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The photoelectrochemical studies on CdS: Cu: Cl electrodes: Importance of photoconductivity effects
Solar Energy Materials 18 (1988) 37-51 North-Holland, Amsterdam
37
THE PHOTOELECTROCHEMICAL STUDIES ON CdS: Cu: C! ELECTRODES: IMPORTANCE OF PHOTOCONDUCFIVITY EFFECTS *
D.P. AMALNERKAR, S. RADHAKRISHNAN Polymer Science and Engineering Group, ChemicalEngineering Division, National ChemicalLaboratory, Pune 411 008, India
H. MINOURA, T. SUGIURA and Y. UENO Department of Synthetic Chemistry, Faculty of Engineering, Gifu University, Yanagido 1-1, Gifu 501-11, Japan Received 6 April 1988; in revised form 9 August 1988 The photoelectrochemical characterisation of highly photosensitive and semi-insulating sintered pellets of CdS doped with Cu and C! has been carried out. In the course of this work, we have observed a complete suppression of the dark current but a pronounced photocurrent under cathodic polarisation. Both these observations indicate an unusual cathodic (i.e. forward bias) behaviour. It has been shown that due to a decrease in series resistance under illumination by an extrinsic photoconductivity effect, the "ideal" cathodic behaviour, i.e. normally found in dark for an n-CdS electrode with low resistivity, can get restored. We have also observed a red shift in the anodic photoelectrochemical spectral reponse of these samples. It is in agreement with our previous photoelectrochemical studies on CdS: Cu: C! el~trodes and can be explained by considering a Cu impurity excitation process. Additionally, we have observed that the background illuminations as such (employed in the "double-beam" technique of spectral response determination) do not alter the overall nature of the spectral response but result in an interesting photocurrent enhancement phenomenon. These photocurrent enhancement observations have been explained by taking into account the photoeffects at Cl-compensated Cu acceptor centers possessing a high capture cross section preferably for photoexcited holes.
1. Introduction
The photoelectrochemical (PEC) studies of semi-insulating materials had been hampered in the past because of the high series resistance problems which affected the measurements, but with the advent of the "double-beam" technique such draw-backs have been almost overcome. For example, Gerischer et al. [1,2] and Husser et al. [3,4] have demonstrated that such studies can be made possible if the internal resistance of the insulating material can be reduced by means of a photoconductivity phenomenon. In their "double-beam" approach, rm additional background illumination with photons of appropriate ~enetration depth makes the bulk of the sample phetoconductive, and in principle, achieves an effect quite * NCL Communication No. 4412.
0165-1633/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
38
D.P. Amalnerkar et ai. / P E C studies on CdS : Cu : Ci electrodes
similar to donor impurity doping required for the formation of a depletion layer at the interface. So far, PEC studies have been reported on GaS, CdS, SnS2, Sn(Seo.~S0.9)2, MnPSe3, CdPSe3, HgI2 and TISbS2 which ai'e all highly resistive photoconducting materials [1-4]. All these studies mostly deal with the "pure", i.e. undoped, form of the sample~, and refer essentially to intrinsic photoconductivity. Normally, photosensitivity of highly resistive photoconducting materials is associated with specific compensated acceptor type of sensitising centers which possess a high capture cross section for minority carders [5]. In fact, an intentional addition of sensitizer impurities - which can introduce such compensated acceptor states in the band-gap of a host - is a weU-estabfished technique to enhance photosensitivity of "pure" photoconducting materials [6]. Despite the importance of such deep-lying states, no serious attempt has been made to perform PEC investigations (with the "double-beam" approach or otherwise) on extrinsically doped highly photoconducting materials. We thought it would be interesting to make such an attempt especially for the following reasons: (i) the compensated acceptor states with preferred recombination kinetics for the photogenerated carders can be expected to influence the photoelectrochemicai reactions and (ii) the photoconduction, being extrinsic in nature, can become quite appreciable through the bulk of the sample by the absorption in a larger part of the visible range of wavelengths. This is highly desirable for applications in solar energy conversion devices. We have chosen the well-known C d S : C u : C I photoconductor for PEC characterisation. As a part of this work, we have recently announced the extension of PEC spectral response towards the desirable red region for electrophoretically deposited CdS: Cu: CI electrodes [7]. This is the first example of impurity sensitisation of CdS electrodes [8]. In the course of further work, we have performed PEC investigations like current-voltage characteristics and spectral response (with single-beam and double-beam methods) on highly resistive and highly photosensitive C d S : C u : C I electrodes in the form of pellets, using aqueous Na2S as an electrolyte. Quite surprisingly, we have observed a pronounced photocurrent under cathodic polarisation. This is an unusual behaviour because nolmally in a n-type semiconductor like CdS, one cannot expect pronounced photocurrent to flow under cathodic bias. We have also observed a complete suppression of dark current, a clear red-shift (not only an extension to red region) in the spectral response peak, and an interesting photocurrent enhancement phenomenon in the "double-beam" method of spectral response determination. In this paper, all these results have been presented and various possibilities pertinent to photoconductivity effects at deep-lying Cl-compensated Cu acceptor [Cu(: C l)] centers have been suggested for their interpretation. 2. Experimenlal
2.1. Preparation of the pellets Pellets of photoconducting C d S : C u : C I were prepared as follows: initially, a mixture containing high purity CdS (99.999~), AR grade CdCI 2 (10 wt~ of CdS)
D.P. Amalnerkar et al. / P E C studies on Cd$ : Cu: C! electrodes
39
and AR grade CuCl 2 (0.1 wt~ CdS) was subjected to prefiring in air at 550 ° C for 2 h (CuCI 2 acts as an activator to enhance the photosensitivity of CdS, and CdC12 acts as a flux or sintering aid). The resulting mass was thoroughly ground in an agate pestle-mortar using distilled acetone and the fine powder of activated C d S : C u : C I obtained in this way was press-moulded in vacuo to get the pellets of desired dimensions (pressure applied -- 400 kg cm-2, total pressing time -- 30 min). Finally, the pellets were fired at different temperatures in the range 500-700 ° C for 30 rain in a stream of purified N2 (to avoid the formation of a surface impurity phase (s) reported earlier [9], the final firing was carried out in N 2 atmosphere). These impurity sensitised pellets showed quite good surface photosensitivity when illuminated with visible light of moderate intensity, which was checked by an independent experiment using silver-paste top contacts. 2.2. Preparation of the electrodes From the fired pellets, the working eicetrc, des were prepared as follows: initially, an ohmic contact was formed on the back surface of the pellets by soldering high purity metallic indium. A copper lead wire was then attached to it with silver paste. Finally, the back surface and the edges of the pellets were covered with insulating epoxy resin, leaving the front surface exposed. 2.3. Photoelectrochemical measurements The photoelectrochemical measurements were performed by adopting a standard potentiostatic configuration with a three-electrode system comprising a CdS : Cu: CI working electrode, a large area platinum foil counter electrode and a saturated calomel reference electrode (SCE). An aqueous solution of 0.SM NaeS was used as the electrolyte. Since the experiments were performed in the ambient atmosphere, the oxygen present in the electrolyte can lead to the formation of elemental sulphur in sulphide solution. Polarisation curvez in dark and under illumination (intensity-100 mW cm-2; source: 500 W xenon lamp) were established by using a Nikko Keisoku potentiostat (NPGS-301) and recorded on an x - y recorder (Watanabe, WX 4402). For spectral response determination, the sample electrodes were illumi~ated by appropriately filtered and chopped monochromatic light. A Nikon monochromator (G 250) equipped with a 300 W tungsten-iodine lamp (primary source) was used for this purpose. Additionally, for the reasons given by Husser et al. [4], spectral response measurements were also performed in a "double-beam" arrangement. In this arrangement, unchopped monochromatic radiation at ~, = 480 nm and ~, = 680 nm (obtained from a JUSCO (CT-10) monochromator equipped with a 500 W xenon lamp : secondary source) were used for "background" illumination of the samples. The resulting photocurrent values were measured by a standard lock-in technique, then normalised by taking into account the corrections arising from spectral intensity distribution of the source and the corrected values were plotted on an x - y plotter interfaced with a NEC computer. All the spectral response measurements were carried out at a potential of - 0 . 3 V (SCE).
