Supplementary Information

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1 Supplementary Information Dye Stabilization and Enhanced Photo-electrode Wettability in Water-based Dye-Sensitized Solar Cells through Post-Assembly, Atomic Layer Deposition of TiO 2 Ho-Jin Son, Chaiya Prasittichai,,+ Joseph E. Mondloch, Langli Luo, Jinsong Wu, Dong Wook Kim, Omar K. Farha,, * and Joseph T. Hupp,, * Department of Chemistry and Argonne-Northwestern Solar Energy Research (ANSER) Center, Northwestern University, Evanston, IL 60208, United States Department of Materials Science and Engineering, NUANCE center, Northwestern University, Evanston, IL 60208, United States Argonne National Laboratory, 9700 South Cass Ave., Argonne, IL 60439, United States Table of Content Section S1. Detailed description for experimental procedures Figure S1. The thickness of TiO 2 ALD layer determined by ellipsometry on flat Si(100) substrate Figure S2. QCM mass gain profiles acquired over 18 cycles of TiO 2 ALD for QCM(Au)/TiO 2 ALD and QCM(Au)/TiO 2 ALD/OrgD Figure S3. Steady-state absorption spectra of TiO 2 :OrgD with increasing the ALD process cycle Figure S4. Photo images for TiO 2 :OrgD films in THF and EtOH; TiO 2 :OrgD films taken out from THF and EtOH; TiO 2 :OrgD and TiO 2 :OrgD/10 TiO 2 ALD (10 cycles of post TiO 2 ALD) films Figure S5. UV-vis absorption spectra of OrgD in EtOH (black line) and basic EtOH (red line) Figure S6. Cyclic Voltammograms of TiO 2 :OrgD and TiO 2 :OrgD/8 TiO 2 ALD film in dichloromethane Figure S7. TEM images of control film (TiO 2 :OrgD) and post-treated (TiO 2 :OrgD/55 numbers of Al 2 O 3 ALD cycles) Figure S8. HR-TEM images of TiO 2 :OrgD/55 Al 2 O 3 ALD (55 numbers of ALD cycle) Figure S9. HR-TEM images of control film (TiO 2 :OrgD) and post-treated film (TiO 2 :OrgD/10 TiO 2 ALD) Figure S10. SEM surface morphological images showing growth of particle size with different ALD cycles Figure S11. UV-visible spectra of basic ethanol (receiver solution) containing dye molecules Figure S12. Dark and illuminated (1 sun) J V curves for TiO 2 :OrgD (control) and TiO 2 :OrgD/8 TiO 2 ALD (post-treated) cell fabricated with water-based electrolyte Figure S13. Dark and illuminated (1 sun) J V curves for TiO 2 :OrgD (control) and TiO 2 :OrgD/8 TiO 2 ALD (post-treated) cell fabricated with acetonitrile-based electrolyte Figure S14. TiO 2 :OrgD (control) and TiO 2 :OrgD/8 TiO 2 ALD (post-treated) devices were aged at 60 C in the dark Figure S15. UV-visible spectra of TiO 2 :N719, TiO 2 :N719/8 TiO 2 ALD, and TiO 2 :N719/16 TiO 2 ALD after exposure in water for 0-24 h Figure S16. Relative A t /A 0 absorbance of TiO 2 :N719, TiO 2 :N719/8 TiO 2 ALD, and TiO 2 :N719/16 TiO 2 ALD at 500 nm after soaking in water Figure S17. A comparison of photo image and J V curve of TiO 2 :N719 and TiO 2 :N719/8 TiO 2 ALD cell Figure S18. A comparison of UV-vis spectra of TiO 2 :Z907 and TiO 2 :Z907/8 TiO 2 ALD films before and after 12 days of soaking in water Figure S19. Dark and illuminated (1 sun) extended J V curves for TiO 2 :Z907 (control) and TiO 2 :Z907/8 TiO 2 ALD (posttreated) cell fabricated with aqueous electrolyte Table S1. Photovoltaic performance of N719 and Z907-based DSCs in water-based electrolyte References S1

