Abstract:
Highly ordered anodic TiO 2  nanotube arrays fabricated by electrochemical anodization and sensitized with dye to yield dye-sensitized TiO 2  nanotube solar cells is described. With inorganic compound (such as TiCl 4 ) treatment, in conjunction with oxygen plasma exposure under optimized conditions, dye-sensitized TiO 2  nanotube solar cells produced using TiO 2  nanotube arrays exhibited a pronounced power conversion efficiency.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. §119 to provisional application Ser. No. 61/377,480 filed Aug. 27, 2010, herein incorporated by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to dye-sensitized solar cells, and more particularly to dye-sensitized TiO 2  nanotube solar cells having markedly enhanced performance as a result of optimized facile surface engineering. 
     BACKGROUND OF THE INVENTION 
     As one of the major renewable energy sources, solar energy has the potential to become an essential component of future global energy production. Dye-sensitized solar cells (DSSCs), represent one of the most promising of several alternative, cost-effective concepts for solar-to-electric energy conversion that has been offered over the past decade to challenge conventional silicon solar cells. The conventional configuration of a DSSC consists of a sintered, wide bandgap semiconductor TiO 2  nanoparticle network film, a ruthenium-based dye (i.e., sensitizer), and an electrolyte. Upon the absorption of photons, the dye generates excitons (i.e., electron-hole pairs). Subsequently, the electrons inject into the TiO 2  photoanode to generate photocurrent; scavenged by a redox couple, holes transport to the cathode. The performance of a DSSC can be improved by optimizing the semiconductor TiO 2  nanoparticle film, sensitizer, and electrolyte. However, much research remains to be done to improve the efficiency and remove the practical problems related to DSSCs. For example, typically, a 10-μm-thick mesoporous TiO 2  (anatase nanocrystals) film with a porosity of 50% is employed as an electron-accepting species in a DSSC. The film is prepared by dispersing 15-30 nm colloidal TiO 2  particles on a conductive glass support resulting in a network of randomly dispersed nanocrystals. While versatile and robust, these sintered three dimensional TiO 2  nanoparticle films lead to enhanced scattering of free electrons and electron trapping at the interfaces, thereby reducing electron mobility and exhibiting less efficient electron transport. The electron and hole transport across several ill-defined, heterogeneous interfaces in TiO 2  nanoparticle films is very complex. 
     In this context, highly ordered, vertically oriented TiO 2  nanotube arrays of different aspect ratios and surface qualities have recently been fabricated as alternative nanoscale architectures to substitute the sintered TiO 2  nanoparticle films in DSSCs. They are produced by potentiostatic anodization of titanium (Ti) foil or Ti thin films that have been sputtered using radio-frequency (RF) sputter deposition on a variety of substrates with fluorine-containing electrolytes. The nanotubular morphology of these arrays offers a large internal surface area with no concomitant decrease in geometric and structural order. The vertical orientation of the crystalline nanotube arrays makes them excellent electron percolation pathways for efficient, vectorial charge transport along the nanotube axis. 
     Solar cells produced from TiO 2  nanotube arrays show enhanced charge collection efficiency and enhanced light scattering over sintered TiO 2  nanoparticle films. The use of TiO 2  nanotubes reduces carrier scattering loss and nonradiative recombination by eliminating unnecessary lateral transport (i.e., hopping between TiO2 nanoparticles) and its resulting recombination loss. 
     Accordingly, it is a primary objective of the present invention to provide a surface treatment for TiO 2  photoanodes to increase the loading of sensitizers on the TiO2 surface and also improve the electronic interaction between the sensitizer (i.e., dye) and the TiO2 surface, thereby yielding improved performance. Thus, by using the process of the present invention, a significant step forward in the amount of dye adsorption and the charge transfer from dye molecules to photoanodes is achieved, and, ultimately, an enhanced PCE obtained. 
     The method of accomplishing this primary objective as well as others will become apparent from the detailed description of the invention which follows hereafter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS.  1 (A)-(D) show the structural characterization and an intensity plot for highly ordered TiO 2  nanotube arrays. 
