Preparation of Titanium Oxide Pillars on Glass Substrates and

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J. Phys. Chem. C 2008, 112, 7886–7894

Preparation of Titanium Oxide Pillars on Glass Substrates and Ultrathin Titanium Oxide Layer using PMMA/PS Blend Films† Dan Liu, Pinyi Yang, and Christine K. Luscombe* Department of Material Science and Engineering, UniVersity of Washington, Seattle, Washington 98195 ReceiVed: December 12, 2007; ReVised Manuscript ReceiVed: February 8, 2008

In this paper, a simple and inexpensive method of preparing nanosized titanium oxide (TiO2) pillars on glass substrates and ultrathin TiO2 layers is presented, utilizing a blend of commercially available polystyrene (PS) and poly(methyl methacrylate) (PMMA). The surface morphology of PS/PMMA blend films is investigated in terms of the processing parameters including solution concentration, blending ratio, and spin-coating speed. For the first time, a phase inversion was revealed for the PS/PMMA blend films spun coated on conducting substrates (ITO and TiO2), with increasing solution concentration. Atomic force microscopy studies show that PMMA forms vertical cylindrical structures in the matrix of PS on glass substrates and on ultrathin TiO2 layers deposited on indium tin oxide (ITO) substrates. After the PMMA phase is etched away by ultraviolet irradiation and acetic acid, TiO2 pillar structures are successfully created on both types of substrates by infiltration of the sol-gel mixture into the nanosized PS templates followed by calcination at an elevated temperature. The size and shape of the TiO2 pillars were found to be affected by the thickness and phase separation of the initial PS/PMMA films, which depend on the solution concentration and blending ratio of the two polymeric components. Introduction Recently, the significance of the active layer morphology in solar cells has been discussed intensely in the literature.1–6 It has been realized that an optimal phase separation is necessary for balanced charge generation, transport and collection.5,6 Although organic fullerene-conjugated polymer blend solar cells reach 5-6% power conversion efficiencies (PCE) through optimization,7,8 dye-sensitized solar cells (DSSCs) have demonstrated relatively high PCE up to 11%.9 Although DSSCs are promising due to their simple structure and use of low cost materials, the liquid electrolyte can cause corrosion and leakage. Hence, conjugated polymers have been introduced into this system, acting as both the light absorber and hole conductor.10–12 Much of the research so far for solid-state DSSCs has utilized nanoporous TiO2 and small molecular organic semiconductors,13 conjugated polymers,11 or solvent-free composite electrolytes consisting of inorganic semiconductors.10,12,14 The composite electrolyte was either prepared by mixing TiO2 nanoparticles with a dye sensitizer and I3-/I- redox couple followed by solvent evaporation,10,14 or in situ synthesized on the TiO2 surface.12 Unfortunately, the above DSSCs did not present high PCEs comparable to those of traditional liquid-electrolyte DSSCs, because of poor interfacial contact between the solid-state electrolyte and TiO2 photoelectrode. In particular, in the case of using conjugated polymers as the solid-state electrolyte, the difficulty of polymer chains to conform to the boundaries of nanoporous TiO2 leads to the formation of voids at the interface, where discontinued electron conduction and/or charge recombination occurs.10,14 Research efforts have also been devoted to the TiO2-conjugated polymer solar cells in the configuration of a bilayer heterojunction with modifications to the TiO2polymer interface aimed to realign the energy levels,15 harvest † Part of the “Larry Dalton Festschrift”. * To whom all correspondence should be addressed. E-mail: luscombe@ u.washington.edu. Phone: 1-206-616-1220. Fax: 1-206-543-3100.

