Increasing Quantum Efficiency of Polymer Solar Cells with Efficient

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Letter

Increasing Quantum Efficiency of Polymer Solar Cells with Efficient Exciton Splitting and Long Carrier Lifetime by Molecular Doping at Heterojunctions Han Yan, Yabing Tang, Xinyu Sui, Yucheng Liu, Bowei Gao, Xinfeng Liu, Shengzhong (Frank) Liu, Jianhui Hou, and Wei Ma ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.9b00843 • Publication Date (Web): 16 May 2019 Downloaded from http://pubs.acs.org on May 17, 2019

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ACS Energy Letters

Increasing Quantum Efficiency of Polymer Solar Cells with Efficient Exciton Splitting and Long Carrier Lifetime by Molecular Doping at Heterojunctions Han Yan,a,* Yabing Tang,a Xinyu Sui,b Yucheng Liu,c Bowei Gao,d Xinfeng Liu,b Shengzhong (Frank) Liu,c Jianhui Hou,d Wei Ma a,*

a

State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an, 710049, P. R. China

b

Division of Nanophotonics

CAS Key Laboratory of Standardization and Measurement for Nanotechnology

CAS Center for Excellence in Nanoscience

National Center for Nanoscience and Technology, Beijing, 100191, P. R. China

ACS Paragon Plus Environment

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c

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Key Laboratory of Applied Surface and Colloid Chemistry, National Ministry of

Education

Shaanxi Key Laboratory for Advanced Energy Devices

Shaanxi Engineering Lab for Advanced Energy Technology

Institute for Advanced Energy Materials, School of Materials Science and Engineering, Shaanxi Normal University, Xi’an, 710119, P. R. China

d

State Key Laboratory of Polymer Physics and Chemistry

Beijing National Laboratory for Molecular Sciences

CAS Research/Education Center for Excellence in Molecular Sciences

Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100049, P. R. China

Corresponding Author * E-mail: [email protected] (H. Yan.); [email protected] (W. Ma)

Abstract


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Optimizing the photovoltaic processes directly by electric technique attracts the exploration of molecular doping in organic photovoltaics (OPVs). However, the inappropriate and inhomogeneous dopant distribution in the bulk heterojunction (BHJ) film inhibits the performance improvement and mechanism understanding in doped OPVs. A strategy to solve these critical problems is reported here. By employing a planar heterojunction (PHJ) device structure, the role of dopant location and photovoltaic performance is clarified. Dopants- tris(pentafluorophenyl)-borane (BCF) distributing at the heterojunction rather than in the single component (poly[(2,6-(4,8bis(5-(2-ethylhexyl)thiophen-2-yl)-benzo[1,2-b:4,5-b’]dithiophene))-alt-(5,5-(1’,3’-di-2thienyl-5’,7’-bis(2-ethylhexyl)benzo[1’,2’-c:4’,5’-c’]dithiophene-4,8-dione))] (PBDB-T) or 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4hexylphenyl)-dithieno[2,3-d:2’,3’-d’]-s-indaceno[1,2-b:5,6-b’]dithiophene

(ITIC))

produces the best performance with quantum efficiency (QE) enhancement. Mechanism studies reveal the facilitated exciton splitting and suppressed bimolecular recombination as the causes for device improvement. Sequential coating procedure is further developed for the doped BHJ cells, and the devices based on this method exhibit a


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short-circuit current (Jsc) increase of 1.3 mA/cm2, which is an impressive value comparing with the previous reports.

TOC GRAPHICS

The QE defining as the ratio of collected electron to incident (absorbed) photon is the key parameter of efficient solar cell. Unlike the inorganic counterparts, the OPVs convert the photons into the Frenkel excitons with large Coulombic interaction, and it requires the donor-acceptor heterojunction to provide extra energy for exciton splitting into free carriers. Meanwhile the heterogeneity aggravates the recombination of mobile carriers, namely bimolecular recombination in the active layer.1-3 These traits increase internal loss pathways for QE in OPVs. Learning from the Onsager-Braun theory,