2 Section S1. Detailed description of experimental procedures Preparation of OrgD-containing photoanodes and post-ald treatment: The organic dye (OrgD) was synthesized and purified as published. S1 Fluorine-doped tin oxide (FTO) coated glass plates (Hartford Glass, TEC 15) were immersed in 40 mm TiCl 4 (aqueous) at 70 C for 30 min. and washed with water and ethanol. A transparent TiO 2 layer on the FTO glass plate was prepared by doctor blade printing commercially available TiO 2 paste (avg. 20 nm, Dyesol, DSL-90-T) and then dried for 2 h at 25 C. The TiO 2 electrodes were gradually heated under flowing air at 325 C for 15 min, at 375 C for 15 min, at 450 C for 15 min, and at 500 C for 15 min. The resulting layer was composed of 6 μm thickness of transparent layer. The active section of the photoelectrode was then geometrically defined by using a razor blade to remove material that would fall outside of the intended illumination area. This step eliminates artifacts (overly large photocurrents) due to light leakage onto portions of the electrode that are not intentionally illuminated. The photoelectrodes were treated again with TiCl 4 at 70 C for 30 min and sintered at 500 C for 30 min. Dye adsorption was accomplished by soaking the TiO 2 films in a 0.3 mm suspension solution (THF/EtOH = 1/4) of OrgD. After 12 h, the films were soaked in clean THF for 1 hour to remove the aggregated/unanchored dye molecules, washed with copious amount of the above solvent, and dried with a nitrogen stream. TiO 2 ALD post-coating was performed with a Savannah 100 instrument (Cambridge Nanotech Inc.), using titanium tetraisopropoxide (TTIP) and distilled water as a titanium and an oxygen source, respectively. TiO 2 was grown at 110 C using reactant exposure times of 8 and 5 s for TTIP and water, respectively, and nitrogen purge times of 30 s between exposures. The growth rate of TiO 2 on flat platforms, as determined by ellipsometry, was 0.96 Å/cycle (see Figure S1); we assume that the growth rate on the TiO 2 nanoparticles is identical with ellipsometry result. Once the ALD deposition was completed, the partially-completed solar cells were cooled in dried nitrogen to near room temperature before their removal from the deposition chamber. The treated samples were kept in vacuum after the TiO 2 ALD deposition until the remainder of the cell construction was completed by adding the electrolyte and the back contact. This handling procedure protected the dye from contamination and/or oxidation by the ambient air. In addition, to clarify the presence of post-ald layer onto TiO 2 photoanodes, Al 2 O 3 ALD post-coating was performed onto TiO 2 :OrgD film using dimethylaluminum isopropoxide (DMAI) and distilled water as a aluminum and an oxygen source, respectively. Al 2 O 3 was grown at 150 C using reactant exposure times of 8 and 5 s for DMAI and water, respectively, nitrogen purge times of 25 s between exposures. The growth rate of Al 2 O 3 on TiO 2 nanoparticles, as determined by HR-TEM, was ~1 Å/cycle (see Figure S8). For Al 2 O 3 post-ald, we made intentionally the Al 2 O 3 layer thicker (more than 50 Å) to demonstrate a distinct difference between post-ald layer and TiO 2 nanoparticles. S2

