Interface Phenomena in Solar Cells and Solar Cell ... - ACS Publications

16 Interface Phenomena in Solar Cells and Solar

Downloaded by NORTH CAROLINA STATE UNIV on December 10, 2012 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/ba-1976-0158.ch016

Cell Development Processes W I G B E R T SIEKHAUS Materials and Molecular Research Division and Energy and Environment Division, Lawrence Berkeley Laboratory, University of California, Berkeley, Calif. 94720

Processes on interfaces critically influence the formation and performance of thin film as well as single crystal silicon solar cells. The present limitations of the single crystal ribbon growth process are resolvable through surface reactions at the gas/liquid silicon interface. The diffusion of carbon substrate material into thin film silicon can be measured as a function of substrate crystallinity by Auger depth profiling. An ultrathin layer of aluminum affects thin film crystallinity. Impurities are distributed between grain boundaries and crystallites in thin films and may affect the chemical vapor deposition mechanism. Electronic states measured by electron loss spectroscopy strongly depend on surface structure and impurities.

T j h o t o v o l t a i c electric power generation w i l l become economical i f the cost of the generating equipment can be brought to ^ $2.40/m times efficiency ( % ) ( 1 ). There are two options available if one wants to achieve that goal (Figure 1 ) : to develop inexpensive concentrators i n conjunction with high cost, high efficiency cells or to develop inexpensive cells. Both options have a multitude of interface problems. F o r the first option, inexpensive optical coatings or lenses must be developed with high reflectivity or transmissivity throughout the solar spectrum. These properties must be preserved for 30 years in an atmosphere made corrosive by air pollution. That is a formidable interface problem. H i g h concentration factors imply collectors tracking the sun. These cannot be built for less than $100/m . Consequently, solar cells must be developed with efficiencies higher than 2 5 % . This appears to be achiev2

2

334 In Radiation Effects on Solid Surfaces; Kaminsky, M.; Advances in Chemistry; American Chemical Society: Washington, DC, 1976.

16.

SJJEKHAUS

335

Interface Phenomena in Sofor Cells

able only b y multilayered cells i n which materials are stacked w i t h increasing band gap energy. A t the layer interfaces of such cells, prob lems of interdiflusion and mismatch of optical properties must be controlled.


ECONOMIC PHOTOVOLTAIC POWER GENERATION $ 1 . 5 0 / m 2 x E f f i c i e n c y (%)

High

Concentration (>Ί000χ)

No

- • T r a c k i n g Concentrators a t $15/m2. S o l a r C e l l Cost Immaterial

(low) Concentration

S i l i c o n , 10% E f f i c i e n c y , $15/m2

Main C o s t i n P u r i f i c a t i o n and C r y s t a l

Growth

X Inexpensive Techniques f o r P u r i f i c a t i o n and C r y s t a l Growth

Figure 1.

T h i n F i l m C e l l Formed by Chemical Vapor D e p o s i t i o n

Cost reduction options

The second option, development of inexpensive solar cells, is our concern. Examples w i l l be restricted to silicon, the material presently receiving the most attention. The present cost of silicon solar at 1 2 % efficiency is $7000/m , and radical changes i n the techniques of cell formation are necessary before the cost can be substantially reduced. T w o techniques are presently under development—growth of thin single crystal ribbons and growth of thin films on various substrates. 2

The EFG Process I n the edge fed growth ( E F G ) ribbon process (Figure 2 ) , l i q u i d silicon rises i n a size-defining capillary slit and forms—with the top of the capillary at the bottom and a solid silicon ribbon at the top—a meniscus from which the ribbon is withdrawn at 2.5 c m / m i n . T h e ribbon is ~ 200/A thick and 2.5 cm wide. There are three interfaces involved i n this process—the liquid silicon-solid silicon interface, the liquid s i l i c o n gas interface, and the capillary-liquid silicon interface. The capillary is usually made of carbon, which goes into solution at the bottom of the capillary and segregates at the meniscus, forming S i C crystallites at the

In Radiation Effects on Solid Surfaces; Kaminsky, M.; Advances in Chemistry; American Chemical Society: Washington, DC, 1976.


