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Enhancement of photoactivity in pyrite (FeS2) interfaces by photoelectrochemical processes
537
Surface Science 204 (1988) 537-554 North-Holland, Amsterdam
ENHANCEMENT OF PHOTOACTIVITY IN PYRITE (Fe&) INTERFACES BY PHOTOELECTROCHEMICAL PROCESSES *, C. PETIENKOFER
LIU Chongyang Hahn - Meitner-Institut
Received
11 March
and H. TRIBUTSCH
Berlin, Bereich Strahlenchemie
1988; accepted
for publication
1000 Berlin 39, Germany
6 June 1988
A temporary photoelectrochemical oxidation of chloride on a pyrite (FeS,) electrode surface leads to a considerable enhancement of photocurrent response and a decrease of the dark current. In contrast, passage of a dark current or of a photocurrent in presence of iodide has no significant effect. The surface transformation is accompanied by a significant modification of the photocurrent spectrum of the pyrite material. The improvement of the interface is caused by a photocorrosive transformation of the interface which reduces the concentration of surface recombination centers and leads to a change in the concentration and redistribution of energy levels in the forbidden energy region. XPS studies identify the surface states in the pyrite interface, which are responsible for its poor (photo)electrochemical behaviour and which can be removed during chloride oxidation as FeS groups.
1. Introduction Pyrite, Fe&, has been shown to be an interesting photoactive semiconductor with an energy gap of EG = 0.95 eV, which has favourable optoelectronic properties and is able to generate photocurrents at high quantum efficiencies [l-5]. With an absorption coefficient of approximately 6 X 10’ cm-i in the entire visible spectral region it could, in principle, become the basis of ultra-thin, flexible solar cells with the advantage of considerable cost reduction, low weight and high density packaging for transport. However, its solid state chemistry, its solid state physics and its interfacial chemistry still involve many non-calculable factors. It has up to now, for example, not been possible to grow FeS, in its ideal stoichiometry or to dope it in a well defined way. The available samples still contain many defects which produce electronic levels within the forbidden energy gap. They seem to be responsible for the low photopotential (< 200 mV) observed up to now which has limited the solar energy conversion efficiency of this material to approximately 3%. The first * One leave from: Dalian People’s Rep. of China.
Institute
of Chemical
Physics,
Chinese
0039-6028/88/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
Academy
B.V.
of Science,
Dalian,
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Liu Chongvang et al. / Enhancement
ofphotoactic~ity m p_yrite interfuces
photoactive MOCVD layers of pyrite have already been reported [6]. However. their optoelectronic and crystallographic properties are quite poor. Since most, charge carriers generated by solar light in pyrite are produced within 200 A of the surface which typically is an important source of recombination and trapping centers, the interfacial chemistry of Fe& is of crucial importance for its optoelectronic performance. Our present knowledge of the solid state properties of pyrite is limited [7-111 and there is nearly no knowledge on its surface chemistry. Recently, first experimental evidence was obtained which demonstrates that interfacial coordination chemistry is involved in photoinduced interfacial electron transfer. High rates of electron transfer and stabilization of the FeS,/electrolyte interface against corrosion could only be accomplished with electron donors which are able to act as ligands to interfacial iron into which photogenerated holes are channelled. When comparing, in rotating ring disc experiments, the photo-oxidation of Br-, Fe(phen):+, and Fe(bipy):+ adjusted to comparable redox potentials, it was observed that under identical experimental conditions only Br- could efficiently stabilize the illuminated pyrite interface [12]. This means that efficient electron transfer is only possible by ligand mediated inner sphere electron transfer. In the meanwhile further evidence is supporting this finding of photoactivated interfacial coordination chemistry [13]. This contribution studies the influence of photoelectrochemical surface modification on the quantum efficiency for photocurrent generation and on the rectifying properties of the pyrite electrolyte interfaces.
2. Experimental Single crystals of n-Fe& were synthesized by the chemical vapor transport (CVT) method [3]. The ohmic back contact was made with a Ga-In alloy. The crystals were fixed by silver paste (3M) onto a copper rod and isolated using epoxy resin (Scotchcast 5241). Mechanical polishing was performed using a diamond paste with successively decreasing grain size (3 to 0.25 pm) followed by rinsing with deionized water and methanol in an ultrasonic bath. A standard three electrode set-up (HERA potentiostat) was used with a platinum electrode as a counter and a saturated calomel as a reference electrode (SCE). The light source was a 240 W W-halogen lamp (Oriel). Photocurrent spectra were obtained with a computer-controlled set-up (monochromator: Kratos GM 252, lock-in amplifier: PAR 9503-SC, chopper: PAR 9479, computer: Hewlett-Packard 85). The second harmonics were cut off using optical filters. Light reflection from the electrode surface was not taken into consideration. For the XPS-experiments the crystals were mounted as grown without polishing or etching in tantalum clamps on stainless steel stubs. Spectra were
Liu Chongvang et al. / Enhancement
of photoactivity
m pyrite interfaces
539
recorded in a conventional VG-Escalab MkII described elsewhere [14]. The electrochemical cell for the chlorine treatment was installed in an oxygen free glove-box and the transfer to the photoelectron spectrometer was done in a special transfer vessel unit without exposing the samples to ambient air. In the electrochemical cell only the surface of the crystal seen by the photoelectron spectrometer had contact with the electrolyte. Great care was taken not to wet the tantalum clamps nor any other metal surface connected to the working electrode output of the potentiostat.
3. Results 3. I. Electrochemical
observations
When a polished n-Fe& electrode is placed in contact with a redox electrolyte such as I-/I;, only a very poor (photo)current-voltage performance is observed (fig. la). Also repeated cycling of the electrode does not improve the quality of the characteristics observed under intermittant illumination. This result is not surprising since earlier experiments [3,4] have shown, that the FeS, surface is not affected by corrosion in presence of I -/I, in the electrolyte. We have, however, observed that a temporary cycling of FeS, in presence of chloride in the electrolyte leads to a considerable improvement of the photoelectrochemical properties of the FeS,/electrolyte interface (“activation” of the electrode). The results of such a temporary cycling, in presence of illumination, in a 3M KC1 containing electrolyte (pretreatment) on the performance of (non-etched) polished pyrite in contact with an electrolyte containing 5M KI with OSM H,SO, is shown in fig. lb. It is interesting to note that during this pretreatment in contact with a Cl- containing electrolyte approximately 73% of the positive charge carriers are going into photoevolution of chlorine, the rest (27%) are lost in some corrosion reaction [4] in which iron chloride is one reaction product. In order to get a better insight into the mechanisms involved we have initiated a systematic study. Fig. 2 shows the dark current-voltage curves of FeS, in contact with a 3M KC1 containing electrolyte during the 1st and the 10th sweep (20 mV s-l). No significant change is observed, which leads to the conclusion that charge transfer processes involved in the generation of dark currents do not lead to an improvement of the FeS, interface. When, however, a similar experiment is repeated under illumination, a clear improvement of the characteristic is observed. Figs. 3a-3c show the (photo)current-voltage curve for FeS, in contact with the Cl- containing electrolyte during the first, an intermediate and the 30th sweep (the intermittant illumination makes dark and photocurrent distinguishable, the light was only turned on during the
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Liu Chongyang et al. / Enhancement
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a) J
0 electrode
0.5 potenhal
/
1 V(SCE)
Fig. 1. Current-potential curves of n-Fe& in contact with 5M KI in 0.5M H2S0, under chopped illumination (sweep rate 20 mV s -I): (a) before activation, (b) after activation.