3 Assembly of photovoltaic cells: Cathode/anode pairs were combined in each case with a 60-μm-thick Surlyn polymer film (Surlyn-1702, Dupont) that served as a spacer and defined the perimeter of a photoelectrochemical sealing. The components were permanently combined by placing them on a hotplate at 90 C and allowing the spacer to melt slightly. Tin-coated copper wires were connected to each electrode with silver epoxy. ~30 μl of the electrolyte-containing solution was drop-casted onto the drilled hole on the back side of the platinized cathode and then vacuum-loaded into the cell. After residual electrolyte around the hole was removed, the hole was sealed by melting a sheet of Surlyn polymer film that was inserted between the backside of the FTO and a micro cover glass slide. Finally, a photo-mask (black tape) as an aperture was applied on top of the active area of each cell. For discussions, see the reports by G.-W. Lee, et al. S2 and S. Ito, et al. S3 Preparation of dye-sensitized TiO 2 film for UV-vis spectroscopic and electrochemical measurement: The samples for UV-vis measurement consist of mesoporous thin films (~3 μm) of TiO 2 nanoparticles. A transparent TiO 2 layer on the ITO or glass plate was prepared by doctor blade printing commercially available TiO 2 paste (avg. 20 nm, Dyesol, DSL-90-T) and then dried for 2 h at 25 C. The TiO 2 electrodes were gradually heated under flowing air at 325 C for 15 min, at 375 C for 15 min, at 450 C for 15 min, and at 500 C for 15 min. The resulting slides were sensitized by submerging the film in a 0.3 mm solution of OrgD, N719, or Z907 for 2 h. The sensitized films were rinsed with THF or CH 3 CN and dried under a stream of N 2 gas. Electrochemical Measurements: Cyclic voltammetry experiments were carried out with a CHI900 instrument (CH Instruments, Inc.) using three electrode cell assemblies. OrgD-sensitized TiO 2 film and Pt wire were used for working and counter electrode, respectively. An Ag/AgNO 3 couple was employed as a reference electrode. Measurements were carried out in nitrogen-saturated CH 2 Cl 2 (10 ml) with tetran-butylammoniumhexafluorophosphate (Bu 4 NPF 6 ) as a supporting electrolyte (0.1 M) at a scan rate of 100 mv/s. Quartz Crystal Microbalance study of TiO 2 Atomic Layer Deposition: Quartz crystal microbalance (QCM) studies were performed on a custom modified Savanah S200 ALD retrofitted with an integral QCM following the work of Riha et al.. S4 The QCM allows for in situ monitoring of mass gain (ng/cm 2 ), and hence ALD growth rate or film thickness. Polished 14 mm gold-coated RC crystals (ColnaTec) were placed into the QCM and coated with 200 cycles Al 2 O 3 (~200 nm) and then 200 cycles of TiO 2 (~10 nm). The Al 2 O 3 was grown at 200 C using alternating pulses of trimethyl aluminum and distilled H 2 O (0.015 s pulses and 8 s N 2 purges) and ensures that the QCM crystals are evenly coated for TiO 2 deposition. A base layer of TiO 2 was grown at 120 C using alternating pulses of titanium(iv) tetraisopropoxide (TTIP) S3

4 and H 2 O (0.1 s TTIP pulse, s H 2 O pulse, 60 s purge). Finally TiO 2 was deposited at 110 C using a 1 s TTIP pulse (7 s N 2 purge) and 0.1 s H 2 O pulse (7.9 s purge) and the mass gain was found to be 67.8 ng/cm 2 over the first eight ALD cycles (i.e., without OrgD, Figure S2). The coated crystals were then removed from the QCM instrument, loaded with OrgD for 12 h and subsequently placed back into the QCM instrument. The same TiO 2 sequence at 110 C was utilized and over the first eight ALD cycles the mass gain was found to be 33.3 ng/cm 2 (i.e., with OrgD, Figure S2). Instrumentation: UV-Vis spectra of samples were recorded on a Varian Cary 5000 UV-VIS-NIR spectrophotometer. The photocurrent-density-to-applied-voltage (J V) curves and IPCE curves of photovoltaic cells were obtained using a home-made set-up which consists of xenon lamp, an AM 1.5 light filter, and a CHI 1202 Electrochemical Analyzer (CHI instruments). The power of filtered light was calibrated with an optical power meter (OPM) and adjusted to 100 mw/cm 2. TEM images were taken from Hitachi H-8100 TEM and HRTEM images were taken from Hitachi HT-7770 and JEOL 2100F S/TEM. SEM images were obtained on a Hitachi SU8030 Field Emission SEM. S4

5 Figure S1. The thickness of TiO 2 layer determined by ellipsometry on flat Si(100) substrate; the deposition rate at this temperature (110 o C) is 0.96 Å/cycle. S5

