Article pubs.acs.org/journal/apchd5
Low-Temperature-Processed Amorphous Bi2S3 Film as an Inorganic Electron Transport Layer for Perovskite Solar Cells Deng-Bing Li,†,‡,§ Long Hu,†,‡,§ Yao Xie,† Guangda Niu,† Tiefeng Liu,† Yinhua Zhou,† Liang Gao,†,‡ Bo Yang,†,‡ and Jiang Tang*,†,‡ †
Wuhan National Laboratory for Optoelectronics (WNLO) and School of Optical and Electronic Information, Huazhong University of Science and Technology (HUST), Wuhan, 430074, People’s Republic of China ‡ Shenzhen R&D Center of Huazhong University of Science and Technology, Shenzhen, 518057, People’s Republic of China S Supporting Information *
ABSTRACT: Organic−inorganic hybrid perovskite solar cells have attracted great attention due to their unique properties and rapid increased power conversion efficiency. Currently, PC61BM is widely used as the electron transport layer (ETL) for inverted hybrid perovksite solar cells. Here we propose and demonstrate that Bi2S3, a ribboned compound with intrinsic high mobility and stability, could be used as the ETL for perovksite solar cells. Through a simple thermal evaporation with the substrate kept at room temperature, we successfully produced a compact and smooth amorphous Bi2S3 ETL with high conductivity. Our NiO/ CH3NH3PbI3/Bi2S3 solar cell achieved a device efficiency of 13%, which is comparable with our counterpart device using PC61BM as the ETL. Moreover, our device showed much improved ambient storage stability due to the hydrophobic and hermetic encapsulation of the perovskite layer by the Bi2S3 ETL. We believe thermally evaporated Bi2S3 is a promising ETL for inverted hybrid perovskite solar cells and worthy of further exploration. KEYWORDS: hybrid perovskite, Bi2S3, low-temperature process, thermal evaporation, electron transfer layer
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Unfortunately, the degradation of inverted perovskite solar cells is a critical issue restricting their applications. Moisture has been reported as one of the major factors affecting the intrinsic stability of perovskite.11 The organic hole transport layer (HTL), such as PEDOT:PSS, could be replaced with NiO or doped NiO, which could reduce the invasion of moisture from the HTL side. However, PC61BM, which was commonly used as an ETL in inverted structures, degraded severely due to the significant decreases in work function and ionization potential when exposed to air or water.12 The penetration of perovskite through the PC61BM layer to contact with the metal electrode also degrades the metal electrode and thus the device performance.13 Moreover, the low carrier mobility of PC61BM also limits further efficiency improvement.14 Ponseca et al. investigated the perovskite/PC61BM interface in detail and represented a mechanism for the low photocurrent of perovskite solar cells.15 To avoid these shortcomings of PC61BM, to date, only presynthesized ZnO nanoparticles16 and SnO2 nanoparticles17 have demonstrated remarkable performance as ETLs in inverted perovskite solar cells. This is mainly because the high-temperature process is necessary for commonly used inorganic compounds as ETL such as ZnCdO,18 TiO2,19
rganic−inorganic hybrid perovskite materials have attracted tremendous attention due to the rapid increased power conversion efficiencies from 3.8% to 22.1% in several years.1,2 This striking development mainly derives from the convenient fabrication process and appealing intrinsic properties, such as extremely high absorption coefficient, ambipolar transport properties, long carrier diffusion length, and outstanding defect tolerance.3 Due to these excellent properties, hybrid perovskites have been utilized as absorbers in various device architectures, including meso-superstructured,4 planar n−i−p,3 and planar p−i−n devices.5 Among them, the inverted planar p−i−n structure attracts increasing interest due to its suppressed hysteresis, high device stability, and excellent performance.6−8 Our group reported a hybrid perovskite solar cell with inverted planar architecture in early 2014, achieving high open-circuit voltage (VOC, 1.05 V) after UV-ozone treatment of a NiO film.9 Nie and co-workers reported an inverted architecture, with a planar p-type PEDOT:PSS at the bottom of the device and PC61BM as the electron collection layer (ETL) above the perovskite layer. They fabricated devices with an efficiency approaching 18% without hysteresis.6 In 2015, Chen and co-workers reported a high certified efficiency (16.2%) with large active area (>1 cm2), with a device structure of NiMgLiO/MAPbI 3/PC61BM/TiNbOx, close to other commercial solar cells in terms of device efficiency.5,10 Therefore, inverted p-i-n architecture is promising and is worthy of further exploration and optimization. © 2016 American Chemical Society
