PHOTOVOLTAIC MATERIALS AND DEVICES FOR TERRESTRIAL SOLAR ENERGY APPLICATIONS

Harold J. Hovel

IBM Thomas J. Watson Research Center

Yorktown Heights, NY 10598

INTRODUCTION

Solar photovoltaics is literally an exploding field as far as inter- est, research effort, and funding are concerned. This wide interest can be attributed to a variety of factors, including the world’s ever-increasing demand for new energy sources, the public’s fascination with the simplicity and cleanliness of solar cells, and the fast rate of progress being achieved technically. A very strong national program exists run by the Department of Energy through its task coordinators at, the Jet Propulsion Lab, Solar Energy Research Institute, Sandia Laboratories, NASA Lewis Research Center, and MIT/Lincoln Lab. In addition, smaller programs exist in Japan, England, France, Germany, the USSR, and other countries. Solar photovoltaics represents a potential new business opportunity for these na- tions and a partial solution to their energy problems as well.

This paper is essentially a review of the status of photovoltaics today and is a sequel to several reviews presented earlier (1,2). It will start with a brief discussion of the economic goals in- volved and end with a description of the various technologies being pursued to meet these goals.

ECONOMIC CONSIDERATIONS

Solar cells work and they work very well. With efficiencies approaching and in some cases exceeding 20%, they are as efficient as many of the conventional means for generating energy, and their performance could double if several of the exploratory technologies being pursued prove viable. Two of the major questions which will eventually determine the future of the photovoltaic concept are : 1) can it be competitive economically with the other future energy alternatives, and 2) will storage be required with these systems or will alternatives to storage be acceptable? The last question is probably the harder to answer, since it involves utility standards of reliabili- ty and down-time, the availability of back-up equipment, the number of systems connected to a grid, and so forth. If solar cell systems are used on individual residences, d.c. storage batteries can be easily incorporated, but tie-in to the utility line through a d.c.-as. inverter is also possible, perhaps with credit for the energy supplied.

The economic question is more amenable to analysis and con- clusion. If valid assumptions can be made about the cost of solar cells, the cost of land and structural members, interest and income tax rates, operating and maintenance costs, and the like, then methodologies exist for projecting what the cost of solar-generated electricity will be. The economic projection techniques normally used by the electric utility industry for examining solar power stations have been outlined in (3) and briefly discussed in (2).

Historically the cost of solar cells themselves has been the dominant cost factor in photovoltaic systems; therefore the levelized busbar cost per kilowatt-hour is often plotted against solar cell cost as shown in Figure 1. The assumptions used are the same as given in (2), and all the direct and indirect costs of the power plant are included. Flat plate systems have solar cell arrays fixed toward the south and tilted at an angie about equal to the local latitude. These systems accept both direct and diffuse light and normally do not require cooling. Copcentrat- ing systems consist of fresnel lenses, parabolic troughs or ar- rays of heliostats which focus light onto small numbers of solar cells at magnifications of 10 to 500 ”suns” and track the diur- nal path of the sun in either one or two axes. These systems can only use the direct component of sunlight and cooling is necessary to prevent excessive temperatures.

For flat plate systems, the solar cells represent more than 50%) of the cost of the power plant as long as the cells cost 50 cents per peak watt ($50/m2 for 10% cells) or more. Solar cells with 18% efficiencies would have to cost less than $50/m2 to reach busbar costs of 5 cents per kWhr, and 10% cells would have to reach $15/m2 for the cell same result (the present cost of Si cells is around $1000/m2). The cell cost is much less important for concentrating systems, which use large areas of relatively inexpensive lenses but small numbers of solar cells. For these systems, the solar cell efficiency has a much larger effect than the cell cost; for example, increasing the efficiency from 15% to 20% of cells which cost $1250/m2 has the same effect as lowering their cost to $125/m2. Increasing the effi- ciency of the concentrating devices, the power inverter, or the cooling has the same benefit as increasing the cell efficiency.

