The surface photovoltage technique: Applications to CdSe films wi

1(t98

Applications of Surface Science 22/23 (1985) 1098-I 105 North-Holland, Amsterdam

THE SURFACE PHOTOVOLTAGE TECHNIQUE: APPLICATIONS TO CdSe FILMS WITH DIFFERENT ELECTROLYTES G.J. S T O R R

a n d D. H A N E M A N

University of New South Wales, P.O. Box 1. Kensington 2033, Australia

Received 27 August 1984; accepted for publication 31 October 1984 The SPV technique is being increasingly used to estimatc the minority carrier diffusion

length Lp in semiconductor films with a Schottky barrier to a contacting liquid. However the effect of the surface contact has not been established. We have applied SPV methods to various CdSe films using both an aqueous alkali electrolyte (NaOH) and an organic electrolyte (quinhydrone). When sufficient bias light is applied, the two quite different interfaces lead to the same values of Lp within measurement accuracy. Sensitivity to the use of proper absorption coefficients is discussed, particularly in using non-mirror surface films.

1. Introduction T h e surface p h o t o v o l t a g e (SPV) m e t h o d for d e t e r m i n i n g the m i n o r i t y c a r r i e r diffusion length in s e m i c o n d u c t o r s is well k n o w n [1,2]. O r i g i n a l l y the m e t h o d was used for h i g h - m o b i l i t y b u l k s e m i c o n d u c t o r s , b u t r e c e n t l y it has b e e n a p p l i e d to s e m i c o n d u c t i n g thin films [3,4], in o r d e r to d e t e r m i n e the diffusion length of h o l e s in u n d o p e d a m o r p h o u s h y d r o g e n a t e d silicon (aS i : H). In the S P V m e t h o d the s u r f a c e of the s e m i c o n d u c t o r b e i n g s t u d i e d is i l l u m i n a t e d with m o n o c h r o m a t i c light of v a r y i n g w a v e l e n g t h s n e a r the a b s o r p t i o n e d g e . A p h o t o v o l t a g e d e v e l o p s b e t w e e n the surface a n d the bulk, d u e to an i n c r e a s e in t h e m i n o r i t y c a r r i e r c o n c e n t r a t i o n at the surface b a r r i e r in t h e s e m i c o n d u c t o r . T h e b a r r i e r a n d r e l a t e d surface field are p r o d u c e d by c h a r g e s e p a r a t i o n in t h e r e g i o n , a n d arise in a v a r i e t y of ways. T h e surface p h o t o v o l t a g e d e t e c t e d b e t w e e n the i l l u m i n a t e d s u r f a c e a n d an u n i l l u m i n a t e d b a c k c o n t a c t is h e l d c o n s t a n t at each w a v e l e n g t h by app r o p r i a t e l y a d j u s t i n g the light intensity. M a n y s e m i c o n d u c t i n g thin films, c o u p l e d with a suitably c h o s e n liquid e l e c t r o l y t e , can b e used to m a k e p h o t o e l e c t r o c h e m i c a l ( P E C ) s y s t e m s of p r a c t i c a l p o t e n t i a l . T h e r e l a t i v e e x p e r i m e n t a l c o n v e n i e n c e that a liquid e l e c t r o l y t e c o n t a c t offers o v e r o t h e r c o n t a c t i n g m e t h o d s [4] has much to d o with its w i d e use. It has b e e n o b s e r v e d e x p e r i m e n t a l l y [5] with a-Si : H, that the p a r a m e t e r , Lp, e x t r a c t e d f r o m a plot of r e l a t i v e light intensity versus 0378-5963/85/$03.30 © E l s e v i e r S c i e n c e P u b l i s h e r s B.V. ( N o r t h - H o l l a n d Physics P u b l i s h i n g D i v i s i o n )