336

RADIATION E F F E C T S O N SOLID SURFACES

Figure 2.

EFG single crystal ribbon growth

top of the capillary which change its and the ribbon s shapes. The crys tallites occasionally detach themselves and are included into the ribbon. This is at the moment a serious, unsolved interface problem w h i c h must be controlled. A t the liquid silicon-solid silicon interface, solidification proceeds rapidly, ^ 10" sec/monolayer, which leaves little time to 6

DxlO

Impurity

cm

5

/sec

Β

33±4

Ρ

27±3

Sb

14±5

Ga

6.6±0.5

In

1.7±0.3

Ca

2.3

Diffusion

Figure 3.

2,

data,

Vol. 3, Vol. 4,

p. p.

86 144

Impurity diffusion coefficients in liquid silicon at 140° C



16.

siEKHAus

Interface Phenomena in Solar Cells

337

correct stacking faults, but w h i c h is slow enough for impurities to segregate and diffuse into the meniscus (Figure 3 ) . I n contrast to the Czochralsky process, these impurities cannot return to the melt, but rather are accumulated i n the meniscus. They influence meniscus surface tension, which changes the shape of the meniscus, and apparently introduce periodic fluctuations i n ribbon shape, ribbon purity, and crystallinity. T o eliminate this, and possibly also the S i C carbide formation problem, we propose to use the liquid silicon-gas interface at the meniscus to remove impurities by gas-surface reactions. Figure 4 shows the process, i n which impurities concentrated i n the meniscus are segregated to the

Figure 4.

Edge fed growth surface segregation purification process

meniscus surface and removed by reactions with a gas. A t a meniscus height of 0.5 cm, a ribbon thickness of 200/x, and a gas-surface reaction time of 1 msec, an impurity concentration of ^ 1% could potentially be removed through the meniscus interface. This surface segregation p u r i fication process may allow crystal growth from low grade silicon, significantly reducing cost. Alternately, it may permit growth of extremely pure, single crystal ribbons w i t h a very low density of crystal imperfections. There are three problem areas i n which research must be carried out before the E F G surface segregation purification process ( E F G - S S P P )


338


An economically competetive solar cell consists of a substrate on which a 100 μ thick silicon layer is deposited, covered by a transparent conductor containing a metallic grid. The problems to which research is directed are identified in the cell by the letters a, b, b\ c.


Transparent Conductor

Substrate Research Areas: a) Low temperature catalysis of the decomposition of SiH4, S1CI4 on a low melting point substrate material (e.g. AI) to form large area silicon sheets. Research tool: molecular beam deposition. b) Catalysis of large grain crystallization by use of suitable impurities. Segregation of impurities onto grain boundaries. Research tool: low energy electron diffraction, Auger spectroscopy. a,b,b',c. Dependence of electronic surface states on impurities. Research tool: photoelectron spectroscopy, energy loss spectroscopy.

Figure 5.

Thin silicon film solar cell design and related research areas

», b 0

5fe

40

60

&

Ï0Ô

_ Ϊ20"

SPUTTERING TIME, MIN

c . Ï4Ô" Tl>6 Ï8Ô" |

Figure 6. Interdiffusion at the carbon-silicon interface. Auger depth profiles of nigh temperature deposited (1150-1200°C) silicon films on (above) (a) edge (prism) plane of pyrolytic graphite, (right) (b) basal plane of pyrolytic graphite, (c) extraded graphite, and (a) glassy carbon.




CO

ο

Ο

r

3"

ϊ f

CO

f

»—'


340

can be perfected—surface segregation of impurities, effect of impurities on surface tension, and g a s / l i q u i d metal impurity surface reactions.