The conclusions of these types of economic analyses are that for flat plate systems the cell cost is the dominant factor as long as the efficiency is maintained at an acceptable value, while for concentrating systems, the cell efficiency, concentra- tor efficiency, and concentrator cost are the major concerns. The efficiencies of cells used in flat plate arrays in the United States must exceed 10 to 12% to be useful for utility power generation and must cost less than around $50/m2 (in 1975 dollars) at the same time. For concentrating systems, efficien- cies in excess of 20% are mandated at costs of several thou- sand dollars/m2 or less depending on the concentration. These conclusions roughly form the goals of the DOE program in this country. (4)

CANDIDATE TECHNOLOGIES

To the non-expert in photovoltaics, the field must seem like a bewildering array of materials, devices, preparation techniques, cell designs, and theories. Previous reviews ( 1 ) have attempted to organize this array into major and minor subheadings, and

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another attempt to do this is shown in Figure 2. The four major categories are Si cells, thin film cells, both for flat plate arrays, concentrator cells, and innovative concepts. Within these categories there are many possible combinations of de- vices, preparation techniques, and substrates that can be made.

SILICON OPTIONS

The first steps in producing Si solar cells involve the reduction of Si ores to metallurgical-grade Si and the purification of this to high purity poly-Si feedstock. At present this is done with distillation of SiHCl, and at a cost which is a factor of five times above the acceptable value. The use of Zn or Na chlo- rides and the use of fluorine compounds as well as several other techniques are being studied as possible methods of reducing the Si raw material cost to $10 per kilogram or less by 1986. After this the Si must be formed into sheets, i.e. substrates, on which to fabricate cells. The Czochralski crystal growth process is well known, but requires wafer sawing, lap- ping, and polishing which waste a large fraction of the Si and add considerable expense. Several methods of forming the sheets directly are being explored. The dendritic web process developed by Westinghouse has produced solar cells with 12% efficiency, while the EFG ribbon silicon approach from Mobil- Tyco and IBM has yielded 11% devices. Multiple ribbons 10 cm wide can now be grown. Another very promising technique for producing Si sheet is the casting approach by which poly- crystalline wafers are directly cast from the purified Si starting material; such polycrystalline silicon has produced 15% effi- cient devices, nearly equal to good single crystal cells.

It is projected that about 1/6 of the cost of Si solar cells will lie in the raw Si starting material and about 1/3 in the sheet production. Another 30% will lie in the device fabrication (with the remainder in the packaging). Various low cost de- vice approaches are being pursued as alternatives to diffusion. One of the first of these used Schottky barriers made with semi-transparent metal layersA but this quickly gave way to MIS cells in which a thin, 20A oxide separates the metal and the silicon. The insulator raises the voltage output of the cell considerably, and efficiencies of 11.9% on single crystals and 8% on polycrystalline wafers have been reported. Even better results have been obtained with heterojunctions of tin oxide or indium-tin oxide on silicon, the so-called SIS approach for semiconductor-insulator-semiconductor. The Sn02 and InSnO, are transparent, high bandgap, high conductivity semiconduc- tors that form good junctions with Si; the insulator in the middle is produced automatically during the film growth and enhances the voltage output as in the Schottky devices. SnOz and InSnO, have been sprayed, sputtered, and evaporated, and have resulted in efficiencies of up to 14% on single crystal Si, 10% on polycrystalline.

Ion implantation is another tool with low cost potential, partic- ularly if laser beam or pulsed electron-beam annealing are used to activate the implant instead of thermal annealing. Spire Corporation, for example, has developed a process which prod- uces 12% cells totally in vacuum and with two minute throughput from blank wafer to finished cell.

In addition to exploring low cost device fabrication, considera- ble effort has also gone into increasing the ultimate efficiency and in developing theoretical models to this end. Figure 3 depicts the evolution of high efficiency cell development. For

many years, the simple n+-p phosphorus diffused planar cell was the mainstay of the space industry. This cell had a 0.5 bm junction depth and was 13-14% efficient. The violet cell was developed in the early 70's; it made use of a very narrow junc- tion and a reduced doping level in the diffused region to elimi- nate most of the "dead region" near the surface which normal- ly destroys the blue response of the cell. The textured ap- proach made use of a serrated surface to almost entirely elimi- nate the reflection loss (after A.R. coated) and also used a very narrow junction. These two cells were 16 to 17% effi- cient.

