J. Phys. Chem. 1993,97, 10769-10773
10769
Photoelectrochemical Behavior of Thin CdSe and Coupled TiOz/CdSe Semiconductor Films Di Liu and Prashant V. Kamat' Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556 Received: April 29, 1993; In Final Form: August I I , 1993"
Photoelectrochemical effects at CdSe thin film electrodes (thickness 30-200 A) have been investigated by monitoring open-circuit voltage and short-circuit current a t varying film thicknesses and incident light intensities. Unlike the situation in conventional photoelectrochemical cells that employ single-crystal or polycrystalline semiconductors, the charge separation in thin CdSe films is not controlled by the space charge layer a t the semiconductor-electrolyte interface, but it is controlled by the differing rates of electron and hole transfer into the solution. Quick decay of the photocurrent, even in the presence of a redox couple such as [Fe(CN)#-/', suggests high degree of charge recombination within the thin CdSe film. It is possible to improve the photocurrent stability of a thin CdSe film by coupling it with a Ti02 particulate film. The improved charge separation in the coupled semiconductor system has a beneficial effect in improving the photocurrent stability of thin semiconductor films.
SCHEME I: Principle of Rectification in a Coupled Semiconductor System
Introduction In recent years considerable interest has been shown in the synthesis of thin semiconductor films by electrochemical and chemical deposition as well as direct deposition of colloidal semiconductors (see, for example, refs 1 and 2). A variety of interesting properties of these thin semiconductor films are being explored in our laboratory and elsewhere.2-1' These include extended photoresponse of large bandgap semiconductor mateelectrochromic effects,M electrochemically assisted photocatalysis,2e and electrochemicalre~tification.~ For example, the porous Ti02 film prepared from a colloidal suspension has been shown to improve the photosensitization efficiency of a ruthenium complex.6 Electrochemical deposition is a convenient technique for preparing thin metal chalcogenide films on electrode surfaces.1J0-12 It has been pointed out that thin films comprised of nanocrystallites (ca. 50 A in diameter) exhibit quantum size effects.I0 Several studies in the past have focused on the electrochemical and photoelectrochemical behavior of electrochemically deposited films of CdS, CdSe, and CdTe.IJ3-l6 Hodes and co-workers" have pointed out that thin films consisting of small semiconductor crystallites exhibit charge recombination losses when they are employed as photoanodes in photoelectrochemical cells. One of the major problems in utilizing thin semiconductor films in photoelectrochemicalcells is the absence of a space charge layer at the electrode/electrolyte interface. Under these circumstances photogenerated charge carriers can move in both directions. For example, the photogenerated electrons in an n-type CdSe thin film electrode either recombine readily with holes or leak out at the electrolyte interface, instead of flowing through the external circuit. In a preceding study3 we have shown that by coating a thin CdSe film on a Ti02 particulate film it is possible to rectify the flow of electrons. The principle of such a concept is illustrated in Scheme I. In such a coupled semiconductor system the photogenerated electrons are quickly transferred from an excited CdSe (ECB= -1 .O V vs NHE) film into the lower-lying conduction band of a Ti02 film (ECB= -0.5 V vs NHE). We now report here the photoelectrochemical behavior of thin CdSe and TiOz/CdSe films coated on an optically transparent electrode (OTE). The role of Ti02 particulate film in improving the photocurrent stability of OTE/TiOz/CdSe electrode is also described. @
Abstract published in Aduonce ACS Abstracts. September 15, 1993.
Experimental Section Materials. Optically transparent electrodes (OTE) were cut from a conducting (indium tin oxide coated) glass plate obtained from Donnelley Corp., Holland, MI. Titanium isopropoxide, CdS04, and SeO2 were obtained from Aldrich. All other chemicals were analytical reagents of highest available purity. Preparation of Ti02 Particulate Films. A transparent colloidal suspension of Ti02 was prepared by a method which has been described earlier? The diameter of these particles was around 100 A. A small aliquot (usually 0.1 mL) of the Ti02 sol was applied to a conducting surface of 0.8 X 5 cm2 of OTE and was dried in air on a warm plate. The TiO2-coated conducting glass plates (referred to as OTE/TiO2) were then baked at 673 K for 1 h. The semiconductor thin films baked at 673 K adhered strongly to the glass surface and were stable in the pH range 1-12. The thickness of the film was determined from the gravimetric analysis. Deposition of CdSe Films. Thin films of CdSe were deposited onto bare OTE and Ti02-colloid-depositedOTE electrodes using a SeO2-based bath.1J2a The thickness of the film was varied by controlling the cathodic charge. The typical thickness of the CdSe film was in the range 30-200 A. The thickness of the film was measured using a Tencor Alpha 200 profilometer. Electrochemical and Photoelectrochemical Measurements. Electrochemical measurements were carried out with a standard three-compartment cell consisting of a Pt wire counter electrode and a saturated calomel electrode (SCE) as reference. A Princeton Applied Research (PAR) Model 173 potentiostat and Model 175 universal programmer or BAS 100 electrochemical analyzer was used in electrochemical measurements. Photocur-
0022-3654/93/2097-10769%04.00/0 0 1993 American Chemical Society
Liu and Kamat
10770 The Journal of Physical Chemistry, VoI. 97, No. 41, 1993 1.4
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V vs. SCE Figure 1. Cyclic voltammograms recorded during the deposition of CdSe film on OTE plates (1-40 scans). The electrolyte consisted of 0.3 M CdS04, 1 mM SeO2,0.4 mM H2S04, and the scan rate was 100 mV/s.