40
D.P. Amalnerkar et al. / PEC studies on CdS : Cu : Ci electrodes
3. R e s u l t s a n d d i s c u s s i o n 3.1. C u r r e n t - p o t e n t i a l curves
The current-potential curves (in dark and under polychromatic illumination) of the C d S : C u : C I electrodes corresponding to firing temperatures 500, 550, 600 and 700 °C are presented in figs. 1A-1D, respectively. It can be observed that: (i) All the sample electrodes - with the exception of those corresponding to a firing temperature of 500"C - exhibit complete suppression of the dark current for both cathodically and anodically appfied biases. (ii) All the sample electrodes surprisingly exhibit pronounced photocurrent upon cathodic polarisation and there is no tendency of photocm'rent saturation for the entire range of applied cathodic biases covered in this experiment (we have polarised the samples only up to -1.45 V (SCE) because there is a possibility of hydrogen evolution a n d / o r cathodic decomposition of the CdS electrode on polarising it beyond - 1 . 7 V (SCE) [10]). (iii) There is a tendency for the photocurrent to saturate in the anodic region and it increases with the increase in firing temperature of the samples. (iv) In general, with the exception of electrodes corresponding to a firing temperature of 500 ° C, the cathodic photocurrent is much higher than the anodic photocurrent for the same electrode (see inset of fig. 1). An observation of a pronounced cathodic current in light is quite unusual in n-type semiconducting electrodes such as CdS. Ordinarily, in such electrodes an application of sufficient cathodic bias leads to a downward band bending and to an accumulation of electrons in the region just below the semiconductor/electrolyte interface [11]. In such a condition, photogenerated electrons contribute only negligibly to the flow of current (residual effect). Ideally, under cathodic polarisation, huge cathodic (forward bias) current is expected to flow even in the dark [11], a fact which we have not observed in the present case. There exist a few reports in the fiterature which deal with similar observations on suppression of cathodic current in dark and its enhancement in light, for example, as reported by Muller et al. [12] in the case of a mechanically damaged CdS electrode/0.1M Fe(CN) 3-/4- electrolyte system. They have shown that this photoenhancement of cathodic current (also referred to as photocathodic effect (PCE) by them) can be explained either by a model based on the presence of a compensated insulating surface layer which is photoconductive in nature, or by considering Fermi level pinning by the surface states° We cannot totally adopt this model to explain PCE in the present case, because there is a basic difference between our CdS: Cu : CI samples and the samples used by Muller et al., viz. our CdS : Cu : Ci samples possess very_ high bulk resistivity in dark ( ~ 107-10 s fi cm) and exhibit extremely high solid state photosensitivity * ( - 103-10 s, see experimental section). Both the characteristics are interrelated and attributed to a compensated acceptor type of sensitising states introduced by Cu:C1 impurities. On the contrary, Muller et al. assume that * Defined as the ratio of photocurrent to dark current.
D.P.
Amulnerkar et ai. / PEC studies on CdS: Cu: Cl decmdes
500 FIRING I
LIOHT
OFF
6QO fE&.
41
OC
UUU
0.0
LIGHT OFF -0*4
LlQHT ON
-1.2
-1.0
-0*0
-0.6
APPLIED
-@4
-02
POTENTIAL
0
0.2
O-4
0.6
(Cl
0.0
fV ISCEI]
Fig. L Current-potential curves for CdS: Cu: Cl pelletised electrodes in OSM Na2S under antermittent illumination (intensity = 100 mW/cm2). (A, B, C, D) Samples fired at 500, 550, 600 and 700°C, respectively. The inset shows dependence of cathodic [at -1.3 v (SE)] and anodic [at 0.8 v @CE)l photocurrent on firing temperature.
mechanical polishing creates a compensated insulating layer (dark resistance in excess of 100 kS2 for a crystal of cross-sectional area 5 mm2) at the surface of CdS electrodes, with the specific resistivity of the order of 1 Q cm in the “virgin” state
42
D.P. Amainerkar et al. / P E C studies on CdS : Cu: C! electrodes
[12]. They further assume that this layer exhibits photoconductivity which is attributable to deep-lying acceptor states introduced by mechanical polishing. Considering all these facts, we suggest the following phenomenological possibilities to interpret our current-voltage results. In order that an externally applied voltage creates a potential drop at the electrode/electrolyte interface, it is necessary that the resistance between this interface and the ohmic contact at the rear face (i.e. the resistance in series with the current generating region, R~) should be as low as possible. The complete suppression of cathodic and anodic current in dark can then be regarded as a consequence of high bulk resistivity of our samples which allows only a minimal potential drop to occur at this interface. However, the highly photoconductive nature of our samples can dramatically alter this situation upon suitable illumination. To explain this, we mainly rely upop the reports on photoconductive a-Si based photovoltaic configurations. For example, it has been recognised in case of photoconductive a-Si (possessing high bulk resistance in dark) that photoconductivity reduces the series resistance (R~) of a photovoltaic cell configuration [13,14]. The effect of the photoconductivity phenomenon becomes particularly significant for the volume-absorbed light since it can induce appreciable photoconduction through the bulk of the sample. In more traditional photoconductive materials like CdS doped with CI and Cu, a similar effect can be expected to occur perhaps to a greater extent owing to extrinsic photoconductivity. We, therefore, believe that the extrinsic photoconduction due to dominance of Cu impurity absorption may have an important contribution in establishing "light" polarisation curves for our CdS : Cu : CI samples. To elaborate further, we quote the solid state photoconductivity studies on CdS: Cu: I crystals reported by Bube and Young [15] which forms the basis to presume the dominance of Cu(: CI) impurity absorption in the present case. They have shown that for a sufficiently high doping level, the excitation of the photoconductive crystals gets dominated by the absorption by Cu impurity centers possessing an absorption constant of the order of 200 cm-1 for 0.1% Cu concentration (same as our samples). The absorption at Cu(:CI) impurity centers has thus two important consequences: (i) the photoexcitation of electrons from such centers to the conduction band makes a larger number of free electrons avaglable for electronic conduction and (ii) the absorption of long wavelength photons, owing to their ~e~ter penetration depth, makes an appreciable fraction of the sample-electrodes conductive under light (for example, roughly 50% for a 0.1 mm thick crystal with 0.1~ Cu concentration). The net result of (i) and (ii) is considerable reduction in series resistance, R~. (The equivalent circuit diagram for our PEC configuration is presented in fig. 2.) In this way, if not most, some potential drop can be expected to appear at the electrode/electrolyte interface (across R j) under illumination. It is now possible to explain the observation of cathodic photocurrent (or more precisely cathodic current in light) as follows: in dark, probably no downward band bending occurs (even for the strong cathodic biases) because of the significant power loss due to high bulk resistivity of the samples and as a result, no forward bias (cathodic) current flows. Upon illumination, the reduced R s presumably leads
D.P. Amainerkar et al. / PEC studies on CdS : Cu: C! electrodes
RS
~Rj
",'VWV~-
~
("s, +Rs2)
I
I:
REL
",NVW~
J
43
]
ci In
CdS: Cu:CI
No2 S
Pt
ConductionB a n d ~ Cu Impurity Band...,. S
Valence Band Elect rolyte
Semiconductor
Fig. 2. Equivalent circuit diagram for C d S : C u : Ci based PEC junction, where: Rj and Cj represent resistance and capacitance of the electrochemical junction, respectively; R EL represents resistance of the eleclroi)te and R~ represents series resistance comprising two components Rsl and Rs2. R~2 is an
effective light dependent resistance.
