Chemical-bath-deposited CdS and CdS: Li films and their use in photoelectrochemical solar cells

Solar Cells, 22 (1987) 163 - 173

163

CHEMICAL-BATH-DEPOSITED CdS AND CdS:Li FILMS AND THEIR USE IN PHOTOELECTROCHEMICAL SOLAR CELLS S. N. SAHU and S. CHANDRA Department o f Physics, Banaras Hindu University, Varanasi 221005 (India)

(Received May 6, 1986; accepted in revised form April 17, 1987)

Summary A chemical bath deposition technique was used to obtain n-CdS and lithium-doped CdS films. Lithium doping apparently decreases the band gap and lowers the resistivity, which are responsible for better performance of photoelectrochemical solar cells with the geometry CdS or CdS :Li/$2-,$22-/ Pt. The CdS-S2-,S22- interface was electrochemically characterized.

1. Introduction Photoelectrochemical (PEC) solar cells are a class of photovoltaic devices which essentially use a semiconductor-electrolyte interface. The aim and scope of such cells are similar to that of their counterpart S c h o t t k y or p - n junction cells. In the case of S c h o t t k y or p - n junction cells, a depletion layer is formed. Under illumination, electron-hole pairs are generated and are separated across the built-in field of the depletion layer. In PEC cells the junction is between a semiconductor and an electrolyte. In this case the formation of a depletion layer is also possible at the interface within the semiconductor. The photoinduced charge transfer at the semiconductorelectrolyte interface is responsible for the photocurrent and photovoltage. The PEC cell has the advantages of low cost, ease of fabrication and the possibility of solar energy storage [1]. An efficiency has already been achieved in PEC solar cells comparable with that of the solid state conventional photovoltaic cells. Some material problems still need to be solved before the large-scale commercial exploitation of PEC cells. Corrosion is the main problem. Another problem which deserves attention is the development of a large-area polycrystalline thin semiconducting photoelectrode yielding a reasonably efficient and stable PEC system. Single-crystal CdS was the first material used for PEC solar cells [2]. Many techniques have been described for obtaining CdS films [3]. The simplest technique is chemical bath deposition developed by Pavaskar e t al. [4] and subsequently used by m a n y workers [5 - 7]. CdS has a band gap of 2.4 eV while the o p t i m u m band gap for PEC cells is approximately 1.6 eV. 0379-6787/87/$3.50

© Elsevier Sequoia/Printed in The Netherlands

164

This results in low efficiency CdS PEC cells. Although the band gap of CdS does not correspond to the ideal band gap for high efficency PEC cells, it is known to be one of the most stable systems in contact with the S 2 ,$22electrolyte. As such it is worthwhile investigating methods by which the absorbance (and photosensitivity) of CdS under solar irradiance can be increased. A few such methods are doping with a divalent impurity or forming solid solutions with different band gap materials. Some attempts have been made by Tsuiki and Minoura [8] to improve the efficiency by doping with lithium in pyrolytically deposited films. The present paper reports a study of CdS and CdS:Li films prepared by chemical bath deposition and their subsequent use in the fabrication of PEC cells.

2. Experimental details The experimental arrangement, which is similar to that previously used by us [5], for preparing CdS and CdS:Li films on stainless steel or glass substrates is shown in Fig. 1. The reaction cell was a 50 ml beaker containing 1 M CdSO4 aqueous solution, 1 M thiourea aqueous solution and 2 M NH4OH in the volume ratio 1:1:1. The substrate was cleaned by washing with an ordinary labwash, dip etching in 0.5 M NaOH for a short time and finally using an ultrasonic cleaner in a trichloroethylene bath. The chemicals used were analytic reagent grade. The substrates were then suspended from a shaft (shown in Fig. 1) in the reaction cell and rotated continuously by a motor (125 rev min--1). Reasonably good films of CdS were obtained in 35 min at a constant temperature (water bath) of 80 °C. The film thickness was 6 #m. The continuous rotation of the substrate resulted in a uniform film deposition on both sides of the substrate. The freshly grown films were thoroughly washed by water jets, dried and kept in a vacuum. To prepare CdS:Li films, 10 -s M Li203 was also added to the reaction cell being used for CdS deposition. The procedure adopted was the same as described for CdS deposition.