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Wavelength, nm Figure 2. Absorption spectra of CdSe films with various thickness: (a) 400 nm). The dependence of photocurrent on the amount of charge employed for CdSe deposition (Figure 3) was similar to that observed for the absorbance dependence in Figure 2 (insert). This shows that the observed photocurrent is directly proportional to the amount of CdSe deposited onto the electrode surface. The mechanism of photocurrent generation in these thin films is considered to be different than that of conventional photoelectrochemical (PE) cells employing single-crystal or polycrystalline semiconductor materials. In conventional PE cells, the space charge layer formed at the electrode/electrolyte interface promotes the charge separation by facilitating the flow of electrons and holes in the opposite directions. Since the CdSe films employed in the present experiments are very thin (thickness smaller than 200 A), the space charge layer is absent at the electrode/electrolyte interface. Charge separation in these flims occurs as a result of differing rates of electron and hole transfer at the electrode/electrolyte interface.2-8J0.11The anodic photocurrent observed in the present experiments suggests that hole injection into the solution is preferred over electron injection (reactions 1 and 2).
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The Journal of Physical Chemistry, Vol. 97, No. 41, 1993 10771
Photoelectrochemical Behavior of Thin CdSe Films
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SCHEME 11: Generation of the Photoelectrochemical Effect at an OTE/TiO2/CdSe Thin Film Electrode
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< \ \ CdSe Ti02 Thus, we expect photogenerated holes to quickly migrate to the film/solution interface and electrons to move toward OTE, thereby causing the CdSe film to behave like an n-type semiconductor (Scheme 11). It is also evident from Figure 3 that the observed photocurrent under back face illumination is greater than the one observed under front face illumination. This effect is more pronounced in thicker films. Similar observations have also been made in an earlier study which addressed the effect of film thickness on the efficiency of photocurrent generati0n.l' Because of higher absorbance of CdSe film at lower wavelengths, the excitation within the film becomes uneven. Most of thelight is thus absorbed near the excitation side of the film. When thicker CdSe films are illuminated from the front face, more charge carriers are generated at the solution interface than at the OTE surface, and these charge carriers are likely to be lost in the recombination process before they can reach the OTE surface (reaction 3).
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CdSe (h + e) CdSe (3) The probability of electron transport for the photocurrent generation is greater when the illumination is carried out at the back face. Effect of Light Intensity. The photoelectrochemical behavior of thin CdSe films was further evaluated by measuring shortcircuit photocurrent (isc) and open-circuit photovoltage (V,) at various incident light intensities (Zinc), The experimental results are presented in Figures 4 and 5 . The logarithmic plot of is,
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Figure 5. Dependence of open-circuit voltage of OTE/CdSe electrode (filmthickness 150 A) on the incident light intensity (electrolyte: 0.5 M Fe[(CN)#-, 0.1 mM Fe[(CN)#-, pH 12). The insert shows the dependence of V, on In Zinc.
