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Boosted Electron Transport and Enlarged Built-in Potential by Eliminating Interface Barrier in Organic Solar Cells Chunyu Liu, Dezhong Zhang, Zhiqi Li, Xinyuan Zhang, Wenbin Guo, Liu Zhang, Liang Shen, Shengping Ruan, and Yongbing Long ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b15631 • Publication Date (Web): 24 Feb 2017 Downloaded from http://pubs.acs.org on February 24, 2017
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Boosted Electron Transport and Enlarged Built-in Potential by Eliminating Interface Barrier in Organic Solar Cells Chunyu Liu1, Dezhong Zhang1, Zhiqi Li1, Xinyuan Zhang1, Wenbin Guo1*, Liu Zhang2*, Liang Shen1, Shengping Ruan1, and Yongbing Long3 1
State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering , Jilin University, 2699 Qianjin Street, Changchun 130012, People’s Republic of China 2
College of Instrumentation & Electrical Engineering, Jilin University, 938 Ximinzhu Street, Changchun 130061, People’s Republic of China
3
School of Electronic Engineering, South China Agricultural University, Guangzhou, 510642, China
ABSTRACT A smart interface modification strategy was employed to simultaneously improve short-circuit
current
density
(J sc )
and
open-circuit
voltage
(V oc )
by
incorporating
Poly[(9,9-bis(3′-(N,N- dimethylamion)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctyl)-fluorene] (PFN) interlayer between TiO 2 film and active layer, arising from that PFN effectively eliminated interface barrier between TiO 2 and fullerene acceptor. The work function (WF) of TiO 2 was apparently reduced, which facilitated effective electron transfer from active layer to TiO 2 electron transport layer (ETL) and suppressed charge carrier recombination between contact interfaces. Electron injection devices with and without PFN interlayer were fabricated to prove the eliminated electron
barrier,
meanwhile
photoluminescence
(PL)
and
time-resolved
transient
photoluminescence (TRTPL) were measured to probe much easier electron transfer from [6, 6]-phenyl C71-butyric acid methyl ester (PC 71 BM) acceptor to TiO 2 ETL, contributing to enhanced J sc . The shift in vacuum level altered the WF of PC 71 BM, which enlarged the internal electrical field at the donor/acceptor interface and built-in potential (V bi ) across the device. Dark current characteristics and Mott-Schottky measurements indicated the enhancement of V bi , benefiting to increased V oc . Consequently, the champion power conversion efficiency (PCE) for device with PFN interlayer of 0.50 mg/mL reached to 7.14 %, which is much higher than the PCE of 5.76 % for control device. KEYWORDS: Interface Barrier, Interface Modification, Work Function, Electron Transport, Built-in Potential 1
1. INTRODUCTION Organic solar cells (OSCs) have attracted much attention to solve the crisis of environmental pollution and energy shortage, which have made great progresses.1-6 While the low power conversion efficiency (PCE) still hastens research process to pursue higher efficiency. Both innovative materials (new donor and acceptor materials) and complex device architecture (cascade and triple-junction solar cells) played important roles for various breakthroughs during the development of OSCs.7-12 But the synthesis of new materials needs to undergo a long period time; fabrications of cascade and triple-junction solar cells require complicated technology. Recently, some maneuverable strategies have been proposed and employed to prepare highly efficient single junction devices.13-16 Upon the absorption of incident photons, charge carriers are generated in active layer and transported through their own pathways, including transport layer, electrode, and all interfaces in between. For more easy electrons transport and collection, cathode and electron transport layer (ETL) with a work function (WF) that is low enough to either inject into or collect electrons from the lowest unoccupied molecular orbital (LUMO) of acceptor are indispensable, as well as good ohmic contact between two layers.17 To meet these requirements, alkaline-earth metals (Ca, Mg) once were used as electrode,18,19 but chemical characteristics of reactive and easily oxidize in the air limited their further application. Nowadays, ITO and Al were generally acted as cathode materials.20,21 Thin transition metal-oxide films (ZnO, TiO 2 ) were employed to be ETL inserting between higher WF electrode and active layer,22,23 which can