Molecular Tuning of Titanium Complexes with Controllable WorkFunction for Efficient Organic PhotovoltaicsRui Wang,†,⊥ Jingjing Xue,†,⊥ Dong Meng,†,⊥ Pei Cheng,† Jun Yuan,‡ Sheng-Yung Chang,†

Shaun Tan,† Boyu Jia,∥ Zhaohui Wang,§ Yingping Zou,‡ Xiaowei Zhan,∥ and Yang Yang*,†

†Department of Materials Science and Engineering and California NanoSystems Institute, University of California, Los Angeles,California 90095, United States§Key Laboratory of Organic Optoelectronics and Molecular Engineering, Department of Chemistry, Tsinghua University, Beijing100084, China‡College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China∥Department of Materials Science and Engineering, College of Engineering, Peking University, Beijing 100871, China

*S Supporting Information

ABSTRACT: Nonfullerene acceptors have recently emerged as promisingcandidates for organic photovoltaics (OPVs) owing to their superioroptoelectronic properties. However, the varied lowest unoccupied molecularorbital (LUMO) levels of the acceptors pose difficulty in aligning their energylevels with existing electron transporting layers (ETLs). The authors proposehere a facile design of the ETL with a tunable work function by simply varyingthe annealing temperature of titanium diisopropoxide bis(acetylacetonate)(TIAA) to change the molecular structure of the titanium complex. The energylevel tuning is realized without changing the morphological features of the activelayer, and as a result, it can enhance the charge extraction efficiency and thusultimately the power conversion efficiency of the nonfullerene OPV. Our designwill provide new important insights for achieving efficient nonfullerene OPVs.

■ INTRODUCTION

Organic photovoltaics (OPVs) have attracted tremendousattention due to their solution processability, low-cost, andflexibility.1−4 So far, the record power conversion efficiency(PCE) of OPVs has reached over 15%.5−8 The typical devicearchitecture (inverted) of OPVs is electrode/electron trans-porting layer (ETL)/active layer/hole transporting layer(HTL)/electrode.9−12 The active layer contributes to thelight harvesting, and the role of the transporting layers is toefficiently extract charge carriers.13−16 To date, mostresearchers focused on the development of the active layer.The active layer normally consists of a donor and an acceptorto form a bulk heterojunction (BHJ).17−19 At the early stagesof OPV development, most of the BHJs reported usedfullerene and its derivatives as the acceptor material to gethigh efficiencies,20−22 and in the meantime, the design of theETL mainly focused to match the lowest unoccupiedmolecular orbital (LUMO) of [6,6]-phenyl C71-butyric acidmethyl ester (PC71BM) (−3.9 to −4.0 eV) to their own workfunctions (WFs).23 Therefore, ETLs with suitable WFs widelyutilized in inverted OPVs include zinc oxide (ZnO),24 titaniumoxide (TiO2),

25 tin oxide (SnO2),26 or poly[(9,9-bis(3′-(N,N-

dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9−dioctyl-fluorene)] (PFN).27 A suitable energy level can provideefficient charge extraction with a reduced energy barrier torealize a device with high PCE.

Very recently, the OPV community has been graduallymoving toward nonfullerene acceptors,28 due to their strongabsorptions, tunable bandgaps to harvest more sunlight to,tunable planarity and crystallinity, and tunable energylevels.29−31 The reported LUMOs of these nonfullereneacceptors varies from −3.7 to −4.2 eV.1 Therefore, traditionalETLs may not align well with the nonfullerene acceptors,which may induce the limited performance of OPVs because ofthe energy barrier.32 Designing new ETLs with a tunable WFthat can satisfy the energy level alignment with mostnonfullerenes is strongly necessary to achieve high perform-ance nonfullerene OPVs.Herein, we report a facile design of the ETL with a

controllable WF by simply changing the annealing temperatureof the titanium diisopropoxide bis(acetylacetonate) (TIAA)film without changing the morphology of the active layer. Byrationally aligning the energy levels of the ETLs with thenonfullerenes with different LUMOs, the best-matched energylevels with the smallest energy barriers realized the highestopen circuit voltage (Voc) and FF and thus the best PCEs.PCEs of 7.1% for the acceptor with a LUMO of −3.69 eV,11.5% for the acceptor with a LUMO of −3.98 eV, and 13.2%

