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Organic Electronic Devices
Reconstructing Space- and Energy-dependent Exciton Generation in Solution-processed Inverted Organic Solar Cells Yuheng Wang, Yajie Zhang, Guanghao Lu, Xiaoshan Feng, Tong Xiao, Jing Xie, Xiaoyan Liu, Jiahui Ji, Zhixiang Wei, and Laju Bu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b14698 • Publication Date (Web): 28 Mar 2018 Downloaded from http://pubs.acs.org on March 30, 2018
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Reconstructing Space- and Energy-dependent Exciton Generation in Solution-processed Inverted Organic Solar Cells Yuheng Wang1, Yajie Zhang2, Guanghao Lu1,*, Xiaoshan Feng3, Tong Xiao1, Jing Xie4, Xiaoyan Liu5, Jiahui Ji5, Zhixiang Wei2,*, Laju Bu3,*
1 Frontier Institute of Science and Technology, Xi’an Jiaotong University, Xi’an 710054, China
2 CAS key laboratory of nanosystem and hierarchical fabrication, Beijing 100190, China
3 School of Science, Xi’an Jiaotong University, Xi’an 710049, China
4 School of Life Science and Technology, Xi’an Jiaotong University, Xi’an 710049, China
5 School of Electrical Engineering, Xi’an Jiaotong University, Xi’an 710049, China
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KEYWORDS: light absorption, small molecule solar cells, organic photovoltaics, inverted devices, vertical phase separation, optical simulation,
ABSTRACT:
Photon absorption induced exciton generation plays an important role in determining the photovoltaic properties of donor: acceptor organic solar cells with an inverted architecture. However, reconstruction of light-harvesting and thus exciton generation at different locations within organic inverted device are still not well resolved. Here we investigate the film-depth-dependent light absorption spectra in a small molecule donor: acceptor film. Including depth-dependent spectra into an optical transfer matrix method allows us to reconstruct both film-depth- and energy-dependent exciton generation profiles, using which short-circuit current and external quantum efficiency of the inverted device are simulated and compared with the experimental measurements. The film-depth-dependent spectroscopy, from which we are able to simultaneously reconstruct light harvesting profile, depth-dependent composition distribution and vertical energy level variations, provides insights into photovoltaic process. In combination with appropriate materials processing methods and device architecture,
the
method
proposed
in
this
work
will
help
optimize
film-depth-dependent optical/electronic properties for high performance solar cells.
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1. INTRODUCTION Organic semiconductors including small molecules and polymeric semiconductors are widely investigated for next generation flexible electronics.1-7 As compared with polymeric semiconductors which are usually multi-dispersed in terms of molecular structure, small organic molecules are of great interests because they have clear chemical structure and could be easily purified for less dispersed performance.8 Currently, organic electronics based on small organic molecules are widely used in organic light emission diodes for display applications. As a comparison, the performance of the state-of-the-art small molecule photovoltaic devices is still too low to be commercially applied. In recent years, the power conversion efficiency of solution-processed small molecule organic photovoltaics has been significantly increased to be higher than 10%, demonstrating a very bright prospect for future energy conversion.8 However, unlike thermally co-evaporated donor: acceptor bulk heterojunction, vertical phase segregation of solution-processed film is commonly observed in donor: acceptor bulk heterojunction,9 which induces a significant variation of light harvesting and thus exciton generation properties along directions of film thickness.10 In order to utilize variable morphology along the vertical direction of the donor: acceptor active layer, two representative device architectures, namely conventional and inverted structures,11-12 are developed to fabricate solar cells. The two types of devices require different materials for cathode, anode, hole/electron transport layers and active layer, which significantly influence the vertical phase evolution of active layer.9 Therefore, for lots of donor: acceptor active layer, inverted
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architecture has higher performance as compared with conventional devices. Moreover, inverted organic solar cells are usually more stable and more compatible with industrial processing than conventional devices. In order to correlate efficiency of inverted device with the vertical variation of composition and optical properties, light-harvesting and thus exciton generation at different location within the inverted device are highly required. The bulk heterojunction here is a composite of two-phase blends consisting of a p-type electron donor and an n-type electron acceptor. The p-type material needs to be connected to the anode and the n-type material needs to be connected to the cathode. The p-type material and the n-type material form an interpenetrating network in the film for effective charge transfer and charge transport, thereby achieving high power conversion efficiency. The intermixture morphology of the donor and acceptor and the compositional ratio along the vertical direction of the film directly affect the power conversion efficiency of the device because these parameters are closely related to exciton generation, energy/charge transfer and charge transport. Therefore, in order to optimize the performance of photovoltaic thin films, one needs to analyze how sunlight is attenuated in thin films, as well as the position of exciton generation, and to establish relationship between deep-resolved optical properties and photovoltaic performance.13-18 In fact, for small molecule donor: acceptor film, the variations of light absorption along film-depth direction are related to the facts as follows: 1) Donor and acceptor are usually not fully compatible, and during solution-processing e.g. spincoating, vertical separation between two components is commonly observed. 2) Molecules at
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different film-depth might be featured with different molecular conformation and crystallization, which have an important effect on conductance/valance levels and thus light absorption spectra. 3) Donor: acceptor is generally semicrystalline, while amorphous
and
crystalline
materials
have
different
optical
properties.
