Photoactive screen-printed pyrite anodes for electrochemical photovoltaic cells

Solar Cells, 31 (1991) 119-141

119

Photoactive screen-printed pyrite anodes for electrochemical photovoltaic cells V. A n t o n u c c i ,

A. S. A r i c o ' and N. G i o r d a n o

CNR Institute for Transformation and Storage of Energy, Salita S. Lucia sopra Contesse 39, 98125 Messina (Italy)

P . L. A n t o n u c c i University of Reggio Calabria, Faculty of Engineering, Via Cuzzocrea, 48, 89100 Reggio Calabria (Italy)

U. R u s s o University of Podova, Department of Chemistry, Via Loredan, 4, 35131 Padova (Italy)

D . L. C o c k e Department of Chemistry, Texas A&M University, College Station, Texas (U.S.A.) F. Crea Universit& della Calabria, Department of Chemistry, 87030 Arcavacata di Rende, Cosenza (Italy) (Received March 9, 1990; in final form March 12, 1990)

Abstract The activation treatments necessary to produce photoactive screen-printed pyrite electrodes for photoelectrochemical applications are described. In particular, air (340 °C), hydrogen (200 °C) and air-hydrogen ( 3 4 0 - 2 0 0 °C) treatments have been selected. Surface and bulk characterization of the electrodes have been carried out by X-ray photoelectron spectroscopy, X-ray diffraction and M6ssbauer spectroscopy. Diffuse reflectance spectroscopy and photoelectrochemical tests in I - / I 3 - solutions allowed us to ascertain the optical absorption and solar energy conversion properties of the differently activated samples. The best performing electrode is the air-hydrogen activated electrode (~ = 5.52%) which shows an optimal combination of the optical absorption characteristics and semiconductor-electrolyte charge transfer properties. The results have been discussed on the basis of the electronic structure of the compounds involved in the interfacial chemistry.

1. I n t r o d u c t i o n T h e p o t e n t i a l o f p y r i t e s e m i c o n d u c t o r s in t h e f i e l d o f t h e p h o t o e l e c t r o c h e m i c a l c o n v e r s i o n o f s o l a r e n e r g y h a s b e e n w e l l o u t l i n e d in t h i s l a s t d e c a d e [ 1 - 6 ] . T h e f a v o u r a b l e o p t o - e l e c t r o n i c p r o p e r t i e s o f F e S z m a k e it a c a n d i d a t e for photoelectrochemical or photovoltaic devices. FeS2 is a c h e m i c a l l y s t a b l e c r y s t a l l i n e s e m i c o n d u c t o r m a t e r i a l w i t h a n energy gap Eg=0.95±0.05 eV [1, 2], v e r y c l o s e t o t h a t o f s i l i c o n (1.1 eV),

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120 but with a m o r e a d v a n t a g e o u s a b s o r p t i o n coefficient ( a = 6 × l 0 b c m - 1 for hv>_ 1.3 eV [3]). This permits the m a n u f a c t u r i n g of the pyrite material as a v e r y thin film, allowing a drastic r e d u c t i o n in the cost of solar cells; this material is also abundant, non-toxic and relatively cheap. The electronic s t r u c t u r e of pyrite r e s e m b l e s that of o t h e r dichalcogenides of transition metals, such as MoS2 or WS2, w h e r e b o t h the valence and c o n d u c t i o n band states derive f r o m d orbitals of the metal atom. Although the p h o t o t r a n s i t i o n does not induce w e a k e n i n g of chemical b o n d s b e t w e e n the metal and the c h a l c o g e n [3], e l e c t r o c h e m i c a l c o r r o s i o n has b e e n o b s e r v e d in the dark on pyrite single crystals [ 7 ] and, m o r e drastically, on polycrystalline pyrite [8]. A suitable choice of the r e d o x couple as well as p r o p e r p r e p a r a t i o n and assembling p r o c e d u r e s of the p h o t o e l e c t r o d e s might minimize this p h e n o m e n o n [9]. Pyrite s e m i c o n d u c t o r s for p h o t o e l e c t r o c h e m i c a l application have b e e n synthesized by chemical v a p o u r t r a n s p o r t [3], metallorganic chemical vap our deposition and s p r a y pyrolysis [11 ]. In this p a p e r a low-cost m e t h o d which utilizes polycrystalline pyrite for the m a n u f a c t u r i n g of p h o t o a n o d e s is presented; it is b a s e d on the screen-printing t e c h n i q u e t h r o u g h which pyrite mixed with an organic binder, i.e. p o l y t e t r a f l u o r o e t h y l e n e (PTFE), is d e p o s i t e d onto a c o n d u c t i v e substrate and thermally activated u n d e r different conditions.

2. E x p e r i m e n t a l details 2.1. Materials Pyrite p o w d e r (99.9%) was supplied by CERAC. The alumina substrate was furnished by General Electrics (96% A1SiMag 860). PdAg c o n d u c t i v e paste was f r o m F e r r o Corp. (lot 3432). The P T F E was D u p o n t 30 N type. Triton X-114 (alkylphenylpolyethyleneglycol) was f r o m BHD Chemicals. 2.2. P y r i t e electrode p r e p a r a t i o n A layer of c o n d u c t i v e PdAg film was s c r e e n printed (325 m e s h ) on an alumina substrate and dried at 150 °C for 15 min, followed by sintering at 850 °C for 15 min. A paste consisting of a m i x t u r e of polycrystalline FeS2 p o w d e r and P T F E (61% w/w) dispersed in a liquid (5% Triton X-114 in HeO), was then printed t h r o u g h a 400 m e s h s c r e e n to partially overlap the conducLive substrate; the final thickness was a b o u t 100 t~m. The " a s - f o r m e d " e l e c t r o d e s were thermally activated in three different e n v i r o n m e n t conditions as follows: Sample A: activation in h y d r o g e n flux (50 cm 3 m i n - 1 ) at 200 °C for 3 h. Sample B: activation in o v e n (air) at 340 °C for 30 min. Sample C: p r e a c t i v a t i o n in air at 340 °C for 30 min and activation in h y d r o g e n flux (50 cm 3 m i n - ' ) at 200 °C for 3 h. After the heat t r e a t m e n t s the e l e c t r o d e s w e r e c o o l e d d o w n slowly ( a b o u t 2 °C m i n - 1 ) . C o p p e r wire was a t t a c h e d to the thick film c o n d u c t o r layer, soldered and c o v e r e d with an insulating silicone r u b b e r (ROTH 5 9 9 0 Silicon-

121

K a u t s c h u k ) to e x p o s e only 0.5 c m 2 (unless o t h e r w i s e specified) of the s e m i c o n d u c t o r area.

2.3. E l e c t r o c h e m i c a l c h a r a c t e r i z a t i o n Resistivity m e a s u r e m e n t s of t h e e l e c t r o d e s w e r e c a r r i e d out at r o o m t e m p e r a t u r e u s i n g the f o u r - p o i n t s y s t e m . The a p p a r a t u s c o n s i s t e d of a Keithley 225 C u r r e n t Source, a Keithley 163 Digital V o l t m e t e r a n d a Keithley 5 3 0 Type-All System. Photoelectrochemical measurements were performed under potentiostatic conditions, u s i n g a c o n v e n t i o n a l t h r e e - e l e c t r o d e cell, c o n n e c t e d with an e l e c t r o c h e m i c a l s e t - u p m a d e of a n AMEL Model 551 p o t e n t i o s t a t , an AMEL m o d e l 631 e l e c t r o m e t e r , a n AMEL Model 567 f u n c t i o n g e n e r a t o r , an AMEL Model 5 6 0 i n t e r f a c e a n d a Keithley M o d e l 197 digital m u l t i m e t e r . A s a t u r a t e d c a l o m e l e l e c t r o d e (SCE) a n d a large a r e a p l a t i n u m disc (10 c m 2) w e r e u s e d as r e f e r e n c e a n d c o u n t e r e l e c t r o d e r e s p e c t i v e l y . An a q u e o u s r e d o x solution c o n t a i n i n g 0.2 M KI, 0.4 M HI a n d 0 . 0 0 5 M I2 w a s u s e d as electrolyte; a 3 0 0 W O s r a m light s o u r c e ( 3 1 0 - 1 0 0 0 n m ) p r o v i d e d 100 m W c m -2 of irradiation. T h e c o n v e r s i o n efficiency w a s m e a s u r e d in a t w o - e l e c t r o d e cell with g r a p h i t e as the c a t h o d e (30 c m 2) c o n n e c t e d to the pyrite p h o t o a n o d e (0.5 c m '~) b y a r e s i s t a n c e d e c a d e box: the v o l t a g e a c r o s s the l o a d w a s m e a s u r e d u n d e r air m a s s (AM) 1 illumination with no a p p l i e d e x t e r n a l bias. T h e e l e c t r o l y t e c o m p o s i t i o n w a s in this c a s e 7 M KI, 0.4 M HI a n d 0.05 M I2.

