Promises of III-V solar cells

Solar Energy Materials 23 (1991) 129-138 North-Holland

•Solar Energy Materials

Promises of I I I - V solar cells J o h n C.C. Fan Kopin Corporation, 695 Myles Standish Boulecard, Taunton, MA 02780, USA

High efficiency solar cells have many practical uses. Concepts for obtaining high-efficiency solar cells using III-V compounds have achieved record efficiencies and are expected to improve even further. Single-junction cells will eventually reach 30% at AM 1.5 at one sun, and two-cell tandem structures will reach 40%. Thin-film I I I - V cells have now reached efficiencies close to double the efficiencies of thin-film cells using other materials. Inexpensive production techniques are now within reach. The future of III-V cells for both space and terrestrial applications is very bright.

1. Introduction

Owing to the area-related balance of photovoltaic systems costs, there are significant advantages for high-efficiency photovoltaic modules versus low-efficiency models. The balance of system costs include land costs, array structure costs and ethers. These costs cannot be reduced easily. Fig. 1 shows the relationship between flat-plate module cost (in 1987 dollars per square meter) and module efficiency as a function of levelized electricity cost [1]. For a levelized electricity cost of $0.06 per kilowatt, the tradeoff between module cost and module efficiency is well illustrated. For example, for a 10% module, the allowable module cost is about $10 per m 2. For a 20% module, the allowable module cost is around $80 per m 2, eight times higher than that for 10% module. The cost advantage will be even greater if the area-related costs incurred during module fabrication, for example, the cost of antireflection coatings and contact fingers, the cost of coverglass, and the cost of adhesive etc. are included in the calculation. It should be noted that fig. 1 is calculated ~:sing only $50 per m 2 for the balance of system costs. If that cost remains high, high-efficiency cells may well be the type that would be most attractive for large-scale terrestrial applications. With regard to space applications, the advantages of high efficiency and lighter modules are even more obvious. It is necessary to provide higher and higher power with lighter-weight photovoltaic modules (i.e. higher power-to-weight ratios). To get higher power-to-weight ratios, one can increase the module efficiency a n d / o r reduce the module weight. A third factor in space application is radiation sensitivity, which we shall discuss later. Another important application for high-efficiency cells is in concentrating sunlight configurations. In these configurations, sunlight is focused down by a lens 0165-1633/91/$03.50 © 1991 - Elsevier Science Publishers B.V. All rights reserved

J.C.C. Fan / Promises of lll--Vsolar cells

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or other means onto solar cells. In this case, the actual cell cost can be much higher because the major cost is in the concentrating optics. It is important that in concentrating configurations the cell efficiency should be as high as possible. In view of the complex tradeoffs between module efficiency and a host of other parameters, including weight, radiation sensitivity, reliability and concentration ratio as well as module cost and balance-of-system cost, it is obvious that no one type of module will be ideal for all photovoltaic applications. It follows also that no one material system will be ideal for all applications. Table 1 shows the most likely configurations and the most probable major applications anticipated [2] for modules with efficiencies of 10%, 15%, 20%, 25% and 30%. In this paper, we shall discuss research efforts directed toward developing high-efficiency cells for producing modules with efficiencies of 20-30% at AM 1. Table 1 Configuration and utilization of photovoltaic modules of various efficlencies Module

Configuration

Utilization

efficiency

Flatplate

Terrestrial cells

Low (10%) Medium (15%) High (20%) Very high (25%) Ultrahigh (30%)

× x x x x

Concentrator

× x x

x x x x ×

Space cells

Consumer electronics

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x x × × ×

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We conclude that I I I - V compounds should play the important role in achieving such modules. Emphasis will be placed on cells for inexpensive fiat-plate modules. However, much of the research is also applicable to concentrator cells. The practical performance limits of single-junction I I I - V cells, including those incorporating materials with different band gaps, will be considered first. These results were obtained based on band-gap theories [2]. Similar results were reported also by various other authors [3]. There are other recent calculations of efficiency limits based on light tr~pping, perfect back-surface reflectors, radiactive-recombination approximations, etc. [4]. For this paper, however, we will focus results calculated from band-gap approximations. Next we will consider multi-junction, multi-bandgap cells (but not multi-junction cells fabricated from a single material). Advanced concepts for achieving inexpensive, high-efficiency I I I - V cells will also be described.