G.J. Storr, D. Haneman / Surface photovoltage technique

1099

inverse absorption coefficient, a, can vary for a given sample when an additional penetrating DC ("bias") light, required to reduce the barrier width, is incident on the semiconductor electrolyte junction. It needs to be demonstrated that for sufficient bias light, the parameter, Lp, represents a bulk parameter independent of the liquid contact. We report here for the first time an experimental determination of the parameter, Lp, by the SPV method using two different electrolyte contacts. Films of CdSe [6] grown by chemical deposition on glass were the semiconductor studied. While the minority carrier diffusion length has been extracted from single crystals of CdSe via the SPV method [7] it has not been previously determined for solution grown films used in PEC solar cells. This laboratory has previously reported considerable success with PEC solar cells [8] utilizing chemically deposited thin films of CdSe and N a O H / N a 2 S / S as the electrolyte. By utilizing this PEC system in a suitable experimental arrangement the extracted Lp values should describe the conditions found in operating cells of this nature. The second electrolyte was quinonehydroquinone in a pH ~ 7 . 0 buffer solution. It should be noted that low concentrations were used so that the liquid was only lightly coloured, and that sufficient amounts of aqueous buffer were required to be incorporated in the electrolyte, for long-term stability of the surface photovoitage.

2. Experimental 2.1. S P V technique

Fig. 1 illustrates the experimental arrangement of the SPV method. The output of a monochromator was chopped at - 4 Hz, and then passed via a lens and a half aluminized mirror onto the semiconductor-electrolyte sample enclosed in a shielded dark box. A reference Si solar cell, used to determine the relative light intensities, received the remaining portion of the chopped light on its surface from a back mirror. Both the reference cell and the film and substrate were mounted in specially made holders containing an " O " ring seal which held a bath of electrolyte over the surfaces. The metal contact was made by a coaxial lead soldered to a platinum wire dipped into the electrolyte. An unilluminated back contact was welded onto the titanium substrate of the film and taken to the earth input of a lock-in amplifier (LIA). The reference Si cell output was fed into a second channel of the LIA via a high gain, low input, chopper-stabilized operational amplifier which enabled a zero volt bias to be maintained, independently of illumination intensity. The A C reference signal was provided by a chopped auxiliary light detector. A strong DC red "bias" light was directed onto the sample by an appropriately placed optical fibre. The intensity of the DC source was varied manually to maintain a constant surface photovoltage.

1100

O.J. Storr, D. Haneman

Surface photovoltage technique

Reference

P

~'f--

+ ::'/

!

~P

(r~'~ Vc~bleuDpCly

~"'~- I

I~

.

'l

optic

I electro yte

F'I

/

"

co 0 o,/

+

Semi- I

D C bios

ol

'

Isi soiQ, cell I

oc, ,o

Oo,reo'

%_ Ref in I =

i

I =

Fig. 1. Schematic diagram of SPV experimental configuration.

The signal from the reference cell at various wavelengths was correctcd for spectral sensitivity using a calibrated irradiance probe and another correction factor took into account the reflection differences between the half aluminized mirror and the back reference mirror. Three films were studied at wavelengths between 600 and 700 nm under " d a r k " (no D C bias light) and "light" (DC bias ~ 15 m W cm -2) conditions using first, the 1 M N a O H : I M N a 2 S : I M S electrolyte and second, the aqueous organic liquid quinone-hydroquinone. The films were produced by the method described previously [6], using two chemical depositions giving a film thickness of approx. 0.85/zm from a calibration using a step height probe. Any differences in the films were due to random variations in ostensibly similar production techniques.

2.2. Absorption coefficient determination CdSe films were chemically deposited on glass slides and annealed. The coating was then removed from one side of a slide. The transmission ( T ) and reflection (R) spectra were measured from 550 to 750 nm using a Cary 17 spectrophotometer. The absorption coefficients for each wavelength were determined from the expressions for T and R of an absorbing film of thickness d, refractive index (n 1 - ik 0, on a (transparent) substrate of RI (n 2- ik2), given by Heavens [9]. It was found essential to use the full expressions and not m a k e approximations, as the results were sensitive to c~. We thus give the full formulae:

G.J. Storr, D. H a n e m a n / Surface photovoltage technique

(g2 + h 2) ed,, + (g~ + h 2) e-d,, + A cos 23' + B sin 2y

1 lOl

(1)

R=

ed. + (g2 + h,)(g2 2 2 + h 22)e-d~ + C c o s 2 3 , + D s i n 2 3 ,

r

[(1 + gl)2+ h2][(1 + gz)2+ h 2} 2 2 + h2) e-d~ + C cos 23' + D sin 2y noea~ + (g2+ hx)(g2

---- n2

(2)

where 2

2

n o- n I-

hi

g ' - (n 0+ nl)2+ k~' n 21 --

2nok i (n o + nl) 2 + k 2

2

k 1

n~+k~

-2n2kl h2 -- (n 1 + n2)2 + k 2 ,

g2 = (n 1 + n2)2+ k~'

2 rrk i d A 27mid (radians) 3' - ~ - -

and A = 2(g]g2 + hlh2),

B = 2(g]h 2 - g2h]) ,

C = 2(g~g 2 - hlhz),

D = 2(g]h2+ g2hl) .

