Monolayer graphene film/silicon nanowire array Schottky junction solar cells

Article abstract of DOI:10.1063/1.3643473

Schottky junction solar cells were constructed by combining the monolayer graphene (MLG) films and the Si nanowire (SiNW) arrays. Pronounced photovoltaic characteristics were investigated for devices with both p-MLG/n-SiNWs and n-MLG/p-SiNWs structures. Due to the balance between light absorption and surface carrier recombination, devices made of SiNW arrays with a medium length showed better performance and could be further improved by enhancing the MLG conductivity via appropriate surface treatment or doping. Eventually, a photoconversion efficiency up to 2.15% is obtained by the means of filling the interspace of SiNW array with graphene suspension.

Full text of DOI:10.1063/1.3643473

Monolayer graphene film/silicon nanowire array Schottky junction solar
cells
Chao Xie, Peng Lv, Biao Nie, Jiansheng Jie, Xiwei Zhang et al.
Citation: Appl. Phys. Lett. 99, 133113 (2011); doi: 10.1063/1.3643473
View online: http://dx.doi.org/10.1063/1.3643473
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APPLIED PHYSICS LETTERS 99, 133113 (2011)
Monolayer graphene film/silicon nanowire array Schottky junction solar cells
Chao Xie,1,2 Peng Lv,1 Biao Nie,1 Jiansheng Jie,1,a) Xiwei Zhang,1 Zhi Wang,1 Peng Jiang,1
Zhizhong Hu,1 Linbao Luo,1,b) Zhifeng Zhu,1 Li Wang,1 and Chunyan Wu1
1School of Electronic Science and Applied Physics, Hefei University of Technology, Hefei Anhui 230009,
People’s Republic of China
2School of Materials Science and Engineering, Hefei University of Technology, Hefei Anhui 230009,
People’s Republic of China
(Received 29 June 2011; accepted 31 August 2011; published online 28 September 2011)
Schottky junction solar cells were constructed by combining the monolayer graphene (MLG) films
and the Si nanowire (SiNW) arrays. Pronounced photovoltaic characteristics were investigated for
devices with both p-MLG/n-SiNWs and n-MLG/p-SiNWs structures. Due to the balance between
light absorption and surface carrier recombination, devices made of SiNW arrays with a medium
length showed better performance and could be further improved by enhancing the MLG
conductivity via appropriate surface treatment or doping. Eventually, a photoconversion efficiency
up to 2.15% is obtained by the means of filling the interspace of SiNW array with graphene
C
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suspension. 2011 American Institute of Physics. [doi:10.1063/1.3643473]
Silicon is the leading material in commercial solar cell
modules, accounting for about 90% of the global photovol-
taic market. Compared with their thin film counterparts, sili-
con nanowires (SiNWs) have exhibited unique properties in
terms of light absorption and electron transport.1,2 By this to-
ken, SiNW-based solar cells have lately received increasing
interest for their potential in the next-generation solar energy
conversion devices.3 Peng et al. reported on the photovoltaic
application of platinum nanoparticles decorated SiNW arrays
synthesized by a HF etching method.4 What is more, hybrid
solar cells consisting of SiNW arrays and P3HT:PCBM with
efficiency of 1.9% were also reported.5
ous device configurations were investigated. A maximum ef-
ficiency of 2.15% was achieved by optimizing the device
structures and further filling the interspace of the SiNW array
with graphene suspension.
The vertical SiNW arrays were prepared via an Ag-
assisted chemical etching method.9 Both n- and p-type Si
wafers (resistivity: 5–15 Xcmꢀ1) were used. After etching,
the as-obtained SiNW arrays were dipped into a diluted
HNO3 and HF solution to dissolve Ag and SiO2 on the
SiNWs surface, respectively. The MLG films were prepared
at 1000 ꢁC by using a mixed gas of CH4 (40 SCCM) and H2
(20 SCCM) via chemical vapor deposition (CVD),10 and
25 lm thick Cu foil was employed as the catalytic substrate.
Nitrogen-doped MLG films were synthesized at the same
conditions except the introduction of NH3 as the dopant.11
After growth, the MLG films were spin-coated with 5 wt. %
The selection of appropriate electrode materials is of
vital importance to the device performance. Generally, high
photoconversion efficiency necessitates transparent electrodes
for solar light transmission and good electrical conduction for
carriers transport. This is contradictory for most of the thin
film transparent electrodes available, as their thickness is nor-
mally proportionate to electrical conduction while inversely
proportionate to transparency. The discovery of highly trans-
parent graphene thin film with carrier mobility as high as
200 000 cm2/Vs provides a solution to this challenge. Li et al.
