Electrochemical preparation and characterization of n-CdSe0.65Te0.35 polycrystalline thin films: Influence of annealing

Solar Energy Materials 19 (1989) 383-393 North-Holland, Amsterdam

383

E L E C T R O C H E M I C A L P R E P A R A T I O N AND C H A R A C T E R I Z A T I O N OF n-CdSeo.6sTeo.35 P O L Y C R Y S T A L L I N E T H I N F I L M S : I N F L U E N C E OF A N N E A L I N G M.T. G U T I I ~ R R E Z and J. O R T E G A Instituto de Energias Renovables (CIEMA T), A vda. Complutense 22, E-28040 Madrid, Spain

Received 1 May 1989; in revised form 15 August 1989

CdSeo.65Teo.35thin films have been prepared by electrodeposition. The films were characterized by X-ray diffracion, optical and photoelectrochemical methods. The influence of annealing treatments on the physical parameters (grain size, d, donor concentration, ND, and hole diffusion length, Lp) determining the photoelectrochemical behaviour of electrodeposited CdSe0.65Teo35 thin films in contact with sulfide/polysulfide electrolytes have been systematically studied.

1. Introduction CdSeo.65Teo.35 with its near-ideal band gap and high optical absorption is a promising solar cell material for low-cost terrestrial applications. Electrodeposition is a simple low-cost technology which has yielded low resistivity and low carrier lifetimes. Electrodeposited CdSeo.65Teo.35 thin films can be utilized in photoelectrochemical cells (PEC's) [1] as well as in heterojunction photovoltaic cells with Cu2Se [2]. For both of these devices the semiconductor characterization by optical and photoelectrochemical methods [1] is very important. In this work we present the results obtained in the electrochemical preparation of n-CdSeo.65Teo.35, as well as structural, optical and photoelectrochemical characterization of this semiconductor in function of the different annealing treatments.

2. Experimental

2.1. Sample preparation

Although different conditions for the electrochemical preparation of CdSe0.65Te0.35 have been reported [1,4], we have used the conditions utilized for CdSe preparation [5,6] adding HETeO 3 to the plating bath. For photoelectrochemical characterization we have electrodeposited CdSe0.65Te0.35 onto titanium substrates [6] and for optical measurements onto glass-SnO 2 substrates [5]. 0165-1633/89//$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

384

M. I2 GutiOrrez. J O r t e g a / P r e p a r a t t o n a n d characterizalion q/ n - ( dSe~; ,, ; l~',, ~~

A systematic study of the stoichiometric variation as function of the electrolyte concentration and the temperature of the bath was made. We can conclude that the conditions to obtain a stoichiometric electrode are: electrolyte composition of 1 M H2SO 4, 65mM 3CdSO 4 • 8H20, 20mM SeO 2, 58nM TeO 2 in the temperature range between 40 and 45°C. The deposition was carried out without stirring under potentiostatic conditions. The counterelectrode was Pt while a satured calomel electrode was used as the reference electrode. The photoelectrodes were removed from the bath and rinsed in deionized water: in flowing Ar containing several p p m of O 2 and in air, during 15 minutes at different temperatures. X-ray diffractograms of the films were obtained with Cu Kc~ radiation. Measurements of the absorption coefficient, c~(X), were performed on CdSe0.~sTeo35 thin films electrodeposited on transparent glass substrates coated with SnO~. 2.2. Photoelectrochemical characterization

The experimental arrangement employed for photoelectrochemical characterization was a classical photoelectrochemical system [17] with a 150 W Xe lamp and high intensity monochromator illumination sytem PTI, a light chopper PAR model 194 A, a potentiostat Tacussel Solea type BI-PAD, a lock-in amplifier PAR model 5206 and a Model 2.000 X - Y recorder Houston Instrument. Photoelectrochemical measurements were made in a three-electrodes cell where the counterelectrode was a platinum foil, the reference electrode was a saturated calomel electrode, SCE, and the electrolyte was an aqueous solution of composition 1M K O H , 1M Na2S and 10-2 M S.

