Page 158 - Multifunctional Photocatalytic Materials for Energy

144 Multifunctional Photocatalytic Materials for Energy

7.4.7 Graphene in Schottky junction solar cells

Schottky junctions have been formed by employing metal and semiconductor mate-
rials; however, graphene has also been used as Schottky junctions for heterojunction-
based solar cells [132,133]. The first graphene-based heterojunction solar cell
was reported by Zhu et al, where graphene and an n-type silicon semiconduc-
tor formed the Schottky junctions and achieved a maximum PCE of 4.35%
[134]. Subsequently, considerable work has been published on graphene -n-type
silicon-based heterojunction solar cells. Graphene sheets not only act as trans-
parent electrode but also achieve electron-hole separation and hole transport. A
new graphene/silicon-based flexible solar cell with a power conversion efficiency
>10% was recently designed. This solar cell is a graphene/silicon Schottky junc-
tion whose performance has been enhanced by depositing a GRO layer on the
graphene sheet [132].
In another study, graphene doped with bis(trifluoromethanesulfonyl)-amide
[((CF 3 SO 2 ) 2 NH)] (TFSA) was used and exhibited lower sheet resistance and enhanced
performance in n-type silicon solar cells. [135] Other semiconducting nanostructures,
such as CdSe, CdS, and GaAs, have also attracted attention among researchers with the
goal of developing the Schottky junction-based solar cells. In this regard, the Schottky
junction solar cells based on HRG-CdSe have achieved a PCE of 1.25%. This junction
facilitates electron-hole separation and also exhibits a built-in potential. In addition,
heterojunction-based solar cells, HRG-CdS nanowires, and nanobelts have been re-
ported. The PCE of HRG-CdS NW and NB-based solar cells was found to be 1.65%
and 1.47%, respectively [136].

7.4.8 Graphene in organic-based solar cells
The photovoltaic effect, which is caused by p-n junctions and forms heterojunction
solar cells that convert solar energy into electrical energy, can be divided into two
types of solar cells: inorganic and organic. The organic heterojunction solar cells have
several advantages over their inorganic counterpart because of the former’s lighter
weight, flexible shape, ability to easily tailor physical properties, easy device fabrica-
tion, solution process ability, and low production cost. Generally, organic solar cells
consist of four main components, namely a transparent anode, an electron transport
layer, a hole transport layer, and a cathode. OPVs are usually fabricated with TiO 2 ,
ZnO, GO, or Cu 2 ZnSnS 4 materials as the electron collection layer and PEDOT:PSS
as the hole-collecting layer [137]. However, the power conversion efficiency of OPV
devices is much lower than that of silicon and other types of solar cells. The graph in
Fig. 7.4 illustrates organic-based solar cells.
Therefore the high absorption, high electron mobility, and the flexible proper-
ties of graphene-based materials mean that they can be effectively employed in the
development of OPV cells to enhance the performance of devices. Several groups
have investigated the use of graphene-based materials for OPVs as transparent
electrodes, electron transport layers, hole transport layers, and cathode materials
in order to increase the power efficiency of solar cells. In Table 8.2 earlier in the