3084
J. Phys. Chem. 1996, 100, 3084-3088
Electron Transport Properties in Dye-Sensitized Nanoporous-Nanocrystalline TiO2 Films Henrik Lindstro1 m, Ha˚ kan Rensmo, Sven So1 dergren, Anita Solbrand, and Sten-Eric Lindquist* Department of Physical Chemistry, UniVersity of Uppsala, Box 532, S-75121, Uppsala, Sweden ReceiVed: May 11, 1995; In Final Form: NoVember 4, 1995X
Spectral response measurements have been performed on dye-sensitized nanoporous-nanocrystalline TiO2 photoelectrodes. The effects of film thickness, electron scavengers in the electrolyte, and surface treatment of the nanocrystalline film were studied by means of action spectra for front- and back-side illumination. Our results show that electron acceptors such as dioxygen and iodine strongly decrease the IPCE. Surface treatment of the electrode with pyridine induces a substantial increase of the photocurrent yields. The observations are discussed in terms of kinetics at the semiconductor-electrolyte interface. IPCE values for sandwich cells were generally much higher than those obtained from three-electrode measurements.
Introduction Since the breakthrough of wet photoelectrochemical (PEC) solar cells by Gra¨tzel and co-workers (e.g., ref 1-3), much attention has been paid to nanoporous-nanocrystalline semiconductor electrodes. Such electrodes are in this paper named nanostructured electrodes. The high internal surface area makes these film electrodes interesting for a wide field of applications (ref 4 and references therein). Many of these require charge transfer across the porous film; hence, studies of electron transport properties, i.e., carrier ranges and mechanisms of carrier losses, are of high importance. The unique property of nanostructured electrode materials is that the electrolyte can penetrate the porous electrode from the surface down to the supporting conductor and contact each grain in the film. At measured doping levels though, the particles constituting the film are too small to form an efficient depletion layer locally within each individual semiconductor particle.2,5 It has been proposed6 that nanostructured PEC cells operate due to charge separation governed by kinetics and dynamics of photoelectrochemical processes at the semiconductor-electrolyte interface (SEI), rather than by a built-in space charge layer normally occurring in photovoltaic or photoelectrochemical cells. Models to describe the photoresponse spectrum and the current-voltage characteristics of nanostructured PEC cells have been derived,7 taking the above considerations into account. In the dye-sensitized cells studied in this paper, the functions of light absorption and charge transport are separated. The positive hole is created directly on the dye molecule at the SEI, and the electron injected by the sensitizer into the TiO2 conduction band travels across the nanostructured film to the conducting glass support working as a current collector. Since the dye-sensitized nanostructured TiO2 electrode acts as a majority carrier device, electron-hole recombination in the “bulk” of the semiconductor is excluded as a path for losing electrons injected into the TiO2 grains. The loss of the electrons must take place via the SEI. Any acceptor in the electrolyte collecting electrons from the semiconductor will decrease the efficiency of the electrode. Oxygen is a well-known electron acceptor and has been used in other studies of nanostructured systems.8-10 It has been shown1,11 that the performance of the colloidal film as a dye-sensitized photoanode is improved by treating the TiO2 surface with organic adsorbates. For the purpose of studying loss mechanisms in dye-sensitized TiO2 electrodes, O2 and pyridine have been adopted in the present investigation. * To whom correspondence should be addressed. X Abstract published in AdVance ACS Abstracts, January 15, 1996.
