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Metal-based semiconductor nanomaterials for thin-film solar cells 161

Fig. 8.6 (A) SEM images of the undoped ZnO nanoparticles and (B, C) their corresponding
high-magnification images.
Reprinted with permission from D. Shao, et al., High responsivity, fast ultraviolet
photodetector fabricated from ZnO nanoparticle-graphene core-shell structures, Nanoscale 5
(2013) 3664–3667.

180°C for 20 h in an autoclave to grow ZnO NPs. In addition, organic or inorganic
additives can be added to control the morphology of ZnO NPs. Fig. 8.6 shows the
SEM images of the undoped ZnO NPs [27]. However, the solution method limits the
large-scale production of ZnO films, and to solve this problem, other methods were
exploited to grow ZnO NPs, such as a vapor deposition method (VD) and spray pyrol-
ysis technology to fabricate large-scale ZnO films.

8.2.2.2 Fabrication of ZnO NRs

Quasi-one-dimensional (quasi-1D) ZnO, a direct wide band gap (3.37 eV) semicon-
ductor with a large excitation binding energy (60 eV), superior near-UV emission,
good transparent conductivity, and piezoelectricity, is one of the most important metal-
oxide semiconductors for thin-film solar cell. For decades, various techniques, in-
cluding solution phase synthesis and gas phase synthesis, were developed to fabricate
highly ordered ZnO NRs/NWs on different substrates. The solution phase synthesis
method is widely used in the fabrication of 1D ZnO nanostructures for various elec-
tronic devices. In solution phase synthesis, growth is carried out in a liquid solution
with controlled temperature and pressure; if aqueous solutions are utilized, the process
is referred to as a hydrothermal growth method. In a typical hydrothermal process,
hexamethylenetetramine (HMT) and Zn salt are the most widely used precursors. In
the first stage, a thin layer of ZnO NPs forms on the substrate, and the HMT in the
2+
aqueous solution releases hydroxyl ions that react with Zn ions to form ZnO. The
whole process can be summarized in the following equations [28]:

+
) N +
(CH 26 4 6 H O « 6 HCHO 4 NH 3 (8.1)
2