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enlarging

the

energetic

offset

at

heterojunction

and

facilitating

intramolecular/intermolecular charge delocalization are the two aspects for efficient exciton

splitting.4

Given

the

complexity

of

different

models

for

bimolecular

recombination, the charge mobility, carrier lifetime, and trap density are taken into account to enhance the free carrier transport.5 In the present OPVs studies, all of these electronic properties are optimized by materials’ design combining with morphology control;6-12 the valid tuning strategy based on electronic technique is still deficient and emergent to be developed. Molecular doping is a powerful mean to control the electric properties of organic semiconductors.13,14 Molecular dopant undergoes a charge transfer reaction (redox or acid-base reaction) with the organic semiconductor and creates charge carriers on it.15,16 Inspired by its merits of tuning the built-in potential at the organic-organic interface,17,18 lowering the activation energy for carrier release,19 enhancing the charge mobility by filling-up tail states,20,21 molecular doping is believed to bring performance improvement for OPVs. The rational molecular doping in BHJ was firstly demonstrated by Deschler et al. in 2011.22 Tunc et al. followed up the work and presented a device


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based study in 2012, where the performance improvement was originated from the enhanced QE, resulting in improved Jsc.23 However, the QE enhancement hasn’t always been the feature in later studies, and the role of molecular doping remained speculative. The initial mechanism study by Deschler et al. demonstrated that molecular doping increased the photoinduced polaron density with a reduced formation of emissive charge-transfer excitons.22 Later, Zhang et al. revealed that an increase of the background charge concentration (charge concentration in dark) by doping and the efficient charge collection were responsible for the QE enhancement.24 In addition, Loiudice et al. suggested that the improved charge transport properties are the reasons for better device performance.25 Furthermore, Shang et al. and we investigated that addition of dopant at low content was able to fill the trap states within OPVs.26,27 Taking advantage of X-ray characterization at synchrotron facility, Xiong et al. and we demonstrated morphology optimization as an important cause for performance enhancement in molecularly doped blend films.28,29 Detailed studies on the effects of solvents and additives with multiple X-ray techniques will provide comprehensive understanding of the doped morphology and guide the further performance optimization.30 Analyzing all these


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reports, we assume that the various impacts and mechanisms of molecular doping may arise from the doped morphology, namely the dopant location in the active layer. Apparently, the depressed charge-transfer state recombination requires the dopants distributing at the donor-acceptor heterojunction. The trap-filling occurs both in bulk and at heterojunction. The generation of background charge, improved charge transport, and morphology optimization will be more effective when the dopant blend or cocrystallize

with

the

semiconducting

materials.31-33

The

inappropriate

and

inhomogeneous dopant distribution in the BHJ film inhibits the favorable features and precise understanding of the doping effect in OPVs. In this contribution, we study the effects of p-type doping by BCF on the performance of polymer solar cell (PSC) in PHJ device structure. A non-fullerene based model system PBDB-T and ITIC were adopted for our investigation. (Figure 1a) We aim at fully exploring the features of dopant distribution on charge dynamics, device physics, and photovoltaic performance of PSCs. By controlling the BCF location in PHJ devices, we demonstrate that the significant QE improvement is achieved when BCF distributes at the PBDB-T/ITIC heterojunction rather than in any single component. Importantly, we


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find that doping at heterojunction leads to efficient exciton splitting with reduced geminate recombination. The increased carrier lifetime under favored interfacial electric field guarantees carriers sweeping out to the electrodes, contributing to the increase of QE value. Learning from the PHJ model, we further develop the sequential doping process for BHJ PSC by triple-layer spin-coating. With the pre-controlled BCF distribution, we obtain a QE improvement in molecular doped cell, and the corresponding Jsc increase is the best among the efficient material systems. PSCs with a PHJ structure were fabricated to investigate the effect of dopant distribution in the PBDB-T, ITIC, and at the heterojunction. To obtain a sharp PBDBT/ITIC interface, the floating-film-transfer method was used to stack the PBDB-T layer on the ITIC layer. We floated the PBDB-T film in water, and used ITIC coated cathode substrate to scoop up the free-standing film. The corresponding photographs are shown in the inset of Figure 1b. We then fabricated the PHJ devices with similar film thickness of 35 nm (half for each layer) after removing the residual water in vacuum at room temperature. The current density versus voltage (J-V) characteristics of PHJ solar cells are tested under AM 1.5G illumination (Figure 1b), as detailed in Table 1and Figure S1.


