The high porosity and strong surface bonding property of the nanostructured semiconductor films facilitate surface modification with organic dyes and organometallic complexes such as bis(2,2'-bipyridine)(2,2'-bipyridine-4,4'dicarboxylic acid)ruthenium(II). The photocurrent response evaluated in terms of photon-to-photocurrent efficiency (IPCE) of a SnO2 nanocrystalline film modified with Ru(II) complex is shown in Figure 2.(23)

The IPCE maximum of the surface-modified SnO2 film closely matches the absorption maximum of the sensitizer. The SnO2 film, which is sensitive only to UV excitation prior to surface modification, responds to visible light (wavelengths greater than 400 nm) as a result of surface modification. This shows that a photosensitization mechanism is operative in extending the photocurrent response of the SnO2 film. When the electrode is illuminated with visible light the sensitizer molecules absorb light and inject electrons into the SnO2 particles. These electrons are then collected at the conducting glass surface to generate anodic photocurrent. The redox couple (e.g., I3-/I-) present in the electrolyte quickly regenerates the sensitizer. By choosing an appropriate sensitizer it is possible to tune the photoresponse of these nanostructured semiconductor films. For example, sensitizing dyes such as chlorophyll a and b,(25,26) squaraines(27) and oxazines(28) can extend the photoresponse of SnO2 films to the red-infrared region. The maximum IPCE in the example discussed in figure 2 (around 50%) shows that nearly half of the injected charge from the excited sensitizer is lost as a result of recombination with the oxidized sensitizer. By optimizing the operating conditions it is possible to improve the performance of IPCE of the sensitizer-based cells. Ru-complex modified TiO2 nanostructured films exhibit IPCE of nearly 90% under optimized light-harvesting conditions.(21) Both experimental and theoretical evaluations of these cells have been carried out and the efficiency limiting factors have been identified.(29,30)

Varying degree of electron accumulation within the semiconductor particles alters the energetics of the quasi-Fermi level and creates a potential gradient within the thin film. Formation of such a potential gradient provides the necessary driving force for the electron transport to the collecting surface of OTE. Although it is difficult to establish the exact nature of this overall potential gradient, the experimental results indicate it to be qualitatively similar to that of the Schottky barrier observed in a single crystal semiconductor system.(23,25,31) Since this potential gradient is not an ideal type of Schottky barrier, significant loss of electrons is encountered during the transit because of recombination at the grain boundaries. This is evident from the relatively high reverse saturation photocurrents observed in these examples.

Three important processes control the efficiency of incident photon-to-charge carrier generation. They include (i) the primary photochemical event of charge injection from excited sensitizer into semiconductor nanocrystallites, (ii) charge transport across the nanocrystalline film and (iii) regeneration of sensitizer with a suitable redox couple. The mechanism and kinetic details of the photoinduced charge injection process is presented in the following discussion.

Interaction between Excited State and Semiconductor Nanoclusters

Extensive efforts have been made in various laboratories to modify the semiconductor surface with various sensitizing dyes. These dyes are directly adsorbed on the semiconductor surface either by electrostatic interaction or by charge transfer interaction. Functional groups such as carboxylate and phosphonates are useful for binding the dyes to oxide surfaces. The interaction between the semiconductor surface and the dye molecules often results in spectral changes which include displacement or broadening of the absorption bands, appearance of new charge transfer bands, and changes in the extinction coefficient of absorption. Ruthenium(II) polypyridyl complexes have so far been proved to be most efficient in sensitizing nanocrystalline semiconductor films. Much interest has recently been directed towards the synthesis and sensitizing properties of Ru(bpy)2(dcbpy)2+ and related ruthenium complexes because of their strong visible absorption and resistance to ligand substitution, as well as their ability to interact with semiconductor surfaces.

Kinetics of Charge Injection Process

Our earlier surface photochemical studies with organic dyes have indicated that the intrinsic semiconductor property of the oxide support plays an important role in controlling the photochemistry of adsorbed molecules.(13,14) Semiconducting oxides such as TiO2 directly participate in the surface photochemical reaction while nonreactive oxides such as SiO2 or Al2O3 do not influence the excited behavior of adsorbed substrate. The sensitizing dyes bound to the semiconductor surface exhibit relatively low emission yields. For example, comparison of the relative quantum yields of Ru(bpy)2(dcbpy)2+* on these oxide surfaces indicated that the fluorescence yields are significantly lower (~5%) on a semiconductor (TiO2, Eg~3 eV) surface than on an insulator surface (alumina, Eg~9 eV). In fact, luminescence decay of the sensitizer on the semiconductor surface is a convenient method to probe the kinetics of charge injection process. Other spectroscopic techniques such as emission, resonance Raman, diffuse reflectance, microwave absorption, and nanosecond and picosecond laser flash photolysis are also useful to probe the interfacial charge transfer between an excited sensitizer and the semiconductor particle.