to an appreciable downward band bending upon application of sufficient cathodic bias and as a result, significant cathodic current flows in the same way as it flows in dark for an n-CdS electrode possessing low-resistivity i.e. in accordance with the following cathodic reaction: S
(electrolyte)
+
2 e-
(electrode)
~
S 2-
(electrolyte) "
(I)
But it is difficult to specify whether "dark" (i.e. injected by an ohmic contact on cathodically biasing) oc photoexcited electrons in the conduction band of CdS participate in this reaction limiting the cathodic current. It is, therefore, plausible to conclude that we might have observed the photomodulation of cathodic current where the effect of illumination lies in reducing the series resistance and thereby, in restoring the ideal forward bias behaviour. A similar argument of photomodulation however, cannot be applied for the anodic case because the interpretation of the red shift in anodic spectral response (to be discussed) requires that the holes - photogenerated in the Cu impurity band - should participate in the anodic oxidation reaction as indicated below: S 2-
(electrolyte)
+
2p (electrode, photogenerated in the impurity band)
~
S (electrolyte)"
(2)
44
D.P. Amainerkar et al. / P E C studies on CdS : Cu : Ci electrodes
It thus leads to "true" anodic photocurrent. This interpretation is partly in agreement with the compensated surface layer model proposed by Muller et al. [12]. It should be noted ~ a t while Muller et al. invoke the presence of deep-lying compensated ac~eptor states of unknown nature [12], we deal with the presence of Cl-compensated Cu acceptor states, the importance of which is rigorously established for CdS photoconductors [16]. It is worth mentioning thvt dark current suppression is more prominent for our CdS:Cu:C1 samples than that for mechanically damaged CdS samples (compare fig, 1 of this report with fig. 1 given in ref. [12]). 3.2. Spectral response (i.e. action spectra) The spectral response of the anodic photocurrent for the samples fired at 500, 550, 600 and 700°C is presented in fig. 3 by curves (A) to (D), respectively. For clarity, curve (A) has been reproduced on the expanded scale in fig. 4. ]t is seen that 100
80
X tZ E Q: u 0 I0 a.
okt Y f
x'
40-
X/
(DI
/
/
".2
/ 2o-
O 400
x
~
""
(A)
500
600
700
coo
900
WAVELENGTH (n m) Fig. 3. Spectral response of the anodic photocurrent (applied pozential -- -0.3 V (SCE)) for CdS: Cu: CI
electrodes in 0.5M Na2S. (A, B, C, D) Samples fired at 500, 550, 600 and 700 o C, respectively. The values of the photocurrent are normalised in such a way that t;he maximum value is 100.
D.P. Amalnerkar et aL / PEC studies on CdS : Cu: C! electrodes
45
q'O
0.8
0'6
0.4
0.2 =
O.I 400
500
600
700
800
900
WAVE LENGTH ( n m) Fig. 4. Curve (A) of fig. 3 on an expanded scale.
the maximum of spectral response occurs in the vicinity of ~ = 490 nm for the samples fired at 5G0 °C (fig. 4), while it shifts towards the red region by occurring in the vicinity of A = 635 nm for the samples fired at and above 550 °C (curves (B), (C) and (D), fig. 3). The spectral response in the former case is in agreement with the previous report [17], and it suggests a band-gap excitation process. The remarkable red shift in the spectral response peak observed for the latter samples is quite peculiar as far as CdS-based PEC cells are considered. It is a novel observation recently made by us for the air-sintered pellets of photoconducting CdS : Cu: CI [18] and also for electrophoretically deposited C d S : C u : C I layers [7,8]. We interpreted this red shift on the basis of photoexcitation of Cl-compensated Cu impurity centers which energetically lie about 0.9 eV above the valence band of the host CdS [7,18]. Such a photoexcitation leaves behind positively charged holes at the impurity centers. These photoexcited holes - on the assumption of formation of impurity bands as a result of sufficient impurity incorporation - can subsequently participate in the photoanodic oxidation of sulphide species giving rise to anodic photocurrent [7] (see reaction (2)). The assumption of formation of impurity bands is necessary to provide the delocalisation of states, a condition under which charge carriers can
46
D.I'. Amalnerkar et al. / P E C studies on CdS : Cu : CI electrodes
drift towards the electrode/electrolyte interface under the influence of an external field. In the absence of such impurity bands, i.e. for the localised states, one has to consider hopping between neighbouring states by more specific mechanisms like thermally activated transfer of trapped charges in tlle Mott model or by tunneling in the Anderson model [19]. Since the impurity excitation requires relatively less energy than the band-gap excitation, the PEC spectral response peak gets shifted to the red region. It is obvious that such a red shift in spectral response is highly desirable from the viewpoint of an efficient solar energy conversion. Furthermore, it directly supports an assumption of dominance of Cu(:Cl) impurity absorption as required for the interpretation of the photoeffect under cathodic polarisation. It should be noted, however, that we could not observe such red shift in the PEC spectral response for the pellets fired at 500 °C (fig. 4). This observation may be ascribable to the relatively low temperature of sintering as it may not be sufficient for an effective/appreciable formation of Cl-compensated Cu impurity sensitising centres and/or impurity band formation associated with them [18]. The extremely feeble photosensitivity exhibited by these pellets (500 ° C) favourably suggests the possibility of an ineffective formation of impurity-induced sensitizing centers. This is clearly reflected in overall suppression of spectral response curve (A) when compared to the other curves in fig. 3.
3.3. Spectral response with additional backgr~und illumination (i.e. "double-beam"
method) Spectral response curves of the anodic photocurrent recorded in the presence and absence of an additional (background) monochromatic illumination at ?~--480 nm (henceforth referred to as supra-band-gap illumination) and ~, = 680 nm (henceforth referred to as sub-band-gap illumination) are shown in figs. 5, 6 and 7 which typically represent the curves for samples fired at 500, 550 and 700 ° C, respectively. These results indicate that the additional background illuminations as such do not alter the overall nature of the spectral response of the samples (except with the appearance of two minor response peaks in the near infra-red region in the case of the samples fired at 500°C, fig. 5), but result in an interesting photocurrent enhancement phenomenon as discussed below. (i) For the samples fired at 500 ° C, the additional background illumination of both the types enhances the photocurrent for the wavelengths corresponding to the fundamental absorption edge (fig. 5). It should be recalled that these samples show the spectral response peak in the fundamental absorption edge of CdS which suggests that the photoexcited holes of the valence band participate in an anodic oxidation process. The photocurrent enhancement due to sub-band-gap illumination (curve (b), fig. 5) can be related to a high hole-capture cross section of Cu(:C1) centers. In the absence of additional sub-band-gap illumination, such Cu(:CI) centers can capture the holes which are photogenerated in the valence band by a band-gap excitation process and thereby make a lower number of holes available for participation in the photoanodic oxidation reaction. In the presence of additional
D.P. Amainerkar et al.
/
P E C studies on CdS : Cu: Ci electrodes
47
100 r - - - - - . ~ v - -
80
IZ tlJ ,v
60
rr
U 0 I-
o
40
z O.
20
0
400
I
500
~ =
I
600
.