An a c t i o n s p e c t r u m of the p h o t o c u r r e n t as a f u n c t i o n of w a v e l e n g t h h a s b e e n c a r r i e d out in a t w o - e l e c t r o d e cell, close to s h o r t - c i r c u i t c o n d i t i o n s (VceH= 50 mV). T h e s e m i c o n d u c t o r e l e c t r o d e w a s l o c a t e d as n e a r as p o s s i b l e to the quartz w i n d o w o f the cell to m i n i m i z e e l e c t r o l y t e a b s o r p t i o n . T h e p h o t o e l e c t r o d e w a s i l l u m i n a t e d with a 100 W W - H a l l a m p followed b y a n Applied P h o t o p h y s i c s f / 3 . 4 M o n o c h r o m a t o r . M e a s u r e m e n t s w e r e o b t a i n e d in the w a v e l e n g t h r a n g e b e t w e e n 1500 a n d 450 n m b y u s i n g two different g r a t i n g s a n d in c o n n e c t i o n with o p t i c a l filters to eliminate s e c o n d h a r m o n i c s . T h e incident light p o w e r w a s m e a s u r e d either b y a m o d e l 1223 G e r m a n i u m m o d e l 5 7 5 p h o t o d y n e s e n s o r h e a d c o n n e c t e d with a m u l t i m e t e r or with a silicon UTD s e n s o r h e a d c o n n e c t e d to a UTD m o d e l 61 r a d i o m e t e r . Calibration w a s a c h i e v e d t h r o u g h the p h o t o d e t e c t o r r e s p o n s i v i t y data. P h o t o c u r r e n t w a s m e a s u r e d with a Keithley m o d e l 197 m u l t i m e t e r a n d n o r m a l i z e d b y dividing b y t h e c o r r e c t e d light p o w e r .

2.4. S o lid state a n a l y s i s C h a r a c t e r i z a t i o n of the pyrite p o w d e r s ( t h e r m a l l y t r e a t e d ) w a s p e r f o r m e d u s i n g a Philips p o w d e r d i f f r a c t o m e t e r with Cu K a r a d i a t i o n ( 1 . 5 4 0 6 /~), c o n t r o l l e d b y a n M 24 Olivetti PC a n d e q u i p p e d with a n a u t o m a t i c p e a k s e a r c h p r o g r a m ; for c o m p a r i s o n the " a s - r e c e i v e d " pyrite p o w d e r w a s also investigated. T h e diffraction p e a k s w e r e a s s i g n e d a c c o r d i n g to the ASTM c a r d s to FeS2 ( 2 6 - 8 0 1 ) , FeogS ( 2 5 - 4 1 0 ) , T-Fe203 (4-755, 15-615), F e 2 O a - H 2 0 (1392), F e S O 4 . H 2 0 ( 2 1 - 9 2 5 ) a n d FegSs (24-73).

122 M 6 s s b a u e r effect s p e c t r a w e r e o b t a i n e d at r o o m t e m p e r a t u r e on a c o n v e n t i o n a l c o n s t a n t a c c e l e r a t i o n s p e c t r o m e t e r utilizing a r o o m t e m p e r a t u r e r h o d i u m m a t r i x 57Co s o u r c e c a l i b r a t e d at r o o m t e m p e r a t u r e with natural aFe foil. The m a t e r i a l s w e r e finely g r o u n d , s u s p e n d e d in Vaseline a n d w r a p p e d in a thin a l u m i n u m foil, u n d e r a r i g o r o u s l y c o n t r o l l e d n i t r o g e n a t m o s p h e r e . The s p e c t r a w e r e fitted to L o r e n t z i a n line s h a p e s b y u s i n g l e a s t - s q u a r e s c o m p u t e r m i n i m i z a t i o n t e c h n i q u e s ; the q u a d r u p o l e d o u b l e t s w e r e fitted as two lines with equal a r e a s and widths, while the lines of the s e x t e t s w e r e c o n s t r a i n e d to h a v e relative intensities 3:2:1:1:2:3 and widths equal in pairs. Surface a n a l y s e s of the finished e l e c t r o d e s w e r e d o n e with a K r a t o s XSAMS00 X-ray p h o t o e l e c t r o n s p e c t r o m e t e r . A Kratos DS 300 or DS 8 0 0 o p e r a t i n g s y s t e m w a s u s e d in c o n j u n c t i o n with a digital P D P 11 m i c r o c o m p u t e r . The Kratos i n s t r u m e n t utilized a c o n c e n t r i c h e m i s p h e r i c a l a n a l y z e r a n d Mg K a radiation ( 1 2 5 3 . 6 eV) at 2 4 0 W. B a s e p r e s s u r e s in the 10 -9 T o r r r a n g e w e r e routinely a c h i e v e d in an ultrahigh v a c u u m s y s t e m . The a n a l y z e r w a s run in a fixed t r a n s m i s s i o n m o d e . All p e a k s w e r e r e f e r e n c e d to the a d v e n t i t i o u s c a r b o n l s p h o t o e l e c t r o n p e a k at 2 8 5 . 0 eV binding e n e r g y [12]. Diffuse r e f l e c t a n c e s p e c t r a w e r e o b t a i n e d b e t w e e n 900 and 200 n m on a Varian UV-Visible s p e c t r o p h o t o m e t e r e q u i p p e d with a 73 m m d i a m e t e r i n t e g r a t i n g s p h e r e to which a p h o t o m u l t i p l i e r w a s a t t a c h e d . The s p h e r e ' s internal s u r f a c e w a s c o a t e d with b a r i u m sulphate. The r e f l e c t a n c e of the s e m i c o n d u c t o r p o w d e r , p u r e or diluted with BaSO4, w a s m e a s u r e d a g a i n s t the s a m e p u r e s t a n d a r d .

2.5. X-ray diffraction (XRD) The diffraction p a t t e r n of the " a s - r e c e i v e d " FeS2 p o w d e r s h o w e d the c h a r a c t e r i s t i c p e a k s of the cubic pyrite s t r u c t u r e (Fig. 1); no e v i d e n c e of m a r c a s i t e (FeS2) or o t h e r iron s u l p h i d e s s u c h as p y r r h o t i t e (Feo.gS) w a s found. In the h y d r o g e n - t r e a t e d s a m p l e (Fig. 2) pyrite prevails t o g e t h e r with a small a m o u n t of pyrrhotite. The XRD s p e c t r u m of the a i r - t r e a t e d pyrite (Fig. 3) also s h o w s FeS2 as the l a r g e s t c o m p o u n d and, in addition, a c o n s i d e r a b l e p r e s e n c e of ~/-Fe203 as an o x i d a t i o n p r o d u c t . F u r t h e r m o r e , s u b s t o i c h i o m e t r i c iron s u l p h i d e s w e r e f o u n d on a c c o u n t of a slight d e c o m p o s i t i o n of FeS2.

1542~ 1159 ~

20

50

40

50

60

TWO-THETA DEGREES

Fig. 1. XRD pattern of "as-received" pyrite powder.