         FIG. 2  shows a schematic for an exemplary configuration of a dye-sensitized TiO 2  nanotube solar cell. 
         FIG. 3  is a plot of current versus voltage characteristics of a dye-sensitized TiO 2  nanotube solar cell. 
       FIGS.  4 (A)-(D) show a plot of current versus voltage characteristics of a dye-sensitized TiO 2  nanotube solar cell assembled with TiCl 4  treatment for different intervals of oxygen plasma exposure. 
       FIGS.  5 (A)-(C) show a plot illustrating the effect of the thickness on the current versus voltage characteristics for a dye-sensitized TiO 2  nanotube for different intervals of oxygen plasma exposure. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     It will be shown, as one method for accomplishing the primary objective of the present invention, that a dye-sensitized TiO 2  nanotube solar cell has enhanced performance (i.e., PCE) based on highly ordered, vertically oriented TiO 2  nanotube arrays after rational surface engineering (i.e., TiCl 4  processing followed by O 2  plasma exposure). According to one aspect of the invention, the TiO 2  nanotube arrays were fabricated by electrochemical anodization and thermally induced crystallization; subsequently, they were impregnated with ruthenium dye N-719 used as a sensitizer and chemically anchored to the surface of the TiO 2  nanotubes via the reaction of carboxylate moieties on the N-719 surface with complementary hydroxyl groups on the TiO 2  surface. Accomplishing another primary object of the present invention, the solar cell of the present invention demonstrated a significant increase in the PCE (from 4.34% to 7.37%) using 14-μm-thick TiO 2  nanotube arrays under simulated AM 1.5 G irradiation of 100 mW/cm 2  in backside illumination mode with TiCl 4  treatment and oxygen plasma exposure on the TiO 2  surface before dye loading. The TiCl 4  and O 2  plasma exposure process, according to one aspect of the invention, increased the photogenerated current and fill factor, and ultimately a higher PCE. This has previously been unable to be accomplished as evidenced by the fact that there are no reports in the technical literature to the Applicant&#39;s knowledge of a PCE of 7.37% for dye-sensitized TiO 2  nanotube solar cells. 
     Fabrication of TiO 2  Nanotube Arrays 
     In a typical fabrication of the arrays, high-purity Ti foil (250 μm thick, 99.7% purity, Sigma-Aldrich) were cut into pieces, each 1 in.×0.5 in. in size, and degreased by ultrasonication for 30 min in a mixture of acetone, methanol, and methylene chloride, followed by a thorough rinse with deionized (DI) water and blow-dried with N 2 . Ethylene glycol (Fisher Scientific) was used as the electrolyte in which a small amount of ammonium fluoride (Sigma-Aldrich) was added. Electrochemical anodization of titanium was performed in a two-electrode cell at room temperature using a EC570-90 power source (Thermo Electron Corporation) at 60 V. A platinum foil was used as the counter electrode. After anodization, the Ti foil with TiO 2  nanotubes grown on one side of its surface was extensively washed with DI water and dried with N 2  gas (the backside of the Ti foil was protected with a layer of insulate resin to prevent oxidation during the anodization process). High temperature annealing was carried out to transform the as-prepared amorphyous TiO 2  nanotube into anatase nanotube. 
     Surface Treatment. 
     Prior to dye adsorption, anatase TiO 2  nanotubes were processed with TiCl 4  by immersing them in 50 mL of 0.2 M TiCl 4  aqueous solution in a beaker sealed with parafilm and kept in a 60° C. oil bath for 1 h, followed by rinsing with ethanol and annealed at 500° C. in air for 30 min. The TiCl 4 -treated TiO 2  nanotubes were further exposed to O 2  plasma at 30 W of power for different times. 
     TiO 2  Nanotube Solar Cell Fabrication. 
     The surface-treated TiO 2  nanotube arrays were then immediately soaked in a 0.2 mM dye/ethanol solution for 24 h to allow complete dye adsorption. The dye used in the study was cis-diisothiocyanatobis(2,2′-bipyridyl-4,4′-dicarboxylato) ruthenium(II) bis(tetrabutylammonium) (N-719, Solaronix). A semitransparent platinum (Pt)-coated ITO glass was used as the counter electrode, prepared by placing a drop of 0.5 mM chloroplatinic acid (H 2 PtCl 6 )/isopropanol solution on ITO glass and subsequently fired at 380° C. for 20 min. The dye-sensitized TiO 2  nanotube arrays were sandwiched between the Ti foil (anode) and the Pt-coated ITO glass (cathode) by applying a 25-μm-thick hot-melt sealing foil as the spacer (SX1170-25, Solaronix). An ionic liquid electrolyte containing 0.60 MBMIM-I, 0.03MI 2 , 0.50MTBP, and 0.10MGTCin acetonitrile/valeronitrile 85/15 (v/v) (No. ES-0004, purchased from io.li.tec, Germany) was injected between two electrodes driven by capillary force through holes on the hot-melt sealing foil. The ionic liquid possesses high thermal stability, negligible vapor pressure, and low toxicity. The backside of the Ti foil was then scratched using sandpaper to remove the thin oxide layer, thereby exposing the underlying titanium that serves as the electrode. 
