DRIFT TYPE PHOTOVOLTAIC EFFECT IN a-Si p-i-n JUNCTION

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DRIFT TYPE PHOTOVOLTAIC EFFECT IN a-Si p-i-n JUNCTION

H. Okamoto, T. Yamaguchi, S. Nonomura, Y. Hamakawa

To cite this version:

H. Okamoto, T. Yamaguchi, S. Nonomura, Y. Hamakawa. DRIFT TYPE PHOTOVOLTAIC EF- FECT IN a-Si p-i-n JUNCTION. Journal de Physique Colloques, 1981, 42 (C4), pp.C4-507-C4-510.

�10.1051/jphyscol:19814108�. �jpa-00220723�

JOURNAL DE PHYSIQUE

CoZZoque

C4,

suppZ6ment au nO1O, Tome

42,

octobre 1981 page

C4-507

D R I F T TYPE PHOTOVOLTAIC EFFECT I N a-Si p-i-n JUNCTION

H. Okamoto, T. Yamaguchi, S. Nonomura and Y. Hamakawa

FacuZty of Engineering

Scielzce,

Osaka University, To'oyonaka, Osaka, Japan

A b s t r a c t . - P h o t o v o l t a i c p r o p e r t y and r e l a t e d processes i n an a-Si:H p - i - n j u n c t i o n have been examined by t a k i n g account t h e i n t e r n a l e l e c t r i c f i e l d d i s t r i b u t i o n . Through t h e t h e o r e t i c a l c o n s i d e r a t i o n s combined w i t h experimental data on t h e c o l l e c t i o n e f f i c i e n c y s p e c t r a , quantum e f f i c i e n c y and e f f e c t i v e h o l e d i f f u s i o n l e n g t h have been evaluated t o be a t l e a s t 0.85 f o r fiw>2.leV and 1000A, r e s p e c t i v e l y . An a n a l y s i s has been a l s o made on t h e p h o t o v o l t a i c performance o f an i n v e r t e d p - i - n j u n c t i o n c e l l , and which c o n f i r m s an

e f f i c i e n t p h o t o v o l t a i c o p e r a t i o n n o t so much d i f f e r e n t from a p - i - n j u n c t i o n .

I n t r o d u c t i o n . - A g r e a t deal o f e x t e n s i v e e f f o r t s p a i d on t h e f a b r i c a t i o n technology o f amorphous s i l i c o n s o l a r c e l l s has a l r e a d y r e a l i z e d a conversion e f f i c i e n c y on

t h e p r a c t i c a l l y f e a s i b l e l e v e l [ l ] . Despite, as regards t h e u n d e r l y i n g p h y s i c s o f t h e p h o t o v o l t a i c e f f e c t , any s u f f i c i e n t understandings have n o t been given y e t . The p h o t o c u r r e n t g e n e r a t i o n g e n e r a l l y i n v o l v e s t h r e e b a s i c processes; l i g h t a b s o r p t i o n , p h o t o - c a r r i e r g e n e r a t i o n and c o l l e c t i o n processes[2]. Owing t o a c o n s i d e r a b l y small c a r r i e r m o b i l i t y i n h e r e n t t o amorphous m a t e r i a l s , t h e l a t t e r two processes a r e q u i t e d i f f e r e n t from those f a m i l i a r i n conventional c r y s t a l l i n e semiconductors[3]. That i s , i t has been b e l i e v e d t h a t n o t o n l y geminate recombination has a s i g n i f i c a n t i n f l u e n c e on t h e p h o t o - c a r r i e r g e n e r a t i o n process[4,5] but a l s o non-geminate recorn- b i n a t i o n becomes a s t r o n g o b s t a c l e t o t h e c a r r i e r c o l l e c t i o n [ 6 ] . Moreover, because o f an e x i s t e n c e o f c o n t i n u o u s l y d i s t r i b u t e d gap s t a t e s , t h e j u n c t i o n p r o f i l e and corresponding i n t e r n a l e l e c t r i c f i e l d d i s t r i b u t i o n would be f a r more complicated compared w i t h t h e case i n c r y s t a l 1 i n e j u n c t i o n s [ 7 , 8 ] .