excitons by resonance energy transfer,16 or adjust the band offsets via the existence of interfacial dipole moments.17 These approaches typically involve synthesis of new functionalized semiconducting small molecules and polymers, which lead to the changes in the open circuit voltage (Voc) and photocurrent density.16–18 While the chemical interactions at the TiO2conjugated polymer interface dictate charge separation, the morphologies of the two components determine the charge conduction pathways to the electrodes. In particular, a morphology consisting of 10-50 nm19 interdigitated structures of TiO2 and conjugated polymer has been targeted in hopes of creating more interfacial area between the electron donor (polymer) and acceptor (TiO2), greatly shortening the distance for the charge carriers to travel to the electrodes. Goh and co-workers constructed nanoporous TiO2 by embossing sol-gel with a poly(methyl methacrylate) (PMMA) mold that was prepared via thermal infiltration into anodic alumina templates.19 They succeeded in obtaining periodically straight pores with the diameter of the pores ranging between 30-70 nm. Koganti et al. developed a generalized method for making nanoporous, twodimensional (2D) hexagonally packed silica and TiO2 films on glass and silicon substrates.20 A cross-linked random copolymer, polyethylene oxide-r-polyphenylene oxide, was employed as a pore-directing agent to which the TiO2 sol-gel was dip coated and calcinated to form vertically aligned cylindrical channels. Kim et al. explored the use of polystyrene-poly(methyl methacrylate) (PS/PMMA) block and functionalized random copolymers to pattern the silicon wafer substrates. Ordered silicon oxide (SiO2) and TiO2 posts were grown from the nanosized pores left by the etching of PMMA phase.21,22 Although nanoporous TiO2 films or TiO2 pillars can be constructed following the above methods, the fabrication processes are rather complex. Block copolymers are expensive to make and require intensive care to achieve desired morphologies.21–23 The alignment of block copolymers to form periodical patterns at various scales is not trivial.10,11,23 While

10.1021/jp711686r CCC: $40.75  2008 American Chemical Society Published on Web 04/30/2008

Preparation of Titanium Oxide Pillars on a local scale, the nanodomains formed by the block copolymers are extraordinarily regular, it is difficult to control their orientation with the respect to the substrate on a more global scale.23 To lower the production cost of a solar cell, simple, inexpensive but effective means of producing the desired active layer structure is preferred. To achieve this, we have developed a new method to form TiO2 structures by using blend films of immiscible PS and PMMA. These two polymers are known to phase-separate to produce ordered surface morphology under easy manipulation.24–29 By spin casting from a common solvent such as toluene, chloroform, and tetrahydrofuran (THF), the sudden extraction of the solvent can result in isolated cylindrical domains, interconnected islandlike phase or pitted topography in the order of 300 nm to 1-2 µm.24–29 The isolated cylindrical domains in PS/PMMA blend films can be selectively etched to form a template, patterning the underlying substrates. Following this path, we have successfully prepared TiO2 pillars on glass substrates that are suitable for making optoelectronic devices. These structures are also useful in the applications of photocatalysis30 and fuel cell systems.31,32 In a similar fashion, the TiO2 pillars were also fabricated on top of an ultrathin TiO2 layer (∼10 nm) on indium tin oxide (ITO) substrates, which fulfills half of the interdigitated TiO2-conjugated polymer photovoltaic device structure. The ultrathin TiO2 layer functions as a hole-blocking interface between ITO and TiO2 pillars, preventing device (solar cell) shorting after filling of p-type conjugated polymers. Experimental Section For all the samples that were studied, 3-4 replicates were made to ensure the reproducibility of our work. Although the AFM images taken are confined to 10 × 10 µm or 5 × 5 µm areas, the patterns observed were present throughout the entire substrate. Substrates Preparation. Glass microslides and ITO sheets (conductivity, 15 ohms/square; thickness, ∼1.1 mm, Colorado Concept Coatings, Inc.) were cut into 1.5 × 1.5 cm squares and sonicated in the sequence of acetone, 2% (by volume) hellmanex II aqueous solution, deionized (DI) water, and isopropyl alcohol (IPA) and dried in air. Dilute TiO2 sol-gel mixture was prepared by mixing titanium ethoxide, ethanol (EtOH), DI water and hydrochloric acid (conc.) in a weight ratio of 5:160:1:0.8 and spun coated on the ITO substrates at a spinning speed of 4000 rpm. After spin coating, the samples were taken into an oven at 70 °C for 12 h and calcinated at a 450 °C furnace for 30 min. The substrates were sonicated in IPA for 20 min to remove dusts and/or carbon residues on the TiO2 surface before the atomic force microscope (AFM) characterization. The thickness of the TiO2 layer was determined as around 10 nm on ITO by use of the AFM. Spin Casting of PS/PMMA Blend Films. The PS particles and PMMA powders with a weight average molecular weight of 280 000 and 120 000, respectively, was purchased from Aldrich. The polymers were used as received and codissolved at room temperature for 24 h in THF with different blending ratios of PS/PMMA (v/v) including 10:90, 30:70, 50:50, 70:30 and 90:10 and an overall concentration of 0.5, 1, 2, and 4% (w/v). The PS/PMMA solution was spin coated onto precleaned glass substrates at a spinning speed of 2000, 4000, 6000 or 8000 rpm. The preparation and spin coating of the PS/PMMA blend on bare ITO substrates was the same as of the samples with glass substrates, except that the solution concentration was 1, 2, and 4% (w/v), and the spinning speed was set as 2000 rpm. PS/PMMA blend films also formed on predeposited TiO2 layer