70

123

i=

I ~ ~

_

50.94 32.96 44.96 T ~ O -TH£T& D £ ~ 1 ~ £ £ ~

20,H

06.93

Fig. 2. XRD pattern of the hydrogen thermal treated pyrite powder.

|g 6

]

0

J H.~

~2.M 44.06 U.M T ~ O -THET& DEGI~EE~

11.93

Fig. 3. XRD pattern of the air thermal treated pyrite powder.

g i

TW0-THEIA0EGREES

Fig. 4. XRD pattern of the air-hydrogen thermal treated pyrite powder.

The air-hydrogen treatment showed effects deriving from the contribution of both the activation procedures (Fig. 4). Cubic pyrite prevails with respect to ~/-Fe203 and Feo.gS originated from the oxidizing and reducing treatments

124

respectively. The amount of the oxidation products is greater than that of pyrrhotite. 2.6. M6ssbauer spectroscopy The M6ssbauer spectrum of the "as-received" FeS2 (Fig. 5(a)), shows only the typical quadrupole split doublet with no trace of substoichiometric sulphides or other oxidation products. The spectra of the thermally treated compounds (Figs. 5 ( b ) - 5 ( d ) ) demonstrate, in full agreement with the XRD results, that the overall structure of the pyrite remained unaffected. The hydrogen activation (Fig. 5(b)) caused a partial desulphuration with concomitant production of pyrrhotite, of which two iron sites at least are well detectable in the spectrum. The hyperfine parameters of the pyrite remain virtually the same as in the "as-received" compound. The air activation produced a more dramatic change in the composition of the starting material (Fig. 5(c)). A second doublet appears in the M6ssbauer spectrum that, according to the XRD results, may be attributed to the presence of a large amount of "/-Fe2Os. As this species is present only in the superparamagnetic

z c LO IG

Z

Z L~ LLI LkJ LL

24 £"f ~. 8

ir [

9;3,#f (d5

B'.O

-4'.@

SOURCE

0.0

VELOCTT v

4.~

B°O

(MM/S)

Fig. 5. M6ssbauer effect spcctra of the "as-received" (a), hydrogen (b), air (c), and air-hydrogen (d) thermal treated pyrite powders.

125 TABLE 1 Mbssbauer parameters for the semiconductor powders ~a (ram s -l)

AEQ (mm s -1)

H~t. (kOe)

A (%)

Attribution

"as-received" FeS2 hydrogen treated FeS2

0.309 0.74 0.73 0.29

0.610 0.09 0.10 0.57

293 263 -

100 14 20 66

pyrite pyrrhotite pyrrhotite pyrite

air treated FeSe

0.40 0.20 1.42 0.71 0.24

0.62 0.60 2.77 0.11 0.61

-

43 57 2 19 79

T-Fe20z pyrite FeSO4 y-Fe203 pyrite

air-hydrogen treated FeS.,

aRelative to room temperature iron foil.

f o r m , i.e. c h a r a c t e r i z e d b y a v e r y ~as~ n u c t u a t i o n of the m a g n e t i z a t i o n v e c t o r , it c a n be d e d u c e d t h a t the a v e r a g e particle size is v e r y small ( d i a m e t e r less t h a n 6 rim) [ 13 ]. T h e i s o m e r shift f o r the r e m a i n i n g FeS2 c o m p o u n d d e c r e a s e d f r o m 0.31 m m s -1 of the original pyrite to 0 . 2 0 m m s -1, s u g g e s t i n g a n i n c r e a s e o f t h e s e l e c t r o n density at t h e iron n u c l e u s d u e to either a partial d e c r e a s e o f the d e l e c t r o n density or to a r e a r r a n g e m e n t of t h e s e electrons. A s u b s e q u e n t r e d u c t i o n of this m a t e r i a l in h y d r o g e n a t m o s p h e r e (Fig. 5(d)) c a u s e d large v a r i a t i o n s in t h e M S s s b a u e r p a r a m e t e r s o f Fe2Oa, indicating t h a t a partial r e d u c t i o n of the Fe a+ h a s t a k e n place. T r a c e s of FeSO4 w e r e f o u n d in the s p e c t r u m . In b o t h the air a n d the a i r - h y d r o g e n t r e a t e d s a m p l e s the d o u b l e t due to FeS2, s y m m e t r i c in the " a s - r e c e i v e d " s a m p l e , p r e s e n t s a large a s y m m e t r y b o t h in the line-width a n d in the a r e a with t h e low v e l o c i t y c o m p o n e n t s m a l l e r t h a n the high v e l o c i t y c o m p o n e n t (F1/2 = 0.74 a n d 0.63, A1/A2 = 0.68 r e s p e c t i v e l y ) . A s y n o p s i s o f the M S s s b a u e r p a r a m e t e r s of the s a m p l e s is g i v e n in T a b l e 1.

2.7. Surface analysis S p e c t r a of the e l e c t r o d e s are s h o w n in Figs. 6 ( a ) - 6 ( g ) , 7 ( a ) - 7 ( h ) , 8 ( a ) - 8 ( f ) a n d 9 ( a ) - 9 ( g ) . A g e n e r a l s u r v e y s p e c t r u m ( 1 2 5 0 eV s c a n ) is r e p o r t e d in Figs. 6(a), 7(a) a n d 9(a). T h e e l e c t r o d e s u r f a c e is m a i n l y c o m p o s e d o f iron, sulphur, c a r b o n , fluorine a n d o x y g e n ; f u r t h e r m o r e iodine a n d silicon w e r e d e t e c t e d in the e l e c t r o d e s u b j e c t e d to t h e e l e c t r o c h e m i c a l test. X-r~{y p h o t o e l e c t r o n s p e c t r o s c o p y (XPS) r e s u l t s s h o w s u r f a c e o x i d a t i o n p r o d u c t s in the a c t i v a t e d e l e c t r o d e s , i.e. Fe2Oa, FeSO4 a n d likely also Fe2(SO4)3. I r o n o x i d e is p r e s e n t b o t h in the air a n d a i r - h y d r o g e n a c t i v a t e d s a m p l e s , s u l p h a t e s in all e l e c t r o d e s . As t h e XPS analysis o f the " a s - r e c e i v e d " pyrite s h o w e d the p r e s e n c e o f v e r y small t r a c e s o f iron s u l p h a t e s , it m a y b e d e d u c e d t h a t a v e r y r a p i d o x y g e n c h e m i s o r p t i o n f r o m t h e a m b i e n t air o c c u r s .

126 ~e2p

o

2000

o N

21:1(11

1500 E 5~

8O0

5OO

400

200

730

0

B~nd~ng Energy [eV]

720

01s

Fls

200

!

7O0

--

710

8,nd,ng Energy lev]

695

69G

5z.O

530

Binding Energy {eV]

ClS

S~o

25O0

300

550

685

Bmd,ng Energy leVI

i

295

290

e

175

285

170

165

160

B~ndlng Energy leVI

8ind,ng Energy [eV] VB

10

S

0

-5

B ~ n g Energy [ev]

Fig. 6. XPS spectra of the hydrogen activated electrode surface.

The hydrogen activated electrode was analysed before (Fig. 6 ( a ) - 6 ( g ) ) and after the electrochemical test (Fig. 7 ( a ) - 7 ( h ) ) . In the first, the o x y g e n signal (Fig. 6(d)) results from a partial surface oxidation of sulphide to sulphate (532.5 eV). Sulphate singles out from the sulphur 2p spectrum at 169 eV of binding energy (B.E.) (Fig. 6(f)) and from the iron 2p spectrum (Fig. 9(b)); the iron 2p 3/2 peak appearing at 711 eV can also be attributed to iron sulphate. The fluorine l s signal at 692 eV deriving from C - F bonds in PTFE is affected by charging, as is evident from the peak broadening and its shifting by 3 eV (Fig. 6(c)). The charging effect can also be seen from the asymmetry of the carbon l s signal (Fig. 6(e)).