H e r e n o is the RI of air and equals 1, the film thickness d = 0.85/zm and n 2 = 1.5. A c o m p u t e r program solved the equations for T and R to obtain best fits, by iteratively searching for the extinction coefficient k~ and refractive index of film n~. T h e absorption coefficient, a, for the films is plotted versus wavelength in fig. 2.

10 5

i

z Ill m U LL LL

10 l' Z rl or" 0 m

10

3

500.0

i

,

i

i

550.0

i

i

,

i

600.0

,

,

~

i

650.0 WAVELENGTH

h

,

,

i

700.0

,

i

i

,

750.0

i

,

i

i

800.0

(nm)

Fig. 2. E x p e r i m e n t a l v a l u e s of a b s o r p t i o n coefficient, a, v e r s u s w a v e l e n g t h , A, for c h e m i c a l l y d e p o s i t e d C d S e using analysis of eqs. (1) and (2) in the text.

G.J. Storr, D. Haneman / Surface photovoltage technique

1102

3. The SPV equations

The original expression given by G o o d m a n [1] gave the intensity as equal to a constant times a functional dependence of a, such that I = const, x (1/a + L) ,

(3)

with the conditions that W
,

(4a)

L ~ d ,

(4b)

n >>A p .

(4c)

W is the depletion width, L the minority carrier diffusion length and d the thickness of the sample, n the majority and A p the minority carrier density. In our films eqs. (4a) and (4b) are not satisfied; however, Moore [5] has shown that the straight line dependence in eq. (3) is observed in regions away from high absorption, using an expression of form similar to eq. (3) from an analysis of current-voltage behaviour in low-mobility solar cells with a Schottky barrier [10]. A more recent analysis with fewer approximations [11] also gives the same form as eq. (3). The correct L is extracted from eq. (3) if the depletion width, W, is sufficiently small, which is achieved by applying a penetrating (red) DC bias light during the experiment.

4. Results and discussion

Table 1 presents results for three CdSe films showing the negative 1/6r intercept under " d a r k " and "light" conditions, using the SPV method. Experiments with varying bias light intensity showed that approx. 15 mW cm -2 was sufficient to cause the 1/a intercept to attain a lower limit. This condition was also found in a-Si : H films [12]. Figs. 3 and 4 illustrate the least squares straight line fits to the experimental intensity versus inverse Table 1 Lp v a l u e s (in m i c r o n s ) f o r C d S e films w i t h d i f f e r e n t e l e c t r o l y t e s Film

NaOH I 2 3

B i a s ~ 15 m W c m

No bias ~)

0 . 3 8 ± 0.07 0.13 + 0.03 0 . 4 2 ± 0.04

a) N a O H : Na2S : S, all 1M.

Quinhydrone

NaOH

a)

0.73 ± 0 . 0 9 0.55 + 0.1 very large

0.30 ± 0.05 0.08 ± 0 . 0 6 0.32 ± 0 . 0 6

2 (DC) Quinhydrone 0.26 + 0.(15 0.10 + 0.05 0.35 + 0.04

G.J. Stotr, D. Haneman / Surface photovoltage technique 1.2

I

,

i

I

'

'

1103

'

CdSe FILM 3 (NaOH, BIAS 1S mWcm -2 )

!/

1.0

0.8

~J ~

0.6

Z

>~ 04 ,..1

/"

/

a: 0.2 IF/ 0.0 /

-0.2

/

./ i

i

i

-0.4

I

-0.2

i

I

i

J

0-0 0-2 INV. O~(xl0"a cm )

i

0.4

0.6

Fig. 3. Plot of incident light intensity I versus inverse absorption coefficient under 15 m W cm 2 bias light for CdSe films in alkaline sulfur redox electrolyte.

