Study of electrochemical and photoelectrochemical properties of nickel phosphide semiconductors

Solar Cells, 26 (1989) 303 - 312

303

STUDY OF E L E C T R O C H E M I C A L AND P H O T O E L E C T R O C H E M I C A L P R O P E R T I E S OF NICKEL PHOSPHIDE SEMICONDUCTORS MAHESHWAR SHARON* and G. TAMIZHMANI

Department o f Chemistry, Indian Institute o f Technology, Bombay 400 076 (India) CLAUDE LEVY-CLEMENT and JACQUES RIOUX

Laboratoire de Physique des Solides, CNRS, 92195 Meudon Principal Cedex (France) (Received February 3, 1988; accepted in revised form January 6, 1989)

Summary For the first time a detailed photoelectrochemical study of n-type nickel phosphide (Ni2P) material (thin film as well as powder) is reported. This material has a hexagonal structure with a band,gap of 1.0 eV. The band edge positions are calculated using electron spectroscopy for chemical analysis. The c o m p o u n d is found to be stable in alkaline and acidic media. The photoconversion efficiency is found to be 0.26%.

1. I n ~ o d u c f i o n In a recent survey by Sharon and Tamizhmani [ 1 ] on the phosphides of unstudied elements, it is suggested that in addition to the well studied InP [2], GaP [3] and ZnP2 [4], nickel phosphide could be a prospective candidate for photoelectrochemical solar cells. In the present paper, we present detailed physical, electrochemical and photoelectrochemical properties of nickel phosphide (Ni2P) material {thin film and pellet) prepared by the m e t h o d described elsewhere [ 1 ]. 2. Preparation of Ni2P

Nickel phosphate p o w d e r was prepared from a mixture of (99.99% pure) nickel p o w d e r and orthophosphoric acid (99.99% pure) by the m e t h o d described elsewhere [1 ]. Pellets were made and reduced for 2 h at 900 °C in a current of hydrogen gas. A thin film of nickel phosphide was also prepared b y allowing a small a m o u n t of orthophosphoric acid to react with the surface o f a nickel substrate at 250 °C for 1 h in an argon atmosphere. Subsequent reduction of the film at 700 °C for 30 min in a hydrogen atmosphere gave a thin film of Ni2P. This film was then fired at 700 °C for *To whom correspondence should he addressed. 0379-6787/89/$3.50

© Elsevier Sequoia/Printed in The Netherlands

304 6 h in an argon atmosphere to increase the crystallinity and the adherence of the film to the substrate. Unlike the pellet, sintering of the film could not be done at 900 °C because the material was found to decompose at this temperature. The surface quality of this film was not very satisfactory and there is a need to improve or develop a different m e t h o d to prepare thin films of this material.

3. Results

3.1. X-ray diffraction study The reduced material (Ni2P) was characterized by an X-ray diffractogram (XRD) using a Philips (model 1140} X-ray diffractometer with Cu K s radiation. The d values of this c o m p o u n d matched with Ni2P [5]. The structure of this c o m p o u n d was investigated at the CNRS laboratory and f o u n d to be hexagonal.

3.2. UV photoelectron spectroscopy study The UV photoelectron spectra were recorded with a V.G. Scientific Ltd., ESCA Mark III using a H e - I source. The spectra were recorded after etching the surface using argon ion sputtering inside the vacuum chamber (10 -9 Torr). The valence band position of the sample was f o u n d out from the sharp onset (see Fig. 1 ) of the photoemission spectra [6, 7 ]. The binding energy values are shown with respect to the Fermi level of the semiconductor. The valence band position is found to be 5.5 eV with respect to the vacuum scale.

