and even more so worldwide. 

Programs to stimulate the use of solar power exist to support homeowners, small businesses, and industry. 

The government has also expanded programs to support local utility companies, state agencies, and even 

international developmental projects (ref. 36). Many other nations are supporting similar programs. The 

UK research council recently funded their largest solar power program to date that will support 6 

universities and 7 companies in the UK (ref. 37). Japan’s largest solar power generator was also recently 

completed in Tsukuba. The system uses 5,600 solar panels and will have the capacity to generate 1 MW 

of power. Figure 11 shows a 1 MW solar power station installed by BP Solar in Toledo Spain. 

4.2 Aerospace 

Solar array designs have undergone a steady evolution since the Vanguard 1 satellite. Early satellites 

used silicon solar cells on honeycomb panels that were body mounted to the spacecraft. Early space solar 

arrays only produced a few hundred watts of power. Satellites today require low-mass solar arrays that 

produce several kilowatts of power. Several new solar array structures have been developed over the past 

forty years to improve the array specific power and reduce the stowed volume during launch. The solar 

arrays presently in use can be classified into six categories: (1) body-mounted arrays; (2) rigid panel 

planar arrays; (3) flexible panel array; (4) flexible roll-out arrays; (5) concentrator arrays; and (6) high 

temperature/intensity arrays. 
NASA/TM—2005-213341 9

In addition, several proposed space missions have put other constraints on the solar arrays. Several 

proposed Earth orbiting missions designed to study the sun require “electrostatically clean” arrays. Inner 

planetary missions and mission to study the Sun within a few solar radii require solar arrays capable of 

withstanding temperatures above 450 °C and functioning at high solar intensities. Outer planetary 

missions require solar arrays that can function at low solar intensities and low temperatures. In addition to 

the near-sun missions, missions to Jupiter and its moons also require solar arrays that can withstand high 

radiation levels. 

The International Space Station will have the largest photovoltaic power system ever present in space 

(see fig. 12). It will be powered by 262,400 (8cm x 8cm) silicon solar cells with an average efficiency of 

14.2 percent on 8 U.S. solar arrays (each ~ 34 m x 12 m). This will generate about 110 kW of average 

power, which after battery charging, life support, and distribution, will supply 46 kW of continuous 

power for research experiments. The Russians also supply an additional 20kW of power to ISS. 

5. Summary and Conclusions 

Zweibel and Green in a recent editorial (ref. 38) observed that: “... photovoltaics is poised to progress 

from the specialized applications of the past to make a broader impact on the public consciousness, as 

well as on the well-being of humanity . . .” We as well as Perlin (this volume, Chapter 1 by Perlin) have 

endeavored to provide a context for the development of organic photovoltaics. While highlighting 

important applications involving uses on Earth, where price per watt will be a critical driver; the 

challenges of aerospace and defense have spurred the development of advanced technologies. We have 

highlighted a number of these in advanced materials, devices, and processing in this overview. It is 

important to keep in mind the fact that solar cells, regardless the material(s) or device structure, are made 

to generate electricity. An important lesson to be learned from a more practical consideration of solar 

cells is that a wide array of power generation applications exist each with its own set of challenges. 

These challenges can be addressed by a variety of technologies that overcome specific issues 

involving available area, efficiency, reliability, and specific power at an optimal cost. The particular 

application may be in aerospace, defense, utility, consumer, grid-based, off-grid (housing), recreational, 

or industrial settings. Organic-based photovoltaics will most likely provide solutions in applications 

where price and/or large area challenges dominate such as consumer or recreational products. Inorganic 

materials will most likely predominate in aerospace and defense uses: satellites, non-terrestrial surface 

power, and planetary exploration. As efficiencies, environmental durability and reliability improve for 

organic photovoltaics, other applications such as off-grid solar villages and utility-scale power generation 

may well be within reach! 

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23

Inorganic Photovoltaics Materials and Devices: Past, Present, and Future

Aloysius F. Hepp, Sheila G. Bailey, and Ryne P. Raffaelle

Copper indium selenide; Poly crystalline; Gallium arsenide; High-efficiency solar cells;

Inorganic photovoltaic materials; Multi-junction devices; Quantum dots; Silicon solar

cells; Amorphous silicon; Thin film(s)

Unclassified -Unlimited

Subject Category: 23 Distribution: Nonstandard

Aloysius F. Hepp and Sheila G. Bailey, NASA Glenn Research Center; and Ryne P. Raffaelle, Rochester Institute of

Technology, Department of Physics and Microsystems Engineering, Rochester, New York 14623. Responsible person,

Aloysius F. Hepp, organization code 5410, 216–433–3835.

This report describes recent aspects of advanced inorganic materials for photovoltaics or solar cell applications. Specific

materials examined will be high-efficiency silicon, gallium arsenide and related materials, and thin-film materials,

particularly amorphous silicon and (polycrystalline) copper indium selenide. Some of the advanced concepts discussed

include multi-junction III-V (and thin-film) devices, utilization of nanotechnology, specifically quantum dots, lowtemperature chemical processing, polymer substrates for lightweight and low-cost solar arrays, concentrator cells, and

integrated power devices. While many of these technologies will eventually be used for utility and consumer applications,

their genesis can be traced back to challenging problems related to power generation for aerospace and defense. Because

this overview of inorganic materials is included in a monogram focused on organic photovoltaics, fundamental issues and

metrics common to all solar cell devices (and arrays) will be addressed.