29. Mor GK, Kim S, Paulose M, Varghese OK, Shankar K, Basham JI and Grimes CA Visible to near- IR harvesting in TiO2 Nanotube Array-P3HT based Heterojunction Solar Cells Nano Letters 2009 9(12) 4250-4257.

Abstract: The development of high-efficiency solid-state excitonic photovoltaic solar cells compatible with solution processing techniques is a research area of intense interest, with the poor optical harvesting in the red and near-IR (NIR) portion of the solar spectrum a significant limitation to device performance. Herein we present a solid-state solar cell design, consisting of TiO2 nanotube arrays vertically oriented from the FTO-coated glass substrate, sensitized with unsymmetrical squaraine dye (SQ-1) that absorbs in the red and NIR portion of solar spectrum, and which are uniformly infiltrated with p-type regioregular poly(3-hexylthiophene-2,5-diyl) (P3HT) that absorbs higher energy photons. Our solid-state solar cells exhibit broad, near-UV to NIR, spectral response with external quantum yields of up to 65%. Under UV filtered AM 1.5G of 90 mW/cm2intensity we achieve typical device photoconversion efficiencies of 3.2%, with champion device efficiencies of 3.8%.

28. Shankar K, Basham JI, Allam NK, Varghese OK, Mor GK, Feng XJ, Paulose M, Seabold JA, Choi KS and Grimes CA Recent advances in the use of TiO2 nanotube and nanowire arrays for oxidative photoelectrochemistry Journal of Physical Chemistry C, 2009113(16) 6327-6359.

27. Shankar K, Feng XJ and Grimes CA Enhanced harvesting of red photons in nanowire solar cells :evidence of resonance energy transfer ACS Nano, 2009 3(4) 788-794.

Abstract: Modern excitonic solar cells efficiently harvest photons in the 350−650 nm spectral range; however, device efficiencies are typically limited by poor quantum yields for red and near-infrared photons. Using Förster-type resonance energy transfer from zinc phthalocyanine donor molecules to ruthenium polypyridine complex acceptors, we demonstrate a four-fold increase in quantum yields for red photons in dye-sensitized nanowire array solar cells. The dissolved donor and surface anchored acceptor molecules are not tethered to each other, through either a direct chemical bond or a covalent linker layer. The spatial confinement of the electrolyte imposed by the wire-to-wire spacing of the close-packed nanowire array architecture ensures that the distances between a significant fraction of donors and acceptors are within a Förster radius. The critical distance for energy transfer from an isolated donor chromophore to a self-assembled monolayer of acceptors on a plane follows the inverse fourth power instead of the inverse sixth power relation. Consequently, we observe near quantitative energy transfer efficiencies in our devices. Our results represent a new design paradigm in excitonic solar cells and show it is possible to more closely match the spectral response of the device to the AM 1.5 solar spectrum through use of electronic energy transfer.

26. Shankar K, Bandara J, Paulose M, Wietasch H, Varghese OK, Mor GK, LaTempa TJ, Thelakkat M and Grimes CA Highly efficient solar cells using TiO2 nanotube arrays sensitized with a donor-antenna dye Nano Letters, 2008 8(6) 1654-59.

Abstract: Donor antenna dyes provide an exciting route to improving the efficiency of dye sensitized solar cells owing to their high molar extinction coefficients and the effective spatial separation of charges in the charge-separated state, which decelerates the recombination of photogenerated charges. Vertically oriented TiO2 nanotube arrays provide an optimal material architecture for photoelectrochemical devices because of their large internal surface area, lower recombination losses, and vectorial charge transport along the nanotube axis. In this study, the results obtained by sensitizing TiO2 nanotube arrays with the donor antenna dye Ru-TPA-NCS are presented. Solar cells fabricated using an antenna dye-sensitized array of 14.4 µm long TiO2 nanotubes on Ti foil subjected to AM 1.5 one sun illumination in the backside geometry exhibited an overall conversion efficiency of 6.1%. An efficiency of 4.1% was obtained in the frontside illumination geometry using a 1 µm long array of transparent TiO2 nanotubes subjected to a TiCl4 treatment and then sensitized with the Ru-TPA-NCS dye. Open circuit voltage decay measurements give insight into the recombination behavior in antenna-dye sensitized nanotube photoelectrodes, demonstrating outstanding properties likely due to a reduction in the influence of the surface traps and reduced electron transfer from TiO2 to ions in solution.

25. Feng X, Shankar K, Varghese OK, Paulose M, LaTempa TJ and Grimes CA Vertically aligned single crystal TiO2 nanowire arrays grown directly on transparent conducting oxide coated glass: synthesis details and applications Nano Letters, 2008 8(11) 3781-86.