0022-3654/96/20100-3084$12.00/0
We have previously5,10,12 presented methods to gain information about fundamental properties of semiconducting thin film electrodes from front-side and back-side illumination of TiO2 electrodes by analysis of action spectra. Hagfeldt et al.5 studied the charge separation in sintered colloidal TiO2 film electrodes by action spectra in the UV region. They concluded that the most efficient charge separation takes place close to the back contact. They also defined a zone in which electrons were effectively collected. The average range of the photogenerated carriers was estimated to be 0.5 µm in aerobic electrolyte solutions. In this paper, we compare previous results from photoelectrochemical measurements on naked nanostructured TiO2 film electrodes in the UV region by extending our investigation to dye-sensitized nanostructured TiO2 electrodes. By illuminating electrodes of different thicknesses with monochromatic light from either side, it is possible to induce charge separation in the nanostructured film electrode at a wide range of distances from the back contact. This gives us the opportunity to get hold of the charge propagation through the nanostructured film in a dye-sensitized TiO2 electrode. The object of the present work is not to optimize the particular studied dye-sensitized nanostructured system in any respect, but to illustrate and highlight the basic features and functioning of nanostructured electrode materials in general. Experimental Section Materials and Apparatus. The sample preparation largely followed the method described by Nazeeruddin et al.1 A paste of 16 g of TiO2 (Degussa P25), 16 mL of MQ water, and 0.6 mL of acetylacetone to prevent reaggregation was ground for 18 h in an Al2O3 ball mill. The paste was diluted with a 1% (w/w) Triton-X water solution to 36% (w/w) with respect to TiO2. Because of the unequal quality between different batches, all electrodes used in this paper were prepared from the same batch during 1 day. The viscous suspension was spread out onto transparent conducting glass sheets (Kappa 16 Ω/square). Using Scotch tape as a frame and spacer, a thin layer was formed by raking off the excess of the colloidal solution with a glass rod. After removing the tape, the samples were sintered at 450 °C in air for 30 min to form a nanostructured film electrode. Thicker films were obtained by repeating this procedure. The thickness of each film was determined using a Tencor Alpha Step profilometer. The porosity was 50 ( 2%, independent of film thickness, and calculated from the film weight (Mettler precision scale, © 1996 American Chemical Society
Electron Transport Properties in TiO2 Films accuracy: (1 µg) the bulk density of TiO2, the electrode area, and the film thickness. The film area was determined by weighing the paper of a magnified projection of the electrode. For the magnification, an overhead projector was employed using a transparent ruler as a scale factor. The TiO2 films consisted of 70% anatase and 30% rutile, and the average particle size was 22 nm and 35 nm, respectively, as determined by X-ray diffraction (Siemens D-5000 powder diffractometer). The line profile half-width was used with Scherrers formula, which gives a rough estimate of the crystallite size. Coating of the TiO2 surface with dye was carried out by soaking the film over several days in a 0.5 mM solution of ruthenium bipyridyl complex, cis-bis(thiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylate)ruthenium(II) in ethanol (cf. ref 1). The electrode was dipped into the dye solution while it was still hot; i.e., its temperature was ca. 100 °C. Some electrodes were also treated in a pyridine bath for at least 24 h immediately after the dye sensitation. Excess pyridine in the cavities of the film was then removed by washing the electrode in an ethanol bath. The electrodes were contacted with copper wires via a silver conductive paint. To prevent dark currents from the electrode, free areas of the back contact were isolated with an epoxy resin. Dyed film electrodes were stored in dry excicators before use. Since the films were highly light scattering, it was not possible to measure the absorption of the dye-sensitized film directly. Therefore, absorption spectra of the electrodes (dye adsorbed on the TiO2 film in propylene carbonate) were recorded with an integrating sphere using a Carry 2000 spectrophotometer. Transmission and reflection were measured for a sample consisting of a sensitized TiO2 electrode soaked with a few drops of propylene carbonate and a quartz plate on top of the nanostructured film. To obtain the absorption of the electrode, the effect from transmission and reflection of the glass substrate and the quartz plate was taken into account. The dye-sensitized electrode was mounted as a working electrode in a conventional three-electrode system enclosed in a Teflon vessel with a quartz window.10 The interior of the vessel was furnished with black Teflon to avoid reflections from the incoming light. Reagent-grade propylene carbonate and potassium iodide were used for preparation of the 0.1 M KI electrolyte solutions. The electrolyte solution in the vessel was either purged with nitrogen or saturated with oxygen. To maintain a constant concentration of the components in the electrolyte at the electrode-electrolyte interface, the solution was heavily stirred during all experiments. The counter electrode was a platinum foil enclosed in a glass tube with a glass frit and the reference electrode a Ag/AgCl in contact with saturated LiCl in ethanol. All potentials are reported vs Ag/ AgCl in ethanol. The light source was a 450-W xenon lamp (Osram XBO). The beam passed through an 80-mm water filter and a cut-off filter (GG370) to avoid light with wavelengths shorter than 370 nm into a high intensity monochromator (Schoeffel GM 252), FWHM ) 10 nm. The power of the defocused beam was measured in the position of the working electrode using an optical power meter (Photodyne Model 44XL) with a silicon photodiode Model 400 AS radiometric sensor head. For the electrochemical measurements, a PAR (EG&G, Princeton Applied Research, Model 173) potentiostat was used. Experimental Procedure. Photon-to-current conversion efficiencies were measured at 420-720-nm monochromatic light. At the beginning of each recording session, the electrode was biased to 0.1 V for a period of about 3-5 h during which time the photocurrent attained a steady-state value. Steady-
J. Phys. Chem., Vol. 100, No. 8, 1996 3085
Figure 1. Absorbance of the desorbed dye in a 1 mM KOH water solution vs film weight for four electrodes of thicknesses 2.6, 7.7, 24, and 38 µm. The absorbance is corrected for the film area (cm2) and the volume (mL) of the decoloration solution.