In about the mid-70's, the BSF (Back Surface Field) device was born. In essence, this added a p+ back surface to the n + - p violet cell. This high-low junction at the hack reduces the recombination rate of minority carriers and raises both the photocurrent and open circuit voltage. Such cells are over 18% efficient today.

Theoretical understanding of the physics of these devices has revealed the importance of such parameters as Auger recombi- nation, bandgap shrinkage, surface recombination, and base lifetime. Epitaxial growth of the junction and/or a high-low top junction in combination with a BSF and high lifetime base are being pursued to further increase the efficiency, and over 19% has been reached already. (The theoretical limit at AM1 is about 23%) .

In summary, the cost of Si solar cells is being addressed in a 3-pronged attack: reducing the cost of the starting material, the production of low cost Si substrates, and the development of simplified device fabrication techniques. The overall goal is to achieve finished devices at about 50 cents per watt and 12% efficiency or higher. There are many approaches being pur- sued simultaneously, and there is no clear indication at this point of which will turn out to be the most promising. On the performance side the best laboratory Si cells are approaching their theoretical limit. Some of the techniques used to ob- tained these higher efficiencies may be applicable for low-cost devices, but others are probably more useful for space cells and concentrator devices.

THIN FILM CELLS

The thin film approach is intended for flat plate systems and therefore has the same near-term goals as the previous Si tech- nologies: cell array costs of < 50 cents/peak watt and effi- ciencies in excess of 10-12%. In the long term, it is hoped that thin film arrays will reach 15-20 cents per watt (1975 dollars). The advantages of the thin film approach are that a minimum amount of the expensive semiconductor material is used, very low-cost substrates can be used (in theory), and automated, large scale flow-through processing of large sheets can be envisioned. The materials under study for thin film arrays were listed in Figure 2, and a summary of their present status is shown in Figure 4.

Flat plate Si cells can be made in two thin film forms, poly- crystalline layers and amorphous layers. Polycrystalline Si layers are being deposited on graphite, moly, alumina, and ceramic substrates using CVD, evaporation, plasma deposition, and electrodeposition. The resulting grain sizes are in the 5-30 pm range unless subsequent recrystallization is carried out,

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which can increase the grain size by more than an order of magnitude. There is a great deal of theoretical and experimen- tal work on the physics of grain boundaries, was well as at- tempts to passivate grain boundary states using hydrogen or oxygen. SIMS, Auger, and ESCA are being used to directly study the chemical nature of grain boundaries. The best poly Si thin film cells have been 9.8% efficient in 10 cm2 areas, using epitaxial growth to form the junction. The MIS, SIS, heterojunction, and ion implantation approaches are also being pursued for thin film Si cells.

Amorphous Si is another form of thin film Si that has excited wide interest lately. The absorption coefficient of a-Si is much higher than crystalline Si above 1.8 eV, and it behaves as a direct gap semiconductor with a 1.6 - 1.7 eV bandgap, requir- ing only a micron of material for the active layer rather than 25 microns or more for crystalline Si. The layers can be de- posited by sputtering, evaporation, electrodeposition, CVD, or glow-discharge plasma CVD, but only the latter has resulted in relatively high efficiencies. Hydrogen incorporation in the 2-10% composition range is necessary in order to eliminate most of the unsatis,fied energy states, and recent results suggest that other elements such as fluorine and oxygen may be benefi- cial in reducing still more such states. The theory of these devices is still poorly understood; the mobilities and diffusion lengths of minority carriers are very low and there appears to be a large electric-field aided drift component to current col- lection, which often causes a poor fill factor in the cells. In any case, encouraging progress is being made. Several years ago 5.5% efficiencies were obtained on small area (10-2cm2) devices, and larger area cells were usually less than 1%. To- day several laboratories in this country, Japan, and Great Britain have achieved 3-4% cells in areas up to 40cm2 , and optimism is height that 7 4 % or more will eventually be reached.