adjust electrons extraction and transport. TiO 2 with the properties of high transparency, good chemical stability, and large electron mobility, has been widely used in the fabrication of OSCs. During the working process, TiO 2 layer actually gets electrons from fullerene acceptor, such as [6, 6]-phenyl C71-butyric acid methyl ester (PC 71 BM). But obvious interface barrier between TiO 2 and PC 71 BM can be observed, which seriously hinders the electron extraction and transport. Careful adjustments and modification of the interface characteristics are necessary and helpful to further facilitate charge carrier transport. Some materials (PEI, PFN-Br, PTMAHT) have been applied to form strong interface and/or molecular dipoles to lower the WF of materials,24-26 boosting efficient electron transport, coated on either electrode or transport layer. PFN have been proved to have excellent performance that can tailor the energy level, not only coating onto film but also doping into 2
materials. He et al. used PFN to tune the WF of ITO offering ohmic contact for photo-generated charge carrier collection.18 Lee et al. blended PFN and ZnO to reduce surface energy and surface defects, obtaining a best PCE of 9.2 %.27 Thus in this work, we incorporated a thin PFN layer between TiO 2 ETL and active layer to further re-modulate the interface properties in inverted OSCs, eliminating interface barrier and facilitating electron transport from the acceptor to TiO 2 . Thus-prepared devices displayed enhanced short-circuit current density (J sc ) and open-circuit voltage
(V oc ),
yielding
high
device
performance
poly[N-9′′-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-
of
7.14
%
based
on
thienyl-2′,1′,3′-benzothiadiazole)
(PCDTBT):PC 71 BM. Detailed analyses were performed to reveal the effect of PFN interlayer on energy level adjustment, not for the one side of TiO 2 film, but also for the other side of fullerene in active layer.
2. RESULTS AND DISCUSSION
Figure 1. (a) The scheme of device structure fabricated in this work. (b) The transmittance spectra of pristine TiO 2 , TiO 2 /P 1 , TiO 2 /P 2 and TiO 2 /P 3 films, as well as the absorption spectra of P 1 , P 2 and P 3 layers.
Inverted OSCs were fabricated with the structure of ITO/TiO 2 /PFN/PCDTBT:PC 71 BM/MoO 3 /Ag and three different concentrations of 0.25, 0.50 and 0.75 mg/mL PFN solution were used in this study. The films prepared with various concentrations of PFN were named as P 1 , P 2 and P 3 , respectively. The scheme of inverted device structure is shown in Figure 1a, which is well known that inverted OSCs possess apparent vertical phase separation,28,29 thus fullerene-rich is near the TiO 2 or PFN, while polymer-rich is near the MoO 3 . Yang et al. investigated the phase separation of polymer/fullerene under different procedures and calculated the weight ratio of fullerene/polymer at the top and bottom surfaces, confirming that higher fullerene concentration is at the bottom surface and higher polymer concentration is at the top surface. Meanwhile, Parnell et al. and Björström et al. all have once proved that several nanometers PCBM-rich phase were 3
observed near the substrate when spin-coating conjugated-polymer/PCBM blends.30,31 Thus, the PFN interlayer mainly contacted with the PCBM-rich phase in this study, which is important for the subsequent analysis. In this experiment, PFN was spin-coated with the speed of 3500 rmp for 40 s and other detailed device fabrication process has been described in previous reports.32,33 Light absorption of active layer is a significant part for the photoelectric conversion process, thus ETL with high transmittance is necessary in inverted OSCs. Transmittance spectra of TiO 2 and TiO 2 /PFN were firstly measured, displayed in Figure 1b, as well as the absorption of P 1 , P 2 and P 3 layers. TiO 2 /PFN composite films possessed the same transmittance with TiO 2 film except a slight loss range from 350 to 420 nm, indicating that majority visible light is allowed to pass through TiO 2 /PFN films and further used by PCDTBT:PC 71 BM active layer. The decreased transmittance of TiO 2 /PFN arises from the absorption of PFN film near the wavelength of 400 nm, retaining part light absorption efficiency of active layer, but it may not actually impact the optical property of devices with PFN interlayer.
Figure 2. AFM images of (a) ITO, (b) ITO/TiO 2 and (c) ITO/TiO 2 /PFN. AFM images of active layer coated on (d) ITO/TiO 2 and (e) ITO/TiO 2 /PFN.
