Recent advances in amorphous silicon solar cells and their technologies

Journal of Non-CrystallineSolids 59 & 60 (1983) 1265-1272 North-Holland Publishing Company

1265

RECENT ADVANCES IN AMORPHOUSSILICON SOLAR CELLS AND THEIR TECHNOLOGIES Yoshihiro HAMAKAWA Faculty of Engineering Science, Osaka University, Toyonaka, Osaka 560, Japan A review is given on recent advances of amorphous s i l i c o n solar c e l l s and t h e i r technologies. F i r s t l y , some unique advantages of amorphous s i l i c o n as a low cost solar c e l l material are pointed out in views of basic physics and p r a c t i c a l technologies. Secondly, some new approaches to improve photov o l t a i c performances by u t i l i z a t i o n of o p t i c a l and c a r r i e r confinement effects with combinations of higher and lower energy gap materials such as a-SiC, uc-Si and a-SiGe, poly c-Si are introduced. In the f i n a l p a r t , recent aspects of the i n d u s t r i a l i z a t i o n s and a p p l i c a t i o n f i e l d are overviewed. I . INTRODUCTION In a recent few years, remarkable progresses have been seen in both the physics and technology of a-Si (hydrogenated amorphous s i l i c o n ) as a new electronic material.l

P a r t i c u l a r l y , i t s s i n g i f i c a n t properties: excellent photo-

conductivity with high optical absorption c o e f f i c i e n t f o r v i s i b l e l i g h t , the a b i l i t y to produce a non-epitaxial growth on any foreign substrates such as glass, metals and even p l a s t i c polymar films and large area f a b r i c a t i o n : make i t a timely technology which matched with a strong social need for the development of a low cost solar photovoltaic system as a new energy resource. Through the wide v a r i e t y of tremendous R&D e f f o r t s , a remarkable progress has been seen in the f i e l d from f i l m growth technology to cell f a b r i c a t i o n processes on the heterojunction and/or m u l t i j u n c t i o n structure solar c e l l s utilizing

new materials such as a-SiC, ~c-Si etc. 2

As the r e s u l t s , more than

12% conversion e f f i c i e n c y has been attained in the small area laboratory phase 3 c e l l with the a - S i / p o l y c-Si stacked junction structure. For the large area solar c e l l s lOxlO cm2 module, more than 7% e f f i c i e n c y are presently quite common elsewhere in the i n d u s t r i a l productions. 4

Market size and i n d u s t r i a l i -

zations of a-Si solar c e l l s are growing quite smoothly. In t h i s paper, the p r i n c i p a l advantages of amorphous s i l i c o n alloys as a low cost solar c e l l material w i l l f i r s t l y

be pointed out together with some progres-

sive technological demonstrations in the a-Si solar cell f a b r i c a t i o n .

Secondly,

recent topics in the f i e l d of device physics f o r the improvement of photovoltaic performances are reviewed and the present status of the performance of various types of amorphous s i l i c o n solar c e l l s are summarized.

F i n a l l y , the current

state of the a r t in the a-Si solar cell a p p l i c a t i o n f i e l d are overviewed 0022-3093/83/0000-0000/$03.00 © 1983 North-Holland/Physical Society of Japan

T Hamakawa/Recentadvancesin amorphoussilicon so~rcel~

1266

together with t h e i r prospects for commercialization. 2. UNIQUE PROPERTYAND ADVANTAGESOF a-Si AND ITS ALLOYS AS SOLAR CELL MATERIALS F i r s t of a l l , the author wishes to point out some unique physical properties and remarkable advantages of a-Si alloys as a low cost solar cell material from both basis physics and technological ~iew points as following: a) High optical absorption and large photoconductivity for solar spectrum As i t has been reported elsewhere, a-Si and uc-Si have more than one order of magnitude larger absorption c o e f f i c i e n t compared to that of single crystal o

s i l i c o n at the maximum solar photon energy region near 5000 A, which is shown by dotted l i n e in the figure. 5 Moreover, a-Si has an excellent photoconductivity

in this v i s i b l e photon energy region.

