Conductive Upconversion Er,Yb-FTO Nanoparticle Coating To Replace Pt as a Low-Cost and High-Performance Counter Electrode for Dye-Sensitized Solar Cells

Abstract

F-doped SnO2 (FTO) nanocrystals modified by Er and Yb with upconversion capability and excellent catalytic property have been designed and fabricated as an economic replacement for Pt for counter electrode (CE) in dye sensitized solar cells. The cost of UC-FTO counter electrode is only about 1/20th of Pt. The upconverted luminescence-mediated energy transfer and the superior catalytic property for I3-/I- circulation overpowered the slight degradation caused by increased CE/electrolyte interface resistance. A 23.9% enhancement of photo-current was achieved with little degradation in photo-voltage, resulting in a 9.12% increase in solar to electric power conversion efficiency. NIR light-to-electricity has been directly observed by SPS and IPCE characterizations, showing the effect of upconversion counter electrode.

Full text

Conductive Upconversion Er,Yb-FTO Nanoparticle Coating To
Replace Pt as a Low-Cost and High-Performance Counter Electrode
for Dye-Sensitized Solar Cells
Liang Li,
†
Yulin Yang,*
,†
Ruiqing Fan,*
,†
Shuo Chen,
†
Ping Wang,
†
Bin Yang,
‡
and Wenwu Cao*
,‡,§
†
Department of Chemistry, Harbin Institute of Technology, Harbin, 150001, People’s Republic of China
‡
Condensed Matter Science and Technology Institute, Harbin Institute of Technology, Harbin, 150080, People’s Republic of China
§
Materials Research Institute, The Pennsylvania State University, University Park, Pennsylvania 16802, United States
*
SSupporting Information
ABSTRACT: F-doped SnO2(FTO) nanocrystals modified by Er and Yb
with upconversion capability and excellent catalytic properties have been
designed and fabricated as an economic replacement for Pt for use as the
counter electrode (CE) in dye-sensitized solar cells. The cost of the UC-FTO
counter electrode is only ∼1/20th of that for Pt. The upconverted
luminescence-mediated energy transfer and the superior catalytic property
for I3
−/I−circulation overpowered the slight degradation caused by increased
CE/electrolyte interface resistance. A 23.9% enhancement in photocurrent
was achieved with little degradation in photovoltage, resulting in a 9.12%
increase in solar-to-electric power conversion efficiency. Near-infrared (NIR)
light-to-electricity has been directly observed by SPS and IPCE character-
izations, showing the effect of the upconversion counter electrode.
KEYWORDS: dye-sensitized solar cells, counter electrode, Pt-free, upconversion, conductive, low cost
■INTRODUCTION
Dye-sensitized solar cells (DSSCs) have brought a revolution
for photoelectrochemical solar cells, because of their environ-
mental friendliness, simple production processes, low cost, and
decent energy conversion efficiency.
1−3
A typical DSSC
consists of a dye-sensitized semiconductor electrode, a redox
electrolyte, and a counter electrode (CE). The CEs in DSSCs
are usually made of platinum, which is one of the world’s
scarcest noble metals, because of its excellent electrocatalytic
activity, high electrical conductivity, and chemical stability.
4,5
Because platinum is the most expensive component in DSSCs,
6
it is highly imperative to develop replacement low-cost CE
materials that can function as well as platinum does. Here, we
report a new CE material, UC-FTO, whose cost is only 1/20th
of Pt and more importantly, the DSSC fabricated using this
new material achieved an overall solar-to-electric power
conversion efficiency of 7.30%, which is even better than the
6.69% for the DSSC using platinum as the CE (pt-CE).
Numerous attempts have been made in the past to exploit
competent substitutes for Pt in order to reduce the overall cost,
while not reduce the overall performance of DSSCs. For
example, carbonaceous materials
7−12
and conductive poly-
mers
13,14
have been proposed for use as the CE to replace Pt in
DSSCs, but their catalytic activities and stability are less
satisfactory. Carbon materials with special size or morphology
have been widely studied considering their high electric
conductivity, corrosion resistance toward I2, high reactivity for
tri-iodide reduction, and low cost.