Abstract: Single-crystal one-dimensional (1D) semiconductor architectures are important in materials-based applications requiring a large surface area, morphological control, and superior charge transport. Titania has widespread utility in applications including photocatalysis, photochromism, photovoltaics, and gas sensors. While considerable efforts have focused on the preparation of 1D TiO2, no methods have been available to grow crystalline nanowire arrays directly onto transparent conducting oxide (TCO) substrates, greatly limiting the performance of TiO2photoelectrochemical devices. Herein, we present a straightforward low temperature method to prepare single crystal rutile TiO2 nanowire arrays up to 5 μm long on TCO glass via a non-polar solvent/hydrophilic substrate interfacial reaction under mild hydrothermal conditions. The as-prepared densely packed nanowires grow vertically oriented from the TCO glass substrate along the (110) crystal plane with a preferred (001) orientation. In a dye sensitized solar cell, N719 dye, using TiO2 nanowire arrays 2−3 μm long we achieve an AM 1.5 photoconversion efficiency of 5.02%.

24. Shankar K, Mor GK, Paulose M, Varghese OK and Grimes CA Effect of device geometry on the performance of TiO2 nanotube array-organic semiconductor double heterojunction solar cells Journal of Non-Crystalline Solids, 2008 354 2767-71. doi:10.1016/j.jnoncrysol.2007.09.070

Abstract: TiO2 nanotube arrays are used as the electron accepting network in hybrid heterojunction excitonic photovoltaic devices. The performance of the photovoltaic devices is studied using two configurations, namely frontside illumination where the illumination occurs from the cathodic side of the device and backside illumination where the illumination is from the anodic side. A blend of regioregular poly(3-hexylthiophene) and a methanofullerene (Phenyl C71-butyic acid methyl ester) is infiltrated into the nanotubes to form the devices. Non-transparent nanotube arrays fabricated on Ti foil substrates were used in backside illuminated solar cells while transparent nanotube arrays fabricated on conducting glass were used to form the frontside illuminated cells. The frontside illumination geometry was found to be superior to the backside geometry due to ease of forming uniform contacts to devices and lower photonic losses due to absorption. In addition to the P3HT–PCBM interface, the P3HT–TiO2 nanotubes interface provides an additional heterojunction for charge separation. Typical backside illuminated solid state solar cells show a short-circuit current density of 3.91 mA/cm2, 324 mV open circuit potential and a 0.43 fill factor, while typical frontside illuminated solar cells show a short-circuit current density of 12.4 mA/cm2, 641 mV open circuit potential and a 0.51 fill factor yielding power conversion efficiencies of 4.1% under AM 1.5 sun.

23. Seabold JA, Shankar K, Wilke RHT, Paulose M, Varghese OK, Grimes CA and Choi KS Photoelectrochemical properties of heterojunction CdTe/TiO2 electrodes constructed using highly ordered TiO2 nanotube arrays Chemistry of Materials, 2008 20(16) 5266-73.

22. Mor GK, Varghese OK, Wilke RHT, Sharma S, Shankar K, LaTempa TJ, Choi KS and Grimes CA p-type Cu-Ti-O nanotube arrays and their use in self-biased heterojunction photoelectrochemical diodes for hydrogen generation Nano Letters, 2008 8(7) 1906-11.

21. Fabregat-Santiago F, Barea EM, Bisquert J, Mor GK, Shankar K and Grimes CA High carrier density and capacitance in TiO2 nanotube arrays induced by electrochemical doping Journal of the American Chemical Society, 2008 130(34) 11312-316.

20. Allam NK, Shankar K and Grimes CA A general method for the formation of crystalline nanotube arrays without the use of thermal annealing Advanced Materials, 2008 20(20) 3942.

19. Mor GK, Prakasam HE, Varghese OK, Shankar K and Grimes CA Vertically oriented Ti-Fe-O nanotube array films: Toward a useful material architecture for solar spectrum water photoelectrolysis Nano Letters, 2007 7(8) 2356-64.

18. Shankar K, Mor GK, Prakasam HE, Varghese OK and Grimes CA Self-assembled hybrid polymer-TiO2 nanotube array heterojunction solar cells Langmuir, 2007 24 12445-449.

17. Mor GK, Shankar K, Paulose M, Varghese OK and Grimes CA High efficiency double
heterojunction polymer photovoltaic cells using highly ordered TiO2 nanotube arrays Applied Physics Letters, 2007 Article 152111.