Another noticeable property in

these hydrogenated t e t r a h e d r a l l y bonded amorphous semiconductors is an existence of valency electron c o n t r o l l a b i l i t y by doping of the substitutional impurity atoms. Effects of impurity doping on the conductivity, optical absorption c o e f f i c i e n t and optical energy gap have been intensively investigated on a-Si by Okamoto et al 6, fluorinated amorphous s i l i c o n (a-Si:F) by Madan et a l . 7 and hydrogenated amorphous s i l i c o n carbide (a-SiC) by Tawada et al 8, and s t i l l now in progress for amorphous s i l i c o n germanium (a-SiGe) by Yukimoto et al 9 I0 and amorphous s i l i c o n t i n (a-SiSn) by Kuwano et al b) Low cost with energy saving mater~aZ On the basis of optoelectronic properties of these materials, systematic calculations have been made on the optimum thickness of solar photovoltaic active region for various solar cell materials. II

These results indicate that

the optimum thickness for the active layer in a-Si solar cells is only 0.5~0.7 um, depending upon the cell structure and the back side r e f l e c t o r material.

a

b

FIGURE 1 Flexible a-Si solar cells. (a) Deposited on the polyimide film (presented by Teijin Limited); (b) deposited on stainless steel cylinder next to an a-Si po~,'eredpocket calculator (presented by Sharp-ECD Solar Ine).

T. Hamakawa / Recent advances in amorphous silicon solar cells

1267

This optimum thickness has been e x p e r i m e n t a l l y well v e r i f i e d , and i t represents the t h i n n e s t one of a l l the s o l a r c e l l m a t e r i a l s under consideration.

The r a t i o

of the material thickness compared to t h a t o f single c r y s t a l l i n e s i l i c o n s o l a r c e l l s is less than 1/500.

I t is evident from t h i s f a c t t h a t a-Si represents

both an energy saving and a resource saving s o l a r cell material f o r a n t i c i p a t e d f u t u r e tremendous demands in s o l a r p h o t o v o l t a i c a p p l i c a t i o n s . c) Large area non-epitaxial growth on any substrate material

Due to i t s amorphous s t r u c t u r e , a-Si can be deposited on any inexpensive substrate, which needs to be treated to only a r e l a t i v e l y low temperature, less than 250-300 C° .

Moreover, i t is possible to form a very wide area s o l a r c e l l ,

because i t can be deposited d i r e c t l y from a kind o f vapor phase growth onto n o n - c r y s t a l l i n e substrates. viewpoint.

This is one big advantage from a technological

In the current technology, glass and s t a i n l e s s steel substrates are

already popular. d) Low balance of system cost

Utilizing

the concent of n o n - e p i t a x i a l deposition technology, i t could be

possible to reduce BOS (balance of system) costs in p h o t o v o l t a i c arrays by the h y b r i d i z a t i o n of a l r e a d y - b u i l t u n i t s .

Solar t i l e

might be useful to r e a l i z e t h i s concept.

and s t i c k e r from solar c e l l

F i g . l shows an example of f l e x i b l e

s o l a r c e l l s deposited on Kapton f i l m and stainless s t e e l . e) Large scale merit with big masspro~ciablity

As has been pointed out in the e a r l y stage of the work, j u n c t i o n formation can be e a s i l y made in the same reaction chamber by mixing o f s u b s t i t u t i o n a l i m p u r i t y gases i n t o SiH 4 or SiF 4.

Moreover, the i n t e r c o n n c t i o n of c e l l s can be

made simultaneousely in the process of a-Si f i l m deposition with a conventional i n t e g r a t e d - c i r c u i t mask technology as shown in Fig.2.

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Combining the advantages

L_ (b) FIGURE 2 Integrated technology available for the cell locations and interconnection. Example of photo-lithographic processin G (a) and consumer application integrated cells

T. Hamakawa / Recent adva.ces in amorphous silicon solar cells

1268

(a) Clean up

(b) TC (c) TC (d) p a-SiC deposition patterning CH 4 SiH4 - - ~ _

I

(e) i a-Si:H SiH 4

(f) n ~c-Si SiH 4

J

t (l) Shipping

(k) Encapsulation

(j) Insolation testing

(i) Laser scriber

(h) Metal contact

(g) Patterning

FIGURE 3 Good mass producibility and large scale merit: the production process sequence for fabricating amorphous silicon monolithic series panels (a), example of the role to role massproduction process (presented by Sharp -ECD Solar Inc.).

discussed in section c) with this technology, automatization of a mass-production l i n e could be e a s i l y accomplished and d i r e c t scale-up can be expected with a-Si solar c e l l s .