positive sweep). Attention should be pointed towards a photocurrent which develops at a photovoltage between 0.3 and 0.5 V (SCE) as a consequence of the “activation” of the pyrite interface. It can be attributed to the oxidation of Fe2+ which is liberated from the electrode due to photocorrosion and the dark reduction of Fe’+. A plot of the change of photocurrent and dark current at 1.5 V (SCE) as a function of the number of performed potential sweeps is shown in fig. 4. In this case the pyrite electrode has previously been etched in HF/CH,COOH/HNO, (1 : 1 : 2 by volume), which is known to liberate the surface from oxides, for one to two minutes. It is clearly seen how the photocurrent density increases while the dark current density decreases. After
Liu Chongyang et al. / Enhancement
of photoactivity
m pyrite interfaces
541
60 -
I 0
L 0.5 electrode
f 1 potential / VISCE)
1 1.5
Fig. 2. Dark current-potential curve of n-Fe& before activation in 3M KC1 in OSM H,SO, first sweep, (- - -) 10th sweep. (sweep rate 20 mV s-l): ( -)
30 cycles in presence of air, nitrogen was bubbled through the electrolyte for 45 min. Arrow (a) shows the first to fifth sweep thereafter. Then the electrolyte was replaced by a fresh solution with the same composition (arrow (b)) and sweep 36 to 40 continued without nitrogen bubbling. Afterwards the electrode was removed from the cell and kept in air for 40 h (arrow (c) in fig. 4). As can be seen there is a significant effect on both the dark current and the photocurrent. Subsequent sweeps, in absence of nitrogen bubbling, again lead to a recovery of the pyrite interface. It has been reported that also anodic photo-oxidation of H,O, is able to improve the (photo)current voltage characteristic of FeS, [3]. This can equally be explained in terms of an anodic photoetching of the electrode surface. The mechanism is, however, difficult to study due to the instability of H,O, on the pyrite surface. Also reduction of protons at the electrode, which leads to hydrogen insertion, hydrogen evolution and the liberation of some H,S leads to an improvement of the photoactive pyrite interface [23]. Fig. 5, which again refers to an unetched electrode as fig. 3, shows photocurrent spectra of pyrite in the near-infrared region where the penetration depth of light is varying between near infinity and as little as 1.6 x lop6 cm. Fig. 5a is the spectrum obtained before “activation” of the pyrite interface, fig. 5b the spectrum during the process of activation and fig. 5c the spectrum after activation of the electrode by photo-oxidizing it in contact with a Cl- containing electrolyte. The spectra are taken in a potential region, where
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LIU Chongvang
et al. / Enhancement
electrode
of photonctwiq
potential
/
in p.yrite interfaces
V(SCE)
Fig. 3. Current-potential curves of n-FeS, in contact with 3M KC1 in OSM H2S04 under chopped illumination (sweep rate 20 mV s _‘): (a) before activation through passage of photocurrent, (h) during activation. (c) after activation.
small photoeffects (due to oxidativc processes [3j at the surface of FeS,) are only detectable by usage of lock-in techniques for recording the photocurrent spectra. It is seen that the spectrum of a “non-activated” pyrite interface shows a pronounced photocurrent peak near 0.95 eV. Light which deeply penetrates the electrode surface and creates holes sufficiently far away from the interface apparently contributes far more to the photocurrents than light absorbed near the electrode surface. With progressive “activation” of the electrode surface charge carriers generated near the FeSJelectrolyte interface contribute increasingly to the
Liu Chongyang et al. / Enhancement
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543
rn pyrite interfaces
60
30
0
0
15 I
the
30 I
sequence
45 I
of cyclic
I
60
J4
sweeping
Fig. 4. Dependence of n-Fe& (photo)current density (etched sample) at 1.5 V (SCE) on the sequence of cyclic voltammetry in 3M KC1 in 0.5M H,SO., (sweep rate: 20 mV s-l): (a) bubbling through the electrolyte with N, for 45 min, (b) electrolyte replaced by a fresh one with the same composition, (c) electrode removed from the cell and kept in air for 40 h.
photocurrent. The photocurrent density produced by photons with higher energy increases to such an extent that it outweighs by far the original photoelectrochemical IR response. The significant change of the photocurrent spectrum near the absorption edge is remarkable. This is even better shown by the plots of figs. 6a-6c which are extrapolations for the determination of indirect (n = 1) and direct (n = 4) energy gaps [25]. It is improbable that photoelectrochemical surface modifications can significantly change the size of the direct energy gap which indicates that extrapolations of electrochemical photocurrents for the determination of the energy gap, as widely practized in the literature, are of questionable value in the case of pyrite. It is seen from the scanning electron micrograph pictures in figs. 7a and 7b that photo-oxidation of chloride at FeS, is accompanied by photoeching. During this process the originally smooth electrode surface is gradually becoming rough and covered by corrosion structures. Even though the absolute surface area increases considerably, the absolute dark current is reduced. The interfacial changes produced during anodic polarization in contact with Cll containing electrolyte interestingly give also rise to a small photocurrent peak at 0.76 eV which is situated at lower energy than the energy gap and can be attributed to characteristic surface states formed during the surface
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I
0.8
I
I
1.0
1.2
of photoactwq
,
I
I.&
in pyrite interfaces
1.6
1.8
photon energy / eV Fig. 5. Spectral
response
of n-Fe& at 0 V (SCE) in 3M KCI in OSM H,S04: (b) during activation, (c) after activation.
(a) before activation,
modification (fig. 8). As already indicated, no “activation” is observed as a consequence of photoanodic polarization of FeS, in contact with a 5M KI solution. A purple-brown film is seen during polarisation but vanishes when the current is turned off. It can be attributed to a polycrystalline film of iodine formed in a I- depleted diffusion layer near the electrode surface.
