Page 150 - Multifunctional Photocatalytic Materials for Energy

136 Multifunctional Photocatalytic Materials for Energy

the dye and electrolyte to facilitate electron injection and dye regeneration, (ii) fast
transport of electrons, and (iii) slow recombination of electrons to facilitate charge
collection. When a photon is absorbed by the dye, the photoexcited electrons can be
injected into the conduction band of a semiconductor, and then it diffuses into the
current collector (FTO) [9,76]. In addition, electrons in the conduction band of the
semiconductor or from the current collector may recombine with the oxidized dye or
electron acceptor species in the electrolyte solution, resulting in a short circuit at the
FTO/electrolyte interface after which a loss of electrons occurs, which reduces the de-
vice’s performance and stability. Thus introducing the barrier layer at the interface of
FTO/electrolytes could prevent the loss of electrons and thereby improve the device.
TiO 2 coatings with thicknesses ranging from 20 to 100 nm are introduced primarily as
a blocking layer between the FTO and the electrolyte [77,78].
In addition, it has been reported that introduction of graphene-TiO 2 composites are
found to increase the performance of the devices. Use of GRO-TiO 2 nanocomposites as
an interfacial layer between the FTO and the nanocrystalline TiO 2 layer with a thickness
of 500 nm was first reported by Kim et al. [79]. This interfacial layer acted as a barrier
that reduced the back contact electrons from the FTO to the electrolyte. Therefore open
circuit voltages increased with the introduction of GRO-TiO 2 nanocomposites, which
greatly enhanced the photoconversion efficiency (PCE) from 4.89% to 5.26% as com-
pared to those without the graphene sheets. This photoanode is the most significant com-
ponent of any solar cell device and must possess suitable band structure, a wide band
gap, corrosion resistance, and ease of structural manipulation for optimal applications.
In another approach, graphene can be incorporated into TiO 2 as composites to improve
the photovoltaic performance by increasing the charge collection efficiency. In DSSCs, the
higher charge collection efficiency is achieved through reduction of the TiO 2 charge recom-
bination, which increases the electron diffusion length. Therefore introducing graphene in
TiO 2 nanomaterials effectively increases the electron transport and reduces the charge re-
combination, which leads to better device performances. The following literature indicates
that the incorporation of graphene into TiO 2 can greatly improve device performances. The
first study reports the addition of 0.6 wt.% HRG into the TiO 2 particle film [62]. A maximum
PCE of 7% was achieved using graphene materials, because of its reduction of the recom-
bination rate in the devices. Subsequently, many studies have been published on achieving
enhanced device performances. Table 7.1 gives the DSSC performances using graphene ma-
terials in semiconductor layers. Graphene materials have been incorporated into semiconduc-
tor films in a variety of ways [62]. For instance, Al 2 O 3 -coated HRG was mixed with TiO 2 to
inhibit sheet aggregation[62]. In another study, thermally reduced GRO was used to prepare
3+
3+
a TRGO–YF4:Er /Yb composite that was incorporated into TiO 2 film and indicated a
gradual increase in the anodic current [62]. With an optimum content of graphene (1 wt.%)
doped into TiO 2 film, the PCE showed a 15% improvement from 5.98% to 6.86%.

7.4.3 Graphene as sensitizer in DSSC
In DSSCs, one of the most important components is the photo sensitizer, which is
anchored to the semiconducting material surface. A suitable sensitizer should ab-
sorb light over the whole visible region and even part of the near-infrared region.