2. Single-junction GaAs solar cells 2.1. Bulk cells

For single-junction cells, as shown in fig. 2, the highest calculated efficiencies are found for energy gaps of about 1.50 eV, which give values of about 27.5% at an operating temperature of 27 °C (fig. 2). The efficiencies decrease with increasing temperature, esoecially for lower band-gap materials such as Si, with Eg = 1.12 eV. For Si, the calculated maximum efficiency at 27 °C is about 24%, and Si cells have recently achieved an efficiency of 23.5% [5]. With advances in material quality and refinements in cell design, Si ceils should improve a little more.

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To achieve much over 20% module efficiency with single-junction cells will require a semiconductor material with Eg between 1.3 and 1.6 eV. Of the binary compounds meeting this requirement, GaAs (Eg --- 1.43 eV) is currently the leading candidate. The other candidates are indhtm phosphide (InP) with a band gap of 1.35 eV and cadmium teiluride (CdTe) of 1.5 eV. GaAs cells have already attained efficiencies of 21-22% using a heteroface structure [6] or a shallow-homojunction structure [7]. Recently, cells with efficiencies over 24% [8] have been obtained by using a GaAs/GaAIAs back-surface-field heterostructure. With further refinements such as a GalnP 2 layer instead of a GaAIAs layer, one-sun efficiencies 25.7% have been obtained [9]. The utilization of GaAs/GaAIAs or GaAs/GalnP: heterostructures is possible because these ternary alloys, whose energy gaps exceed that of GaAs, are lattice-matched to GaAs. Fig. 2 was calculated for conventional homojunction structures, and does not reflect the improvement that may resull from advanced hetero~tructures; including doping gradients, etc. Fig. 3 shows the measured one-sun conversion efficiencies of GaAs and Si ir successive years; these efficiencies are primarily based on the values measured al Solar Energy Research Institute in those years [10]. Silicon started in the mid-fif. ties at only about 5% and kept improving. In !988 silicon cells have reached aboul 20% at one sun, and in later 1989 they reached 23.5% at AM 1.5. GaAs performance started below the efficiency of silicon until the early sixtie~ when it crossed over. Since then, GaAs has always been more efficient than silicon In 1988 GaAs was pushing around 24% at one sun. The efficiency has just reache( 25.7%. Further increases in record efficiencies are expected. It is expected tha single-junction cells will reach 30% at AM 1.5.

2.2. Thin-film cells There are significant advantages if GaAs solar cells can be composed of thil layers of GaAs; e.g. lower manufacturing cost, lower material utilization, lighte weight and suitability of multi-junction applications. Two approaches ale beinl investigated for such cells; growth on reusable GaAs substrates and giowth o~ inexpensive substrates such as silicon. Important advances have been achieved il both areas.

/C.C. Fan / Promises of l l l - - V s o l a r cells

SINGLE-CRYSTAL MOLD

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Fig. 4. Schematic diagram showing the peeled-film technology.

2.21. CLEFT technique The C L E F T process permits the growth of thin single-crystal GaAs films by CVD on reusable GaAs substrates. Since many films can be obtained from one substrate, this process should permit a marked reduction in material usage and cost; since single-crystal films are used, cell efficiencies should remain high and, since the films are only a few microns thick, such cells can have very high power-to-weight ratios. The C L E F T process is a peeled-film technique. The basic idea of peeled-film technology is to grow a thin single-crystal epilayer on a single-crystal mold, to separate the epilayer from the mold and then to use the mold again (fig. 4). The key element of the CLEFT process [11] is the use of lateral epitaxial growth. If a mask with appropriately spaced stripe openings is deposited on a GaAs substrate, the epitaxial growth initiated on the GaAs surface exposed through the openings will be followed by lateral growth over the mask, eventually producing a continuous single-crystal film that can be grown to any desired thickness, and with any desired cell structure. The upper surface of the film is then bonded to a secondary substrate of some other material. If there is poor adhesion between the mask material and the GaAs, the film will be strongly attached to the GaAs substrate only at the stripe openings. The film can be cleaved from the GaAs substrate without degradation of either. The C L E F T procedure has been developed so that conversion efficiencies have improved to over 23% at one sun AM 1.5 for 4 cm 2 total area cells. This represents the highest truly thin-film solar cell efficiency ever reported and is about a factor of two higher than any single-junction thin-film cells of materials other than I I I - V materials. In addition, the successful reuse of GaAs substrates has also been reported using 3" diameter GaAs wafers [12]. Furthermore, for the first time, monolithic thin-film GaAs submodules of four 1 cm 2 interconnected together have been fabricated, having a total area efficiency at AM 1.5 of 21% [13] (see fig. 5), the highest thin-film submodule efficiency ever reported. Using reusable substrates, a rough estimate of $10/watt for GaAs thin-film solar cells has been obtained if the production volume is substantial (5000 kilowatt per year). Another technique of peeling off thin-film process was reported where thin-film GaAs solar cell layers grown by MBE with intermediate AIGaAs layers as etching layers were separated from GaAs substrates. Solar cells were fabricated using this