I n t h i s paper, under t h e r e c o g n i t i o n o f these p e c u l i a l i t i e s , an c a r e f u l exa- m i n a t i o n i s made an t h e p h o t o v o l t a i c p r o p e r t y o f amorphous hydrogenated s i l i c o n

(a-Si:H) i n a p - i - n j u n c t i o n s t r u c t u r e through t h e t h e o r e t i c a l c o n s i d e r a t i o n s com- bined w i t h v a r i o u s experimental data. I n t h e course o f these a n a l i s e s , p h y s i c a l parameters r e l a t i n g t o p h o t o - c a r r i e r g e n e r a t i o n and c o l l e c t i o n processes a r e d e t e r mined. The p h o t o v o l t a i c p r o p e r t y i n an i n v e r t e d p - i - n j u n c t i o n c e l l w i l l a l s o

be discussed i n comparison w i t h a p - i - n j u n c t i o n c e l l .

P h o t o v o l t a i c e f f e c t i n a-Si:H D - i - n j u n c t i o n c e l l s . - The p h o t o c u r r e n t J can be expressed as a f u n c t i o n o f t h e photon energy 3, and a p p l i e d v o l t a g e V ~ [ ! ' , ] ] ,

where t h e i n t e g r a t i o n w i t h respect t o t h e p o s i t i o n X i s proceeded w i t h i n t h e range o f an a c t i v e l a y e r ( i - l a y e r ) from x=O t o di. @(x,hw) i s a photon f l u x d i s t r i b u t i o n and i s o b t a i n e d f o r a s e t o f geometrical and o p t i c a l parameters o f l a y e r s composing t h e c e l l . P (x,hw;V ) i s a p h o t o - c a r r i e r g e n e r a t i o n p r o b a b i l i t y , t h a t i s , quantum e f f i c i e n c y For p h o t g - c a r r i e r generat ion. According t o t h e Onsager's t h e o r y , t h i s f a c t o r can be represented i n terms o f t h e i n t e r n a l e l e c t r i c f i e l d E(x;V ) , Onsager r a d i u s r (46A f o r a-Si :H) and a thermal i z a t i o n d i s t a n c e r (3w) o f p h o t g e x c i t e d e l e c t r o n % o l e p a i r s [ 9 ] . While, a p h o t o - c a r r i e r c o l l e c t i o n E r o b a b i l i t y P (x;V ) i s g i v e n as an extended formula o f o u r p r e v i o u s one[3] by assuming a complete sank f o r C

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19814108

JOURNAL DE PHYSIQUE

h o l e s a t x=O w h i c h corresponds t o t h e i n t e r f a c e between t h e p- and i - l a y e r ;

r

here, f k ( x ) = e x p [;

J ~ X

^L g ' ( ~ ) d f l , q

L

gi(x)=&+:(x), y(x)=+ E(x;va)

L denotes an e f f e c t i v e h o l e d i f f u s i o n l e n g t h w h i c h t a k e s i n t o account t h e a m b i p o l a r t e a n s p o r t w i t h a p r o b a b l e i n f l u e n c e o f t r a p s .

As i s e a s i l y accepted, one s h o u l d examine f i r s t l y t h e i n t e r n a l e l e c t r i c f i e l d E(x;Va) i n t h e i - l a y e r w h i c h i s e s s e n t i a l l y c o r r e l a t e d w i t h t h e d e n s i t y o f s t a t e s i n t h e gap because i t would p l a y an i m p r o t a n t r o l e i n d e t e r m i n g t h e p h o t o v o l t a i c p r o p e r t y t h r o u g h t h e p h o t o - c a r r i e r g e n e r a t i o n and c o l l e c t i o n processes. F i g u r e l (a ) and ( b ) d i s p l a y examples o f space charge p ( x ) and i n t e r n a l e l e c t r i c f i e l d d i s t r i b u t i o n E(x;O), r e s p e c t i v e l y o b t a i n e d by s o l v i n g P o i s s o n l s e q u a t i o n w i t h r e a l i s t i c boundary c o n d i t i o n s and p h y s i c a l parameters summerized i n t h e f i g u r e . Here, an e x p o n e n t i a l l y d i s t r i b u t e d gap s t a t e s i s assumed w i t h a minimum gap s t a t e s d e n s i t y g in and c h a r a c - t e r i s t i c energy ^E [ 1 0 ] . One o f t h e n o t i c e a b l e f e a t u r e s found i n E ( X ; O ~ I S an e x i s t e n c e o f a h i g h i n t e r n a ? e l e c t r i c f i e l d i n excess o f 104v/cm t h r o u g h o u t t h e i - l a y e r i n an a-Si :H p - i - n j u n c t i o n c e l l o f a c t u a l d i m e n s i o n [ l l ] . P h o t o - c a r r i e r g e n e r a t i o n and c o l l e c t i o n p r o b a b i l i t y c o r r e s p o n d i n g t o t h i s E(x;O) a r e demonstrated i n F i g . 2 ( a ) and (b)