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The most impressive result is that the Jsc significantly rises when BCF distributes at the PBDB-T/ITIC interface. Adding BCF layer between the two components by spin-coating 0.01 mg/mL methanol solution produces a Jsc increase from 2.25 mA/cm2 to 2.81 mA/cm2. The Jsc enhancement is supported by the external quantum efficiency (EQE) measurement, which resulted in the respective values of 2.20 mA/cm2 and 2.79 mA/cm2 for control and BCF at heterojunction devices. (Figure 1c) We notice that the EQE enhances through the whole response range, suggesting that both layers benefit from the heterojunction doping. Regardless of the slightly reduced open-circuit voltage (Voc) and fill factor (FF), the overall power conversion efficiency (PCE) increases from 1.20% to 1.39%. Although adding 0.5 wt% of BCF in PBDB-T layer also increases the Jsc to 2.46 mA/cm2, the PCE is tied due to lower Voc and FF. Adding BCF in ITIC makes no enhancement in device performance. To provide a better overview of the effect of BCF distribution on the solar cell performance, the detailed parameters are summarized in Figure 1d and e. As already apparent in the J-V and EQE curves, doping at heterojunction is most effective for QE enhancement. According to the coincided trends


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of PCE and Jsc, the improvement in efficiency can be clearly attributed to the QE increase.

Figure 1. (a) Scheme of PHJ device structure and chemical structures of PBDB-T, ITIC, and BCF. (b) Photovoltaic performances of PBDB-T/ITIC in PHJ device structure. The insets are the photos of PBDB-T, ITIC, and PHJ films. (c) Corresponding EQE curves. (d) Plots of Voc and FF under different doping conditions in PHJ device structure. (e) Plots of Jsc and PCE under different doping conditions in PHJ device structure. The doping conditions are defined as follows: control (undoped PHJ devices), BCF in PBDBT (PHJ devices with 0.1 wt% addition of BCF in PBDB-T), BCF in ITIC (PHJ devices


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ACS Energy Letters

with 0.1 wt% addition of BCF in ITIC), and BCF at heterojunction (PHJ devices with spin-coating 0.01 mg/mL BCF solution between PBDB-T and ITIC).

Table 1. Photovoltaic performances of PBDB-T/ITIC in PHJ device structure. Average value of 15 devices fabricated under identical conditions ±1 standard deviation. The control devices define as the undoped PHJ solar cells.

Materials Control 0.1 wt% BCF in 0.5 wt% PBDB-T 1 wt% 0.1 wt% BCF in 0.5 wt% ITIC 1 wt% 0.005 mg/mL BCF at 0.01 mg/mL heterojunction 0.05 mg/mL

Voc max (V)

FFmax (%)

Jsc max (mA/cm2)

Jsc avg (mA/cm2)

PCEmax (%)

PCEavg (%)

0.85 0.84 0.81 0.78 0.84 0.83 0.82 0.83 0.81 0.76

62.6 64.9 60.7 59.1 61.7 60.3 55.2 61.3 60.4 42.7

2.25 2.30 2.46 2.12 2.26 2.08 1.81 2.55 2.81 2.01

2.15±0.09 2.21±0.07 2.41±0.08 2.03±0.06 2.16±0.05 1.98±0.07 1.79±0.06 2.47±0.07 2.73±0.06 1.92±0.06

1.20 1.25 1.21 0.99 1.18 1.05 0.82 1.30 1.39 0.66

1.14±0.04 1.18±0.03 1.17±0.05 0.92±0.03 1.13±0.03 0.98±0.04 0.81±0.05 1.23±0.04 1.31±0.04 0.60±0.04

The PHJ device provides us an opportunity to study the photophysics and device physics of molecular doping on the QE enhancement. In our previous report, small amounts of BCF can improve photovoltaic performance by optimizing film morphology.29


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Here, we do not observe a morphology change with BCF in PBDB-T/ITIC or on PBDB-T surface characterizing by grazing incident wide angle X-ray scattering (GIWAXS). (Figure S2 and S3, Table S1 and S2) In addition, the overlapped absorption profiles of the PHJ samples show that the exciton yield could neither account for the QE improvement. (Figure S4) Then we study the mechanism by the photoluminescence (PL) spectroscopy. (Figure 2a) We find that both of the PHJ films show moderate PL quenching due to interfacial charge transfer, and BCF doping at heterojunction increases the quenching extent. Hence the PL measurements correlate the QE variations with the exciton splitting at PBDB-T/ITIC interface. We perform the ultrafast transient absorption (TA) spectroscopy measurement to study the charge transfer dynamics. For the neat PBDB-T, the ground-state bleaching (GSB) signal appear in the range of 500~680 nm, (Figure S5) The GSB signal stems from the singlet excitons, and these excitons leak to photoinduced absorption (PIA) over 680 nm. In the neat PBDB-T, we do not see an isobestic point, the zero-crossing redshifts at longer delay time due to exciton relaxation from higher energy state.34 Limited by the detecting range, we only observe the GSB signal over 560 nm in the neat ITIC