3. Results and discussion Fig. 1. presents the stoichiometric variation of the samples with the bath temperature and the species concentration in the electrolyte. The deposition potential was 0.65 V versus SCE [6]. From this figure we can conclude that the conditions to obtain a stoichiometric electrode were: 65mM 3CdSO 3- 8H20, 1M H2SO 4, 20mM SeO3, 5.8mM TeO 2 in the temperature range 40-50 ° C. Fig. 2 presents the X-ray diffractograms obtained for an electrode of CdSe0.65Te035 annealed at different temperatures in Ar with p p m of 02 during 15 min. We can see that at 4 0 0 ° C and 500 °C, cubic as well as hexagonal phases are present in the films. At 6 0 0 ° C only the hexagonal phase is present. This is interpreted as a recrystallization process accompanied by phase transformation from cubic into hexagonal. The composition of the films was calculated from Vegard's rule [9] expressed as follows: a C d T e X q- (1 -- X ) a c d s e = aobtained ,

(1)

CCdTeX-l- (1

(2)

-

X ) C C d T e = Cobtained,

M.T. Guti~rrez, J. Ortega /Preparation and characterization of n-CdSeo.6sTeo 35 Se Te/cd

385

SeTe/cd '

(a)

(b)

1.2k

11 1 09

o,8~

08

o.6t

0.7

o./-.,~ o.2l

06

I

I

I

I

1

2

3

20

[H~so~] (.) SeTe4d

SeTe/c d l

1,2

1.21

I.I

1.11

I

I

I

4O 6O (oc)

1 0.9

091

08

0,81

[so04:20m.

o71

0.7

1"

I

10

I

I

20

[SeO~](~M)

30

1

I

2

I

I

4 6 [TeOz] (raM)

I

8

Fig. 1. Stoichiometry variation of the samples with the deposition parameters.

where a and c are the lattice constants obtained from [10]: 1 _ 4(h 2+hk+k

d2

3a 2

2) + - -l 2

(3)

c2 '

where d is the distance between lattice planes and h, k, l the Miller indices. The composition obtained from eq. (1) is: CdSe0.63Te0.37 while the composition calculated from eq. (2) is CdSeo.67Teo.33. The crystal sizes were estimated by Scanning Electron Microscopy (Philips SEM mod. 500), obtaining grain sizes of 2,3 and 5 /~m for 400, 500, and 600°C respectively. Fig. 3 shows the SEM pictures for samples annealed at different temperatures. Measurements of the absorption coefficient a(?~) were performed on CdSe0.65Teo.35 thin films electrodeposited on transparent glass substrates coated with SnO2, using the methods reported by Goodman [11] and Lubberts et al. [12]. The results obtained are presented in fig. 4. We show in fig. 5 the quantum efficiency, 77, versus potential plots under monochromatic illumination of 560 nm, for CdSeo.65Teo.35 at different ambients and temperatures in sulfide-polysulfide.

386

,r~4.T. Gutidrre:, J. Ortega

I%'paratmn and characlerl:ation q/ n-( dSc,, :,~ : ,

(110)

'~d Se o ~.s Yeo ~5

I

'7

,~oo

u-J\

50

I '

o

"

(002;, ~

:,o:) I I I

c

A

40

, just: 30

20

I

I

500°C

d

A 600°C

A

Fig. 2. X-ray d i f f r a c t o g r a m s of the n-CdSeo.65Te0 3s thin films a n n e a l e d at different temperatures.

The quantum efficiency is defined as [13]: = Iph/qQo,

(4)

where Iph is the steady state photocurrent (i.e. the current under illumination minus the current at the dark), q is the electronic charge and Qo is the photon flux absorbed by the photoelectrode surface in contact with the electrolyte. As we can see in this fig. 3, the maximum efficiency is reached with electrodes annealed in flowing Ar containing several ppm of 0 2. On the other hand, for V> - 0 . 3 5 V versus SCE, ~ increases slowly with the potential indicating that G~irtner's model [14] can be applied in this region. It has been shown [15] that in most cases G~irtner's model of the metal semiconductor junction [141 can also provide a successful physical description of the semiconductor-electrolyte junction. The requirements and the equations of this model for n-CdSe have been reported elsewhere [16] obtaining this equation: ln(1 - r#) = --~(2~%/qND) 1/2 ( V - Vrb)l/z -- ln(1 + o~Lp),

(5)

M.T. Guti~rrez,J. Ortega / Preparation and characterization of n-Cdgeo.65Teo.35

387

Fig. 3. SEM pictures for samples annealed at different temperatures. Each line division is equal to 10 #m. (a) 400 o C, (b) 500 ° C, (c) 600 ° C.