3.2. Photoelectron
spectroscopy
XPS investigations on pyrite surfaces have shown that parts of the airoxidized surface were covered with FeS, iron oxides and iron hydroxides [3,15,16]. These surface layers of FeS should contribute to the high dark currents of untreated FeS, crystals in electrochemical solar cells by giving rise to defect or surface states in the gap. Since iron d-states are forming the edge
Liu ChongVang et al. / Enhancement
1
0.8
of photoactivity
I
1
1.0
1.2
1
1.4
in pyrite interfaces
,
1.6
545
I
1.8
photon energy / eV Fig. 6. Plot of hv versus (q hv)“/* from activation, (b) during
n = 4, (- fig. 5: ( -) activation, (c) after activation.
-)
n = 1; (a) before
of the valence band of pyrite, transformation of interfacial FeS, to FeS must necessarily involve a shift of Fe d-states into the forbidden energy region. It has been shown, that FeS, decomposes to FeS by Ar sputtering [7,19]. Fig. 9 shows the core level spectra of an air-exposed FeS, crystal (exposure time < 1 h) before and after the Ar sputtering for the Fe2p and S2p levels. The tail at the higher binding energy side in the Fe2p spectrum of the sputtered sample is due to FeS in the topmost surface layers [19]. Even more distinct is the change due to the chemical shift in the S2p levels, for FeS one finds a binding energy of 161.4 eV [20] significantly shifted from the value for FeS, 162.5 eV [15,21]. Fig. 10 displays the spectra after cycling the crystal 15 times from 0 to 1.5 V versus SCE in 3M KC1 in 0.5M H,SO, under illumination. The crystal was removed at 1.5 V after the last cycle from the
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Fig. 7. Scanning
electron
micrographs
of ph0toactioit.y in pyrite interfaces
(SEM) of n-FeS, activation.
electrode:
(a) before
activation.
(b) after
electrolyte and dried by N, purging before transfer. The spectra show clearly that FeS is removed from the sample. The components of the electrolyte (Cl-, K+, SO,‘-) are found on the samples (hump at 169 eV BE is the signal from SO:-). Beside the removal of FeS from the surface it is remarkable, that nearly all oxides or hydroxides bound to the iron are removed as can be deduced from the change in the 0 1s emission displayed in fig. 11. This is due
Liu Chongyang et al. / Enhancement
L
photon spectral
response
of n-Fe& activation,
in pyrite interfaces
541
I
I
0.7
Fig. 8. Subgap
of photoactivity
0.9
0.8 energy
/
eV
at 0 V (SCE) in 3M KC1 in OSM H,SO,: (b) after activation.
(a) before
to the low pH of the electrolyte and the solubility of the oxides at lower pH values.
4. Discussion The XPS results suggest a connection between the presence of FeS at the electrode surface and at least a part of the electrochemical dark current. Removing this FeS layer in a photoelectrochemical process improves the characteristics of the pyrite/redox electrolyte interface. The S 2p spectra show, that the removal of FeS is nearly complete, but on the other hand the linewidth and shape of the sulfur line compared to the initial one suggests, that the sulfur stoichiometry still differs from the expected behavior of a clean FeS, surface. Goslowsky et al. [22] found on etched CuInS, samples significant amounts of sulfur in molecular aggregations up to S,,. From the linewidth and structure of the Fe2p spectra, which show a rather well defined shape and did not differ very much from spectra obtained from vacuum cleaved samples [19], such conclusions cannot be drawn. Buckley et al. reported [15], that on air-exposed pyrite the main emission in the 01s region is obtained around 531.6 eV by chemisorbed water or hydroxide. The shoulder at 530 eV on the unsputtered sample belongs to iron oxides. By sputtering
Liu Chongyang et al. / Enhancement ofpholoactivity in pyriteinterfaces
548 1600
-
a
1500 l&O0 -
800 700
-
600 700
705
715
710
720
725
730
BindingEnergy EIEfeV
600 : 500 -t: t/l t400 .* 2 2 300 c 200 100
0. 155
157
159
161
163
BindingEnergy
165
167
169
BEf eV
Fig. 9. Fe2p level (a) and S2p level (b) from an air-exposed (lower curves) and Ar-sputtered (4 min 600 V, 2 x 10C5 mbar Ar) pyrite single crystal. 200 W MgKa, 10 V pass energy. In the insert and 2p,,, sulfur level for the sputtered sample is given for clarity. a deconvolution of the 2p,,, Solid line: sulfur as in FeS, dotted line: sulfur as in FeS,.
Liu Chongyang et al. / Enhancement
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1100 1050 "?OOO 2 > 950 u Ir"900 c" 2 850 -c 800 750 700' 700
450.
720 715 710 BindingEnergy BE, eV
705
725
730
b
400 ^ 350
x200 .k t" 2 150
c
100
155
157
159
165 163 161 BindingEnergy BEF eV
167
169
Fig. 10. Fe2p level (a) and S2p level (b) recorded after the photoelectrochemical treatment under the same conditions as in fig. 9; the hump at 169 eV in the sulfur spectrum is due to SO:-.
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ut pyrite interfaces
700 650
350’
_-II_
?I20 522
520
52L 526 528 530 532 531, 536 BindingEnergy 6EF eV
522 52L 526
528
530
532
53L
536
538
510
538
540
BindinaEnergy BEF eV Fig. 11. 01s spectra for the sputtered sample (top), the initial air exposed sample (middle) and the electrochemically treated crystal (bottom). Same spectrometer settings and sample orientation as in fig. 9. The arrow indicates the position for the oxide.
Liu Chongvang et al. / Enhancement
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Fig. 12. Energy level diagrams for n-Fe& showing the electron transfer process in the dark and during illumination. SS denotes surface states. Ox and Red indicate the distributions of oxidized and reduced species. 0.76 belongs to a transition probably from iron-chlorine complexes at the surface, whereas 0.9 is due to a transition in the bulk, taking the larger penetration depth of light for this energy into account.
