Nanotubes

A geometry which has generated remarkable scientific interest during the past years is the nanotube array. Similar to nanowires, tubular structures provide excellent path-ways for directed charge transport. In addition, nanotubes exhibit almost twice the surface area as rod-like structures of similar dimensions. This makes metal oxide nan-otubes especially interesting for application in HSCs, where large surface areas for dye adsorption and large donor-acceptor interfaces are demanded.

Figure 3.13: SEM cross-sectional view of typical TiO₂nanotubes on an ITO substrates.The nanotube array was fabricated by anodization of a sputtered Ti film in a NH4F solution in ethylene glycol.

By far the most common metal oxide for tubular structures is TiO₂. Progress in the field of these structures is summarized in the reviews of Mor et al. and Ghicov and Schmuki.[184, 206] Utilizing self-organization processes, TiO₂ nanotubes are usually formed by anodization of metallic Ti in fluoride ion containing electrolytes, either aque-ous HF solutions or NH₄F containing ethylene glycol baths.[207, 208] For the latter, 360µm long nanotubes with aspect ratios above 2000 have been realized when an-odizing high purity Ti foils.[209] The tubes are hexagonally packed and are forming at growth velocities of 15µm h⁻¹. TiO₂ nanotubes can also be grown directly on trans-parent conducting glass substrates by sputtering high quality Ti films and subsequent anodization.[210] A cross-sectional SEM micrograph of a typical TiO₂ nanotube array on ITO glass is shown in Figure 3.13. This offers the advantage that conventional cell

Chapter 3. Working Mechanisms of Nanostructured Hybrid Solar Cells geometries with front illumination (through the glass substrate) can be realized, which is especially interesting for solid state HSCs. Paulose et al. reported on backside illu-minated DSCs assembled from Ti foils and found a significant loss in light intensity and performance due to the partial opaqueness of the Pt covered counter electrodes.[211]

By adjusting the anodization process parameters, good control over the tube length, diameter, wall thickness and spacing can be achieved.[148]

Remarkable efficiencies above6 %have been reported for liquid electrolyte DSCs based on anodized Ti foils.[208, 212] Although this is lower than performances of DSCs with mesoporous TiO₂films, tubular structures are considered to offer great potential. Aside from the non-ideal backside illumination, the lower efficiencies are mainly attributed to a lower dye uptake due to the smaller surface area. However, the favorable charge trans-port properties of ordered tubular structures may give rise to record efficiencies DSSs in the future. In contrast to single crystalline nanowires, nanotubes from anodization of Ti are polycrystalline with typical grain sizes of30−40 nmand have to be annealed at temperatures above450^◦Cto yield anatase crystallinity.[213] Due to crystal domain sizes being only slightly larger than in mesoporous TiO₂, only a small difference in the electron mobility is expected. However, ordered structures enhance electron life-times due to optimized percolation pathways and therefore reduce recombination losses.

Future research may also yield nanotubular structures with enlarged grain size.

One of the most impressive examples of the high potential of TiO₂ nanotubes was presented by Mor et al. in 2006.[214] Using only360 nmlong TiO₂ tubes on conducting glass they realized DSCs with efficiencies of2.9 %andJ_SC of7.9 mA cm⁻². The overall dye absorption is relatively low in such small structures, which limits the photocurrent.

However, the impressive efficiencies could be directly attributed to increased electron lifetimes and optimized transport pathways compared to mesoporous TiO₂.

To our knowledge, there are no systematic studies on SS-DSCs based on nanotubular TiO₂ yet. However, TiO₂ nanotubes have been used in highly efficient HSCs with conjugated polymers. In 2009 Mor and co-workers reported on TiO₂-P3HT HSCs based on nanotubular films of a few100 nmon FTO glass.[101] By using a dye with absorption in the near infrared and utilizing nanotube geometries that almost match the exciton diffusion length of P3HT, they were able to achieve efficiencies up to3.8 %, which is the highest performance reported so far for metal oxide-conjugated polymer HSCs. These high efficiencies were possible only after TiCl₄ treatment of the nanotubes and the use oftBP. TiO₂ nanotube arrays infiltrated with blends of P3HT and PC₇₁BM still show slightly higher efficiencies, suggesting that additional surface engineering and further optimization of the TiO₂ geometry is necessary if the organic acceptor is completely replaced by the TiO₂.[215]

In addition to the anodization of Ti foil, other methods for the fabrication of TiO₂ nanotubes are available. Foong and co-workers reported on a template-directed growth of TiO₂ nanotubes using atomic layer deposition (ALD) onto AAO membranes.[168]

The method is sketched in Figure 3.14 a). AAO membranes on thin TiO₂ blocking layers on ITO are grown via anodization of sputtered Al films (1-2). Using atomic layer deposition, homogenous coatings of a few nm thick TiO₂ are grown on these

3.6 Nanostructured Metal Oxides for Solid-State Dye-Sensitized and Hybrid Solar cells

Al

TiO₂ TCO

a) b)

(1)

(2)

(3)

(4) (5)

Figure 3.14: Growth of TiO₂ nanotubes on TCO via atomic layer deposition (ALD) onto anodized aluminum oxide (AAO) membranes. a)Schematic of the synthesis route. (1) A layer of aluminum is sputter deposited onto TiO2-coated TCO and (2) anodized to yield a nanoporous structure. (3) The structure is coated with TiO2 via ALD and (4) the overlayer of TiO2 on top of the AAO is removed via Ar sputtering. (5) Finally, the AAO is removed in sodium hydroxide solution to yield free-standing nanotubes. b)SEM top view of TiO2nanotubes grown via ALD into AAO (scalebar corresponds to 500 nm). For this experiment, ALD and subsequent Ar etching has been carrier out by Robert Zierold in the group of Prof. Nielsch at UHH Hamburg, Germany.

structures (3) and the resulting overlayer of TiO₂ is removed via reactive ion etching or bombardment with Ar plasma (4). Finally, the AAO membrane is dissolved in NaOH solution, yielding free-standing TiO₂ nanotubes directly on the dense TiO₂ blocking layer (5). An SEM top view image of TiO₂ nanotubes grown via this synthesis route is shown in Figure 3.14 b).

Optimized anodization conditions for AAO membranes allow the formation of highly ordered hexagonally packed nanostructures.[216] Thus, nanotubular TiO₂ thin films could be realized in the future. Additionally, ALD allows the fabrication of tubes with small wall thicknesses below5 nm.

Another interesting method of tube synthesis was presented by Na et al. in 2008.[217]

Via electrodeposition onto ITO, a template of ZnO nanorods can be fabricated. TiO₂is then deposited onto this template from a sol-gel and then undergoes a heat treatment.

Finally, the ZnO is removed, resulting in free standing TiO₂ nanotubes on conducting glass. Sol-gel deposition also results in a thin compact TiO₂ layer which makes the structures highly interesting for use in HSCs.

Compared to TiO₂there are only very limited studies on nanotubular structures of ZnO.

Similar to TiO₂, nanotubes are fabricated via self-organization processes. Using a low temperature liquid phase method, hexagonal ZnO nanotubes have been fabricated on Zn foil.[218] By choosing appropriate conditions, ultrathin ZnO nanowires have also been synthesized on a ZnO compact layer on Si in a hydrothermal process.[219] Martinson et al. reported on liquid electrolyte DSCs based on ZnO nanotubes.[220] These structures were fabricated by ALD onto AAO membranes. Even though only moderate efficiencies

Chapter 3. Working Mechanisms of Nanostructured Hybrid Solar Cells of 1.6 % have been achieved, their results show that ZnO nanotubes provide excellent and almost loss-free electron transport over several µm.