     Characterization. 
     Scanning electron microscopy (SEM) imaging was performed with a JEOL Model LV 5800 microscope. Raman spectroscopy measurements were performed using a Renishaw in Via Raman microscope excited with a 488-nm Ar +  laser at 5 mW with an acquisition time of 10 s. The current-voltage (I-V) curves were measured using a Keithley Model 2400 multisource meter. A solar simulator (SoLux Solar Simulator) was used to simulate sunlight, with an illumination intensity of 100 mW/cm 2  (calibrated with Daystar Meter). The photoactive area for all solar cells was 0.125 cm 2 . 
     EXAMPLE 
     Highly ordered TiO 2  nanotube arrays according to one aspect of the invention were fabricated as hereinbefore described. The SEM images of as-prepared TiO2 nanotube arrays are shown in FIGS.  1 (A)-(C). The nanotubes had an average inner diameter of 90 nm and an interpore distance of 120 nm (see  FIGS. 1(A) and 1(C) ). The membrane thickness can be readily controlled by the anodization time; the cross section of a 14-μm-thick TiO 2  membrane obtained after 2 hours of anodization is shown in  FIG. 1(B) . The TiO 2  nanotube arrays with thicknesses of 22 and 29 μm were obtained after 3 and 4 hours of anodization, respectively. The as-prepared TiO 2  nanotubes were amorphous. To take advantage of nanotubular structures for use in solar cells, they should be converted to crystalline forms, i.e., anatase or rutile phase. The crystalline phase, induced by thermal annealing at 500° C. in air for 3 hours, was confirmed by Raman spectroscopy. A broad spectrum was seen on amorphous TiO 2  nanotubes. By contrast, specific Raman peaks at 145, 196, 399, 516, and 640 cm −1  were observed from annealed TiO 2  nanotube arrays, signifying the formation of anatase TiO 2  (see  FIG. 1(   d )). The TiO 2  nanotubular structures were maintained after high-temperature annealing. 
     Anatase TiO 2  nanotube arrays were then utilized as photoanodes to assemble dye-sensitized nanostructured solar cells (see  FIG. 2 ). Prior to dye adsorption, the nanotubes were processed with TiCl 4  using 0.2 M TiCl 4  aqueous solution. Subsequently, the TiCl 4  treated TiO 2  nanotube arrays were exposed to O 2  plasma for different time. The N-719 dye-sensitized TiO 2  nanotube solar cells were assembled and the performance was measured. 
       FIG. 3  shows the effects of TiCl 4  treatment on the I-V characteristics of dye-sensitized TiO 2  nanotube solar cells. The thickness of TiO 2  nanotube arrays was 14 μm. For a solar cell without subjecting to TiCl 4  processing, the measurement yielded an open circuit voltage (V OC ) of 0.70 V, a short circuit current density (J SC ) of 12.16 mA/cm 2 , and a fill factor (FF) of 0.51; thus, the power conversion efficiency (PCE), which is defined as 
               P   ⁢           ⁢   C   ⁢           ⁢   E     =         I   SC     ⁢     V   OC     ⁢   F   ⁢           ⁢   F       P   inc             
is 4.34% (see solid line in  FIG. 3 ). In contrast, when TiCl 4 -treated TiO 2  nanotube arrays were used, the performance sharply increased, yielding values of V OC =0.75 V, J SC =12.48 mA/cm 2 , FF=0.68, and PCE=6.36% (see dash-dotted line in  FIG. 3 ), which represents a 47% increase in PCE, when compared to its non-TiCl 4  treated counterpart (see dashed line in  FIG. 3 ). The performance improvement is described as follows. Upon high-temperature annealing, there existed structural defects and cracks on the nanotubes, as well as on the barrier layer (i.e., structural disorders), which made it possible for the electrolyte to have direct contact with the Ti electrode on the bottom, leading to decreased shunt resistance and higher chance for charge recombination. By contrast, with TiCl 4  treatment, a thin blocking layer of TiO 2  can be formed on the uncovered region of Ti foil, thus reducing or eliminating the structural disorders. As a consequence, the shunt resistance increased and series resistance decreased; this improved the FF value, and, ultimately, a higher PCE is obtained. With TiCl 4  treatment, the increased electron transport through a TiO 2  nanotube wall to the bottom Ti electrode by decreasing the surface charge recombination and the increased dye adsorption by facilitating improved bonding between TiO 2  and dye molecules provided enhanced performance.
 