127 Fo2p

/

_u

3000 6OO

c

10o0 1000

2~ 800

600

.~0

200

0

0

730

7=)0

BindpngEnergy{eV} Fls

d 695

690

685

~)

_c

5ix)

6~0

~

2000 680

530

5~0

Binchng Energy [eV]

Binding Energy [eVl

13d

Cls

1500 ~

700

01s

2~ 0

710

l~ndmg Energy leV}

520

•

630

620

610

295

290

285

Binding Energy {eV]

Bir~(:llfK~ Energy [eV}

S,?.p

~B

280

15~.

200 5(1o. 175

100 170

165

Binding Er'~'gy [eVJ

160

0

10

5

0

-5

Binding Energy [eV}

Fig. 7. XPS spectra of the hydrogen activated electrode surface after the photoelectrochemical test in 7 M KI, 0.4 M HI and 0.05 12 solution. The h y d r o g e n - a c t i v a t e d e l e c t r o d e f r o m the p h o t o e l e c t r o c h e m i c a l effic i e n c y e x p e r i m e n t in 7 M KI, 0.4 M HI and 0 . 0 5 M I2 s o l u t i o n w a s r e p e a t e d l y w a s h e d with distilled w a t e r and dried for 30 m i n in a v a c u u m a t m o s p h e r e at 1 2 0 °C. In the s u r v e y s p e c t r u m (Fig. 7(a)) the silicon 2s ( 1 5 3 eV) and silicon 2p 3/2 ( 1 0 2 eV) p e a k s are due to the e l e c t r o d e s e a l i n g within an insulating s i l i c o n e rubber; the o x y g e n i s signal (Fig. 7(d)) c a n also be attributed in this c a s e to iron sulphate. The sulphur 2p s p e c t r u m (Fig. 7 ( g ) ) s h o w s t w o peaks: the first ( 1 6 4 eV) attributable to iron sulphide, whilst the s e c o n d at 171 eV derives f r o m iron sulphate. B o t h p e a k s are shifted by 2 eV, p r o b a b l y as a c o n s e q u e n c e o f a c h a r g i n g effect. The iron 2p 3/2 signal (Fig. 7(b)) is quite b r o a d e n e d , s u g g e s t i n g the p r e s e n c e o f iron sulphate,

128 ~:e2p

730

720

Fls

710

70(3

6~5

Energy {eV]

690

01s

540

530

520

295

2~

~

1~

295

280

Energy [eel

S2p

170

680

Cls

Energy [eV]

0

~5

B4ncl~',gEr~gy [ev}

vB

160

Energy [eV]

755

10

5 ~

0

-5

E~n~y levi

Fig. 8. XPS spectra of the air activated electrode surface.

m o n o s u l p h i d e and disulphide with binding e n e r g i e s of 712 eV, 710.5 eV and 709 eV respectively; f r o m s u c h e v i d e n c e it is inferred t h a t s u l p h a t e on the s u r f a c e dissolved into the solution owing to its high solubility ( 2 9 6 g 1-1 FeS04 at 25 °C) [14] and thus it did n o t significantly c o n t r i b u t e to the e l e c t r o n i c s during the p h o t o e l e c t r o c h e m i c a l e x p e r i m e n t s . The fluorine l s signal (Fig. 7(c)) is c h a r a c t e r i z e d b y the s a m e b r o a d e n i n g and shifting effects s e e n a b o v e for the f r e s h electrode. T h e s e effects are p r o b a b l y due to c h a r g i n g originated f r o m insulating P T F E chains s u r r o u n d i n g the pyrite particles. L a c k of differential c h a r g i n g in the two e l e c t r o d e s a c c o u n t s f o r a h o m o g e n e o u s distribution of P T F E on the surface. The iodine 3d signal w h i c h a p p e a r s in the e l e c t r o c h e m i c a l l y t e s t e d e l e c t r o d e at binding e n e r g y v a l u e s of 632 eV a n d 6 2 0 eV for the 3d 3/2 and 3d 5/2 e l e c t r o n i c s t a t e s (Fig. 7(e)) m a k e s it evident t h a t s t r o n g iodide and iodine a d s o r p t i o n h a d o c c u r r e d , s u g g e s t i n g a specific role of the a d s o r b e d s p e c i e s in the e l e c t r o c h e m i c a l p r o c e s s .

129

~2p

I 8O0

60O

/.00

200

0

730

720

710

Bwn(hng Energy [eV]

Binding Energy leVI

Fls

01s

695

690

685

680

5~0

530

520

~r~dtr~ Energy [eVl

Bwcitn~l Energy [eV}

Cls

S2p

295

29

285

280

B~llng Energy leVI

170

165

7DO

160

155

BndLng E~'gy {eV]

VB

10

5

0

-5

Brw;qng Energy leVI

Fig. 9. XPS spectra of air-hydrogen activated electrode surface. XPS spectra of the air activated electrode are shown in Fig. 8. The o x y g e n l s spectrum (Fig. 8(c)) shows a peak at 532 eV due to.surface iron sulphate. The shoulder at higher binding energy ( 5 3 4 eV) likely derives from ether-type oxygens in the Triton. The o x y g e n l s peak is more broadened towards the low binding energy region with respect to the corresponding peak in the hydrogen treated electrode. Iron oxide (Fe203) may be responsible for this, as its B.E. value in the o x y g e n l s spectrum is 530 eV. The iron 2p 3/2 peak (Fig. 8(a)) is quite broadened towards low B.E. values. The simultaneous presence of sulphates, sulphides (including pyrite) and oxides of iron may determine such a broadening. The sulphur 2p spectrum (Fig. 8(e)) shows the presence of iron sulphides (162 eV) at low B.E. and iron

130

0.1urn 15.0kV

4.06E2

5396/14

A29

Fig. 10. Scanning electron microscopy or" the air treated electrode surface.

sulphate (169 eV) at high B.E., confirming the evidence f r o m the iron 2p s p e c t r u m . The fluorine l s signal (Fig. 8(b)) for the air t r e a t e d e l e c t r o d e a p p e a r s to be sharp, suggesting a g r e a t e r surface c o n c e n t r a t i o n of fluorine than in the h y d r o g e n activated electrodes. The main p e a k is c h a r g e shifted by 3 eV; the s h o u l d e r at 689 eV is related with C - F b o n d s and not affected by charging. The p r e s e n c e of a differential charging is also evident in the c a r b o n l s s p e c t r u m (Fig. 7(d)). Here two p e a k s are present; one is related to c a r b o n c o n t a m i n a t i o n (285 eV) and the other, shifted by 3 eV, to fluorine b o n d e d c a r b o n (295 eV). P r o b a b l y o x y g e n b o n d e d carbon, s u c h as the e t h e r - t y p e g r o u p of Triton, m a y also c o n t r i b u t e to this latter peak. The s p e c t r a of the a i r - h y d r o g e n activated e l e c t r o d e (Figs. 9 ( a ) - 9 ( g ) ) r e s e m b l e those of the air t r e a t e d electrode. Contribution by the h y d r o g e n t r e a t m e n t only i n c r e a s e s the sulphide c o n t e n t on the surface, as f u r t h e r confirmed by the increase of the peak at 162 eV in the sulphur 2p s p e c t r u m (Fig. 9(f)). The p r e s e n c e of a differential charging is also evident in the fluorine l s and c a r b o n l s s p e c t r a (Figs. 9(c) and 9(e)); the differential charging as well as the m o r e m a r k e d evidence of C - F surface b o n d s in the air and a i r - h y d r o g e n activated e l e c t r o d e s suggest a different PTFE surface distribution a n d / o r structure. As air t r e a t m e n t at 340 °C induces melting of the P T F E [15] it is likely that assembling into islands and migration towards the surface should occur, as the surface distribution of PTFE on the air and a i r - h y d r o g e n activated samples, less h o m o g e n e o u s than in the h y d r o g e n activated electrode, a p p e a r s to show. Scanning e l e c t r o n m i c r o s c o p y of the air t r e a t e d e l e c t r o d e surface does in fact s h o w PTFE in the f o r m of islands at the e d g e s of the polycrystalline s t r u c t u r e (Fig. 10).