J. Phys. Chem. C, Vol. 112, No. 21, 2008 7887 on ITO, spin coated from 2, 4, and 6% solutions at 2000 rpm with a fixed blending ratio of 90:10. To investigate the effect of thermal energy on feature sizes of the blend films, part of the samples on bare ITO was annealed in a vacuum oven at 80 °C for 2 h. Removal of PMMA Phase. The films with isolated cylindrical PMMA-rich domains were exposed to the 254 nm UV light for 2-4 h to etch away the PMMA phase, which degrade the PMMA and partially cross-linked the PS.33–35 The amount of deep UV irradiation time necessary to degrade and completely remove the PMMA phase was calculated based on the UV dosage threshold (>3.4 J/cm2) suggested by Reiser et al. and Russell et al.33–35 The films were then submerged in glacial acetic acid for 35 min, DI water for 10 min, and then air-dried. Fabrication of the TiO2 Pillars. Before infiltration, a regular batch of TiO2 precursor was made by mixing titanium ethoxide, EtOH, and DI water in the weight ratio of 5:40:1 with HCl added to ensure a pH value of 1. The prepatterned substrates from the previous step were submerged in the sol-gel mixture for 72 h. At the end of submersion, the substrates were taken out of the sol-gel mixture, sprayed with DI water 3-4 times. Almost identical thermal treatments with the substrates preparation step were carried out to the samples to convert the sol-gel to TiO2 pillars. The only difference lies in the calcination time, which was extended from 30 min to 1 h. Because the calcination temperature was well above the decomposition temperature of PS (∼345 °C),36 the PS templates were considered to have been completely removed after calcination. Surface Characterizations. The AFM was a Nanoscope II scanning probe microscope from Digital Instruments and applied in the tapping mode. The silicon tips have a spring constant of 42 N m-1. AFM imaging was conducted for the films on glass, ITO substrates, and ultrathin TiO2 layers after each manufacturing step, to reveal the surface morphology and also to determine the film thickness after the surface was scratched with a razor blade (which is not hard enough to scratch the glass, ITO, and TiO2 substrates). In this context, film thicknesses correspond to the average depth of the scratches relative to the mean surface plane. The feature sizes extracted from AFM images was reported as the best estimates by averaging data collected for samples with 2∼3 replicates. Water contact angle measurements were performed on a clean glass substrate, ITO substrate and TiO2 surface to compare their surface free energy. For each of these three substrates, a NRL (Navel Research Laboratory) contact angle goniometer (Rame-Hart, Inc.) was utilized to obtain the average contact angle, which was the arithmetic mean of contact angles measured at three different locations on the same substrate. Results and Discussion Methodology. The scheme designed to make TiO2 pillars on glass or ultrathin TiO2 layer deposited on ITO substrates is shown in Figure 1. The polymer blend of PS and PMMA is first deposited onto the precleaned substrates. After the PMMA is etched away by UV-irradiation and acetic acid submersion, an evenly distributed PS nanoporous structure is formed on the substrates. The TiO2 precursor is infiltrated into the pores by soaking the substrates in the sol-gel mixture. Calcination of the TiO2 sol-gel mixture at 450 °C not only enables the conversion of the sol-gel into TiO2 crystalline pillars but removes the polymer templates. Dependence of the Blend Film Topography on Blending Ratio, Spinning Speed, Solution Concentration, and Type of Substrates. It has been revealed that the surface structure of multicomponent polymer blend films may be fairly different

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Figure 1. The processes of patterning glass substrates and ultrathin TiO2 layers using PS/PMMA blend, and the formation of TiO2 pillars after sol-gel mixture infiltration and calcination. The ultrathin TiO2 layer acts as a hole blocker and prevent shorting of devices. Note that this schematic drawing does not scale with the actual sizes of the features in the films.