     O 2  plasma treatment is demonstrated as one approach to saturate the metal oxide surface with a hydroxyl group, which increases the hydrophilicity at the metal oxide surface. In this regard, to further improve the performance of dye-sensitized nanostructured solar cells, O 2  plasma exposure is performed on the TiCl 4  treated TiO 2  nanotubes prior to infiltration of dye N-719. Specifically, the aforementioned TiCl 4  treated, 14-μm-thick TiO 2  nanotube arrays were placed in O 2  plasma cleaner (Harrick Plasma Model PDC-001; a PlasmaFlo gas meter was used to control the O 2  flow in the reaction chamber). The plasma condition was optimized by tuning the O 2  flow and vacuum until the brightest fluorescence was observed, i.e., with the O 2  flow of 1 SCFH and a vacuum of 500 mTorr. After plasma exposure for different periods of time, TiO 2  nanotubes were immersed in the 0.2 mM N-719/ethanol solution for 24 hours. All other steps for the construction of solar cells and the performance measurements were the same. The I-V curves of resulting solar cells are shown in FIG.  4 (A)-(D). In comparison to the dash-dotted line in  FIG. 3 , in which TiO 2  nanotubes were not subjected to O 2  plasma (PCE=6.36%), the PCE increased to 6.48% after 1 min O 2  plasma treatment. With longer time exposure to O 2  plasma, the PCE increased; the highest PCE value of 7.37% was achieved from the 10-min O 2  plasma-treated sample (see Table 1 below for complete device characterization). The enhancement in PCE can be attributed to a large increase in the photocurrent density (J SC ), while the photovoltage (V OC ) experienced little change and the fill factor (FF) slightly decreased. Upon O 2  plasma treatment, the hydroxyl group on the TiO 2  surface increased and eventually saturated; this promoted the adsorption of the N-719 dye by forming interfacial bonding between N-719 and the TiO 2  via the coupling reaction of terminal carboxylic acid groups on N-791 with complementary hydroxyl groups on the TiO 2  surface, thereby substantially regulating the interfacial charge transfer from N-719 dye to TiO 2  and, therefore, the ultimate photophysical properties at the nanoscale. 
     Note that, upon lengthy O 2  plasma exposure (e.g., 20 min; see  FIG. 4(D)  and Table 1), the device performance decreased, exhibiting a largely reduced J SC  value, compared to the sample after 10 min of O 2  plasma exposure (see  FIG. 4(D)  and Table 1). This may be because the TiO 2  surface became less hydrophilic after a longer-time plasma exposure than that under the optimized condition. Consequently, the dye loading was reduced, which in turn resulted in decreased device performance. Similar trends were seen in solar cells assembled using 22- and 29-μm-hick TiO 2  nanotube arrays, that is, the PCE increased with exposure time when nanotubes were subjected to O 2  plasma for less than 10 min; otherwise, the PCE was reduced (see Table 1). 
     The present invention contemplates increasing the thickness of TiO 2  nanotube arrays to improve the performance of solar cells by allowing more dye molecules to be adsorbed on the TiO 2  nanotube surface, thereby obtaining a higher light harvesting efficiency. To this end, thicker TiO 2  nanotube arrays 22 and 29 μm in thickness were prepared and used to fabricate solar cells; they were processed with TiCl 4  and oxygen plasma. The use of thicker TiO 2  nanotube arrays yielded progressively decreased performance (PCE=6.29% for 22-μm-thick nanotubes and PCE=6.05% for 29-μm thick nanotubes; see Table 1) compared to the device produced using 14-μm-thick nanotubes (PCE=7.37%). The I-V characteristics of solar cells assembled using 14-, 22-, and 29-μm-thick TiO 2  nanotubes are shown in  FIGS. 5(A) ,  5 (B), and  5 (C), respectively. For the 22-μm-thick TiO 2  nanotube arrays, although the device performance increased as a function of O 2  plasma exposure time (see Table 1), the PCE value was less than that using 14-μm-thick TiO 2  nanotubes. When even thicker TiO 2  nanotube arrays were used (i.e., 29 μm), the PCE value decreased further. The decrease in PCE with increased TiO 2  thickness may be due to the limitations of the current device configuration, as depicted in  FIG. 2 . In the backside illumination mode, the incident photons had to pass through semitransparent platinum-coated ITO glass and the ionic liquid electrolyte (i.e., ES-0004). Thus, a portion of incident photons were lost before reaching the sensitizer N-719 dye and the photoanode TiO 2  nanotube arrays. Therefore, as the thickness of nanotubes increased, the remaining photons may be fully absorbed by the dyes that were anchored on the top part of TiO 2  nanotube; however, the dyes on the bottom part of nanotubes are not excited. Accordingly, the present invention contemplates the use of freestanding TiO 2  nanotube arrays (i.e., detached from Ti foil after anodization) or TiO 2  nanotubes directly grown on transparent conductive substrate in a front-side illumination mode, where the dye-adsorbed TiO 2  nanotube arrays are directly illuminated from the TiO 2  nanotube arrays before incident photons encounter the electrolyte and the counter electrode, thereby ensuring good light harvesting efficiency. 
     