Received: June 27, 2019Revised: August 6, 2019Published: August 6, 2019

Article

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■ EXPERIMENTAL SECTION

Unless stated otherwise, solvents and chemicals were obtainedcommercially and used without further purification. PBDB-T,IT-M, PBDB-T-SF, IT-4F, and PC71BM were purchased fromSolarmer Inc. HFAQx‑T and IDT-2BR were home synthesized.Titanium diisopropoxide bis(acetylacetonate), titanium iso-propoxide, 1-Butanol, 1,8-diiodooctane (DIO), chloroform(CF) and chlorobenzene (CB) were obtained from Sigma-Aldrich Inc. Titanium acetylacetonate was obtained from TCIinc.Organic photovoltaics were fabricated with the following

structures: indium tin oxide (ITO)/titanium diisopropoxidebis(acetylacetonate) (TIAA)/active layer/molybdenum triox-ide (MoO3)/silver (Ag). The ITO glass was precleaned in anultrasonic bath of acetone and 2-propanol, and treated in anultraviolet−ozone chamber (Jelight Company, USA) for 15min. A thin layer (ca. 20 nm) of titanium diisopropoxidebis(acetylacetonate) was spin-coated onto the ITO glass andbaked at different temperatures for 10 min in the air. A mixtureof HFAQx-T/IDT-2BR was dissolved in CF/DIO (100:0.5, v/v) mix solvent (D:A = 0.6:0.9, 15 mg mL−1 in total) withstirring 30 min (60 °C). A mixture of PBDB-T/IT-M wasdissolved in CB/DIO (100:0.75, v/v) mix solvent (D:A = 1:1,18 mg mL−1 in total) with stirring overnight (90 °C). Amixture of PBDB-T-SF/IT-4F was dissolved in CB/DIO(100:0.75, v/v) mix solvent (D:A = 1:1, 17 mg mL-1 in total)with stirring overnight (80 °C). Then, these blend solutionswere spin-coated on the ZnO or TIAA layer to form active

layers. The thickness of active layer was ca. 100−115 nm. AMoO3 (ca. 10 nm) and Ag layer (ca. 100 nm) was thenevaporated onto the surface of the photosensitive layer undervacuum (ca. 10−5 Pa) to form the back electrode. The activearea of the device was 0.1 cm2.J−V characteristics of photovoltaic cells were taken using a

Keithley 2400 source measure unit under a simulated AM 1.5Gspectrum, with an Oriel 9600 solar simulator. EQEs weremeasured using an integrated system (Enlitech, Taiwan) and alock-in amplifier with a current preamplifier under short-circuitcondition. Thin-film (on quartz substrate) transmittancespectra were recorded on a 4100 Hitachi spectrofluoropho-tometer. The samples for X-ray photoelectron spectroscopy(XPS) analysis were spin-coated on substrates. XPS measure-ments were carried out on an XPS AXIS Ultra DLD (KratosAnalytical). An Al Kα (1,486.6 eV) X-ray was used as theexcitation source. UPS measurements were carried out todetermine the work function of the materials, and a Hedischarge lamp, emitting ultraviolet energy at 21.2 eV, wasused for excitation. All UPS measurements of the onset ofphotoemission to determine the workfunction were performedusing standard procedures with a −9 V bias applied to thesamples. Clean gold was used as reference. XRD patterns (θ−2θ scans) were obtained from samples of TIAA deposited onsubstrates using a double-axis X-ray diffractometer (Bede D1)equipped with a focusing graded X-ray mirror withmonochromatic CuKα (λ = 1.5405 Å) radiation source.Scans were taken with a 0.5 mm-wide source and detector slitsand with X-ray generator settings of 40 kV and 30 mA.

Figure 1. (a) Chemical structures of the donors and acceptors. (b) Work-functions of TIAA films with different annealing temperatures. (c) Energydiagrams of the acceptor materials.