I
--l.e.a.---q..q~...,~.___~
700
800
900
WAVELENGTH (rim)
Fig. 5. "Double-beam" spectral response of the anodic photocurrent for CdS:Cu:CI samples fired at 500 o C. (a) Usual spectral response without additional background illumination; (b, c) spectral response with additional (background) monochromatic illumination at ?~-- 680 nm and )~ ffi 480 nm, respectively. Applied potential, electrolyte and photocurrent normalisation as in fig. 3.
sub-band-gap illumination, the high density of localised holes - caused by promotion of electrons from such impurity states to the conduction band on the absorption of sub-band-gap photons - perhaps exerts a coulombic repulsion for photoexcited holes of the valence band. This process thus provides more free (mobile) holes generated by bandgap excitation from primary source - for participation in a photoanodic oxidation reaction and, thereby enhances photocurrent for the wavelengths corresponding to the fundamental absorption edge only. Alternatively, it can also be explained by the production of additional space charge in the bulk of CdS: Cu:C1 samples [8]. A photocurrent enhancement due to supra-band-gap illumination (curve (c), fig. 5) can be explained by considering superimposition of photons of the fundamental absorption edge (of the two sources). It may lead to high intensity in a critical range, where no direct recombination of free charge carriers takes place, and thus the photocurrent is enhanced. (ii) For the samples fired at and above 550°C, the additional sub-band-gap ;!lumination enhances the photocurrent for the wavelengths corresponding to the -
D.P. Amalnerkar et al. / P E C studie,~ on CdS : Cu: C! electrodes
48 100
80-
Z w E lg u o I-
o
a.'l"
60-
!
40-
1~,1
20
400
500 ,
600 WAVELENGTH
700
800
900
(nm)
Fig. 6. "Double-beam" spectral response of the anodic photocurrent for C d S : C u : C ! samples fired at 550 o C. (a, b, c) Notations and details as in fig. 5.
fundamental absorption region (curve (b) of figs. 6 and 7), while the additional supra-band-gap illumination enhances the photocurrent for the wavelengths corresponding to the red-shift region (curve (c) of figs. 6 and 7). The photocurrent enhancement in the former case can be associated with a high hole-capture'cross section of Cu(:CI) centers as discussed before. The photocurrent enhancement in the latter case (i.e. by additional supra-band-gap illumination) is quite pro n~inent and can be explained qualitatively as follows. The background of supra-band-gap illumination - due to generation of holes in the valence band - can be thought to lower the quasi-Fermi-level for holes towards the top of the valence band. This situation embraces more Cu( : CI) impurity states between the two quasi-Fermi-levels and therefore ensures theh sensitising nature as proposed by Rose [20]. In effect, the supra-band-gap illumination increases the number of Cu(: CI) centers which, upon excitation with red region photons (by the primary source) provide more holes for an additional contribu',ion to anodic phot0current. A similar situation can also be visualised by considering the capture of holes (photogenerated in the valence band by background illumination) at Cu(:CI) impurity centers which have been previously occupied with electrons. Such impurity centers upon excitation with red region
49
D.P. Amalnerkar et al. / P E C studie,~ on CdS : Cu : Ci electrodes
100
80
IZ ILl
(.) 0 I0 Z o.
60
40
400
t riO0
600
700
800
900
WAVEL[NGTH ( n m ) Fig. 7. "Double-beam" spectral response of the anodic photocurrent for CdS:Cu:CI sa~aples fired a~ 700 o C. (a, b, c) Notations and details as in fig. 5.
photons can additionally contribute to anodic photocurrent. For tile sampies fired at and above 550 o C, it is necessary to presume the "mobile" nature of holes for the interpretation of the red shift in the spectral response and, therefore, we discard the alternative mechanism based on an additional space-charge layer in this case.
4. Conclusions We have illustrated the importance of photocon,tuctivity effects associated with deep-lying Cl-compensated Cu acceptor states in the evaluation of highly photosensitive and semi-insulating C d S : C u : C I as PEC electrodes. Such states, upon absorption of long wavelength photons, can make an appreciable fraction of the sample conductive by a photoconductivity effect and, thus, permit the flow of cathodic current in light which is, otherwise, suppressed in dark due to a high bulk resistivity of the sample. In this way, the cathodic photocurrent can be shown to be a photomodulat~ed majority carrier current. The dominance of absorption at such CI-compensated Cu centers also causes a remarkable red shift in the spectral
50
D.P. Amalnerkar et al. / P E C studies on CdS : Cu: CI electrodes
response which is highly desirable for an efficient photovoltaic conversion of solar energy. However, doping with Cu - an acceptor - also results in an increase in the resistivity of CdS. Although the high resistivity is favourable for enhancing the photosensitivity (the ratio of resistivity in dark to that in tight) of the solid state photoconductive devices, it is detrimental to PEC devices. To overcome this problem, at least partially, it is imperative to use thin films/layers in such photoconductor-based PEC devices. Furthermore, oxygen chemisorption, which is wellknown to produce similar enhancement in photosensitivity of photoconducting CdS should be taken care of; for example, the high bulk resistivity of the samples can be reduced by photo- or thermal desorption of chemisorbed oxygen [16]. The Cl-compensated Cu centers also lead to an enhancement in photocurrent for the particular wavelength region depending upon the type of additional monochromatic illumination (i.e. sub-band-gap or supra-band-gap). This photocurrent enhancement phenomenon ,:~n be explained by considering the preferential recombination kinetics for the charge carriers of such states. In certain cases, it can also be explained by an alternative mechanism based on the additional space-charge formation. However, to ascertain the validity of this model, we emphasize the need of studying the spectral response of the cathodic photocurrent (in a double beam configuration) because one caunot expect any space-charge formation under cathodic polarisation. All such studies are now under progress.
Aeknow|edgements This work was supported in part by a Grant-in-Aid for Energy Research from the Ministry of Science, Education and Culture (MONBUSHO), Government of Japan. One of us (D.P.A.) thanks MONBUSHO for the award of a research scholarship wl~ch enabled him to carry out experiments of this work at Gifu University, Japan. He also thanks the Scientific Pool Scheme of CSIR, India for the current financial support. D.P.A. and S.R. are grateful to Dr. R.A. Mashelkar, Deputy Director and Head, Chemical Engineering Division, National Chemical Laboratory, Pune, for the constant encouragement.
References [1] [2] [3] [4] [5]
H. Gerischer, M. Lubke and B. Bressel, J. Electrochem. Soc. 130 (1983) 2112. H. yon Kanel, E. Kaldis, P. Wachter and H. Gerischer, J. Electrochem. Soc. 131 (1984) 77. O.E. Husser, H. yon Kanel and F. Levy, J. Electrochem. Soc. 132 (1985) 810. O.E. Husser, F. Hulliser and H. yon Kanei, J. Electrochem. Soc. 134 (1987) 477. R.H. Bube, in: Photoconductivity and Related Phenomena, Eds. J. Mort and D.M. Pai (Elsevier, Amsterdam, 1976) ch. 4. [6] R.H. Bube, in: Physics and Cl,emis~ry o f II-'vi Compounds, Eds. M. Aven and J.S. Prener (North-Holland, Amsterdam, 1967) ch. 13. [7] H. Minoura, T. Sugiura, E. Suzuki, D.P~ Amalnerkar and Y. Ueno, Work Presented at 172nd Meeting of the Electrochemical Society, Honolulu, Hawaii, October 1987. [8] H. Minoura, E. Suzuki, D.P. Amalnerkar, T. Sugiura and Y. Ueno, J. Electrochem. Soc., submitted.