reported the few-layer graphene (FLG)/Si substrate Schottky
junction solar cells with power conversion efficiency of
2.2%.6 Fan et al. also demonstrated the FLG/SiNW solar cells
with efficiency of 2.86%, as a consequence of enhanced light
trapping and faster carrier transport.7 In spite of these pro-
gresses, the utilization of monolayer graphene (MLG), which
possess higher transparency leading to more light absorption
in photovoltaic devices and inherits most of the striking prop-
erties of graphene, as building block for SiNW-based solar
cells remains less explored and understood.8
In this work, we extensively studied the photovoltaic
characteristics of the solar cells composed of MLG films and
SiNW arrays, and totally eight types of solar cells with vari-
FIG. 1. (Color online) (a) Schematic illustration of the fabrication process
for the MLG film/SiNW array Schottky junction solar cell. (b) Reflection
spectra of SiNW arrays obtained from 10, 20, and 40 min etching. Inset
shows cross-sectional SEM image of SiNW array with PMMA-supported
MLG film on it. (c) Raman spectrum of the MLG film. Inset shows the SEM
image of the isolated MLG film on SiO2/Si substrate.
a)Author to whom correspondence should be addressed. Electronic mail:
jason.jsjie@gmail.com.
b)Electronic mail: luolb@hfut.edu.cn.
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99, 133113-1
2011 American Institute of Physics
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133113-2
Xie et al.
Appl. Phys. Lett. 99, 133113 (2011)
Further Raman spectrum in Fig. 1(c) is found to be com-
posed of two sharp peaks, i.e., 2D-band peak at ꢂ2695 cmꢀ1
and G-band peak at ꢂ1581 cmꢀ1. The intensity ratio of I2D
:
IG ꢃ 2.2, combined with the weak D-band scattering at
ꢂ1343 cmꢀ1, confirms the high crystal quality of the mono-
layer graphene film.12
First, different Schottky junction solar cells composed
of the MLG, generally known to exhibit week p-type con-
ductivity,12 and n-SiNWs were evaluated [Figs. 2(a) and
2(b), and devices A1-A4 in Table I]. By virtue of the forma-
tion of Schottky barrier, excellent rectification characteristics
are observed [Fig. 2(a)]. Moreover, these devices exhibit
pronounced photovoltaic effects upon white light illumina-
tion. As shown in Fig. 2(b), the device made of n-Si wafer
(A0) shows the worst performance, with a power conversion
efficiency (˛) of 0.04%. In contrast, substantial increase of ˛
to 0.10% is observed, when silicon wafer was replaced with
SiNW array etched for 10 min (A1). The conversion effi-
ciency continues to increase to a maximum value of 0.40%
for device made of 20 min etched SiNW array (A2), near ten
times of the initial value for Si wafer. Nevertheless, further
prolongation of the etching time (40 min, A3) results in the
degeneration of the device performance. Interestingly,
enhancement of p-type conductivity of the MLG via HNO3
treatment can considerably increase the ˛ value to 0.95%.13
Remarkably, good rectification and photovoltaic effects
are observed in the n-MLG/p-SiNW Schottky junctions (B1
and B2) as well [Figs. 2(c) and 2(d)]. It can be found that the
device performance increases with the N doping level, which
could be attributed to the enhanced n-type conductivity of
the MLG film at higher N doping concentration. However,
we note that the overall performance of B1 and B2 is still
worse than A2 and A4. The lower sheet conductivity of the
N-doped graphene compared to pristine graphene might be
an important reason.11,14
FIG. 2. (Color online) Dark I-V curve of device A4 (a), device B2 (c). Inset
in (a), (c), and (e) shows the schematic illustration of p-MLG/n-SiNW
Schottky junction, n-MLG/p-SiNW solar cells, and graphene suspension
filled solar cell, respectively. J-V curves of p-MLG/n-SiNW solar cells for
devices A0, A1, A2, A3, and A4 (b), n-MLG/p-SiNW solar cells for devices
B1 and B2 (d), device C1 (f).
polymethylmethacrylate (PMMA) in chlorobenzene, and the
underlying Cu foils were removed in Marble’s reagent solu-
tion (CuSO4:HCl:H2O ¼ 10 g:50 ml:50 ml). To construct the
solar cells, the PMMA-supported MLG films were directly
transferred onto the top of the SiNW arrays. For insulating
purpose, sellotape was stuck at the four edges of the mother
Si substrates before the preparation of SiNW arrays. Ag
paste contacts formed at the corners of the MLG films and
the back side of the Si substrates acted as the top and back
electrodes, respectively. The step-wise process for the device
construction was illustrated in Figure 1(a). The photovoltaic
characteristics of the MLG/SiNW solar cells were evaluated
by Keithley 4200 SCS under the white light illumination
(350 lWcmꢀ2).
To understand the work mechanism of the MLG/SiNW
Schottky junction solar cells, photoresponse characteristics
of both p-MLG/n-SiNW and n-MLG/p-SiNW devices were
investigated [Figs. 3(a) and 3(b)]. The observed high sensi-
tivity to the light illumination with a large Ion/Ioff ratio >103,
along with the fast response speed, signifies that the
electron-hole pairs could be efficiently generated and sepa-
rated in the MLG/SiNW solar cells, which helps facilitate
the Schottky junctions to harvest solar light more effectively.