Tbm Mm Cells The other option to reduce silicon solar cell cost is to develop thin film solar cells. There are interface problems encountered there also. Figure 5 shows a thin film cell which may soon become economical and lists the interface problem areas to which research must be directed. The cell consists of a conducting substrate upon which silicon is deposited, preferably by chemical vapor deposition, forming crystallites comparable i n size with the film thickness ( 100/A). The p-n junction is formed during deposition by changing the dopant i n the carrier gas. A transparent (semi) conducting layer is deposited on top of the silicon film. This layer contacts each grain so that current transport across grain bounda ries is minimized. A metallic grid is imbedded i n the transparent layer to reduce its sheet resistance. In an alternate cell design, the transparent conductor is one part of a silicon-transparent conductor heterojunction. Interface Diffusion. A serious difficulty i n thin film development is the diffusion of substrate material into the silicon layer. A high tempera ture material suitable as a substrate is carbon. Figure β summarizes the silicon-carbon interdiffusion for various allotropie forms of carbon ( 2 ) ; carbon diffuses deeply into silicon, and for a l l subtrates except glassy carbon, silicon also diffuses into carbon. I n both cases, a high resistivity S i C layer is formed, which reduces cell efficiency. Since many attempts to b u i l d diffusion barriers have failed, it is highly desirable to investigate whether chemical vapor deposition and growth of large crystal grains can be accomplished at a lower tempera-

/////Si/////

SiH -*[SiH -]-»Si + 2H 4

SiCl

3

4

+ ZH -»[SiCl -]-*Si 2

3

1100°C

2

+

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S i H + B ^ - ^ I S i R j - j - ^ S i + Β + 5H. 4

Figure 7.

1200°C 300-600°C

Chemical vapor deposition reactions


16.

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«• 1155

1060 977 903

838

779 727

T'C

679

626

ΔΒ Η

1 0


341


2

596

560

527

B/Si=376x10'

A

6

Journal of the Electrochemical Society

Figure 8. Influence of deposition temperature on the growth rate of polyerystalline silicon films without and with additional doping of AsH , PH , and B,H s

S

6

ture. Figure 7 shows the reaction equations of two reactants, S i H and S1CI4, and points out that addition of diborane ( B H ) to the chemical vapor allows deposition at a substantially lower temperature. The actual reaction mechanism is now being investigated ( 3 ) , b u t much more research using modulated molecular beam spectroscopy is needed before it can be determined how the deposition mechanism is influenced b y impurities i n the gas stream. Figure 8 shows that a p-type impurity increases the deposition rate while a η-type dopant decreases the deposi tion rate. Since both S i H and S1CI4 are tetrahedral molecules carrying a slight negative excess charge on the hydrogen atoms, i t is tempting to propose (Table I ) (5) that the p-dopant forms a charge layer at the surface which attracts such molecules, whereas η-dopants form a repulsive layer. Table I shows that a prediction of the model, the reduction of the deposition rate i n the presence of B H of diamond from C B U ( a molecule w i t h a slight positive excess charge on the hydrogen atoms), is verified by the literature. Experiments to study the influence on the deposition rate of surface charges induced b y electric fields are presently carried out i n our laboratory. There is, i n summary, justifiable optimism that C V D deposition rates may be catalyzed to proceed at lower temperatures. 4

2

e

4

2

e


342


Low Temperature Crystal Growth. Crystal growth at temperatures much below the melting point of silicon can be achieved i f the silicon surface is covered during deposition by a thin layer of a low-melting point liquid alloy of silicon (vapor-liquid-solid ( V L S ) techniques). Experiments have shown that both on S i 0 (6) and pyrolytic graphite (7), crystal grain size and crystal orientation is significantly improved if Si deposition is preceded by the deposition of a thin aluminum layer (-500 A). Downloaded by NORTH CAROLINA STATE UNIV on December 10, 2012 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/ba-1976-0158.ch016

2

Figure 9. Electron diffraction micrographs of (a) silicon film deposited on quartz at 600°C and (b) silicon film deposited on quartz at 600°C with prior coating of a Si-AC-Si layer Figures 9a and b compare diflEraction electron micrographs of Si films deposited at 600°C without (a) and with (b) the predeposition. Figure 10, an Auger depth profile of such a film on quartz, confirms that


16.