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■ RESULTS AND DISCUSSION

To investigate the role of our strategy, three donor/acceptorpairs were systematically selected. The chemical structures ofmaterials HFAQx-T,33 IDT-2BR,34 PBDB-T,35 IT-M,36 PBDB-T-SF, and IT-4F6 are shown in Figure 1a. The work-functionsof the TIAA with different annealing temperatures determinedby ultraviolet photoelectron spectroscopy (UPS) are shown inFigure 1b. An almost linear distribution of WFs versusannealing temperatures was observed. The LUMOs andhighest oc-cupied molecular orbitals (HOMOs) of IT-4F,IT-M and IDT-2BR are displayed in Figure 1c. To confirm thework-function tunability of TIAA will enhance the chargecollection, three annealing temperatures of 75 °C (lowtemperature (LT), WF = −3.7 eV), 125 °C (mediumtemperature (MT), WF = −3.9 eV), and 200 °C (hightemperature (HT), WF = −4.1 eV) of TIAA films wereemployed in this study. Figure S1 shows the selected UPScurves of the cutoff energy.The chemical components of the as-prepared films at

different temperatures were characterized by XPS. As shown inFigure S2 all the samples showed two peaks at 458.3 and 464.0eV, corresponding to Ti 2p3/2 and Ti 2p1/2 respectively,which agree with the binding energy values of Ti (IV) in thetitanyl complex. XPS of O 1s for the film annealed at 75, 125,and 200 °C are shown in Figure 2a−c. All the samples showeda peak splitting at 529.8 and 531.4 eV. The peak at a lowerbinding energy position of 529.8 eV can be attributed to theoxygen component in the metal oxide, i.e. TiOx, while the peakat 531.4 eV corresponded to the oxygen in the organic ligands.Considering the fact that Ti(IPA)4 suffers from superfasthydrolysis once exposed under ambient conditions, let alonehigh temperature annealing, the peak of O 1s at the higherbinding energy position can be attributed to the oxygen in theAcAc ligands The atomic ratio calculated from the area of O 1s

(529.8 eV) and O 1s (531.4 eV) showed a gradual increasingvalue of 0.63, 0.72, and 0.80 for the samples annealing at 75,125, and 200 °C respectively, while that of O 1s (529.8 eV)gradually increased as the annealing temperature increased.This component evolution in the films suggested that thehigher the annealing temperature, the less AcAc ligandsremained in the film (the corresponding chemical structureevolution is shown in Figure 2d). The chemical componentchanges in the film annealed at different temperatures wasfurther investigated using FTIR. As shown in Figure S3, thepeak at 1600 cm‑1 that can be assigned to the CO stretchingvibration mode exhibited a decrease in intensity as thetemperature increased, further confirming the gradual decom-position of AcAc at elevated temperatures. Therefore, thetunable Fermi energy of the films annealed at differenttemperatures can be attributed to the amount of AcAc ligandsthat remained in the titanium complex, where the localized π-bond in the AcAc ligands might overlap with the electronorbital of titanium to enable the change of the electronicstructure of the resultant TiOx.

37 This hypothesis was furtherconfirmed by examining the Fermi levels of the films preparedfrom pure Ti(AcAc)4 and pure Ti(IPA)4 respectively. Asshown in Figure S4, the films prepared from pure Ti(IPA)4with various annealing temperatures of 75, 125, and 200 °Cexhibited the same Fermi energy of −4.3 eV. In contrast, thefilms prepared from pure Ti(AcAc)4 showed shallower Fermienergies when annealed at higher temperatures, which isconsistent with the trend of the films prepared from TIAA(Figure S5). Noticeably, despite the tunable work function ofthe films fabricated from pure Ti(AcAc)4, the photovoltaicdevices based on these films generally showed poor FFs, whichcan be attributed to the relatively large amount of residualAcAc that hindered the effective electron transport (FigureS6). Therefore, the IPA ligands in TIAA enabled it to be an

Figure 2. XPS spectra of O 1s scans of (a) LT TIAA, (b) MT TIAA and (c) HT TIAA films. (d) Chemical structure evolution of TIAA film uponheating.