6 Figure S2. QCM mass gain profiles acquired over 18 cycles of TiO 2 ALD. QCM(Au)/TiO 2 ALD (black) and QCM(Au)/TiO 2 ALD/OrgD (red). The gradual increase of TiO 2 ALD mass over time onto QCM(Au)/TiO 2 ALD/OrgD (OrgD-sensitized QCM crystal) supports that the post TiO 2 ALD layer indeed does grow on the dye-coated electrodes, although the increasing rate of TiO 2 mass on OrgDsensitized QCM crystal is reduced by half compared to QCM(Au)/TiO 2 ALD (without OrgD) due to a blocking of nucleation side by the presence of the sensitized OrgD molecules. QCM(Au)/TiO 2 ALD/OrgD films were prepared by dye loading for 12 h, after 200 cycles of TiO 2 (~10 nm) ALD on the QCM (Au) crystal surface for efficient dye adsorption. S6

7 Figure S3. Steady-state absorption spectra of TiO 2 :OrgD with increasing the ALD process cycle. The thickness of each film was 6 μm. S7

8 Figure S4. Photos of TiO 2 :OrgD films in THF (left, a) and EtOH (right, a); TiO 2 :OrgD films taken out from THF (left, b) and EtOH (left, b); TiO 2 :OrgD (left, c) and TiO 2 :OrgD/10 TiO 2 ALD (10 cycles of post TiO 2 ALD) films (right, c). These data show that the color of TiO 2 :OrgD film is changed slightly as the polarity of the immersion solvent used. Films exposed ethanol (ε = 24.5) are darker (deep red) than those exposed to THF (ε = 7.6; orange) before the complete evaporation of the solvent remaining after washing (see panel b). Enshrouding the dye with TiO 2 (ε = 83~100) via ALD induces a further deepening of the red color, consistent with the spectral shifts shown in Figure S3. S8

9 Figure S5. UV-vis absorption spectra of OrgD in EtOH (black line) and basic EtOH (red line), and photos of OrgD in EtOH (left) and basic EtOH (right). S9

10 Bare TiO 2 on FTO substrate TiO 2 :OrgD film TiO 2 :OrgD/8 TiO 2 ALD film 100 µa Potential (V vs NHE) Figure S6. Cyclic voltammograms of TiO 2 :OrgD and TiO 2 :OrgD/8 TiO 2 ALD films in CH 2 Cl 2 solution containing 0.1 M Bu 4 NBF 4 with a scan rate (v) of 0.1 V s 1. Measurement conditions are described in Section S1. S10

11 Figure S7. TEM images of (a) control (TiO 2 :OrgD, left) and (b) post-treated (TiO 2 :OrgD/55 numbers of Al 2 O 3 ALD cycles, right). In this measurement, Al 2 O 3 are post-coated onto sensitized TiO 2 NP film through ALD to show the distinct difference between post-ald layer and TiO 2 NP. Section S1 contains a detailed description for post-al 2 O 3 layer growth. S11

12 Figure S8. HR-TEM images of TiO 2 :OrgD/55 Al 2 O 3 ALD (55 deposition cycles). S12

13 Figure S9. HR-TEM images of (a) control (TiO 2 :OrgD, left) and (b) post-treated (TiO 2 :OrgD/10 TiO 2 ALD, right). S13

14 Figure S10. SEM surface morphological images showing growth of particle size with ALD cycles: (a) TiO 2 :OrgD (control), (b) TiO 2 :OrgD/30 TiO 2 ALD (30 numbers of TiO 2 ALD cycles), (c) TiO 2 :OrgD/20 Al 2 O 3 ALD (20 numbers of Al 2 O 3 ALD cycles), and (d) TiO 2 :OrgD/55 Al 2 O 3 ALD (55 numbers of Al 2 O 3 ALD cycles). S14

15 Figure S11. UV-visible spectra of basic ethanol (receiver solution) containing dye molecules detached from TiO 2 :OrgD (0 cycle) and TiO 2 :OrgD/8 TiO 2 ALD films. Photo image of the receiver solutions after 24 h soaking (inset). S15