both signals are reduced. After the electrochemical treatment the iron oxides and hydroxides are removed and the main emission in the 0 Is region appears at 532.2 eV. This is the value for physisorbed water and sulfate which are components of the electrolyte. The following conclusions can be drawn from (photo)electrochemical experiments: Since dark currents and photocurrents in presence of a redox system like iodide/iodine, which suppresses corrosion, do not cause an “activation” of the pyrite surface yielding improved photoactivity, photocorrosion in presence of chloride ions has to be made responsible for the improvement of the interface. An oxidation of Cl- on the FeS, surface is also produced in the dark, however without improvement of the pyrite interface. This means that the path of holes during dark oxidation and during photooxidation of Cl- is not identical (fig. 12). Responsible for the dark currents are apparently FeS surface states in the range for the forbidden energy region. Oxidation of the pyrite surface in air considerably increases the number of these FeS states (as well as treatment of the pyrite surface with a variety of chemical compounds). When an electron donor is oxidized in the dark, electrons are apparently transferred by way of these states into the conduction band of Fe&. Only when photogenerated holes are reacting from the valence band in the pyrite surface, and become involved in a photocorrosion reaction, an improvement of the photoactivity is possible together with a decrease of the dark
current. Both mechanisms cannot easily be correlated. For an etched sample (fig. 4) the dark current density decreases, for example. by a factor of nine while the photocurrent density only doubles. The ratio is less pronounced for unetched samples (fig. 3). Surface states which are acting as recombination centers for photogenerated electron-hole pairs are therefore only partially identical with those surface states which are responsible for the dark current. The variability of the electronic structure near the band edge deduced from the photocurrent spectra (fig. 6) is an indication for the participation of surface states. The mentioned small photocurrent peak observed at 0.76 eV (fig. 8) indicates surface states which are situated approximately 0.76 eV below the conduction band. It is formed during the “activation” of the pyrite surface, and probably a Fe-Cl, surface state and may thus participate in the electron exchange with the electrolyte just above the valence band. Experimental evidence has in the meanwhile been obtained, which confirmes that the electron donor Cll is indeed interacting with interfacial Fe atoms when transferring electrons to pyrite in an inner sphere mechanism [12]. The surface of the electrode is heterogeneous on a microscopic scale with kink sites, steps and terraces. An iron atom located in the neighbourhood of an edge may be exposed to a local potential which is sizably different from that of a bulk atom and that of a planar surface atom. The local fields have the effect of causing different d-orbital splitting and different d-orbital occupation. Also the sulfur ligands of such atoms may be subject to a deviating electronic situation. The surface states which are responsible for surface recombination processes as well as for the dark current will be formed by such atoms on preferential sites. XPS measurements have in addition shown that even on UHV-cleaved FeS, interfaces [19] a surface reorganization of atoms is observed and up to three sulfur species are detected, among them probably FeS-sulfur [24]. According to our XPS data surface bound FeS will be preferentially involved in photocorrosion mechanisms due to the specific reaction pathway of photogenerated holes and due to weaker bonds to the bulk material. By releasing such surface atoms, which are the source of surface states, photocorrosion is able to improve the photoelectrochemical quality of the pyrite/electrolyte interface. However. the small photocurrent peak ohtained at 0.76 eV (fig. 8) also indicates that photo-oxidation of chloride at the Fe& interface leads to a certain chemical modification of the pyrite surface. Filled states are apparently formed above the valence band of Fe&. from which electrons can be excited into the conduction band. It is, however. difficult to decide whether these levels also play a role as transition states for those charge carriers generated further inside the crystal. An analysis of the effect of surface activation on the spectral response of Fe& (shown in fig. 5) confirms that with increasing surface quality there is an increasing contribution to the photocurrent from holes generated close to the electrode surface. Since the absorption coefficient of FeS, in the near infrared
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and visible spectral region reaches 6 x lo5 cm ‘, the absorption of light occurs 0 within 200 A from the electrode surface. It has to be concluded that electron hole pairs generated within such a thin surface layer are largely lost when the surface is not photoelectrochemically pretreated (“activated”) and thus essentially cleaned of FeS groups. This means that both holes and electrons, interact with surface states and finally recombine under energy loss. The physically smooth polished surface structure (fig. 7a) of FeS, is photoelectrochemically less efficient than the photoetched, “activated” surface which has a quite rough appearance. Interestingly, the considerable loss of photogenerated charge carriers is also observed when a higher positive electrode potential is applied to the unetched pyrite electrode (fig. 5a) where an electric field should facilitate charge separation. The described results emphasize the importance, due to its high absorption coefficient for light, of the interfacial chemistry of pyrite for its optoelectronic and photoelectrochemical behaviour. They also underline the necessity for further research on the mechanism of surface recombination.
Acknowledgement The authors would like to thank their colleagues, especially Dr. A. Ennaoui, Dr. H.-M. Ktihne and Dr. H.-J. Lewerenz for discussion and Dr. W. Hofmann for the SEM measurements.
References [l] A. Ennaoui and H. Tributsch, Solar Cells 13 (1984) 197. [2] A. Ennaoui. S. Fiechter, H. Goslowsky and H. Tributsch, J. Electrochem. Sot. 132 (1985) 1579. [3] A. Ennaoui, S. Fiechter, W. Jaegermann and H. Tributsch, J. Electrochem. Sot. 133 (1986) 97. [4] A. Ennaoui and H. Tributsch, J. Electroanal. Chem. 204 (1986) 185. [5] A. Ennaoui and H. Tributsch, Solar Energy Mater. 14 (1986) 461. [6] G. Chatzitheodorou, S. Fiechter, R. Konenkamp, M. Kunst, W. Jaegermann and H. Tributsch, Mater. Res. Bull. 21 (1986) 1481. [7] A. Schlegel and P. Wachter, J. Phys. C 9 (1976) 3363. [8] MS. Seehra and S.S. Seehra, Phys. Rev. B 18 (1979) 6620. [9] K. Sato, Progr. Crystal Growth Characterization 11 (1985) 109. [IO] W. Folkerts, G.A. Sawatzky, C. Haas, R.A. de Groot and F.U. Hillebrecht J. Phys. C (Solid State Phys.) 20 (1987) 4135. (111 E.K. Li, K.H. Johnson, D.E. Eastman and J.L. Freeouf, Phys. Rev. Letters 32 (1974) 470. [12] X.-P. Li, N. Alonso Vante and H. Tributsch, J. Electroanal. Chem. 42 (1988) 255. [13] B. Schubert, N. Mgoduka and H. Tributsch, in preparation. [14] W. Jaegermann, D. SchmeiBer and J. Lilie, in preparation. [15] A. Buckley and R. Woods, Appl. Surface Sci. 27 (1987) 437.
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G. Panzner and B. Egert, Surface Sci. 144 (1984) 651. Y.C. Lee and P.A. Montano, Surface Sci. 143 (1984) 442. J. Carver, G.K. Schweitzer and T.A. Carlsson J. Chem. Phys. 57 (1972) 973. C. Pettenkofer, to be published. C. Pettenkofer, unpublished. H. Binder, 2. Naturforsch 28b (1973) 255. H. Goslowsky, H.M. Kiihne, H. Neff, R. K&z and H.J. Lewerenz Surface Sci. 149 (1985) 191. [23] N. Alonso Vante, G. Chatzitheodorou, S. Fiechter, N. Mgoduka, I. Poulios and H. Tributsch. Solar Energy Mater., in press. [24] C. Pettenkofer, W. Jaegermann and H. Kuhlenbeck, in preparation. [25] J.I. Pankove, in: Optical Processes in Semiconductors (Dover, New York, 1975) pp. 34. 56. [16] [17] [18] [19] [20] [21] [22]
Surface Science 204 (1988) 537-554 North-Holland, Amsterdam
ENHANCEMENT OF PHOTOACTIVITY IN PYRITE (Fe&) INTERFACES BY PHOTOELECTROCHEMICAL PROCESSES *, C. PETIENKOFER
LIU Chongyang Hahn - Meitner-Institut
Received
11 March
and H. TRIBUTSCH
Berlin, Bereich Strahlenchemie
1988; accepted
for publication
1000 Berlin 39, Germany
6 June 1988
A temporary photoelectrochemical oxidation of chloride on a pyrite (FeS,) electrode surface leads to a considerable enhancement of photocurrent response and a decrease of the dark current. In contrast, passage of a dark current or of a photocurrent in presence of iodide has no significant effect. The surface transformation is accompanied by a significant modification of the photocurrent spectrum of the pyrite material. The improvement of the interface is caused by a photocorrosive transformation of the interface which reduces the concentration of surface recombination centers and leads to a change in the concentration and redistribution of energy levels in the forbidden energy region. XPS studies identify the surface states in the pyrite interface, which are responsible for its poor (photo)electrochemical behaviour and which can be removed during chloride oxidation as FeS groups.