       
         
               
             
               
               
               
               
               
               
             
               
             
               
               
               
               
               
               
             
               
             
               
               
               
               
               
               
             
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Summary of the Device Performance of Dye-Sensitized  
               
               
                 TiO 2  Nanotube Solar Cells 
               
             
          
           
               
                   
                 O 2  plasma 
                   
                   
                   
                 Power  
               
               
                   
                 exposure 
                 Open circuit 
                 Short-circuit 
                 Fill 
                 conversion 
               
               
                   
                 time 
                 voltage, 
                 current density,  
                 factor, 
                 efficiency,  
               
               
                 TiCl 4   
                 (min) 
                 V OC  (V) 
                 J SC  (mA/CM 2 ) 
                 FF 
                 PCE (%) 
               
               
                   
               
             
          
           
               
                 TiO 2  Nanotube Thickness = 14 μm 
               
             
          
           
               
                 no 
                  0 
                 0.70 
                 12.16 
                 0.51 
                 4.34 
               
               
                 yes 
                  0 
                 0.75 
                 12.48 
                 0.68 
                 6.36 
               
               
                 yes 
                  1 
                 0.81 
                 11.76 
                 0.68 
                 6.48 
               
               
                 yes 
                  2 
                 0.74 
                 13.36 
                 0.68 
                 6.72 
               
               
                 yes 
                 10 
                 0.77 
                 15.44 
                 0.62 
                 7.37 
               
               
                 yes 
                 20 
                 0.76 
                 12.45 
                 0.62 
                 5.87 
               
             
          
           
               
                 TiO 2  Nanotube Thickness = 22 μm 
               
             
          
           
               
                 yes 
                  0 
                 0.73 
                 8.10 
                 0.63 
                 3.73 
               
               
                 yes 
                  1 
                 0.67 
                 8.48 
                 0.67 
                 3.81 
               
               
                 yes 
                  2 
                 0.75 
                 9.30 
                 0.69 
                 4.29 
               
               
                 yes 
                  5 
                 0.72 
                 9.2 
                 0.71 
                 4.70 
               
               
                 yes 
                 10 
                 0.77 
                 11.84 
                 0.69 
                 6.29 
               
               
                 yes 
                 20 
                 0.72 
                 9.6 
                 0.70 
                 4.84 
               
             
          
           
               
                 TiO 2  Nanotube Thickness = 29 μm 
               
             
          
           
               
                 yes 
                 10 
                 0.75 
                 11.52 
                 0.70 
                 6.05 
               
               
                   
               
             
          
         
       
     
     As can be seen from the examples of the present invention, the Applicants have assembled ruthenium dye (N-719)-sensitized nanostructured solar cells, using highly ordered anodic TiO 2  nanotube arrays as the photoanode. Rational surface engineering on TiO 2  nanotubes with TiCl 4  treatment together with O 2  plasma activation under optimized condition dramatically enhanced the performance of resulting devices. A solar cell in backside illumination mode produced using 14-μm-thick TiO 2  nanotube arrays after surface treatments with TiCl 4  and O 2  plasma yielded a power conversion efficiency (PCE) of 7.37%, which is the highest promising efficiency for solar cells made of TiO 2  nanotube arrays reported in the literature. In addition to ruthenium-based organic dyes, in principle, the invention contemplates that conjugated homopolymers (e.g., regioregular poly(3-hexyl thiophene)), conjugated block copolymers, and semiconducting nanocrystals (e.g., quantum dots) can be employed as alternative photosensitizers to create a variety of dye-sensitized TiO 2  nanotube solar cells. 
     As can be seen from the above specification and illustrative examples, the invention accomplishes at least its primary objective, but its limitation is based upon the claims which follow. 
     REFERENCES 
     The reference listed below, is herein incorporated by reference in its entirety.
     J. Wang and Z. Lin, “Dye sensitized TiO2 nanotube solar cell with markedly enhanced performance via rational surface engineering”, Chemistry of Materials 22, 579-584 (2010)(Featured on the Cover of Chemistry of Materials).