442 IEIE E, TRANSACTIONS ON ELECTRON DEVICES, VOL,. ED-24, NO. 4, APRIL 1977

[3] W. Hooper and H. J. Queisser, Bull. Am. Phys. Sol., vol. 7, I~ 211, 1975. 1962. [7] R. Buhs and H. Gochermanrx, “German activities in the field of ter-

[4] W. Shockley, Electron and Holes in Semiconductors. Van N0.A rand, restrical application of solar cell arrays,” in Conf. Rec. 12th Photo- 1950, p. 218. uoltaic Spec. Coni. (Nov. 15-18,1976, Baton Rouge, LA).

[5] S. I. Soclov and P. A. Iles, “Grain boundaries and impurity :Cfects [SI Deutsche Offenlegungsschrift 250883, “Neuartige Siliziuankristalle in low cost silicon solar cells,” in Conf. Rec. 21th Photouoltai, ,‘Spec. und Verfahren zu ihrer Herstellung,” Wacker-Chemitronic, Gesell- Conf. (May 6-8,1975, Scottsdale, AZ). schaft fur Elektronik-Grundstoffe m.b.H., Inventor: B. A. Authier,

[6] H. J. Hovel, Solar Cells, vol. XI of Semiconductors and Semi:.netals. US-Application Ser. No.: 652359.

Silicon Solar Cells on il.lnidirectionally Recrystallized Metallt.rgical Silicon

Abstract-The deposition of a silicon layer containing 8. p-n large-scale manufacturing operations. However, the major junction on a metallurgical silicon substrate has been used f’’or the disadvantage of thin-film silicon solar cells at present is fabrication of solar cells. The substrate was prepared by th15 uni- directional solidification of purified metallurgical silicolr~ on a the relatively low conversion efficiency due to grain graphite plate, and the active region of the solar cell was dep llsited boundaries and other defects in silicon films on foreign bv the thermal reduction of trichlorosilane with hydrogel:, con- substrates. Furthermore, silicon is an indirect gap material taining appropriate dopants. The current-voltage character.i stics of a number of solar cells were measured in the dark and untll er il- lumination. The AM1 efficiency of large-area cells (30 em2) was up to 5.5 percent. When a large-area cell was divided into small! -area (5-cmZ) ones, the conversion efficiency was found to correlrille di- rectly with the dark current-voltage characteristics of, a1:I.d the structural properties of silicon in, each cell.

. -

I I. INTRODUCTION

T IS WIDELY recognized that for large-3cale terrestrial power generation by photovoltaic solx~~* en-

ergy conversion, the present cost of solar cell arrays must be substantially reduced. Several cost-reductior ap- proaches for silicon solar arrays have been under in.wsti- gation during the past few years. In one approach, E thin film of silicon on a low-cost substrate is used for the! fab- rication of solar cells. This thin-film approach rec:Iuces substantially the material and processing costs o f the present single crystalline silicon cells and is amena‘11Le to

with a very gradual absorption edge, and more than 10 ym of silicon films are required for the efficient absorption of solar radiation as compared with 2-3 ym for direct gap materials, such as gallium arsenide.

During the early sixties, a thin.,film technique has been used for the fabrication of small-area polycrystalline silicon solar cells [l]. In that work, a polycrystalline silicon film of 25-50-ym thickness was deposited on silicon substrates at 900°C by the hydrogen reduction of trichlorosilane; silicon substrates were used for convenience to eliminate the cracking of silicon deposited on other substrates. Solar cells of about 1 cm2 area were then fabricated by the suc- cessive diffusion of gallium and phosphorus to form a p-n junction about 2.5 ym below the surface. The maximum open-circuit voltage obtained was about 0.3 V, and the highest efficiency was about 0.9 percent.