1. Introduction Pyrite, Fe&, has been shown to be an interesting photoactive semiconductor with an energy gap of EG = 0.95 eV, which has favourable optoelectronic properties and is able to generate photocurrents at high quantum efficiencies [l-5]. With an absorption coefficient of approximately 6 X 10’ cm-i in the entire visible spectral region it could, in principle, become the basis of ultra-thin, flexible solar cells with the advantage of considerable cost reduction, low weight and high density packaging for transport. However, its solid state chemistry, its solid state physics and its interfacial chemistry still involve many non-calculable factors. It has up to now, for example, not been possible to grow FeS, in its ideal stoichiometry or to dope it in a well defined way. The available samples still contain many defects which produce electronic levels within the forbidden energy gap. They seem to be responsible for the low photopotential (< 200 mV) observed up to now which has limited the solar energy conversion efficiency of this material to approximately 3%. The first * One leave from: Dalian People’s Rep. of China.
Institute
of Chemical
Physics,
Chinese
0039-6028/88/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
Academy
B.V.
of Science,
Dalian,
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Liu Chongvang et al. / Enhancement
ofphotoactic~ity m p_yrite interfuces
photoactive MOCVD layers of pyrite have already been reported [6]. However. their optoelectronic and crystallographic properties are quite poor. Since most, charge carriers generated by solar light in pyrite are produced within 200 A of the surface which typically is an important source of recombination and trapping centers, the interfacial chemistry of Fe& is of crucial importance for its optoelectronic performance. Our present knowledge of the solid state properties of pyrite is limited [7-111 and there is nearly no knowledge on its surface chemistry. Recently, first experimental evidence was obtained which demonstrates that interfacial coordination chemistry is involved in photoinduced interfacial electron transfer. High rates of electron transfer and stabilization of the FeS,/electrolyte interface against corrosion could only be accomplished with electron donors which are able to act as ligands to interfacial iron into which photogenerated holes are channelled. When comparing, in rotating ring disc experiments, the photo-oxidation of Br-, Fe(phen):+, and Fe(bipy):+ adjusted to comparable redox potentials, it was observed that under identical experimental conditions only Br- could efficiently stabilize the illuminated pyrite interface [12]. This means that efficient electron transfer is only possible by ligand mediated inner sphere electron transfer. In the meanwhile further evidence is supporting this finding of photoactivated interfacial coordination chemistry [13]. This contribution studies the influence of photoelectrochemical surface modification on the quantum efficiency for photocurrent generation and on the rectifying properties of the pyrite electrolyte interfaces.
2. Experimental Single crystals of n-Fe& were synthesized by the chemical vapor transport (CVT) method [3]. The ohmic back contact was made with a Ga-In alloy. The crystals were fixed by silver paste (3M) onto a copper rod and isolated using epoxy resin (Scotchcast 5241). Mechanical polishing was performed using a diamond paste with successively decreasing grain size (3 to 0.25 pm) followed by rinsing with deionized water and methanol in an ultrasonic bath. A standard three electrode set-up (HERA potentiostat) was used with a platinum electrode as a counter and a saturated calomel as a reference electrode (SCE). The light source was a 240 W W-halogen lamp (Oriel). Photocurrent spectra were obtained with a computer-controlled set-up (monochromator: Kratos GM 252, lock-in amplifier: PAR 9503-SC, chopper: PAR 9479, computer: Hewlett-Packard 85). The second harmonics were cut off using optical filters. Light reflection from the electrode surface was not taken into consideration. For the XPS-experiments the crystals were mounted as grown without polishing or etching in tantalum clamps on stainless steel stubs. Spectra were
Liu Chongvang et al. / Enhancement
of photoactivity
m pyrite interfaces
539
recorded in a conventional VG-Escalab MkII described elsewhere [14]. The electrochemical cell for the chlorine treatment was installed in an oxygen free glove-box and the transfer to the photoelectron spectrometer was done in a special transfer vessel unit without exposing the samples to ambient air. In the electrochemical cell only the surface of the crystal seen by the photoelectron spectrometer had contact with the electrolyte. Great care was taken not to wet the tantalum clamps nor any other metal surface connected to the working electrode output of the potentiostat.
3. Results 3. I. Electrochemical
observations
When a polished n-Fe& electrode is placed in contact with a redox electrolyte such as I-/I;, only a very poor (photo)current-voltage performance is observed (fig. la). Also repeated cycling of the electrode does not improve the quality of the characteristics observed under intermittant illumination. This result is not surprising since earlier experiments [3,4] have shown, that the FeS, surface is not affected by corrosion in presence of I -/I, in the electrolyte. We have, however, observed that a temporary cycling of FeS, in presence of chloride in the electrolyte leads to a considerable improvement of the photoelectrochemical properties of the FeS,/electrolyte interface (“activation” of the electrode). The results of such a temporary cycling, in presence of illumination, in a 3M KC1 containing electrolyte (pretreatment) on the performance of (non-etched) polished pyrite in contact with an electrolyte containing 5M KI with OSM H,SO, is shown in fig. lb. It is interesting to note that during this pretreatment in contact with a Cl- containing electrolyte approximately 73% of the positive charge carriers are going into photoevolution of chlorine, the rest (27%) are lost in some corrosion reaction [4] in which iron chloride is one reaction product. In order to get a better insight into the mechanisms involved we have initiated a systematic study. Fig. 2 shows the dark current-voltage curves of FeS, in contact with a 3M KC1 containing electrolyte during the 1st and the 10th sweep (20 mV s-l). No significant change is observed, which leads to the conclusion that charge transfer processes involved in the generation of dark currents do not lead to an improvement of the FeS, interface. When, however, a similar experiment is repeated under illumination, a clear improvement of the characteristic is observed. Figs. 3a-3c show the (photo)current-voltage curve for FeS, in contact with the Cl- containing electrolyte during the first, an intermediate and the 30th sweep (the intermittant illumination makes dark and photocurrent distinguishable, the light was only turned on during the
540
Liu Chongyang et al. / Enhancement
ofphotoactivity in pyrite interfaces
a) J
0 electrode
0.5 potenhal
/
1 V(SCE)
Fig. 1. Current-potential curves of n-Fe& in contact with 5M KI in 0.5M H2S0, under chopped illumination (sweep rate 20 mV s -I): (a) before activation, (b) after activation.