3.3. Resistivity and Hall measurement Silver was found to give an ohmic contact. The resistivity of the hydrogen-reduced Ni2P pellet was found to be 0.017 ~2 cm at room temperature by the four-probe van der Pauw technique. The qualitative Seebeck study shows that the material is an n-type semiconductor. No Hall voltage could be detected, even by changing the applied current (0.5 - 9 mA) and magnetic field strengths (3 - 13 kGauss). This suggested that the mobility of carriers in Ni2P must be too low to be detected during Hall measurements. This may be one of the reasons w h y Ni2P showed a low photoconversion efficiency, as discussed in Section 3.10.

3.4. Chemical stability o f Ni2P 0.1 g of Ni2P powder was added to each of the 100 ml solutions of 0.1 M H2SO4, 0.1 M NaOH and distilled water. These solutions were frequently stirred and kept for 10 days. On the eleventh day they were filtered and the filtrates were analysed by the atomic absorption technique for the presence of metal ions. Concentrations of metal ions in the respective solutions were f o u n d to be 3.8, 0.68 and less than 0.001 #g m1-1. The low concentration of nickel ions in these solutions suggests t h a t Ni2P is insoluble in H2SO4,

305

padded

T

scale

c

~n g) (Jo

I EF=O

I 2

I

I 4

I

I I I I I I 6 B 10 Binding Energy (eV)-~P

I 12

I

t 14

Fig. 1. Curve A: UV photoelectron spectrum of Ni2P; the binding energy is plotted with respect to the Fermi level of the sample. Curve B: the same graph with an expanded scale showing the position of E v.

N a O H and in neutral water. However, this does not exclude the possibility of either electrochemical or photoelectrochemical corrosion of this material in these media.

3.5. Band gap measurement by the photocurrent method For this purpose, the photocurrent of a cell of the t y p e Ni2P/Ce 3+, Ce4+/ Pt was measured by illuminating the photoelectrode by a light source with different wavelengths. No external bias was applied to the electrode for this purpose. A tungsten halogen quartz lamp (250 W) was used as a power source and its intensity was normalized to obtain the same intensity of light at each wavelength. The intensity of light was measured with an Eppley thermopile (model 23344). An Oriel m o n o c h r o m a t o r (model 7240) was used to measure the wavelength of the incident light on the photoelectrode. A graph of (Jhv) 21" vs. hv was plotted, which gave a linear plot [8] (Fig. 2) with n = 4, suggesting Ni2P as an indirect band gap (1.1 eV) semiconductor. This value is different from the values obtained [1] with the reflectance m e t h o d (0.9 eV). Hence an average of t w o values has been taken as a correct band gap, i.e. 1.0 eV for Ni2P.

306

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hv to calculate the band gap of Ni2P.

3.6. F l a t b a n d p o t e n t i a l m e a s u r e m e n t

The capacitance C of the cell Ni2P/Na2SO4(PH = 7)/Pt was measured at different applied potentials with a VLCR-23 digital capacitance meter at a fixed frequency of 1.0 kHz. The potential of the Ni2P electrode was measured with respect to a reference calomel electrode using a digital voltmeter. Na2SO4 was used as an electrolyte for this measurement. A linear plot of C -2 vs. the potential (SCE) of the working electrode (Fig. 3) was observed from which the flatband potential was found to be +0.155 V (SCE). From the slope of the graph, the doping concentration was found to be 3.90 × 102° cm -3, assuming the dielectric constant of Ni2P to be 10.0. The flatband potential of Ni2P with different redox electrolytes was also determined b y this m e t h o d and the results are given in Table 1. These results suggest that the flatband potential of Ni2P depends on the redox potential of the electrolyte and the pH of the solution. The flatband potential determined by the differential capacitance, however, is always questionable unless the frequency dispersion experiment is carried o u t to find the frequency-independent capacitance region. However, this experiment could n o t be carried out owing to lack of facilities. Therefore, these results naturally need to be confirmed. 3.7. P h o t o c u r r e n t a n d p h o t o p o t e n t i a l m e a s u r e m e n t s