Figure 2. Absorption coefficient vs wavelength for a dye-sensitized electrode in propylene carbonate.
state values of photocurrents were then recorded for a period of at least 5 min whereupon the wavelength of the beam was changed. Dark currents were measured before and after the recording of the action spectra and ranged from 0 to 10 nA/ cm-2. The photocurrents were typically in the µA/cm-2 range. Results and Discussion Dye Sensitization. The amount of adsorbed dye was determined by desorbing the dye from the films (with thicknesses 2.6, 7.7, 24, and 38 µm) into a 1 mM KOH water solution and measuring the absorption spectra. Figure 1 shows that the absorbance (540 nm) of the dye solution is linear with film weight, indicating that the films are homogeneously dyed at all thicknesses. The small variance of the porosity together with homogeneous dye sensitation shows that the TiO2 films are smooth and homogeneous. The average number of sensitizers per colloid (using a colloid diameter of 22 nm) was estimated to be 200. Neglecting the area loss due to necking between particles and assuming that the adsorbed dye molecule covers 1 nm2, this number corresponds to a total surface coverage of approximately 50%. Absorption Spectra. An absorption spectrum of surfaceadsorbed dye on a nanostructured TiO2 film soaked in propylene carbonate is displayed in Figure 2. Beginning at 400 nm, the absorption curve declines to a minimum at approximately 480 nm. The maximum at 520 nm is followed by an absorption tail extending into the red region. For a 5-µm-thick electrode, 13% of the incident light is transmitted through the whole film at 520 nm and 62% at 650 nm. For a 20-µm-thick electrode, practically no light is transmitted at 520 nm, but at 650 nm, still 16% of the light passes through the film.
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Figure 3. Action spectra at SE and EE illumination of a 5.4-µm-thick electrode.
Effect of Bias and Intensity on the Photocurrent. The photocurrent at 520 and 680 nm for front- and back-side illumination was independent of the applied potential between 0 and 0.3 V (film thickness: 30 µm). Higher biases yielded small anodic dark currents but did not raise the photoinduced currents. The photocurrent at front- and back-side illumination was linear with light intensity at 520 and 680 nm for a 30-µmthick electrode. This assures linearity of the photocurrent vs light intensity for all recorded action spectra at all wavelengths. Action Spectra for SE and EE Illumination. The IPCE was registered for both front-side and back-side illumination. We define front-side illumination as the case where light first meets the projected electrode/electrolyte (EE) interface and back-side illumination as the case where the light first hits the supporting substrate/electrode (SE) interface. The IPCE is defined as the number of collected electrons divided by the number of incident photons that hit the TiO2 electrode area exposed to the electrolyte and is calculated by
IPCE )
1241iph[µA] P[µW]λ[nm]
where iph and P are the photocurrent and the effect of the incident radiation per unit area, respectively. λ is the wavelength of the monochromatic light. No corrections were made for the absorption and reflection in the glass substrate. Three-Electrode Measurements for Front- and Back-Side Illumination. The low IPCE values obtained in this study, compared to literature values, are associated with the geometry of the experimental setup and will be discussed below. Comparing the SE action spectrum (Figure 3) of a relatively thin electrode (5.4 µm) with the absorption spectrum of the surface-adsorbed dye (Figure 2), the action spectrum resembles the shape of the absorption spectrum. If the same electrode is illuminated from the other direction (EE illumination), we note that the SE and EE photoresponse curves have almost the same shape (though there is a small red shift in the EE spectrum). However, higher IPCE values are obtained for SE illumination than for EE illumination. Knowing that the generation of current in the nanostructured film electrode is most efficient close to the back contact,5,6,13,14 the similarities in the absorption spectra and the SE action spectra of the electrodes can be explained by considering where light is absorbed in the electrode: Upon SE illumination, photons with wavelengths at high absorption coefficients are effectively absorbed close to the back contact, and as a consequence, the electrons injected into the semiconductor will have a higher probability of reaching the back contact, compared to photoelectrons created in the outer layers of the nanostruc-
Lindstro¨m et al.