~

otor

Suhstrates

~Z__-~=---==-~c,M

L--Z--i~-

-~ =~

CdSO~

+ ~M thiourea

Constant temperature water bath

Chem~cat bath deposition arrangement.

Fig. 1. Experimental arrangement for the deposition of CdS and CdS :Li films.

165 Structural identification was carried out using electron diffraction patterns taken on a Philips TEM 200 electron microscope. The CdS films were first scratched from the steel substrate and then lifted on a copper grid dipped in formvar. The grid was then fitted in the grid holder. The d values were calculated and compared with standard d values from the ASTM card. Optical absorption spectra were recorded with a Cary-14 spectrophotometer. For optical absorption studies, CdS and CdS:Li films were deposited on a glass substrate. A bare glass substrate was put in front of the reference beam while the glass plate with chemical-bath-deposited CdS or CdS:Li film was kept in front of the sample beam and the spectra were recorded at 300 K. For M o t t - S c h o t t k y studies and electrochemical characterization, a three,electrode (semiconductor film electrode, platinum counterelectrode and the saturated calomel reference electrode (SCE) with respect to which all the potentials were measured) one-compartment cell containing 1 M NaOH, 1 M Na2S and 1 M S was used. The space-charge capacitance at various d.c. bias was measured at various frequencies (500 Hz - 5 kHz) with a Marconi universal bridge TF 2700. Measurements were made in the dark to avoid photoeffects. Current density-voltage ( J - V ) characteristics of the interface were drawn in the dark and under illumination to study the charge transfer mechanism. For this purpose, the three~lectrode one-compartment cell configuration as described above was used. The potentials were measured with respect to a saturated calomel electrode. Finally, PEC solar cells were fabricated using the cell configurations steel/CdS/S2-,S22-/pt and steel/CdS:Li/S2-,S22-/pt. To obtain the J - V characteristics of the solar cell, the current and voltage obtainable from a cell at different loads were measured under illumination. A tungsten filament lamp was used for illumination and the light intensity at the semiconductor electrode was adjusted to 35 mW cm -2. 3. Results and discussions

3.1. Film formation The mechanism of CdS film formation was given by Pavaskar et al. [4]. On similar lines, the mechanism of CdS:Li film formation can be assumed to proceed as follows: (i) formation of a complex with cadmium CdSO4 + 2NH3

> [Cd(NH3)4]SO4

(ii) diffusion of [Cd(NH3)4] 2+, OH- and thiourea to the catalytic surface of CdS; (iii) dissociation of thiourea in alkaline media H2N--C--NH 2 + OHI S

> CH2N 2 + H20 + HS-

166 (iv) f o r m a t i o n o f a s u l p h i d e i o n HS- + OH

~ S2- + H20

(v) Li2CO 3 + 2 N H 4 O H (vi) L i O H

> 2 L i O H + (NH4)2CO3

> Li ÷ + O H

(vii) [ C d ( N H 3 ) 4 ] 2+ + x L i ÷ + S 2-

> C d S ' x L i + 4NH31"

T h e f o r m a t i o n o f CdS f i l m w a s c o n f i r m e d b y t h e e l e c t r o n d i f f r a c t i o n p a t t e r n s h o w n i n Fig. 2. T h e c a l c u l a t e d d v a l u e s are l i s t e d i n T a b l e 1 w i t h t h e s t a n d a r d d v a l u e s f r o m t h e A S T M c a r d c o r r e s p o n d i n g t o t h e w u r t z i t e (W) o r

Fig. 2. Electron diffraction pattern of a chemical-bath-deposited CdS film.

