Photoelectrochemical study of the layered compound In23Pse3

Mat. Res. Bull., Vol. 17, pp. 579-584, 1982. Printed in the USA. 0025- 5408/82/050579-06503.00/0 Copyright (c) Pergamon Press Ltd.

PHOTOELECTROCHEMICAL STUDY OF THE LAYERED COMPOUND In2/3PSe 3

M. ETMAN , A. KATTY *

, C. LEVY-CLEMENT

and P. LEMASSON

Laboratoire d'Electrochimie Interfaciale du CNRS I, Place ~ristide Briand, 92190 MEUDON, France. ** Laboratolre de Physique des Solides du CNRS i, Place Aristide Briand, 92190 iWEUDON, France.

(Received February 18, 1982; Communicated by A. Wold)

ABSTRACT - The layered compound In2/sPSe3 is studied by photoelectrochemical technique~ Twb modes of transition, indirect ~ 1.55 eV, and direct ~ 1.80 eV are evidenced. ~n estimation of the hole diffusion length is given through the use of G~rtner model (L ~ 4 um). The flat band positions of the Junction In2/~PSes-electrolyte at different pH values are determined in an electrochemical scale.

Introduction Hyposelenophosphate !n2/3PSe 3 is a layered compound of the series MPX3 which contains indium vacancies. It may be represented by the complete stoichiometric formula In2/3 H1/sPSes. Until now, it has not received many attention and only-few-~ndi~ations concerning its electrical and optical properties are available (1). The electrochemical technique permits to obtain in a simple way general indications on the physical properties of an unknown compound. For this~t is provided that some requirements concerning the electrolyte-semiconductor Junction are fulfilled : (i) a large potential range exists where there is no detectable dark current (ii) under adequate monochromatic illumination, a photoeffect takes place. This has been checked for the compound studied and new informations are given.

579

580

M. ETMAN, et a].

Vo]. 17, No. 5

Experimental In2/3PSe 3 single crystals are g r o w n by chemical yap our transport a b - t e m p e r a t u r e s r a n g i n g b e t w e e n 500 and 700 '~ C (i). Crystals grow as thin red plates. For the e l e c t r o c h e m i c a l measurements, ohmic contacts are made by means of silver paste or indium amalgam on the rear face and a gold wire is soldered on these contacts, qhen, crystals are glued in an e p o x y resin and mounted on a teflon rod in such a w a y that o n l y the front face be in contact w i t h the electrolyte. The e l e c t r o c h e m i c a l cell and apparatus are classical. They have a l r e a d y been described elsewhere (2). Cyclic v o l t a m e t r y is performed in the dark and under m o n o c h r o m a t i c i l l u m i n a t i o n but impedgnce m e a s u r e m e n t is impossible, due to the high r e s i s t i v i t y (> lO ° ~ cm) of our samples. Solutions are p r e p a r e d from h i g h p u r i t y grade chemicals and p u r i f i e d (Millipore) water. E l e c t r o lytes used are 0.i M H 2 S O 4 (pH = i), 1 M KCI (pH = 6.4) and i M KOH (pH = 14). Potentials are referred to an e l e c t r o c h e m i c a l scale whose origin is c o n s t i t u t e d by a m e r c u r y - m e r c u r o u s sulphate electrode immersed in a saturated s o l u t i o n of p o t a s s i u m sulphate (i~SE). Hesults The dark current and p h o t o c u r r e n t (wavelength 660 n m - 1.88 eV) vs. potential c h a r a c t e r i s t i c s in acidic medium are p r e s e n t e d in Fig.1. The dark current is n e g l i g i b l e in a wide range of p o t e n t i a l (+3 to -3 V/MSE). P h o t o c u r r e n t changes sign (anodic to cathodic) at a potential of - 0.8 V/MSE but, even for illuminations as long as one hour, its shape and magnitude r e m a i n unchanged. W h e n pH changes from 1 to 14, the intercept of the p h o t o c u r r e n t c h a r a c t e r i s t i c w i t h the potential axis varies by ca. 300 m V (pH = I, - 0.8 V ; pH = 6.4, - 0.92 V ; pH = lh, - i.i V).

i 71 4

i

.