versus Zinc was found to be linear with a slope of 1.17. This value which is close to unity indicates that photogeneration of charge carriers which collectively yield anodic photocurrent is a monophotonic process. For a photoelectrochemical cell operating on a Schottky barrier principle,19 one can correlate V, and is, by the expression (4)
where k , T ,andq are respectively the Boltzmannconstant,absolute temperature, and electric charge and n and io are respectively the diode quality factor and the reverse saturation current. The dependence of V, on In is, (and hence also on In Zinc, see insert in Figure 5) shows that expression 4 can be used to characterize the photoelectrochemical parameters of CdSe thin film electrodes. Similar behavior has been observed by us in the case of thin film electrodes prepared from TiO2, ZnO, and CdS colloids.2 The values of n and io obtained from the data in Figure 5 for CdSe film electrode are 5.7 and 8.9 pA/cm2, respectively. The high value of reverse saturation current (io) further highlights the problem of rectifying the flow of charge carriers in thin semiconductor films. TiOz/CdSe Coupled Semiconductor Films. Photosensitization of Ti02 Using ElectrochemicallyDeposited CdSe. The coupling of two semiconductor colloids or films is useful in improving the performance of the photoelectrochemical The advantages of using such coupled films are twofold: (i) to extend the photoresponse of large bandgap semiconductors and (ii) to retard the recombination of photogenerated charge carriers by injecting electrons into the lower-lying conduction band of the large bandgap semiconductor such as Ti02 (Ecs = -0.5 V vs NHE). Thechargeseparation in a coupled semiconductor system is indicated by the reaction CdSe (h
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Charge separation in several different coupled semiconductor systems has been investigated both in colloidal suspensions and in coupled particulate films (see, for example, refs 2, 3, and 7). The photocurrent action spectra of OTE/Ti02,OTE/CdSe, and OTE/TiOz/CdSe electrodes are shown in Figure 6. The incident photon to photocurrent efficiency (IPCE) wasdetermined by measuring the photocurrent of these three electrodes at various
Liu and Kamat
10772 The Journal of Physical Chemistry, Vol. 97, No. 41, 1993
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excitation wavelengths and from the expression2q7
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The Ti02 film shows the onset of photocurrent generation a t wavelengths below 350 nm. Since Ti02 is a large bandgap semiconductor ( E g = 3.2 eV), it can only be excited with the UV light. When coupled with CdSe film, its photoresponse extends into the visible (up to 700 nm). Under visible light irradiation, only CdSe is excited, and the photogenerated electrons are transported efficiently through the Ti02 particulate film. The photocurrent action spectra recorded in Figure 6 are further indicative of the fact that the Ti02 particulate films are suitable for accepting electrons from the excited CdSe and transporting them to the OTE surface to generate anodic photocurrent. The IPCE values for OTE/TiOz/CdSe were higher (10-20%) than OTE/CdSe a t excitation wavelengths 400-600 nm. Suppression of charge recombination in coupled semiconductor systems is likely to improve the efficiency of photocurrent generation. Pbotocurrent Stability. The photocurrent-time profiles recorded under front face and back face illumination of OTE/ CdSe with visible light are shown in Figure 7. Both these profiles exhibit a quick decay of the photocurrent with increasing time
Figure 8. Influence of Ti02 particulate film thickness on the stability of the photocurrent generation at an OTE/TiOZ/CdSe electrode (CdSe film thickness was kept constant around 150 A; electrolyte: 0.5 M Fe[(CN)6IC, 0.1 mM Fe[(CN)6]&,pH 12). Theordinatc scale represents the fraction of short-circuit photocurrent sustained after irradiating for a period of 20 s.
