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Improved efficiency and short-term stability of the planar heterojunction perovskite solar cells with a polyelectrolyte layer

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Phys. Status Solidi A, 1700281 (2017) / DOI 10.1002/pssa.201700281

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Jian Xiong*,1, Zhen He1, Shiping Zhan2, Xiaowen Zhang1, Liangzhu Long1, Junliang Yang3, Bingchu Yang3, Xiaogang Xue**,1, and Jian Zhang***,1 1

Guangxi Key Laboratory of Information Materials, School of Materials Science and Engineering, Guilin University of Electronic Technology, Guilin 541004, P.R. China 2 College of Physics and Electronic Science, Hunan University of Science and Technology, Xiangtan 411201, P.R. China 3 Institute of Super-Microstructure and Ultrafast Process in Advanced Materials, School of Physics and Electronics, Central South University, Changsha, Hunan 410083, P.R. China Received 7 May 2017, revised 27 June 2017, accepted 28 June 2017 Published online 26 July 2017 Keywords degradation, perovskite solar cells, polyelectrolyte, short-term stability author: e-mail [email protected], Phone: þ86-733-23322375 (office), þ86-13471275449, Fax: þ86-0773-2290129 e-mail [email protected], Phone: þ86-18172666419, Fax: þ86-0773-2290129 e-mail [email protected], Phone: þ86-733-2316372 (office), þ86-15296806930 (Mobile), Fax: þ86-0773-2290129 * Corresponding

The power conversion efficiency and the short-term stability of the perovskite solar cells (PSCs) are very important for their commercialization. The decay of the planar heterojunction PSCs (PHJ-PSCs) based on p-i-n structure (ITO/ PEDOT:PSS/CH3NH3PbI3/PCBM/Al) dramatically took place when they were exposed in air. We confirmed that the electrode bubble was responsible for this quick degradation and revealed a preliminary physical mechanism as well. A polyelectrolyte layer (ethoxylated polyethylenimine, PEIE) was inserted into the interface between the active layer and the cathode to enhance the interface contact and facilitate the charge carrier extraction. The power

conversion efficiency (PCE), open circuit voltage (Voc), and fill factor (FF) were found to be enhanced with the PEIE insertion. In addition, the PEIE layer largely improved the interface stability. The PEIE insertion could prohibit the formation of the electrode bubbles, which dramatically enhance the short-term air stability of the devices. The T50 (time for performance to decay to 50% of its initial measured value) of the devices with PEIE layer was 90 min, which was enhanced by 9 times compared with that without any modification layer. These results provide important information for understanding the decay of the PHJ-PSCs, which may benefit the application of PSCs.

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1 Introduction Since the traditional energy source such as coal and fossil fuel will be used up soon, the solar energy becomes a promising candidate to solve the energy crisis for it is abundant, renewable, and clean [1–4]. In the application of solar energy, the photovoltaics devices are required to be highly efficient, stable, and costeffective [5–7]. In recent years, the organic–inorganic halide perovskite materials have attracted considerable attention in solar cells research since their unique properties of high carrier mobility and absorption coefficient, small

exciton binding energy [8–11]. Meanwhile, the power conversion efficiency (PCE) of perovskite solar cells (PSCs) enhanced rapidly from the initial value of 3.8 [12] to over 20% [13, 14] by now, which demonstrates the enormous potential of PSCs for commercial application [15, 16]. Up to now, the structure of the PSCs can be divided into the n-i-p configuration and the p-i-n configuration [17, 18]. The n-ip configuration devices usually takes the compact TiO2 as electron transport layer (ETL) in modifying the transparent cathode and possesses rather high performance [19]. However, ß 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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J. Xiong et al.: Planar heterojunction perovskite solar cells with a polyelectrolyte layer