Received: August 6, 2016 Published: October 12, 2016 2122
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SnO2,20 CdS,21 and CdSe.22 Yang and co-workers established a presynthesized ZnO particle film as ETL in inverted structures with a typical power conversion efficiency (PCE) of 14.6%, and improved stability was observed for 60 days with negligible degradation in ambient air.16 Conversely, Kelly’s group observed proton transfer reaction at the ZnO/CH3NH3PbI3 interface, causing accelerated degradation of perovskite.23 Accordingly, a suitable inorganic ETL for inverted perovskite solar cells to improve the performance and stability is of high urgency. Therefore, we sought to replace PC61BM or ZnO with a stable and low-temperature-processed ETL material. Bismuth trisulfide (Bi2S3), an inorganic material with intrinsically high electron concentration and carrier mobility,24,25 was explored as an ETL in perovskite solar cells for the first time herein. Bi2S3 was chosen for the following reasons: (i) intrinsically n-type defect properties. According to the first-principles theoretical calculations, Bi2S3 possesses intrinsically n-type defects whenever the dominant defects are sulfur vacancies (VS) or interstitial sulfur (Si),19 guaranteeing high carrier concentration in the Bi2S3 film regardless of the fabrication process. (ii) Bi2S3 possesses a much higher carrier mobility (257 cm2/(V s))24 compared to PC61 BM (1 × 10 −3 cm 2 /(V s)) 14 and presynthesized ZnO nanoparticles (0.066 cm2/(V s)).26 (iii) Bi2S3 is composed of one-dimensional (Bi4S6)n ribbons. When processed into an amorphous state, the ordering of these ribbons is ruined, yet the dangling bonds are limited, as in an amorphous chained polymer, thus resulting in low defect density. Our recent published work on Sb2Se3, with a similar crystal structure to Bi2S3, indicated that the grain boundaries of a one-dimensional material will be terminated by the intrinsically benign surfaces and the recombination loss can be minimized.27 Conceivably, Bi2S3 film as an electron collection layer would promise high electron collection efficiency with low carrier recombination loss. Herein, a simple thermal evaporation process was employed to fabricate amorphous Bi2S3 (a-Bi2S3) film as ETL in hybrid perovskite solar cells. We chose thermal evaporation over solution processing because of the binary composition and high vapor pressure of Bi2S3 facilitating simple evaporation. Furthermore, the substrate temperature is kept at room temperature during the thermal evaporation process, which could minimize possible damage to the vulnerable hybrid perovskite film and enable the construction of flexible devices. In addition, thermal evaporation is free of solvents, which could minimize potential contamination and variations otherwise introduced by the processing solvent. Although appealing, many grand challenges should be envisaged because Bi2S3 has never been reported as an ETL. The foreseeable problems can be listed as follows: (i) whether proper band alignment could be formed and efficient charge separation could occur at a-Bi2S3/perovskite interfaces? This is the precondition for high efficiency solar cells; (ii) whether aBi2S3 could preserve its intrinsically high carrier mobility and electron concentration as crystalline Bi2S3 (c-Bi2S3)? This is the sticking point for desired performance in a-Bi2S3-based devices; (iii) how stable is the resultant device? Fortunately, our experiment results provide positive answers to all questions listed above: thermally evaporated a-Bi2S3 films possess high carrier mobility and high carrier concentration with only little decrease compared with those of its crystalline counterpart; proper band alignment enables efficient carrier separation, resulting in a NiO/CH3NH3PbI3/a-Bi2S3 solar cell with a
maximum PCE of about 13.1%; the device also demonstrates exceptional stability due to the hydrophobic and hermetic Bi2S3 encapsulation. Our results indicated that thermally evaporated a-Bi2S3 is a promising ETL for inverted perovskite solar cells worthy of further exploration and optimization.