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Fig. 5.21% efficient GaAs thin-film monolithic submodule.

technique with conversion efficiencies up to 15% reported [14]. However, this technique was not developed further, because of the difficulty of removing largearea layers and of controlling the etching process. Very recently, a major improvement in this chemical etching technique was reported [15] and using a wax-banding technique, the chemical etchant is encouraged to enter the etching channel and layers as large as 4 cm .',<4 c.m. have be.en successfully set~arated. This technique must be fitrther developed before it has any potential of producing large-area ceils. Z2.2. GaAs single-junction cells on Ge and Si substrate As we have described earlier, GaAs solar ceils can be made lighter with Si substrates (Si is about a factor of two less dense than GaAs and thin Si wafers are much stronger than GaAs wafers of the same thickness), a very important advantage for space applications. In addition, currently Si bulk wafers are much cheaper u,,,, ~ , , ~ bulk wafers. Since all advanced GaAs solar cell structures require heterostructure epitaxial growth, it is logical to use 5i substrates for GaAs cells, if efficient cells can be made. Finally, the material system of GaAs and Si is important for multi-junction tandem cells (we will discuss this later). The material challenge of this heteroepitaxy, system is very severe, however. The lattice constants of GaAs and Si are different by 4%, and the thermal coefficients of expansion of GaAs and Si are different by a factor of two. These two physical differences cause many misfit dislocations, and stress-related problems. The first GaAs-on-Si solar cells ever fabricated were reported in 1981 and they were grown with a thin Ge interface layer between GaAs and Si. Small area cells with one-sun efficiency over 10% were obtained [16], and by 1984, much larger-area cells (up to 0.5 cm2), and more efficient cells (up to about 14%) were obtained [17]. The Ge interface was introduced because the lattice constants of GaAs and Ge are almost the same, and Ge surfaces are easier to keep cleaa for GaAs growth. However, with further development in Si surface cleaniag, efficient GaAs solar cells have been successfully grown directly on Si substrates. In fact, the GaAs-on-Si solar cells have been improved so that one-sun efficiency of 18% A M 0 have been recently obtained [19]. These cells were grown with elaborate defect-reduction techniques, such as thermal cycling [18] and strained superlattices [19]. With further improvements in material quality, even higher efficiencies will be reached. The GaAs-on-Si solar cells potentially can be useful for space applications.