Performance of OSCs is remarkably influenced by film quality, especially for the solution-process films.34-36 Thus, typical atomic force microscopy (AFM) images were measured to investigate film morphology, shown in Figure S1 (Supporting Information) and Figure 2. The AFM images of ITO/P 1 , ITO/P 2 and ITO/P 3 (Figure S1) have been firstly measured, showing lower root mean square (RMS) compared with that of ITO surface (2.05 nm), which indicates fantastic film-forming characteristics of PFN. The AFM images of ITO, ITO/TiO 2 and 4
ITO/TiO 2 /PFN films were investigated and given in Figure 2a, b and c. Meanwhile, the effect of PFN interlayer on the morphology of active layer film should be inquired. With a scale of 1 μm×1 μm, the RMS for TiO 2 is 1.25 nm, and it can be found that TiO 2 film consists of nanoparticles coated on ITO glass. While covered by PFN, the RMS drops to 0.94 nm, suggesting that PFN surface is smooth enough to form well contact with active layer. Figure 2d and e depict the surface morphologies of PCDTBT:PC 71 BM coated on ITO/TiO 2 and ITO/TiO 2 /PFN films, a neglectable difference in RMS (0.441 to 0.433 nm) was observed, implying that the insertion of PFN film will not damage the morphology of active layer.
Figure 3. (a) J-V characteristics curves and (b) EQE spectra comparing the devices with and without PFN interlayer.
Table 1. Photovoltaic Properties of the Devices Without and With PFN Interlayer, Including V oc , J sc , FF and PCE. Device
V oc (V)
J sc (mA/cm2)
FF (%)
PCE (%)
TiO 2 TiO 2 /P 1 TiO 2 /P 2 TiO 2 /P 3
0.856±0.002 0.887±0.003 0.889±0.002 0.888±0.003
11.40±0.13 12.17±0.13 13.35±0.14 12.89±0.15
59.03±0.21 60.20±0.16 60.12±0.18 59.11±0.20
5.76±0.10 6.50±0.11 7.14±0.11 6.77±0.12
Figure 3a is the current density vs voltage (J-V) characteristics of all devices, including control device without PFN interlayer, and the relevant photovoltaic performance parameters were summarized in Table 1. It can be noted that after incorporating PFN thin layer to adjust interface characteristics, J sc were significantly improved and V oc presented visible increase. The control devices showed the PCE of 5.76 %, with a J sc of 11.40 mA/cm2, a V oc of 0.856 V, and a FF of 59.03 %. While for optimal device with P 2 interlayer, it exhibited a J sc of 13.35 mA/cm2, a V oc of 0.889 V, and a FF of 60.12 %, which yielded a PCE up to 7.14 %. For devices with P 1 and P 3 , the performances are both reduced compared with optimal device with P 2 . It can be deduced that for device with P 1 , extraction and transport of electrons from acceptor to TiO 2 are still limited due to 5
the slight impact on WF of TiO 2 . But for the device with P 3 , thicker PFN lengthens the transport distance, which will increase the recombination probability of electrons in PFN layer, and increases the series resistance (from 7.79 to 8.84 Ω·cm2 for devices with P 2 and P 3 ), which inevitably decreases the J sc and device performance. The detailed analyses and descriptions will be provided in subsequent study. Meanwhile, the devices with the 1.00 and 1.25 mg/mL PFN have also been fabricated and shown in Figure S2 (Supporting Information), demonstrating ulterior deviation from the optimal efficiency of device with P 2 , which obviously proved that thicker PFN layer blemish the performance of devices. Figure 3b compares the external quantum efficiency (EQE) spectra of devices without and with PFN interlayer, demonstrating evident increase from 350 to 650 nm. The device with P 2 exhibited a maximum EQE response of 79 % at 485 nm while the device without PFN showed a lower value of 67 % at 490 nm, which is a realistic estimation for the increased J sc .
Figure 4. (a) The UPS spectra of pristine TiO 2 , TiO 2 /P 1 , TiO 2 /P 2 and TiO 2 /P 3 ; (b) The energy level diagrams of devices with different WF from TiO 2 and TiO 2 /P 2 ; (c) Energy level diagrams of TiO 2 and PCBM without and with P2 interlayer; (d) The structure and (e) I-V characteristics of electron injection devices without and with PFN interlayer.