Figure 2. The AFM height images of PS/PMMA blend films (spin coated from 1% solution) with a blending ratio of (A) 90:10, (B) 70:30, and (C) 50:50 and (D) the section analysis for image B. The scan size of the images was 10 × 10 µm.

from that in the bulk, due to the rapid evaporation of solvent leading to energy states far from thermodynamic equilibrium.37,38 The phase separation processes in blend films are usually involved with several surface phenomena such as demixing, wetting, and dewetting.36 For thin PS/PMMA blend films spun cast on a substrate, the morphology has been demonstrated to be strongly influenced by both the polymer-air and polymersubstrate interfacial free energies, the solubility of the solvent, and the blending ratio.24–29 Before generating the nanosized patterns on glass, ITO, and on ultrathin TiO2 layers, an investigation of the PS/PMMA blend film morphology under various preparation conditions is necessary to substantiate the feasibility of the designed scheme (Figure 1).

Figure 2 compares the AFM height images of the blend films on glass substrates spin coated from 1% solution at 2000 rpm, varying only the blending ratio. When the blending ratio was 90:10 and 70:30, isolated cylindrical domains protruded from the matrix, as shown in the section analysis in Figure 2D. These domains were assigned as the PMMA-rich phase,24,28 the surface fraction and size of which increased along with the increase of PMMA bulk fraction. As the blending ratio reached 50:50, the films represented an interconnected islandlike structure (Figure 2C). This observation is consistent with those reported by others.24,28 Although PS has a slightly lower surface free energy than PMMA, the blend films exhibited protruding PMMA on the surface. There are several explanations provided in the

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TABLE 1: The Film Thickness and Feature Size of PS/PMMA Blend Films Spin Coated on Glass at Different Solution Concentration, Spinning Speed, and Blending Ratio solution conc (%)

blending ratio

FTa at 2000b (nm)

FT at 4000 (nm)

FT at 6000 (nm)

FT at 8000 (nm)

diameter of cylinders (nm) at 2000

blend viscosity (dL/g)

0.5

90:10 70:30 50:50 90:10 70:30 50:50 90:10 70:30 50:50 90:10 70:30 50:50

56 ( 3 46 ( 3 50 ( 4 109 ( 2 105 ( 12 105 ( 10 218 ( 9 202 ( 7 200 ( 6 570 ( 10 563 ( 13 504 ( 11

44 ( 3 38 ( 4 52 ( 4 145 ( 12 105 ( 6 78 ( 12 161 ( 8 141 ( 6 186 ( 7 485 ( 14 434 ( 12 393 ( 9

44 ( 2 41 ( 1 60 ( 7 96 ( 5 102 ( 4 78 ( 6 166 ( 5 161 ( 6 157 ( 5 482 ( 13 466 ( 12 425 ( 10

49 ( 6 37 ( 4 61 ( 7 122 ( 3 107 ( 8 70 ( 5 183 ( 8 169 ( 7 163 ( 4 537 ( 17 403 ( 11 372 ( 8

137 ( 14 234 ( 19 312 ( 28 196 ( 17 360 ( 22 585 ( 35 254 ( 12 429 ( 26 1110 ( 42 352 ( 18 625 ( 27 1855 ( 49

0.457 0.312 0.263 0.876 0.589 0.492 1.737 1.140 0.938 3.600 2.284 1.846

1 2 4

a

FT denotes film thickness. b 2000, 4000, 6000, and 8000 refer to spinning speed in rpm.

Figure 3. The AFM height images and section analysis of PS/PMMA blend films on bare ITO with a blending ratio of 90:10 spin cast from (A) 2%, and (B) 4% solution. The scan size was 5 × 5 µm.