.

.

.

.

.

.

.

.

.

.

.

.

.

-

.

-1

0 i

F

, v,b

.

,

t

I

T i

I

VOLTAGE/V/MSE Figure 1 Photocurrent vs. potential c h a r a c t e r i s t i c at 660 nm in O.1 M H 2 S O 4 solution. The dark current is indicated by a dashed line.

Vol. 17, No. 5

In2/3PSe 3

581

In Fig.2, the anodic and cathodic p h o t o c u r r e n t vs. w a v e l e n g t h c h a r a c t e r i s t i c s at + 0.2 V and - 1.8 V / M S E in acidic medium are presented. We observe that the w a v e l e n g t h range in w h i c h the spectral r e s p o n s e exists is n a r r o w and pH and p o t e n t i a l independent. i 1.95

ENERGY/eV I I 1.80 1.75

I ] 1.~ X~

I 1.70

i 1.65

I 1.60

I 1.55

u

u.l Ix

-5

-10

65O I

I

I

l

WAVELENGTH/nm

Figure

2

P h o t o c u r r e n t vs. p h o t o n w a v e l e n g t h c h a r a c t e r i s t i c at + 0.2 V / M S E (~ and - 1.8 V / M S E ~ . Discussion We assume that such a s e m i c o n d u c t o r may likely be d e s c r i b e d u s i n g a band model. Such an a s s u m p t i o n was g e n e r a l l y made in p h o t o e l e c t r o c h e m i c a l studies of layered compounds (3,4). From the iph vs. V characteristic, we deduce the flat band p o t e n t i a l value Vfb. We assume that w h e n V = Vfb, iph changes sign. Vfb is s l i g h t l y pH d e p e n d e n t and changes by ca. 20 mV/pH unit. We analyze now the anodic part of the iph vs. V characteristic by means of G~rtner model (5). For this purpose, we use the value of the incident p h o t o n flux ~o at the electrode surface, m e a s u r e d d i r e c t l y ~ o ~ 5 x i012 photon, cm -2 at 660 nm. We express

the p h o t o c u r r e n t

by

iph : e@oS [! - e x p ( - a W ) / l

+ aL7

w h e r e e is the absolute value of the e l e c t r o n charge, S the illuminated surface area of semiconductor, ~ the m o n o c h r o m a t i c optical a b s o r p t i o n coefficient, W the space charge region w i d t h

582

M. ETMAN, et al.

and L the hole d i f f u s i o n length. e q u a t i o n may be r e w r i t t e n in (I - iph/imax)

Vol. 17, No.

Setting e¢oS = imax,

= - In (I + a L )

the latter

- aW

1

withaW

= aWoV ~

and W o = (2Eo ~i/eno)½

V s is the band bending in the space charge region, ~i the static d i e l e c t r i c constant of the s e m i c o n d u c t o r p e r p e n d i c u l a r to the layers, and n o is the bulk c o n c e n t r a t i o n of electrons. By plotting in ( I - iph/imax) vs. Vs~ (~ = 660 nm), aL and aW o are d i r e c t l y obtained and we have

L/W o "- O. 3 By reasQnable assumption, W o may be evaluated. C o n d u c t i v i t y is ca. l0 -~ - 10-9 ~ -i cm-1. The f o l l o w i n g r e l a t i o n holds nO

=

(~/~£e

where ~i is the e l e c t r o n mobility, p e r p e n d i c u l a r to layers. has been found v e r y low in In2/3PSe 3 (1). We take

~i

i ~ i0-2 cm2. v-l.s-I We therefore have n o ~ 1013 cm-3 ~long w i t h this value of no, we take ~A = 15 and calculate W o ~ 12 ~m L This value of L seems

=

4 ~m

in good agreement w i t h literature

(6).