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easy source of TiOx because of its relatively easy hydrolysisprocess, guaranteeing the effective charge transport within thefilm, while the AcAc ligands with a temperature-sensitivedecomposition process enabled the controllable amount oftitanium complexes in the film,38 resulting in the work functiontunability of the as-fabricated film via simple control of theannealing temperature.Enhanced Photovoltaic Performance and Charge

Collection. The photovoltaic devices were made by adoptingthe n-i-p structure. The TIAA layers with different annealingtemperatures (LT, MT, and HT) were employed as the ETLs.All of the TIAA are amorphous, as shown in Figure S7. Thetransmittance spectra of the TIAA films with various annealingtemperatures are displayed in Figure S8, showing over 95%average transmittances in the visible light region, whichindicated that all three of the TIAA films could be suitableETL candidates. HFAQx-T/IDT-2BR, PBDB-T/IT-M, andPBDB-T-SF/IT-4F were employed as the three active layers.

Molybdenum oxide (MoO3) and Ag were used as the holetransporting layer and top electrode, respectively. The J−Vcurves and external quantum efficiency (EQE) spectra of thethree different active layers based on TIAA with variousannealing temperatures are displayed in Figure 3. Table 1summarizes the average and best device data based on theHFAQx-T/IDT-2BR, PBDB-T/IT-M, and PBDB-T-SF/IT-4Fwith TIAA films annealed at different temperatures as theETLs. The averages were calculated from 10 individual devices.For the devices based on HFAQx-T/IDT-2BR, the devices

with LT-annealed TIAA exhibited an average PCE of 6.9 ±0.2%. The highest Voc of 1.03 ± 0.01 V and the highest FF of56.7 ± 0.4% than the devices based on MT- (1.02 ± 0.01 Vand 53.8 ± 0.5%) and HT- (0.96 ± 0.01 V and 54.2 ± 0.6%)annealed TIAA may have resulted from the well-matched WFbetween the ETL and the LUMO of the acceptor. Similartrends were observed for the devices based on PBDB-T/IT-Mand PBDB-T-SF/IT-4F. The LUMO of IT-M is −3.98 eV,

Figure 3. J−V curves of devices based on (a) HFAQx-T/IDT-2BR, (b) PBDB-T/IT-M, and (c) PBDB-T-SF/IT-4F and EQE spectra of devicesbased on (d) HFAQx-T/IDT-2BR, (e) PBDB-T/IT-M, and (f) PBDB-T-SF/IT-4F with TIAA flims annealed at different temperatures under theillumination of an AM 1.5G solar simulator, 100 mW cm−2.

Table 1. Average and Best Device Data Based on HFAQx-T, IDT-2BR, PBDB-T: IT-M, and PBDB-T-SF, and IT-4F with TIAAFilms Annealed at Different Temperatures as the ETLs

PCE (%)

active layer anneal temperature of ETLs VOC (V) JSC (mA cm‑2) calculated JSC (mA cm‑2) FF (%) average best

HFAQx-T; IDT-2BR low 1.03 ± 0.01 11.8 ± 0.3 11.4 56.7 ± 0.4 6.9 ± 0.2 7.1HFAQx-T; IDT-2BR medium 1.02 ± 0.01 10.0 ± 0.4 9.5 53.8 ± 0.5 5.5 ± 0.2 5.7HFAQx-T; IDT-2BR high 0.96 ± 0.01 10.7 ± 0.4 10.6 54.2 ± 0.6 5.6 ± 0.3 5.8PBDB-T: IT-M low 0.91 ± 0.01 15.5 ± 0.4 15.1 65.3 ± 0.3 9.2 ± 0.3 9.4PBDB-T: IT-M medium 0.93 ± 0.01 16.8 ± 0.4 16.3 72.3 ± 0.4 11.3 ± 0.3 11.5PBDB-T: IT-M high 0.92 ± 0.01 16.3 ± 0.5 16.1 67.0 ± 0.3 10.0 ± 0.3 10.2PBDB-T-SF: IT-4F low 0.83 ± 0.01 17.5 ± 0.3 17.4 74.7 ± 0.4 10.9 ± 0.2 11.1PBDB-T-SF: IT-4F medium 0.83 ± 0.01 19.2 ± 0.3 19.1 74.0 ± 0.4 11.8 ± 0.2 12.0PBDB-T-SF: IT-4F high 0.86 ± 0.01 19.8 ± 0.3 19.3 75.7 ± 0.4 12.9 ± 0.2 13.2