Current-voltage characteristics of an Ni2P/electrolyte/Pt cell were measured in darkness and after 30 s illumination with a halogen quartz lamp (60 mW cm-2). The photocurrent (with no load) and the photopotential (with 1 M~2 resistance) were measured (Table 2). These measurements were made with different electrolytes: Fe(CN)64-/Fe(CN)63- (0.01 M in 0.01 M NaOH), I2/I - (0.01 M in water), Fe2+/Fe 3. (0.01 M in 0.5 M H2SO4) and Ce3+/Ce 4* (0.01 M in 0.5 M H2SO4). Results show that Ce3+/Ce 4+ (0.01 M in 0.5 M H2SO4) gives the highest photoresponse with this material.

307

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Fig. 3. Differential capacitance measurement showing the linear relationship between C-2 us. electrode potential in an electrolyte of 1 M Na2S04. TABLE 1 Flatband potential obtained with different electrolytes Electrolytes

Flatband potentials (V vs. SCE)

Fe(CN)63-/Fe(CN)64-, 0.01 M in 0.01 M NaOH I 2 / I - , 0 . 0 1 M in water of p H = 7.0 Fe3+/Fe2÷, 0.01 M in 0.5 M H2SO4 Ce3+/Ce4+, 0.01 M in 0.5 M H2SO4 1.0 M Na2SO4 in water of pH = 7.0

--0.02 +0.02 --0.29 --0.25 +0.16

3.8. P h o t o c u r r e n t vs. t i m e o f i l l u m i n a t i o n

An Ni2P/Ce3+, Ce 4+ (0.01 M in 0.5 M H2SO4)/Pt cell was i l l u m i n a t e d w i t h a halogen q u a r t z lamp ( 2 5 0 W) a n d t h e p h o t o c u r r e n t was m e a s u r e d w i t h a P A R p o t e n t i o s t a t ( m o d e l s 173 a n d 176) vs. t i m e o f illumination. T h e results w e r e p l o t t e d w i t h an X Y ( t ) r e c o r d e r ( O m n i g r a p h i c 2 0 0 0 ) . T h e p h o t o c u r r e n t was m e a s u r e d u n d e r c o n s t a n t i l l u m i n a t i o n (Fig. 4(b)) and b y c h o p p i n g t h e light s o u r c e m e c h a n i c a l l y (Fig. 4(a)). Since t h e p h o t o c u r r e n t w i t h t h e Ni2P e l e c t r o d e takes a b o u t 4 0 s t o r e a c h t h e d a r k c u r r e n t value a f t e r t h e light has b e e n s w i t c h e d off, it can be c o n c l u d e d t h a t t h e surface o f Ni2P m u s t have large c o n c e n t r a t i o n s o f surface states. T h e s e surface states m i g h t be releasing t r a p p e d carriers at a slow rate. H o w e v e r , t h e r e p r o d u c i b i l i t y o f

308 TABLE 2 Photocurrent and photopotential in different electrolytes Electrolyte

Photocurrent (pA cm -2)

Photopotential (V)

Fe(CN)b4-/Fe(CN)63(0.01 M in 0.01 M NaOH)

(i) (ii)

68.0 61.0

I2/I(0.01 M in H20 )

(i) (ii)

103.0 14.0

--

FeZ+/Fe 3+ (0.01 M in 0.5 M H2SO4)

(i) (ii)

178.0 25.0

0.014 --

Ce3+/Ce

(i) (ii)

1768.0 22.0

0.183 --

4+

(0.01 M in 0.5 M H2SO4)

0.013 0.005 0.007

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Fig. 4. PhotocuzTent us. time of illumination: (a) Ni2P (pellet) showing the effect of chopped light (mechanical) and continuous illumination; (b) Ni2P (film) showing the effect of chopped light and continuous illumination. The continuous illumination reaches a maximum value after 8 - 10 rain of irradiation (not shown in the figure).