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3. Multi-junction solar cells For any tandem device consisting of two single-crystal cells whether its structure is monolithic or hybrid and whether it has two- or four-terminal connections, for maximum overall efficiency our calculations [2] show that the energy gaps should be 1.75-1.80 eV for the top-cell material and 1.0-1.1 eV for the bottom-cell material. For these values the calculations give a maximum overall efficiency of 36%-37%, with the top cell con'~ributing about two thirds of the power output. The highest practical module efficiency that can be expected is therefore about 32%-33%. Because of its well developed solar cell technology and low cost, single-crystal silicon is the material of choice for the bottom cell in a tandem structure. Alternatively, a thin-film CulnSe z solar cell, because of its stability and its potential low cost, can also be a good choice for the bottom cell. The most promising materials for the top cell are two ternary I I I - V alloys based on GaAs: Ga0.x.Ad0.3ms and GaAs0.TP0. 3. 3.1. Mechanically stacked cells 3.1.1. GaAs mechanically stacked on Si Mechanically stacked GaAs on Si cells have been experimentally pursued by a number of investigators. The respective band gaps are 1.43 and 1.11 eV. The first report of a G a A s / S i tandem was made in 1984, and consisted of the stacking of a thin GaAs cell formed by C L E F T on a conventional Si cell [20]. Since then, more optimized improvements were made to this combination, so that in 1988, Gee and Virshup [21] reported a combined four-terminal efficiency of GaAs top cell and Si bottom cell of 31% at AM 1.5 under a concentration of 347 suns. No one-sun efficiencies were reported. The thin GaAs top cell was prepared with a chemical etching technique. 3.1.2. GaAs mechanically stacked on CulnSe 2 Highly efficient mechanically stacked celF, intended for space power have been formed from the combination of CulnSe 2 and GaAs [22]. An attractive aspect of the use of CulnSe z is the high radiation resistance of such cells [23]. Briefly, a GaAs heteroface cell formed by CLEFT having a thickness of 5-10/.tm, is stacked on top of a polycrystalline CulnSez/CdZnS cell that is formed on a glass substrate. 3.2. Monolithic multi-junction cells Two-terminal monolithic cells not only require current matching, as discussed earlier, but also require that the interface between the cells have negligible resistance. In a monolithic tandem, one forms a p / n junction at the interface between cells. To overcome the series resistance that would ordinarily be introduced by this junction, research has focused on metal [26] tunneling [27] or composition interconnects [28] to obtain high conduction in forward and reverse bias.

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J.C.C. Fan / Promises of HI--Vsolar cells

We will select a few examples on monolithic multi-junction cells composed of the I I I - V and column IV materials. 3.2.1. AIGaAs grown on Ga,4s

Significant progress has been made by using an A I G a A s / G a A s multi-junction [29]. The top A10.37Ga0.63Ascell has a band gap of 1.93 eV; the bottom cell is GaAs (1.43 eV). The cell is grown using OMCVD on top of GaAs substrates. For AM 1.5, the two-terminal tandem structures have efficiencies as high as 27.6% at ol~e sun. The intercell reverse p / n junction resistance has been overcome by using a metal interconnect that effectively shorts this junction. 3.2.2. G a l n P grown on GaAs

Olson et al. [30] have recently reported the use of Ga.,.In~_xP for top cell tbrmation. The band gaps available in this system are in the range of 1.34 to 2.25 eV. One advantage of this ternary is that since no AI is present in the active layers, the minority carrier properties are less susceptible to water and oxygen contamination. A heteroface structure can be formed by using A10.5In0.sP that is lattice constant matched to GaAs. The devices made thus far comprise two-terminal Ga0.~In~sP/GaAs tandem structure. A GaAs tunnel diode is used to connect the two subcells. The resultant one-sun AM 1.5 efficiency is 27.3% [30].

4. Single-junction InP solar cells

We have reviewed single-junction and multi-junction cells primarily using Gabased compounds. Recently, lnP cells are becoming very interesting, primarily for space applications. The room temperature band gap of InP is 1.3 eV and so it is in the useful range for efficient solar energy conversion. An attribute of InP that is particularly interesting for space solar cells is its radiation oamage and annealing properties. A characteristic of importance in InP cell design is the high optical absorption coefficient (greater than 1 p.m-~), similar to GaAs. To obtain high performance, the junction must therefore be quite shallow, or alternatively, the surface must have a low recombination velocity. Spitzer at al. [31] have reported on highly efficient cells with a junction depth of only 400 A with AM 0 efficiency of 18%. Yet higher efficiency was obtained with an ion-implanted emitter [32]. Since these emitters are thin, they are characterized by high sheet resistance and therefore require a fine-line front surface grid. The most interesting feature of InP cells is its radiation resistance. Most of the radiation work was reported by Yamaguchi et al. [33]. In essence, InP solar cells, under electron and X-ray radiation, have less radiation damage than GaAs cells, which in turn have smaller radiation damage than Si. This phenomenon is presumed to be a self-annealing effect in InP that anneals out radiation damage close to room temperatures. This advantage can have important implication for certain space systems.