On the other hand, such photoresponse behavior is highly re-
versible and reproducible, implying that our devices can also
function as high-performance photodetectors that could
Figure 1(b) depicts the reflection spectra of SiNW arrays
synthesized via 10, 20, and 40 min etching, in which, light
absorption was observed to increase with length increase.
TABLE I. Device performances of MLG film/SiNW array Schottky junction solar cells.
Sample
V
OC(V)
J
SC(lA/cm2)
FF
g(%)
A0
A1
A2
A3
A4
B1
B2
C1
n-Si pristine MLG
0.25
0.15
0.24
0.15
0.35
0.13
0.16
0.19
3.0
10.3
22.3
5.3
0.19
0.24
0.26
0.24
0.23
0.19
0.23
0.25
0.04
0.10
0.40
0.05
0.95
0.02
0.27
2.15
10 min n-SiNWs pristine MLG
20 min n-SiNWs pristine MLG
40 min n-SiNWs pristine MLG
20 min n-SiNWs p-MLG (HNO3 treated)
20 min p-SiNWs n-MLG (10SCCM NH3)
20 min p-SiNWs n-MLG (20SCCM NH3)
20 min n-SiNWs pristine MLG (grapheme Suspension filled)
41.2
2.8
25.3
154.5
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133113-3
Xie et al.
Appl. Phys. Lett. 99, 133113 (2011)
Undeniably, the photoconversion efficiency of the pho-
tovoltaic devices is relatively low. We attribute this to the
small Schottky junction area as well as the long distance the
photogenerated carriers have to travel, which are pre-
determined by the device configurations. In this connection,
graphene suspension that compose of small graphene sheets
(0.5–5 lm) was placed at the interspace of the SiNW array
before the transfer of MLG film [Fig. 2(e)], in order to facili-
tate carrier separation and transport by enlarging the contact
area. Indeed, C1 exhibits a much improved short-circuit cur-
rent density (Jsc) of 154.5 lA cmꢀ2 [Fig. 2(f)], nearly seven
times of that of A2. Accordingly, a large ˛ value of 2.15% is
achieved for this device. It should be mentioned that the fill
factors (FF) of our MLG/SiNW devices (0.19-0.26) are still
much lower than the commercial Si solar cells (ꢂ0.80). The
load resistance comes from the MLG film and the electrode
contact might be responsible for the low value.
FIG. 3. (Color online) Photoresponse of devices A4 (a) and B2 (b) at zero
bias. (c) and (d) are energy diagrams of p-MLG/n-SiNW and n-MLG/p-
SiNW Schottky junctions upon light illumination, respectively. UG/USi and
EFG/EFSi denote the work functions and Fermi energy levels of graphene/
SiNW. vSi is the electron affinity of silicon. EC and EV are the conduction
band and valence band of silicon, respectively.
In conclusion, Schottky junction solar cells composed of
MLG films and SiNW arrays were fabricated. Devices with
both p-MLG/n-SiNW and n-MLG/p-SiNW structures were
investigated and they show pronounced photovoltaic effects
under the light illumination. Optimization to the devices
leads to a photoconversion efficiency of 2.15%. The simple
and low-cost fabrication process of the MLG/SiNW Schottky
junctions makes it a promising candidate for future high-
performance solar cell applications.
operate at zero bias voltage. Besides the fast rise edge, a
slow rise edge is observed for device B2. Doping induced
defects in the graphene is likely responsible for this result
since the trap centers have to be filled upon light illumination
before the system can finally reach equilibrium.14
Figs. 3(c) and 3(d) show the energy band diagrams of
p-MLG/n-SiNW and n-MLG/p-SiNW, respectively. As a
result of the formation of Schottky barrier at the MLG/SiNW
interface, partial carriers in SiNWs tend to move to the gra-
phene side and consequently the energy levels near the
SiNW surface will bend upward (for n-SiNW) or downward
(for p-SiNW), causing the formation of space-charge region
and built-in electric field near the MLG/SiNW interface.
Upon light illumination, the photogenerated electron-hole
pairs will be separated within the built-in field region, and
the resulted free electrons and holes will move towards
opposite directions, which results in the generation of the
photocurrent. This model suggests that the MLG films serve
not only as transparent electrodes but also as important
active layers in the devices.
This work was supported by the National Natural Sci-
ence Foundation of China (Nos. 60806028, 51172151,
20901021, 61106010, 21101051), the Program for New Cen-
tury Excellent Talents in University of the Chinese Ministry
of Education (NCET-08-0764), the Major Research Plan of
the National Natural Science Foundation of China (No.
91027021), and National Basic Research Program of China
(No. 2012CB932400).
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