Figure 4. Action spectra at SE and EE illumination of a 12.6-µmthick electrode.
Figure 5. Action spectra at EE illumination of electrodes with different thicknesses.
tured film. This explains why the position of the absorption maximum of the electrode and the photoresponse maximum of action spectra coincide. At wavelengths with low absorption coefficients, the photoresponse decreases since a substantial amount of light is transmitted through the whole electrode. In addition, since the extinction coefficient of the dye is small in the red region, charge carriers are generated more uniformly over the whole film thickness. Thus, a large fraction of the photoinjected electrons have to travel over a long distance within the film before reaching the current collector. Due to indirect electron recombination processes with, e.g., I3-, at the SEI, a fraction of the charge carriers generated at a distance from the conducting glass are lost, thus decreasing the IPCE. Upon illumination of the electrode from the SE direction at a certain wavelength, light will on average be absorbed (creating charge carriers) closer to the back contact than for the case of EE illumination. The general lowering of the IPCE for EE ilumination compared to SE illumination is therefore explained by recombination losses during transport of electrons from the outer part of the nanostructured film to the back contact. For a thicker film (Figure 4), the SE action spectrum again mimics the absorption spectrum of the dye, the maximum peak having the same spectral position independent of film thickness, but the IPCE for wavelengths with low absorption coefficients is increased for thicker films because more light is absorbed in the electrode. It can also be observed that the EE spectrum is less intense and has a red-shifted maximum. The increased red shift in EE action spectra for the thicker electrode is explained by the fact that longer wavelengths penetrate deeper into the nanostructured film and the light is absorbed closer to the back contact. If this interpretation is correct, we would expect an increased red shift of the EE maximum with increasing film thickness. This is also observed experimentally (Figure 5).
Electron Transport Properties in TiO2 Films
Figure 6. Effect of N2 and O2 in the electrolyte on the IPCE efficiency for a 20-µm-thick electrode (logarithmic scale).
Figure 7. Action spectra of a 20-µm-thick pyridine-treated electrode in N2-purged and O2-saturated electrolyte (logarithmic scale).
Effects of Dissolved Dioxygen. Figure 6 illustrates the effect of N2 and O2 in the electrolyte (film thickness: 20.4 µm). A comparison of the N2 and O2 photoresponse curves shows that O2 drastically lowers the IPCE at all wavelengths independent of the direction of the incident light. The SE IPCE (N2 purging) at 500 nm is 13% compared to 1.5% under O2-saturation conditions. Furthermore, O2 saturation of the electrolyte induces stronger red shifts in the EE action spectra than N2 purging does; the EE maximum for N2 purging lies at 595 nm compared to 620 nm for O2 saturation. In addition, the SE IPCE for O2 saturation decreases relatively more at longer wavelengths. These observations can be rationalized by considering the fact that O2 is an efficient electron scavenger and collects electrons from the conduction band or surface states at the semiconductor particles. The process promotes indirect recombination via redox processes in the electrolyte which is observed as a decrease in photocurrent yields. Similar effects were previously observed on bare TiO2 electrodes.10 However, it cannot be excluded that dissolved O2 also acts as a quencher of the excited dye molecules, thereby decreasing the quantum yield for electron injection. Because long wavelengths penetrate deeper into the film upon SE illumination, the relative differences in the SE photoresponse between N2 and O2 saturation typically increase at wavelengths where the film absorption is low, e.g., the SE IPCE ratio at 500-nm light (absorption maximum) for N2 and O2 saturation is 8.5, but at 650 nm, the ratio is 23. Effects of Pyridine Treatment. The IPCE for a 20.4-µmthick pyridine-treated electrode in N2-purged and O2-saturated electrolyte is typically more intense than for an untreated electrode of the same film thickness (Figure 7). Similar effects have been observed by other workers.1,11 The effect can be seen, for example, in the presence of N2 where the SE and EE
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Figure 8. Effect of iodine addition to the electrolyte. The curve shows the ratio of the IPCE for 0.1 M KI in propylene carbonate and after addition of 10 mM I2 to the bulk electrolyte. The film thickness was 11 µm.