of illumination, with photocurrent under back face illumination (trace b) decaying faster than the one recorded under front face illumination (trace a). This poor photocurrent stability is in contrast to the behavior of conventional photoelectrochemical cells that employ a single-crystal CdSe semicond~ctor.~~J* The redox couple, Fe[(CN)6I3-/”, employed in these experiments is expected to stabilize the photocurrent by scavenging photogenerated holes, thereby reducing the probability of direct electronhole recombination (reactions 2 and 3). As recommended in earlier s t ~ d i e s , ’ ~the J ~ concentration of K3[Fe(CN)6] in the electrolyte was kept very low (0.1 mM) for achieving maximum efficiency of photocurrent generation. By increasing the concentration of K3[Fe(CN)6] to 0.1 M,the photocurrent stability can be improved, but the maximum photocurrent decreases dramatically (by a factor of 20). Because of the absence of a space charge layer at the semiconductor/electrolyteinterface, the charge carriers generated in thin semiconductor films move in both directions. In the experiment described in Figure 7, the anodic photocurrent generation is controlled by scavenging of holes by Fe[(CN)#-, which is present at the semiconductor/electrolyteinterface. With increasing irradiation time, the oxidized product of reaction 2, Fe[ (CN)6l3-, accumulated near the electrode makes the hole scavenging process less efficient. This in turn increases the rate of charge recombination within the CdSe film. Therefore, the photocurrent decrease observed during steady- state illumination can be attributed to the increased rate of charge recombination with increasing time of irradiation. When the light is turned off, a reversal of current is seen. The concentration gradient resulting from the buildup of K3[Fe(cN)6l3- causes the electrons to flow in the opposite direction. The rectification of charge flow in thin CdSe film is discussed in our previous comm~nication.~ Photocurrent Stabilization in TiOz/CdSe Coupled Semiconductor Films. We have further evaluated the photocurrent stability of TiOz/CdSe films by keeping the thickness of CdSe film constant and varying the thickness of the Ti02 film. The photocurrent stability of the TiOz/CdSe electrode is determined by monitoring the photocurrent immediately and 20 s after visible light irradiation. The results of these experiments are summarized in Figure 8 and Table I. The open-circuit voltage of these films was independent of film thickness and remained constant around -650 mV (Table I). This value of open-circuit potential is comparable to the values reported in earlier studies.14 This shows that the photoelectrochemical effect is initiated by the excitation of CdSe, and there is no direct contribution from the Ti02 film in initiating the photoelectrochemical effect. The Ti02 film has a significant effect on the short-circuit c u m n t observed immediately after the light irradiation. A decrease in
The Journal of Physical Chemistry, Vol. 97, No. 41, 1993 10773
Photoelectrochemical Behavior of Thin CdSe Films TABLE I: Photocurrent Stability of TiOZ/CdSe Semiconductor Films. thickness of V,, &i =Oh i,,(t=20 s)/ Ti02 film (rm)
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-0.670 -0.600 -0.430 -0.395 -0.265 -0.145 -0.115
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17.2 29.1 29.1 35.4 50.9 56.6 60.0
a The thickness of CdSe film coated on Ti02 film was maintained constant around 150A. The photoelectrochemical cell consisted of OTE/ TiOZ/CdSe working electrode and Pt counter electrode immersed in an 0.1 mM Fe[(CN)#- (pH 12) electrolyte. aqueous 0.5 M Fe[(CN)6]’, V, represents open-circuit photovoltage, and isc(t=O) and iSc(t=20 s ) represent short-circuit photocurrents measured immediately and 20 s after the visible light irradiation of OTE/TiOt/CdSe electrode.
short-circuit current was observed with increasing thickness of the Ti02 film. This decrease indicates that the flow of electrons through the Ti02 particles is hindered in thicker Ti02 films. However, coupling with the Ti02 particulate film has a beneficial effect in improving the stability of the photocurrent (Figure 8). In the absence of Ti02 particulate film, only 13% of the photocurrent is sustained after 20-sirradiation, but when coupled with a Ti02 film nearly 60% of the initial short-circuit photocurrent is sustained. The rectification properties of the coupled semiconductor films that regulate the flow of electrons improve the stability of the photocurrent in the thin semiconductor films. By employing the laser flash photolysis technique, we have shown that charge separation in CdS colloids can be greatly improved by coupling with Ti0Z2I and ZnOZ2colloids. We expect a similar mechanism to be operative in thin coupled semiconductor films. In order to employ thin semiconductor films in photoelectrochemical cells, it is necessary to avoid the effects of charge recombination and rectify the flow of charge carriers. We have presented here an example of coupled semiconductor films for the purpose of attaining the photocurrent stability. Conclusions Thin semiconductor