16. Shankar K, Mor GK, Prakasam HE, Yoriya S, Paulose M, Varghese OK and Grimes CA Highlyordered TiO2 nanotube arrays up to 220 μm in length: use in water photoelectrolysis and dye-sensitized solar cells Nanotechnology 2007 18(6) Article no 065707. doi: 10.1088/0957-4484/18/6/065707

Abstract: The fabrication of highly-ordered TiO2 nanotube arrays up to 134 µm in length by anodization of Ti foil has recently been reported (Paulose et al 2006 J. Phys. Chem. B 110 16179). This work reports an extension of the fabrication technique to achieve TiO2 nanotube arrays up to 220 µm in length, with a length-to-outer diameter aspect ratio of ≈1400, as well as their initial application in dye-sensitized solar cells and hydrogen production by water photoelectrolysis. The highly-ordered TiO2 nanotube arrays are fabricated by potentiostatic anodization of Ti foil in fluoride ion containing baths in combination with non-aqueous organic polar electrolytes including N-methylformamide, dimethyl sulfoxide, formamide, or ethylene glycol. Depending upon the anodization voltage, the inner pore diameters of the resulting nanotube arrays range from 20 to 150 nm. As confirmed by glancing angle x-ray diffraction and HRTEM studies, the as-prepared nanotubes are amorphous but crystallize with annealing at elevated temperatures.

15. Ong KG, Varghese OK, Mor GK, Shankar K and Grimes CA Application of finite-difference time domain to dye-sensitized solar cells: The effect of nanotube-array negative electrode dimensions on light absorption Solar Energy Materials and Solar Cells 2007 91(4) 250-257.

14. Prakasam HE, Shankar K, Paulose M, Varghese OK and Grimes CA A new benchmark for TiO2 nanotube array growth by anodization Journal of Physical Chemistry C, 2007 111(20) 7235-41.

13. Shankar K, Mor GK, Fitzgerald A and Grimes CA Cation effect on the electrochemical formation of very high aspect ratio TiO2 nanotube arrays in formamide-water mixtures Journal of Physical Chemistry C, 2007 111(1) 21-26.

12. Mor GK, Shankar K, Paulose M, Varghese OK and Grimes CA Use of highly-ordered TiO2 nanotube arrays in dye-sensitized solar cells Nano Letters 2006 39 2498-2503.

11. Paulose M, Shankar K, Varghese OK, Mor GK and Grimes CA Application of highly-ordered TiO2 nanotube-arrays in heterojunction dye-sensitized solar cells J. Phys. D: Appl. Phys, 2006 39(12) 2498-2503.

10. Paulose M, Shankar K, Varghese OK, Mor GK, Hardin B and Grimes CA Backside illuminated dye-sensitized solar cells based on TiO2 nanotube electrodes Nanotechnology 17(5) 144-6-48, 2006.

9. Shankar K, Tep KC, Mor GK and Grimes CA N-doped and N,F-codoped TiO2 photoelectrodes :An electrochemical strategy to incorporate anionic dopants J. Phys. D: Appl. Phys 39 2361-2366, 2006 .

8. Anodic growth of highly ordered TiO2 nanotube arrays to 134 μm in length. Paulose M, Shankar K, Yoriya S, Prakasam HE, Varghese OK, Mor GK, LaTempa TJ, Fitzgerald A and Grimes CA, Journal of Physical Chemistry B 110 16179-16184 2006.

7. Paulose M, Mor GK, Varghese OK, Shankar K and Grimes CA, Visible light photoelectrochemical and water-photoelectrolysis properties of titania nanotube arrays, Journal of Photochemistry and Photobiology A-Chemistry, 178(1) 8-15, 2006.

6. Yoriya S, Prakasam HE, Varghese OK, Shankar K, Paulose M, Mor GK, LaTempa TJ and Grimes CA. Initial studies on the hydrogen gas sensing properties of highly-ordered high asepct ratio TiO2 nanotube-arrays 20 um to 200 um in length, Sensor Letters 4 334-339, 2006.

5. Mor GK, Shankar K, Paulose M, Varghese OK and Grimes CA, Enhanced photocleavage of water using titania nanotube-arrays Nano Letters 5 191-195, 2005.

4. Mor GK, Shankar K, Varghese OK and Grimes CA, Photoelectrochemical properties of titania nanotubes, Journal of Materials Research 19(10) 2989-96, 2005.

3. Mor GK, Varghese OK, Paulose M, Shankar K and Grimes CA, Effect of anodization bath chemistry on photochemical water splitting using titania nanotubes, 2004 Materials Research Society Symposium Proceedings Vol. 836, 29-34 2005.

2. Paulose M, Varghese OK, Mor GK, Shankar K and Grimes CA, Photoelectrochernical properties of highly-ordered titania nanotube-arrays, 2004 Materials Research Society Symposium Proceedings Vol. 837, 65-70 2005.