Film-depth-dependent crystallinity contributes to the film-depth-dependent light absorption. 4) Molecules at the layer-layer interface might have a different molecular or crystal orientation from that inside the active layer, leading to the variable absorption spectra along film-depth. However, correlation the photovoltaic performance of such small molecules organic devices with light harvesting profiles inside the device is still not well resolved. In recent years, film-depth-dependent light absorption spectra have been used to study film-depth-dependent composition, charge transport and light propagation in polymer field-effect transistors and polymer solar cells with conventional architecture. 4, 15-17
UV-Vis absorption spectroscopy is a commonly used analytical technique to
characterize the organic solar cell. This method is easily accessible and can provide insight into the morphology and composition of the film. Oxygen plasma contains a variety of reactive components, including O2 +, O2-, O3, O, O+, O-, ionized ozone, and metastable oxygen excited states, and these highly reactive components can etch the material from the surface of the organic film. Therefore, if the oxygen plasma pressure is high, these active components will diffuse below the surface of the film,15 which may cause chemical reaction with the internal components of the film and change the optical absorption characteristics of the material inside the film, leading to
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many difficulties for resolving the absorption spectrum of sub-layer film. It was found that at low pressure (< 30 Pa), oxygen plasma could incrementally etch the organic film without damaging the materials under the surface.15-16 Therefore, the light absorption spectra of the film could be monitored during the plasma etching. From Beer-Lambert’s law, the light absorption spectra at different film-depth can thus be derived. In this work, we investigate the film-depth-dependent light absorption spectra in a small molecule donor: acceptor film which is subsequently used in inverted solar cells with a high power-conversion efficiency approximately 8%. Using such spectra, composition profile along the vertical direction is obtained. Taking the optical properties of different layers including electrodes and hole/electron transport layers into account, we include the film-depth-dependent absorption spectra of the active layer into an optical interference model to simulate the film-depth-dependent and energy-dependent exciton generation profiles, which were subsequently used to simulate short-circuit current and external quantum efficiency. Our numerical simulations are well consistent with the experimentally measured photovoltaic performance.
2. EXPERIMENTAL SECTION 2-(thiophen-2-yl)thieno[3,2-b]thiophene
as
p-bridges
and
end-capped
with
1H-indene-1, 3(2H)-dione (abbreviated as BTID) was synthesized as reported in our previous work.8 Inverted devices with a structure of glass/ITO/ZnO/active layer/MoOx/Ag is
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prepared.8 A layer of ZnO precursor was spin-coated onto the ITO. The mixture of BTID (Figure 1) and PC71BM with total concentration ca. 18 mg/ml was used for subsequent spin-coating at 40 oC. Finally, 5 nm MoOx and 100 nm Ag layer were evaporated to complete the device fabrication. The J-V curves were measured using Keithley 2400 under AM 1.5G illumination (Newport Thermal Oriel 91159A solar simulator). EQEs were performed with an Oriel Newport system (Model 66902). The film-depth-dependent light absorption spectra were measured in a home-made setup. We integrated the plasma etching machine (PT-15S, SANHOPTT 300 W) with the ultraviolet-visible (UV-Vis) optical fiber spectrometer (PG2000-Pro, Ideaoptics) to ensure that the light is vertically aligned with the film plane and that the in-situ absorption spectra can be acquired during incremental etching. The size of observed region during the absorption measurement is 1 cm (diameter), and we use condenser lens to collect the transmitted light and scattered light. Atomic force microscopy (AFM) with tapping mode (INNOVA, Veeco) is used to characterize the photovoltaic film before and after etching (Figure S1). Raman spectra were acquired at the excitation wavelength 532 nm (LabRAM HR Evolution, HORIBA) (Figure S2). 3. RESULTS AND DISCUSSION In this work we reconstruct the light-induced exciton generation at different positions inside the inverted solar cells where both material and optical properties are very variable along the vertical direction of the donor: accept active layer. The