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RESULTS AND DISCUSSION Bi2S3 is a binary compound with a single-phase composition below 775 °C.24 Furthermore, Bi2S3 has a low melting point and high evaporation pressure.24 Thus, simple thermal evaporation was used to prepare Bi2S3 film in our experiments. Purchased Bi2S3 powder (99.999% purity) was used as the evaporating source without any further treatment. The deposition rate was controlled by tuning the current of a tungsten filament and monitored by a microcrystalline quartz balance (MQB). Substrates were kept at room temperature during the evaporation of a-Bi2S3. X-ray photoelectron spectroscopy (XPS) measurements were employed to characterize the chemical state of thermally deposited a-Bi2S3 film. For clarity, detailed deconvolution of Bi 4f and S 2p high-resolution spectra is shown in Figure 1a. The result revealed that the
Figure 1. Characterization of a-Bi2S3 film: (a) XPS spectrum of Bi 4f and S 2p; (b) AFM image; (c) photosensitivity curves (glass substrate, illuminated at 530 nm with a power density of 430 μW/cm2, 40 V driving voltage); (d) fitting curve of photocurrent decay. Two exponential decay components with time constants of 29 and 1.2 ms were obtained.
binding energies of Bi 4f5/2 and 4f7/2 were 164.1 and 158.8 eV, respectively, consistent with the previous report for Bi2S3.28 Also, the binding energy peak at 162.7 eV (Bi 4f) originated from Bi element was observed, indicating partial decomposition of Bi2S3 during the thermal deposition process. Figure S1 shows the full survey spectra of a-Bi2S3, indicating no other impurity except a trace amount of metallic Bi in the film. It should be noted that Bi contamination due to Bi2S3 decomposition accelerates as the evaporating temperature increases; thus a proper evaporating temperature and hence a proper evaporation speed should be engineered. For our system, an optimized evaporation speed of 2.5 Å/s is obtained. The potential effect of metallic Bi in the film will be discussed later. The morphology of a-Bi2S3 was characterized by atomic force microscopy (AFM) to confirm the full coverage of the underneath perovskite film (Figure 1b). The a-Bi2S3 film is compact and flat without pinholes and cracks, with an 2123
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estimated film roughness of 0.87 nm. Note that no grain boundaries can be observed because of its amorphous state. This is the key difference from the sintered film or presynthesized particle film. In those films, a large amount of grain boundaries exist in the film made from presynthesized nanoparticles, which often act as channels for oxygen and water diffusion from the outside environment catalyzing the decomposition of the perovskite films. Hence, our a-Bi2S3 could effectively prevent the infiltration of oxygen and moisture by the hybrid perovskite layer, promising better device stability. Photosensitivity of the a-Bi2S3 film was evaluated by depositing Au on an evaporated a-Bi2S3 film as electrode to build a photodetector. The device area was determined by the electrode length (3 mm) and electrode space (0.2 mm). The test was carried out inside an electrically and optically shielded box using a light-emitting diode (530 nm wavelength, 430 μW/ cm2) as the excitation source, which was controlled by a functional generator. The linear current−voltage curve (Figure S2) indicated that ohmic contact was formed between the Au electrode and a-Bi2S3. A strong and reversible photoresponse was observed as shown in Figure 1c, suggesting the high quality of the a-Bi2S3 film. When biased at 40 V, the devices showed a dark current of 8 nA and a photocurrent of 788 nA, corresponding to a photocurrent to dark current ratio of 98.5. Furthermore, transient photocurrent decay measurement was carried out to estimate the lifetime of trapped carriers by fitting the tail of the decay curves with two exponential functions: y = y0 + A1e−(x − x0)/ t1 + A 2 e−(x − x0)/ t2
Table 1. Hall Characterization Results of a-Bi2S3 and the Corresponding Reported Parameters of c-Bi2S3, PC61BM, and ZnO Film n-type material a-Bi2S3 c-Bi2S324 PC61BM32,33 ZnO16,26
Ne (cm−3) 4.5 6 1 1
× × × ×
16
10 1019 1020 1014−19
μe (cm2/(V s)) 49 257 1 × 10−3 6.6 × 10−2
Neμe (cm−1/(V s)) 2.2 1.5 1 6.6
× × × ×
1018 1022 1017 1012−17