15
However, low-cost carbon
is worse in power conversion efficiency, compared to Pt-CE.
Hard carbon spherule CE for DSSC produced a 5.7% overall
power conversion efficiency (PCE), which is slightly less than
the 6.5% PCE observed for Pt-CE used for DSSC.
16
Other
materials, such as carbon nanotubes
9,17
and graphene,
18−20
were reported to perform much worse than Pt in terms of
photovoltaic performance of DSSCs. Although some improve-
ment were made using conductive polymers (e.g., polyaniline,
21
polypyrrole
22
), the increased fabrication cost, low thermal
stability, and short lifetime make them less attractive.
Gratzel et al. found that cobalt sulfide has excellent catalytic
activity for the iodine-based redox couple,
23
certain inorganic
compounds have recently been studied as CE materials for
DSSCs, including sulfides,
24−27
nitrides,
4
carbides,
28,29
and
oxides.
30,31
Fluorine-doped tin oxide (FTO), which is a low-
cost oxide, is an excellent conductive material. SnO2, which is
the base material of FTO, is a good catalyst with excellent
chemical stability. Thus, modified FTO possesses the potential
to be a low-cost and high-efficiency CE material for DSSCs.
Photons of lower energy, such as near-infrared (NIR) light,
cannot be absorbed by the DSSC; hence, they do not
contribute to the electrical output. Although many novel dyes
Received: February 16, 2014
Accepted: May 8, 2014
Published: May 8, 2014
Research Article
www.acsami.org
© 2014 American Chemical Society 8223 dx.doi.org/10.1021/am5009776 |ACS Appl. Mater. Interfaces 2014, 6, 8223−8229

with wide absorption spectra have been reported in the
literature,
32,33
the high cost, complicated production process,
and low yield limited their usage potential in DSSC. A useful
method for reducing the transmission loss of lower-energy
photons (especially NIR light) is through upconversion (UC),
which converts lower-energy photons to higher-energy photons
that are in the absorption spectrum of the DSSC, so that the
efficiency of solar cells can be increased.
34
Here, we report a
successful fabrication of a novel FTO with very low cost (1/20th
of the cost of chloroplatinic acid of equal quantity) that has
excellent conductivity, decent catalytic properties, and UC
capability.
■EXPERIMENTAL SECTION
Preparation of UC-FTO. A sample of 15 g of SnCl2·2H2O with
0.188 g of Er(NO3)3and 1.718 g of Yb(NO3)3as dopant was dissolved
into 50 mL of distilled water. Commercial ammonia then was added
into the above solution dropwise, adjusting the pH to 7. The dropping
rate must be well-controlled for chemical homogeneity. The product
(mainly Sn(OH)2) was washed with distilled water and centrifuged.
Aqueous hydrofluoric acid (∼1%) was dropped to the above mixture,
and a sol−gel mixture of Sn(OH)2and SnF2was obtained. The
mixture was transferred into a Teflon-lined stainless-steel vessel, and
heated at 200 °C for 24 h after sealing. Then, it was further heated in a
muffle in nitrogen at 400 °C for half an hour and natural-cooled to
room temperature to produce Er, Yb-FTO nanoscale powder. Slurry of
UC-FTO was prepared by mixing UC-FTO and ethyl cellulose in
terpinol. Counter electrodes (CEs) were obtained by screen printing
on FTO glasses and calcined at 450 °C.
Characterization. Powder X-ray diffraction (XRD) patterns were
recorded in the 2θrange of 10°−70°using Cu Kαradiation by
Shimadzu Model XRD-6000 X-ray diffractometer. Transmission
electron microscopy (TEM) micrographs were taken using FeiTecnia
G2-STWIN equipment. The X-ray photoelectron spectroscopy (XPS)
result was obtained from a Thermo ESCALAB-250 spectrometer with
an Al Kαsource, under an ultrahigh vacuum (UHV) at 3.5 ×10−7Pa.
