Atomic Layer Deposition of TiO2 on Mesoporous nanoITO: Conductive

May 20, 2014 - Core–shell structures consisting of thin shells of conformal TiO2 deposited on high surface area, conductive Sn-doped In2O3 nanoparti...
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Atomic Layer Deposition of TiO2 on Mesoporous nanoITO: Conductive Core−Shell Photoanodes for Dye-Sensitized Solar Cells Leila Alibabaei,† Byron H. Farnum,† Berç Kalanyan,‡ M. Kyle Brennaman,† Mark D. Losego,‡ Gregory N. Parsons,‡ and Thomas J. Meyer*,† †

Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599, United States Department of Chemical and Biomolecular Engineering, North Carolina State University, Raleigh, North Carolina 27695, United States

‡

S Supporting Information *

ABSTRACT: Core−shell structures consisting of thin shells of conformal TiO2 deposited on high surface area, conductive Sn-doped In2O3 nanoparticle. Mesoscopic films were synthesized by atomic layer deposition and studied for application in dye-sensitized solar cells. Results obtained with the N719 dye show that short-circuit current densities, open-circuit voltages, and back electron transfer lifetimes all increased with increasing TiO2 shell thickness up to 1.8−2.4 nm and then decline as the thickness was increased further. At higher shell thicknesses, back electron transfer to −RuIII is increasingly competitive with transport to the nanoITO core resulting in decreased device efficiencies. KEYWORDS: Dye-sensitized solar cells, core−shell structure, atomic layer deposition, tin-doped indium oxide, TiO2

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with insulating ZrO2 cores. The results of these measurements, and those from electrochemical impedance spectroscopy and open-circuit voltage decay measurements, provide clear evidence for the value of the thin outer shell in facilitating rapid electron transport to the conducting nanoITO core, decreasing the effects of BET and substantially enhancing device performance.4 These results augment the earlier observation of visible light water splitting in a core−shell based dye-sensitized photoelectrosynthesis cell (DSPEC) with a surface-bound chromophore-catalyst assembly for light absorption and water oxidation.23 Core−shell films were synthesized by ALD using TiCl4 and H2O at 120 °C, which gave thin, conformal layers of TiO2 on 3.2 μm nanoITO and 3.6 μm ZrO2 films. Figure 1 presents a series of transmission electron micrograph (TEM) images of TiO2 on top ∼50 nm particles of nanoITO. The TiO2 layers were homogeneous over a range of thicknesses from 0.6−3.6 nm obtained by varying the number of ALD cycles from 10 to 60. TiO2 film thickness was also measured on flat silicon substrates as a reference and confirmed by TEM. The ALD growth rate of 0.6 Å/cycle is similar to that reported previously.16 The nanometer thickness of the TiO2 shell material precluded the use of X-ray diffraction to determine the crystal phase, but the TiO2 layers are most likely amorphous24 given the 120 °C deposition temperature with an earlier report that at least 150 °C is required to produce anatase TiO2 layers.25

ye-sensitized solar cells (DSSCs) have matured considerably with solar-to-electrical power conversion efficiencies of >12% achieved.1 In these devices light absorption by molecular dyes attached to high surface area, wide band gap semiconductors such as TiO2 is followed by electron injection, oxidation of the chromophore, and its reduction by an added redox mediator, typically I3−/I−.2−4 Output and efficiencies are controlled by a series of microscopic events, such as electron injection, electron transport through TiO2, and back electron transfer (BET). As in any solar energy conversion strategy, from Photosystem II to a PV device, overcoming recombination by BET is a key to performance and high energy conversion efficiencies. For DSSCs, multiple strategies have evolved to minimize BET, such as organic dielectric layers5,6 and chemical7−10 or atomic layer deposition11−16 of insulating oxides. Here, we describe a unique approach, which exploits a core−shell strategy with a thin shell of TiO2 used to minimize the time scale for transferring injected electrons to an underlying core of a transparent conducting oxide. The conducting oxide is tindoped indium oxide (ITO) which has a greatly enhanced conductivity compared to typical oxide semiconductors.17−22 The core−shell structures are fabricated by atomic layer deposition (ALD) with controlled growth of thin layers of TiO2 on preformed mesoscopic, nanoparticle films of ITO (nanoITO). As shown in Figure S1, the ALD TiO2 is derivatized with the surface-attached, RuII polypyridyl metal complex dye N719, [N(n-C 4 H 9 ) 4 ] 2[cis-Ru(4,4′-(CO 2− )bpy)2(NCS)2]. DSSC, current−voltage, and transient absorption (TA) characteristics of the derivatized core−shell films in acetonitrile with added I3−/I− are compared with analogous core−shells © 2014 American Chemical Society

Received: February 18, 2014 Revised: April 30, 2014 Published: May 20, 2014 3255

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Figure 1. TEM of TiO2 layers deposited by ALD on nanoITO cores: (A) 10 cycles, (B) 20 cycles, (C) 30 cycles, (D) 40 cycles, (E) 50 cycles, and (F) 60 cycles of TiO2 on nanoITO.

films of the same thickness. For both, a slight decrease in the extent of surface loading was observed as the TiO2 shell thickness was increased, Figure S2, due to the decrease in internal surface area in the cavities of the mesoscopic structures. Nanosecond TA measurements were used to investigate injection/BET dynamics. TA difference spectra in 0.1 M LiClO4 acetonitrile from 400 to 850 nm following excitation at 532 nm are shown in Figure S4. The difference spectra are consistent with excitation followed by rapid electron injection (3 nm, BET occurs 3259

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through low-energy trap states in the shell which limits performance in DSSC devices. Maximized device performance is reached for 2.4 nm TiO2 shells. For ZrO2/TiO2 with an inert core in the potential range of the excited state, injected electrons are constrained to the TiO2 shell, and device efficiencies are decreased by ∼4 providing an additional illustration of the value of the nanoITO/TiO2 core− shell structure. The use of conductive core−shell photoelectrodes provides a practical alternative for maximizing solar cell efficiencies, in this case, by controlling local dynamics to minimize BET and enhance electron transport through the mesoporous film. Improvements in device performance are being investigated based on increasing the available surface area within the nanoITO films, controlling the crystallinity of the TiO2 shell layer, and exploring other oxide materials. Enhanced microscopic performance with minimized BET is particularly important in DSPEC applications where the storage of multiple redox equivalents for H2O oxidation or CO2 or H+ reduction requires the accumulation of multiple redox equivalents at interfacial sites for catalysis.

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ASSOCIATED CONTENT

* Supporting Information S

More detailed instructions for the experimental details, Figures S1− S9, and Tables S1−S3. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

L.A. and T.J.M. conceived the concept. L.A. and B.H.F. designed the experiments. B.K. performed the ALD deposition under supervision of M.D.L. and G.N.P. L.A. fabricated DSSC and carried out most experiments. B.H.F. measured TA. L.A., B.H.F, M.K.B., and T.J.M wrote the manuscript. All authors discussed the results and commented on the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported primarily by the UNC EFRC: Center for Solar Fuels, an Energy Frontier Research Center supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under award DE-SC0001011, supporting L.A., B.H.F., and M.K.B. M.D.L. was supported by the Research Triangle Solar Fuels Institute. B.K. (G.N.P) was supported by the National Science Foundation under award no. CBET-1034374.

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