Recently, a thin film of polycrystalline silicon containing a shallow p-n junction has been deposited on low-cost substrates for the fabrication of solar cells [2]-[6]. Steel is the most economical choice for a large-area substrate; however. a diffusion barrier. such as borosilicate, is re-

Manuscript received September 27,1976; revised November 22,1976. quired t; prevent the diffusion of iron from the substrate This work was prepared for the Division of Solar Energy of the US I Snergy Research and Development Administration under Contract ElO4-3)- into silicon at high temperatures [2]. Solar cells deposited 1285. on borosilcate/steel substrates by the pyrolysis of silane 75275. and appropriate dopants were found to have poor electrical

The authors are with the Southern Methodist University, Dall a 3 , T X

CHU et d . : Si SOLAR CELLS ON METALLURGICAL SILICON 443

characteristics and low conversion efficiencies due to the small size of silicon crystallites [3]. Graphite is more compatible with silicon in properties than steel. Silicon deposited on graphite or carbon-coated graphite substrates by tlhe conventional chemical vapor deposition technique has been found to consist of crystallites ranging from a few micrometers to tens of micrometers in size with an average of 20-30 pm in better materials [4], [5]. Solar cells of the configuration n+-silicon-p-silicon-p+-silicon-substrate Fig* 1. have AM0 efficiencies (without antireflection coatings) of about 1.5 percent. This low efficiency is due predomin-antly to thie recombination of light-generated carriers a t grain boundaries. To improve the conversion efficiency of solar cells, the grain boundaries must be passivated to reduce the carrier recombinations or the size of silicon crystallites must be increased substantially.

Metallurgical grade silicon at a cost of about $l/kg has also lbeen used as a substrate for the deposition of the ac- tive region of silicon solar cells [6] . Metallurgical silicon substrates prepared by the casting of the pulverized ma- terial in a boron nitride container consist of elongated crystallites up to several millimeters in length. The con- version efficiencies of solar cells of a few cm2 area deposited on such substrates have been found to be on the order of 3 percent.

In this work, the unidirectional solidification of chem- ically treated metallurgical silicon has been used for the preparation of substrates, and the active region of solar cells .has been deposited by the thermal reduction of tri- chlorosilane containing appropriate dopants in a one-step process. Because of the improved chemical and structural perfection of the substrate, large-area (30-cm2) solar cells with an AMP efficiency of up to 5.5 percent have been obtained. The experimental procedures for the preparation of substrates and solar cells and the characteristics of solar cells are discussed in this paper.

11. PREPARATION OF METALLURGICAL SILICON SUBSTRATES

The use of metallurgical silicon as a substrate for the deposition of solar cells has two major problems: high impurity content, and unavailability of metallurgical sil- icon plates. Metallurgical silicon contains iron and alu- minum at concentratiom of a few tenths of one percent and other metallic impurities (such as copper, vanadium, nickel!, etc.) at concentrations of up to several hundred parts per million. The transition metals have deep levels in silicon and act as traps or recombination centers with larger cross sections for capture of minority and majority carriers. These impurities diffuse readily from the sub- strate into the deposited film during the deposition process and reduce 'the minority carrier lifetime. Metallurgical silicon may be partially purified by the treatment of the melt with halogens or hydrogen halides [7] and by the unidirectional solidification of the melt [6] .

Metallurgical silicon is manufactured in large electric furnaces, and molten silicon is cast in steel molds. Such

RF COIL,

0 0 0

FUSED SILICA TUBE SILICON ON GRAPHITE

Schematic diagram ofthe apparatus for the unidirectional so- lidificatian of purified metallurgical silicon.

Fig. 2. Optical photograph of the as-recrystallized surface of purified metallurgical silicon on a graphite plate.

material is highly porous and must Ibe recrystallized before use. The recrystallization of silicon is complicated by the chemical reactivity and large surface tension (about 725 dynedcm) of the melt. In this work, the unidirectional solidification of chemically treated metallurgical silicon on a graphite plate, as shown schernatically in Fig. 1, was used for the preparation of metallurgical silicon substrates. Metallurgical silicon on a graphite plate was placed in a fused silica tube in a hydrogen atmosphere, and graphite was heated by an RF generator. The temperature profile of the graphite plate was controliled by adjusting the spacings between the turns of the RF coil to yield a uni- directional temperature gradient (of 15-2OoC along the length of the specimen. The entire silicon charge was first melted, and the power was reduced for the solidification to take place from one end of the specimen to the other. This unidirectional solidification is essential because of the higher density of liquid silicon. The regions which so- lidify last will be grossly polycrystalline with a protruded surface due to the expansion of silicon on solidification. Also, the solidification process should initiate from the surface of the melt and proceed inward to minimize the effects of random nucleations at thle silicon-graphite in- terface. Fig. 2 shows an optical photograph of the as-re- crystallized surface of a typical specimen; its surface is not planar but has many ridges and valleys. The larger crys- tallites are elongated, several centimeters in length and several millimeters in width. Nearly all faces of the speci- mens exhibited a two-fold symmetry as indicated by the X-ray Laue back reflection technique. The valleys appear to be high-angle grain boundaries.