In this study, the J sc of 13.35 mA/cm2 for the device with P 2 interfacial layer is much larger than 11.40 mA/cm2 for the control device, which attributes to more efficient electron extraction and transport from PC 71 BM to TiO 2 due to the decreased WF of TiO 2 . It can be concluded that the decreased work function of TiO 2 attributes to the formation of interfacial dipole, which have been presented by some previous literatures.37,38 Thus, UPS measurements (with a He I of 21.2 eV) 6
were carried out to obtain the WF of TiO 2 , TiO 2 /P 1 , TiO 2 /P 2 , and TiO 2 /P 3 . Figure 4a is the UPS spectra in view of the high binding energy region, which reveals the WF of pristine TiO 2 film (~4.7 eV), TiO 2 /P 1 (~4.2 eV), TiO 2 /P 2 (~4.1 eV), and TiO 2 /P 3 (~4.1 eV). With further increased concentration of PFN, the WF of TiO 2 will not be continuously decreased. On the basis of these data, energy level alignments of devices (without and with P 2 ) are displayed in Figure 4b, wherein the WF of PC 71 BM is ~4.4 eV, as determined by UPS measurement. The energy level structures for TiO 2 and PC 71 BM without and with PFN interlayer are given in Figure 4c. There is a difference between the WFs of TiO 2 and PC 71 BM, so when pristine TiO 2 layer contacts with PC 71 BM, unified Fermi level will be established and the energy bands of PC 71 BM will be bended, leading to an apparent electron barrier occurring in the side of PC 71 BM (left of Figure 4c), which blocks the electron transport from PC 71 BM to TiO 2 . While the TiO 2 /P 2 contacts with PC 71 BM (right of Figure 4c), the interface electron barrier in PC 71 BM will disappear, removing the restriction on the outward transfer of electrons, which promotes electron transport from PC 71 BM to TiO 2 . To further verify this effect, the electron injection device with the structure of ITO/TiO 2 /PC 71 BM/Ag and ITO/TiO 2 /PFN/PC 71 BM/Ag were fabricated. The device schematic is shown in Figure 4d that electrons were injected from the Ag electrode. There is no barrier to influence electron injection from Ag electrode to PC 71 BM (transport from -4.3 to -4.3 eV). Seen from the I-V characteristics demonstrated in Figure 4e, for the device with pristine TiO 2 , electrons have to jump the barrier (qɸ) in PC 71 BM, and the barrier will not be broke through till the applied bias exceeding ca. 0.2 V. While the P 2 device exhibited the I-V characteristics with well ohmic property, due to the disappearance of electron barrier. It can be also found that device with TiO 2 /P 2 demonstrated higher electric conductivity than device with TiO 2 /P 1 and TiO 2 /P 3 . We conclude that the WF of TiO 2 /P 2 is lower than that of TiO 2 /P 1 , which is more beneficial for electron transport. Though TiO 2 /P 3 possesses the same WF with TiO 2 /P 2 , larger film thickness will decrease the conductivity of TiO 2 /P 3 . All of these could also provide the reason why the J sc of the devices with TiO 2 /P 1 and TiO 2 /P 3 are lower than the optimal device.
7
Figure 5. (a) PL and (b) TRTPL spectra of TiO 2 /PC 71 BM and TiO 2 /P 2 /PC 71 BM composite films.
