Janus Ga
2
SeTe: A Promising Candidate for Highly Efficient
Solar Cells
Jin Wang, Hui Guo, Junjun Xue,* Dunjun Chen,* Guofeng Yang, Bin Liu, Hai Lu,
Rong Zhang, and Youdou Zheng
The electronic and optical properties of Janus Ga
2
SeTe monolayer are calculated
using first-principles calculations and it is found that it has potential in solar cells.
It is found that ultrathin cross-plane pn-junctions are obtained by stacking
Ga
2
SeTe structures. The graphene-Ga
2
SeTe-graphene sandwich-structured solar
cells are configured to explore the device performance of Ga
2
SeTe solar cells. The
photocurrent and the power conversion efficiency of the Janus Ga
2
SeTe solar cells
are evaluated. The results show that multilayer Janus Ga
2
SeTe solar cells give rise
to a photocurrent exceeding that of thin-film silicon devices, indicating that
Ga
2
SeTe is a potential material that could be used in photovoltaics devices.
People have been looking for highly efficient, low-cost, reliable,
and green sources of energy to replace petroleum fuel. Solar
cells, which convert sunlight directly into electricity, have been
extensively researched and used.
[1–4]
Most of the solar cells
currently on the market are based on silicon. Over the past few
decades, many promising 2D materials have been demonstrated
for fabricating solar cells in both theory and experiment.
[5–7]
Recently, the utilization of 2D materials as photovoltaics has
attracted more attention. Thanks to their low thickness, 2D mate-
rials can shorten the migration distance of photogenerated
electrons and holes, thus effectively reducing the probability
of electron–hole recombination, which is more conducive to
the preparation of efficient solar cells.
[8]
In addition, 2D materials
are more suitable for making wearable solar cells because of their
better flexibility than traditional silicon.
Currently, solar cells based on 2D mate-
rials are usually fabricated by stacking two
p-type and n-type semiconductors which
can drive the separation of photogenerated
electron–hole pairs. Many groups have
demonstrated that 2D heterojunctions can
be used to fabricate high-efficient solar cells,
such as graphene/BN, graphene/MoS
2
,
WSe
2
/MoSe
2
, and so on.
[9–13]
The group-III
chalcogenide GaX (X ¼S, Se, Te) has
superior electronic and optical properties,
which can absorb light from visible to
far-ultraviolet wavelengths and hence is
an excellent alternative in solar energy con-
version, nanoelectronics, and optoelectronic applications.
[14–17]
More recently, it has been reported that by stacking Janus mate-
rials we can obtain cross-plane pn-junctions with high homoge-
neous doping,
[18]
which is a new opportunity for solar cells.
These have inspired us to explore the feasibility of solar cells
based on Janus group-III chalcogenide materials. Among them,
the monolayer Janus Ga
2
SeTe is a direct semiconductor with a
bandgap equal to 1.2 eV, which is suitable for solar radiation
absorption. In addition, it has been reported that the Janus
Ga
2
SeTe monolayer has both a small effective mass of
electron (Γ !K, 0.31 m
e
*; Γ !M, 0.17 m
e
*) and hole (Γ !K,
0.74 m
h
*; Γ !M, 0.46 m
h
*),
[19]
implying that the photogener-
ated carriers would transfer quickly to the electrodes after absorp-
tion of photons. Thereby, we first investigated the potential of
Janus Ga
2
SeTe as a new alternative in photovoltaic devices.
All results presented in this article are obtained with the
QuantumATK software based on density functional theory.
[20]
We use the Perdew–Burke–Ernzerhof (PBE) function of the gen-
eralized gradient approximation (GGA) as the exchange-correla-
tion potential, together with double-ζ polarized for expanding the
electronic density.
[21,22]
As the GGA-PBE usually underestimates
the bandgap, the GGA-1/2 method has been employed for a
more accurate bandgap, which corrects the density functional
theory (DFT) self-interaction error by defining an atomic self-
energy potential that cancels the electron–hole self-interaction
energy.
[23]
We use Grimme’s DFT-D3 dispersion correction
approach for van der Waals correction as it shows higher
accuracy as compared with DFT-D2.
[24]
Optimizing geometry
is performed using the limited memory Broyden–Fletcher–
Goldfarb–Shannon scheme until all interatomic forces acting
on every atom are below 0.02 eV Å
1
.
[25]
The calculated lattice
constant of Ga
2
SeTe monolayer is 3.98 Å, which is quite consis-
tent with previous literature.
[26,27]
The vacuum region is set
to 20 Å at least in the out-of-plane direction with Neumann
Dr. J. Wang, Dr. H. Guo, Prof. D. Chen, Prof. B. Liu, Prof. H. Lu,
Prof. R. Zhang, Prof. Y. Zheng
Key Laboratory of Advanced Photonic and Electronic Materials
School of Electronic Science and Engineering
Nanjing University
Nanjing 210093, China
E-mail: djchen@nju.edu.cn
Prof. J. Xue
School of Electronic Science and Engineering
Nanjing University of Posts and Telecommunications
Nanjing 210023, China
E-mail: jjxue@njupt.edu.cn
Prof. G. Yang
School of Science
Jiangnan University
Wuxi 214122, China
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/solr.201900321.
DOI: 10.1002/solr.201900321
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