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which is well-aligned with the WF of TIAA with MT annealing,and hence the average PCE of devices based on PBDB-T/IT-M with MT TIAA achieved 11.3 ± 0.2% with a Voc of 0.93 ±0.01 V, short-circuit current (Jsc) of 16.8 ± 0.4 mA/cm2 and

FF of 72.3 ± 0.4%, which is higher than the one with LT (9.2± 0.3%) and HT (10.0 ± 0.3%) TIAA. Notably, the averagePCE of the deep LUMO level system, PBDB-T-SF/IT-4F,with HT TIAA reached 12.9 ± 0.2% with a Voc of 0.86 ± 0.01

Figure 4. Corrected photocurrent data as a function of the potential difference V0 − V. Data are presented for the devices based on (a) HFAQx-T/IDT-2BR, (b) PBDB-T/IT-M, and (c) PBDB-T-SF/IT-4F.

Figure 5. AFM height images of the TIAA films with the following annealing temperatures: (a) low temperature (75 °C), (b) medium temperature(125 °C), and (c) high temperature (200 °C). AFM height images of the PBDB-T/IT-M active layers on top of TIAA films with the followingannealing temperatures: (d) low temperature (75 °C), (e) medium temperature (125 °C), and (f) high temperature (200 °C). 2D GIWAXSpatterns of the PBDB-T/IT-M active layers on top of TIAA films with the following annealing temperatures: (g) low temperature (75 °C), (h)medium temperature (125 °C), and (i) high temperature (200 °C).

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V, Jsc of 19.8 ± 0.3 mA/cm2 and FF of 75.7 ± 0.4%, which ishigher than the ones with LT (10.9 ± 0.2%) and MT (11.8 ±0.2%) TIAA. The best PCE attained for the HT TIAA-baseddevice is as high as 13.2%. It could be argued that the overallPCE was enhanced by employing an ETL with a well-matchedWF with the LUMO of the acceptor. As shown in parts d−f ofFigure 3, the trend observed in the EQE spectra was like theone seen for the Jsc, and the calculated Jsc values obtained fromthe integration of the EQE spectra with the AM 1.5G referencespectrum closely matched the measured values (the averagemismatch is smaller than 5%, Table 1). Additionally, the OPVdevice with TIAA as the ETL showed excellent stability inambient conditions for over 3 months (Figure S8).To further investigate the charge collection efficiencies in

devices based on HFAQx-T/IDT-2BR, PBDB-T/IT-M andPBDB-T-SF/IT-4F with TIAA annealed at different temper-atures, corrected photocurrent measurements were carried out.According to the literature,39 the corrected photocurrent isdefined as the device current under illumination as a functionof voltage after subtraction of the corresponding dark current(Jph = Jlight − Jdark). If the effective applied voltage (V0 − V,where V0 is the voltage at which the currents measured underillumination and in the dark are equal) is large enough, allcarriers generated by light absorption in the active layer will beswept out, leading to a saturated current (Jsat). The ratio of Jphto Jsat represents the product of the charge separation,transportation and extraction efficiencies. Figure 4 shows theJph/Jsat ratios for devices based on HFAQx-T/IDT-2BRemploying LT, MT and HT TIAA over a range of biases(V0 − V). The Jph/Jsat of the device with LT TIAA at lowelectric fields was much higher than that of the devices withMT and HT TIAA, which was attributed to the enhancedcharge collection efficiencies. Similarly, the PBDB-T/IT-Mdevices with MT TIAA and the PBDB-T-SF/IT-4F deviceswith HT TIAA exhibited better charge collection efficienciesthan the ones with other annealing temperatures. Theenhanced charge collection efficiencies were hypothesized tobe caused by the reduced energy barrier between the acceptorand ETL, a direct result of the well-matched WFs of the ETLswith the LUMO levels of the acceptors. We further comparedthe electron mobility of the TIAA films with differentannealing temperatures with the device structure of Ag/TIAA/Ag, a similar level of the electron mobilities wasobserved in the three films. (Figure S10)Morphology of the ETLs and the Influence on the