the compact TiO2 ETL also needs a relatively higher temperature annealing (450–500 8C) to ensure its good semiconductor property. However, such high temperature treatment is not good for the PSCs compatible with flexible substrate and to reach the low cost of mass production [20, 21]. Furthermore, the PSCs based on this configuration shows anomalous hysteresis phenomenon, when the devices was measured with different scan speed and direction [21]. While for the p-i-n configuration based on planar heterojunction of perovskite/fullerence prepared under low temperature, which can be easily fabricated with the advanced production technologies, such as roll to roll, doctor blading, and so on [22–24]. In addition, the hysteresis phenomenon also can be resolved using this devices configuration [24, 25]. Therefore, more and more attention has been paid to this p-i-n planar heterojunction PSCs (PHJ-PSCs). Unfortunately, the PCE of p-i-n PHJ-PSCs is still lower than that of the n-i-p PSCs. Meanwhile, some reports also pointed out that the efficiency of such structure quickly drops within only several minutes [26, 27] while the related reasons were still uncovered [18]. In order to overcome these disadvantages, the cathode interface modification is introduced into the p-i-n-based devices. Such interface modification can improve the interface contact, obstacle the interface chemical reactive, or interface diffusion and also can enhance the carrier extraction, which can be applied to improve the stability and the performance of organic solar cells [28]. Recently, the cathode modification treatment was also used to achieve better PHJ-PSCs performance. The first p-i-n configuration PHJ-PSCs used a thin bathocuproine (BCP) as interface and achieved a PCE of 3.9% [22]. Then J. Huang’s group reported the double fullerence and BCP interface, and achieved a PCE over 10%, which also paved a way for achieving considerable PCE of p-i-n PHJ-PSCs by cathode modification [24]. Later, Y. Yang’s group combined the moisture annealing and the PFN modification and achieved a higher PCE of 17.1% and a higher fill factor (FF) of 80% [29]. Besides, some thiol-functionalized cationic surfactant [30], N,N’-dimethyl-3,4,9,10-tetracarboxylic perylene diimide (PTCDI) [1], Cr2O3/Cr [1], C60 [24], LiF [31], Ti(Nb)Ox [9], ZnO [32, 33], TiO2 [34], PEIE [35, 36], and other interface materials [30] insertion between perovskite/fullerence and cathode have been proved to be effective in enhancing the performance and stability of the devices based on the smooth and stable interface. In this work, the CH3NH3PbI3/PCBM was introduced as the active layer of the p-i-n configuration PHJ-PSCs. Firstly, we explored the main reasons for the fast decay of the p-i-n configuration PHJ-PSCs exposed in air. In view of this quick performance decay of devices, a kind of polyelectrolyte (polyethylenimine ethoxylated, PEIE) was inserted between the active layer and the cathode. The PCE, open circuit voltage (Voc) and FF of PHJ-PSCs were all improved by the PEIE modification. Meanwhile, the PEIE insertion largely improves the interface stability and prohibit the formation of the electrode bubble when the devices was exposed in air. Those results provided some important information to understand the ß 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

interface decay of the p-i-n configuration PHJ-PSCs and the improvement of its interface contact and stability. 2 Experimental 2.1 Materials The perovskite precursor solution was made with mixing methylammonium iodide (CH3NH3I, 99%, Jingge, Wuhan) and lead iodide (PbI2, 99%, Zhengpin, Shanghai) at a molar ratio of 1:1 in N,NDimethylformamide (Super dry, DMF, J&KSeal) with the concentration of 33 wt.%. The precursor solution was stirred over 8 h at 40–60 8C, and was filtered with a 0.22 mm PVDF filter before the deposition. Fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM, 99.5%, American Dye Source) was dissolved in anhydrous chlorobenzene (Super dry, CB, J&KSeal) at 15 mg ml1. PEIE was purchased from Sigma–Aldrich, it was diluted in isopropyl alcohol (AR, IPA Sinopharm) with 0.2 or 0.4 wt.% before used. 2.2 Device fabrication The devices configuration is shown in Fig. 1a and the preparation process is as follows. Patterned indium tin oxide (ITO) glass substrates were ultrasonically cleaned in acetone, detergents, distilled water, and IPA for 15 min, respectively, then dried by hot air and treated by UV-ozone for 15 min. The PEDOT:PSS solution (Baytron, PVP AI 4083) was filtered and spin-coated at 3000 rpm onto the patterned ITO substrate and dried on hot plate at 150 8C for 15 min in air. The deposition of CH3NH3PbI3 layer was performed in a glove box (both H2O and O2 < 1.0 ppm) and the solvent engineering method was introduced to get high-quality films as previously reported [25]. Firstly, CH3NH3PbI3 solution (50 ml) was first dropped onto a PEDOT:PSS coated ITO substrate, of which the size is 1.5 cm 1.5 cm. The substrate was then spun at 4000 rpm. When the precursor roll out, CB solvent (60 ml) was quickly dropped onto the center of the substrate. The color of just deposited CH3NH3PbI3 layer was changed from transparent to light brown. The samples were subsequently treated at 100 8C for 10 min. The 290 nm black perovskite film was formed finally. After cooling to room temperature, the PCBM solution was spin-coated onto the CH3NH3PbI3 layer with 3000 rpm for 30 s. After that, the film was annealed at 100 8C for 10 min. For the interface modification devices, PEIE solution was spin-coated at 5000 rpm for 30 s. To ensure the destructive of the solvent, the IPA and methanol was spin coated 5000 rpm for 30 s. Finally, a bar-like defined Al electrode (100 nm) by mask was deposited by thermal evaporation under the vacuum of 4 106 mbar, resulting in an active area of 0.09 cm2. For testing the stability of PHJ-PSCs, the unencapsulated devices were exposed to air (humidity 45 RH %) and measured their performance at a specified time. 2.3 Characterization The morphology was characterized by atomic force microscopy (AFM, Agilent Technologies 5500 AFM/SPM System, USA) with tapping-mode. The topographic characteristics of electrode www.pss-a.com

