Effect of synthesis time on the structural and opto-electronic properties of hydrothermally grown ZnO nanowires

Highlight

·      Vertically aligned ZnO nanowires (NWs) were synthesized and characterized

·      FESEM images show that the nanowires are distributed vertically and uniformly

·      GXRD and SAED patterns show hexagonal Wurtzite crystal structure

·      UV-Vis spectroscopy shows enhanced transmittance up to 96% at 755 nm

·      XPS reveals no reshaping of the conduction band minimum.  

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Graphical Abstract

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Abstract

Vertically aligned ZnO nanowires (NWs), grown on a ZnO seeded substrates and used as an electron transport layer (ETL) in thin absorber solar cells (TASCs), have shown promise in improving the carrier transport and overall performance of the underlying solar cell. In this study, the synthesis time was systematically increased from 2, 4, 8 to 22 hours during a facile hydrothermal process, aimed at changing the physical dimensions and distribution of ZnO NWs during growth. The NWs were grown on fluorine-doped tin oxide (FTO) coated glass substrates, with a 150 nm thick ZnO seed layer pre-deposited using spin-coating. Grazing incidence X-ray diffraction (GXRD) and selected area electron diffraction (SAED) patterns show that the synthesized nanowires have a hexagonal Wurtzite crystal structure with lattice constants, a = 0.325 and c = 0.519 nm, yielding a c/a ratio of 1.596, which closely resembles the theoretical value of 1.633 and shows that the material is highly crystalline. Field emission scanning electron          microscopy (FESEM) images show that the nanowires are distributed vertically and uniformly, with diameters and lengths corresponding to 2, 4, 8 and 22 hours are respectively 34.69 ± 4.81 nm and 524 ± 0.24 nm, 35.86 ± 4.89 nm and 631 ± 5.89 nm, 36.1 ± 3.89 nm and 745 nm ± 2.87, 37.01 ± 2.12 nm and 1.02 ± 1.1 μm. UV-Vis spectroscopy shows that the light transmittance improves to 96% at 755 nm when synthesized at 22 h, with an indirect band gap of approximately 3.30 eV. X-ray photoelectron spectroscopy (XPS) reveals no reshaping of the conduction band minimum, as oxygen atoms exclusively fill vacancies, with no substitutional replacement of lattice oxygen, ultimately leading to O-Zn-O bond formation. The improvement in light transmission with an increase in growth time, coupled with the high structural purity of the resultant nanowire arrays, make these materials potential candidates as electron transport media in different photovoltaics.

 

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