388

M. 72 Guti#rrez. J. Ortega / Preparation and characterization o! n - C d S e , ~li:,

10 6

0 00 O00

10 ~

0 0 0 0

O0 0 0 0 0

0 00

(J

0 0 0

10 4 O 0

0 0

10 3 I

I

I

I

I

I

/~00

500

600

700

800

900

~, ( n m )

Fig. 4. Evolution of the absorption coefficient, a, with the wavelength, A, for n-CdSeo 65Teo.35.

80 [] A r

X= 560 m m

• Ar+ppm

of 02

A AIR 60

0 -1.3

-10

I -0.5

I 0.0

0.5

Vvs SCE

Fig. 5. Dependence of the quantum efficiency on the applied potential for the n-CdSeo.65Te0.3~ films at different annealing treatments.

389

M. T. Guti~rrez, J. Ortega / Preparation and characterization of n-CdSe o.os Te o.3~

.~,1~

~, LJ

a

[]Ar

4~

,Ar+ppmo,

- -0.36 []

~

.

• -072

[

6OO

500

L

I

I

0.35

07

1.05

14

(V -Vfb )t#

Fig. 6. G~rtner's model plot for the data of fig. 3.

with

W= (2c%/qNt)) 1/2 ( V - Vfb)1/2,

(6)

where W is the depletion layer width, N D is the donor concentration, a is the optical absorption coefficient, Vfb is the flatband potential, Lp is the hole diffusion length, and c and c o are the dielectric constant and the vacuum permittivity, respectively. According to (5) a plot of ln(1 - ~/) versus ( V - Vfb)1/2 should be linear with a slope and an intercept with the vertical axis that yield ND and Lp respectively when a and Veo are known. The Vfb was obtained experimentally from photocurrent transient experiments performed with chopped light. The potential at which the initial transient photocurrent, corresponding to the electron hole separation, becomes zero, is considered as a reliable measurement of Vfb. We have obtained a value of - 1 . 2 5 V versus SCE for Vfb of the n-CdSe0.65Te0.35/ sulfide-polysulfide junction. We have applied eq. (5) to the results of fig. 5 obtaining fig. 6. From the linear plots in this fig. 6 we can obtain the donor concentration, N D, and the hole diffusion length, Lp, from the slope and the intercept with the vertical axis, respectively. In figs. 7 and 8 we can see the evolution of these parameters as function of the annealing temperature for different ambients. On the other hand, fig. 9 shows the quantum efficiency evolution as function of the annealing temperatures for the three different ambients. Taking into account the figs. 8 and 9, we can correlate the maximal L v values with a maximum quantum efficiency for the electrodes annealed in Ar containing several ppm of 0 2.

390

M. 72 Guti~rre-,

Ortega / Preparation and characterication ~)1n-('dS,'e,

J.

10 )9

~,~ I~'

,

........................ Q

Ar

•

Ar + p p m

A

A

~-- 10 m E u

of 02

I

zS.

2

1017

I 400

I 500

I 600

T (°C)

Fig. 7. Dependence of N D on the annealing temperature in different ambients.

10 -5

~" 10 -6 O. _J

[] Ar

10 - 7

• A r + ppm of 02 A AIR

10-8

J

I

I

I

400

,

,

,

I

,

,

,

I

,

,

J

5O0 600 T (°C) Fig. 8. Dependence of Lp on the annealing temperature in different ambients.

M.T. Guti~rrez,J. Ortega / Preparation and characterization of n-CdSeo.6sTeo.~5

391

/\ E~

40

30

v

20

n - C d Seo.6s Tee3 s ~, = 560 n m V = 0 5 Vvs ECS

10

[ ] Ar

• Ar+ppm

of 02

A AIR

~'

I

I

I

400

500

600

T (°C) Fig. 9. Evolution of quantum efficiency with the annealing temperature in different ambients.

These results agree with those obtained for n-CdSe [13]. Therfore, we think that the variation of N D values, shown in fig. 7, is explained by the annealing treatments generating selenium-tellurium vacancies (Vseve) and N D increases with the temperature. If the annealing ambient is air, the selenium-tellurium vacancies (Vseve) can be occupied by 0 2 and, as we can see in fig. 7, the N D values decrease. When the annealing ambient is Ar with several ppm of O 2, the Vseve are partially occupied by O2; and in Ar ambient without 0 2, the vacancies are not neutralized. As in CdSe [16] electronic conduction mechanisms in CdSe0.65Te0.35 are associated to Se and Te vacancies (VseT~) which are k n o w n to behave both as shallow donor centers and deep electron traps [17].