309

results after various dark and illuminated conditions suggests that though the photoresponse is delayed owing to trapping of minority carriers, the process of releasing the trapped carriers is reversible. The fact that this material takes about 8.0 min (Fig. 4(b)) to attain its maximum photocurrent (not shown in the figure) further supports this view.

3.9. Cyclic uoltammetry study A cyclic voltammogram using a PAR potentiostat (models 173 and 176), a potential programmer (model 175) and an XY(t) recorder (Omnigraphic 2000) was studied with four different redox electrolytes (Table 2). These studies were made with platinum electrodes, i.e. when both electrodes were platinum (Fig. 5 part X) as well as with an Ni2P/electrolyte/Pt system (Fig. 5 part Y). The cyclic voltammograms were also obtained under illuminated condition (i.e. conditions similar to those of Fig. 5 part Y). Since

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Potentio!(Vvs. SCE) Fig. 5. Cyclic voltammogram of Fe(CN)64-/Fe(CN)63- (0.01 M in 0.01 M NaOH) at an Ni2P electrode: X, with platinum electrodes (i.e. using both the anode and cathode as platinum electrode) at different sweep rates; Y with an Ni2P electrode (i.e. Ni2P as an anode and platinum as a cathode) in the dark (d) and under illumination (L); inset Z shows that the peak current is proportional to the square root of the scan rate.

310 the nature of the voltammograms obtained with all four redox electrolytes was almost identical, results with the Fe2+/Fe 3+ electrolyte are shown in Fig. 5. The plot of peak current vs. scan rate (mV s-1) gave a straight line (i.e. for Fig. 5 part X), suggesting that both the anodic and the cathodic currents are diffusion controlled [9]. The shift in the cyclic voltammogram with Ni2P (i.e. from dark to illuminated condition, Fig. 5 part Y) to negative potential, also suggests this material to be an n-type semiconductor. The presence of both the anodic and the cathodic current with Ni2P (dark and photocurrent) is an indication of unsatisfactory junction formation with these electrolytes. 3.10. P o w e r conversion efficiency The solar conversion efficiency with an Ni2P electrode using an Ni2P/ Ce 3+, Ce4+/Pt cell was determined (Fig. 6). The intensity of light falling on the electrode was measured using an Eppley thermopile (model 23344). The solar energy conversion efficiency was found to be 0.26% with a fill factor of 0.48. 1.8 ¸ i I

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Fig. 6. J - V curve for calculating the power efficiency of Ni2P (pellet) in Ce3+/Ce4+ electrolyte (0.01 M in 0.5 M H2SO4).

4. D i s c u s s i o n

Nickel phosphide has been characterized by the XRD to have a hexagonal structure with the formula Ni2P. The band edge positions of Ni2P were calculated using the flatband potential (0.16 V SCE equal to the Fermi level position) as well as the valence band edge position (5.5 eV) obtained from the UV photoelectron spectroscopy study. Taking the band gap of Ni2P as 1.0 e V , t h e value o f t h e c o n d u c t i o n band edge c o m e s t o almost 0.0 V with respect to the normal hydrogen electrode. Energy levels of this material, along with t h e e n e r g y levels of different redox electrolytes studied

311 VQCUUm

"~- Eox

-4

Eox

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i~red=~601 -6

~ - red l~red N.20 -7

(a)

(b)

(c)

(d)

-t" Ered (e)

-8

-9

Fig. 7. Band diagram of Ni2P along with the reorganization energy of different electrolytes; Eox and Ere d are the most probable energy levels of the oxidizing and reducing agents respectively; Eredo x is the redox potential of the electrolyte found from cyclic voltammogram; (a) band positions of Ni2P; (b) Fe(CN)6a-/Fe(CN)64-; (c) iodine/ iodide; (d) Fe2+/Fe3+; (e) cea+/ce 4+.