16 Figure S12. a) Dark and illuminated (1 sun) J V curves for TiO 2 :OrgD (control: black) and TiO 2 :OrgD/8 TiO 2 ALD (post-treated: red) cell fabricated with water-based electrolyte. (b), (c), and (d) show the IPCE plots, current transient data, and the charge lifetimes from open-circuit photovoltage decays of the control and post-treated cells, respectively. S16

17 Figure S13. a) the IPCE plots for TiO 2 :OrgD (control: black) and TiO 2 :OrgD/8 TiO 2 ALD (post-treated: red) cell fabricated with acetonitrile-based electrolyte and b) the charge lifetimes from open-circuit photovoltage decays of the control and post-treated cells, respectively. S17

18 Figure S14. TiO 2 :OrgD (control) and TiO 2 :OrgD/8 TiO 2 ALD (post-treated) devices were aged at 60 C in the dark. (a), (b), and (c) show the long-term stabilities of the efficiency, J SC, and V OC, respectively. S18

19 Figure S15. UV-visible spectra of TiO 2 :N719 (control) (a), TiO 2 :N719/8 TiO 2 ALD (b), and TiO 2 :N719/16 TiO 2 ALD (c) after exposure in water for 0 24 h. The thickness of each film was 3 μm. S19

20 Figure S16. Relative A t /A 0 absorbance of TiO 2 :N719 (black), TiO 2 :N719/8 TiO 2 ALD (red), and TiO 2 :N719/16 TiO 2 ALD (green) at 500 nm after soaking in water. S20

21 Figure S17. A comparison of photo image of TiO 2 :N719 (upper left, A) and TiO 2 :N719/8 TiO 2 ALD (down left, A) cell in 5 minutes after aqueous electolyte is added to the sandwich cell. J V curve of TiO 2 :N719 (black, B) and TiO 2 :N719/8 TiO 2 ALD (red, B) cell. All cells were measured within 5 minutes after the water electrolyte is added. The lower J SC and V OC of TiO 2 :N719 cell compared to the post-treated cell shows that the desorption of N719 molecules is relatively significant in control cell. S21

22 Figure S18. A comparison of UV-vis spectra of TiO 2 :Z907 (black) and TiO 2 :Z907/8 TiO 2 ALD (red) films before and after 12 days of soaking in water. S22

23 Figure S19. a) Dark and illuminated (1 sun) extended J V curves for TiO 2 :Z907 (control: black) and TiO 2 :Z907/8 TiO 2 ALD (post-treated: red) cell fabricated with aqueous electrolyte. Evolution of (b) current density (ma/cm 2 ) of control and post-treated cell. These devices were aged at 60 C in the dark. S23

24 TABLE S1. Photovoltaic performance of N719 and Z907-based DSCs in water-based electrolyte.* Sample J sc [ma/cm 2 ] V oc [V] FF η [%] TiO 2 :N TiO 2 :N719/8 TiO 2 ALD TiO 2 :Z TiO 2 :Z907/8 TiO 2 ALD *All cells were fabricated with water-based electrolyte and measured within 5 minutes of cell assembly. S24

25 Reference S1. Justin Thomas, K. R.; Hsu, Y.-C.; Lin, J. T.; Lee, K.-M.; Ho, K.- C.; Lai, C.-H.; Cheng, Y.-M.; Chou, P.-T. Chem. Mater. 2008, 20, S2. Lee, G.-W.; Kim, D.; Ko, M. J.; Kim, K.; Park, N.-G. Solar Energy 2010, 84, 418. S3. Ito, S.; Nazeeruddin, M. K.; Liska, P.; Comte, P.; Charvet, R.; Péchy, P.; Jirousek, M.; Kay, A.; Zakeeruddin, S. M.; Grätzel, M. Prog. Photovoltaics 2006, 14, 589. S4. Riha, S. C.; Libera, J. A.; Elam, J. W.; Martinson, A. B. Rev. Sci. Instrum. 2012, 83, S25