Figure 3 illustrates the principle of the charge injection from excited singlet (S1) and triplet (T1) states into a semiconductor nanocrystallite.

The excited state lifetime of the sensitizer adsorbed on a semiconductor surface is significantly lower than that adsorbed on an insulator surface such as alumina. The kinetic evaluation of the multiexponential luminescence decay of excited Ru(II) polypyridyl complex adsorbed on SnO2 suggests that multiple injection/adsorption sites exist on the surface of a semiconductor nanocrystallite. The rate constants for heterogeneous electron transfer between excited Ru-complex and semiconductor crystallites such as SnO2, TiO2 and ZnO are in the range of 107-109 s-1.(23,32,33) Independent microwave absorption and luminescence measurements have been carried out to monitor the charge injection from excited Ru(bpy)2(dcbpy)2+ into SnO2, ZnO and TiO2 nanocrystallites.34 The growth of microwave absorption was delayed from the laser pulse by a process showing a similar rate constant to the fast decay portion of the luminescence. The appearance of microwave conductivity at rates corresponding with the luminescence directly confirms the fast component of the heterogeneous electron rate constant to be in the range of 1-3X10^8 s-1.

The charge injection from singlet excited sensitizer into the conduction band of a large bandgap semiconductor is usually considered to be an ultrafast process occurring in the picosecond time domain. Charge injection process in the case of organic dyes such as anthracene carboxylate,(35) squaraines(36) and cresyl violet(37,38) has been shown to occur within 20ps. Similar fast electron transfer has also been noted for Ru(H2O)2[2-] on TiO2 surface at very low coverage.(37,38) On the contrary, relatively smaller charge injection rate constants (10^8-10^9 s-1) have been reported by several research groups investigating the photophysical behavior of ruthenium complexes adsorbed on various semiconductor surfaces.(33,34,39) Similarly, the charge injection from the triplet excited dyes into TiO2 and ZnO colloids has also been shown to occur on a slower time scale.(40,41) Our kinetic measurements suggest that the electron transfer from the excited Ru(bpy)2(dcbpy)[2+] occurs with a relatively slower rate than the singlet excited organic dyes but is comparable to the triplet excited dyes. It should be noted that the excited state of Ru(bpy)2(dcbpy)[2+] involves metal to ligand charge transfer state and implications are that such an electronic configuration of the excited state plays an important role in controlling the electron injection rates. Trapping and detrapping processes can also play a major role in influencing the kinetics of the charge injection process.

Transient Absorption Spectroscopy of Dye Modified Nanocrystalline SnO2 Thin Films

Transient absorption spectroscopy is a convenient method to monitor the photochemical changes that occur at the nanocrystalline semiconductor surface under the influence of an externally applied electrochemical bias.(42) The spectroelectrochemical measurements were carried out using a thin layer cell of a 2 or 5 mm path length quartz cuvette. The spectroelectrochemical cell shown in Figure 4 has two side arms for inserting reference (Ag/AgCl) and counter (Pt gauze) electrodes. The thin nanostructured film (~1 mm thick) cast on a conducting glass plate and modified with the sensitizing dye was the working electrode.

The preparation and characterization of highly porous nanocrystalline semiconductor film from colloidal suspension are illustrated elsewhere.(23) The design of the cell employed in this study is such that we can insert it into the sample compartment of the nanosecond laser flash photolysis set-up and carry out transient absorption measurements under the influence of an applied bias.

Time-resolved transient absorption spectra of nanocrystalline SnO2 films modified with Ru(II) were recorded following 532 nm laser pulse excitation using the cell described above. The difference absorption spectra recorded at +0.2 V and -0.7 V are shown in Figure 5. The spectra recorded at early times exhibit spectral features (absorption maximum at 370 nm and an isosbestic point around 400 nm) similar to the spectrum of Ru(II)* as observed on a neutral oxide surface such as SiO2. However, the applied bias influences the spectral characteristics of transients recorded at longer times. The formation of Ru(III) is evident from the bleaching observed in the spectrum recorded at +0.2 V bias. The absence of Ru(III) and the appearance of excited Ru(II) (Abs. Max. at 380 nm) as the only observable transient at -0.7 V indicates the failure of Ru(II)* to participate in the charge injection process. At potentials more negative than the flat band potential of SnO2, charge injection process is suppressed thus extending the lifetime of Ru(II)*. On the other hand, the applied positive bias facilitates electron transfer quenching of Ru(II)* on the SnO2 surface. The applied positive bias shifts the pseudo-Fermi level of the SnO2 nanocrystallites in such a way that it provides the necessary driving force for the heterogeneous electron transfer. These examples demonstrate that it is possible to modulate the heterogeneous electron transfer at the semiconductor interface using an externally applied electrochemical bias.

