Supporting Information. Solar Cells and the Improvement on Open-Circuit. Voltage

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1 Supporting Information Zn 2 SnO 4 Nanowires as Photoanode for Dye Sensitized Solar Cells and the Improvement on Open-Circuit Voltage Jiajun Chen, Liyou Lu, and Wenyong Wang* Department of Physics and Astronomy, University of Wyoming, P. O. Box 3905, Laramie WY 82071, USA wwang5@uwyo.edu 1. Dependence of nanowire phases on synthesis conditions Figure S1(a) shows XRD data of Zn 2 SnO 4 nanowires synthesized with different oxygen flow rates. At low oxygen flow rate such as 1 sccm, oxygen was deficient and the Sn phase was found in the product. At high oxygen flow rates (5 sccm or 8 sccm), the SnO 2 phase was formed. Figure S1(b) shows the effect of oxygen injection temperature. It had a small influence on the phases of the final products; however, it affected the amount of nanowires synthesized in the process. With a low injection temperature, the source material was oxidized before they could evaporate, which lowered the fabrication yield. Figure S2 shows the energy dispersive x-ray spectroscopy (EDS) analysis of the tip and body of a Zn 2 SnO 4 nanowire. Particles that consist of Au, Zn, Sn, and O elements were found on the tips of the 1

2 nanowires, which suggested a VLS growth mechanism. However, the sizes of the catalyst particles were larger than the diameters of the nanowires, indicating a high interfacial tension between the catalyst and nanowire during the growth. S1,S2 The formation of Zn 2 SnO 4 nanowires periodic structures could be attributed to a competition between the nanowire surface energy and the catalyst-nanowire interfacial energy during the growth process, and a detailed study is needed to further clarify these mechanisms. S3 2. Stability of Zn 2 SnO 4 nanowires in dye solution A comparison study of nanowire stability in dye solution was carried out in this research. Specifically, DSSCs with Zn 2 SnO 4 and ZnO nanowires were fabricated using the same 0.05 mm N719 dye solution. Figure S3 shows the characterization results of DSSCs fabricated with Zn 2 SnO 4 and ZnO nanowires. For Zn 2 SnO 4 nanowire-dsscs, the device performance has improved after the dye loading time becomes longer as discussed previously. For ZnO nanowire-dsscs, however, severe device degradation was observed under same fabrication and characterization conditions. The short-circuit current density of the cell dropped from 5.53 to 0.61 ma/cm 2 after a 24-hour dye loading, and the efficiency also decreased significantly. Further SEM examination revealed etching of the ZnO nanowires by the dye solution. Compared to the as-grown ZnO nanowires, after soaking in the dye solution the nanowires were shortened and the hexagonal facets disappeared. In fact, ZnO were dissolved in the acidic dye solution and formed Zn 2+ -dye complex, which deteriorated the cell performance. S4,S5 3. Quasi-Fermi level of Zn 2 SnO 4 nanowire-photoanodes The quasi-fermi level (V F ) in a photoanode under illumination can be expressed by the following equation using the redox potential of the electrolyte (V red ) and V OC : S6 V V V, F red OC where V red can be calculated using the Nernst equation: S6,S7 2