PFN was employed to modify the interface between TiO 2 film and active layer, evidently eliminating the electron transport barrier, which will dramatically promote electrons transfer from PC 71 BM acceptor to TiO 2 ETL. The photoluminescence (PL) spectra of TiO 2 /PC 71 BM and TiO 2 /P 2 /PC 71 BM were measured to further investigate the enhanced electron or energy transfer between PC 71 BM and TiO 2 , displayed in Figure 5a. Generally, if energy transfer exists between two different materials, weaker PL emission intensity will be observed for charge possessor.39-41 In Figure 5a, TiO 2 /P 2 /PC 71 BM exhibited weak PL emission intensity with an emission peak at 710 nm compared with TiO 2 /PC 71 BM, indicating the decrease of charge carrier recombination due to occurrence of more effectively electron transport in the contact interface.42,43 To deeply demonstrate the much easier electron transfer process from PC 71 BM to TiO 2 , corresponding time-resolved transient photoluminescence (TRTPL) spectra were measured by monitoring the emission peak at 710 nm. Seen from Figure 5b, with the incorporation of P 2 interfacial layer, the 710 nm emission decay time is slightly decreased from 1.21 to 1.08 ns, suggesting that electron transport from PC 71 BM to TiO 2 was improved,44 which also contributes to the suppression of charge carrier recombination in accepter. In other words, exciton dissociation in active layer was strengthened, which can be examined by exciton dissociation probabilities (P(E, T)) (Supporting Information Figure S3), related to the electric field (E) and temperature (T) for solar cells. In practice, only portion photo-generated excitons can dissociate into free carriers for OSCs, others will recombine in the transport process before collected by electrodes. The value of P(E, T) can be given by the plot of the normalized photocurrent density (photocurrent density (J ph ):saturation current density (J sat )) with respect to effective voltage (V eff ).45 Figure S3 reveals that the P(E, T) increased from 94.5% (for the control device) to 95.6% (for the device with PFN) under the short-circuit conditions (V a = 0 V), indicating excitons dissociation in active layer was facilitated 8
originating from PFN interlayer. Figure S4 (Supporting Information) is the J-V characteristics of electron-only device with the structure of ITO/TiO 2 /PFN/PCDTBT:PC 71 BM/BCP/Ag, where BCP is hole blocking layer. The current density could truly reflect the impact of PFN on electron transport in devices. It can be noted that the electron-only device with PFN displays higher current density than device without interlayer, which is consistent with the tendency of photocurrent, confirming that electron transport property was substantially enhanced due to the eliminated barrier between PC 71 BM and TiO 2 . Corresponding increase in electron mobility from 3.54×10-4 cm2/Vs to 4.98×10-4, 8.78×10-4 and 6.03×10-4 cm2/Vs were extracted, respectively.
Figure 6. Dark-current density curves of device with pristine TiO 2 film and TiO 2 /P 2 composite films.
Figure 6 is the J-V characteristics of control device and device with P 2 interlayer measured in dark under linear and semilog coordination systems. Under semilog coordination system, the dark current density in negative bias from the device with P 2 interlayer is lower than that of control device, which indicates that charge carrier recombination is suppressed by P 2 interfacial layer, apparently decreasing the leakage of current and increasing the photocurrent.46,47 Furthermore, device with lower electrical leakage have potential advantage to obtain an increased V oc by reducing the loss of charge.48 In positive bias, it's worth noting from the Figure 6 that the turn-on voltage of device with P 2 interlayer is slightly higher than that of control device, which can be intuitively compared under linear coordinate system, implying increased built-in potential (V bi ) across the device upon utilization of P 2 interfacial layer. The increased V bi could attribute to the consequence that the insertion of PFN between TiO 2 film and active layer induces a vacuum-level shift at the interface and modifies the WF of the PC 71 BM, which fatefully results in an enlarged V oc .49,50 9
Figure 7. The UPS spectra of PC 71 BM coated on TiO 2 and TiO 2 /P 2 , the inset is the energy level diagrams of PCDTBT and PC 71 BM.