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continuous Beer-Lambert law and the simplified optical model are used to provide insights into thin-film morphology and light-material interactions, which finally determines the device performance. According to Beer-Lambert's law, the absorbance of the film is defined as
A = log( I / I 0 ) , where I and I 0 are intensity of transmitted and incident light, respectively. We can assume that the whole film is composed of n layers (Figure 1), and each layer has an absorbance A i (i = 1 , 2 , 3 , L , n ) . Therefore, the intensity of incident light after passing through n sublayers is obtained as: n
n
I = I 0 ∏ 10 − Ai = I 0 10
−
∑ Ai i =1
(1)
i =1
This shows that the total absorbance of the film is equal to the linear superposition of the absorbance of each layer. As is shown in Figure 2(a), through surface-selective etching without underneath damage (Figure S2),15 plasma acts on the film surface and the film thickness decreases gradually. Afterwards, we get in the etching process a series of spectra according to the absorbance of the surface and the reduced amount of each sublayer that is further analysis of each layer component. A series of absorption spectral data obtained before and after etching are subtracted layer by layer, getting each layer’s absorption intensity data as ∆ Ai = Ai +1 − Ai (i=1, 2, 3, L , n-1) , which is shown in Figure 2(b). When the absorbance of each layer is subtracted, the peak dislocation is obviously observed, and the effect of vertical distribution of the component on the active layer is further verified. The variation of the absorption peak along film-depth direction is due to the different composition or crystallinity at different position. The proportion distribution of donor and acceptor in each layer is
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then calculated by least square method. Figure 2(c) describes vertical distance distribution of blend composition, obtained from the absorbance of pure samples and non-etched film as shown in Figure 2(d). This vertical phase separation of acceptor and donor reveals the exciton formation site indirectly. Figure 2(e) shows the dependence of absorption on etching time for different wavelengths (330 nm, 480 nm and 600 nm, respectively). Figure 2(f) shows the approach for fitting the sublayer absorption spectrum of the blend by the spectra of pure BTID and pure PC71BM. In principle, the thickness of the film is positively correlated with the intensity of the absorption spectrum. Therefore, we can establish a quantitative relationship between the intensity of the absorption spectrum and the thickness of the film by using a numerical method, according to the change of the absorption spectrum during etching. This means that we do not need a separate film to measure the film thickness after each etching. Since the sample thickness is known, the spectral constants can be used to calculate the optical constants of the samples. For the films, the refractive index and extinction coefficient are satisfied with K-K consistency principle.19 Because of the interference effect, the interference peaks existing in the transmission and reflection spectra (T and R) of the films, the curve T/(1-R) shows a monotonic increasing trend. The absorption coefficient α of the films can be calculated by the monotonicity. Thus, when the wavelength of the incident light is λ, the absorption intensity A and extinction coefficient k of the film are approximately linear with the film thickness d as equations (2)-(5) describe.