literature, most of which considered the PMMA in a “frozen” state after spin coating.24,27,28 Because PMMA is less soluble in THF, it demixes from PS and solidifies first, while the swollen PS phase collapses to a level that lies below the PMMA-air interface.28 In addition to the variation in blending ratio, a test matrix was developed to examine the relationship between the features of PS/PMMA blend films on glass substrates and the processing parameters. The change in film thickness and average diameter of the cylindrical features with overall solution concentration (w/v), blending ratio, and spinning speed was measured. The results of blend film thickness, domain size as well as solution viscosity are summarized in Table 1. The factor that affected the thickness the most was the overall solution concentration and hence the solution viscosity with a relatively linear relationship between the film thickness and viscosity. On the other hand, the diameter of the protruded PMMA cylinders was altered with both the solution concentration and blending ratio at a constant spinning speed of 2000 rpm. It is expected that when the PMMA fraction is increased, more chains are available to segregate and form larger domains. For thicker films spin cast from a higher solution concentration, a longer drying time

is needed allowing for extended chain diffusion period, hence giving rise to a higher degree of domain-coarsening that is reflected by the considerable increase in diameter in the PMMA features.28 In solar cells, the use of conducting substrates such as ITO is required for the purpose of charge collection. The PS/PMMA blend film morphology on conducting substrates has never been studied in the past but is crucial information to produce interdigitated TiO2-conjugated polymer structure via the polymer blend approach. Targeting for the isolated cylindrical domains to be in the range of 150-200 nm, the blending ratio of the polymer blend was selected at 90:10. Although 150-200 nm may be a little bit large compared to the ideal periodicity of TiO2 and conjugated polymer phase (notice that both the diameter of TiO2 pillars and the gap between the pillars matters) that favors exciton dissociation (∼10-50 nm), they are reasonable numbers as the diameters of TiO2 pillars to start with in the sense of possible enhancement in light scattering and photon absorption. Chou et al. recently reported that the PCE of DSSCs with zinc oxide (ZnO) photoelectrodes can be increased from 0.6 to 3.5% by having a hierarchical structure of ZnO with ∼300 nm diameter colloidal spheres made from ∼20 nm nanopar-

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Figure 4. The schematic of phase inversion in PS/PMMA blend films spun coated on bare ITO substrates, as the solution concentration increased from 2 to 4%.

Figure 5. The AFM height images of PS/PMMA blend films on ultrathin TiO2 layer depostied on ITO with a blending ratio of 90:10 spin cast from (A) 2%, (B) 4%, and (C) 6% solution. (D-F) The corresponding images of film in panels A-C after the PMMA phase was etched away. (G-I) Section analysis of panels D, E, F, respectively. The scan size was 5 × 5 µm. The height scale in panels G, H, and I was (75, (75, and (150 nm, respectively.

TABLE 2: The Height/Depth and Width of the Isolated Cylindrical Domains and PS Pores in 2, 4 and 6% 90:10 PS/PMMA Blend Films As Well As the Surface rms Roughness before and after Etchinga height/depth (nm) 2% 4% 6%

width (nm)

rms roughness (nm)

before etching

after etching

before etching

after etching

before etching

after etching

-42 ( 3b 6.8 ( 1.2c 5.6 ( 0.9

-19 ( 2 -78 ( 5 -111 ( 8

175 ( 11 235 ( 14 318 ( 17

382 ( 21 234 ( 16 402 ( 19

9.7 ( 0.2 2.1 ( 0.1 1.7 ( 0.1

14.1 ( 0.2 15.6 ( 0.3 16.3 ( 0.3

a

The films were spun cast on ultrathin TiO2 layer deposited on ITO. b Negative signs indicate that the values are average depths of pits in nanoporous structures. c Positive signs indicate that the values are the average heights of protruded domains.

ticles.39 The hierarchically structured ZnO films demonstrated an additional absorption peak due to the light scattering from the 300 nm colloids, resulting in a 10-fold increase in the photocurrent density.39 Meanwhile, higher overall polymer solution concentrations were employed to prepare the blend

films, as to increase the thicknesses of succeeding configurations so that taller TiO2 pillars (∼100 nm) could be achieved at the end of construction. It should be noted that the scan size of the AFM images for 90:10 films was reduced to 5 × 5 µm to better illustrate the topographical features. As shown in Figure 3A,

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Figure 6. The binary phase diagrams of PS/PMMA blend films spun cast on glass and ITO substrates with respect to solution concentration and blending ratio.