111. PREPARATION OF SOLAR CELLS Since the chemically treated and unidirectionally re-

crystallized metallurgical silicon substrates are usually p-type with an electrical resistivity of 0.005-0.05 fl - cm,

444 IEEE TRANSACTIONS ON ELECTRON DEVICES, APRIL 1977

c I I I I I I

0 5 10 I5 20

THICKNESS ABOVE SUBSTRATE SURFACE, J1M.

Fig. 3. Resistivity profile in a polycrystalline silicon solar cell, excel.~l for the n+-layer.

solar cells prepared in this work were of the configuratic!ri n+-silicon-p-silicon-metallurgical silicon-graphite. The low resistivity substrate serves to minimize the conta.ct resistance between silicon and graphite and also providw a back-surface field. Immediately following the recryj- tallization of the substrate, the active region of the so1;ri.r cell was deposited on the substrate at 1100-1150OC by tl IC

thermal reduction of trichlorosilane with hydrogen, t h most commonly used process for the deposition of silicm [SI. The conductivity type and electrical resistivity of dl?- posited silicon were controlled by using diborane :IC phosphine as a dopant, and the p-n junction was formc (1 during the deposition process by varying the compositic III

of the reactant mixture. Typically, 20-30 pm of 0.2-1- 52 cm p-type silicon was deposited at a rate of about I pmlmin followed by the deposition of 0.3-0.6 pm If 0.001-0.004-Q - cm n-type silicon at a rate of about C 1 , I pm/min. The deposit is usually epitaxial with respect ;I>

the substrate as shown by metallographic examinatiom The sheet resistance of the n+-layer, measured by t i l e four-point probe technique, was 30-50 Wsquare. The r e - sistivity profile in the device was determined by using a spreading resistance probe along the angle-polished S U I -

face, and an example of the profile is shown in Fig. 3. In tl iils specimen,, the metallurgical silicon substrate was crf 0.005-52 cm resistivity, the p-layer was of 20-pm thicknws and 0.5-52 - cm resistivity, and the n+-layer was removl?d inadvertently during the angle-polishing process.

The graphite plate serves as the ohmic contact to It ole p-region. The grid contact of about lo00 A of titanium a11d 3-5 pm of silver was evaporated onto the n+-surface, fbd- lowed by annealing at about 50OOC in a hydrogen atrrl o- sphere. Antireflection coatings were applied by chemili:;d vapor deposition; approximately 1000 A of silicon dioxide was deposited on the cell surface at 400°C by the oxidati 'cm of silane in a hydrogen atmosphere [9]. Silicon dioxida is not an optimum material for antireflection coatings cn silicon solar cells because of its low refractive index; how- ever, it was used here for convenience.

I I I I I 0. 2 0. 4 0. 6 0. 8 I. 0

VOLTAGE, VOLTS

Fig. 4. Dark current-voltage characteristics of a silicon dioxide-n+- silicon-p-silicon-chemically treated metallurgical silicon (unidirec- tionally recrystallized)-graphite solar cell.

IV. CHARACTERISTICS OF SOLAR CELLS

Solar cells prepared in this work were usually of 30 cm2 in area. Their current-voltage characteristics were mea- sured at room temperature in the dark and under illumi- nation. The dark current-voltage relation of a solar cell is directly related to its performance. Fig. 4 shows an example of the dark current-voltage characteristics of a solar cell of about 28 cm2 area. Both the forward and reverse char- acteristics are far from ideal. In the forward direction, the value of n, the empirical parameter in the diode equation, is about 5 at voltages between 0.1 and 0.4 V and is about 3 at higher voltages. These high n values are due pre- dominately to the recombination of carriers at grain boundaries, and shunting currents may also be a contrib- uting factor. The junction also exhibits high reverse cur- rents, about A/cm2 at l V. Fig. 5 shows the current- voltage characteristics of this cell under illumination with a quartz-halogen lamp equivalent to AM1 conditions. The open-circuit voltage, short-circuit current density, and fill factor are 0.52 V, 17.5 mA/cm2, and 60 percent, respec- tively, corresponding to a conversion efficiency of 5.4 percent. The effective minority carrier diffusion length in this and similar yells measured by the surface photovoltage method is 15-25 pm [lo].