131

2.8. Optical a n a l y s i s Diffuse r e f l e c t a n c e s p e c t r a have b e e n m a d e to ascertain w h e t h e r chemical modifications i n d u c e d by the different t h e r m a l activations m a y have c a u s e d variations in the optical density in the w a v e l e n g t h r a n g e of interest for solar e n e r g y conversion. The radiation reflected f r o m the finely g r o u n d p o w d e r s is the s u m of its regular (specular) p a r t (i.e. the light w i t h o u t t r a n s m i s s i o n t h r o u g h the crystals) and the diffuse part of the light. This latter derives f r o m the radiation which has p e n e t r a t e d into crystals loosing intensity a c c o r d i n g to Lamb e r t - B e e r ' s law and r e a p p e a r i n g at the surface as multiple scatterings. A dilution m e t h o d was u s e d to minimize the regular part of the ree m i t t e d radiation [16]. The m e t h o d c o n s i s t e d of mixing and grinding the sample p o w d e r with an inactive n o n - a b s o r b i n g s t a n d a r d (BaSO4) and m e a s u r i n g the reflected light against the s a m e p u r e standard. The dilution with BaSO4 was m a d e up to a 0.33 mol fraction to obtain g o o d a c c u r a c y for the ree m i t t e d light m e a s u r e m e n t . However, s u c h a p r o c e d u r e has given rise to a flattening of the optical density signal with c o r r e s p o n d i n g p o o r l y resolved spectra. T h e s e p h e n o m e n a are clearly i n t e r p r e t e d if we c o n s i d e r that the regular part of the r e - e m i t t e d light d e p e n d s on the a b s o r p t i o n index as well as on the refractive index, a c c o r d i n g to the Fresnel f o r m u l a [17]. P u r e crystals selectively reflect at well defined a b s o r p t i o n bands; as the s a m e p h e n o m e n a o c c u r during o p e r a t i o n in a solar cell, diffuse reflectance s p e c t r a of the differently t r e a t e d p o w d e r s (without any dilution) are also p r e s e n t e d in Figs. 1 1 ( a ) - 1 l ( d ) . The s p e c t r a are well s t r u c t u r e d and s h o w c o n s i d e r a b l e values of optical density even at 900 nm. Optical density i n c r e a s e s t o w a r d s lower wavelengths, r e a c h i n g a m a x i m u m at a b o u t 830 nm. The " a s - r e c e i v e d " pyrite, as well as the h y d r o g e n t r e a t e d pyrite sample, shows a b r o a d a b s o r p t i o n b a n d in the visible r e g i o n b e t w e e n 500 and 350 nm, and a m o r e s h a r p e n e d a b s o r p t i o n b a n d in the n e a r UV region (Figs. 1 l ( a ) and 1 l ( b ) ) . The a b s o r p t i o n characteristics of the h y d r o g e n t r e a t e d sample are s o m e w h a t g r e a t e r than pyrite, p r o b a b l y due to an increase in the particle size as a c o n s e q u e n c e of the t h e r m a l t r e a t m e n t . However, air t r e a t m e n t c a u s e s substantial c h a n g e s in the visible region of the s p e c t r u m (Figs. 11 (c) and 11 (d)). A new b r o a d a b s o r p t i o n b a n d with a m a x i m u m at a b o u t 550 n m indicates that the air t r e a t m e n t has i n t r o d u c e d s o m e modifications in the e l e c t r o n i c s t r u c t u r e of the solid. This a b s o r p t i o n b a n d has b e e n a t t r i b u t e d to the f o r m a t i o n of ~/-Fe203 in a g r e e m e n t with the XRD as well as with the MSssbauer spectra. The a b s o r p t i o n b a n d s in the n e a r IR ( 8 3 0 nm) and in the n e a r UV r e g i o n (275 nm) s h o w the same profile as in the " a s - r e c e i v e d " pyrite. No p a r t i c u l a r c h a n g e s on the a i r - h y d r o g e n t r e a t e d p o w d e r s have b e e n r e c o r d e d with r e s p e c t to the p r e v i o u s s p e c t r u m , confirming that the a b s o r p t i o n characteristics are not modified b y the h y d r o g e n t r e a t m e n t . 2.P. E l e c t r o c h e m i c a l m e a s u r e m e n t s The r o o m t e m p e r a t u r e resistivity of the electrodes, m e a s u r e d b y the f o u r - p o i n t system, r a n g e d b e t w e e n 0.5 and 2 ~ c m for the h y d r o g e n t r e a t e d

132 ®

o.e2 o.N ILII4 O.IM 0.71 0.72 0.00

i

L

i

i

i

!l

1.00

o.a 0.~ 0.1HI

0.0k 0.70

I I

0.71

© 1.0~ L64 t.0C O.N~092~

~

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i

O001~

/. I tzoJ-

i,

I

1.15~

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/ ~ - ~

/ '

0.~

~, (NMiOHI[TER)

Fig. 11. Diffuse reflectance spectra of the "as-received" (a), hydrogen (b), air (c) and air-hydrogen (d) thermal treated pyrite powders.

samples and between 5 and 10 Ft cm for the air or a i r - h y d r o g e n treated samples. An n-type behaviour was found in all cases by thermoelectrical power measurements, also confirmed by photoelectrochemical tests (see below). Electrochemical properties have been evaluated in terms of corrosion, conversion efficiency of the solar energy radiation and interfacial charge transfer. Pyrite corrosion is known to be inhibited by acidic environments [7, 18]. As pH values close to pH 1 or even lower are particularly suitable, we have chosen an acidic environment of pH 1 (aqueous solution of H2SO4 0.17 M) to test the electrochemical behaviour of the finished electrodes. Cyclic voltammograms have been carried out (Figs. 12 and 13) starting from the rest potential and moving first in the anodic direction. In comparison with the results r epor t e d under similar conditions [7] on pyrite single crystal,

133

2,56

0.64

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O20~

E

J

0A 0,5 G6 0~7 0 8 0,9

VOLTAGE,V (S~)

0,4

S

J

u

J

,', VOLTA6E,V

Fig. 12. Corrosion behaviour of the hydrogen activated pyrite electrode in 0.17 M H2SO 4 solution. Fig. 13. Corrosion behaviour of air activated pyrite electrode in 0.17 M H2SO4 solution.