the PS/PMMA blend film (2%, 90:10) spun cast on bare ITO exhibited a pitted surface morphology, which is the opposite morphology to what was found on glass. The pits were approximately 85 nm in depth and 150 nm in diameter. Walheim et al. observed a similar morphology for PS/PMMA blend films on octadecylmercaptan (ODM) treated silicon wafers.28 The X-ray photoelectron spectroscopy (XPS) study revealed that a PMMA/PS bilayer was formed on the ODM with PS lying next to the substrate. The top PMMA layer was punctured by holes that are partially filled by the PS-rich phase. Here, similar to ODM, ITO is a much more hydrophobic substrate compared to glass (∼8°) with a water contact angle of 62°. The preferential adsorption of PS to ITO dominated the film morphology, where dewetting of the PMMA occurred on the top of the PS layer.26 Annealing of the film in an 80 °C vacuum oven for 2 h enlarged the holes to 275 nm in diameter and 105 nm in depth, which complies with the predictions using the above dewetting theory. Surprisingly, the pitted morphology displayed by the 2% 90: 10 blend film on ITO transformed to a dissimilar phase separation by simply changing the solution concentration. In Figure 3B, the surface of the film spin cast from 4% PS/PMMA solution on ITO become almost flat with small peaks of only 5-10 nm tall protruding from the matrix. These small peaks or slightly protruded domains were assigned to the PMMA-rich phase, as confirmed by the etching experiments discussed in more detail in the next section. One possible explanation for the disappearance of pitted topography is that when the solution concentration and hence the film thickness doubled, the preferential adsorption of PS to the substrate was not strong enough to hold all of the PS polymers directly on top of the substrate, giving rise to a vertically phase-separated morphology as depicted in the model (Figure 4). A similar model has been suggested by Ton-That et al. who studied PS/PMMA blend films on mica27 and has been confirmed by XPS.24,27 One other possible explanation is that during spin coating of the more concentrated PS/PMMA blend, an increase in the rate of evaporation of the solvent is observed. A faster drying rate would move the system away from thermodynamic equilibrium causing vertical phase separation to occur where both PS and

PMMA are in contact with the substrate. In contrast, a more dilute solution reduces the rate of evaporation, allowing the system to become closer to thermodynamic equilibrium. In this case, the more hydrophobic PS adsorbs more strongly to the ITO, creating a laterally phase-separated structure. This conversion of morphology also happened to the blend films spin cast on the ultrathin TiO2 layer deposited on ITO (Figure 5). The water contact angle on TiO2 layer was measured as 82.5°. It is expected that TiO2 demonstrates a similar type of behavior with ITO as a substrate, due to their similar hybrophobicity. The height/depth and width of the isolated cylindrical domains and PS pores in 2, 4, and 6% 90:10 PS/PMMA blend films as well as the root-mean-square (rms) roughness of the surface before and after etching are reported in Table 2. The size of PMMA domains was seen to have augmented with the increase of solution concentration. To better illustrate the effects of substrate hydrophicility (water contact angle), solution concentration, and blending ration on the PS/PMMA blend film morphology, two binary phase diagrams are constructed in Figure 6. The binary phase diagrams revealed the presence of protruded isolated cylindrical domains, pitted morphology as well as islandlike domains observed for PS/PMMA films under various conditions. These different types of phase separation can be considered as the comprehensive results of demixing processes in immicible blends, formation of wetting/dewetting layers and confinement/preferential adsorption of polymers at specific interface after the sudden extraction of solvent. The phase diagrams provide a useful tool to achieve desired surface morphology in the applications of polymer nanolithrography. Formation of Nanoporous PS Network and TiO2 Pillars on Glass Substrates and Ultrathin TiO2 layer on ITO. The height image and section analysis of a 1% 70:30 blend film on glass substrate after PMMA phase was etched away is shown in Figure 7. The images clearly represent the porous structure left behind by the removal of isolated PMMA cylindrical domains (Figure 2B). The PS phase partially cross-links under UV radiation, which facilitates its adhesion to the glass substrate. The average pore size was approximately 80 nm deep and 150

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Figure 7. The AFM height images and section analysis of PS template on glass sustrate after the PMMA phase was etched away from the PS/PMMA film spin cast from 1% 70:30 solution. The scan size was 10 × 10 µm.