positive sweep). Attention should be pointed towards a photocurrent which develops at a photovoltage between 0.3 and 0.5 V (SCE) as a consequence of the “activation” of the pyrite interface. It can be attributed to the oxidation of Fe2+ which is liberated from the electrode due to photocorrosion and the dark reduction of Fe’+. A plot of the change of photocurrent and dark current at 1.5 V (SCE) as a function of the number of performed potential sweeps is shown in fig. 4. In this case the pyrite electrode has previously been etched in HF/CH,COOH/HNO, (1 : 1 : 2 by volume), which is known to liberate the surface from oxides, for one to two minutes. It is clearly seen how the photocurrent density increases while the dark current density decreases. After
Liu Chongyang et al. / Enhancement
of photoactivity
m pyrite interfaces
541
60 -
I 0
L 0.5 electrode
f 1 potential / VISCE)
1 1.5
Fig. 2. Dark current-potential curve of n-Fe& before activation in 3M KC1 in OSM H,SO, first sweep, (- - -) 10th sweep. (sweep rate 20 mV s-l): ( -)
30 cycles in presence of air, nitrogen was bubbled through the electrolyte for 45 min. Arrow (a) shows the first to fifth sweep thereafter. Then the electrolyte was replaced by a fresh solution with the same composition (arrow (b)) and sweep 36 to 40 continued without nitrogen bubbling. Afterwards the electrode was removed from the cell and kept in air for 40 h (arrow (c) in fig. 4). As can be seen there is a significant effect on both the dark current and the photocurrent. Subsequent sweeps, in absence of nitrogen bubbling, again lead to a recovery of the pyrite interface. It has been reported that also anodic photo-oxidation of H,O, is able to improve the (photo)current voltage characteristic of FeS, [3]. This can equally be explained in terms of an anodic photoetching of the electrode surface. The mechanism is, however, difficult to study due to the instability of H,O, on the pyrite surface. Also reduction of protons at the electrode, which leads to hydrogen insertion, hydrogen evolution and the liberation of some H,S leads to an improvement of the photoactive pyrite interface [23]. Fig. 5, which again refers to an unetched electrode as fig. 3, shows photocurrent spectra of pyrite in the near-infrared region where the penetration depth of light is varying between near infinity and as little as 1.6 x lop6 cm. Fig. 5a is the spectrum obtained before “activation” of the pyrite interface, fig. 5b the spectrum during the process of activation and fig. 5c the spectrum after activation of the electrode by photo-oxidizing it in contact with a Cl- containing electrolyte. The spectra are taken in a potential region, where
542
LIU Chongvang
et al. / Enhancement
electrode
of photonctwiq
potential
/
in p.yrite interfaces
V(SCE)
Fig. 3. Current-potential curves of n-FeS, in contact with 3M KC1 in OSM H2S04 under chopped illumination (sweep rate 20 mV s _‘): (a) before activation through passage of photocurrent, (h) during activation. (c) after activation.
small photoeffects (due to oxidativc processes [3j at the surface of FeS,) are only detectable by usage of lock-in techniques for recording the photocurrent spectra. It is seen that the spectrum of a “non-activated” pyrite interface shows a pronounced photocurrent peak near 0.95 eV. Light which deeply penetrates the electrode surface and creates holes sufficiently far away from the interface apparently contributes far more to the photocurrents than light absorbed near the electrode surface. With progressive “activation” of the electrode surface charge carriers generated near the FeSJelectrolyte interface contribute increasingly to the
Liu Chongyang et al. / Enhancement
ofphotoactivity
543
rn pyrite interfaces
60
30
0
0
15 I
the
30 I
sequence
45 I
of cyclic
I
60
J4
sweeping
Fig. 4. Dependence of n-Fe& (photo)current density (etched sample) at 1.5 V (SCE) on the sequence of cyclic voltammetry in 3M KC1 in 0.5M H,SO., (sweep rate: 20 mV s-l): (a) bubbling through the electrolyte with N, for 45 min, (b) electrolyte replaced by a fresh one with the same composition, (c) electrode removed from the cell and kept in air for 40 h.
photocurrent. The photocurrent density produced by photons with higher energy increases to such an extent that it outweighs by far the original photoelectrochemical IR response. The significant change of the photocurrent spectrum near the absorption edge is remarkable. This is even better shown by the plots of figs. 6a-6c which are extrapolations for the determination of indirect (n = 1) and direct (n = 4) energy gaps [25]. It is improbable that photoelectrochemical surface modifications can significantly change the size of the direct energy gap which indicates that extrapolations of electrochemical photocurrents for the determination of the energy gap, as widely practized in the literature, are of questionable value in the case of pyrite. It is seen from the scanning electron micrograph pictures in figs. 7a and 7b that photo-oxidation of chloride at FeS, is accompanied by photoeching. During this process the originally smooth electrode surface is gradually becoming rough and covered by corrosion structures. Even though the absolute surface area increases considerably, the absolute dark current is reduced. The interfacial changes produced during anodic polarization in contact with Cll containing electrolyte interestingly give also rise to a small photocurrent peak at 0.76 eV which is situated at lower energy than the energy gap and can be attributed to characteristic surface states formed during the surface
544
Liu Chongyang et al. / Enhancement
I
0.8
I
I
1.0
1.2
of photoactwq
,
I
I.&
in pyrite interfaces
1.6
1.8
photon energy / eV Fig. 5. Spectral
response
of n-Fe& at 0 V (SCE) in 3M KCI in OSM H,S04: (b) during activation, (c) after activation.
(a) before activation,
modification (fig. 8). As already indicated, no “activation” is observed as a consequence of photoanodic polarization of FeS, in contact with a 5M KI solution. A purple-brown film is seen during polarisation but vanishes when the current is turned off. It can be attributed to a polycrystalline film of iodine formed in a I- depleted diffusion layer near the electrode surface.
3.2. Photoelectron
spectroscopy
XPS investigations on pyrite surfaces have shown that parts of the airoxidized surface were covered with FeS, iron oxides and iron hydroxides [3,15,16]. These surface layers of FeS should contribute to the high dark currents of untreated FeS, crystals in electrochemical solar cells by giving rise to defect or surface states in the gap. Since iron d-states are forming the edge
Liu ChongVang et al. / Enhancement
1
0.8
of photoactivity
I
1
1.0
1.2
1
1.4
in pyrite interfaces
,
1.6
545
I
1.8
photon energy / eV Fig. 6. Plot of hv versus (q hv)“/* from activation, (b) during
n = 4, (- fig. 5: ( -) activation, (c) after activation.