Under illumination, the device with P 2 shows a V oc of 0.889 V, which is higher than control device of 0.856 V. The origin of the enhancement in V oc in this study can be attributed to enlarged built-in electric field with the PFN interfacial layer. As point out previous, the shift in vacuum level at the interface could alter the WF or surface potential of materials.51 It has been referred before that active layer in inverted OSCs exhibits obvious vertical phase separation, implying that TiO 2 film and TiO 2 /PFN films will mainly connect with PC 71 BM phase. Thus the interface characteristics between TiO 2 and PC 71 BM are inevitably varied due to the modification of PFN. TiO 2 covered by PFN thin film showed a decreased WF compared with bare TiO 2 layer, which has been proved by Figure 4a. When connecting with TiO 2 /PFN, the Fermi level of PC 71 BM near the interface will be draw higher than that of connecting with bare TiO 2 . In this study, the WFs of PC 71 BM were measured by UPS with the structure of TiO 2 /PC 71 BM (15 nm) and TiO 2 /P 2 /PC 71 BM (15 nm), shown in Figure 7. It implies that the WF has a variation of ca. 0.3 eV, which results in a larger internal electrical field at the donor/acceptor blend film, contributing to the exciton dissociation at the donor/acceptor interface and increasing the effective charge carrier density. Furthermore the enlarged electrical field is also responsible for the increased V bi across the device and ultimately enlarged V oc ..52 It is worth mentioning that better contact at electron transport interface and eliminated electron transport barrier also play vital roles on the enhancement of V oc . Revealed by the AFM images, the surface of TiO 2 becomes smoother after coated by the PFN interlayer, forming better contact with active layer. Meanwhile, electron transport barrier between acceptor and electron transport layer was eliminated due to the reduced work function of TiO 2 with the PFN interface layer, thus decreasing the loss of V oc . 10
Figure 8. (a) C-V and Mott−Schottky curves of control device and device with P 2 interlayer. (b) Nyquist plots of devices without and with PFN interlayer measured with frequency range from 20 Hz to 1 MHz in dark.
Interface modification of PFN facilitates electron transport and decreases bimolecular recombination, which contributes to the increase of charge carrier concentration. To obtained carrier concentration, we measured the capacitance-voltage (C-V) characteristic of the devices without and with P 2 interlayer, as well as the Mott−Schottky curves, shown in Figure 8a. V bi can be attained from the Mott−Schottky curves, and then charge carrier density (n) will be given by
n=
1 Aed
∫
Vbi
0
C (V )dV (1)
where A is the device area (0.064 cm2), e is the elementary charge, d is the thickness of the active layer (about 100 nm), and C is the chemical capacitance.53,54 For control device, we extract a V bi of 0.841 V and the value of n is 7.60 × 1016 cm-3. While for device with P 2 , the V bi is 0.872 V and n is 1.02 × 1017 cm-3. It can be noted that the improvement of the V bi is well agreement with the enlarged V oc . Meanwhile, the higher charge carrier concentration for device with PFN will inevitably lower the device resistance. Thus, the Nyquist curves of the impedance spectra for all devices were measured in frequency range from 20 Hz to 1 MHz, shown in Figure. 8b. Apparent decrease for device resistance were observed, being attributed to increased charge carrier concentration, reduced electron barrier, and improved electron mobility.55
3. CONCLUSIONS In summary, we have successfully demonstrated high efficiency OSCs of 7.14 % using PFN as interface modification layer between TiO 2 film and active layer, leading to a 24.0 % PCE enhancement, which is mainly attributed to the increase of J sc due to the reduced electron barrier and facilitated electron transfer from PC 71 BM to TiO 2 , and the improvement of V oc due to the enlarged internal electrical field at the donor/acceptor interface and V bi across the device. PFN 11
with simple fabrication technology could induce vacuum-level shift and adjust the WF, which effectively tailor charge extraction and transport, suppressing bimolecular recombination. This study detailedly analyzes underlying mechanisms on improvement of J sc and V oc from the perspective of energy level, providing an easy way to eliminate interface barrier and fabricate high-performance solution-process OSCs with bright application prospects.
ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Figure S1 is the AFM images of PFN films coated on ITO glass; Figure S2 is the J-V characteristics of devices fabricated with 1.00 and 1.25 mg/mL PFN. Figure S3 is the P (E, T) spectra of devices without and with P 2 interlayer; Figure S4 is the J-V characteristics of hole-only devices.
AUTHOR INFORMATION Corresponding Author *E-mail: W. B. Guo,
[email protected]; L. Zhang,
[email protected]. Notes The authors declare no competing financial interest.
ACKNOWLEDGMENTS The authors are grateful to National Natural Science Foundation of China (61370046, 11574110), the Opened Fund of the State Key Laboratory on Integrated Optoelectronics (IOSKL2013KF10), Guangdong Natural Science Funds for Distinguished Young Scholar (Grant No.2014A030306005), Foundation for High-level Talents in Higher Education of Guangdong Province, China (Yue Cai-Jiao [2013]246, Jiang Cai-Jiao[2014]10) for the support to the work.
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