Figure 8. The AFM profile taken for (A) a 1% 70:30 blend film on glass, (B) nanoporous PS structure after PMMA etching, and (C) TiO2 pillars formed on glass after infiltration and calcination. The scan size was 10 × 10 µm and the Z-range of the profile was 100, 120, and 100 nm respectively.

nm in diameter. Similarly, for PS/PMMA blend films spin coated from 4 and 6% solution on ultrathin TiO2 layers the isolated PMMA domains were removed by etching, revealing clean porous structures shown in Figure 5E,F. In the case of the 2% 90:10 blend film, etching resulted in the presentation of larger circular shapes on the surface (Figure 5D). This provides direct evidence to the PMMA/PS bilayer model for pitted topography in which PMMA dewets from the underlying PS layer.26 If the pitted topography were enriched with a PS-rich layer on top, the pits would at least partially be kept in shape after etching, instead of exposing shallow circular rings that might be the bases of the pits (which is what occurs in our case). The surface rms roughness of 2% PS/PMMA blend film did not change significantly after the removal of PMMA phase, whereas the roughness of 4 and 6% films increased 8-10 fold as a consequence of the formation of nanoporous PS network. The infiltration of TiO2 sol-gel mixture into nanoporous PS templates by gravity and followed by thermal treatment successfully created single layers of TiO2 pillars on glass substrates and ultrathin TiO2 layers. Figures 8 and 9 illustrate the pillar fabrication processes by including the AFM profile images of original PS/PMMA blend films, PS templates, and resulting TiO2 pillars after calcination. The 12 h annealing process in a 70 °C oven converts the pore-filling sol-gel into amorphous TiO2 via a hydrolysis reaction.40 The amorphous TiO2 is then converted to anatase crystalline phase by calcination at 450 °C for 1 h. It can be seen from Figures 8C and 9C that the TiO2 pillars aligned

themselves vertically with slightly narrower ends pointing to the free air. This successful formation of pillar arrays implies that the sol-gel mixture has connected to the glass or TiO2 substrate at the bottom of the PS template through voids or free volume, behaving as the base for the pillars. The pores in the PS template can be considered as a polymer mold that not only embraces the sol-gel mixture, but separate the sol-gel evenly within certain distances. There was a distribution of height and width for the pillars. The average dimensions of these pillars are given in Table 3. The heights and widths of corresponding blend films and PS templates in Table 3 indicated that 4% 90:10 PS/PMMA gave rise to wider TiO2 pillars (∼350 nm) than the target value (150-200 nm), implying that the PS pores on the ultrathin TiO2 layer may not be as cleanly defined as on the glass substrates. It should be emphasized here that it is the combination of PS/ PMMA blending ratio and solution concentration that manipulates the feature sizes in blend film morphology. When a small PMMA fraction and high solution concentration is applied, the longer drying time of the film required during spin coating (due to smaller solvent gradient) seemed to overcome the larger PS to PMMA volume ratio, causing the film to further phase segregate. Meanwhile, the depth of the PS template remained close to that of the 1% 70:30 film. More UV irradiation may be required to completely decompose the PMMA. The TiO2 pillars were both ∼30 nm in height, about half of the desired value (∼70 nm) to fabricate good interdigitated PV (photovola-

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Figure 9. The AFM profile taken for (A) a 4% 90:10 blend film on ultrathin TiO2 layer deposited on ITO, (B) nanoporous PS structure after PMMA etching, and (C) TiO2 pillars formed on the ultrathin TiO2 layer after infiltration and calcination. The scan size was 5 × 5 µm and the Z-range of the profile was 50, 83 and 75 nm respectively.

TABLE 3: The Height/Depth and Width of the Isolated Cylindrical Domains, PS Pores and TiO2 Pillars Made from 1% 70:30 PS/PMMA Blend Film on Glass and 4% 90:10 PS/PMMA Blend Film on Ultrathin TiO2 Layer Deposited on ITO 1% 70:30 height/depth (nm) width (nm)

4% 90:10

blend film

PS template

TiO2

blend film

PS template

TiO2

28 ( 2 286 ( 18

-80 ( 8a 312 ( 22

33 ( 3b 204 ( 17

8(2 283 ( 15

-77 ( 12 234 ( 14

28 ( 7 350 ( 19

a Negative signs indicate that the values are average depths of pits in nanoporous structures. b Positive signs indicate that the values are the average heights of protruded domains.

