APPLIED PHYSICS LETTERS 98, 263509 (2011)
Enhanced infrared light harvesting of inorganic nanocrystal photovoltaic
and photodetector on graphene electrode
Chih-Cheng Lin,1 Di-Yan Wang,1 Kun-Hua Tu,1 You-Ting Jiang,2 Meng-Hsiang Hsieh,1
Chia-Chun Chen,2,3 and Chun-Wei Chen1,a)
1
Department of Materials Science and Engineering, National Taiwan University, Taipei 10617, Taiwan
Department of Chemistry, National Taiwan Normal University, Taipei 11677, Taiwan
3
Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 10617, Taiwan
2
(Received 18 April 2011; accepted 4 June 2011; published online 30 June 2011)
We demonstrate an enhancement of infrared light harvesting of inorganic PbS nanocrystal
photovoltaic and photodetector devices based on the transparent graphene electrode. Due to high
infrared transparency of the graphene electrode with respect to indium tin oxide (ITO), the infrared
photoresponse of the graphene-based device is superior to the ITO-based counterpart, in spite of a
higher sheet resistance of the graphene electrode. The outstanding infrared characteristics of the
devices based on the graphene electrode make it a promising candidate for infrared optoelectronic
C 2011 American
applications such as solar cells, imaging and sensing, or optical communication. V
Institute of Physics. [doi:10.1063/1.3605682]
Since one third of the Sun’s power reaching the Earth
lies beyond 1000 nm, the solution to harvest the incident
photons with wavelengths in the infrared (IR) spectral region
becomes crucial to further improve the photovoltaic efficiencies. In the past decade, many advances have been realized
in the synthesis of efficient infrared photoactive materials of
organic polymer1,2 or inorganic nanocrystals (NCs)3–6 in the
applications of IR solar cells or IR imaging and sensing.
Nevertheless, the use of the conventional transparent conductive electrode of indium tin oxide (ITO) is still problematic in IR photovoltaic or photodetector devices due to its
poor transparency with wavelengths beyond 1000 nm,7
which largely limits the light harvesting efficiency. In addition, the price of ITO has increased by an order of magnitude
over the last decade due to the scarcity of In. Therefore, an
alternative electrode material to replace the ITO with high
IR transparency is very important in the development of
infrared optoelectronic devices.
Graphene, a two dimensional (2D) monolayer of sp2bonded carbon atoms arranged in a honeycomb lattice has
demonstrated an excellent carrier transport arising its unique
2D energy dispersion.8 It has shown substantial advantages,
such as high transparency and excellent electrical and thermal conductivity, and superior mechanical flexibility and has
been proposed as the potential material for the next-generation large-scale multifunctional transparent electrode.9 One
of the unique properties of graphene over ITO as a transparent electrode is that it can persist very high transparency
over a wide range of wavelengths from visible to IR
regions.10 To date, many reports have been focused on the
applications of the transparent graphene electrode in the visible wavelength range while the exploration of the graphene
electrode in IR optoelectronics is still lacking. In this article,
we would like to demonstrate that the IR photoresponses of
photovoltaic or photodetector devices using the inorganic
PbS nanocrystals deposited on the graphene electrode. The
a)
Author to whom correspondence should be addressed. Electronic mail:
chunwei@ntu.edu.tw. Telephone: 886-2-3366-5205; Fax: 886-2-2363-4562.
0003-6951/2011/98(26)/263509/3/$30.00
outperforming IR photoresponse of the device based on the
graphene electrode due to its high IR transparency makes it a
promising candidate for IR optoelectronic applications.