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Figure 1 (a) The typical device structure of PHJ-PSCs; (b) the optical microscopic topography of the electrode when the devices exposed in air for 1 h; (c) the cross-sectional image of the PHJ-PSCs devices; (d) the typical current–voltage (J–V) characteristic of PHJ-PSCs at initial and exposed in air for 1 h.

were measured by an optical microscope (Caikang DMM200C, Shanghai). The cross-sectional image was tested by scanning electron microscopy (SEM, FEI Helios Nanolab 600i, USA). Current density–voltage (J–V) characteristics of PHJ-PSCs were measured by digital Source Meter (Keithley, model 2420, USA). A solar simulator (Newport 91160s, AM 1.5G, USA) was used for the PCE test. Light intensity was 100 mW cm2 calibrated by a standard silicon solar cell. The thickness of films was measured with surface profiles (Dektak 150, Veeco, USA) and AFM. 3 Results and discussion The p-i-n configuration PHJ-PSCs can be easily prepared, however, the lifetime of the devices is extremely short with only dozens of minutes when been exposed in air. That decay will eventually makes the device inefficient, which had already been reported by some other researchers and us [26, 27]. In this work, the electrode easily become rough even after the devices was exposed in air for several minutes. Figure 1b is the optical microscopic image of the Al cathode after the devices have been exposed to air for 1 h, where plenty of electrode bubbles can be found. Figure 1c shows the crosses-sectional SEM of the devices, it is clearly observed that the cathode bubbles resulted into an apparent space gap between the Al electrode and the active layer. The interfacial debonding may lead the devices to quickly lose their efficiency. Figure 1d is the typical J–V curve of the PHJ-PSCs at initial and after 1 h exposure in air. A typical J–V curve was achieved for the devices prepared under N2 atmosphere. However, the normally J–V curve quickly change into the open circuit curve as insulator after the devices only exposed in air for short time. The devices completely lost their efficiency only after being exposed in air for 1 h, which also was found in our previous work [34]. In our previous works, we have attempted to uncover the mechanism of this www.pss-a.com

degradation [34, 37]. We discovered that the quick degradation of the perovskite solar cells mainly resulted from the interface issues rather than the attenuation of perovskite thin film itself. The CH3NH3PbI3 material exposed to air would react with the water at the presence of light and result in the production of H2 [38]. The detail reaction processes are shown below as Eqs. (1) and (4). The schematic diagram of the devices degradation is shown in Fig. S1 (see Supporting Information). The breakdown of very few perovskite materials could not impact largely on its photovoltaic property, while the small product of H2 gas will lead the interface completely stripping. This stripping will lead disastrous effect on its performance of the devices. H2 O

CH3 NH3 PbI3 $ CH3 NH3 I þ PbI2

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CH3 NH3 I $ CH3 NH2 þ HI

ð2Þ

4HI þ O2 $ 2I2 þ 2H2 O

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hn

2HI$H2 " þI2

ð4Þ

However, the decay of the device relates with many factors and conditions, which are complex and our explanation is preliminary. Some further studies will be needed to uncover the details mechanism. In order to overcome this rapid interface degradation of the PHJ-PSCs devices and to enhance its performance, we introduced the PEIE as interface modification layer. The PEIE can form surface dipole and thus decrease the work function of the cathode, which is widely confirmed by many previous works [39, 40]. The modified work function of ß 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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electrode will minimize electrical losses upon injection or extraction of electrons. Meanwhile, the surface dipole form with PEIE is more stable than other similar materials in air exposure despite the hydrophilic nature of that film [41]. According to the previous works, the rapid interface decay is attributed to the electrode bubbles which originated from the reaction between the water and CH3NH3PbI3 [34]. Thus, the compact PEIE film may only benefit the short-term stability due to its hydrophilic property. Preparation of the longstable devices with PEIE interlayer must require some other hydrophobic materials. This issue is also important and will be addressed in the future research. In order to figure out the function of PEIE layer in the performance of the PHJ-PSCs devices, the PEIE was coated on the PCBM film to modify the interface between CH3NH3PbI3/PCBM and Al. Figure 2a is the CH3NH3PbI3 film deposited via solvent engineering method. This film shows low root-mean-square (RMS) roughness of 2.18 nm, and the coverage is ultrahigh. It is well-known that the preparation process of upper layer may affect the lower film if the solvent is not appropriate. Here, the destructive effect of two kinds of solvents (methanol and IPA) on CH3NH3PbI3 film were studied. The results are shown in Fig. S2 (see Supporting Information). It is clearly seen that some parts of the CH3NH3PbI3 film turning from dark brown to light brown after treated with methanol, indicating the damage to the perovskite layer. IPA solvent treatment does not obviously change as shown in Fig. S2b (see Supporting Information). Figure 2b is the micromorphology of the CH3NH3PbI3 film after the IPA treatment, which proved that the IPA has no obvious damage to the perovskite film. The RMS of the CH3NH3PbI3 film is about 2.18 nm