Luminescence spectra and fluorescence lifetimes were measured by the
Edinburgh Model FLSP920 combined steady-state fluorescence and
phosphorescence lifetime spectrometer, using a 980-nm laser as the
excitation source. The SPS instrument was assembled by Jilin
University, monochromatic light was obtained by passing light from
a 500-W xenon lamp (Beijing Changtuo Co., Model CHF-XQ500W,
China) through a double-prism monochromator (Zolix, Model
SBP300, China), and the signals were collected by a Stanford Model
SR830 DSP lock-in amplifier. IPCE was measured on a Newport
EQE/IPCE spectral response system. Electrochemical impedance
spectra were recorded using an electrochemical analyzer (Model
CHI660D) from Beijing Chenhua Co., China. The sample was
sandwiched between two FTO glass electrodes. Optically transparent
electrodes were made from an FTO-coated glass plate purchased from
Acros Organics, Belgium. The photoanode films were immersed in 0.3
mM N719 (Solaronix SA, Switzerland) in absolute ethyl alcohol for 24
h at room temperature. The electrolyte composed of 0.05 M I2, 0.5 M
LiI, and 0.1 M TBP in 1:1 (volume ratio) acetonitrile−propylene
carbonate was admitted by capillary action. Photocurrent−photo-
voltage curves were recorded using a CH Instruments, Model
CHI660D electrochemical analyzer. The light intensity of AM1.5G
global sunlight from a filtered 500-W xenon lamp (Newport, Model
CHF-XM500, Changtuo, China with an AM1.5G global filter) was
calibrated using a standard Si solar cell (calibrated at National Institute
of Metrology, PRC). All characterizations were carried out under
ambient pressure and temperature.
Based on the photocurrent density−photovoltage (J−V) curve, the
fill factor (FF) is defined as
=
×
×
JV
JV
FF max max
SC OC (1)
where Jmax and Vmax are the photocurrent density and photovoltage for
maximum power output, JSC is the short-circuit photocurrent density,
and VOC is the open-circuit photovoltage. The overall power
conversion efficiency (PCE) is defined as
=
××JV
P
P
CE FF
SC OC
in (2)
where Pin is the power of incident light, which, in this work, is 100 mW
cm−2.
■RESULTS AND DISCUSSIONS
The morphology of UC-FTO particles was investigated by
TEM, as shown in Figure 1a. The particle size distribution is
∼7−8 nm and the as-prepared nanoparticles are single domain
crystallites (Figure 1b). The interplanar spacing of d101 derived
from the lattice fringes are 2.6 Å, which is in good agreement
with the theoretical values (d101 = 2.64 Å). The crystal phase
was confirmed through XRD pattern (see Figure S1 in the
Supporting Information). To investigate the composition of
UC-FTO nanoparticles, XPS, EDS, and element distribution
mapping were carried out (see Figures S2 and S3 in the
Supporting Information for details).
Figure 2a shows the luminescence emission spectra of UC-
FTO nanoparticles under 980-nm laser excitation with different
pump powers. The photoluminescence spectrum exhibits three
Figure 1. TEM images of UC-FTO nanoparticles with lattice fringes.
Figure 2. UC luminesce spectra of UC-FTO with different pump
power ranging from 0.40 W to 1.00 W under 980-nm excitation.
(Magnifying spectra from 300 nm to 600 nm are shown as the inset.
Peaks centered at 522, 545, and 660 nm correspond to Er3+:2H11/2 →
4I15/2,4S3/2 →4I15/2, and 4F9/2 →4I15/2 transitions, and the peak at 660
nm is the strongest peak. Peaks centered at 408, 455, and 488 nm
correspond to Er3+:2H9/2 →4I15/2,4F5/2 →4I15/2, and 4F7/2 →4I15/2
transitions, respectively.)
ACS Applied Materials & Interfaces Research Article
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distinct Er3+ emission bands, weak in green and much stronger
in red. The peaks centered at 522, 545, and 660 nm correspond
to Er3+:2H11/2 →4I15/2,4S3/2 →4I15/2, and 4F9/2 →4I15/2
transitions, respectively.