Figure 3 shows an exapmple of integrated solar cell deposited

on the stainless steel substrate with the role to role mass production l i n e . 3. PROGRESSOF THE CELL PERFORMANCE IMPROVEMENTS TECHNOLOGY a) Heterojunction solar ceIZ

Figure 4 shows the t r a n s i t i o n s of c e l l e f f i c i e n c y for various types of a-Si solar c e l l s since 1976.

As can be seen from t h i s f i g u r e , a s t e p - l i k e increase

of the cell e f f i c i e n c i e s is seen in the region -1981, while the slope A before 1981 corresponds to the improvement of the f i l m q u a l i t y and routine cell f a b r i cation processes.

The key technologies that produced the steep slope change

from A to B at 1981, were development of heterojunction solar c e l l s with a-SiC (hydrogenated amorphous s i l i c o n carbide) 8 and a-SiGe (hydrogenated amorphous s i l i c o n germanium) 9.

The wide gap window e f f e c t , an increase of the b u i l t - i n

T. Hamakawa I Recent advances in amorphous silicon solar cells

13.O

p o t e n t i a l and the

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tn FIGURE 4 Progress of a-S| solar cell efficiencies for various types junction structures as of April 1983. It is seen a steep slope change with appearance of new amorphous materials such as a-SiC, a-SiGe and ~c-Si around 1980.

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area p-t-n cell

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CALENOAR YEAR S ~ 8 the carbon f r a. c t i o n in a - . i , U. - x ) ,~ x and of the opzimum design theory 15 based upon the concept of the d r i f t type photovoltaic process, this

r.5. V = 881.0 ( m Y }

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type of heterojunction solar cell e f f i c i e n c y has continued to improve i t s e f f i c i e c n y with the slope B.

The recent world record

10% e f f i c i e n c y by the RCA group in 1982 is also obtained in this type of a-SiC/a-S| heterojunction 16 solar c e l l . b) Utilization of optical confinement effect

One important remining area f o r a f u r t h e r improvement of a-S| solar c e l l e f f i c i e n c y is more e f f i c i e n t c o l l e c t i o n of longer

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J-V characteristics and cell structure of the inverted p-i-n solar cell with the optical confinement electrode TiO2/Ag/SUS.

1270

72 Hamakawa / Recent advances in amorphous silicon solar cell~

wavelength photon's energy just above the band edge of a-Si, because the penetration depth of 1.8 eV phton, for example, is the order of 5 um17, While, the thickness of a-Si solar cell is only 0.6 ~Jm. A concept of e f f i c i e n t c o l l e c t i o n of long wavelength photon energy by a highly r e f l e c t i v e random surface has been f i r s t l y proposed by Boer et al in 198118, and its theoretical basis was estab19 lished by Exxon group. This concept has been extended to more e f f i c i e n t u t i l i z a t i o n of optical and c a r r i e r confinement in the multi-layered heterostructure junction. 20 Recently, Fujimoto et a121 have developed a practical technology with the cell structure of ITO/n uc-Si/i p a-Si/TiO2/A 9 plated semi-textured stainless steel. An example of J-V characteristics showing an efficiency of 9.17% is given in Fig.6.

As can be seen from the inserted c o l l e c t i o n efficiency

curve, more than 20% of Jsc improvement has been obtained by this treatment. The moon symbol 0 plotted in Fig.4 is the e f f i c i e n c y of this type c e l l . c) a-Si/poly c-S Stacked soZar cells

Another way to c o l l e c t the longer wavelength photon energy is the absorption with the stacked junction of the lower energy gap semiconductor. Because the energy gap in a-Si, 1.7-I.8 eV, is higher than that of c r y s t a l l i n e solar cell semiconductor, e.g. 0.66 eV for Ge, I . I eV for Si and 1.43 eV for GaAs, the c a r r i e r c o l l e c t i o n e f f i c i e n c y for solar radiation spectrum in a-Si solar cell is

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considerably low as compared

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with that of the crystalline basis solar cells. On the other

hand, the fabrication Of large

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on wide variety of classical semiconductors with CVD, MOCVD, MBE, Sputtering and ion plating etc..

By combining these well 0.0

developed technology with the low temperature a-Si solar cell deposition technology, i t is possible to make more high e f f i ciency a-Si basis solar cell with low cost. Quite recently, Osaka University group have

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J-V characteristics of the stacked solar cell and its junction structure, which has a longer wavelength photon absorber layer made of polycrystalline silicon.

