Supporting Information

Nanowire/Nanotube Array Tandem Cells for Overall Solar Neutral Water Splitting

Alireza Kargar,1 Jirapon Khamwannah,2 ChinHung Liu,2 Namseok Park,1 Deli Wang,1,2,4* Shadi A. Dayeh,1 Sungho Jin,2,3*

1Department of Electrical and Computer Engineering, University of California-San Diego, 9500 Gilman Drive, La Jolla, California 92093, United States

2Materials Science and Engineering Program, University of California-San Diego, 9500 Gilman Drive, La Jolla, California 92093, United States

3Department of Mechanical and Aerospace Engineering, University of California-San Diego, 9500 Gilman Drive, La Jolla, California 92093, United States4Qualcomm Institute (QI), University of California-San Diego9500 Gilman Drive, La Jolla, California 92093, United States

*Corresponding authors: jin@ucsd.edu, d.w.wang@ieee.org

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Figure S1. (a) Cross-sectional and (b,c) tilted view (different magnification) SEM images of bare p-Si NW array.

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Figure S2. Optical images of FeOOH, A-Fe2O3, and N-Fe2O3 NRs grown on the FTO substrates for 3 hrs.

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Figure S3. (a-e) Elemental mapping analysis of p-Si/TiO2/A-Fe2O3 csh-NW array; (a) SEM image of spot used for the elemental mapping, (b) Si map, (c) Ti map, (d) Fe map, and (e) O map. The scale bar for (b-e) is the same as that in (a).

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Figure S4. Current density under illumination versus normal hydrogen electrode (NHE) of bare p-Si NW, p-Si/TiO2/A-Fe2O3 csh-NW, and p-Si/TiO2/N-Fe2O3 csh-NW arrays measured in the neutral pH water. The dashed line shows the water reduction potential.

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Band diagram of p-Si/n-TiO2/n-Fe2O3 csh-NW heterojunction

The band gap of anatase TiO2 layer deposited by ALD was considered to be 3.2 eV [1, 2]. The considered Si and Fe2O3 band gaps were 1.11 eV and 2.1 eV, respectively. To draw/simulate the band diagram, the Si NW average radius, TiO2 thickness, and Fe2O3 NR average length were considered as 130 nm, 25 nm, and 212 nm, respectively, based on the measurements from the SEM images and ALD cycles number. The electron affinities of Si, TiO2 (anatase and ALD deposited), and Fe2O3 were considered as 4.05 eV, ~4.3 eV [1, 3], and ~4.7 eV [3], respectively. The doping concentration of p-Si is in the range of 6.7×1014−1.5×1016 cm−3 based on its resistivity of 1−20 Ωcm (was considered 1015 in the simulation), and of ALD-deposited anatase TiO2 is in the range of 1017 cm−3 [4] (was considered 1017 in the simulation). The considered doping concentration for Fe2O3 was 1019 cm−3 [5]. Figure S5 shows the approximate energy band diagram of the p-Si/n-TiO2/n-Fe2O3 csh-NW heterojunction at equilibrium condition and at dark, which was simulated using SCAPS (version 3.1.02) numerical simulation software.

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Figure S5. Approximate simulated energy band diagram of p-Si/n-TiO2/n-Fe2O3 csh-NW heterojunction at thermal equilibrium condition and at dark. Ec, Ev, Efp, and Efn are conduction band, valance band, hole quasi-Fermi level, and electron quasi-Fermi level, respectively. E fp=Efn

for the thermal equilibrium condition simulated above. Energy levels are plotted with reference to vacuum level. Three different regions including p-Si, n-TiO2, and n-Fe2O3 are labeled in the band diagram.

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Figure S6. Cross-sectional SEM images of (a) bare p-Si NWs, (b) p-Si/TiO2 cs-NWs, and (c) p-Si/TiO2 cs-NWs with N2 annealing. Insets in (b,c) show the high-magnification images. Top-view SEM images of (d) bare p-Si NWs and (e) p-Si/TiO2 cs-NWs with N2 annealing. (f-h) Low-magnification images of p-Si/TiO2 cs-NWs with N2 annealing.

As shown above, the ALD TiO2 shell uniformly covers the entire length of long Si NW cores due to gas phase nature of ALD deposition. And the coating is within the entire area of sample as can be realized by the low magnification images in (f-h). The surface of ALD TiO2 shell is not smooth as can be seen by the inset images in (b,c). The N2 annealing does not provide any significant morphological change for the cs-NWs.

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Figure S7. (a) Current density at dark and under illumination of bare p-Si NWs, p-Si/TiO2 cs-NWs, and p-Si/TiO2 cs-NWs with N2 annealing measured in the neutral pH water. (b) Incident photon-to-current efficiency (IPCE) of p-Si/TiO2 cs-NWs with N2 annealing measured in the neutral pH water at both reverse and forward biasing potentials.

As shown above, the p-Si/TiO2 cs-NW array exhibits higher current than the bare p-Si NW array, however, its photocathodic current is not that significant without any post annealing. The post annealing under N2 atmosphere improves its performance which can be due to improved conductivity of TiO2 shell and enhanced interfaces. The photocathodic performance of p-Si/TiO2

cs-NWs with N2 annealing is much lower than that of p-Si/TiO2/N-Fe2O3 csh-NWs (comparing this figure with Figure 3a in the main text) revealing the significant effect of Fe2O3 NRs. Consistent with observation in the J-V measurement in (a), the IPCE of p-Si/TiO2 cs-NWs with N2 annealing is higher at negative potential than that at positive potential. Note that the IPCE magnitude at both negative and positive potentials is small around 400-500 nm. At short wavelengths, light absorption and carrier generation mostly happen near the Si wire surface [6], thus possibly due to surface recombination coming from the long and dense Si NW cores, the IPCE is small.

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Figure S8. Incident photon-to-current efficiency (IPCE) of p-Si/TiO2/N-Fe2O3 csh-NW array measured in the neutral pH water at -0.636 V versus RHE. Note that the IPCE magnitude is small around 500 nm, consistent with that of p-Si/TiO2 cs-NWs with N2 annealing (Figure S7b), likely due to surface recombination coming from the long and dense Si NW backbones discussed in Figure S7b. The measured IPCE for sample with shorter and less dense Si NWs (Si NW array made with nanoimprinting and dry etching) showed a broader spectrum starting from around 400 nm.

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Figure S9. (a-d) SEM images (different magnification) of TiO2/TiO2 cs-NTs exhibiting the unsmooth surface of the ALD-deposited TiO2 coating.

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