3 V RT I ln. 2F I 0 3 red V 3 V 0 is the formal potential and can be assumed to be V for I / 3 I redox electrolyte.s6 Using these two equations we could estimate the quasi-fermi level in the Zn 2 SnO 4 nanowire photoanode, and its dependence on the illumination intensity is shown in Figure S4. The obtained quasi-fermi level values were close to those of TiO 2 nanoparticle photoanode reported previously. S6 4. Performance of Zn 2 SnO 4 nanoparticle-dsscs Figure S5(a) shows the XRD data of Zn 2 SnO 4 nanoparticles synthesized in this research. The inset is a SEM image of the Zn 2 SnO 4 nanoparticles. Figure S5(b) shows the J-V curves of the Zn 2 SnO 4 nanoparticle-dssc obtained under different illumination intensities. The nanoparticle film thickness was 20 m and the dye loading time was 24 hours. 5. Oxygen plasma treatment of Zn 2 SnO 4 nanowires and its effects on dye loading Previous studies suggested that oxygen plasma treatment could increase dye loading on photoanodes based on TiO 2 nanoparticles. S8,S9 In order to study the possibility of improving dye loading on Zn 2 SnO 4 nanowires, we also carried out oxygen plasma treatment in this work. The O 2 plasma treatment on Zn 2 SnO 4 nanowires was performed in an Oxford Instrument PlamsaLab 80Plus Reactive Ion Etcher (RIE). The RF power, pressure, and O 2 flow rate were set to be 100W, 50 Torr, and 50 sccm, respectively. The treatment time was 2 to 20 min. Figure S6 shows the N719 dye absorbance data before and after the treatment, and as a function of the treatment time. The dye loading time was 24 hr. It is clear from Figure S6 that the N719 dye loading on Zn 2 SnO 4 nanowire photoanodes was increased due to the O 2 plasma surface treatment. However, the corresponding cell performance including J SC, V OC, FF, and degraded afterwards (Figure S6). Similar results have been reported previously, and the decreased DSSC performance after prolonged O 2 plasma treatment was suggested to be caused by increased 3

4 oxygen vacancies that enhanced the recombination process. S9 However, the exact mechanism of this effect is still unclear, and future study is needed to fully understand this issue. 4

5 Figure S1. (a) XRD data of nanowires synthesized with different oxygen flow rates. Star symbols indicate the diffraction peaks of Sn. (N 2 flow rate: 100 sccm; O 2 injection temperature: 800 o C; source material ratio: Zn:Sn = 3:1) (b) XRD data of nanowires grown with different oxygen injection temperatures. (N 2 flow rate: 100 sccm; O 2 flow rate: 3 sccm; source material ratio: Zn:Sn = 3:1) 5

6 Figure S2. EDS analysis of the tip and body of a Zn 2 SnO 4 nanowire. Au was detected in the particle attached on the tip of the nanowire, suggesting a VLS growth mechanism. 6

7 Figure S3. Comparison study of DSSCs based on Zn 2 SnO 4 and ZnO nanowires. The performance of Zn 2 SnO 4 cells were improved after 24-hr dye loading, while the ZnO cells degraded significantly. The insets are SEM images of ZnO nanowires before (a) and after (b) dye loading, revealing the etching of ZnO nanowires in the acidic dye solution. 7

8 Figure S4. Dependence of the quasi-fermi level in Zn 2 SnO 4 nanowire photoanode on light intensity. 8

9 Figure S5. (a) XRD data of Zn 2 SnO 4 nanoparticles. Inset shows the SEM image of the synthesized Zn 2 SnO 4 nanoparticles. (b) J-V curves of the Zn 2 SnO 4 nanoparticle-dssc under different illumination intensities. 9

10 Absorbance (arb. u.) V OC (V) J SC (ma/cm 2 ) FF (arb.u.) (%) Plasma Time (min) Figure S6. The effects of the oxygen plasma treatment on dye loading and DSSC performance. REFERENCES (S1) Wang, Z. W.; Li, Z. Y. Nano Lett. 2009, 9, (S2) Schmidt, V.; Senz, S.; Gösele, U. Nano Lett. 2005, 5, (S3) Shim, H. W.; Zhang, Y.; Huang, H. J. Appl. Phys. 2008, 104, (S4) Keis, K.; Lindgren, J.; Lindquist, S.-E.; Hagfeldt, A. Langmuir 2000, 16, (S5) Law, M.; Greene, L. E.; Radenovic, A.; Kuykendall, T.; Liphardt, J.; Yang, P. J. Phys. Chem. B 2006, 110, (S6) Zhang, C.; Chen, S.; Tian, H.; Huang, Y.; Huo, Z.; Dai, S.; Kong, F.; Sui, Y. J. Phys. Chem. C 2011, 115, (S7) Boschloo, G.; Hagfeldt, A. Acc. Chem. Res. 2009, 42, (S8) Wang, J.; Lin, Z. Chem. Mater. 2009, 22, (S9) Liu, R.; Yang, W.-D.; Qiang, L.-S. J. Power Sources 2012, 199,