Active Layers. To further confirm that the enhanced PCEsobserved for all three OPV devices resulted from the enhancedcharge extraction efficiencies due to the well-aligned WFs ofthe ETLs with the LUMOs of the different acceptors, weconducted morphology analysis of the TIAA films withdifferent annealing temperatures and their influence on theactive layers to eliminate any morphological effects.40,41 Figure5a-5c show the atomic force microscopy (AFM) images of theLT, MT, and HT TIAA films, respectively. The surfaces of theTIAA films were uniform and smooth, and the root-mean-square (RMS) roughness values of the LT, MT, and HT TIAAfilms were 3.73, 3.21, and 2.51 nm, respectively. Although themorphological features of the HT TIAA film are different fromthose of the TIAA films with other annealing temperatures, wespeculate that the difference may not affect the finalmorphology of the active layer. Therefore, we conductedfurther AFM analysis to study the effect of the TIAA film onthe active layer, and we chose PBDB-T/IT-M as the object of

this study. As displayed in parts d−f of Figure 5, the RMSroughness values of the active layer on top of LT, MT, and HTwere 1.05, 1.04, and 1.01 nm, respectively, which are consistentwith previously reported values for typical OPV materials. Noobvious morphological differences of the surface were observedamong the active layers on the TIAA with various annealingtemperatures. The influence of the TIAA film on themorphology of the active layer was then investigated via twodimensional (2D) grazing incidence wide-angle X-ray scatter-ing (GIWAXS). GIWAXS is a powerful tool that can provideinformation regarding the molecular packing of the active layer.Figure 5g-5i show the 2D GIWAXS patterns of the PBDB-T/IT-M on top of TIAA films with various annealing temper-atures. The molecular packing behavior in all three films wassimilar in terms of their molecular crystallite orientation andaggregation. These suggest that all the active layers on top ofthe TIAA films with different annealing temperatures couldmaintain the optimized morphological features. Therefore, wecan speculate that the enhanced photovoltaic properties stemmainly from the unique energy alignments between the WFs ofthe ETLs and the LUMO levels of the acceptors.

■ CONCLUSIONIn summary, we have demonstrated a facile strategy to tune thework function of the ETL by simply varying the annealingtemperature to tune the chemical structure of the titaniumcomplex, which can be designed to match well with the LUMOlevels of the nonfullerene acceptors. The unique energy levelalignment is realized without any changes in the morphologicalfeatures of the active layer, and as a result, it can enhance thecharge collection efficiencies and thus ultimately the PCEs ofthe nonfullerene OPVs. Our design will provide new crucialinsights for realizing efficient nonfullerene OPVs.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.jpcc.9b06141.

XRD, FTIR, UPS, XPS, and UV−vis information on theTIAA films (PDF)

■ AUTHOR INFORMATIONCorresponding Author*(Y.Y.) E-mail: yangy@ucla.edu.ORCIDDong Meng: 0000-0001-6776-0707Pei Cheng: 0000-0002-1012-749XZhaohui Wang: 0000-0001-5786-5660Yingping Zou: 0000-0003-1901-7243Xiaowei Zhan: 0000-0002-1006-3342Yang Yang: 0000-0001-8833-7641Author Contributions⊥These authors contributed equally to this work.Author ContributionsR.W., J.X., and D.M. contributed equally to this work. R.W.,J.X., and D.M. deigned the experiments and performed thedata analysis. R.W. and P.C. performed the device fabrication.S.-Y.C., S.T., and Z.W. performed the characterization. J.Y. andY.Z. synthesized the HFQx-T. B.J. and X.Z. synthesized theIDT-2BR. Y.Y. supervised the project. All authors discussed theresults of the manuscript.

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NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSY.Y. acknowledges the Air Force Office of Scientific Research(AFOSR) (Grant No. FA2386-18-1-4094), the Office of NavalResearch (ONR) (Grant No. N00014-17-1-2,484), and theUC Solar Program (Grant No. MRPI 328368) for theirfinancial support. All the authors thank Mr. Yuqiang Liu, Mr.Hao-Wen Cheng, and Mr. Tianyi Huang for helpfuldiscussions during this project.

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