a slight c h a n g e in the cyclic v o l t a m m o g r a m profile for the h y d r o g e n t r e a t e d e l e c t r o d e and a c a t h o d i c shift of 100 mV for the c o r r o s i o n o n s e t have b e e n m o n i t o r e d . As for the a i r t r e a t e d pyrite e l e c t r o d e , the c o r r o s i o n o n s e t lies 100 mV m o r e positive than that of the h y d r o g e n sample. A c o r r o s i o n p e a k is well m o n i t o r e d at 0.5 V (SCE). As the a i r - h y d r o g e n t r e a t e d s a m p l e s h o w e d a b e h a v i o u r similar to the air activated e l e c t r o d e , its v o l t a m m o g r a m has not b e e n r e p o r t e d . It is s t r e s s e d that the o n s e t of the e l e c t r o c h e m i c a l c o r r o s i o n lies, in all cases, at potential values outside o f the p h o t o e l e c t r o c h e m i c a l range of interest (less t h a n 0.25 V (SCE)). C o n v e r s i o n efficiency m e a s u r e m e n t s have b e e n carried out in a twoe l e c t r o d e cell containing KI 7 M, HI 0.4 M and I2 0.05 M solution u n d e r AM 1 illumination with the e l e c t r o d e s c o n n e c t e d by an e x t e r n a l load (see E x p e r i m e n t a l details). The p o w e r c h a r a c t e r i s t i c s of the p h o t o e l e c t r o c h e m i c a l cells are s h o w n in Fig. 14 (curves A, B and C). Main p h o t o e l e c t r o c h e m i c a l p a r a m e t e r s of the cells are s u m m a r i z e d in Tables 2 and 3 t o g e t h e r with t h o s e of the e l e c t r o d e m a d e of the " a s - r e c e i v e d " FeS2 sample. This latter s h o w e d an o p e n - c i r c u i t voltage (Voc) of 220 mV with negligible c u r r e n t s and c o n v e r s i o n efficiency. The air activated sample is c h a r a c t e r i z e d by a Voc of 0.27 V with a p p r e c i a b l e short-circuit c u r r e n t values (Isc) of 3.9 m A c m -2. The s h a p e of curve B in Fig. 14 shows s o m e kinetic limitation for the c h a r g e t r a n s f e r at the e l e c t r o d e - e l e c t r o l y t e interface. The c u r r e n t s e e m s in fact to r e a c h a limiting value at the cell potential o f 0.1 V, t h e n increases n e a r the Isc region. As the low value of Vo¢ s e e m s to be in c o n t r a s t with the e v i d e n c e of iron oxide f o r m a t i o n (Eg= 2.2 eV), this is i n t e r p r e t e d as being due to r e c o m b i n a t i o n p h e n o m e n a . In fact, as p r e v i o u s l y s h o w n [19], r e c o m b i n a t i o n sites l o c a t e d at the s e m i c o n d u c t o r - e l e c t r o l y t e interface m a y influence the c u r r e n t o n s e t at low b a n d b e n d i n g values. Activation in h y d r o g e n a t m o s p h e r e leads to a significant increase of b o t h Voc (0.46 V) and I~¢ (18.4 mA c m - 2 ) , resulting in a c o n v e r s i o n efficiency

134

20 0

E

16 12

3

.

3

~ C

8 4 0

B 0'.1

0;2

013

0'.4

015 0:6 0:7 VOLTAGE/V

0.8

0.9

Fig. 14. Power characteristics of the photoelectrochcmical cell with hydrogen (A), air (B) and air-hydrogcn (C) activatcd pyrite photoanodes in contact with a 7 M KI, 0.4 M HI and 0.05 M I2 solution. TABLE 2 Preparative conditions of FeS2 samples Sample

Activation temperature (°C)

Atmosphere

A B C

200 340 340 200 -

hydrogen air air hydrogen -

D

(3 h) (30 min) (30 min) (3 h)

TABLE 3 Photoelectrochemical characteristics of FeS2 activated samples Sample

A B C D

Vc,¢ (V)

Isc ( m A c m -2)

Fill factor

0.46 0.27 0.80 0.22

18.4 3.9 12.0 0.12

0.39 0.25 0.57 0.004

(%) 3.30 0.26 5.52 0.0001

v a l u e o f 3 . 3 % ( c u r v e A). A s is e v i d e n t f r o m t h e s o l i d s t a t e a n a l y s i s ( s e e above), the hydrogen treatment induces surface modifications through the f o r m a t i o n o f s u b s t o i c h i o m e t r i c p y r i t e s i t e s ( a l s o i n h i b i t i n g i r o n o x i d e form a t i o n ) ; it is t h e n i n f e r r e d t h a t a r e d u c t i o n o f t h e r e c o m b i n a t i o n p h e n o m e n a at t h e i n t e r f a c e h a s o c c u r r e d , c a u s i n g a s i g n i f i c a n t i m p r o v e m e n t e i t h e r in Vo, o r p h o t o c u r r e n t .

135

Such e v i d e n c e m a y be i n t e r p r e t e d in t e r m s o f s t r o n g iodide a d s o r p t i o n on the e l e c t r o d e surface, as also c o n f i r m e d b y XPS analysis. The a d s o r b e d iodide, in fact, has b e e n s h o w n to cause passivation of the surface r e c o m bination sites in m a n y of the metal c h a l c o g e n i d e s e m i c o n d u c t o r s , thus improving the c h a r g e t r a n s f e r [20]. As the best p e r f o r m i n g e l e c t r o d e is the a i r - h y d r o g e n activated sample, displaying Voc and Isc values of 0.8 V and 12 mA c m -2 respectively, with ~? m a x = 5 . 5 2 % , it a p p e a r s r e a s o n a b l e to associate this high Voc value with ~/-Fe203 in c o n j u n c t i o n with a r e d u c t i o n of the r e c o m b i n a t i o n p h e n o m e n a at the interface, i n d u c e d b y the h y d r o g e n t r e a t m e n t . In fact, as s e e n a b o v e f r o m the diffuse reflectance s p e c t r o s c o p y analysis, the optical a b s o r p t i o n c h a r a c t e r of the air or a i r - h y d r o g e n t r e a t e d pyrite p o w d e r is c o n s i d e r a b l y i m p r o v e d by these treatments. The e x i s t e n c e of two distinct phases, i.e. FeS2 and Fe203, contributes, t h r o u g h their c o r r e s p o n d i n g b a n d gaps (theoretically 0.95 and 2.2 eV), to the o b s e r v e d s e m i c o n d u c t i v e behaviour. F u r t h e r insights into the c h a r g e t r a n s f e r characteristics have b e e n o b t a i n e d f r o m the polarization curves. To distinguish the effect of the iodide a d s o r p t i o n (already s h o w n to strongly influence the cell p e r f o r m a n c e ) the c o n c e n t r a t i o n of iodide was r e d u c e d f r o m 7.4 to 0.06 M (Fig. 15). U n d e r t h e s e conditions, the differences at the open-circuit voltage: Vredo×- V~ight as well a s Y d a r k - - Y l i g h t , are significantly less t h a n the open-circuit potentials o f the c o r r e s p o n d i n g samples in Fig. 14. The m e a s u r e m e n t of dark currents, at V
16.o

i

96I 3.2 -02

Vredox / /

i 0

0.2

64

VOLTA6e.V(~)

Fig. 15. Polarization curves in the dark (a, b, c) a n d u n d e r AM 1 illumination (A, B, C) o f the hydrogen, air and a i r - h y d r o g e n activated pyrite p h o t o a n o d e s respectively, in c o n t a c t with a 0.2 M KI, 0.4 M HI and 0.005 I2 M solution.

136

for the hydrogen activated electrode as a function of time is reported in Fig. 16. The photocurrent increases as the dark current decreases. Steady state values of 3.2 mA in phot oc ur r e nt and 0.4 mA in dark current, after a period of about 1 h, were reached. Results of the stability test for the hydrogen activated sample, in a two-electrode cell under illumination, are shown in Fig. 17. A smooth decrease of the cell voltage and cell current values has been recor de d during a time of about 60 rain under constant ohmic load (210 ~), until an almost constant output power has been reached. The dependence of normalized p h o t o c u r r e n t from the wavelength in polyiodide electrolyte is shown in Fig. 18. A particular trend observed in this action spectrum is the slow decrease of normalized phot ocurrent at wavelengths corresponding to the electrolyte absorption onset (650 nm). ut

//

/

Ul W Ul

t, ,I LZ E

210

" ..... b

N u

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aid

Time. n ~ Time, rrdn. Fig. 16. P h o t o - (a) a n d dark (b) c u r r e n t v s . t i m e c u r v e s o f t h e h y d r o g e n t r e a t e d e l e c t r o d e in a 0.2 M KI, 0.4 M HI a n d 0 . 0 0 5 M I2 s o l u t i o n with a n a p p l i e d e l e c t r o d e p o t e n t i a l of - 0 . 2

V (SCE) Fig. 17. Cell v o l t a g e a n d c u r r e n t as a f u n c t i o n of t i m e for t h e h y d r o g e n a c t i v a t e d e l e c t r o d e (0.1 c m 2) in a 7 M KI, 0.4 M HI a n d 0.05 M Ie s o l u t i o n u n d e r AM 1 i l l u m i n a t i o n with a n e x t e r n a l load o f 2 1 0 ft.