J.C.C. Fan / Promises of III~Vsolar celis

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lnP solar cells have application mainly in space power system, owing to the radiation annealing properties described above. Efficiencies competitive with GaAs cells have been demonstrated; however, it is still necessary to improve the cell technology further. In addition, the cost of InP cells must be reduced to levels comparable to Si and thin-film GaAs (GaAs needs about 50 times less material than Si to absorb the same ratio of sunlight [2]). At the present time, lnP substrates are approximately three times more expensive than GaAs, and about 40 times more expensive than Si. Work is therefore in progress to form lnP heteroepitaxially on alternative substrates, and in particular, on GaAs [34] and Si [35]. Another interesting approach to this technology is the development of I n P / InGaAs tandem structures [36]. If these approaches can yield material of suitable quality, then InP cells in terms of efficiency and cost while being superior in radiation resistance. 5. Conclusion

We have examined the status of high-efficiency solar cells. In order to obtain high conversion efficiency, advanced solar cell structures must be used, and the photovoltaic material must be carefully chosen. The ideal material should be highly absorbing in the active solar spectrum range so that it can be used in thin-film cells. It should also have large majority-carrier mobility and long minority-carrier diffusion length. The material should form excellent homojunctions, or better yet, heterojunctions with another lattice-matched material. The material should be stable, inexpensive in thin-film configuration, and should have a broad technology base (in other words, the material should also be very useful for products other than photovoltaic cells). In addition, the cells made from this material should be useful for fiat-plate, concentrating, space, and tandem applications. Finally, for the maximum efficiency the single-junction cells should be composed of a material with a band gap Eg = 1.3-1.5 eV and for a two-cell tandem structure, the top cell should be composed of a material with Eg --- 1.5-1.8 eV and the bottom cell should be composed of a material with Eg --- 1.0-1.3 eV. The III-V materials, in particular GaAs-based materials, satisfy almost all the requirements of an ideal photovoltaic material, and in this paper we have reviewed the status of I I I - V solar cell developments. Progress in those cells have been excellent, and the future of these cells are vet3' bright. References [1] USA National Department of Energy 5-Year PLan (1987). [2] J.C.C. Fan and B.J. Palm, Solar Cells 12 (1984) 401. [3] M.B. Prince, Solar Cells (IEEE, New York, 1076) p. 89;-J.J. Loferski, ibid, p. 96; J.J. Wysocki and P. Rappaport, ibid, p. 110. [4] T. Tiedje, E. Yablonovitch, G.D. Cody and B.G. Brc,oks, IEEE Trans. Electron Devices ED-31 (1984) 711; G.L. Araujo, Physical Limitations to Pho~.ovoltaic Energy Conversion, Eds. A. Luque and G.L. Araujo (Adam Hilger, 1990) p. 107.

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J.C.C. Fan / Promises of l l l - - V solar cells