35,36
Taking the peak at 660 nm as an
example, the intensity increases with the laser power, following
the relationship of IUC ∝Pn,
37,38
where nis the pump photons
required to excite rare-earth (RE) ions from the ground state to
the emitting excited state. The nvalues for 660 and 545 nm are
2.02 and 2.20, respectively; both nvalues correspond to the
two-photon process (confirmed by Figure S4 in the Supporting
Information). Figure 2b shows the enlarged luminescence
emission spectra of UC-FTO from 400 nm to 600 nm. The
peaks centered at 408, 455, and 488 nm correspond to
Er3+:2H9/2 →4I15/2,4F5/2 →4I15/2, and 4F7/2 →4I15/2 transitions,
respectively. The possible mechanism of the UC process is
discussed in the Supporting Information (see Figure S5).
The catalytic properties and reaction kinetics of the UC-FTO
and Pt electrodes were investigated simultaneously by cyclic
voltammetric (CV) measurements, and the results are plotted
in Figure 3a. Similar to the Pt electrode,
23
for the UC-FTO
electrode, there exist two anodic current peaks (Ipa1:3I
−=I
3
−+
2e−, and Ipa2:2I
3
−=3I
2+2e
−) and two cathodic current peaks
(Ipc1:I
3
−+2e
−=3I
−, and Ipc2:3I
2+2e
−=2I
3
−).
39−41
Here, we
focus on peaks Ipa1 and Ipc1, since the CE is responsible for the
reduction of I3
−in DSSCs. The result indicated that the
electrolyte of I−/I3
−can circulate on either surface of the Pt or
UC-FTO electrodes. The peak-to-peak separation (Epp) and
peak current density, which is negatively correlated with the
standard electrochemical rate constant of a redox reaction, are
two critical parameters used to quantify electrocatalytic
activities of different CEs.
42
The Epp value in the UC-FTO
(705 mV) is close to that in Pt (643 mV), indicating a total
reserve of high reversibility of I3
−/I−circulation in Pt CE.
Furthermore, the UC-FTO CE has even higher redox peaks
toward I3
−/I−catalysis, indicating its superior catalytic ability,
relative to Pt-CE. In general, smaller size nanoparticle UC-FTO
layer provides larger interfacial surface area with I−/I3
−redox
electrolyte, resulting in increased catalytic activity. The
enhanced electrocatalytic activity can be attributed to its higher
specific surface and more rapid electron transfer nature from
the FTO, and the intrinsically excellent electrocatalytic activity
after doping with Er and Yb. In addition, the diffusion
coefficient (Dn) in the Randles−Sevcik equation is proportional
to the peak current density:
43
υ=
i
Kn AD c
pe
0.5 0.
5
1.5 (3)
where K= 2.69 ×105is a constant, neis the number of
electrodes contributing to the charge transfer, Ais the electrode
Figure 3. Cyclic voltammograms for Pt and UC-FTO electrodes in 0.01 M LiI, 0.001 M I2, and 0.1 M LiClO4acetonitrile solutions at a scan rate of
50 mV s−1: (a) CVs for Pt and UC-FTO electrodes; (b) 100 consecutive CVs for Pt-CE; and (c) 100 consecutive CVs for UC-FTO-CE.
ACS Applied Materials & Interfaces Research Article
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area, crepresents the bulk concentration of I3
−species, Dis the
diffusivity, and υis the scanning rate. The diffusivity in the UC-
FTO CE is 5.51 ×10−6cm−2s−1, which is 82% larger than that
in the Pt CE (3.02 ×10−6cm−2s−1), presumably arising from its
ultrasmall nanostructure, along with a greatly increased active
surface area.
Figures 3b and 3c show 100 successive CV cycles of Pt-CE
and UC-FTO-CE, respectively. In the consecutive 100 CV
tests, the CVs change little. All redox peak current densities of
UC-FTO-CE retain stable with increasing the cycle number,
which is similar to Pt-CE. This indicates that UC-FTO has
good chemical stability and is tightly bound to the FTO glass
surface for the same composition.
As shown in Figure 4, there are mainly three parts in a
DSSC: a photoanode, an electrolyte, and a UC-photocathode.