tic) devices. The reduction in height against PS pores can be accounted for by two reasons: (1) the difficulty of sol-gel infiltration into the pores because of air blockage inside the channel41 and (2) the volume of the sol-gel shrinks to a large extent after calcination due to the evaporation of solvent and the hydrolysis reaction. The above analysis of the results suggested that to make the height and width of TiO2 pillars satisfiable for PV applications (i.e., ∼100 nm tall and 50-200 nm wide), the overall concentration of the PS/PMMA solution as well as its blending ratio needed to be tailored to find the balance. Future work will involve the testing of various solution concentration/blending ratio combinations, adjusting UV irradiation duration, and enhancing the efficiency of sol-gel infiltration. Conclusions In this paper, nanosized TiO2 pillars were constructed on both glass substrates and ultrathin TiO2 layer (∼10 nm) deposited on ITO via a simple and inexpensive approach that utilizes PS/ PMMA blend films. The surface morphology of the blend films was found to be strongly related to the solution concentration, blending ratio, spinning speed, and especially the hydrophilicity/ hydrophobicity of the substrates. The films formed isolated cylindrical or interconnected PMMA-rich domains protruding from the PS matrix, when spun cast on glass substrates. A pitted topography was observed for PS/PMMA blend films on bare ITO or ultrathin TiO2 layer. Instead of demixing and phase segregating vertically, PMMA-rich phase packed on top of the PS layer lying next to the substrate due to the preferential adsorption between PS and the hydrophobic substrate. The pitted structure was converted back to the morphology of PMMArich phase protruding from the PS matrix, as the solution concentration was increased to 4 and 6%. Although the PS nanoporous template and TiO2 pillars have been realized by PMMA etching and sol-gel mixture infiltration, their dimen-

sions will be improved for interdigitated solar cell fabrication. Currently, work is being focused on adjusting the solution concentration and blending ratio of the PS/PMMA solution for spinning coating on ultrathin TiO2 layer. Once TiO2 pillars with the right dimensions are available, conjugated p-type polymer such as poly(3-hexylthiophene) (P3HT) can spin cast on top to form a TiO2-P3HT interdigitated structure. An improvement in PCE is anticipated for this type of p-n heterojunction. Acknowledgment. We acknowledge the generous help from Dr. Alex K.-Y. Jen and his group. We also thank NSF Science and Technology Center for materials and devices for information technology research (NSF DMR-0120967). References and Notes (1) Hoppe, H.; Glatzel, T.; Niggemann, M.; Hinsch, A.; Lux-Steiner, M. C.; Sariciftci, N. S. Nano Lett. 2005, 5, 269. (2) Hoppe, H.; Niggemann, M.; Winder, C.; Kraut, J.; Hiesgen, R.; Hinsch, A.; Meissner, D.; Sariciftci, N. S. AdV. Funct. Mater. 2004, 14, 1005. (3) Hoppe, H.; Sariciftci, N. S. J. Mater. Chem. 2006, 16, 45. (4) Kim, Y.; Choulis, S. A.; Nelson, J.; Bradley, D. C.; Cook, S.; Durrant, J. R. Appl. Phys. Lett. 2005, 86, 063502. (5) Liu, J.; Shi, Y.; Yang, Y. AdV. Funct. Mater. 2001, 11, 420. (6) Shaheen, S. E.; Brabec, C. J.; Sariciftci, N. S. Appl. Phys. Lett. 2001, 78, 841. (7) Kim, K.; Liu, J.; Namboothiry, M. A. G.; Carrolla, D. L. Appl. Phys. Lett. 2007, 90, 163511. (8) Ma, W.; Yang, C.; Gong, X.; Lee, K.; Heeger, A. J. AdV. Funct. Mater. 2005, 15, 1617. (9) Gra¨tzel, M. Inorg. Chem. 2005, 44, 6841. (10) Han, H.; Liu, W.; Zhang, J.; Zhao, X.-z. AdV. Funct. Mater. 2005, 15, 1940. (11) Kwong, C. Y.; Choy, W. C. H.; Djurisi′c, A. B.; Chui, P. C.; Cheng, K. W.; Chan, W. K. Nanotechnology 2004, 15, 1156. (12) Wang, Y.; Yang, K.; Kim, S.-C.; Nagarajan, R.; Samuelson, L. A.; Kumar, J. Chem. Mater. 2006, 18, 4215. (13) Hal, P. A. v.; Wienk, M. M.; Kroon, J. M.; Janssen, R. A. J. J. Mater. Chem. 2003, 13, 1054.

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