in this paper, are shown in Fig. 7. The reorganization energies of redox electrolytes are taken from the literature [10 - 12]. Ni2P has given both anodic and cathodic photocurrents and dark currents. This can happen if there is a poor junction formation. For a satisfactory junction formation, it is desirable that only the oxidation level of the redox electrolyte overlaps with the conduction band of an n-type semiconductor, and the reduction level should lie m u c h below the valence band position. But it will be noticed that the band edge positions of Ni2P lie in between the oxidized and reduced levels of all these redox electrolytes (Fig. 7), with the exception of the caesium electrolyte. As a result of this condition, both oxidation and reduction processes are expected to occur during illumination of the semiconductor-electrolyte interface. Values for the photopotential with different electrolytes are also almost identical.This is an indication of pinning of the Fermi level [13]. The low photoresponse values with the thin film, compared with the pellet, m a y be due to the low crystallinity(i.e.less than that of powder) and non-ohmic nature of the back contact (nickel) with Ni2P, because the work function of nickel (5.04- 5.35 eV) is m u c h lower than the Fermi level position of Ni2P (4.88 eV). However, a pellet of Ni2P gave an ohmic contact (accumulation layer) with silver because the work function of silver (4.26 4.74 eV) is above the Fermi level of Ni2P [14].

312 H o w e v e r , t h e p h o t o e l e c t r o c h e m i c a l b e h a v i o u r o f t h i n films o f this m a t e r i a l m u s t b e studied, a f t e r o b t a i n i n g a g o o d m e t h o d o f p r e p a r i n g thin films o f Ni2P.

Acknowledgments We t h a n k P r o f e s s o r O. P. A g n i h o t r i , I n d i a n I n s t i t u t e o f T e c h n o l o g y , Delhi, a n d Dr. B. M. A r o r a , t h e T a t a I n s t i t u t e o f F u n d a m e n t a l R e s e a r c h , B o m b a y , f o r e x t e n d i n g t h e i r facilities t o c a r r y o u t r e f l e c t a n c e a n d Hall studies r e s p e c t i v e l y . We are also t h a n k f u l t o t h e D e p a r t m e n t o f A t o m i c E n e r g y f o r t h e s u p p o r t o f a f e l l o w s h i p t o o n e o f us (G.T.).

References 1 2 3 4 5

6 7 8 9 10 11 12 13 14

M. Sharon and G. Tamizhmani, ,i.. Mater. Sci., 21 (1986) 2193. A. A. K. Vervaet, W. P. Gomes and F. Cardon, J. Electroanal. Chem., 91 (1978) 133. Lun-Shu Ray Yeh and N. Hackerman, J. Phys. Chem., 82 (1978) 2719. H. yon Kanel, L. Cantert, R. Hauger and P. Wachter, Int. J. Hydrogen Energy, 10 (1985) 821. Inorganic Phases Powder Diffraction File, International Center for Diffraction, Pennsylvania, 1981. N. J. Shevcheit, J. Tereda, M. Cardona and D. W. Langer, Phys. Status Solidi B, 59 (1973) 87. A. Q. Contractor and J. O'M. Bockris, Electrochem. Acta, 29 (1984) 1427. M. Sharon and B. M. Prmmd, Sol. Energy Mater., 8 (1983) 457. C. A. Koval and R. L. Anstermann, J. Electrochem. Soc., 132 (1985) 2656. K. W. Frese, Jr., J. Phys. Chem., 85 (1981) 3911. R. Memming, in A. J. Bard (ed.), Eiectroanalytical Chemistry - - A Series o f Advances, Marcel Dekker, New York, 1979. C. Gutierrez and P. Salvador, J. Elecrochem. Soc., 133 (1986) 924. A. J. Bard, B. Bocarsly, F.-R. F. Fan, E. G. Walton and M. W. Wrighton, J. A m . Chem. Soc., 102 (1981) 3671. L. V. Azaroff, Introduction to Solids, Tata McGraw-Hill, Bombay, 1960, p. 343.