MINORITY CARRIER TRANSPORT IN DEPLETION LAYERS OF nip a

JOURNAL DE PHYSIQUE CoZZoque C4, suppZ6ment au nO1O, Tome 42, octobre 1981

M I N O R I T Y C A R R I E R TRANSPORT I N D E P L E T I O N LAYERS OF n-.i-p

page C4-491

a-Si:

H

SOLAR C E L L S V. Dalal and F. Alvarez

I n s t i t u t e of Energy Conversion, University o f DeZaware, Newark, Delaware 19712, U.S.A.

Abstract, Transport p r o p e r t i e s o f e l e c t r o n s and h o l e s i n i l a y e r s of p i n a-Si s o l a r c e l l s a r e important i n determining t h e c u r r e n t c o l l e c t i o n e f f i c i e n c y o f t h e s e c e l l s . In t h i s paper, we d e s c r i b e a new technique f o r measuring t h e (v-r) product of minority c a r r i e r s i n t h e undoped l a y e r of an a-Si:H c e l l . I t i s shown t h a t t h e t r a n s p o r t p r o p e r t i e s i n d e p l e t i o n l a y e r s o f s o l a r c e l l s can be very d i f f e r e n t from t h e t r a n s p o r t i n v i r g i n i l a y e r s , and t h a t under c e r t a i n circumstances, e l e c t r o n s can become t h e minority c a r r i e r s , i n s t r o n g c o n t r a s t t o v i r g i n i l a y e r s , where h o l e s a r e t h e minority c a r r i e r s , 7.

I n t r o d u c t i o n . - The c u r r e n t c o l l e c t i o n e f f i c i e n c y of a-Si:H s o l a r c e l l s i s l i m i t e d by t h e t r a n s p o r t p r o p e r t i e s , p a r t i c u l a r l y (PT) products and d i f f u s i o n lengths L, and of e l e c t r o n s and h o l e s . Generally speaking, i t i s assumed t h a t t h e ( y ~ )product o h o l e s i s much smaller than t h a t of e l e c t r o n s (1-3), and hence, h o l e s a r e t h e l i m i t i n g However, no r e l i a b l e evidence e x i s t s t h a t such i s o r minority c a r r i e r s (4,5)_, indeed t h e case i n t h e i l a y e r of a p i n c e l l . In t h i s paper, we d i s c u s s a new technique f o r i d e n t i f y i n g t h e l i m i t i n g c a r r i e r and f o r e s t i m a t i n g t h e (PT) product and d e p l e t i o n width o f t h e junction i n p i n c e l l s . We s h a l l show t h a t under c e r t a i n circumstances, t h e e l e c t r o n s may have a lower ( y ~ )product than holes and may s e r v e a s minority c a r r i e r s . I t i s p o s t u l a t e d t h a t such a condition can a r i s e because of cross-contamination between t h e p+ and i l a y e r s during th'e growth of t h e c e l l .

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Basic Model. - The b a s i c model f o r analyzing t h e c u r r e n t c o l l e c t i o n e f f i c i e n c y of p i n c k l l s ' i s shown i n Figure 1 (5). The model s t a t e s t h a t t h e r e a r e two junctions i n a n+ip* a-Si:H s o l a r c e l l , t h e f r o n t n + i junction and t h e back i p + T n c t i o n . Thus, t h e r e a r e two d e p l e t i o n r e g i o n s , separated by a n e u t r a l region o f low f i e l d . The s p a t i a l e x t e n t of t h e f i e l d regions depends upon t h e d e n s i t y of s t a t e s ( 5 ) . I f t h e d e n s i t y of s t a t e s i n t h e mid-gap region i s high, t h e d e p l e t i o n t h i c k n e s s e s t l and t g a r e small, and n e u t r a l region t h i c k n e s s t a i s l a r g e . I f t h e mid-gap d e n s i t y i s low, t h e d e p l e t i o n t h i c k n e s s e s overlap and t h e r e i s no n e u t r a l r e g i o n . In t h i s c a s e , t h e p o t e n t i a l i n t h e i l a y e r can be approximated by a l l o t t b a r r i e r ( 6 ) , but with sharp p o t e n t i a l p r o f i l e s a t t h e edges. Using t h e b a s i c model of Figure 1, we can p r e d i c t t h e behavior of t h e c o l l e c t i o n e f f i c i e n c y of a-Si s o l a r c e l l s with a p p l i e d v o l t a g e . A s t h e a p p l i e d v o l t a g e i n t h e forward d i r e c t i o n i n c r e a s e s , t h e d e p l e t i o n r e g i o n s w i l l s h r i n k , and t h e n e u t r a l region w i l l i n c r e a s e . Consequently, i f t h e d i f f u s i o n length of c a r r i e r s i s small, t h e 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 w i l l d e c l i n e . Conversely, f o r r e v e r s e applied v o l t a g e , t h e c o l l e c t i o n e f f i c i e n c y w i l l i n c r e a s e . S i m i l a r c o n s i d e r a t i o n s hold f o r t h e Mott-barrier c a s e , where a forward v o l t a g e reduces t h e f i e l d , and a r e v e r s e v o l t a g e i n c r e a s e s t h e f i e l d . Since t h e c a r r i e r s r e l y on f i e l d a s s i s t e d t r a n s p o r t , i f t h e range of c a r r i e r s (p-rE) is comparable t o t h e f i l m t h i c k n e s s , t h e c a r r i e r c o l l e c t i o n q f f i c i e n c y w i l l i n c r e a s e with r e v e r s e v o l t a g e and decrease with forward v o l t a g e . Therefore, a measurement of c o l l e c t i o n e f f i c i e n c y with b i a s v o l t a g e provides a t o o l f o r measuring (W) products. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19814104

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Analysis. - E l e c t r i c F i e l d P r o f i l e : The e l e c t r i c f i e l d and p o t e n t i a l p r o f i l e s i n t h e i l a y e r depend upon t h e d e n s i t y of s t a t e s i n t h e mid;.gap ( 7 ) . Recent r e s u l t s on t h e measurement of d e n s i t y of s t a t e s i n a-Si:H by DLTS (8,9) and by m o b i l i t y t r a n s p o r t (10) show t h a t t h e d e n s i t y of s t a t e s i n a-SizH i s well represented by Figure 2 , w i t h sharp t a i l s near t h e band edges and a f l a t d e n s i t y 'L 1 0 ' ~ / c m ~ - e vi n t h e middle. In such a case, f o r a t h i n i l a y e r (?. 0.4 pm), t h e e l e c t r i c f i e l d extends over t h e e n t i r e i l a y e r ; i . e . , a Mott b a r r i e r e x i s t s . For s i m p l i c i t y , we can assume t h a t t h e f i e l d i s uniform, given by E:(v~ + V ) / t i , where V i s a p p l i e d v o l t a g e , VD t h e d i f f u s i o n v o l t a g e , and t i t h e t h i c k n e s s of i l a y e r . VD i s given by: VD = Eg - A Ef,, - A Ef,b where Eg is t h e bandgap, A Ef,n = (Ec - Ef,n) on t h e n - s ~ d eA Ef,p = (Ev - hf,P) on t h e p-side. Typically, we measure A Ef,, = 0.25V, A E f , p = 0:4V and Eg = 1.7V. T h u s , V ~= 1.05V and E 6 2 . 5 X 104v/cm i n our sample a t zero b i a s . For s i m p l i c i t y , we can a l s o assume t h a t oniy one kind of c a r r i e r i s important; i . e . , (UT) product f o r one c a r r i e r