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T. Hamakawa / Recent advances in amorphous silicon solar cells

1271

developed a new type of the stacked a-Si s o l a r c e l l deposited on p-type poly crystalline silicon. Fig.6.

The c e l l s t r u c t u r e and J-V c h a r a c t e r i s t i c s are shown in

As can be seen in t h i s f i g u r e , more than 12% e f f i c i e n c y is e a s i l y

obtained.

Although a series of systematic studies on the material s e l e c t i o n ,

and economical f e a s i b i l i t y

are now in progress, more than 15% e f f i c i e n c y would

be obtained with the stacked a-Si solar c e l l on only 1.0 L~m of GaAs or Ge t h i n f i l m deposited on s t a i n l e s s steel substrates. In the case of p o l y c r y s t a l l i n e S i l i c o n f i l m , about 20-30 um t h i c k w i l l be required f o r the conventional CVD deposition method from SiHCl 3 or SiH 4. I t is suggested here t h a t the proposed stacked s o l a r c e l l might become a d e f i n i t e promised technology f o r the high e f f i c i e n c y low cost p h o t o v o l t a i c system with a good cooperation of amorphous and c r y s t a l l i n e f i e l d s c i e n t i s t s . 4. ASPECTS OF APPLICATION SYSTEM DEVELOPMENTS On the technological task of the enlargement of c e l l area.

Sanyo22, Mitsu-

bishi 23 and Fuji 24 have achieved 7.58%, 8.25% and 7.30% r e s p e c t i v e l y with the I00 cm2 s o l a r c e l l s .

For 1 cm2 class s o l a r c e l l s , 10% by RCA16 and 7.9% by

Fuji 24 are c u r r e n t e f f i c i e n c y record on non-stacked D-i-n based s o l a r c e l l s . Another important technical task f o r the f u t u r e mass production is to increase the deposition r a t e .

Various kinds of attemptes are in progress from

source gas 25, furnace construction 26 to the arc discharge mode. 27

Hayashi's

group of ETL28 reported r e c e n t l y about 7 A/sec orowth rate by using a higher order s i l a n e gas glow with c e r t a i n c o n d i t i o n and 5.4% e f f i c i e n c y solar c e l l s . In p r i n c i p l e , amorphous m a t e r i als can be deposited onto any foreign substrate without cons t r a i n t s of the a t o m i s t i c l a t t i c e p e r i o d i c i t y so t h a t i t is possible to make a large area f i l m .

Utili-

zing t h i s c h a r a c t e r i s t i c advantage, a series o f unique t r i a l s

are in

progress a t several l o c a t i o n s .

For

example, T e i j i n company has a t t a i n e d 6.36% e f f i c i e n c y in an a-Si s o l a r c e l l deposited on f l e x i b l e polymide f i l m 29 as shown in F i g . l ( a ) .

The

s o l a r p h o t o v o l t a i c system with t h i s s o l a r c e l l does not take BOS cost by u t i l i z a t i o n structions.

of already b u i l t con-

Recently, Sharp-ECD Solar

FIGURE 7 Example of consumer applications: battery charger, portable type recorder, portable television and headphone type recorder (presented by Sanyo Electric Co. Ltd.).

T. Hamakawa / Recent advances in amorphous silicon solar cells

1272

Inc. has also announced f l e x i b l e a-Si solar c e l l s deposited on t h i n s t a i n l e s s steel sheet having the e f f i c i e n c y of 8.3% as shown in F i g . l ( b ) . A wide v a r i e t y of a p p l i c a t i o n systems are developed in a recent few years particularly, very rapidly.

consumer e l e c t r o n i c s a p p l i c a t i o n s as shown in Fig.7 are expandin! For i n s t a n c e ,

about 2.7 m i l l i o n

sets/month

o f a-Si d r i v e d p o c k e t a b l e calculators Japan.

are f a b r i c a t e d

In the f i e l d

applications,

in

o f power

several e x p e r i -

mental s o l a r houses and r o o f

I" /

top a r r a y t e s t are in operat i o n now.

F i g . 8 shows an exa-

mple o f a-Si power module

F,

o p e r a t i n g f o r the power conditioning

test,

~E 8 test o f a-Si p o w e r Fuji E l e c t r i c Co. Ltd.