In the sun radiation energy distribution, there is ∼7%
ultraviolet light, ∼50% visible light, and ∼43% NIR light.
When sunlight goes through the photoanode, the UV light is
absorbed by the TiO2and visible light is absorbed by N719,
which is coated on the surface of TiO2nanoparticles. The UC-
photocathode is excited by the NIR light and then emits red
light by the UC-FTO. Finally, the upconverted red light
reflected from the CE excites N719 from the reverse direction.
In addition, the UC-photocathode can catalyze I−/I3
−
circulation and accelerate I−regeneration, to increase the
concentration of I−with reducibility.
The electron-process in DSSC with UC-FTO is illustrated in
Figure 5. Under the illumination of sunlight, the Dye goes to
the excited state (Dye*) and injects one electron into the
conductive band of TiO2for each excited-dye molecule. Dye*
loses one electron and becomes oxidized Dye+. The injected
electrons diffuse into the external circuit and finally into the
back contact. To regenerate free-state Dye, I−ions in
electrolyte reduce Dye+back to Dye, while itself becomes
oxidized I3
−. The number of I3
−can be reduced by electrons
from the back contact. UC-FTO can catalyze this process,
resulting in the increase of I−in the electrolyte.
While pursuing photovoltaic efficiency in the NIR region, it
is important to maintain the overall performance of solar cells.
In order to study the effect of UC nanocrystals on the overall
performance of solar cells, surface photovoltage (SPV)
responses of cells using Pt or UC-FTO were tested as CEs,
and the results are shown in Figure 6. Both DSSCs utilized
commercial P25 as photoanodes. The SPV analysis for UC-
FTO samples demonstrates three characteristic response peaks
in the entire spectrum of sunlight from 300 nm to 1000 nm,
which are marked as peaks A, B, and C, respectively. In the
ultraviolet (UV) region, a typical characteristic response peak
from 300−380 nm (peak A) can be assigned to the band−band
electronic transition in TiO2. For the same photoanode, the
two samples showed almost the same level response in the UV
region. The broad peak from 400 nm to 700 nm can be
assigned to the dye sensitization of N719. Compared with cells
using Pt, an overall increasing intensity of peak B was found in
cells using UC-FTO, which indicates an enhanced circulation of
I−/I3
−.MorenewlybornI
−can efficiently and rapidly
regenerate oxidized dye molecules, which help improve the
SPV response and enhance the conversion efficiency of DSSCs.
In addition, a new band of response appeared in the NIR region
from 750 nm to 1000 nm (peak C), which confirmed the effect
and LET mechanism of upconversion. The two NIR responses
correspond to Er:4I9/2 and Er:4I11/2 and Yb:2F5/2. NIR light of
Figure 4. Illustration of the photon process of a working dye-
sensitized solar cell (DSSC) with a UC-FTO photocathode.
Figure 5. Electron process of a working DSSC with a UC-FTO
photocathode.
Figure 6. Surface photovoltage (SPV) spectra of a DSSC with Pt or
UC-FTO as the counter electrode from 300−1000 nm. Inset
highlights the near-infrared (NIR) region.
ACS Applied Materials & Interfaces Research Article
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∼800−980 nm can be absorbed by UC-FTO nanoparticles,
which emit visible light and are being utilized.
Photocurrent density−voltage (J−V) curves under simulated
solar light irradiation of 100 mW cm−2(AM1.5G global) are
shown in Figure 7 for DSSCs using CEs made of UC-FTO and
Pt. For reproducibility, each value for cell performance was an
average result from three samples. The photocurrent density of
DSSC using UC-FTO CE is J= 18.44 mA cm−2, which is a
23.9% increase, compared with the photocurrent density of
14.88 mA cm−2from a DSSC using Pt CE. The voltage value of
DSSC with UP-FTO maintained 97.89% of that of DSSC with
Pt, which is 0.744 V, while the total performance PCE value
increased from 6.69% to 7.30%. More importantly, the cost of
UC-FTO is only ∼1/20th of that for Pt (an equivalent amount
of Pt contained in chloroplatinic acid, see Table S2 in the
Supporting Information for details).