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1 d
α =- ln(
α=
T ) 1− R
4π k
λ
I ≈ I 0e−α d
∆Ai (λ ) = − lg(e −4π ki d / λ )
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(2) (3) (4) (5)
For organic solar cells, the influence of the refractive index n on the absorption intensity is not large.20 The extinction coefficient k of the sublayer in the film fluctuates in the film thickness direction, which is due to the inhomogeneous distribution of the film in the film-depth direction.6,20 Therefore, we use the transfer matrix method more precisely for the optical simulation of the device. According to the fluctuation of extinction coefficient at each thickness of the film, the active layer film is divided into several sublayers. Each sublayer has a different extinction coefficient k, and we assume that the interior of each sublayer is homogeneous.16 This is the premise of optical analysis and numerical simulation. After the film is divided into multiple sublayers, we assume that each sublayer has a homogeneous medium and is isotropic. Since the equation of the electric field propagation is linear, and the tangential component of the electric field is continuous, the propagation of the photoelectric field in the layered structure of the parallel plane can be described by a 2*2 matrix.6 As is shown in the Figure 1(c), the active layer is divided into a plurality of sublayers, each having a thickness of dj. The optical properties of each layer are determined by the complex refractive index Nj=nj+ikj. Complex refractive index is a complex parameter consisting of refractive index n and
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extinction coefficient k. The photoelectric field at any point in the layer j consists of two components, one component propagating forward along the X axis and the other propagating back along the X axis, denoted respectively as Ej+ and Ej-. The incident matrix for each interface is: 6
I jk =
1 t jk
1 r jk
r jk 1
(6)
where tjk and rjk are the final transmission coefficient and reflection coefficient between layers j and k, respectively. The phase shift occurs when the light passes through the layered interface, and the phase shift matrix of the light at layer j is:
e −ξ j d j Lj = 0
0 ξ d e j j
(7)
where, ξj = (2π/λ)Nj, ξjdj is the phase change of light passing through layer j, which is positively related to the layer thickness. Thus, the incident matrix and the phase shift matrix of the light are known, and the total transfer matrix from the first layer to the substrate is denoted as S, then the photoelectric field of the whole device can be expressed as: E 0+ E m+ + 1 − = S − E0 E m +1
(8)
The total transfer matrix S is multiplied by the incident matrix and the phase shift moment of each layer:
S S = 11 S 21
S12 m = Π I (ν −1)ν Lν ⋅ I m ( m +1) = 1 ν S 22
(9)
In order to calculate the internal electric field in layer j, the whole layer system can
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be divided into two parts separated by layer j, one from the external environment to the j layer, and one from the substrate to the j layer. Then the total system transfer matrix can be written as a product of both sides:
S = S'j Lj S''j
(10)
S' S 'j = 'j11 S j 21
S 'j12 j −1 L = ΠI S 'j 22 ν =1 (ν −1)ν ν
S ''j11 S = '' S j 21
S ''j12 m L ⋅ I = Π I S ''j 22 ν = j +1 (ν −1)ν ν m ( m +1)
'' j
⋅ I (j−1) j
(11)
(12)
Finally, we obtain the intensity of the photoelectric field at any point x in the layer j represented by an electric field formed by the incident wave. −iξ j (d j −x)
Ej (x) =
S''j11 ⋅ e
−iξ j d j
S S ⋅e ' '' j11 j11
iξ j (d j −x)
+ S''j21 ⋅ e
iξ j d j
+ S S ⋅e ' '' j12 j 21
E0+
(13)
Simulation devices were fabricated with a structure of glass/ITO/ZnO/active layer/MoOx/Ag. The active layer is BTID: PC71BM blend (Figure 1). The length parameter and the optical characteristics of other parts are obtained from the optical database. The absorption spectra of the sublayers data measured by etching apply to numerical simulation of photovoltaic devices. We make further efforts to ensure that the result is accurate. Specifically, we put the sublayer spectra obtained by etching into the model to analyze, and then obtain more accurate characteristics of photovoltaic devices by calculation. The experimental design, structural design and construction, and process simulation algorithm provide some references for structural
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design of solar cell materials. All in all, the vertical distribution characteristics of the optical constants of materials are fitted by K-K relation,19 and the photoelectric field distribution in the device is calculated by transfer matrix method according to the optical constants. Here, we directly plug in the normal solar test spectrum (AM1.5). Based on the distribution of the photoelectric field, the energy distribution inside the active layer and the density distribution of the exciton are calculated. Then, the short-circuit current density Jsc, the reflection of the device and the absorption of the internal layers and the external quantum efficiency (EQE) of the device are calculated, experiment measured data and simulation result are shown in the Figure 3(b). At around 700 nm, there is some light scattering although we used a series of condenser lens to collect the light, leading to a background value of the simulated EQE. Through the comparison of the experimental values of the device performance parameters Jsc and EQE (Figure 3), the results are basically consistent, so that the accuracy of the simulation is verified by using the multi-layer structure model. Figure 4(a) is TMM simulation of depth-dependent active inner layer light distribution, and Figure 4(b) is energy dissipation contour. Simulation is used to obtain the variation law of the square of the normalized field intensity modulus with the depth distribution of the thin-film device at different wavelengths of light. 6 The calculation of the distribution of light field inside the optical device plays an important role in the study of the internal mechanism of the device. In the film, exciton generation and dissociation occur, resulting in the conversion of solar energy to electrical energy. The absorbed photonic energy is a function of the optical field,