REFERENCES l) for example, AMORPHOUSSEMICONDUCTOR,TECHNOLOGIES & DEVICES1982 (OHM-Sha & North-Holland, Tokyo, Amsterdam, ]982) ed. by Y. Hamakawa. 2} Y. Hamakawa: Solar Energy Mat., 8 (1982) lOl. 3) K. Okuda, H. Okamoto and Y. HamaEawa: to be published. 4) Y. Uchida, H. Sakai, M. Nishiura and H. Haruki: Proc. 15th IEEE Photovo]taic Specialists Conf., Florida (1981) 922. 5) Y. Hamakawa: Solar Energy Mat., 8 (]982) ]01. 6) H. Okamoto, Y. Nitta, T. Yamaguc~i and Y. Hamakawa: Solar Energy Mat., 2 (1980) 313. 7) A. Madan, S.R. Ovshinsky and E. Benn: Phil. Mag., B4O (1979) 259. 8) Y. Tawada, M. Kondo, H. Okamoto and Y. Hamakawa: App-~F. Phys. Lett., 39 (198l) 237. 9) Y. Higaki, M. Kato, M. Alga and Y. Yukimoto: Proc. 4th EC Photovolta~Solar Energy Conf., Stresa (]gS2). lO)S. Tsuda, N. Nak~mura, Y. Nakashima, H. Tarui, H. Nishiwaki, M. Ohnishi and Y. Kuwano: Jpn. J. Appl. Phys., 21 (1982) suppl. 22-2, 251. II)Y. Hamakawa: Surface Sci., 86 (1979) 444. 12)Y. Tawada, K. Tsuge, M. Kon~, H. Okamoto and Y. Hamakawa: J. Appl. Phys. 53 (1982) 5273. 13)S. Nonomura, H. Okamoto, K. Fukumoto and Y. Hamakawa: App]. Phys. A Inpress. 14)H.Okamoto, H. Kida, K.Fukumoto and Y. Hamakawa: J. Appl. Phys. 54 (1983) 3236. 15)H. Okamoto, H. Kida, S. Nonomura and Y. Hamakawa: Solar Cells, ~-(]g83) 317. 16)T. Catalano, A. Firestar and B. Fanghaman: Proc. 16th IEEE Photovoltaic Specialists Conf., San Diego (1982) 16-A-6. 17)F. Yamaguchi, H. Okamoto and Y. Hamakawa: Jpn. J. Appl. Phys. 20 (]980) Suppl. 20-2, ]95. ]8)W. Den Boer and R.M. Van Strijp: Proc. 4th Photovoltaic Solar ~ergy Conf., Stresa (1982) 764. 19)E. Yablonovitchi and G.D. Cody: IEEE Trans. on Electron Devices, ED-Zg (1982) 300. 20)Y. Hamakawa, Y. Tawada, K. Nishimura, K. Tsuge, M. Kondo, K. F u j i ~ S . Nonomura and H. Okamoto: Proc. 16th IEEE Photovoltai¢ Specialists Conf., San Diego (1982). 21)K. Fujimoto, H. Kawai, H. Okamoto and Y. Hamakawa: to be published in Solar Cells. 22)Y. Kuwano, M. Ohnishi, S. Nakano, T. Fukatsu, H. Nishiwaki and S. Tsuda: Proc. Japan Appl. Phys. Soci. Spring Meeting (1983) 7a-A-ll. 23)Y. Higaki, M. Kato, M. Alga and Y. Yukimoto: ibid (1983) 7p-A-6. 24)Y. Uchida, H. Sakai, M. Nishiura and H. Haruki: ibid (1983) 7a-A-3. 25)I. Shimizu: Report of Special Research Project on Amorphous MateFials 3 (198]) 146. 26)Y. Kuwano, M. Ohnishi, H. Nishiwaki, S. Tsuda, H. Shibuya and S. Nakano: Proc.15th IEEE Photovoltalc Specialists Conf., Florida (198l) 698. 27)Y. Uchida, O. Kobayashi and M. Matsumura: Proc. 3rd Photovoltaic Sci. & Enq. Conf. in Japan (]982) 85. 28)M. Yamanaka, Y. Hayashi, F. Fujino and M. Umeno: Abst. of App]. Phys., Tokyo (1982l 520. 29)H. Okanlwa, M. Asano, K. Nakatani, M. Yano and K. Suzuki: Proc. 3rd Photovoltalc Sci. & Eng. In

Japan (1982).