D.P. Amalnerkar et al. / P E C studies on CdS : Cu: CI electrodes
51
[9] D.P. Amalnerkar, E. Suzuki, T. Sugiura, H. Minoura and Y. Ueno, J. Mater. Sci. Letters 6 (1987) 481. [10] H. Minoura and M. Tsuiki, Chem. letters (1978) 205. [11] H. Gerischer, in: Physical Chemistry (An Advanced Treatise), Vol. 9A, Eds. H. Eyring, D. Henderson and W. Jost (Academic Press, New York, 1970) ch. 5. [12] N. Muller, G. Hodes and B. Vainas, J. Electroanal. Chem. 172 (1984) 155. [13] S.J. Fonash and S. Ashok, Appl. Phys. Letters 35 (1979) 535. [14] C.R. Wronski, IEEE Trans. Electron Devices ED-24 (1977) 351. [15] R.H. Bube and B. Young, J. Appl. Phys. 35 (1964) 462. [16] D.P. Amalnerkar, PhD Thesis, University of Poona, Pune, India (1982), and references therein. [17] H. Minoureh M. Tsuiki and T. Old, Ber. Bunsenges. Phys. Chem. 81 (1977) 588. [18] D.P. Amalnerkar, E. Suzuki, T. Sugiura, H. Minoura and Y. Ueno, J. Mater. Sci. Letters 6 (1987) 1307. [19] F.A. Lindholm and C.T. Sah, IEEE Trans. Electron Devices ED-24 (1977) 299. [20] A. Rose, Concepts in Photoconductivity and Allied Problems (Interscience, New York, 1963) ch. 3.
37
THE PHOTOELECTROCHEMICAL STUDIES ON CdS: Cu: C! ELECTRODES: IMPORTANCE OF PHOTOCONDUCFIVITY EFFECTS *
D.P. AMALNERKAR, S. RADHAKRISHNAN Polymer Science and Engineering Group, ChemicalEngineering Division, National ChemicalLaboratory, Pune 411 008, India
H. MINOURA, T. SUGIURA and Y. UENO Department of Synthetic Chemistry, Faculty of Engineering, Gifu University, Yanagido 1-1, Gifu 501-11, Japan Received 6 April 1988; in revised form 9 August 1988 The photoelectrochemical characterisation of highly photosensitive and semi-insulating sintered pellets of CdS doped with Cu and C! has been carried out. In the course of this work, we have observed a complete suppression of the dark current but a pronounced photocurrent under cathodic polarisation. Both these observations indicate an unusual cathodic (i.e. forward bias) behaviour. It has been shown that due to a decrease in series resistance under illumination by an extrinsic photoconductivity effect, the "ideal" cathodic behaviour, i.e. normally found in dark for an n-CdS electrode with low resistivity, can get restored. We have also observed a red shift in the anodic photoelectrochemical spectral reponse of these samples. It is in agreement with our previous photoelectrochemical studies on CdS: Cu: C! el~trodes and can be explained by considering a Cu impurity excitation process. Additionally, we have observed that the background illuminations as such (employed in the "double-beam" technique of spectral response determination) do not alter the overall nature of the spectral response but result in an interesting photocurrent enhancement phenomenon. These photocurrent enhancement observations have been explained by taking into account the photoeffects at Cl-compensated Cu acceptor centers possessing a high capture cross section preferably for photoexcited holes.
1. Introduction
The photoelectrochemical (PEC) studies of semi-insulating materials had been hampered in the past because of the high series resistance problems which affected the measurements, but with the advent of the "double-beam" technique such draw-backs have been almost overcome. For example, Gerischer et al. [1,2] and Husser et al. [3,4] have demonstrated that such studies can be made possible if the internal resistance of the insulating material can be reduced by means of a photoconductivity phenomenon. In their "double-beam" approach, rm additional background illumination with photons of appropriate ~enetration depth makes the bulk of the sample phetoconductive, and in principle, achieves an effect quite * NCL Communication No. 4412.
0165-1633/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
38
D.P. Amalnerkar et ai. / P E C studies on CdS : Cu : Ci electrodes
similar to donor impurity doping required for the formation of a depletion layer at the interface. So far, PEC studies have been reported on GaS, CdS, SnS2, Sn(Seo.~S0.9)2, MnPSe3, CdPSe3, HgI2 and TISbS2 which ai'e all highly resistive photoconducting materials [1-4]. All these studies mostly deal with the "pure", i.e. undoped, form of the sample~, and refer essentially to intrinsic photoconductivity. Normally, photosensitivity of highly resistive photoconducting materials is associated with specific compensated acceptor type of sensitising centers which possess a high capture cross section for minority carders [5]. In fact, an intentional addition of sensitizer impurities - which can introduce such compensated acceptor states in the band-gap of a host - is a weU-estabfished technique to enhance photosensitivity of "pure" photoconducting materials [6]. Despite the importance of such deep-lying states, no serious attempt has been made to perform PEC investigations (with the "double-beam" approach or otherwise) on extrinsically doped highly photoconducting materials. We thought it would be interesting to make such an attempt especially for the following reasons: (i) the compensated acceptor states with preferred recombination kinetics for the photogenerated carders can be expected to influence the photoelectrochemicai reactions and (ii) the photoconduction, being extrinsic in nature, can become quite appreciable through the bulk of the sample by the absorption in a larger part of the visible range of wavelengths. This is highly desirable for applications in solar energy conversion devices. We have chosen the well-known C d S : C u : C I photoconductor for PEC characterisation. As a part of this work, we have recently announced the extension of PEC spectral response towards the desirable red region for electrophoretically deposited CdS: Cu: CI electrodes [7]. This is the first example of impurity sensitisation of CdS electrodes [8]. In the course of further work, we have performed PEC investigations like current-voltage characteristics and spectral response (with single-beam and double-beam methods) on highly resistive and highly photosensitive C d S : C u : C I electrodes in the form of pellets, using aqueous Na2S as an electrolyte. Quite surprisingly, we have observed a pronounced photocurrent under cathodic polarisation. This is an unusual behaviour because nolmally in a n-type semiconductor like CdS, one cannot expect pronounced photocurrent to flow under cathodic bias. We have also observed a complete suppression of dark current, a clear red-shift (not only an extension to red region) in the spectral response peak, and an interesting photocurrent enhancement phenomenon in the "double-beam" method of spectral response determination. In this paper, all these results have been presented and various possibilities pertinent to photoconductivity effects at deep-lying Cl-compensated Cu acceptor [Cu(: C l)] centers have been suggested for their interpretation. 2. Experimenlal
2.1. Preparation of the pellets Pellets of photoconducting C d S : C u : C I were prepared as follows: initially, a mixture containing high purity CdS (99.999~), AR grade CdCI 2 (10 wt~ of CdS)
D.P. Amalnerkar et al. / P E C studies on Cd$ : Cu: C! electrodes
39
and AR grade CuCl 2 (0.1 wt~ CdS) was subjected to prefiring in air at 550 ° C for 2 h (CuCI 2 acts as an activator to enhance the photosensitivity of CdS, and CdC12 acts as a flux or sintering aid). The resulting mass was thoroughly ground in an agate pestle-mortar using distilled acetone and the fine powder of activated C d S : C u : C I obtained in this way was press-moulded in vacuo to get the pellets of desired dimensions (pressure applied -- 400 kg cm-2, total pressing time -- 30 min). Finally, the pellets were fired at different temperatures in the range 500-700 ° C for 30 rain in a stream of purified N2 (to avoid the formation of a surface impurity phase (s) reported earlier [9], the final firing was carried out in N 2 atmosphere). These impurity sensitised pellets showed quite good surface photosensitivity when illuminated with visible light of moderate intensity, which was checked by an independent experiment using silver-paste top contacts. 2.2. Preparation of the electrodes From the fired pellets, the working eicetrc, des were prepared as follows: initially, an ohmic contact was formed on the back surface of the pellets by soldering high purity metallic indium. A copper lead wire was then attached to it with silver paste. Finally, the back surface and the edges of the pellets were covered with insulating epoxy resin, leaving the front surface exposed. 2.3. Photoelectrochemical measurements The photoelectrochemical measurements were performed by adopting a standard potentiostatic configuration with a three-electrode system comprising a CdS : Cu: CI working electrode, a large area platinum foil counter electrode and a saturated calomel reference electrode (SCE). An aqueous solution of 0.SM NaeS was used as the electrolyte. Since the experiments were performed in the ambient atmosphere, the oxygen present in the electrolyte can lead to the formation of elemental sulphur in sulphide solution. Polarisation curvez in dark and under illumination (intensity-100 mW cm-2; source: 500 W xenon lamp) were established by using a Nikko Keisoku potentiostat (NPGS-301) and recorded on an x - y recorder (Watanabe, WX 4402). For spectral response determination, the sample electrodes were illumi~ated by appropriately filtered and chopped monochromatic light. A Nikon monochromator (G 250) equipped with a 300 W tungsten-iodine lamp (primary source) was used for this purpose. Additionally, for the reasons given by Husser et al. [4], spectral response measurements were also performed in a "double-beam" arrangement. In this arrangement, unchopped monochromatic radiation at ~, = 480 nm and ~, = 680 nm (obtained from a JUSCO (CT-10) monochromator equipped with a 500 W xenon lamp : secondary source) were used for "background" illumination of the samples. The resulting photocurrent values were measured by a standard lock-in technique, then normalised by taking into account the corrections arising from spectral intensity distribution of the source and the corrected values were plotted on an x - y plotter interfaced with a NEC computer. All the spectral response measurements were carried out at a potential of - 0 . 3 V (SCE).