At present, the unidirectional solidification technique cannot produce large-area substrates with uniform structural characteristics. The nonuniformity is readily visible; however, the degree of nonuniformity may vary considerably from specimen to specimen. To determine the effects of structural characteristics of silicon crystallites on the properties of solar cells, the solar cell described above was divided into six cells of 4-5 cm2 in area by masking and etching techniques. The current-voltage characteristics of each cell were then measured in the dark

CWU el: al.: Si SOLAR CELLS ON METALLURGICAL SILICON

F l L l FACTOR : 6 0 % CELL AREA : 28 on2

A M I EFF : 5.4%

t \ I I I I I I \ J

0. I 0.2 0. 3 0. 4 0.5

VOLTAGE, VOLTS

Fig. 5. Current-voltage characteristics of a silicon dioxide-n+-silicon- p-silicon-chemically treated metallurgical silicon (unidirectionally recrycital1ized)-graphite solar cell under illumination with a quartz- halogen lamp equivalent to AM1 conditions.

and under illumination. Cells with visibly larger and more unifor:m crystallites had distinctly better dark current- voltage characteristics and higher conversion efficiencies. Fig. 6(a) and (b) show the dark current-voltage charac- teristics of the best and the worst cells, respectively. The forward characteristics shown in Fig. 6(a) are reasonably good; two regions with n values of about 2.5 and 1.4, re- spectively, may be distinguished. The data were fitted by the least square method to the two exponential model [ll]:

I = Iol[exp (qV/hT) - 11 V

.t Ioz[exp (qVIA2hT) - 11 + - RSh

where the first two terms represent the contributions of the diffusion and the recombination currents, respectively, and R s h is the shunt resistance. The saturation currents, 4.6 X and 5.1 X A/cm2 for 101 and 102, respec- tively, ,and the shunt resistance, lo5 ohms, of this cell are compalrable to those of single crystalline cells. Its conver- sion efficiency is about 6.8 percent, considerably higher than thLat of the large-area cell. The characteristics shown in Fig. 16(b) are considerably inferior to those in Fig. 6(a): the least square fitting of the current-voltage data pro- duced no meaningful results, and the conversion efficiency of this cell is about 5.2 percent.

'v. DISCUSSION AND SUMMARY

The conversion efficiency of a polycrystalline silicon solar cell is determined predominately by the extent of recombination of light-generated carriers at grain boundaries. This loss may be estimated from the junction depth in the device and the minority carrier diffusion length in the lightly doped region. Qualitatively, two re-

VOLTAGE, VOLTS

(a)

415

0. 2 0. 4 0. 6 0. 8

VOLTAGE, VOLTS

I

(b) Fig. 6. Dark current-voltage characteristics of the best (a) and the worst

and 5. (b) small-area solar cells isolated from the solar cell shown in Figs. 4

about twice the junction depth.and the inner region. In the surface region, carriers generated at distances less than one junction depth form a grain boundary will be lost through recombinations. The effect of grain bloundaries is thus to reduce the effective area of the cell by an amount equal to the product of the grain boundary length and twice the junction depth. Since solar cells havie shallow junctions, 0.5 Fm or less, the effective area of solar cells is not ap- preciably reduced, about 1 percent for each rneter of grain boundaries per cm2 of cell. Accordingly, the spectral re- sponse in the short wavelength region may only be slightly affected. In the inner region, the diffusion of carriers be- comes important, and the carriers generated at distances

gions may be distinguished: surface region with a depth 'less than one diffusion length from a grain boundary will

446 IEEE TiANSACTIONS ON ELECTRON DEVICES, VOL. ED-24, NO. 4, APRIL 1977

be lost through recombinations. The minority carrhr diffusion length in a polycrystalline solar cell is equal to or less than the thickness of the lightly doped region, 20- 3D pm. Thus the effective area of the solar cells is more tic!-

verely reduced, about 1 percent for each 2 cm of grain boundaries per cm2 of cell. Furthermore, the amount of solar radiation absorbed in the active region of a pol, y- crystalline cell is 85-90 percent of that absorbed in a sing1,e crystalline cell. In order that the conversion efficiency of a polycrystalline cell may approach 75-80 percent of ta1.e efficiency of a single crystalline cell, only 10-20 cm of electrically active grain boundaries per cm2 of cell can tre tolerated. This is probably an upper limit because of It.t;,e over simplification of the estimates here.