siEKHAus

343


Table I. Predicted and Observed Dopant Gas Effects on the Deposition Rates of Silicon and Carbon Chemical Vapors 0

SiH

SiCli

t

Bonding characters B2H6


PH

Si*-HI I' D D* D D>

3

AsH

3

ecu c*-cr I

D

C--H* D D* I

D

I

D

si*-α ϊ

V

D

I and D indicate an increase and decrease, respectively, in deposition rate. T h e first row for each dopant gas is for the predicted effects, the second row for the observed effects. * Data from Ref. 9. Data from Ref. 10. β

9

the aluminum initially deposited onto the quartz surface is carried along during deposition and concentrated on top of the silicon layer. Figure 11 shows a similar deposition at 600°C on a graphite surface. Aluminum is apparently distributed uniformly throughout the silicon layer and even diffused into the graphite substrate. A clear silicon-graphite interface is formed w i t h little interdiffusion, and no S i C formation is observable i n the Auger spectrum. The depositions reported i n Refs. 6 and 7 were done using silicon evaporation. Further work w i l l show whether this technique can be used at a l l during silicon vapor deposition from silicon.

/

x

\

X

' - V..:.

-ν·

-I Ο

Al5 X

V·*"".".·/·.. .

I 20

L 40

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I 00

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\

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_

l 100

Ι 120


Figure 10. Auger depth profile of a silicon film deposited at 600°C onto a thin aluminum layer on a quartz substrate


344



C2.5X

AI5X

Si Ο

20

40

_ J 60

I 80

I 100

Ι 120


Figure 11. Auger depth profile of a silicon film deposited at 600°C onto a graphite substrate covered with a thin aluminum layer Grain Boundary Effects. It is likely that even under the best condi tion, thin films w i l l contain grain boundaries, although probably substan tially fewer than today. T o assess their influence on the electrical per formance and the time-stability of thin films, one needs to know the distribution of impurities at the grain boundaries. Experiments i n our laboratory to determine the impurity distribution b y Auger spectroscopy of surfaces created b y fracture along grain boundaries have not been successful thus far because silicon cleaves easily. There is, however, indirect evidence that at least doping impurities are to be found i n high concentration along grain boundaries of films formed b y chemical vapor deposition. Figure 12 (8) shows that while there is an almost linear relationship between the donor concentration i n the solid and the dopant concentration during chemical vapor deposition, the donor concentration i n polycrystalline films falls abruptly b y five orders of magnitude as the dopant concentration is reduced below one part i n 10,000. T h e authors show then ( 8 ) , that the total concentration i n the chemical vapor: N

T

D

= phosphorous/silicon 0.35 Χ 1 0

22

(1)

and, starting from the assumption that a l l grain boundaries are covered with dopants before any dopant is incorporated into the grain, that the electrically active dopant concentration:


16.

siEKHAus

345

Interface Phenomena in Solar Cells = iV

Ν

Ώ

D T

- N

(2)

GB

can be fitted nicely to a relationship (Figure 13) : N


D

= Ν

ΌΎ

4.05 Χ 10 S

-

(3)

20

where S (S — 0.024) is the fraction of the surface atoms covered or associated with an electrically inactive dopant atom. A n average grain size of cubic shape is assumed to define the number of surface atoms. Surface States. T o analyze surface and interface states associated w i t h all boundries i n cells, polycrystalline or single crystal, we use elec tron loss spectroscopy, which, i n conjunction w i t h uv spectroscopy, allows definition of empty and filled surface states. Figure 14 shows the loss spectra of the silicon (111 — 2 X 1) surface as a function of oxygen exposure. O n the clean surface, bulk and surface plasmons were observed at loss energies of 16.5 eV and 10.6 eV, respec tively. The 5-eV transition is possibly a bulk transition. Three surface

