Page 179 - Multifunctional Photocatalytic Materials for Energy

Metal-based semiconductor nanomaterials for thin-film solar cells 165

The low-temperature hydrothermal method is used extensively to fabricate nanoscale
SnO 2 . V 2 O 5 shows excellent catalytic properties, low production costs, and consider-
able stability, both physical and chemical method could fabricate nanoscale V 2 O 5 , many
groups have developed sol-gel method to grow V 2 O 5 NPs to further apply them into
solar cells. ZrO 2 benefits from having a high-melting-point, high electrical resistivity, a
high index of refraction, and a low coefficient of thermal expansion that could make it
a great choice for solar cells. The high heat resistance and 5 eV to 7 eV band gap make
it a good match for various solar cells. The precipitation method can quickly produce a
large quantity of nanoscale ZrO 2 NPs. ZrOCl 2 , 8H 2 O, and polyethylene glycol (PEG)
are normally used as reactants, with the addition of ammonium hydroxide to precipitate
out raw ZrO 2 , followed by annealing at high temperature to obtain pure ZrO 2 NPs. CeO 2
is a yellowish-white powder that can also be applied in solar cells. It is not water soluble,
which means that it costs more than other metal-based semiconductor nanomaterials.
However, the great stability and high melting point and boiling point makes it a good
match for many solar cell systems and can also prolong the working life of batteries.
What is more, CeO 2 can be fabricated in high temperatures by oxidizing Ce 2 O 3 [15].

8.3 Semiconductor nanomaterials as interfacial
materials for solar cells

8.3.1 Electron-transporting materials
For both OSCs and PSCs, the use of metal-based semiconductor nanomaterials as
electron-transporting materials (ETMs) requires low WFs to match the LUMO levels
of the acceptor materials, which benefits charge transportation, produces nice com-
patibility, and reduces the risk of interface defects. At the same time, to effectively
harvest solar energy, ETM layers need to be stable in order to prevent the diffusion of
metal electrodes in conventional OSCs/PSCs and to achieve the high light transpar-
ency needed for transmitting light in inverted OSCs/PSCs. Several semiconducting
metal oxides and low WF metals, including ZnO, TiO 2 , and SnO 2 , have been devel-
oped to achieve the ETMs necessary to improve a device’s performance. Fig. 8.10
shows a band energy diagram of different ETMs with respect to perovskite. Several
works have systemically surveyed the design, fabrication, and applications of various
electron-transporting materials for OSCs [12]. In this section, we discuss the optimi-
zation of ETMs needed to attain efficient PSCs.
Owing to their easy fabrication, favorable band gap, long electron lifetimes, and
tunable morphology, TiO 2 nanomaterials have been one of the most popular ETL ma-
terials used in PSCs [42–44]. Different methodologies, including sol-gel processing,
nanoparticle approaches, vacuum deposition, and ALD, have been developed to fabri-
cate TiO 2 ETM films and to balance their transmittance, electron mobility, and interfa-
cial properties, which is critical for high-performance PSCs. Kim et al. used the ALD
method to fabricate ultrathin amorphous TiO x on an inexpensive polyethylene naph-
thalate (PEN) substrate as the ETM for flexible PSCs [45]. Compared with the ETM
based on the mesoporous TiO 2 , such a thin film provided uniformity and homogeneity
in short range order, yielding 12.2% power conversion efficiencies (PCEs) with high