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film. (Figure S6). The TA spectra in the PHJ films show distinct features from the neat films. (Figure 2b and c) Here, we observe an isobestic point at ~665 nm in both of control and heterojunction doped PHJ films, suggesting the excitons directly split at higher energy of singlet state.35,36 In the short delay time within 1 ps, we observe a larger extent of GSB decay in the heterojunction doped film, revealing faster charge transfer at the interface. This is further supported by the shorter lifetime of the photoinduced signals at 677 nm, it decreases from 678 fs to a value over the time resolution (~130 fs) after heterojunction doping. (Figure 2d and e) Moreover, the interfacial recombination can be estimated from the long-lived component of GSB signals.35,37 The fitted lifetime enlarges from 6.74 ns to 9.18ns (see supporting information for details), demonstrating that the interfacial recombination is hindered with BCF addition (Figure 2f). In the following we analyze more closely the dynamics of exciton splitting by timeresolved PL spectrum. (Figure 2g-i) As shown in Figure 2a, the PHJ blends show distinct emission peaks of PBDB-T at 678 nm, and ITIC at 775 nm, thus we study the radiative decays separately in both components. The donor-acceptor heterojunction works as a new recombination center for each component, and the rate equations


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describing the radiative recombination at heterojunction are shown in the following equations. (Equation 1-4)

PBDB-T: kheterojunction = kcontrol ― kPBDB ― T

kBCF + heterojunction = kBCF at heterojunction ― kPBDB ― T

ITIC: kheterojunction = kcontrol ― kITIC

kBCF + heterojunction = kBCF at heterojunction ― kITIC

(1)

(2)

(3)

(4)

where kheterojunction and kBCF+heterojunction are the radiative recombination rates at PBDBT/ITIC interface before and after BCF doping, kcontrol and kBCF at heterojunciton are the total radiative recombination rates in the PHJ films, kPBDB-T and kITIC denote the radiative recombination rates in neat films. The values in neat and PHJ films are fitted according to the decay dynamics in Figure 2g-i. The calculated radiative recombination rates at heterojunction are 1.10×10-10 s-1 for PBDB-T and 9.47×10-10 s-1 for ITIC. After BCF doping at heterojunction, the recombination rates slow down to 0.64×10-10 s-1 and 8.95×10-10 s-1, demonstrating lower interfacial recombination. We assign these radiative


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ACS Energy Letters

decays to geminate recombination rather than bimolecular recombination according to its time-scale.38 We note that the lifetimes here are consistent with those from the GSB at 635 nm, confirming that doping BCF at heterojunction facilitates the exciton splitting by suppressing the geminate recombination. We carried out the same photophysical studies on the PHJ sample with 0.1 wt% BCF in PBDB-T. (Figure S7) Its dynamics is quite similar to the control one, and we do not observe doping benefits from it.


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Figure 2. (a) Steady state photoluminescence spectra recorded using 400 nm excitation. (b-f) Transient absorption spectra: (b, c) transient absorption spectra of PHJ samples using 400 nm laser pulse excitation; (d, e) decay curves of PHJ samples probing at 677 nm (PIA singnal of PBDB-T); (f) decay curves of PHJ samples probing at 635 nm (GSB signal of PBDB-T). (g-i) Transient PL spectra: (g) decay curves of PBDBT (678 nm) and ITIC (775 nm); (h) decay curves of PHJ samples probing at 678 nm (emission peak of PBDB-T); (i) decay curves of PHJ samples probing at 775 nm (emission peak of ITIC). To evaluate the role of BCF doping at heterojunction, we examined the light intensity dependence of Jsc, and the results are shown in Figure 3a. The extracted slopes of the control and BCF at heterojunction samples are 0.87 and 0.96, demonstrating that the bimolecular recombination is suppressed by the heterojunction doping.39 Supposing that the surface doping doesn’t change the charge mobility in bulk, we then examine the carrier lifetime by performing transient photovoltage (TPV). (Figure 3b) At a light intensity of 95 mW/cm2, the carrier lifetime increases from 5.65 μs to 8.27 μs (see supporting information for details). The longer carrier lifetime reduces the bimolecular