concentration (Ne) and mobility (μe), a parameter representing film conductivity, is much higher than that of PC61BM and ZnO (Table 1). This product could be further enhanced by increasing the processing temperature to facilitate the crystallization of Bi2S3, leading to much improved carrier concentration and hence an impressive Neμe product of 1.5 × 1022 cm−1/(V s). Of course, careful attention should be paid to avoid damaging the underneath hybrid perovskite layer, which is beyond the scope of this work. Nonetheless, such a high conductivity in our a-Bi2S3 layer promises its capability of reducing the series resistance when employed as the ETL in organic−inorganic hybrid perovskite solar cells. Besides the appealing electronic properties, proper band alignment is another crucial requirement for high-quality heterojunctions. For this case, we carried out UV−vis−IR transmittance and ultraviolet photoemission spectroscopy (UPS) measurements to characterize the band structure of aBi2S3 film. A UV−vis−IR transmittance test (Figure S3) was employed to determine the band gap of a-Bi2S3. By plotting (αhν)2 versus hν we extrapolated the linear fitting line with the x-axis and obtained an optical edge of 1.59 eV (Figure 2a). This
(1)
The fitting yields two time constants, 29 and 1.2 ms, respectively (Figure 1d), indicating two types of defects with varied depth in a-Bi2S3 film. As we know, the photocurrent decay kinetics strongly correlates to the defect depth and shallow defects generally result in a fast photoresponse.29 Considering a 60 ms photoresponse time corresponds to a trap depth of 0.09 eV below the conduction band in a lead sulfide (PbS) colloidal quantum dot photoconductive photodetector,30,31 a much shallower trap depth can be expected in our aBi2S3. An amorphous film generally has many defects such as atom vacancies and dangling bonds acting as recombination centers. However, Bi2S3, as mentioned above, is composed of one-dimensional (Bi4S6)n ribbons stacking together through van der Waals forces. Hence, a-Bi2S3 possesses very few dangling bonds since it is like a disordered chain polymer without many broken bonds. This is confirmed by the strong and fast photoresponse of our a-Bi2S3-based photodetector, an intrinsic advantage of Bi2S3 that promises application as an ETL. The Hall effect measurement is known as an effective characterization method to illustrate film electronic properties. We fabricated an a-Bi2S3 film on a quartz substrate and applied four Au electrodes as the contacts. All the data of a-Bi2S3 were obtained directly from the test system (Ecopia HMS-550), and the Van der Pauw method was employed for data analysis. In order to ensure data credibility, the results were averaged from five regions of the a-Bi2S3 films, as shown in Tables 1 and S1. The results revealed that the a-Bi2S3 film showed an electron mobility of μe = 49 cm2/(V s), which is much higher than that of PC61BM (1 × 10−3 cm2/(V s))14 and presynthesized ZnO film (6.6 × 10−2 cm2/(V s)).26 Despite having a low doping density of only 4.5 × 1016 cm−3, the product of electron
Figure 2. Characterization of a-Bi2S3 film: (a) UV−vis−IR transmittance spectrum; (b) UPS spectrum. (c) Band diagram sketches in CH3NH3PbI3/a-Bi2S3 devices. (d) Photoluminescence spectra of CH3NH3PbI3 films underneath a-Bi2S3 or PC61BM ETL.
value is in good agreement with the reported 1.60 eV for aBi2S3 film.34 As shown in the UPS spectra (Figure 2b), the Fermi level of a-Bi2S3, 4.45 eV, was established by subtracting the cutoff value from the excitation source of 21.2 eV. Here, we would like to discuss the potential effect of a trace amount of metallic Bi in a-Bi2S3 as observed by XPS measurement. Metallic Bi has a similar work function (∼4.4 eV) to the Fermi level of a-Bi2S3. Furthermore, metallic Bi possesses a much higher conductivity than a-Bi2S3. For this perspective, the 2124
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possesses a higher fill factor (FF, 77.5%) than that with PC61BM as the ETL (71.4%). This can be contributed to the larger conductivity and hence smaller series resistance of the aBi2S3 ETL. Unfortunately, the VOC decreased from 989 meV for the PC61BM control to 941 meV in the device with a-Bi2S3 as the ETL, sacrificing device efficiency. This can be ascribed to the higher work function of a-Bi2S3 (4.45 eV) than PC61BM (4.30 eV), resulting in lower built-in potential and hence lower VOC.39 Finally, we obtained a comparable PCE using a-Bi2S3 (12.3%) and PC61BM (12.1%) as ETLs. It is worth mentioning that the scanning hysteresis of devices was depressed as a-Bi2S3 was used as ETL instead of PC61BM. The hysteresis of hybrid perovskite solar cells was intensively discussed in terms of several origins, including ion diffusion or charge blockage in the interfaces,40 ferroelectric polarization of CH3NH3PbI3,41 capacitive charge,42 interfacial charge extraction, and charge