For the fabrication of the graphene electrode, we first
synthesized the grapheme films on copper (Cu) foils following the chemical vapor deposition processes.11 Graphene
films were transferred from Cu foils to the glass substrate by
using the roll-to-roll production method reported by Bae et
al.9 Since a single layer of graphene does not have sufficiently high sheet conductivity, we have to stack multilayers
of graphene on the glass substrate to obtain the electrode
with optimal conductance and transparency. To further
improve the conductivity of graphene, p-type doping was
performed by soaking the graphene film into nitric acid (65
wt. %) solution for 3 min and drying it with nitrogen flow.9
The sheet resistances of the graphene films were found to
decrease with increasing number of layers (one to eight
layers), as shown in Fig. 1(a). The thin films consisting of
one, two, and three layers of graphene deposited on the glass
substrate were also shown in the inset of Fig. 1(a). The optical transmittance of the graphene electrodes with different
stacked layers were shown in Fig. 1(b) and high transparency
of graphene from 400 nm to 1800 nm can be clearly
observed. The transmittance is reduced about 2–3% as each
layer of graphene was added.12 In addition, the surface
roughness of the graphene electrodes were also found to
increase with increasing number of transferred graphene
layers (not shown), which may deteriorate the device performance. Upon optimization of sheet resistance, transmittance, and surface roughness of the graphene electrodes, we
used the three-layer graphene, which has sheet resistance of
165 X/sq. and a transmittance of 80% at 500 nm and of 85%
at 1500 nm for the following fabrication of IR photovoltaic
and photodetector devices. For the referenced ITO electrode
with a sheet resistance of 15 X/sq., the transmittance is about
87% at 500 nm while it decreased significantly with wavelengths beyond 1000 nm, showing a transmittance of 42% at
1500 nm. Although the sheet resistance of the graphene electrode is still higher than that of ITO, the excellent infrared
98, 263509-1
C 2011 American Institute of Physics
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263509-2
Lin et al.
FIG. 1. (Color online) (a) Sheet resistances as a function of number of graphene layers and (b) optical transmittance for ITO and graphene electrodes
composed of different stacked layers. (c) AFM and (d) SKPM images for
three layers of graphene films on the silicon substrate. The arrow indicates a
folded edge during the roll-to-roll production.
transparency of the graphene electrode provides the opportunity to enhance light harvesting of IR optoelectronic devices.
The atomic force microscope (AFM) topological image of
the three-layer graphene film was shown in Fig. 1(c) and the
corresponding workfunction mapping image examined by
the scanning Kelvin probe microscope (SKPM) was also
shown in Fig. 1(d), with an average workfunction value of
about 4.86 eV, close to that of a typical ITO electrode.13
For the fabrication of IR photovoltaic and photodetector
devices, we used the low band-gap PbS quantum dots as the
photoactive layer material. The solution-processed solar cells
consisting of lead chalcogenide (PbX) quantum dots have
recently demonstrated an excellent infrared photovoltaic performance with unique advantages of tunable absorption characteristics by the quantum confinement effect14 and enabling
the large-area low-cost fabrications.15–17 The device consisting of a Schottky-junction structure of Al/PbS/PEDOT:PSS/
graphene electrode is shown schematically in Fig. 2(a). The
Appl. Phys. Lett. 98, 263509 (2011)
FIG. 3. (Color online) (a) The J-V characteristics (AM 1.5, 100 mW/cm2)
(inset) and EQEs of PbS solar cells fabricated on the ITO and graphene electrodes. The solid line exhibits the absorption spectrum of PbS NCs. The photovoltaic performances of the devices under monochromatic illumination (b)
500 nm (2.519 mW/cm2) and (c) 1500 nm (0.368 mW/cm2).
thickness of the PbS NC active layer is about 100 nm. The
PEDOT:PSS layer was incorporated into the NCs solar cell
to reduce the interfacial roughness between the anodic electrode and active layer.18 For comparison, the device using
the ITO electrode was also fabricated as the referenced cell.
Since the as-synthesized PbS quantum dots were usually
capped with insulating oleic acid ligands, we performed the
ligand exchange treatment by using the 1,2-ethanedithiol to
remove the oleic acid ligand.19,20 Fig. 2(b) shows the transmission electron microscopy (TEM) image of uniformly distributed PbS quantum dots with a size of about 6 nm in
diameter and the high resolution TEM (HRTEM) image
(inset) revealed that the d-spacing value was 0.296 nm corresponding to the (200) plane of the rock salt structure of
PbS. Next, the cyclic voltammetry measurement21,22 was
performed to obtain the lowest unoccupied molecular orbital
level of PbS nanocrystals as shown in Fig. 2(c). The corresponding highest occupied molecular orbital level of PbS
NCs was further obtained by subtracting the measured band
gap of 0.86 eV, resulting from a well-defined excitonic
absorption peak centered at 1530 nm as shown in the inset of
Fig. 2(c). The corresponding energy levels of the respective
layers of the device with a structure of Al/PbS/PEDOT:PSS/
graphene electrode were thus summarized in Fig. 2(d), where
the graphene electrode acts as an anode to collect holes and
the cathode of Al is for collecting electrons.