,

r e s p e c t i v e l y . From F i g . 2 (b)

,

a " d r i f t t y p e " p h o t o v o l t a i c b e h a v i o r o f a-Si :H j u n c t i o n s [ 8 1 can be c o n f i r m e d , where a h i g h i n t e r n a l e l e c t r i c f i e l d e f f e c t i v e l y enhances a d r i f t component o f t h e p h o t o c u r r e n t and r e s t r i c s non-geminate r e c o m b i n a t i o n l o s s a r i s e n f r o m a v e r y s m a l l c a r r i e r d i f f u s i o n l e n g t h .

F i g u r e 3 ( a ) and ( b ) show v a r i a t i o n s o f t h e i n t e r n a l e l e c t r i c f i e l d E(x;v ) i n t h e i - l a y e r c o r r e s p o n d i n g t o t h o s e o f t h e i - l a y e r t h i c k n e s s and f o r w a r d b i a s v81 t a g e V

,

r e s p e c t i v e l y . These r e s u l t s p u t f o r w a r d a new r e c o g n i t i o n t h a t t h e concept o f d g p l e t i o n r e g i o n c o n s i d e r e d i n c r y s t a l l i n e j u n c t i o n s n o l o n g e r h o l d s good i n t h e case o f a - S i j u n c t i o n s , i n s t e a d , an o v e r a l l change i n E(x,V ) s h o u l d be t a k e n i n t o account i n response t o t h e v a r i a t i o n o f c e l l demension o r beas v01 tage.

1

r. = l s o a

loo&

-

S O B

-

L.- 2 5 8

( a ) rc= 46 A

-

-

^{- 1 0 -}

2

^no

Eg(opt)=t 7 8 e V ( b ) I E ~ - E F ~ = O 8 5 e V I

g

AEF" = 0 6 5 e V

I

& E F p = - 0 4 3 e V

-

9

I

^{0 1 0 7 m 3 m 40W SW0}

W POSITION X ( A )

-P ,-- i

:;.

^n-

^Fig.

²

Plots of photo-carrier generation

PG

( a ) and collection probability

PC

(bl Fig.1 Ezamples of space charge density as parmeters of a t h e m a l i z a t i o n dis- p(z;o) ( a ) m d internal e l e c t r i c f i e l d tance ro and e f f e c t i v e hole diffusion E(z;O)

( b )

i n m a-Si:H p-i-n junction. length

LP,

respectively.

(a)

0

2

-

U @

E 0 3

-

2

-

m 2 0 6 0 a 2 0 4 0

,-

X g 0 2

5

0

Fig.

3

Vaariations of internal e Zectric field

E(x; Va)

corresponding t o those o f i-Zcger thickness 4 ( a ) and fomard bias voltage

Va

( b ) .

-

Geminate and non-qeminate recombination i n a-Si:H.- I n t h e f o l l o w i n g d i s c u s s i o n , f i r s t l y , t h e p h o t o - c a r r i e r g e n e r a t i o n p r o b a b i l i t y i s assumed t o be constant through-

O S , o u t t h e i - l a y e r i n t h e e x t e n t o f t h e cases encountered i n t h e usual o p e r a t i o n o f an a-Si:H p - i - n j u n c t i o n c e l l o f a c t u a l d i m e n s i o n [ l l ] . Coupling t h e r e s u l t i n Fig.3 w i t h eqs. ( l ) and ( 2 ) , an i - l a y e r t h i c k n e s s dependence o f t h e c o l l e c t i o n e f f i c i e n c y f o r monochromatic l i g h t i s d e r i v e d , which i s i l l u s t r a t e d i n Fig.4 w i t h a parameter o f e f f e c t i v e h o l e d i f f u s i o n l e n g t h L

.