40
D.P. Amalnerkar et al. / PEC studies on CdS : Cu : Ci electrodes
3. R e s u l t s a n d d i s c u s s i o n 3.1. C u r r e n t - p o t e n t i a l curves
The current-potential curves (in dark and under polychromatic illumination) of the C d S : C u : C I electrodes corresponding to firing temperatures 500, 550, 600 and 700 °C are presented in figs. 1A-1D, respectively. It can be observed that: (i) All the sample electrodes - with the exception of those corresponding to a firing temperature of 500"C - exhibit complete suppression of the dark current for both cathodically and anodically appfied biases. (ii) All the sample electrodes surprisingly exhibit pronounced photocurrent upon cathodic polarisation and there is no tendency of photocm'rent saturation for the entire range of applied cathodic biases covered in this experiment (we have polarised the samples only up to -1.45 V (SCE) because there is a possibility of hydrogen evolution a n d / o r cathodic decomposition of the CdS electrode on polarising it beyond - 1 . 7 V (SCE) [10]). (iii) There is a tendency for the photocurrent to saturate in the anodic region and it increases with the increase in firing temperature of the samples. (iv) In general, with the exception of electrodes corresponding to a firing temperature of 500 ° C, the cathodic photocurrent is much higher than the anodic photocurrent for the same electrode (see inset of fig. 1). An observation of a pronounced cathodic current in light is quite unusual in n-type semiconducting electrodes such as CdS. Ordinarily, in such electrodes an application of sufficient cathodic bias leads to a downward band bending and to an accumulation of electrons in the region just below the semiconductor/electrolyte interface [11]. In such a condition, photogenerated electrons contribute only negligibly to the flow of current (residual effect). Ideally, under cathodic polarisation, huge cathodic (forward bias) current is expected to flow even in the dark [11], a fact which we have not observed in the present case. There exist a few reports in the fiterature which deal with similar observations on suppression of cathodic current in dark and its enhancement in light, for example, as reported by Muller et al. [12] in the case of a mechanically damaged CdS electrode/0.1M Fe(CN) 3-/4- electrolyte system. They have shown that this photoenhancement of cathodic current (also referred to as photocathodic effect (PCE) by them) can be explained either by a model based on the presence of a compensated insulating surface layer which is photoconductive in nature, or by considering Fermi level pinning by the surface states° We cannot totally adopt this model to explain PCE in the present case, because there is a basic difference between our CdS: Cu : CI samples and the samples used by Muller et al., viz. our CdS : Cu : Ci samples possess very_ high bulk resistivity in dark ( ~ 107-10 s fi cm) and exhibit extremely high solid state photosensitivity * ( - 103-10 s, see experimental section). Both the characteristics are interrelated and attributed to a compensated acceptor type of sensitising states introduced by Cu:C1 impurities. On the contrary, Muller et al. assume that * Defined as the ratio of photocurrent to dark current.
D.P.
Amulnerkar et ai. / PEC studies on CdS: Cu: Cl decmdes
500 FIRING I
LIOHT
OFF
6QO fE&.
41
OC
UUU
0.0
LIGHT OFF -0*4
LlQHT ON
-1.2
-1.0
-0*0
-0.6
APPLIED
-@4
-02
POTENTIAL
0
0.2
O-4
0.6
(Cl
0.0
fV ISCEI]
Fig. L Current-potential curves for CdS: Cu: Cl pelletised electrodes in OSM Na2S under antermittent illumination (intensity = 100 mW/cm2). (A, B, C, D) Samples fired at 500, 550, 600 and 700°C, respectively. The inset shows dependence of cathodic [at -1.3 v (SE)] and anodic [at 0.8 v @CE)l photocurrent on firing temperature.
mechanical polishing creates a compensated insulating layer (dark resistance in excess of 100 kS2 for a crystal of cross-sectional area 5 mm2) at the surface of CdS electrodes, with the specific resistivity of the order of 1 Q cm in the “virgin” state
42
D.P. Amainerkar et al. / P E C studies on CdS : Cu: C! electrodes
[12]. They further assume that this layer exhibits photoconductivity which is attributable to deep-lying acceptor states introduced by mechanical polishing. Considering all these facts, we suggest the following phenomenological possibilities to interpret our current-voltage results. In order that an externally applied voltage creates a potential drop at the electrode/electrolyte interface, it is necessary that the resistance between this interface and the ohmic contact at the rear face (i.e. the resistance in series with the current generating region, R~) should be as low as possible. The complete suppression of cathodic and anodic current in dark can then be regarded as a consequence of high bulk resistivity of our samples which allows only a minimal potential drop to occur at this interface. However, the highly photoconductive nature of our samples can dramatically alter this situation upon suitable illumination. To explain this, we mainly rely upop the reports on photoconductive a-Si based photovoltaic configurations. For example, it has been recognised in case of photoconductive a-Si (possessing high bulk resistance in dark) that photoconductivity reduces the series resistance (R~) of a photovoltaic cell configuration [13,14]. The effect of the photoconductivity phenomenon becomes particularly significant for the volume-absorbed light since it can induce appreciable photoconduction through the bulk of the sample. In more traditional photoconductive materials like CdS doped with CI and Cu, a similar effect can be expected to occur perhaps to a greater extent owing to extrinsic photoconductivity. We, therefore, believe that the extrinsic photoconduction due to dominance of Cu impurity absorption may have an important contribution in establishing "light" polarisation curves for our CdS : Cu : CI samples. To elaborate further, we quote the solid state photoconductivity studies on CdS: Cu: I crystals reported by Bube and Young [15] which forms the basis to presume the dominance of Cu(: CI) impurity absorption in the present case. They have shown that for a sufficiently high doping level, the excitation of the photoconductive crystals gets dominated by the absorption by Cu impurity centers possessing an absorption constant of the order of 200 cm-1 for 0.1% Cu concentration (same as our samples). The absorption at Cu(:CI) impurity centers has thus two important consequences: (i) the photoexcitation of electrons from such centers to the conduction band makes a larger number of free electrons avaglable for electronic conduction and (ii) the absorption of long wavelength photons, owing to their ~e~ter penetration depth, makes an appreciable fraction of the sample-electrodes conductive under light (for example, roughly 50% for a 0.1 mm thick crystal with 0.1~ Cu concentration). The net result of (i) and (ii) is considerable reduction in series resistance, R~. (The equivalent circuit diagram for our PEC configuration is presented in fig. 2.) In this way, if not most, some potential drop can be expected to appear at the electrode/electrolyte interface (across R j) under illumination. It is now possible to explain the observation of cathodic photocurrent (or more precisely cathodic current in light) as follows: in dark, probably no downward band bending occurs (even for the strong cathodic biases) because of the significant power loss due to high bulk resistivity of the samples and as a result, no forward bias (cathodic) current flows. Upon illumination, the reduced R s presumably leads
D.P. Amainerkar et al. / PEC studies on CdS : Cu: C! electrodes
RS
~Rj
",'VWV~-
~
("s, +Rs2)
I
I:
REL
",NVW~
J
43
]
ci In
CdS: Cu:CI
No2 S
Pt
ConductionB a n d ~ Cu Impurity Band...,. S
Valence Band Elect rolyte
Semiconductor
Fig. 2. Equivalent circuit diagram for C d S : C u : Ci based PEC junction, where: Rj and Cj represent resistance and capacitance of the electrochemical junction, respectively; R EL represents resistance of the eleclroi)te and R~ represents series resistance comprising two components Rsl and Rs2. R~2 is an
effective light dependent resistance.