The grandfather of thin film cells is the CdS/Cu2S device first discovered in 1954. These cells are normally made by evapo- rating 20pm thick films of CdS onto Zn coated Cu foil, then dipping into a copper chloride solution to form the Cu2S by exchange reaction, and finally grid contacting and plastic en- capsulating (an alternative technique is to evaporate the CuCI2 onto the CdS instead of dipping). These cells were investigat- ed for a number of years for use in the space program and were about 5-6% efficient. Very careful analysis of all the losses in the conventional CdS cell at the University of Dela- ware has resulted in efficiencies up to 9.1%. Further theoreti- cal work has shown that the interface states and conduction band discontinuity at the CdS/Cu2S heterojunction will limit the efficiency of this device to 10-11%; therefore CdZnS is being explored as a replacement to CdS in order to improve the lattice and electron affinity matches, and 8.7% devices have been obtained with this material.

An alternate technique of fabricating CdS/Cu2S cells has been developed which uses glass substrates and sprayed CdS and Cu2S films. Although the efficiencies have been low so far (4-5%), the cost projections are also low and the use of the already existing float glass technology should make scale-up fairly easy.

Another version of the CdS cell uses copper ternaries as the active semiconductor rather than Cu2S. CuInSe2, for example, can be sputtered or evaporated onto CdS substrates, and the

resulting heterojunctions have been over 12% efficient in single crystal form, 5.7% in thin film polycrystalline form.

Poly Si, amorphous Si, and the CdS-like cells are all of interest because they use relatively low cost materials and low cost processing. Most of the work going on is in ways to increase their efficiency and ensure their stability. In contrast, the 111-V materials are expensive but they have produced cells with very high effkiencies, above 20% in single crystal form. The effort with these materials is in reducing the amount of semi- conductor to a bare minimum while finding low cost growth techniques, all without reducing the efficiency much below the demonstrated high values.

An interesting experimental result has been obtained by Lin- coln Lab recently in which a 20% efficient thin film GaAs cell was obtained, using 4 microns of GaAs grown by vapor epitaxy on single crystal Ge. No response was obtained from the Ge, which acted as a passive substrate. This serves as an existence theorem that very high efficiencies can be obtained with very thin films. Most thin film GaAs cells have been grown on moly, tungsten, graphite, or tantalum substrates however, with poly GaAs films 5-10 pm thick and grain sizes of 5-20 pm. Grain boundary effects seem to be more severe in GaAs than in Si, and even though good photocurrents are routinely ob- tained with these films, the open circuit voltages are unexpect- edly low (0.2-0.5 volt) compared to single crystal counterparts (0.9-1.0 volt). Theoretical work is underway to study the grain boundaries in GaAs, and some success in grain boundary passivation has been obtained with both plasma hydrogenation and anodic oxidation. Experimentally, efficiencies of 5-69'0 have been obtained without AR coatings in several laboratories using graphite or moly substrates and metal-insulator- semiconductor junctions. The results are very similar for InP cells, which often use CdS layers to form heterojunctions. Both CdS/InP and InSnO,/InP "SIS" cells have been 14% efficient in single crystal form.

Finally, several types of organic materials have been explored as thin film cells, including phthalocyanine, chlorophyll, mero- cyanine, squarylium, and polyacetylene. These materials share many of the same features as amorphous Si, particularly high trap levels and poor diffusion lengths. Their best efficiencies have been less than 1%.

CONCENTRATOR CELLS

As outlined earlier, sunlight concentrating systems use large numbers of collecting lenses but small numbers of solar cells; therefore relatively expensive cells can be used but they must be as efficient as possible. Cooling is required to keep the cells at an acceptable temperature, but it may be possible to use this. low grade heat (a hybrid photovoltaic-photothermal system) and thereby derive extra value.

The earliest concentrator cells were conventional, planar Si cells of the n+/p variety with extra grid metallization to carry the high current. This is the type that can be bought from several vendors today. They are generally 13-15% efficient at 10-40 "suns" and 28OC. These and other cells are listed in Figure 5.

A number of theoretical studies of solar cell behavior under elevated concentrations and temperatures have been per-

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formed, both for the high efficiency structures discussed in the flat plate section of this paper and for several novel structures. Some of the most important conclusions to emerge from these analyses include the need for very high lifetime, high purity Si base material (hundreds of microseconds), the fact that con- ductivity modulation will almost surely take place in the highest efficiency cells, the increase at high concentrations of the theoretical limit efficiency from 23% to nearly 30%, and the dominant role played by the series resistance of the cell. The high lifetime provision may require a change in the way these cells are made, since impurity gettering becomes much more important and lifetime-killing thermal treatments must be avoided.