IPCE values at the absorption maximum are increased 2 and 5 times, respectively. Pyridine treatment also blue shifts the EE action spectra when N2 purging. Since pyridine is a well-known adsorbate to TiO2, cf. ref 15, it is expected to be coadsorbed at vacant sites on the TiO2 surface between the preadsorbed dye molecules. Higher IPCE values for SE and EE illumination can therefore be explained in terms of pyridine blocking the TiO2 surface (sterically or by deactivating recombination sites at the TiO2), thus preventing the encounter of conduction band electrons with electrolyte species at the SEI. This also explains the higher photovoltages reported in ref 1. The blue shift in the EE spectrum (N2 purging) for a pyridinetreated electrode indicates that electrons excited far away from the conducting support are being more efficiently collected, compared with nontreated electrodes, cf. Figure 6. This is reasonable since the probability of recombination increases with the distance to the back contact. Since surface adsorbed pyridine, even at O2 saturation, strongly improves the IPCE, we propose that the main loss of electrons to O2 occurs via the TiO2 surface. Effect of Iodine Addition to the Electrolyte. Figure 8 shows the ratio of the SE spectral photoresponse for an electrode (film thickness: 11 µm) before and after addition of 10 mM I2 to the bulk electrolyte solution. To avoid light absorption of the electrolyte, a special cell compartment was designed. The conducting glass served both as substrate for the film electrode and entrance window for the reaction vessel.10 The electrolyte consisted of 0.1 M KI in propylene carbonate, and the bias was 0.1 V vs Ag/AgCl in ethanol. It can be seen that I2 lowers the photoresponse at all wavelengths. The effect increases at wavelengths where the film absorption is low. The photoresponse behavior upon I2 addition is very similar to that of O2 saturation. The explanation for the decrease in IPCE is that the introduction of I2 yields species, e.g., I3-, that capture photogenerated electrons. Comparison of Two- and Three-Electrode Measurements. To assure that the electrodes operated as those used in solar cell prototypes, SE action spectra were registered for twoelectrode arrangements (sandwich-type cells). The counter electrode consisted of an electrochemically platinized conducting glass, and the electrolyte was 0.1 M KI and 10 mM I2 in propylene carbonate. Figure 9 shows the photoresponse for a 11-µm-thick electrode. The action spectrum is similar to those obtained by other workers.1 Figure 4 displays the SE action spectrum of an electrode (12.6 µm) in a three-electrode compartment using convection in the
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Lindstro¨m et al. Conclusions
Figure 9. Action spectrum of a sandwich electrode. The film thickness was 11 µm.
bulk electrolyte solution. Strikingly, the IPCE values are generally much lower than those of sandwich-type cells. The maximum value at 500 nm is 8% compared to values up to 50% for the two-electrode setup. The important observation is that the regenerative two-electrode device may exhibit higher efficiencies than the conventional three-electrode system (at the same donor concentration and at higher acceptor concentrations). Only a small part (around 5%) of the difference in IPCE can be explained by absorption of the electrolyte and reflection losses of the quartz window and the electrolyte/substrate interface. In addition, reflection by the Pt counter electrode is not significant since the amount of transmitted light through the film is negligible in the wavelength range of interest (see Absorption Spectra subsection above). Thus, optical losses do not explain the low IPCE values when measuring in the threeelectrode mode. However, the different results obtained from two- and three-electrode measurements may be explained by considering the geometry of these two systems: e.g., in the regenerative three-electrode system, for each electron collected at the back contact, a new donor must be transported to the film by diffusion from the bulk solution to keep the donor concentration outside the film surface constant. On the other hand, in the sandwich-type cell, the Pt counter electrode (on top of the film) regenerates a new donor for each electron collected at the back contact. Due to regeneration of I- from I3- at the Pt counter electrode, the donor and acceptor concentrations in the two-electrode geometry will certainly be higher and lower, respectively, compared to the three-electrode geometry. Thus, the recombination losses of photogenerated electrons to I3- will be smaller for the two-electrode system. As a consequence, the IPCE values will be higher when measuring in the two-electrode mode. The above measurements indicated that this particular dye-sensitized regenerative twoelectrode device is very insensitive to electron acceptors. However, the effects from the photocurrent losses to acceptors in solution are clearly seen in the three-electrode measurement.