~-2

r

,

r

'

i

,

i

,

4

i

b

CdSe FILM 3 (QUIN.-HYDROQuIN,

,

]

r

i

,

BIAS 15mWc2

1-0

0.8

j/

u~

z

0.6

Z

uJ _> 0-4 U.I

r,- 0.2

j

O.C~

J/ -0.2 -0.6

i ~

-0.4

-0.2

t

,

i

J

L

L

l

0,0 0.2 0.4 INV. c( (xlO -c cm )

Fig. 4. Same as fig. 3 but using quinhydrone electrolyte.

,

J

l

0.6

0.8

1 I04

G.J. Storr, D. Haneman / Surface photovoltage technique

absorption coefficient data points. The dashed lines on either side of the best fit line represent one standard deviation from it. It should be noted that a complete set of data points (not displayed) shows the characteristic turnup of the line at high absorptions, observed with a-Si: H and explained by Moore [5] as due to back diffusion of minority carriers in the depletion region. In all cases the bias light reduced the intercept values significantly. Under sufficient bias light conditions, the best fit intercepts are taken to be a measure of the minority carrier diffusion length. For both redox electrolytes the results (under bias) are close, and within m e a s u r e m e n t error. The large differences under no bias conditions are due to breakdown of conditions (4a) and (4b), which are different for the two redox couple surface barriers when the depletion region is large. This width is collapsed under bias conditions and the intercepts tend to converge if a true bulk p a r a m e t e r is being observed. We note that some previous authors have not shown the errors in their straight line fits to data points but this should be done due to the extent of the extrapolation to the I / a axis. The actual values of L ( 0 . 1 - 0 . 3 ~ m ) are comparable with the range of values reported in the literature (0.1-5/.tin, measured for single crystals only [7,13]). Films 1 and 3 have similar v a l u e s - t h e lower value in fihn 2 is due to unknown variations in the chemical growth process.

5. Conclusion

The results show that the parameter, L, obtained for CdSe films using the SPV method, has the same value for completely different contacting electrolytes. Provided one applies bias light that is sufficiently intense to collapse the depletion region at the semiconductor-electrolyte interface, and provided the absorption coefficient is accurately determined for the particular film, the parameter, L, is closely related, or equal, to the minority carrier diffusion length in the surface region, depending on the reliability of the theoretical analysis. Formulae that differ in their assumptions and algebraic forms [5,11] both yield L under the SPV method of analysis, lending confidence to its representing the true minority carrier diffusion length. It is possible to extract other parameters such as surface recombination velocity and depletion width by fitting the full versions of eq. (3) from ref. [5] or [11] to the data. This is a more complex procedure and the level of reliability of such further analysis has yet to be assessed.

References

[1] A.M. Goodman, J. Appl. Phys. 32 (t961) 255(I.

G.J. Storr, D. Haneman / Surface photovoltage technique

11(15

[2] R.O. Bell and G.M. Freedman, in: Proc. 13th IEEE Photovoltaic Specialists Conf., 1978, p. 89. [3] A.R. Moore, App. Phys. Letters 40 (1982) 403. [4] J. Dresner, D.J. Szostak and B. Goldstein, App. Phys. Letters 38 (1981) 998. [5] A.R. Moore, J. Appl. Phys. 54 (1983) 222. [6] R.C. Kainthla, J.F. McCann and D. Haneman, Solar Energy Mater. 8 (1983) 491. [7] S. Mora, N. Romeo and L. Tarricone, Nuovo Cimento 60 (1980) 97. [8] D. Haneman, G.H.J. Wantenaar and R.C. Kainthla, Solar Energy Mater. 10 (1984) 69. [9] O.S. Heavens, Optical Properties of Thin Solid Films (Butterworths, London, 1955) pp. 76-77. [10] J. Reichman, App. Phys. Letters 38 (1981) 251. [11] J.F. McCann and D. Haneman, J. Electrochem. Soc. 129 (1982) 1134. [12] T.J. McMahon, in: Proc. 16th IEEE Photovoltaic Specialists Conf., 1982, p. 1389. [13] A. Etchebery, M. Etman, B. Fotouhi, J. Gautron, J. Sculfort and P. Lemasson, J. Appl. Phys. 53 (1982) 8867.