TABLE 1 Experimentally observed d values and standard d values from the ASTM card of the chemical-bath-deposited CdS films

hk l

Standard d (A)

Observed d (1~)

10 20 1I 31 00 22 10 30 21 44

3.160(W) 2.900(ZB) 1.761(W) 1.755(ZB) 1.679(W) 1.680(W) 1.520(W) 1.194(W) 1.158(W) 1.028(ZB)

3.00 2.85 1.77 -1.67 -1.51 -1.15 1.03

1 0 2 1 4 2 4 0 3 0

W, wurtzite; ZB, zinc blende.

167

zinc blende (ZB) structures. It is obvious from Table 1 that the chemicalbath-deposited film of CdS exists as a mixed phase (W + ZB). Further, the nature of the electron diffraction pattern (Fig. 2) indicates that the deposit is polycrystalline. 3.2. Optical absorption CdS is a direct band gap semiconductor for which the absorption coefficient a at various photon energies hv is given by (by

= const. ×

-

Eg)l/2

-

hv

So, the value of Eg can be evaluated from a plot of (odor) 2 vs. hv. The optical absorption spectra of CdS and CdS:Li are shown in Fig: 3(a) and the corresponding (ahv) 2 vs. hv plots are given in Fig. 3(b). A shift in absorption towards higher wavelength is observed as result of lithium doping. The

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0"7

~

0.6

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700

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650 600 550 500 450 PHOTON WAVE LENGTH~.(nm)

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400

75 70

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(b)

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1.9

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2.5

2.7

Fig. 3. (a) Optical absorption spectra for chemical-bath-deposited CdS ( ) and CdS:Li ( - - - - - - ) films and (b) (~hv) 2 vs. h v where a is the absorption coefficient in arbitrary units and h v is the photon energy in electronvolts.

168 apparent band gap of the CdS:Li film is found to be slightly (but not significantly} lower (approximately 2.35 eV) than that of CdS (2.42 eV). The difference in absorption spectra in the two cases may be due either to scattering by impurity centres or to a slight lowering of the band gap.

3.3. Electrochemical characterization 3.3.1. M o t t - S c h o t t k y plot The semiconductor-electrolyte interface [9, 10] can be treated as a series combination of space charge capacitance Cs¢ and Helmholtz layer capacitance CH. The latter is neglected for the case of the concentrated electrolyte. Cs¢ at different applied bias V can be expressed as [9, 11] 1 _ 2 (V_Vfb_ Csc 2 eoeseN D

kT) e

(1)

where e0, es, ND and V~ are respectively the permittivity of free space, relative dielectric constant, donor concentration of the semiconductor and fiat band potential. From the plot between 1/Cs¢: vs. V, the values of V~D and ND can be calculated. The other parameters such as the energy band positions E¢ and Ev, band bending Vb, depletion width W, and density of states N¢, in the conduction band can also be calculated from standard formulae [12, 13]. Figure 4(a) gives the M o t t - S c h o t t k y plots at various frequencies for CdS film using the $2-,$22- redox couple. Frequency-dependent capacitance has been observed in the present study where all the slopes converge to a c o m m o n point. This enables us to determine the fiat band potential but the determination of N D is unreliable. Assuming es to be frequency dependent, the space-charge capacitance given in eqn. (1) can be expressed as [14]

Cs 2

eoe ~(f)eN D\

-- Vfb -- m

(2)

where e~(f) is the dielectric constant at frequency f and e(0) is the static dielectric constant. We have calculated the values of es by measuring the capacitance at various frequencies {and zero bias) using a sandwich geometry A1/CdS/A1. The e s vs. f plot is given in Fig. 4(b). The important parameters calculated from M o t t - S c h o t t k y plots at 1 kHz for CdS film are summarized in Table 2. The values for CdS:Li film are also given in Table 2 for which a typical M o t t - S c h o t t k y plot at 1 kHz is shown in Fig. 4(c). Figure 5 shows the energy band schemes for the C d S - S 2 - , S : : - interface drawn from Table 2. The energy band schemes clearly show that the depletion region is formed within the semiconductor film.