Action spectra analysis for front-side (EE) and back-side (SE) illumination can be used for qualitative analysis to get hold of the propagation of electrons through dye-sensitized nanostructured semiconducting materials. It can for this reason be used as a tool to characterize dye-sensitized nanostructured solar cell devices. Dioxygen and iodine have a severe effect on the IPCE. Most probably they work as indirect mediators for current losses at the TiO2-electrolyte interface. Surface treatment of the electrode with pyridine induces a substantial increase of the photocurrent yields blocking the TiO2 surface. There is a difference in geometry between two- and three-electrode measurements. Part of the higher IPCE values obtained from sandwich cell measurements can probably be explained by higher and lower concentrations of donors and acceptors, respectively, in the cavities of the film electrode when measuring in the two-electrode mode. Acknowledgment. We thank Professor Gra¨tzel and coworkers for supplying us with the sensitizer. This work has been supported by the Swedish Research Council for Engineering Sciences (TFR), the commission of the European Community Joule II program, the Swedish Board for Technical Development (NUTEC), and Go¨ran Gustavssons Foundation. References and Notes (1) Nazeeruddin, M. K.; Kay, A.; Rodicio, I.; Humphry, B. R.; Mueller, E.; Liska, P.; Vlachopoulos, N.; Graetzel, M. J. Am. Chem. Soc. 1993, 115, 6382-90. (2) O’Regan, B.; Moser, J.; Anderson, M.; Graetzel, M. J. Phys. Chem. 1990, 94, 8720-6. (3) O’Regan, B.; Graetzel, M. Nature 1991, 353, 737-40. (4) Hagfeldt, A.; Graetzel, M. Chem. ReV. 1995, 95, 49-68. (5) Hagfeldt, A.; Bjo¨rksten, U.; Lindquist, S. E. Sol. Energy Mater. Sol. Cells 1992, 27, 293-304. (6) Hodes, G.; Howell, I. D. J.; Peter, L. M. J. Electrochem. Soc. 1992, 139, 3136-40. (7) So¨dergren, S.; Hagfeldt, A.; Olsson, J.; Lindquist, S. E. J. Phys. Chem. 1994, 98, 5552-6. (8) Vinodgopal, K.; Hotchandani, S.; Kamat, P. V. J. Phys. Chem. 1993, 97, 9040-4. (9) Hagfeldt, A.; Lindstro¨m, H.; So¨dergren, S.; Lindquist, S.-E. J. Electroanal. Chem. 1995, 381, 39-46. (10) Rensmo, H.; Lindstro¨m, H.; So¨dergren, S.; Willstedt, A.-K.; Solbrand, A.; Hagfeldt, A.; Lindquist, S.-E. J. Electrochem. Soc., submitted. (11) Kay, A.; Graetzel, M. J. Phys. Chem. 1993, 97, 6272-7. (12) Lindquist, S. E.; Finnstroem, B.; Tegner, L. J. Electrochem. Soc. 1983, 130, 351-8. (13) Alonso-Vante, N.; Nierengarten, J. F.; Sauvage, J. P. J. Chem. Soc., Dalton Trans. 1994, 11, 1649-54. (14) Bedja, I.; Hotchandani, S.; Kamat, P. V. J. Phys. Chem. 1994, 98, 4133-40. (15) Primet, M.; Pichat, P.; Mathieu, M.-V. J. Phys. Chem. 1971, 75, 1221.
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