50

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:) O 0 I.,

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,

¢

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aV

Fig. 18. S p e c t r a l r e s p o n s e o f n o r m a l i z e d p h o t o c u r r e n t in a t w o - e l e c t r o d e cell in 4 M KI a n d 0.5 m M I solution, u n d e r t h e r e v e r s e bias of 50 inV.

137 A f t e r w a r d s , p h o t o c u r r e n t s m o o t h l y i n c r e a s e s due to the p r e s e n c e of a f u r t h e r transition; this latter o p p o s e s the a b s o r p t i o n effect of the electrolyte. As p r e v i o u s l y s e e n in diffuse r e f l e c t a n c e s p e c t r a (Fig. 11), this t r a n s i t i o n is a t t r i b u t e d to t h e p r e s e n c e of iron oxide. In this c a s e the t r a n s i t i o n o n s e t (1.8 eV) also a p p e a r s shifted t o w a r d s higher w a v e l e n g t h s with r e s p e c t to the Fe203 e n e r g y g a p (2.2 eV). W e h a v e a t t e m p t e d to d e t e r m i n e the n a t u r e and the value of the b a n d g a p b y p l o t t i n g ( I p h h v ) n/2 as a f u n c t i o n of h~ [3]. A well defined linearity r e g i o n for n = 4 is s h o w n in Fig. 19 giving a direct b a n d g a p of 1.1 eV, w h i c h is in a g r e e m e n t with t h a t f o u n d for single crystal pyrite [3]. H o w e v e r , linearity a p p e a r s in a s h o r t r a n g e for n = 1 (indirect g a p ) giving an i n t e r c e p t close to 0.72 eV. In this c a s e the s h o r t linearity r a n g e d o e s n o t allow a true a s s e s s m e n t . This effect m a y b e a t t r i b u t e d to the s u r f a c e r o u g h n e s s a n d / o r to the p r e s e n c e of d e f e c t s in the polycrystalline m a t e r i a l resulting in tailing of the a b s o r p t i o n e d g e due to s u b b a n d g a p levels. U n f o r t u n a t e l y , it is also i m p o s s i b l e to e v a l u a t e the e n e r g y g a p c o r r e s p o n d i n g to the Fe203 transition, due to the e l e c t r o l y t e a b s o r p t i o n in t h a t w a v e l e n g t h range.

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

The " a s - r e c e i v e d " p o l y c r y s t a l l i n e pyrite h a s b e e n s h o w n to be v e r y p o o r l y p h o t o a c t i v e ; t h e r m a l a c t i v a t i o n is n e e d e d to significantly i m p r o v e the p h o t o c o n v e r s i o n p r o p e r t i e s . In s u c h a c o n t e x t , c h e m i c a l m o d i f i c a t i o n s occurring during t h e r m a l t r e a t m e n t s as well as the p a r t i c u l a r p r e p a r a t i v e p r o c e d u r e play a f u n d a m e n t a l role. Fe203 f o r m a t i o n o c c u r r i n g at the s u r f a c e of the s e m i c o n d u c t o r is a d r a w b a c k in t e r m s of c h a r g e transfer. F o r m a t i o n of pyrite s u b s t o i c h i o m e t r i c c o m p o u n d s , i . e . FeS2_~:, h a s p o s i t i v e effects which, on a c c o u n t of lack of v a r i a t i o n s in the a b s o r p t i o n s p e c t r a a n d t h e i r low density (XRD), are to b e a t t r i b u t e d only to m o d i f i c a t i o n s of the surface. T h e p r e s e n c e of FeS2_x h a s +

I.

+

,/ 0.7 m

O.gO

1.02 ENIlWQY,

1.26

1.60

1.74

eeV

Fig. 19. Plot of (Iphhv) "~2 vs. hv, with n = 1 and n = 4 in the case of an indirect or direct band gap respectively.

138

allowed the a c h i e v e m e n t of significant c u r r e n t values u n d e r illumination also as a result of b e t t e r interfacial p r o p e r t i e s [22]. In fact, the e x i s t e n c e of s o m e c h e m i c a l i n t e r a c t i o n s b e t w e e n the iron sulphide a n d the iodide s p e c i e s in a q u e o u s solution has b e e n m a d e evident f r o m XPS on s a m p l e s d i s c h a r g e d f r o m the e l e c t r o c h e m i c a l test. This h a s s u p p o r t e d the e x i s t e n c e of a strict c o r r e l a t i o n b e t w e e n the iodide a d s o r p t i o n on the Fe 2+ or Fe 3+ s u r f a c e sites and the kinetics of the c h a r g e transfer. Such a p h e n o m e n o n c a n b e s e e n in t e r m s of the S - F e - I interfacial chemistry. During cell o p e r a t i o n , c h a r g e s e p a r a t i o n in the s e m i c o n d u c t o r occurs, the electrolyte acting as carrier a n d r e s e r v o i r of electrons. The n a t u r e of the i n t e r a c t i o n b e t w e e n t h e s e two p h a s e s is d e t e r m i n e d b y their e l e c t r o n i c structure. Pyrite h a s a cubic crystalline s t r u c t u r e w h e r e iron Fe 2+ cations o c c u p y the o c t a h e d r a l sites and s u l p h u r $2 a - anions o c c u p y the t e t r a h e d r a l sites. Fe 2+ h a s six e l e c t r o n s outside closed shells. The electronic configuration n e a r the F e r m i level s h o w s an e n e r g y s e p a r a t i o n b e t w e e n the T 2 g and Eg* m o l e c u l a r orbitals due to the crystal field e x e r t e d by $22- [23]. T h e s e orbitals h a v e low spin configuration as the T 2 g are entirely o c c u p i e d b y the six d e l e c t r o n s and the e n e r g y s e p a r a t i o n b e t w e e n the T 2 g and Eg* b a n d s is the e n e r g y g a p of the semiconductor. F o r m a t i o n of s u b s t o i c h i o m e t r i c FeS2 -x s u r f a c e sites p r o d u c e s a d e c r e a s e of the crystal field, thus p r o m o t i n g e l e c t r o n s to the Eg* levels. The n e w configuration m a y likely be d e p i c t e d as a quintet state. W h e n iodide a d s o r p t i o n on t h e s e u n s a t u r a t e d Fe 2÷ a n d / o r Fe 3+ sites occurs, the crystal field is n o t greatly i m p r o v e d . Iodide, in fact, is k n o w n to h a v e a low c a p a c i t y to p r o d u c e splitting of the d m e t a l orbitals, a c c o r d i n g to the ligands' s p e c t r o e l e c t r o c h e m i c a l series. A c c o r d i n g to the c o v a l e n c y a p p r o a c h , as I - s p e c i e s s u b s t i t u t e one or m o r e of the six n e a r e s t - n e i g h b o u r i n g s u l p h u r ions of the F e 2 ÷ sites at the e d g e s of the o c t a h e d r o n , t h e y o v e r l a p their v a l e n c e shell orbitals with t h o s e of the metal. In the s t o i c h i o m e t r i c pyrite structure, the ~ orbitals deriving f r o m the s u l p h u r 3p s t a t e s o v e r l a p prevailingly with the iron 3d Eg* orbitals a n d m u c h less with the iron 3d T 2 g orbitals, leading in this latter case to an essentially n o n - b o n d i n g T 2 g b a n d [23 ]. H o w e v e r , the e l e c t r o n s in the T 2 g orbitals are not purely m e t a l e l e c t r o n s w h e n the i n t e r a c t i o n o c c u r s with the pw orbitals of I ligands. In fact, the I - ions h a v e filled v a l e n c e shell p orbitals with e n e r g y l o w e r t h a n the m e t a l T 2 g orbitals. In t h e s e cases, the i n t e r a c t i o n destabilizes the T 2 g with r e s p e c t to the Eg* orbitals, thus diminishing the s e p a r a t i o n [24]. The d e c r e a s e of the crystal field and the quintet s t a t e f o r m a t i o n are likely a c c o m p a n i e d b y a c o n s i d e r a b l e e x p a n s i o n of the d e l e c t r o n cloud due to the o v e r l a p p i n g with the ligand orbitals, I ions having one of the g r e a t e r n e p h e l a u x e t i c effects e x p e c t e d in a ligand. With this p i c t u r e in mind, we e x p e c t e d that, w h e n p h o t o t r a n s i t i o n of e l e c t r o n s f r o m the T 2 g orbital v a l e n c e b a n d to the Eg* orbitals occurs, the a d s o r b e d iodide s p e c i e s m a y easily s h a r e e l e c t r o n s with the s e m i c o n d u c t o r v a l e n c e band. In fact, o v e r l a p p i n g of the Fe 2+ with the I - ions' orbitals likely i n c r e a s e s , the m e a n free p a t h of the e l e c t r o n s in the T 2 g b a n d thus d e c r e a s i n g the i n t e r e l e c t r o n i c repulsion; a f t e r w a r d s , d e s o r p t i o n of iodide