[5] M.A. Green (1990), Si solar cell measured in SERI, personal communication by L.L. Kazmerski; and Proc. 21st IEEE PV Specialists Conf., to be published. [6] J.G. Werthen, G.F. Virshup, C.W. Ford, C.R. Lewis and H.C. Hamaker, in: Proc. 20th IEEE PV Specialists Conf. (IEEE, New York, 1988) p. 300. [7] C.O. Bozler and J.C.C. Fan, Appl. Phys. Lett. 31 (1977) 629. [8] R.P. Gale, J.C.C. Fan, G.W. Turner and R.L. Chapman, in: Proc. 20th IEEE PV Specialists Conf. (IEEE, New York, 1988) p. 446. [9] J.M. Olson, S.R. Kurtz and A.E. Kobber (1989), Cells measured at SERi, private communication by L.L. Kazmerski; and Proc. 21st IEEE PV Spec Conf., to be published. [10] Values obtained from J.P. Benner of Solar Energy Research Institute. [11] R.W. McClelland, C.O. Bozler and J.C.C. Fan, Appl. Phys. Lett. 37 (1980) 560. [12] R.P. Gale, R.W. McCleiland, B.D. King and J.V. Gormley, in: Proc. 20th IEEE PV Conf. (IEEE, New York, 1988) p. 446. [13] R.W. McClelland, B.D. King, R.P. Gale and J.C.C. Fan, 21st IEEE PV Specialists Conf., 1990, to be published. [14] M. Konagai, M. Sagimoto and K. Takahashi, J. Cryst. Growth 45 ~1978) 277. [15] E. Yablonovitch, T. Gmitter, J.P. Harbison and R. Bhat, Appl. Phys. L~tt. 51 (1987) 2222. [16] R.P. Gale, .I.C.C. Fan, B.-Y. Tsaur, G.W. Turner and F.M. Davis, IEEE Electron Dev. Lett. EDL-2 (1981) 169. [17] B.-Y. Tsaur, J.C.C. Fan, G.W. Turner, B.D. King, R. McCleiland and G.M. Metze, in: Proc. 17th IEEE PV Conf. (IEEE, New York, 1984) p. 440. [18] B.-Y. Tsaur, J.C.C. Fan G.W. Turner, F.M. Davis and R.P. Gale, in: Proc. 16th IEEE PV Conf. (IEEE, New York, i982) p. 1143. [19] Okamoto, H.Y. Kadota, Y. Watanable, Y. Fukuda, T. O'Hara and Y. Ohmachi, in: Proc. 20th IEEE PV Spec. Conf. (IEEE, New York, 1988) p. 475. [20] J.C.C. Fan, B.-Y. Tsaur and B.J. Palm, in: Proc. 16th IEEE PV Spec. Conf. (IEEE, New York, 1982) p. 692. [21] J.M. Gee and G.F. Virshup, in: Proc. 20th IEEE PV Spec. Conf. (IEEE, New York, 1988) p. 754. [22] N.P. Kim, R.M. Burgess, B.J. Stanbery, R.A. Mickelsen, J.E. Avery, R.W. McClelland, B.D. King, M.J. Boden and R.P. Gale, in: Proc. 20th 1EEE PV Spec. Conf. (IEEE, New York, 1988) p. 457. [23] R.M. Burgess, W.S. Chen, W.E. Delaney, D.H. Doyle, N.P. Kim and B.J. Stanbery, in: Proc. 20th IEEE PV Specialists Conf. (IEEE, New York, 1988) p. 909. [24] N.P. Kim, B.J. Stanbery, R.P. Gale and R.W. McCleUand (1988), in: Proc NASA Space Photovoltaic Research and Technology Conf., NASA-Lewis Research Enter, in press. [25] L.M. Fraas, J.E. Avery, J. Martin, V.S. Sundaram, G. Girard, V.T. Dinh, T.M. Davenport, J.W. Yerkes and MJ. O'Neill, IEEE Trans. Electron Devices ED-37 (1990) 443. [26] M.J. Ludowise, R.A. Larue, P.G. Borden, P.E. Gregory and W,T. Dietze, Appl. Phys. Lett. 41 (1982) 550. [27] M.L. Timmons and S.M. Bedair, J. Appl. Phys. 52 (1981) 1134. [28] M.L. Timmons and P.K. Chiang, in: Proc. 19th 1EEE PV Spec. Conf. (IEEE, New York, 1989) p. 102. [29] H.F. Macmillan, H.C. Hamaker, G.F. Virshup and J.G. Werthen, in: Proc. 20th IEEE PV Spec. Conf. (IEEE, New York, 1988) p. 48. [30] J.M. OIson, S.R. Kurtz and A.E. Kibber, in: Proc. 20th IEEE PV Spec. Conf. (IEEE, New York, 1988) p. 777. [31] M.B. Spitzer, C.J. Keavney, S.M. Vernon and V.E. Haven, Appl. Phys. Lett. 51 (1987) 364. [32] C.J. Keavney and M.B. Spitzer, Appl. Phys. Lett. 52 (1988) 1439. [33] M. Yamaguchi, Y. Yamamoto, J. Appl. Phys. 55 (1984) 1429. [34] H. Horikawa, Y. Ogawa, Y. Kawai and M. Sakuta, Appl. Phys. Lett. 53 (1988) 397. [35] A. Yamamoto, N. Uchida and M. Yamaguchi, Optoelectronics Devices and Technologies 1 (1986) 41. [36] M.W. Wanlass, T.A. Gessert, G.S. Horner, K.A. Emery and T.J. Courts, in: Proc. Space PV Res. and Tech. Conf., NASA-Lewis Res. Ctr., in press.