As can be seen in the incident photon to current (IPCE)
spectra (Figure 8), the photon-to-current conversion efficiency
obviously increased in the visible region. This indicates that the
UC-FTO nanoparticles enhanced the catalytic performance of
the electrolyte, which produced better circulation of I−/I3
−,
hence, enhancing the conversion efficiency of DSSCs. Besides
the enhanced visible IPCE, a small peak in the NIR region can
be found in Figure 8.
Electric impedance spectra (EIS) is a powerful method for
investigating internal resistance that is attributed to the charge-
transfer properties of DSSCs. Impedance at low frequencies
(0.05−1 Hz) corresponds to the Nernst diffusion of I3
−/I−
within the electrolyte. The capacitance and charge-transfer
resistance at the CE|I3
−/I−electrolyte interface are associated
with impedance at higher frequencies (1−100 kHz). The
response in the range of 1−100 Hz corresponds to the dyed-
TiO2|I3
−/I−electrolyte interface.
44
Figure S6 in the Supporting
Information shows the EIS of DSSCs with UC-FTO and Pt as
CE materials. The sinusoidal perturbation was set to −0.8 V.
Table S1 in the Supporting Information summarizes parameters
of the equivalent circuit RS[C1(R1O1)](R2Q1), and the results
are discussed in detail in the Supporting Information.
■CONCLUSIONS
In summary, a conducive layer made of upconverted fluorine-
doped tin oxide (UC-FTO) nanoparticles was successfully
synthesized, which has good conductivity, decent catalytic
properties, and the capability to upconvert near-infrared (NIR)
light to visible light, to increase the total efficiency of solar cells.
It can substantially reduce the cost of the counter electrode
(CE) by replacing the expensive platinum (∼1/20th of that for
Pt). By using this novel material as the CE in dye-sensitized
solar cells (DSSCs), the overall power conversion efficiency
(PCE) of the DSSC was increased to 7.30%, which represents
an enhancement of more than 9.12%, compared with DSSC
using a platinum counter electrode (denoted as Pt-CE; 6.69%).
To date, this is the only known material that not only
substantially reduced the cost of the CE, but also effectively
increased the overall performance of DSSCs. Our results
suggest a promising strategy of using conductive upconversion
materials as the CE to further reduce the cost and
simultaneously enhance the overall efficiency of solar cells.
■ASSOCIATED CONTENT
*
SSupporting Information
XRD and XPS of UC-FTO samples. Details concerning the UC
process and mechanism. EIS and cost summary of DSSCs. This
material is available free of charge via the Internet at http://
pubs.acs.org.
■AUTHOR INFORMATION
Corresponding Authors
*Tel.: +86 451 86413710. Fax: +86 451 86418270. E-mail:
ylyang@hit.edu.cn (Y. L. Yang).
*Tel.: +86 451 86413710. Fax: +86 451 86418270. E-mail:
fanruiqing@hit.edu.cn (R. Q. Fan).
*Tel.: +1 814 8654101. Fax: +1 814 8652326. E-mail: dzk@
psu.edu (W. W. Cao).
Notes
The authors declare no competing financial interest.
■ACKNOWLEDGMENTS
This work was supported by National Natural Science
Foundation of China (Grant Nos. 21171044 and 21371040)
and the National Key Basic Research Program of China (973
Program, No. 2013CB632900). The work was also supported
by the Fundamental Research Funds for the Central
Universities (Grant No. HIT. IBRSEM. A. 201409), Program
Figure 7. Photocurrent density−voltage (J−V) curves of DSSCs with
UC-FTO or Pt.
Figure 8. IPCE curves of solar cells with Pt or UC-FTO as the CE.
ACS Applied Materials & Interfaces Research Article
dx.doi.org/10.1021/am5009776 |ACS Appl. Mater. Interfaces 2014, 6, 8223−82298227

for Innovation Research of Science in Harbin Institute of
Technology (PIRS of HIT No.A201416 and B201414), and
National Key Technology Research and Development Program
of China No. 2013BAI03B06.
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