3.3.2. Current density-voltage characteristics and charge transfer mechanism at the interface The J - V characteristics of the as-deposited CdS film with the cell configuration steel/CdS/S:-,S22-/Pt in the dark and under illumination are

169

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--3

Hz " z . i KNZ 3-5 ~HZ i.$oo

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t

30

CdS:

L,

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t 25

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1

2

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5

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6

F (FREQUENCY)

.

.

7

8 tN

.

.

9

lo

11

(KHz)

Fig. 4. (a) Mott-Schottky plots for CdS at different frequencies (curve 1 , 5 0 0 Hz; curve 2, 1 kHz; curve 3, 5 KHz), (b) relative dielectric constant vs. frequency for a CdS film and (c) Mott-Schottky plot for a CdS:Li film at 1 KHz.

TABLE 2 Summary of the results obtained from Mott-Schottky plots for the CdS-S2-,S22- and CdS :Li-S2-,S22- interfaces Parameter

CdS

C d S :L i

Flat band potential, Vfb(V(SCE)) Conduction band edge, Ec(V(SCE)) Valence band edge, Ev(V(SCE)) Position of Fermi level Ef below the conduction band (V(SCE)) Donor concentration N D (cm -3) Density of states in the conduction band (cm -3) Depletion width W (/.tin) Carrier type Band bending Vb(V ) Band gap (eV) from optical studies

--0.78 --0.88 1.54

--0.84

0.10 6 × 1014

6 X 1014

3.6 X 1018 0.050

0.062

n

n

0.16 2.42

0.24 2.35

170 VscE

,.o~- E~o/

-

0.4~

I0.0

0.0--

+

i

0.4 - -

2.42~v *

0.8 - -

+1.2 ,1.6

--

;E(~n~..t.

Fig. 5. Energy band positions Ec, Ev, Ef, zedox and Ef in the SCE scale for CdS film and $2-,$22- electrolyte. INTENSITY OF ILLUMINATION = 35m W/cm 2

o

-I.~

..

i -I.0

i6I

~ ~

~

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i!

g

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~-

Under illur~In~tlon

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- O.~

On 0

VOLTAGE

I

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I

O •~

O ,4

O •{

V (VscE)-----~-

-0.02

004

Fig. 6. J - V characteristics in the dark (1) and under illumination (z~) with d.c. bias. The potentials are measured with respect to SCE.

shown in Fig. 6. The dark cathodic current gradually increases for voltages from - - 0 . 9 V(SCE) to - - 0 . 6 V(SCE) whereas the dark anodic current increases steadily w i t h o u t any sign of saturation in the region - - 0 . 6 - +0.2 V(SCE). The anodic current curve under illumination is also w i t h o u t any sign of saturation but is marked by being higher than that in dark. This can be interpreted as being due to hole transfer across the semiconductorelectrolyte interface with significant contribution from the surface states and deep traps [5, 12]. The anodic photocurrent curve and the dark cathodic current curve intercept the voltage axis at - - 0 . 8 V(SCE) and - - 0 . 6 V(SCE)

171

respectively. The difference between these intercepts approximately gives the open-circuit voltage Vo¢ obtainable from a PEC cell with similar cell configuration. In our case we expect Voc ~ 200 mV. 3.4. Photoelectrochemical solar cells PEC solar cells have been fabricated with cell configurations (i) steel/ CdS/S2-,S22-/Pt and (ii) steel/CdS:Li/S2-,S22-/pt. The cell characteristics are drawn for different cells when illuminated with a light intensity of 35 mW cm -2 from a tungsten filament lamp. Figure 7 shows the J - V characteristics of PEC cells using CdS films of various thicknesses obtained with various chemical bath deposition times. The best performance is obtained for the CdS film photoelectrode deposited in 35 min (approximate thickness 6 pm). The poor cell performance of the low thickness film can be attributed to low p h o t o n absorption and short circuiting by voids present in the film. The high series resistance of the thicker films may be responsible for their poor cell performance. Figure 8 shows the J - V characteristics of CdS:Li film PEC cells for films obtained at different Li2CO3 concentrations in the chemical bath. The film deposition time was 35 min. The important cell parameters obtained from Figs. 7 and 8 are listed in Table 3. From Figs. 7 and 8 and Table 3, it is obvious that lithium doping improves the cell performance. This may be attributed to the lowering of resistivity as a result of lithium doping. There is an apparent decrease in the band gap from

I t.6 ¢4

2! ~,4, ' 4

,.o , . , , , \ \ 0.8 0.6

~ ~ l l I

uO.4

I

'

10,0| 40

t

!