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ACS Energy Letters

recombination at the heterojunction. We further probe the interfacial electronic structures to explore the mechanism for lower bimolecular recombination with ultraviolet photoelectron spectroscopy (UPS). As shown in Figure 3c, the energetic gap between Fermi level (EF) and highest occupied molecular orbit (HOMO) becomes narrower from 0.78 eV to 0.60 eV due to the p-type doping increased hole concentration, which is absent in the 0.1 wt% BCF doped film (Figure S8). Combining with the high energy part (Figure 3e), the EF of PBDB-T moves from -3.79 eV to -4.05 eV, accompanying with the HOMO shifting from -4.57 eV to -4.65 eV when adding BCF on its surface. As expected, depositing BCF on ITIC surface does not change the surface energetic structure (Figure 3d and f). Based on these data, the schematic band diagrams of the PBDB-T/ITIC heterojunction are illustrated in Figure 3g and h, where the EF and HOMO level of ITIC are set at -3.93 eV and -5.25 eV. It is clearly shown that the band bending of PBDBT/ITIC heterojunction reverses after molecular doping. The direction of built-in electric field is obviously positive for charge separation and transport after BCF doping at heterojunction. As the absolute value of the band bending merely changes from 0.14 eV


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to 0.12 eV, the charge transport at the heterojunction is not influenced by the space charge limited region.

Figure 3. (a, b) Device studies of control and BCF at heterojunction PHJ samples: (a) Jsc plots against light intensity; (b) TPV curves. (c-f) UPS spectra of the neat and BCF on PBDB-T or ITIC films under -10 V bias: (c) low and (e) high energy part of PBDB-T films; (d) low and (f) high energy part of ITIC films. (g, h) The schemes of band diagrams according to the UPS spectra: (g) undoped; (h) BCF at heterojunction.


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Having studied the heterojunction doping mechanism, we continue searching for its feasibility in BHJ device. The BHJ device can be fabricated in two techniques: coating the blend solution in one-step and coating the neat solution sequentially. 40,41 We start from the one-step spin-coating by blending BCF with PBDB-T/ITIC in solution. (Figure 4a, Table 2) The addition of 0.1 wt% BCF leads to PCE increase from 9.52% to 10.03%. However, we barely see the QE enhancement in Figure 4c, accompanying with minor Jsc increase from 15.26 mA/cm2 to 15.59 mA/cm2. Higher BCF content reversely reduces all parameters and produces a lower PCE of 9.51%. In contrast, we design to control the BCF distribution by sequential coating. We spincoat the ethanol solution of BCF on PBDB-T, and then spin-coat ITIC in CB on top of BCF layer. We assume that the pre-separated BCF with PBDB-T and ITIC helps form the heterojunction doping in the BHJ active layer. Agreed with our assumption, when we spin-coat the 0.0001 mg/mL BCF solution as a middle layer, the Jsc sufficiently increases from 15.33 mA/cm2 to 16.67 mA/cm2. (Figure 4b, Table 2 and Figure S9) The increased Jsc is well supported by the EQE plot, and the improvement spreads over response region from 480 nm to 750 nm. (Figure 4d) The morphology optimizing is


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excluded by GIWAXS characterization (Figure S10), where the crystalline is disturbed with BCF addition. Although higher BCF concentration of 0.01 mg/mL further increases the Jsc to 16.87 mA/cm2, the losses of Voc and FF harm the overall efficiency. To further support to the controlling of BCF distribution, we co-blend the BCF with PBDB-T in solution. Under this procedure, the EQE curve even decreases a bit in the short wavelength range, producing a slightly lower Jsc of 15.20 mA/cm2. We plot Jsc and PCE resulting from different doping techniques, it demonstrates that the QE improvement with Jsc increase can be achieved by pre-controlling the dopant location. (Figure 4e) We compare the Jsc in this work with the previous reports on doped BHJ OPVs, (Figure 4f) and we highlight that we obtain the best value among the efficient material systems.23,2529,42-48