A comparison be- - tween experimental data[2,11] and t h y s c a l c u l a t e d one

y i e l d s an e v a l u a t i o n f o r L o f about 1000A. A s i m i i a r - procedure i s a l s o a p p l i e d 8 i t h t h e r e s u l t i n ~ i ~ . 3 ( b ) 50oA t o g e t t h e p h o t o c u r r e n t versus forward b i a s v o l t a g e V

durves. F i g u r e 5 shows these dependencies f o r two a Ob 0; o i oS6 o'8 1 0 d i f f e r e n t i l l u m i n a t i o n wavelengthes w i t h corresponding

I-LAYER THICKNESS dl (pm) experimental data o b t a i n e d i n s e v e r a l e f f i c i e n t p - i -n

fi9.4 Relation b e b e e n an i-

j u n c t i o n c e l l s [ l 2 ] under v a r i o u s b i a s l i g h t c o n d i t i o n s .

l a y e r thickness 4 m d

photo-This examination a l s o supports an L o f around 1000A.

-rent

J L

as a parmeter of

I t i s added here t h a t an experiments1 J - V curve f o r

e f f e c t i v e h o Z e d i f f u s i o n

a n i ~ l u m i n a t i o n o f f u r t h e r s h o r t w a v e l e k g t ~ l i g h t ~ ~ ~ l d

length $.

n o t be f i t t e d by L o f lOOOA and t h i s m i g h t imply an

e f f e c t i v e l o w e r i g g o f LP near t h e p / i i n t e r f a c e . On t h e o t h e r hand, through t h e same way desc-

1.0 3 = 5 5 o o d r i bed i n o u r p r e v i o u s papers [2,3], t h e p h o t o - c a r r i e r g e n e r a t i o n p r o b a b i l i t y ~ ~ ( f i w , E ) can be e s t i m a t e d by

-

a t t r i b u t i n g a discrepancy observed between experimen-

0 ,209 - t a l and c a l c u l a t e d c o l l e c t i o n e f f i c i e n c y s p e c t r a t o

> geminate recombination. The r e s u l t i s summerized i n

-

^-

-. ^-

Fig.6. That i s , z e r o - f i e l d quantum e f f i c i e n c y PG(Tw,O)

Lp(A)

0 8 - a 1500 . d e f i n e d by e x p [ - r / r ] e x h i b i t s a sharp r i s e from

b 1200 the photon energychi;her than t h e o p t i c a l gap energy

-

^{c I O W} - Theory

d 900 --- Exper~rnents and exceeds about 0.85 f o r X w > 2 . l e V . Here, a s l i g h t

a e 700 decrease o f PG i n t h e photon energy r e g i o n o f h i g h e r

:2=630bA

than 2.2eV might i n d i c a t e an e x t r a non-geminate t i o n l o s s i n t h e v i c i n i t y o f p / i i n t e r f a c e n o t taken i n t o c o n s i d e r a t i o n i n t h e present

P - i - n j u n c t i o n c e l l . - I n t h i s s e c t i o n , we i n e how a small h o l e d i f f u s i o n l e n g t h would

t h e p h o t o v o l t a i c performance o f an i n v e r t e d

c: l 0 0 0

- d . 900 -Theory c t i o n c e l l where m s t o f photo-holes a r e

t e . 7 0 0

--

Expertrnents

, ^6,

^'

^Fig.

⁵

Photocurwnt

JL

versus fo~Ward bias voltage

Va

APPLIED VOLTAGE va (Y)

e w e s for i ZZwnination wavelengthes

of 6300 and 5500A.

g

E l - Layer Thickness =5000h

=105-

- .

E

3 .

: i-Layer Thickness

0 2 0.3 ~ ~O.L 0.5 0.6 0.8-

i 2

-

v

W .

=

. ( a ) , ^'

v

=O

:

10; I

2000 4000 6 0 0 BOM)

d

( b )

POSITION x ( A ) lo30 I&O 2doo 3600 40bo 5000

POSlTlON X ( A )

C4-510 JOURNAL DE PHYSIQUE

1.0 ^I generated near t h e i / n i n t e r f a c e and SO

...

..

