Environmental Engineering Reference
In-Depth Information
other. A thin ZnO i lm at the base of the nanowires is deposited to prevent
hole recombination, while α-NPD between PbSe QDs and Au top contact
is used to block PbSe-to-Au electron transfer. Aydil and coworkers have
shown that signii cant improvement in photocurrent and power conver-
sion can be achieved by increasing nanowire length [190].
Recently carbon nanoparticles or graphene quantum dots have also been
applied to sensitize the ZnO nanowire, and demonstrated cell functioning
[10]. Li-doped ZnO nanowire has shown improved short-circuit current
density (Jsc) and open-circuit voltage (Voc) [191]. Reduction in radiative
trap sites on the ZnO surface is attributed to more ei cient charge extrac-
tion and improves the ei ciency. Presence of Zn(OH) 2 on the surfaces of
nanorods and its decomposition products also govern the photovoltaic
performance [192]. Defects in ZnO strongly af ect the transport properties;
hence they will alter the solar cell performance. Nanowire i lms have all
the desired properties such as high electron transport, larger open surface
area and single-crystalline structure for electron transport with minimum
grain boundaries for highly ei cient devices. Yet, the best solar cell perfor-
mance for applying nanostructures of ZnO in DSC currently reach around
7.2% ei ciency and 2.5% for vertically-aligned nanowire. h erefore, fur-
ther investigation is required in this direction, and defect engineering of
ZnO nanowire i lms might be one important path to improve the quality
and ei ciency of PV devices [10, 192-194].
16.6.2
Water Splitting/Solar Hydrogen Generation
Hydrogen is a clean, energy ei cient and abundant fuel, which can be an
excellent energy source in the future; it has a very high energy value per
gram mass (122 kJ/g). Among the various hydrogen generation methods,
water splitting is a very economical and clean process to produce hydrogen
[195]. h e decomposition of water into hydrogen and oxygen is a chemi-
cal reaction with large Gibbs free energy (ΔG= 237 kJ/mol). Water split-
ting by photocatalysis can be achieved via two dif erent approach, either
by photochemical cell or photoelectrochemical (PEC) cell. A suspended
particle solution of photocatalyst is used in the i rst method; one can
appreciate the simplicity of the process, which uses the high reactive sur-
face of catalyst [195]. However, the PEC method has several other advan-
tages; for example, internal bias can be used to enhance the photocatalystic
activities. Multiple material i lms can utilize and increase the absorption
of visible light [196]. Oxidation and reduction processes occur simultane-
ously and can perform separately in photoelectrochemical cell; they can
separately store oxygen and hydrogen, which is a relatively safer process.
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