What are the Controlling Factors?

The energy difference between the conduction band of the semiconductor and the oxidation potential of the excited sensitizer is the major driving force for the excited state charge transfer. A simple energy level diagram is illustrated in Figure 6.

Different approaches have been considered to establish the energy gap dependence of the photosensitization efficiency.(41,43-47) For example the energy gap can be varied by changing the pH of the medium since the conduction band of metal oxide semiconductor shifts 59mV per pH unit. Since the pseudo-Fermi level of the SnO2 nanocrystalline film can be varied by an externally applied electrochemical bias it is possible to alter DE and hence the kinetics of heterogeneous electron transfer. A more detailed discussion on this topic can be found in our forthcoming paper.(42)

The dependence of log(ket) on the applied potential is shown in Figure 7. The apparent values of ket were determined from the spectroelectrochemical experiments described above. The apparent rate constant for the charge injection process was constant at potential greater than -0.2 V with a maximum ket of 4X10^8 s-1. At potentials more negative than -0.2 V, a sharp decrease (more than 3 orders of magnitude) in ket is observed. At -0.7 V the charge injection process is almost completely suppressed as the deactivation of the excited state is dominated only by the radiative route (i.e., ket becomes smaller than kr).

It should be noted that the applied potential at which we observe this effect is close to the oxidation potential of excited sensitizer (0.72 V vs Ag/AgCl). Application of external bias alters the pseudo-Fermi level of the nanocrystalline film. At negative bias the electrons are accumulated within the SnO2 nanocrystallites thus shifting the pseudo-Fermi level to more negative potentials. Under these conditions the difference in energy (DE) between E'F and E0Ru(II*/III) that acts as a driving force for the heterogeneous electron transfer decreases. The heterogeneous electron transfer rate constant is maximum at potentials greater than 0.0 V vs. Ag/AgCl. The failure to observe the inverted region at positive applied potentials is attributed to the semiconducting nature of the acceptor which is composed of continuously distributed electronic energy levels. Although the electron transfer theory48-51 predicts that electron transfer rates should decrease with increasing thermodynamic driving force this phenomenon has been rarely demonstrated for interfacial or electrochemical processes.(52)

Back Electron Transfer Process

Back electron transfer between the injected charge and the sensitizing dye is a process that results in recombination losses.52,53 To achieve long term stability the forward reaction steps must be 106 times faster than irreversible degradation steps. Quick regeneration with a redox couple such as I3-/I- is an essential step for the stability of the photochemical solar cells.

The reverse electron transfer is usually monitored from the recovery of the sensitizer or from the decay of oxidized sensitizer. The reverse electron transfer is often a multiexponential process with a range of rate constants and suggests inhomogeneity of trap and/or surface sites that control the heterogeneous electron transfer. In the case of Ru(II) polypyridyl complex/TiO2 or SnO2 system the reverse electron transfer rate constant is 3-6 orders of magnitude lower than the rate constant of the charge injection process. The slow back electron transfer is particularly remarkable since the forward electron transfer (charge injection) is very fast as discussed in the previous section and contrasts relatively fast reverse electron transfer observed in organic dye systems. This is apparently a unique characteristic of metal-oxide semiconductor/Ru(II) polypyridyl complex systems that permits efficient operation of photochemical solar cell.

Future Challenges

Now that the feasibility of operating photochemical solar cell has been demonstrated in several laboratories, the task still remains to elucidate various photoinduced processes at the semiconductor/sensitizer interface. Novel sensitizers such as supramolecular sensitizers composed of donor acceptor moieties and composite semiconductors which promote efficient charge separation deserve careful consideration. The basic understanding of the photophysical and photochemical properties gained from the photochemical studies of sensitizers on semiconductor surfaces is crucial in the design and development of such cells for solar energy conversion. Another area that could benefit from the photochemical understanding of nanostructured semiconductor film is the area of photocatalysis. One such example would be to use a photosensitization approach for degrading colored contaminants such as textile dyes over semiconductor particles. The nanostructured semiconductor films also provide a convenient way to carry out bimolecular reactions in a confined reaction space of the nanopores.

Acknowledgments

The work described herein was supported by the Office of Basic Energy Sciences of the US Department of Energy. This is SR No. 178 from the Notre Dame Radiation Laboratory. I would like to thank many researchers (Drs. R. W. Fessenden, S. Hotchandani, X. Hua, G. Lappin, K. L. K. Patterson, Vinodgopal) and students (Mr. Idriss Bedja, Di Liu, C. Nasr) who have made a significant contribution in the research work presented here.

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