.O' o... ~ , = r o o o ~

O r ,Y

0 8

io6-

5

5 0 4 - t

3 0 2 -

o b l iged t o pass through whole t h e i - l a y e r .

0.9

-

--.

-

F i g u r e 7 shows t h e c a l c u l a t e d c o l l e c t i o n

no e f f i c i e n c y s p e c t r a f o r v a r i o u s p - i - n and

m

i n v e r t e d p - i - n j u n c t i o n c e l l s . Here, Ae and Aa denote t h e Fermi l e v e l s h i f t s dopingFfn n- and p - l a y e r , r e s p e c t i v e l y .

' These c a l c u l a t i o n s a r e c a r r i e d o u t w i t h r e a l i s t i c c e l l c o n s t r u c t i o n parameters by

-

t a k i n g account o f t h e a b s o r p t i o n losses i n

I Moll *l a1

.

p - and n - l a y e r s [ l l ] . As can be seen i n t h e

g

W ^{0 5 -} - * - g ( E . 0 ) f i g u r e , an i n v e r t e d p - i - n j u n c t i o n c e l l

~ , ( o P I ) ...-o....

Crandall PG(E=Eminl e x h i b i t s an i n f e r i o r p h o t o c u r r e n t t o t h a t

I o f a p - i - n j u n c t i o n c e l l so f a r as a s l i g h t l y 0.4

l'

_{1.8 2.2 2.6} ^I

I

310

' d o p e d i - l a y e r i s employed. However,a s u f -

PHOTON ENERGY 3rd ( e V ) f i c i e n t l y doped p - l a y e r would improve t h e p h o t o c u r r e n t o f an i n v e r t e d c e l l , a t l e a s t ,

Fig.6 Phoh-caprier generation Poba-

up t o t h e same degree as i n a p - i - n j u n c t i o n

b i l i t ~

P G ( m , E )

as a fwzction o f

c e l l . While, o u r p r e l i m i n a r y a n a l y s i s on

t a t i o n photon energy a.

_J -V curves i n d i c a t e s t h a t an i n v e r t e d p-

i - n j u n c k i o t c e l l e s s e n t i a l l y i n v o l v e s a lower f i l l f a c t o r as compared w i t h p - i - n j u n c t i o n c e l l s .

0 ~ ' 4 " 0 5 " 0 6

WAVELENGTH X (pm) 0 7 0.85 and i s enhanced up t o 0.95-0.98 by t h e i n t e r n a l e l e c t r i c f i e l d f o r t h e e x c i t a t i o n photon energy f i g . 7

Conp?apison o f coZlection

h i g h e r than 2.leV. Instead, a probable e x i s t e n c e

e ffi'iciency spectra among

o f e x t r a non-geminate recombination i n t h e v i c i n i t y

various p-i-n and inverted

o f the p / i ' i n t e r f a c e would r e s t r i c t t h e p h o t o v o l -

p-i-n junction c e l l s .

t a i c performance o f t h e present a-Si:H p - i - n

j u n c t i o n c e l l s . While, an a n a l y s i s on i n v e r t e d p- i - n j u n c t i o n c e l l s c o n f i r m s a s i m i l a r l y e f f i c i e n t o p e r a t i o n , i f w e l l designed, as compared w i t h p - i - n j u n c t i o n c e l l s .

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' L~-IOOOA

T A € , , = O73eV ..

-

q-

^{,,' ,.-p - ,'}

^..-

NORMAL p-I-n .

^-

I----

^{INVERTED pin}

AE~n:065ey

Conclusion,-

I n t h e p h o t o v o l t a i c e f f e c t o f a-Si:H p - i - n j u n c t i o n c e l l s , b o t h geminate and non- geminate recombination processes a r e considered t o o p e r a t i o n , p l a y a s i g n i f i c a n t non-geminate r o l e . recombination However, i n t h e l o s s a c t u a l might be almost saved by a h i g h i n t e r n a l e l e c t r i c f i e l d i n t h e a c t i v e i - l a y e r . Moreover, an i n f l u e n c e o f geminate recombination i s a l s o thought t o be r e l a t i v e l y small f o r h i g h e r photon energy r e g i o n s i n c e i n a w e l l prepared a-Si:H f i l m quantum e f f i c i e n c y f o r photo- c a r r i e r generat i o n exceeds