-)
n = 1; (a) before
of the valence band of pyrite, transformation of interfacial FeS, to FeS must necessarily involve a shift of Fe d-states into the forbidden energy region. It has been shown, that FeS, decomposes to FeS by Ar sputtering [7,19]. Fig. 9 shows the core level spectra of an air-exposed FeS, crystal (exposure time < 1 h) before and after the Ar sputtering for the Fe2p and S2p levels. The tail at the higher binding energy side in the Fe2p spectrum of the sputtered sample is due to FeS in the topmost surface layers [19]. Even more distinct is the change due to the chemical shift in the S2p levels, for FeS one finds a binding energy of 161.4 eV [20] significantly shifted from the value for FeS, 162.5 eV [15,21]. Fig. 10 displays the spectra after cycling the crystal 15 times from 0 to 1.5 V versus SCE in 3M KC1 in 0.5M H,SO, under illumination. The crystal was removed at 1.5 V after the last cycle from the
546
Liu Chongyang et al. / Enhancement
Fig. 7. Scanning
electron
micrographs
of ph0toactioit.y in pyrite interfaces
(SEM) of n-FeS, activation.
electrode:
(a) before
activation.
(b) after
electrolyte and dried by N, purging before transfer. The spectra show clearly that FeS is removed from the sample. The components of the electrolyte (Cl-, K+, SO,‘-) are found on the samples (hump at 169 eV BE is the signal from SO:-). Beside the removal of FeS from the surface it is remarkable, that nearly all oxides or hydroxides bound to the iron are removed as can be deduced from the change in the 0 1s emission displayed in fig. 11. This is due
Liu Chongyang et al. / Enhancement
L
photon spectral
response
of n-Fe& activation,
in pyrite interfaces
541
I
I
0.7
Fig. 8. Subgap
of photoactivity
0.9
0.8 energy
/
eV
at 0 V (SCE) in 3M KC1 in OSM H,SO,: (b) after activation.
(a) before
to the low pH of the electrolyte and the solubility of the oxides at lower pH values.
4. Discussion The XPS results suggest a connection between the presence of FeS at the electrode surface and at least a part of the electrochemical dark current. Removing this FeS layer in a photoelectrochemical process improves the characteristics of the pyrite/redox electrolyte interface. The S 2p spectra show, that the removal of FeS is nearly complete, but on the other hand the linewidth and shape of the sulfur line compared to the initial one suggests, that the sulfur stoichiometry still differs from the expected behavior of a clean FeS, surface. Goslowsky et al. [22] found on etched CuInS, samples significant amounts of sulfur in molecular aggregations up to S,,. From the linewidth and structure of the Fe2p spectra, which show a rather well defined shape and did not differ very much from spectra obtained from vacuum cleaved samples [19], such conclusions cannot be drawn. Buckley et al. reported [15], that on air-exposed pyrite the main emission in the 01s region is obtained around 531.6 eV by chemisorbed water or hydroxide. The shoulder at 530 eV on the unsputtered sample belongs to iron oxides. By sputtering
Liu Chongyang et al. / Enhancement ofpholoactivity in pyriteinterfaces
548 1600
-
a
1500 l&O0 -
800 700
-
600 700
705
715
710
720
725
730
BindingEnergy EIEfeV
600 : 500 -t: t/l t400 .* 2 2 300 c 200 100
0. 155
157
159
161
163
BindingEnergy
165
167
169
BEf eV
Fig. 9. Fe2p level (a) and S2p level (b) from an air-exposed (lower curves) and Ar-sputtered (4 min 600 V, 2 x 10C5 mbar Ar) pyrite single crystal. 200 W MgKa, 10 V pass energy. In the insert and 2p,,, sulfur level for the sputtered sample is given for clarity. a deconvolution of the 2p,,, Solid line: sulfur as in FeS, dotted line: sulfur as in FeS,.
Liu Chongyang et al. / Enhancement
ofphotoactivityin pyrite
interfaces
549
1100 1050 "?OOO 2 > 950 u Ir"900 c" 2 850 -c 800 750 700' 700
450.
720 715 710 BindingEnergy BE, eV
705
725
730
b
400 ^ 350
x200 .k t" 2 150
c
100
155
157
159
165 163 161 BindingEnergy BEF eV
167
169
Fig. 10. Fe2p level (a) and S2p level (b) recorded after the photoelectrochemical treatment under the same conditions as in fig. 9; the hump at 169 eV in the sulfur spectrum is due to SO:-.
550
Liu Chongyang
et ai. / Enhancement
ofphocoacrioiy
ut pyrite interfaces
700 650
350’
_-II_
?I20 522
520
52L 526 528 530 532 531, 536 BindingEnergy 6EF eV
522 52L 526
528
530
532
53L
536
538
510
538
540
BindinaEnergy BEF eV Fig. 11. 01s spectra for the sputtered sample (top), the initial air exposed sample (middle) and the electrochemically treated crystal (bottom). Same spectrometer settings and sample orientation as in fig. 9. The arrow indicates the position for the oxide.
Liu Chongvang et al. / Enhancement
of photoactivity in pyrite interfaces
551
Fig. 12. Energy level diagrams for n-Fe& showing the electron transfer process in the dark and during illumination. SS denotes surface states. Ox and Red indicate the distributions of oxidized and reduced species. 0.76 belongs to a transition probably from iron-chlorine complexes at the surface, whereas 0.9 is due to a transition in the bulk, taking the larger penetration depth of light for this energy into account.