10 ι

1

1

1

1

1

Γ

P/Si ATOM RATIO IN VAPOR

Figure 12. Donor concentration, N vs. Ρ7Si atom ratio in vapor for poly Si samples deposited on thermal SiO substrates. Deposited at 650°C from SiH. (Δ), deposited at 840°C from SiBr -H (A). Phosphorusdoped, single-crystal silicon. D

s

A

2


346


state transitions were observed, S i at 2.5 eV, S at 7.4 eV, and S at 14.5 eV. The S transition at 0.6 eV, as seen i n high resolution energy loss spectroscopy, is lost i n the elastic background. However, according to recent theoretical calculations, the S and S i transitions have similar initial states and are therefore directly correlated. B y exposing the surface to oxygen, the S and S transitions are effectively removed at a coverage of 0.1 monolayer while S increases. A t the same time, a loss peak at 3.3 e V appears and becomes well established at monolayer coverage. This transition lies far below the bandgap and is not important to the electrical performance of S i cells. It is apparent that interfaces introduce a multitude of electronic states and that these can be controlled by adsorption or segregation of suitable impurities onto such interfaces. W e are continuing to investigate the effect of impurities likely to be found i n practical cells onto electronic 2

3

0

0

±

3


2

10

20

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Ε

Ζ

H O 10",19 U

ζ ο ι-

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ιο

18

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ο

/ ID ο χ CL CO

ο χ

ir

10

17

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id

10

3

P/Si ATOM RATIO IN VAPOR

Figure 13. Total phosphorus concentration, N , and donor concentration, N , vs. P/Si atom ratio in vapor for poly Si deposited from SiH at 650°C. Total phos phorus concentration (A), donor concentrations (Δ)— see Equation 1, calculated from Equation 3. T

D

D

A



16.

siEKHAus

347

Interface Phenomena in Solar CeUs

Figure 14. Effect of oxygen adsorption on the strength and position of energy loss peaks as a function of Θ, oxygen coverage. (Θ=1 represents one monolayer). The most effective trapping state S (tied to S ) close to the top of the valence band is clearly eliminated by oxygen adsorption. The abscissa gives the loss energy in eV, measured from the elastic peak, and the shaded bands are absorp tion bands as measured by uv transmission spectroscopy of S 0 molecules. t

0

t

interface states. T h e following interface problems are encountered i n thin film cells. ( 1 ) Catalysis of chemical vapor deposition (2) Catalysis of crystallization ries

(3) Impurity segregation on surfaces, interfaces, and grain bounda (4) Control of surface and interface states

( 5 ) Control of interface diffusion Research i n our and other laboratories is directed toward solving or circumventing them. . « · »

American Chemical Society Library

St.. N.W. Kaminsky, M.; In Radiation1155 Effects16th on Solid Surfaces; Advances in Chemistry; American Chemical Society: Washington, DC, 1976. Wishing**. DC. 20036

348



Literature Cited 1. Wolf, Martin, IEEE Photovoltaic Specialist Conf. 9th, 1972. 2. J. Appl. Phys. (1975) 46, 3402. 3. Farrow, R. F. C., J. Electrochem. Soc., Solid-State Sci. Technol. (1974) 121 (7), 889. 4. Eversteyn, F. C., Put, Β. H., J. Electrochem. Soc., Solid-State Sci. Technol. (1973) 120 (1), 107. 5. Chang, Chin-An, J. Electrochem. Soc. (1976) 123 (8), 1245. 6. Chang, Chin-An, Siekhaus, W. J., Kaminsky, T., Appl. Phys. Lett. (1975) 26 (4), 178. 7. Chang, Chin-An, Siekhaus, W. J., Appl. Phys. Lett. (1976) 29 (3), 208. 8. Cowher, M. E., Sedgwick, T. O., J. Electrochem. Soc., Solid-State Sci. Technol. (1972) 11, 1565. 9. Cotton, F. Α., Wilkinson, G., "Advanced Inorganic Chemistry," p. 466, John Wiley & Sons, New York, 1966. 10. Poferl, D. J., Carducer, N. C., Augus, J. C., J. Appl. Phys. (1973) 44, 1428. RECEIVED January 5, 1976. Work supported by U.S. Energy Research and velopment Administration.


De

Interface Phenomena in Solar Cells and Solar Cell ... - ACS Publications

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