We discuss now the ioh v s . ~ characteristics. In Fig.2, both responses are d i r e c t l y b o m p a r a b l e as they c o r r e s p o n d to identical values of band bending (I V) in opposite c o n f i g u r a t i o n s : bands bending downwards at + 0.2 V / M S E and upwards at - 1.8 V/MSE. '[~e n a r r o w spectral response in both cases is likely a t t r i b u t e d to the influence of p h o t o g e n e r a t e d electrons (anodic) or holes (cathodic) (7). We analyze the shape of the iph vs. k characteristics. this purpose, we use a(h~) =

(Eg - h~) n/2

w h e r e n = 1 for a direct gap and n = 4 for an indirect gap If we assume that IDh is d i r e c t l y p r o p o r t i o n a l r e l a t i o n is equival~nt to iph

For

= (Eg - h~) n/2

to a,

(8).

the latter

5

Vol. 17, No. 5

In Fig. 3 ~

In2/3PSe 3

½

2

and 3 ~ , ~

583

the iph and iph vs. h~ plots are reported.

®

®

,iiii.iii,~i_ ~ iil,, l ,,,, ,

i;2

O-

t':""'-.

"',,

r z

z

n~ DO o 1-

B

1.55 , d,

1.60 ,,,

I ,,

~~ ENERGY/eV 1.65

ik

I ....

130

[,,,,

135

1.80

....

I

i

1.90

L J i l I L L i J ENERGY/eV

Figure 3 i 2 (Photocurrent) ~ Q and (photocurrent) Q vs. photon energy plots. Energy gap is determined by extrapolation of a straight line to iph = O. ~: vV = + v/MsE V/MSE From t~ese we deduce that In2/3PSe 3 presents an indirect gap at ca. 1.55 eV and a direct gap a~ ca] 1.80 eV. These values are PH and potential independent, provided the potential is kept cathodic. For anodic potentials, we obtain E~(direct) ~ 1.83 eV. Such a difference seems likely to be due to ~ strong interaction between semiconductor and electrolyte giving rise to an intercalation phenomenon (9). Summary Photoelectrochemical measurements contribute to the knowledge of some physical properties of hyposelenophosphate of indium which can hardly be found in the literature. The hole diffusion length is evaluated to be 4 w m. The first fundamental band-band transition at 1.55 eV is indirect whereas the second at 1.80 eV is direct. Again with the flat band potential of - 0.8 V/MSE at pH = I, this makes In 2 3L PSe3 potentially interesting for solar energy conversion. However, both the low electron mobility and the low electron concentration of the samples limit up to now investigations in this direction.

584

M. ETMAN, et al.

Vol. 17, No. 5

References 1. A. Katty, S. Soled and A. Wold : Mat. Res. Bull. 12, 663 (1977). 2. J. Gautron, P. Lemasson, F. Rabago and R. Triboulet : J. Electroehem. Soc. 126, 1868 (1979). 3. W. Kautek, J. Gobrecht and H. Gerischer Phys. Chem. 84, 1034 (1980).

: Ber. Bunsenges.

4. F.R.F. Fan and A.J. Bard : J. Electrochem. Soc. 128, 945 (1981). 5. W.W. G~rtner

: Phys. Rev. I16, 84 (1959).

6. W. Kautek, H. Gerischer and H. Tributsch : J. Electrochem. Soc. 127 , 24yi (1980). 7. M. Lavagna, J.P. Pique and Y. Marfaing : Solid State Electron. 20, 235 (1977). 8. J. Pankove : Optical processes in semiconductors, P.36, Dover Publications, New-York (1975). 9. W. Kautek, H. Gerischer and H. Tributsch : Ber. Bunsenges. Phys. Chem. 8~, I000 (1979).