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Figure 4. (a, c) Photovoltaic performances of PBDB-T/ITIC fabricated by one-step spincoating: (a) J-V curves; (c) corresponding EQE curves. (b, d) Photovoltaic performances of PBDB-T/ITIC fabricated by sequential spin-coating: (b) J-V curves; (d) corresponding EQE curves. (e) Plots of Jsc and PCE under various fabricating processes. (f) Summary of Jsc improvement in different reports. The doping conditions are defined as follows: BHJ_Control (undoped BHJ devices), BHJ_Doped (0.1 wt% addition of BCF in BHJ devices), Seq_Control (undoped devices using sequential coating procedure), Seq_Bulk doping (devices with 0.1 wt% addition of BCF in PBDB-T solution using sequential


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coating procedure) and Seq_Heterojunction doping (devices with spin-coating 0.0001 mg/mL BCF solution between PBDB-T and ITIC using sequential coating procedure).

Table 2. Photovoltaic performances of PBDB-T/ITIC in BHJ structure. Average value of 10 devices fabricated under identical conditions ±1 standard deviation. The BHJ_Control devices define as the undoped BHJ solar cells. The Seq_Control devices define as undoped devices using sequential coating procedure.

Conditions BHJ

Seq BHJ

BCF in PBDB-T BCF at Heterojunc tion

0 wt% 0.1 wt% 0.5 wt% 0 wt% 0.1 wt% 0.5 wt% 0.0001 mg/mL 0.01 mg/mL 0.1 mg/mL

Voc max (V)

FFmax (%)

Jsc max (mA/cm2)

Jsc avg (mA/cm2)

PCEmax (%)

PCEavg (%)

0.92 0.91 0.90 0.92 0.91 0.89 0.88 0.87 0.86

64.5 66.8 65.9 63.4 64.3 62.9 62.5 60.8 43.0

15.41 15.90 15.69 15.33 15.20 15.21 16.67 16.87 11.02

15.26±0.15 15.59±0.28 15.17±0.27 15.25±0.08 15.23±0.09 15.23±0.10 16.52±0.09 16.85±0.07 10.59±0.50

9.52 10.03 9.51 9.32 9.33 8.66 9.65 9.34 4.04

9.39±0.09 9.74±0.14 9.26±0.15 9.13±0.07 9.20±0.12 8.58±0.07 9.59±0.04 9.22±0.10 3.87±0.15

In conclusion, molecular doping with confined dopant distribution has been investigated in PHJ devices so as to re-envisage its effect on the QE in PSCs. When


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the p-type dopant BCF locates at the PBDB-T/ITIC heterojunction, the Jsc receives a considerable increase from 2.25 mA/cm2 to 2.81 mA/cm2 due to stronger QE over the whole response region. With the heterojunction doped PHJ structure, we observe faster decay of the photoinduced absorption and slower geminate recombination rate, which indicates that the ultra-thin doping layer can efficiently facilitate the exciton splitting while suppress reverse recombination at the heterojunction. We also find that the heterojunction doping strategy produces longer carrier lifetime due to favoured interfacial electric field which further depresses the bimolecular recombination before charge collection. Motivated by these results, we use sequential coating procedure to pre-control the BCF distribution in BHJ PSCs. This specific procedure leads to the highest Jsc increase among the efficient material systems. We are convinced that these findings are important because we have answered three critical questions of the active layer doped PSCs: the favored dopant distribution, the device working mechanism, and the feasible fabricating procedure in BHJ devices. We believe that promoting the electronic properties by molecular doping together with the design of novel materials,


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the optimization of morphology, and the inclusion of efficient interfacial layer49,

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50

will

continue driving the development of PSCs in the future.

ASSOCIATED CONTENT

Supporting Information.

Experimental procedures and additional results.

AUTHOR INFORMATION

E-mail: [email protected] (Prof. Han Yan); [email protected] (Prof. Wei Ma)

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors thank for the support from Ministry of Science and Technology of China (2016YFA0200700), National Natural Science Foundation of China (51803162, 21875182), and Natural Science Foundation of Shaanxi Province (2018JM5007). UPS data were collected at Instrument Analysis Center of Xi'an Jiaotong University. X-ray


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data were acquired at beamlines 7.3.3 and 11.0.1.2 at the Advanced Light Source, which was supported by the Director, Office of Science, Office of Basic Energy Science, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. The authors thank Chenhui Zhu at beamline 7.3.3 and Cheng Wang at beamline 11.0.1.2 for assistance with data acquisition.

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