139 species as I3- can occur, completing the s e m i c o n d u c t o r - e l e c t r o l y t e c h a r g e transfer. Furthermore, as polycrystalline pyrite possesses a great number of deep lying states beyond the surface electronic levels induced by the iodide chemisorption, a direct tunnelling mechanism of electrons from these states into the s e m i c o n d u c t o r band gap (without the illumination requirement) likely also contributes to the overall charge transfer; this may especially occur when the Fermi level at the bulk of the semiconductor goes down the level corresponding to these energetic states in the band gap. The resulting current which flows through the cell during the first time of operation may be considered as the sum of the p h o t o c u r r e n t and dark current. As for Fe203, the electronic structure is very complex and a detailed analysis of its interaction with the electrolyte is beyond the scope of this work. In fact, iron ions are randomly distributed over the tetrahedral and octahedral sites of the spinel-like structure. The crystal field of the surrounding 0 2 - ligands may result in a splitting into seven states for the 3d electrons of the octahedral coordinated Fe 3 + ion [25]. However, Fe203 semiconductors have been largely studied with respect to the photoelectrolysis of water. Although they are characterized by an interesting band gap (2.2 eV) and fiat-band potential ( - 0 . 6 v s . SCE in 1 NaOH) [26], it has been shown that the photoelectrochemical performances are low. Studies by Dare-Edwards e t a l . [26] have shown that the p o o r photoresponse of n-Fe203 semiconductors derives from unfavourable surface effects rather than their bulk properties. In the Fe203 electronic structure the highest occupied orbitals are the half-occupied Eg* levels. Holes reaching the surface come into the iron Eg* band levels lying above the top of the oxygen 2p 6 band. According to Dare-Edwards e t al., the hole capture by these levels at or near the surface leads to the observed recombination drawbacks. From the above it is reasonable to assume that a mixed thermal treatment, i . e . preactivation in air followed by activation in hydrogen, represents the best compromise to obtain a photoelectrode with optimal bulk and surface characteristics. The spectral dependence of p h o t o c u r r e n t for the a i r - h y d r o g e n activated electrode (Fig. 18) shows that both pyrite and iron oxide appear to contribute to the photoelectrochemical behaviour. Fe203 transition produces a beneficial effect in the wavelength region where the electrolyte adsorption b e c o m e s stronger ( 6 5 0 - 4 0 0 nm). The low absorption threshold appears to be contradictory with the observed Voc value of the same electrode. This can be explained taking into consideration the following aspects. The a i r - h y d r o g e n activated electrode containing FeS2 and ~/-Fe203 shows different compositions varying with the position (passing from the semiconduct o r - e l e c t r o l y t e interface to the rear ohmic contact) as shown by surface (XPS) and bulk (XRD, Mbssbauer) analyses. The material propert y variations with position determine a continuous change in the energy gap value, i . e . in the conduction band edges and in the electron affinity through the overall s e m i co n d u cto r structure. As the contributions to the photovoltaic action in a solar cell arise from effects of the built-in electrostatic field at the junction, the effective force fields and the Dember potential, the theoretical Voc is

140 d e t e r m i n e d b y t h e s e t h r e e factors. In fact, p h o t o v o l t a g e s g r e a t e r t h a n t h o s e deriving f r o m the g a p e n e r g i e s h a v e b e e n r e p o r t e d in the l i t e r a t u r e [27, 28]; t h e y are c a u s e d m a i n l y b y the D e m b e r t e r m or f r o m a n o p t i m a l c o m b i n a t i o n of the o t h e r s o u r c e s . F o r the F e S 2 - F e 2 0 3 h e t e r o s t r u c t u r e , the g r a d i e n t in the e l e c t r o n affinity IAXI p r o d u c e d b y the c o m p o s i t i o n a l g r a d i n g m a y b e close to or g r e a t e r t h a n 0.2 eV f r o m the difference b e t w e e n the two fiat-band p o t e n t i a l s [3, 26] a n d a s s u m i n g a d i s t a n c e of 0.05 eV of the F e r m i level, in b o t h s e m i c o n d u c t o r s , f r o m the b o t t o m of the c o n d u c t i o n b a n d as o c c u r s for highly d o p e d s e m i c o n d u c t o r s . Hence, AX m a y be an additional s o u r c e of p h o t o v o l t a g e . R e g a r d i n g the D e m b e r c o n t r i b u t i o n to Vo¢, we c a n n o t e x a m i n e its effect as this implies k n o w l e d g e of the light-induced c h a n g e s in hole a n d e l e c t r o n p o p u l a t i o n (Ap and An r e s p e c t i v e l y ) , a n d b o t h hole a n d e l e c t r o n mobilities [29]. A s e c o n d effect m a y arise f r o m the i n c r e a s e of the d a r k c u r r e n t with iodide c o n c e n t r a t i o n w h i c h results in a significant n e g a t i v e shift of the light c u r r e n t o n s e t (Fig. 15). The m e a s u r e d Voc in the light m a y be s t r o n g l y influenced b y the e l e c t r o n injection in the s e m i c o n d u c t o r c o n d u c t i o n b a n d o c c u r r i n g at the F e S 2 - I - interface. The m a i n d r a w b a c k in the p r e s e n t e x p e r i m e n t s is the rapid d e c r e a s e of the p e r f o r m a n c e after the first 20 min of o p e r a t i o n (Fig. 17). This is not a t t r i b u t a b l e to c o r r o s i o n as it has b e e n s h o w n t h a t the P T F E a c t s b o t h as a b i n d e r and as a c o r r o s i o n inhibiting a g e n t b y i m p a r t i n g slightly h y d r o p h o b i c p r o p e r t i e s to the e l e c t r o d e surface. In the air and a i r - h y d r o g e n t r e a t e d s a m p l e s , the s u r f a c e p r e s e n c e of C - F g r o u p s a p p e a r s localized into discrete islands, w h e r e a s in the h y d r o g e n a c t i v a t e d e l e c t r o d e C - F g r o u p s are m o r e h o m o g e n e o u s l y distributed; accordingly, a different c o r r o s i o n b e h a v i o u r is m o n i t o r e d . H o w e v e r , as the c o r r o s i o n c u r r e n t o n s e t is outside the p h o t o e l e c t r o c h e m i c a l r a n g e of e n e r g y c o n v e r s i o n in the I - / I 3- solution, this d e m o n s t r a t e s t h a t the o b s e r v e d 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 c a n be entirely a t t r i b u t e d to the s e m i c o n d u c t o r c h a r a c t e r i s t i c s and, especially, to the s e m i c o n d u c t o r - e l e c t r o l y t e interface.

Acknowledgments The a u t h o r s are i n d e b t e d to Dr. Razzini for his help a n d for the use of his p h o t o e l e c t r o c h e m i c a l set-up for the s p e c t r a l d e p e n d e n c e of p h o t o c u r r e n t . T h e y are also grateful to Mr. M. Minutoli, Mr. G. Monforte a n d Miss E. Modica for their c o o p e r a t i o n .

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