TABLE I. Photovoltaic performances of PbS solar cells fabricated on the
different anodic electrode under various illuminations.
FIG. 2. (Color online) (a) The device structure of Al/PbS/PEDOT:PSS/graphene electrode. (b) TEM and HRTEM (inset) images of the PbS NCs. (c)
Cyclic voltammetry graph and absorption spectrum (inset) of PbS NCs. (d)
The corresponding energy levels of the device.
ITO electrode (AM 1.5)
ITO electrode (500 nm)
ITO electrode (1500 nm)
Graphene electrode (AM 1.5)
Graphene electrode (500 nm)
Graphene electrode (1500 nm)
Jsc(mA/cm2)
Voc(V)
FF(%)
PCE(%)
8.76
0.48
0.05
6.65
0.25
0.09
0.29
0.21
0.11
0.30
0.20
0.11
40
48
40
31
45
42
1.02
2.28
0.55
0.61
0.97
1.20
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263509-3
Lin et al.
FIG. 4. (Color online) (a) Infrared responsivity R of the devices using ITO
and graphene electrodes at the applied bias of 0, 0.2 and 1.0 V. (b) The
photocurrent density of the devices as a function of light power at 1530 nm
under zero bias.
The current density-voltage (J-V) characteristics and
external quantum efficiencies (EQEs) of the PbS Schottkyjunction solar cells based on the graphene and ITO electrodes were plotted in Fig. 3(a). Although the ITO-based control device exhibits a higher power conversion efficiency
(PCE) of 1.02% than 0.61% of the graphene-based device
under simulated AM 1.5 (100 mW/cm2) illumination (inset),
the two devices demonstrate different EQE spectral
responses at different ranges of incident wavelengths. As the
illuminated wavelengths were shorter than 1060 nm, the
ITO-based PbS solar cell has higher EQE values than the
graphene-based device, mainly attributed to a lower sheet resistance of ITO. By contrast, the graphene-based PbS solar
cell shows larger EQE values at wavelengths beyond 1060
nm due to the high IR transparency of graphene, in spite of
its higher sheet resistance. The result can be further supported from the measured current density-voltage characteristics of these devices under monochromatic illumination at
wavelengths of 500 and 1500 nm, respectively. For the devices under visible light illumination of 500 nm (incident
power 2.519 mW/cm2), the ITO-based PbS solar cell yields a
better power conversion efficiency of 2.28% compared to
0.97% of the graphene-based PbS solar cells [Fig. 3(b)].
While the devices were illuminated at wavelength of 1500
nm (0.368 mW/cm2), the graphene-based PbS solar cell
exhibits a PCE of 1.20%, which is higher than the 0.55% of
the ITO-based solar cell as shown in Fig. 3(c). Table I summarizes the photovoltaic performances of the PbS solar cells
fabricated on the different anodic electrodes. The outperforming IR photovoltaic response of the graphene-based device further suggests its potential application in IR
photodetection or sensing. Fig. 4(a) shows the responsivity R
deduced from the EQE spectra between 1100 and 1800 nm
of the graphene-based and ITO-based PbS photodiodes operated at different reverse bias conditions. The responsivity R
of the graphene-based and ITO-based PbS devices are 0.112
and 0.076 A/W at k ¼ 1530 nm and increase to 0.69 and 0.50
Appl. Phys. Lett. 98, 263509 (2011)
A/W at bias of 1 V, respectively. The photocurrent
response of the devices under IR illumination (k ¼ 1530 nm)
were found to increase linearly with light power as shown in
Fig. 4(b). The high IR transparency of the graphene electrode
thus makes it an excellent candidate in the IR optoelectronic
device applications. Optimization in the conductivity of the
graphene electrode by chemical doping, defect reduction,
etc., will lead to a further improvement of the device
performance.
This work is supported by National Science Council,
Taiwan (Project Nos. NSC 98-2119-M-002-020- and 992119-M-002-012).
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