both signals are reduced. After the electrochemical treatment the iron oxides and hydroxides are removed and the main emission in the 0 Is region appears at 532.2 eV. This is the value for physisorbed water and sulfate which are components of the electrolyte. The following conclusions can be drawn from (photo)electrochemical experiments: Since dark currents and photocurrents in presence of a redox system like iodide/iodine, which suppresses corrosion, do not cause an “activation” of the pyrite surface yielding improved photoactivity, photocorrosion in presence of chloride ions has to be made responsible for the improvement of the interface. An oxidation of Cl- on the FeS, surface is also produced in the dark, however without improvement of the pyrite interface. This means that the path of holes during dark oxidation and during photooxidation of Cl- is not identical (fig. 12). Responsible for the dark currents are apparently FeS surface states in the range for the forbidden energy region. Oxidation of the pyrite surface in air considerably increases the number of these FeS states (as well as treatment of the pyrite surface with a variety of chemical compounds). When an electron donor is oxidized in the dark, electrons are apparently transferred by way of these states into the conduction band of Fe&. Only when photogenerated holes are reacting from the valence band in the pyrite surface, and become involved in a photocorrosion reaction, an improvement of the photoactivity is possible together with a decrease of the dark
current. Both mechanisms cannot easily be correlated. For an etched sample (fig. 4) the dark current density decreases, for example. by a factor of nine while the photocurrent density only doubles. The ratio is less pronounced for unetched samples (fig. 3). Surface states which are acting as recombination centers for photogenerated electron-hole pairs are therefore only partially identical with those surface states which are responsible for the dark current. The variability of the electronic structure near the band edge deduced from the photocurrent spectra (fig. 6) is an indication for the participation of surface states. The mentioned small photocurrent peak observed at 0.76 eV (fig. 8) indicates surface states which are situated approximately 0.76 eV below the conduction band. It is formed during the “activation” of the pyrite surface, and probably a Fe-Cl, surface state and may thus participate in the electron exchange with the electrolyte just above the valence band. Experimental evidence has in the meanwhile been obtained, which confirmes that the electron donor Cll is indeed interacting with interfacial Fe atoms when transferring electrons to pyrite in an inner sphere mechanism [12]. The surface of the electrode is heterogeneous on a microscopic scale with kink sites, steps and terraces. An iron atom located in the neighbourhood of an edge may be exposed to a local potential which is sizably different from that of a bulk atom and that of a planar surface atom. The local fields have the effect of causing different d-orbital splitting and different d-orbital occupation. Also the sulfur ligands of such atoms may be subject to a deviating electronic situation. The surface states which are responsible for surface recombination processes as well as for the dark current will be formed by such atoms on preferential sites. XPS measurements have in addition shown that even on UHV-cleaved FeS, interfaces [19] a surface reorganization of atoms is observed and up to three sulfur species are detected, among them probably FeS-sulfur [24]. According to our XPS data surface bound FeS will be preferentially involved in photocorrosion mechanisms due to the specific reaction pathway of photogenerated holes and due to weaker bonds to the bulk material. By releasing such surface atoms, which are the source of surface states, photocorrosion is able to improve the photoelectrochemical quality of the pyrite/electrolyte interface. However. the small photocurrent peak ohtained at 0.76 eV (fig. 8) also indicates that photo-oxidation of chloride at the Fe& interface leads to a certain chemical modification of the pyrite surface. Filled states are apparently formed above the valence band of Fe&. from which electrons can be excited into the conduction band. It is, however. difficult to decide whether these levels also play a role as transition states for those charge carriers generated further inside the crystal. An analysis of the effect of surface activation on the spectral response of Fe& (shown in fig. 5) confirms that with increasing surface quality there is an increasing contribution to the photocurrent from holes generated close to the electrode surface. Since the absorption coefficient of FeS, in the near infrared
Liu Chongyung
et al. / Enhancement
of photoactivity
in pyrite
interfaces
553
and visible spectral region reaches 6 x lo5 cm ‘, the absorption of light occurs 0 within 200 A from the electrode surface. It has to be concluded that electron hole pairs generated within such a thin surface layer are largely lost when the surface is not photoelectrochemically pretreated (“activated”) and thus essentially cleaned of FeS groups. This means that both holes and electrons, interact with surface states and finally recombine under energy loss. The physically smooth polished surface structure (fig. 7a) of FeS, is photoelectrochemically less efficient than the photoetched, “activated” surface which has a quite rough appearance. Interestingly, the considerable loss of photogenerated charge carriers is also observed when a higher positive electrode potential is applied to the unetched pyrite electrode (fig. 5a) where an electric field should facilitate charge separation. The described results emphasize the importance, due to its high absorption coefficient for light, of the interfacial chemistry of pyrite for its optoelectronic and photoelectrochemical behaviour. They also underline the necessity for further research on the mechanism of surface recombination.
Acknowledgement The authors would like to thank their colleagues, especially Dr. A. Ennaoui, Dr. H.-M. Ktihne and Dr. H.-J. Lewerenz for discussion and Dr. W. Hofmann for the SEM measurements.
References [l] A. Ennaoui and H. Tributsch, Solar Cells 13 (1984) 197. [2] A. Ennaoui. S. Fiechter, H. Goslowsky and H. Tributsch, J. Electrochem. Sot. 132 (1985) 1579. [3] A. Ennaoui, S. Fiechter, W. Jaegermann and H. Tributsch, J. Electrochem. Sot. 133 (1986) 97. [4] A. Ennaoui and H. Tributsch, J. Electroanal. Chem. 204 (1986) 185. [5] A. Ennaoui and H. Tributsch, Solar Energy Mater. 14 (1986) 461. [6] G. Chatzitheodorou, S. Fiechter, R. Konenkamp, M. Kunst, W. Jaegermann and H. Tributsch, Mater. Res. Bull. 21 (1986) 1481. [7] A. Schlegel and P. Wachter, J. Phys. C 9 (1976) 3363. [8] MS. Seehra and S.S. Seehra, Phys. Rev. B 18 (1979) 6620. [9] K. Sato, Progr. Crystal Growth Characterization 11 (1985) 109. [IO] W. Folkerts, G.A. Sawatzky, C. Haas, R.A. de Groot and F.U. Hillebrecht J. Phys. C (Solid State Phys.) 20 (1987) 4135. (111 E.K. Li, K.H. Johnson, D.E. Eastman and J.L. Freeouf, Phys. Rev. Letters 32 (1974) 470. [12] X.-P. Li, N. Alonso Vante and H. Tributsch, J. Electroanal. Chem. 42 (1988) 255. [13] B. Schubert, N. Mgoduka and H. Tributsch, in preparation. [14] W. Jaegermann, D. SchmeiBer and J. Lilie, in preparation. [15] A. Buckley and R. Woods, Appl. Surface Sci. 27 (1987) 437.
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G. Panzner and B. Egert, Surface Sci. 144 (1984) 651. Y.C. Lee and P.A. Montano, Surface Sci. 143 (1984) 442. J. Carver, G.K. Schweitzer and T.A. Carlsson J. Chem. Phys. 57 (1972) 973. C. Pettenkofer, to be published. C. Pettenkofer, unpublished. H. Binder, 2. Naturforsch 28b (1973) 255. H. Goslowsky, H.M. Kiihne, H. Neff, R. K&z and H.J. Lewerenz Surface Sci. 149 (1985) 191. [23] N. Alonso Vante, G. Chatzitheodorou, S. Fiechter, N. Mgoduka, I. Poulios and H. Tributsch. Solar Energy Mater., in press. [24] C. Pettenkofer, W. Jaegermann and H. Kuhlenbeck, in preparation. [25] J.I. Pankove, in: Optical Processes in Semiconductors (Dover, New York, 1975) pp. 34. 56. [16] [17] [18] [19] [20] [21] [22]