Another prediction that came out of the computer simulations was that p+/n cells would be slightly preferable to n f / p be- cause an aiding Dember voltage would appear in the former as a result of conductivity modulation. Figure 5 shows that slightly better cells were indeed obtained this way. Back- surface-field cells of either the p+/n/n+ or n+/p/pf variety are even better because of the carrier confinement and reduced recombination in the base. The ultimate version of the con- ventional design may be the high-low emitter, back surface field cell where the minority carrier losses are minimized in both the emitter and base regions of the cell. Efficiencies of over 19% have been obtained with these cells, and 20% or higher is expected shortly.

Several novel Si cell designs have been investigated also. One of these is the vertical multijunction device in which a number of p-n junctions are connected in electrical series and illumi- nated parallel to the junctions. These cells have limit efficien- cies of 30% but have been only 8% in practice. Another version of this device uses the acronym GVJ for grooved verti- cal junction. In this structure deep vertical grooves are etched into high lifetime starting material and the p' regions are formed on the sides of these grooves, with the n+ region of the junction made uniformly over the back. This design leads to extremely low series resistance and the capability of operation at very high intensities. Efficiencies of 19% have been ob- tained at 600 suns.

Two other Si structures for concentration applications are the interdigitated back contact and the V-grooved horizontal multi- junction (VGMJ). The IBC has alternating n and p type regions on the back of the cell and a very low surface recombi- nation velocity interface at the front. The high lifetime, con- ductivity modulated base and the heavily doped junction re- gions combine to result in low series resistance, and efficiencies of 16-17% have been measured at 200 suns. The VGMJ structure uses wafers of Si bonded to glass and anisotropically etched into individual triangular prisms. Ion implantation is used to form p-n junctions, which are then connected in elec- trical series. These cells have the lowest series resistance of any of the designs, and have been operated at 300 suns with an open circuit output of 30.2 volts and with 12.2% efficiency.

The highest efficiency of any single junction cell has been obtained from GaAs cells made with GaAlAs window layers. At 1 sun intensities, these devices have improved from the original plain GaAs efficiencies of 11 Yo prior to 1971 to 22% in 1977. The efficiencies under concentration are even higher, 23-24Oh at 10-400 suns, while the high bandgap of GaAs

makes these devices capable of high temperature operation, retaining 14% efficiency at 2OO0C and 19% at 6OoC.

The final concentrator design, and the one with the highest potential payoff, is the multi-color concept, which divides the solar spectrum into two or more wavelength ranges and directs each portion onto a different cell having the optimum bandgap for that wavelength range. In this way the two major losses in the photovoltaic process, insufficient photon energy and exces- sive photon energy, are circumvented and the limit efficiency rises from 25% for one cell to 40% for two cells and 50% for 3 cells. There are two versions of this concept, the first in which the spectrum is split by a dichroic mirror which reflects one range of wavelengths and transmit the other, and the sec- ond where the cells are mounted in optical tandem and each one absorbs light above its bandgap and transmits the rest. The spectrum splitting approach has been reduced to practice at Varian Associates using Si and GaAlAs cells and a dichroic mirror with a cut-off at 1.6 eV, resulting in 28% efficiency for the combination at 150 suns. The tandem concept has been demonstrated by Research Triangle Institute using a tunnel junction to ohmically connect GaAs and GaAlAs cells together, obtaining 2.2 open circuit volts for the combination.

EMERGING MATERIALS

Emerging materials and innovative concepts include the class of high risk, high payoff ideas as well as several experiments interesting from a physics point of view. The only require- ments for a candidate in this category are that the approach be capable of 10% efficiency or higher and 50 cents per watt or less. Several old and new materials are being studied as photo- voltaic contenders, including, CdTe, Cu,O, Zn,P,, ZnSiAsz, CSiAs2, and BAS. CdTe films are prepared by sputtering, spraying, evaporating, and electrodeposition, and 8% efficient thin film cells have been made. CuzO has been explored for a number of years and is a very low cost material, but the effi- ciencies to date have always been less than 2%, and work is underway to determine if this is a fundamental limitation or if it can be overcome.