In this work, polycrystalline silicon solar cells have bem prepared by the chemical vapor deposition of the active region on recrystallized metallurgical silicon substrat’js. Because of the relatively large size of silicon crystallit 35,

AM1 efficiencies of higher than 7 percent have been ~ 1 ) - tained in cells of 5 cm2 area. Further improvements in the size and uniformity of silicon crystallites in the substr;:ite will increase the conversion efficiency. The collection ;f- ficiency may also be increased by optimizing the dopmt

distribution in the active region of the cell. Thus the use of low-cost materials and simplified processing in this work has potentials for producing low-cost solar cells. However, further research and development are necessary before a realistic economic analysis can be made.

REFERENCES [I] J. D. Heaps, 0. M. Tufte, and A. Nussbaum, IEEE Trans. Electron

[2] T. L. Chu, H. C. Mollenkopf, and Shirley S. Chu, J . Electrochem.

[3] T. L. Chu e t al., Solar Energy, vol. 17, p. 229,1975. [4] T. L. Chu, H. C. Mollenkopf, and Shirley S. Chu, J . Electrochem.

SOC., vol. 123, p. 106, 1976. [5] T. L. Chu et al., in Chemical Vapor Deposition, Fifth International

Conference. Princeton, NJ: The Electrochemical SOC. 1975, p. 653.

[6] T. L. Chu and K. N. Singh, Solid-State Electronics, vol. 19, p. 837, 1976.

[7] G. A. van der Leeden, H. I. Yoo, and T. L. Chu, Proc. National Workshop on LOLL Cost Polycrystalline Silicon Solar Cells (Dallas, TX, May 1975).

Deuices, vol. ED-8, p. 560, 1961.

Soc., vol. 122, p. 1681,1975.

[8] T. L. Chu, J . Electrochem. SOC., vol. 113, p. 717, 1966. [9] T. L. Chu, J. R. Szedon, and G. A. Gruber, Trans. Met. SOC. A I M E ,

vol. 243, p. 532, 1968. [lo] E. D. Stokes and T. L. Chu, in IEDM Tech . Dig., i976, p. 478. [ll] R. J. Stirn, in Proc. 9th ZEEE Photovoltaic Specialists Conference

(Silver Spring, May 1972), p. 72.

Fabrication and Characterizatiisn of Thin-Film Silicon Solar Cells on Alurr’ina Ceramic

TERUNORI WARABli3AKO AND TADASHI SAITOH

Abstract-Dendritic silicon layers are prepared on alumina ceramic by melting and regrowing a CVD Si layer with a BSC e’n- capsulation. Thin-film solar cells fabricated by srlccessive dt! 110- sition of p- and n+-Si layers on this dendritic silicon exhibit a cctn- version efficiency of 2.6 percent under AM1 illumination.

I. INTRODUCTION

T HE THIN-FILM approach is known to have ,an essential advantage in realizing lowrcost solar cells, for

large-scale terrestrial application. Several attempts h lave

of Industrial Science and Technology, MITI, Japan, as a part of *;he Manuscript received. This work was supported in part by the Agt’ncy

National Research and Development Program “Sunshine.” The authors are with the Central Research Laboratory, Hitachi l:d,d.,

Kokubunji, Tokyo, Japan.

been made to fabricate solar cells on various materials such as steel [l] or graphite [2].

Another possible choice for the substrate is ceramic material. Recently, polycrystalline silicon thin films with a comparably large area have been obtained on alumina ceramic by CVD [3] and/or by dipping a substrate into molten silicon [4].

A large-area, dendritic silicon film has also been obtained by melting and regrowing a chemically deposited silicon layer with glass film encapsulation, in conjunction with the use of a titanium layer to improve the wettability between the silicon and the alumina ceramic [5]. This correspon- dence describes the feasibility of fabricating solar cells on such a dendritic silicon film.