392

M. "1~ Gutidrrez. J. Ortega / Preparation and characterization of n-('d),'e. ~,~7")_'~ ~

Se and Te vacancies are generated as a result of partial electrode evaporatio~ during annealing. The higher the annealing temperature the greater Vse.~ and so ND, as we can see in fig. 5. For an efficient control of Vse~e and Nr~ the annealing treatment has to be performed in Ar with a few p p m of 02, in order to facilitate oxygen chemisorption. Chemisorbed 02 behaves as an efficient electron acceptor [18] able to compensate the excess concentration of free electrons, which results in an efficient control of N D. With respect to the experimental data of fig. 8, we can see that the dependence of Lp on the temperature is similar to that observed with ~ (fig. 9), presenting a maximum at 500°C, except for the samples annealed in air for which negative values of Lp have been obtained. In ref. [19] there is a detailed explanation of these results that are due to recombination in the depletion layer.

4. Conclusions In order to obtain an efficient control of WSeTeand N D in n-CdSe065Te035 the annealing treatment has to be performed in an inert atmosphere (e.g. Ar) containing a few p p m of O:, to facilitate oxygen chemisorption. Summing up, the main effects of electrode annealing in this atmosphere are: (i) The increase of the crystallite size, so that d > I / a , which means that the photons of 560 nm (c~ = 7.1 × 10 4 cm -1) are practically absorbed within the first grain's layer and so recombination at grain boundaries is negligible. (ii) The generation of Se and Te vacancies behaving as donor centers. Above 500 o C these vacancies begin to be partially compensated by Cd vacancies and partially neutralized by 02 which is able to diffuse into the CdSe and CdTe lattices occupying Se and Te vacancies [20].

Acknowledgments The authors wish to thank Dr. A. Bayon for his help and use of X-ray facilities, and also thank Dr. T. N o v o for his help and use of SEM in C I D A laboratories.

References [1] [2] [3] [4] [5] [6] [7] [8] [9]

Y. Mirovsky, R. Tenne, G. Hodes and D. Cahen, Thin Solid Films 91 (1982) 349. R. Noufi and R. Tenne, Appl. Phys. Comm. 4 (1985) 241. S. Licht and J. Manassen, J. Electrochem. Soc. 117 (1970) 1420. R.N. Bhattacharya, J. Appl. Electrochem. 16 (1986) 168. M.T. Guti6rrez and J. Ortega, Journ6es d'Electrochimie '85, Florence, 1985. M.T. Guti6rrez and J. Ortega, J. Electrochem. Soc. 136 (1989) 2316. N. Chandra, J.K. Leland and A.J. Bard, J. Electrochem. Soc. 134 (1987) 76. M.T. Guti6rrez and J. Ortega, Thin Solid Films 174 (1989) 295. R.W. Cahn and P. Haasen, Eds., Physical Metallurgy, Part I (North-Holland, Amsterdam, 1974) p. 178.

M.T. Guti~rrez, J. Ortega / Preparation and characterization of n-CdSeo.65Teo.35

393

[10] M.F. Ladd and R.A. Palmer, Eds., Structure Determination by X-Ray Crystallography (Plenum Press, New York, 1964) p. 71. [11] A.M. Goodman, Appl. Opt. 17 (1978) 2779. [12] G. Lubberts, B.C. Burkey, F. Moser and E.A. Trabaka, J. Appl. Phys. 52 (1981) 6870. [13] M.T. Guti&rez, J. Ortega and P. Salvador, MELECON'85, Vol. IV, p. 59. [14] W.W. G~irtner, Phys. Rev. 116 (1959) 84. [15] M.A. Butler, J. Appl. Phys. 48 (1977) 1914. [16] M.T. Guti6rrez and P. Salvador, Solar Energy Mater. 15 (1987) 99. [17] L.L. Kazmerski, W.B. Berry and C.W. Allen, J. Appl. Phys. 43 (1972) 3515. [18] G.A. Somorjai, J. Phys. Chem. Solids 24 (1963) 175. [19] P. Salvador, J. Appl. Phys. 55 (1984) 2977. [20] G. Ross6, F. Raoult and B. Fortin, Thin Solid Films 111 (1984) 175.