to an appreciable downward band bending upon application of sufficient cathodic bias and as a result, significant cathodic current flows in the same way as it flows in dark for an n-CdS electrode possessing low-resistivity i.e. in accordance with the following cathodic reaction: S
(electrolyte)
+
2 e-
(electrode)
~
S 2-
(electrolyte) "
(I)
But it is difficult to specify whether "dark" (i.e. injected by an ohmic contact on cathodically biasing) oc photoexcited electrons in the conduction band of CdS participate in this reaction limiting the cathodic current. It is, therefore, plausible to conclude that we might have observed the photomodulation of cathodic current where the effect of illumination lies in reducing the series resistance and thereby, in restoring the ideal forward bias behaviour. A similar argument of photomodulation however, cannot be applied for the anodic case because the interpretation of the red shift in anodic spectral response (to be discussed) requires that the holes - photogenerated in the Cu impurity band - should participate in the anodic oxidation reaction as indicated below: S 2-
(electrolyte)
+
2p (electrode, photogenerated in the impurity band)
~
S (electrolyte)"
(2)
44
D.P. Amainerkar et al. / P E C studies on CdS : Cu : Ci electrodes
It thus leads to "true" anodic photocurrent. This interpretation is partly in agreement with the compensated surface layer model proposed by Muller et al. [12]. It should be noted ~ a t while Muller et al. invoke the presence of deep-lying compensated ac~eptor states of unknown nature [12], we deal with the presence of Cl-compensated Cu acceptor states, the importance of which is rigorously established for CdS photoconductors [16]. It is worth mentioning thvt dark current suppression is more prominent for our CdS:Cu:C1 samples than that for mechanically damaged CdS samples (compare fig, 1 of this report with fig. 1 given in ref. [12]). 3.2. Spectral response (i.e. action spectra) The spectral response of the anodic photocurrent for the samples fired at 500, 550, 600 and 700°C is presented in fig. 3 by curves (A) to (D), respectively. For clarity, curve (A) has been reproduced on the expanded scale in fig. 4. ]t is seen that 100
80
X tZ E Q: u 0 I0 a.
okt Y f
x'
40-
X/
(DI
/
/
".2
/ 2o-
O 400
x
~
""
(A)
500
600
700
coo
900
WAVELENGTH (n m) Fig. 3. Spectral response of the anodic photocurrent (applied pozential -- -0.3 V (SCE)) for CdS: Cu: CI
electrodes in 0.5M Na2S. (A, B, C, D) Samples fired at 500, 550, 600 and 700 o C, respectively. The values of the photocurrent are normalised in such a way that t;he maximum value is 100.
D.P. Amalnerkar et aL / PEC studies on CdS : Cu: C! electrodes
45
q'O
0.8
0'6
0.4
0.2 =
O.I 400
500
600
700
800
900
WAVE LENGTH ( n m) Fig. 4. Curve (A) of fig. 3 on an expanded scale.
the maximum of spectral response occurs in the vicinity of ~ = 490 nm for the samples fired at 5G0 °C (fig. 4), while it shifts towards the red region by occurring in the vicinity of A = 635 nm for the samples fired at and above 550 °C (curves (B), (C) and (D), fig. 3). The spectral response in the former case is in agreement with the previous report [17], and it suggests a band-gap excitation process. The remarkable red shift in the spectral response peak observed for the latter samples is quite peculiar as far as CdS-based PEC cells are considered. It is a novel observation recently made by us for the air-sintered pellets of photoconducting CdS : Cu: CI [18] and also for electrophoretically deposited C d S : C u : C I layers [7,8]. We interpreted this red shift on the basis of photoexcitation of Cl-compensated Cu impurity centers which energetically lie about 0.9 eV above the valence band of the host CdS [7,18]. Such a photoexcitation leaves behind positively charged holes at the impurity centers. These photoexcited holes - on the assumption of formation of impurity bands as a result of sufficient impurity incorporation - can subsequently participate in the photoanodic oxidation of sulphide species giving rise to anodic photocurrent [7] (see reaction (2)). The assumption of formation of impurity bands is necessary to provide the delocalisation of states, a condition under which charge carriers can
46
D.I'. Amalnerkar et al. / P E C studies on CdS : Cu : CI electrodes
drift towards the electrode/electrolyte interface under the influence of an external field. In the absence of such impurity bands, i.e. for the localised states, one has to consider hopping between neighbouring states by more specific mechanisms like thermally activated transfer of trapped charges in tlle Mott model or by tunneling in the Anderson model [19]. Since the impurity excitation requires relatively less energy than the band-gap excitation, the PEC spectral response peak gets shifted to the red region. It is obvious that such a red shift in spectral response is highly desirable from the viewpoint of an efficient solar energy conversion. Furthermore, it directly supports an assumption of dominance of Cu(:Cl) impurity absorption as required for the interpretation of the photoeffect under cathodic polarisation. It should be noted, however, that we could not observe such red shift in the PEC spectral response for the pellets fired at 500 °C (fig. 4). This observation may be ascribable to the relatively low temperature of sintering as it may not be sufficient for an effective/appreciable formation of Cl-compensated Cu impurity sensitising centres and/or impurity band formation associated with them [18]. The extremely feeble photosensitivity exhibited by these pellets (500 ° C) favourably suggests the possibility of an ineffective formation of impurity-induced sensitizing centers. This is clearly reflected in overall suppression of spectral response curve (A) when compared to the other curves in fig. 3.