80 Celt

120 160 200 240 voltage p V ! II)V)

Fig. 7. J - V characteristics of PEC cells using CdS films prepared at different chemical d e p o s i t i o n times: curve 1, 15 rain; curve 2, 30 rain; curve 3, 35 rain; curve 4, 50 m i n ; curve 5, 60 rain.

28:

172

24

0.4

0.8

---~'-CELL

1.2

16

2.0

24

2.8

3"2

3-6

CURRENTOENSITY d ( m A / c m 2}

Fig. 8. J - V characteristics of PEC cells for CdS and CdS:Li films at different molar concentrations of Li2CO3; curve 1, 0.0 M; curve 2, 0.01 M; curve 3, 0.005 M; curve 4, 0.0001 M; curve 5, 0.00005 M; curve 6, 0.00001 M.

TABLE 3 Photoelectrochemical solar cell parameters for CdS and CdS :Li films Cell configuration

Band bending

Voc (mY)

Jsc (mA cm ~ )

Fill factor

Efficiency (%)

185 254

1.7 3.4

0.256 0.281

0.3 0.8

(mV) Steel/CdS/S 2 ,$22 -/Pt Steel/CdS :Li/S2-,S22-/Pt

176 236

Intensity of illumination is 35 mW cm -2.

2.42 eV to 2.35 eV because of lithium doping which may increase the photosensitivity. Further, lithium is known to introduce additional acceptor states in CdS [15] which may help in additional charge transfer at the electrode-electrolyte interface.

References 1 J. Manasseh, G. Hodes and D. Cahen, J. Electrochem. Soc., 124 (1977) 532. 2 H. Gerischer, J. Electroanal. Chem. Interfacial Electrochem., 58 (1975) 263. 3 R. Hill, in T. J. Coutts (ed.), Thin Solid Film Active and Passive Devices, Academic Press, New York, 1978, p. 487. 4 N. R. Pavaskar, C. A. Menzes and A. P. B. Sinha, J. Electrochem. Soc., 124 (1977) 743. 5 S. Chandra, R. K. Pandey and R. C. Agrawal, J. Phys. D, 13 (1980) 1757.

173 6 R. C. Kainthala, D. K. Pandya and K. L. Chopra, J. Electrochem. Soc., 127 (1980) 277. 7 C. D. Lokhande, M. D. Uplane and S. H. Pawar, Solid State Commun., 43 (1982) 623. 8 M. Tsuiki and H. Minoura, Chem. Lett., 1 (1979) 113. 9 R. Memming, in A. J. Bard (ed.), Charge Transfer Processes at Semiconductor Electrodes, Electroanalytical C h e m i s t r y - - A Series o f Advances, Vol. 11, Dekker, New York, 1979. 10 V. A. Myamlin and Yu. V. Pleskov, Electrochemistry o f Semiconductors, Academic Press, New York, 1967. 11 N. F. Mott, Proc. R. Soc. London, Ser. A, 1 71 (1939) 27. W. Schottky, Z. Phys., 113 (1939) 367. 12 S. Chandra, D. P. Singh, P. C. Srivastava and S. N. Sahu, J. Phys. D, 17 (1984) 2115. 13 S. Chandra and S. N. Sahu, Phys. Status Solidi A, 89 (1985) 321. 14 F. Cardon and W. P. Gomes, J. Appl. Phys., 11 (1978) L63. 15 M. Savelli and J. Bougnot, in B. O. Seraphin (ed.), Topics in Applied Physics, Vol. 31, Solar Energy Conversion, Springer, Berlin, 1979, p. 214.