trapping/detrapping.6 For our devices, the reason may arise from the suppressed charge trapping between hybrid perovskite and a-Bi2S3 due to the smaller interfacial energy barrier and thereby photogenerated propagation through the interface without accumulation.6,43 Nevertheless, the origin of hysteresis is complicated. Xu et al. found an insoluble and pinhole-free [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) layer characteristic of a highly efficient carrier extraction property can successfully suppress the hysteresis of perovskite solar cells.44 They also found that perovskite−PC61BM hybrid devices show enhanced hysteresis suppression compared to bilayer devices by reducing anion migration through defects at grain boundaries.43,45 We thus believe a-Bi2S3 probably plays a similar role and hence reduces hysteresis of the device. We further optimized the device performances by varying the a-Bi2S3 film thickness (Figure 3d). As the thickness of the aBi2S3 film increased from 50 nm to 80 nm, the devices showed the highest PCE of 13.1% in forward scanning, mainly attributed to the improvement in JSC (from 18.0 to 18.6 mA/ cm2), FF (from 73.4% to 74.2%), and VOC (from 939 to 949 meV). As the thickness of the a-Bi2S3 film further increased to 120 and 200 nm, the device performance decreased rapidly below 10% mainly through the FF loss possibly due to the larger-than-optimal thickness increasing the series resistance. Besides high power convention efficiencies, stability is a topic of wide concern for organic−inorganic hybrid perovskite solar cells. We therefore investigated the stabilities of our fabricated devices based on a-Bi2S3 and PC61BM, respectively. The devices were stored and characterized in an ambient environment without encapsulation (25 °C, 50−75% humidity). The results are shown in Figure 4. The performance of the device with Bi2S3 remained at about 80% of its initial efficiency when stored for 30 days. In sharp contrast, the performance of the devices using PC61BM as the ETL degraded dramatically in several days and was close to zero in only 10 days, similar to others’ observations.16,17 We now study the mechanism for improved device stability. One probable reason is the better stability of a-Bi2S3 compared to PC61BM. As is reported, organic PC61BM degraded quickly when exposed to an ambient environment.12,27 It was found that the lowest unoccupied conduction band increased when exposed in ambient air, which could result in an even larger energy barrier for electron transport.15 Under this consideration, we investigated the stability of our a-Bi2S3 films by monitoring J−V characterization as shown in Figure 5a, indicating negligible conductivity variation upon ambient
presence of Bi in our a-Bi2S3 film is acceptable or even beneficial. The distance between the Fermi level and valence band maximum (VBM) of a-Bi2S3 can be obtained by fitting the long tail region of the UPS spectra. The conductive band minimum (CBM) can be established by combining the UPS fitting result and the established optical edge from the transmittance measurement. The VBM and CBM were thus estimated as 5.45 and 3.86 eV, respectively. The energy level of CH 3NH 3 PbI3 was adopted from previous results. 35 A conduction band offset of 0.07 eV was observed from the band diagram of CH3NH3PbI3 and a-Bi2S3 in Figure 2c, enabling efficient separation of photogenerated carriers.36,37 This is further supported by photoluminescence measurements (Figure 2d). For the measurement, the sample was prepared by depositing a CH3NH3PbI3 film on precleaned soda lime glass, and then a-Bi2S3 and PC61BM were deposited above by thermal evaporation and conventional spin-coating processes, respectively. The results show that CH3NH3PbI3 with a-Bi2S3 on top demonstrated nearly complete photoluminescence quenching, to a degree that is comparable to PC61BM, verifying efficient charge separation between CH3NH3PbI3 and a-Bi2S3. Having resolved the first three problems mentioned in the introduction, we fabricated devices by evaporating a-Bi2S3 (50 nm thickness) above CH3NH3PbI3 films (Figure S4). A p-i-n structure was employed, and a schematic demonstration of the device architecture is shown in Figure 3a. NiO film was first
Figure 3. (a) Schematic demonstration of our inverted ITO/NiO/ CH3NH3PbI3/a-Bi2S3/Au device. (b) J−V curves of devices with aBi2S3 and PC61BM as electron transport layer measured in reverse and forward modes under standard AM 1.5G illumination and (c) the corresponding EQE spectra. (d) J−V characteristics of NiO/ CH3NH3PbI3/a-Bi2S3 devices with different a-Bi2S3 film thicknesses.