3.3. Spectral response with additional backgr~und illumination (i.e. "double-beam"
method) Spectral response curves of the anodic photocurrent recorded in the presence and absence of an additional (background) monochromatic illumination at ?~--480 nm (henceforth referred to as supra-band-gap illumination) and ~, = 680 nm (henceforth referred to as sub-band-gap illumination) are shown in figs. 5, 6 and 7 which typically represent the curves for samples fired at 500, 550 and 700 ° C, respectively. These results indicate that the additional background illuminations as such do not alter the overall nature of the spectral response of the samples (except with the appearance of two minor response peaks in the near infra-red region in the case of the samples fired at 500°C, fig. 5), but result in an interesting photocurrent enhancement phenomenon as discussed below. (i) For the samples fired at 500 ° C, the additional background illumination of both the types enhances the photocurrent for the wavelengths corresponding to the fundamental absorption edge (fig. 5). It should be recalled that these samples show the spectral response peak in the fundamental absorption edge of CdS which suggests that the photoexcited holes of the valence band participate in an anodic oxidation process. The photocurrent enhancement due to sub-band-gap illumination (curve (b), fig. 5) can be related to a high hole-capture cross section of Cu(:C1) centers. In the absence of additional sub-band-gap illumination, such Cu(:CI) centers can capture the holes which are photogenerated in the valence band by a band-gap excitation process and thereby make a lower number of holes available for participation in the photoanodic oxidation reaction. In the presence of additional
D.P. Amainerkar et al.
/
P E C studies on CdS : Cu: Ci electrodes
47
100 r - - - - - . ~ v - -
80
IZ tlJ ,v
60
rr
U 0 I-
o
40
z O.
20
0
400
I
500
~ =
I
600
.
I
--l.e.a.---q..q~...,~.___~
700
800
900
WAVELENGTH (rim)
Fig. 5. "Double-beam" spectral response of the anodic photocurrent for CdS:Cu:CI samples fired at 500 o C. (a) Usual spectral response without additional background illumination; (b, c) spectral response with additional (background) monochromatic illumination at ?~-- 680 nm and )~ ffi 480 nm, respectively. Applied potential, electrolyte and photocurrent normalisation as in fig. 3.
sub-band-gap illumination, the high density of localised holes - caused by promotion of electrons from such impurity states to the conduction band on the absorption of sub-band-gap photons - perhaps exerts a coulombic repulsion for photoexcited holes of the valence band. This process thus provides more free (mobile) holes generated by bandgap excitation from primary source - for participation in a photoanodic oxidation reaction and, thereby enhances photocurrent for the wavelengths corresponding to the fundamental absorption edge only. Alternatively, it can also be explained by the production of additional space charge in the bulk of CdS: Cu:C1 samples [8]. A photocurrent enhancement due to supra-band-gap illumination (curve (c), fig. 5) can be explained by considering superimposition of photons of the fundamental absorption edge (of the two sources). It may lead to high intensity in a critical range, where no direct recombination of free charge carriers takes place, and thus the photocurrent is enhanced. (ii) For the samples fired at and above 550°C, the additional sub-band-gap ;!lumination enhances the photocurrent for the wavelengths corresponding to the -
D.P. Amalnerkar et al. / P E C studie,~ on CdS : Cu: C! electrodes
48 100
80-
Z w E lg u o I-
o
a.'l"
60-
!
40-
1~,1
20
400
500 ,
600 WAVELENGTH
700
800
900
(nm)
Fig. 6. "Double-beam" spectral response of the anodic photocurrent for C d S : C u : C ! samples fired at 550 o C. (a, b, c) Notations and details as in fig. 5.
fundamental absorption region (curve (b) of figs. 6 and 7), while the additional supra-band-gap illumination enhances the photocurrent for the wavelengths corresponding to the red-shift region (curve (c) of figs. 6 and 7). The photocurrent enhancement in the former case can be associated with a high hole-capture'cross section of Cu(:CI) centers as discussed before. The photocurrent enhancement in the latter case (i.e. by additional supra-band-gap illumination) is quite pro n~inent and can be explained qualitatively as follows. The background of supra-band-gap illumination - due to generation of holes in the valence band - can be thought to lower the quasi-Fermi-level for holes towards the top of the valence band. This situation embraces more Cu( : CI) impurity states between the two quasi-Fermi-levels and therefore ensures theh sensitising nature as proposed by Rose [20]. In effect, the supra-band-gap illumination increases the number of Cu(: CI) centers which, upon excitation with red region photons (by the primary source) provide more holes for an additional contribu',ion to anodic phot0current. A similar situation can also be visualised by considering the capture of holes (photogenerated in the valence band by background illumination) at Cu(:CI) impurity centers which have been previously occupied with electrons. Such impurity centers upon excitation with red region
49
D.P. Amalnerkar et al. / P E C studie,~ on CdS : Cu : Ci electrodes
100
80
IZ ILl
(.) 0 I0 Z o.
60
40
400
t riO0
600
700
800
900
WAVEL[NGTH ( n m ) Fig. 7. "Double-beam" spectral response of the anodic photocurrent for CdS:Cu:CI sa~aples fired a~ 700 o C. (a, b, c) Notations and details as in fig. 5.
photons can additionally contribute to anodic photocurrent. For tile sampies fired at and above 550 o C, it is necessary to presume the "mobile" nature of holes for the interpretation of the red shift in the spectral response and, therefore, we discard the alternative mechanism based on an additional space-charge layer in this case.
4. Conclusions We have illustrated the importance of photocon,tuctivity effects associated with deep-lying Cl-compensated Cu acceptor states in the evaluation of highly photosensitive and semi-insulating C d S : C u : C I as PEC electrodes. Such states, upon absorption of long wavelength photons, can make an appreciable fraction of the sample conductive by a photoconductivity effect and, thus, permit the flow of cathodic current in light which is, otherwise, suppressed in dark due to a high bulk resistivity of the sample. In this way, the cathodic photocurrent can be shown to be a photomodulat~ed majority carrier current. The dominance of absorption at such CI-compensated Cu centers also causes a remarkable red shift in the spectral
50
D.P. Amalnerkar et al. / P E C studies on CdS : Cu: CI electrodes
response which is highly desirable for an efficient photovoltaic conversion of solar energy. However, doping with Cu - an acceptor - also results in an increase in the resistivity of CdS. Although the high resistivity is favourable for enhancing the photosensitivity (the ratio of resistivity in dark to that in tight) of the solid state photoconductive devices, it is detrimental to PEC devices. To overcome this problem, at least partially, it is imperative to use thin films/layers in such photoconductor-based PEC devices. Furthermore, oxygen chemisorption, which is wellknown to produce similar enhancement in photosensitivity of photoconducting CdS should be taken care of; for example, the high bulk resistivity of the samples can be reduced by photo- or thermal desorption of chemisorbed oxygen [16]. The Cl-compensated Cu centers also lead to an enhancement in photocurrent for the particular wavelength region depending upon the type of additional monochromatic illumination (i.e. sub-band-gap or supra-band-gap). This photocurrent enhancement phenomenon ,:~n be explained by considering the preferential recombination kinetics for the charge carriers of such states. In certain cases, it can also be explained by an alternative mechanism based on the additional space-charge formation. However, to ascertain the validity of this model, we emphasize the need of studying the spectral response of the cathodic photocurrent (in a double beam configuration) because one caunot expect any space-charge formation under cathodic polarisation. All such studies are now under progress.
Aeknow|edgements This work was supported in part by a Grant-in-Aid for Energy Research from the Ministry of Science, Education and Culture (MONBUSHO), Government of Japan. One of us (D.P.A.) thanks MONBUSHO for the award of a research scholarship wl~ch enabled him to carry out experiments of this work at Gifu University, Japan. He also thanks the Scientific Pool Scheme of CSIR, India for the current financial support. D.P.A. and S.R. are grateful to Dr. R.A. Mashelkar, Deputy Director and Head, Chemical Engineering Division, National Chemical Laboratory, Pune, for the constant encouragement.
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