films synthesized by electrochemical and chemical deposition methods are quite attractive for designing systems for electrooptics and photoelectrochemical conversion of solar enery. The photoelectrochemicalinvestigationof thin CdSe films described here highlights some of the problems encountered in utilizing thin semiconductor films as electrode materials. The mechanism of charge separation in these thin films is governed by the differing rates of electron and hole transfer at the semiconductor/electrolyteinterface. Increased charge recombination is a major problem in attaining good photocurrent stability. Even redox couples such as Fe[(CN)6I3-/” are not capable of stabilizing the photocurrent at thin CdSe film electrodes. By coupling CdSe thin film with a Ti02 particulate film, it is possible to inject photogenerated electrons into the conduction band of Ti02 and thus retard the charge recombination
within the CdSe film. Coupling with thin Ti02 particulate film improves the charge separation and thus has a beneficial role in improving the photocurrent stability. Microwave absorption studies are currently being carried out to confirm the beneficial role of Ti02 layer in improving the charge separation in Ti02/ CdSe coupled semiconductor films. Acknowledgment. It is a pleasure to thank Dr. Surat Hotchandani for helpful discussions and Ms. Z. H. Lin for her help in measuring film thickness. The work described herein was supported by the Office of the Basic Energy Sciences of the US. Department of Energy. This is Contribution No. 3603 from the Notre Dame Radiation Laboratory. References and Notes (1) (a) Rajeshwar, K. Adu. Mater. 1992, 4, 23. (b) DeMattei, R. C.; Feigelson, R. S. In Electrochemistry of Semiconductors and Electronics; McHardy, J., Ludwig, F., Eds.; Noyes Publications: Park Ridge, NJ, 1992, pp 1-52. (2) (a) Hotchandani, S.;Kamat, P. V. Chem. Phys. Lert. 1992,191,320. (b) Hotchandani, S.;Kamat, P. V. J . Electrochem. SOC.1992,139, 1630. (c) Hotchandani, S.; Kamat, P. V. J. Phys. Chem. 1992, 96, 6834. (d) Hotchandani, S.;Bedja, I.; Kamat, P. V. Langmuir, in press. (e) Vinodgopal, K.; Hotchandani, S.;Kamat, P. V. J. Phys. Chem. 1993, 97,9040. (3) Liu, D.; Kamat, P. V. J . Electroanal. Chem. 1993, 347, 451. (4) O’Regan, B.; Moser, J.; Anderson, M.; Gratzel, M. J . Phys. Chem. 1990, 94, 8720. ( 5 ) (a) ORegan, B.; Moser, J.; Gratzel, M., Fitzmaurice, D. Chem. Phys. Lett. 1991, 183, 89. (b) ORegan, B.; Moser, J.; Gritzel, M.; Fitzmaurice, D. J. Phys. Chem. 1991,95, 10525. (c) Rothenberger, G.; Fitzmaurice, D.; Gritzel, M. J . Phys. Chem. 1992, 96, 5983. (6) ORegan, B.;Gratzel, M., Nature 1991, 353, 737. (7) (a) Vogel, R.; Pohl, K.; Weller, H. Chem. Phys. Lett. 1990,174,241. (b) Ennaoui, A.; Fiechter, S.;Tributsch, H.; Giersig, M.; Vogel, R.; Weller, H. J . Electrochem. SOC.1992, 139, 2514. (8) Sakahora, S.;Tickanen, L. D.; Anderson, M. A. J . Phys. Chem. 1992,96, 11086. (9) Hodes, G.; Albu-Yaron, A.; Decker, F.; Motisuke, P. Phys. Reu. B 1987.36, 4215. (IO) (a) Hodes, G.; Albu-Yaron, A. Proc. Electrochem. Soc. 1988,88-14, 298. (b) Golan, Y.; Margulis, L.; Rubinstein, I.; Hodes, G. Lungmuir 1992, 8, 749. (1 1) Hodes, G.; Howell, I. D. J.; Peter, L. M. J. Electrochem. SOC.1992, 139, 3136. (12) (a) Kressin, A. M.; Doan, V. V.; Klein, J. D.; Sailor, M. J. Chem. Mater. 1991, 3, 1015. (b) Cerdeira, F.; Torrani, I.; Motisuke, P.; Lemos, F. Decker, F. Appl. Phys. A 1988.46, 107. (13) (a) Hodes, G. Isr. J . Chem. 1993, 33, 95. (14) (a) Chandra, S.;Pandey, R. K. Phys. Status Solid A 1980,59,787. (b) Liu, C. J.; Olsen, J.; Saunders, D. R.; Wang, J. H. J . Electrochem. Soc. 1981,128,1224. (c) Xiao, X.-R.; Tien, H. T. J . Electrochem. Soc. 1983,130, 55. (d) Szabo, J. P.; Cocivera, M. J . Electrochem. Soc. 1986,133,1247. (e) Gutibrrez, M. T.; Salvador, P. Sol. Energy Mater. 1987, 15, 99. (15) (a) Wei, C.; Mishra, K. K.; Rajeshwar, K. Materials 1992,477. (b) Bicelli, L. P. J . Phys. Chem. 1992,96,9995. (c) Colyer, C. L.; Cocivera, M. J. Electrochem. SOC.1992, 139, 406. (16) Baranski, A. S.;Fawcett, W. R.; Gatner, K.; McDonald, A. C. J . Electrochem. Soc. 1983, 130, 579. (17) Licht, S.J. Phys. Chem. 1986, 90, 1096. (18) (a) Guiterrez, M. T.; Ortega, J. J . Electrochem. SOC.1989, 136, 2316. (b)Arent,D.J.;Rubin,H.D.;Chen,Y.;Bocarsly,A.B.J.Electrochem. SOC.1992, 139, 2705. (19) Memming, R. Top. Curr. Chem. 1988, 143, 81. (20) Nozik, A. J. Appl. Phys. Lett. 1977, 30, 567. (21) Gopidas, K. R.; Bohorquez, M.; Kamat, P. V. J . Phys. Chem. 1990, 94, 6435. (22) Hotchandani, S.; Kamat, P. J . Phys. Chem. 1992, 96, 6834.