Zn,P , ZnSnP2, ZnSiAs2, and CdSiAs, are all semiconductors with iandgaps of 1.5 eV k 0.2 and some degree of promise as solar cells. Very little is known about these materials, and fundamental properties such as absorption coefficients, growth techniques, doping capability, and junction formation are being examined. The same is true for boron arsenide and amorphous boron. Polymeric sulfur nitride (SN,) is a highly conducting material that is being explored as a replacement to thin metal films jn Schottky barrier cells.

Photoelectrochemical cells are formed with one or more semi- conductor electrodes and solid or liquid electrolytes. They are potentially low in cost because they can make use of single crystal, polycrystal, or amorphous semiconductors with equal ease and because the "junctions" are formed automatically with plentiful, low cost electrolytes. The capability of in-situ energy storage is also present. The chief disadvantage lies in the instability of the semiconductor electrodes when illuminat- ed in contact with most electrolytes. Electrochemical cells have yielded efficiencies of 12% on single crystal and 5% on polycrystal GaAs using selenium based electrolytes. Reutheni- um coatings on the GaAs surface have been used to improve the stability, and several redox couples have been partially

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successful for this purpose also. Polycrystalline films of CdTe and CdSe have produced 8 % efficiencies in electrochemical cells.

Finally, one unique type of concentrating device is being ex- plored which can concentrate both direct and diffuse light and which does not need to track the sun. The device consists of a plastic sheet doped with a fluorescent dye and with solar cells mounted at the ends. Light absorbed by the dye is re-emitted at longer wavelengths and most of this light is trapped inside the plastic sheet by total internal reflection until it reaches the solar cell. at the ends. The chief concerns with this approach are low optical efficiency and the longevity of the plastic mate- rials and organic dyes in sunlight.

SUMMARY AND CONCLUSION

The photovoltaic program can be divided roughly into four areas, Si flat plate arrays, thin film devices, concentration systems, and innovative concepts. The goal of the flat plate Si approach is to bring the cost of Si arrays down to SO cents per peak watt, and even though very high efficiency, sophisticated Si devices have been designed, it is believed that the cost goal will probably be met by one of several simple sheet and device fabrication techniques which yield only 12-15% efficient de- vices. The thin film and emerging materials/innovative con- cepts approaches are intended for farther in the future, princi- pally because the goals for these approaches of 10-14% effi- ciencies and 15-20 cents per peak watt are expected to be difficult to accomplish and require long range research and development. Concentrating photovoltaic systems are capable of using today's expensive Si cells or any one of several novel designs and are expected to reach their cost goal of $1-2 per peak watt relatively soon. Since the solar cells are only a small part of the overall concentrating system, the cost of the cells is not as important as their efficiency, and if a breakthrough is

obtained in reducing the concentrator cost, this approach may even compete with flat plate Si in reaching the 50 cents per watt figure.

Flat plate arrays are modular in form and can be used in sizes from 10 watts to hundreds of kilowatts and in applications from boats to weather stations to residences and up to shop- ping centers and central power stations. They utilize both 'direct and indirect sunlight and can be used anywhere in the country. Concentrating systems require direct sunlight and must track the sun in one or two axes; they are probably more applicable in the West and Southwest and for central station power plants rather than for individual residences and other small applications.

REFERENCES

1. H. 3. Hovel, "Solar Cells for Terrestrial Applications", Solar Energy 19, 605 (1977).

2. H. J. Hovel, "Novel Materials and Devices for Sunlight Concentrating Systems", ZBM J. Res. and Dev. 22, 112, (1978).

3. J. W. Doane, R. P. O'Toole, R. G. Chamberlain, P. B. Bos, and P. D. Maycock, "The Cost of Energy From Utility-Owned Solar Electric Systems", JPL 5040- 29, ERDA/JPL-1012-76/3 , June 1976.

4. Economic analyses for residential photovoltaic systems may relax these requirements somewhat, since homeown- ers can use roof tops to mount the systems and can per- form their own maintenance. Some opinions are that efficiencies of 6-8% may be acceptable for residences and perhaps even lower values may be acceptable in develop- ing countries where alternate energy sources are much more expensive.

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