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and the equation of which is: 21-22
Q ( x) =
2πε 0ckn E ( x)
2
λ
(14)
where |E(x)|2 is the square of the optical field modulus, n is the refractive index, k is the extinction coefficient, λ is the wavelength of incident light, c is the speed of light in vacuum, and ɛ0 is the dielectric constant. The equation for the rate of exciton production is: 21-22
G (x) =
∫
800 nm 300nm
λ hc
Q ( x, λ )d λ (15)
According to the equation, the distribution of exciton production rate is calculated, as shown in the Figure 5(a). Subsequently, the short-circuit current density (Jsc) of the solar cell is calculated. For clarity, here we assume that one photon generates one exciton and is immediately translated into one free electron and one free hole, both of which are finally collected by both side electrodes. Internal quantum efficiency (IQE) is set as 1 for each wavelength.23 The device is assumed to be working under AM 1.5 condition and is consistent with the actual situation. The short-circuit current calculation equation is:
Jsc = ηIQE q ∫
100nm
0
G( x)dx
(16)
Where, q is elementary charge. The as-calculated Jsc for the device with 100 nm active layer is 16.0 mA/cm-2, which is close to the measured value 14.0 mA/cm-2.7 We explain that this tiny difference between simulated and measured Jsc attributed to the solar spectrum does not match the solar simulator. Meanwhile, the EQE of the device is emulated. Since IQE is set to be 1 for each wavelength, EQE is then the number of
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absorbed photons in active layer that occupies the total incident photons number proportion, as shown in the Figure 3(b) for comparison with experimentally measured data (Table S1). On the other hand, because light absorption spectra are correlated to electronic excitation from HOMO to LUMO levels, the depth-dependent light absorption profiles as shown in Figure 2b mean that the distributions of such energy levels are also film-depth dependent. From the depth-dependent light absorption spectra shown in Figure 2b, we found that the BTID component at different film-depth is featured with different absorption peak varying from ca. 500 nm at top part of the film to ca. 600 nm in the middle region and ca. 550 nm at the bottom surface. This variation is correlated to the variation of energetic distribution of localized states and thus definitely influences the exciton diffusion and free charge transport after charge transfer from donor to acceptor.24,25 Moreover, the light absorption of BTID at the top part of the film is much broader, as compared with the well-evolved absorption peaks in the bottom part of the film, implying a significant distribution of energetic disorder of localized states at each film-depth (lateral) in the top part of the film. The energetic disorder is found to induce low open-circuit voltage and fill factor.24,25 Optimizing the vertical and lateral distribution of localized states could be helpful to improve the photovoltaic performance. The exciton distribution shown in this manuscript is not the optimal case. Optimizing the sample preparation method (solvent type and its evaporation rate, processing temperature, thermal/solvent annealing or other post-treatment26) and
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device architecture could be used to tune the exciton distribution, which could not only tune the light harvesting behavior, but also help optimize energy level distributions to facilitate charge transport. It can be considered that the plasma in-situ etching and absorption spectra can be used to analyze the sublayer of the film. Compared with other existing methods, it has the advantages of simple operation and accurate results. Sublayer absorption spectra and the optical model for active layer, compared to the traditional method based on the heterogeneity within the active layer, can make the results more accurate; meanwhile, the design of photovoltaic devices, which is expected to analyze optical multilayer structure organic solar cells, helps to further enhance the performance of the device to provide a theoretical basis and reference value. Our innovation lies in the continuous etching of the films to obtain continuous spectroscopic data of the films, and further analysis of the thin films is based on this continuous detection process. Compared to the traditional model, the innovation point of the new optical model lies in considering the heterogeneity within the active layer, thus improving the calculation accuracy. Therefore, the internal mechanism can be qualitatively and quantitatively analyzed, and the internal structure of the organic film is diagnosed. Thus, the optical analysis of multilayer structures of photovoltaic devices and the improvement of device performance of organic solar cells are solved.