fabricated by spin coating on precleaned indium-doped tin dioxide (ITO) glass as a hole transport layer.9 Then, on the NiO film, hybrid perovskite CH3NH3PbI3 was deposited by an antisolvent process as in a previous report.38 The crosssectional and top-view SEM images of the full device are shown in Figures S5 and S6, indicating complete coverage of a-Bi2S3 on perovskite. The devices based on PC61BM and a-Bi2S3 as ETL demonstrated a comparable short-circuit current density (JSC, 17.9 and 18.1 mA/cm2 for a-Bi2S3- and PC61BM-based devices, respectively), which was further confirmed by external quantum efficiency (EQE) measurement as shown in Figure 3b and c. Not surprisingly, the device using a-Bi2S3 as the ETL 2125
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solar cells. X-ray diffraction (XRD) was employed to further study the evolution of the perovskite layer in devices during ambient storage with a-Bi2S3 and PC61BM as ETL layers (Figure S7). The degree of degradation can be identified by the increase of the diffraction peak of PbI2. The results show that perovskite films with thermally deposited a-Bi2S3 decompose much less than those with PC61BM as ETL. This further confirms that the dense a-Bi2S3 film could fully cover perovskite, encapsulating the vulnerable hybrid perovskite layer.
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CONCLUSIONS Amorphous bismuth trisulfide was first introduced into inverted hybrid perovskite solar cells as an electron transport layer. Simple thermal evaporation produced a compact amorphous Bi2S3 layer with well-controlled thickness. The high carrier mobility and conductivity of a-Bi2S3 and the efficient charge separation at the a-Bi2S3/CH3NH3PbI3 interface together resulted in comparable device performances to the control using PC61BM as the ETL. Furthermore, the high stability and hydrophobicity nature of the a-Bi2S3 layer, as well as its clean vacuum processing and full coverage, lead to a device with much improved storage stability compared to that with PC61BM as the ETL. Through further doping density improvement and interface optimization, we believe a-Bi2S3 would be the top choice as the ETL in inverted hybrid perovskite solar cells.
Figure 4. Normalized device stabilities of solar cells with a-Bi2S3 and PC61BM as ETL, respectively.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsphotonics.6b00582. Additional information (PDF)
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AUTHOR INFORMATION
Corresponding Author
Figure 5. Stability of (a) a-Bi2S3 and (b) PC61BM films exposed to ambient air for 60 days.
*E-mail:
[email protected]. Author Contributions §
D.-B. Li and L. Hu contributed equally.
storage for 10 days, in sharp contrast with the PC61BM case, where a significant conductivity drop was observed (Figure 5b). We also investigated the hydrophobicity of an a-Bi2S3 film along with a PC61BM film on soda lime glass (Figure 5c and d). The results indicate that water shows a much larger contact angle on a-Bi2S3 than PC61BM, indicating better hydrophobic nature of a-Bi2S3, which could block moisture diffusion through the ETL layer and retard the degradation of hybrid perovskite solar cells by water attack more effectively. Furthermore, the non-solventinvolved thermal evaporation process should also help stability. In the traditional spin-coating process, the usage of organic solvents is inevitable and the impurities in the solvent could trigger unexpected reactions with hybrid perovskite and accelerate its decomposition. The high-vacuum deposition environment would minimize possible contamination by the water, oxygen, or organic solvent introduced due to the ETL fabrication, thus improving heterojunction quality and device stability. One additional reason is that the thermally evaporated a-Bi2S3 layer is smooth and compact without any pinholes and cracks, enabling a conformal encapsulation of the hybrid perovskite. We believe the above four factors together resulted in much improved device stability compared with the counterpart using PC61BM as the ETL in hybrid perovskite
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by the “National 1000 Young Talents” Project, the Major State Basic Research Development Program of China (2016YFA0204000, 2016YFB0700700), the National Natural Science Foundation of China (61322401), the Fundamental Research Funds for the Central Universities, HUST (2016JCTD111), and the Special Fund for Strategic New Development of Shenzhen, China (JCYJ20160414102210144). The authors thank the Analytical and Testing Center of HUST and the facility support of the Center for Nanoscale Characterization and Devices, WNLO.
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REFERENCES
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ACS Photonics
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DOI: 10.1021/acsphotonics.6b00582 ACS Photonics 2016, 3, 2122−2128