4. CONCLUSION In summary, we investigated the film-depth-dependent light absorption spectra in a
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small molecule donor: acceptor film with a high photovoltaic efficiency (~8%). Depth-dependent absorption spectra are subsequently utilized to study vertical phase segregation behavior. Including film-depth-dependent spectra of active layer into an optical interference method, with the inputs of optical properties of electrodes and hole/electron transport layers, allows us to reconstruct the film-depth-dependent and energy-dependent exciton generation profiles in an inverted solar cell, from which short-circuit current and external quantum efficiency of the device are simultaneously simulated. Our numerical simulations are well consistent with the experimentally measured photovoltaic performances. The film-depth-dependent spectroscopy, from which we are able to simultaneously reconstruct light harvesting profile, depth-dependent composition distribution and vertical variations of energy levels, provides insights into photovoltaic process. In combination with optimizing materials processing method, the approach proposed in this work helps improve the power conversion efficiency upon finely optimizing the film-depth-resolved photon harvesting profiles and simultaneously constructing appropriate vertical-variation of energy levels.
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(a)
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(b)
BTID
PC71BM
(c)
Figure 1. (a, b) Chemical structures of thiophene-substituted benzodithiophene (BTID) and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM). (c) Device configuration of the small molecule solar cell with inverted configuration. The donor: acceptor active layer is assumed to be composed of n sublayers.
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(a)
(b)
0.6
A
A
0.4
0.2
0.0 200
400
600
800
Wavelength (nm)
(c)
400
BTID PC71BM
800
0.6 0.5
0.6 A
0.4
0.4
0.3 Blend BTID PC71BM
0.2 0.2 0.0
600
Wavelength (nm) 0.7
0.8 content (vol %)
top to bottom
nm 0-8 8-16 16-30 30-38 38-57 57-74 74-86 86-92 92-100 0 200
(d)
1.0
0.1 0
20
40 60 Depth (nm)
80
0.0
100
(e)
400
600 Wavelength (nm)
800
(f)
0.7
Blend BTID PC71BM
0.6 0.5
Fit
A
0.4 A
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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0.3 0.2 0.1 0.0
330 nm 480 nm 600 nm 0 20 40 60 80 100 120 140 160 180 Time (s)
400
600 Wavelength (nm)
800
Figure 2. (a) Real time absorption spectra of blend film (100 nm) during etching by oxygen plasma, where the etching step is 20 s. (b) The calculated absorption spectrum of each sublayer, and the film-depth range for each sublayer is shown. (c) Distribution of BTID and PC71BM along the vertical direction as obtained from the absorption spectra. (d) Absorption spectra of BTID, PC71BM and BTID: PC71BM blends. (e) Dependence of absorption on etching time at different wavelength (330 nm, 480 nm and 600 nm) as obtained form (a). (f) Fitting the sublayer absorption spectrum of the blend by the spectra of pure BTID and pure PC71BM films. The black line is from the blend film after etching 60 s, and the line is shifted vertically for comparison.
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(a)
(b) 80
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5 EQE (%)
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Figure 3. Measured photovoltaic characteristics of BTID: PC71BM solar cell with inverted structure. (a) Measured J-V curves. (b) Measured and simulation EQE profile.
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(a)
(b)
Figure. 4 (a) Distribution of |E|2 inside the active layer. The incident solar light is from the left side. (b) Energy dissipation Q contour. (unit, mWcm-3nm-1 at its position and wavelength).
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MoOX ZnO
Ag
Figure. 5 Simulated exciton generation rate (s-1cm-3) via reconstruction of film-depth-dependent and energy-dependent exciton generation profile.
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ASSOCIATED CONTENT
Supporting Information.
Atomic Force Microscopy height images and Raman spectroscopy of the active layer before and after etching; Comparison of simulated and experimented results at specific wavelengths.
AUTHOR INFORMATION Corresponding Author *
[email protected]
*
[email protected]
*
[email protected]
Notes The authors declare no competing financial interest.
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Funding Sources This work is financially supported by Natural Science Foundation of China (Grant No. 21574103) and China Postdoctoral Science Foundation (2015M580841 and 2016T90910). L.B. thanks the Fundamental Research Funds for the Central Universities. G. L. thanks Cyrus Tang Foundation.
ACKNOWLEDGMENT The authors are grateful to Dongfan Li, Zihao Wang, Jinde Yu, Tianzhu Yao, Prof. Dingxin Liu and Prof. Yongquan Qu for fruitful discussions. The authors thank Yu Wang in Instrument Analysis Center of Xi'an Jiaotong University for assistance with the Raman experiments, and Nan Zhu and Kun Gao in State Key Laboratory for Manufacturing Systems Engineering for Infrared Spectroscopy and Atomic Force Microscopy measurements. We also sincerely thank Zexin Deng for the Scanning Electron Microscopy measurement.
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