that is close to that of initial perovskite film (2.14 nm). Thus, IPA is chosen to dilution the commercial PEIE solution for preparation interface layer. Figure 2c is the micromorphology of the PCBM coated upon the CH3NH3PbI3 film, the film RMS changes not obviously with value of 2.44 nm. The PCBM film is not uniform and some pinholes in it as shown in Fig. 2c. This pinhole may lead the CH3NH3PbI3 contact with Al electrode directly, which is not beneficial to the performance and the stability of the devices. Figure 2d is the AFM image of the PEIE layer coated upon the PCBM layer. The PEIE layer dramatically reduce the roughness of the PCBM layer and the RMS value is reduced to 0.64 nm. Some pinholes of PCBM is improved via a PEIE coating comparing with the initial PCBM layer, which may result in a high quality interface contact and will improve the performance of the devices. Usually, the thickness of the interlayer is important for the performance of the devices. The PEIE interlayer is prepared from 0.2 wt.% solution, and the PCE is the average of the devices based on this PEIE layer. The weight concentration of the PEIE solution is usually 0.4 wt and 0.2 wt.% [35, 36, 42, 43]. Here, we also prepared the PEIE interlayer following this concentration standard. The results were shown in Table S1 (see Supporting Information), where the best performance appeared when the PEIE was prepared from 0.2 wt.% weight concentration. For the lower concentration of the PEIE precursor, the PCE of the devices shows obvious variation, which may mislead to understand the function of the PEIE interlayer. Thus, the PEIE was fixed at 0.2 wt.% in this work. Figure 3 is the typical J–V characteristics of PHJ-PSCs without interface layer (reference device) and with PEIE

Figure 2 The AFM imags of different films. (a) CH3NH3PbI3 film coated on ITO/PEDOT: PSS substrate; (b) CH3NH3PbI3 film coated on ITO/PEDOT:PSS treated with IPA; (c) PCBM coated on the CH3NH3PbI3 film; (d) PEIE film coated on the PCBM layer. ß 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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Figure 3 The typical J–V characteristics of PHJ-PSCs without interface layer (black square), treated with IPA (red circle) and with PEIE interface layer (blue triangle) under illumination of an AM 1.5G solar simulator (100 mW cm2).

interface layer under illumination of an AM 1.5G solar simulator (100 mW cm2). The detailed device parameters are summarized in Table 1. The reference devices show average PCE of 11.19%, Voc of 0.92 V, FF of 66%, and Jsc of 18.49 mA cm2. In order to confirm that the IPA was harmless to the CH3NH3PbI3 film, we also fabricated the devices with the IPA treated CH3NH3PbI3. Here no performance decay was observed in the IPA treated devices, and the average PCE of 11.41%, Voc of 0.94 V, FF of 66%, and Jsc of 18.37 mA cm2 were achieved. The performance of PHJ-PSCs with IPA treated PCBM showed a little bit of improvement. While for PHJ-PSCs modified with PEIE layer, the PCE is enhanced from 11.19 to 12.10% and all of the performance parameters were found to be improved. The PCE of the best device with PEIE layer is beyond 13%. The Voc is enhanced from 0.92 to 1.00 V, which is higher than many reports values based on this devices structure [1, 22–24]. The FF was also enhanced via the PEIE layer which improved from 66 to 69%. This enhancement may ascribe to the high quality of the interface contact according to AFM analysis. It is worth to point out that the PCE of the PHJ-PSCs is lower than mainstream PCE in recent reports. The reasons for this lower PCE can be attributed to the processing of the perovskite film. Here, we used the solvent-induced-fast-crystallization deposition methods to prepare the CH3NH3PbI3 film, which lead to a smaller

perovskite crystalline in the film (as shown in the left image of Fig. S3a, see Supporting Information). The substantial perovskite crystalline in the film may result in many grain-boundary defects and lowered photoelectric current [44]. Thus, the processing of the perovskite film forming plays an important role in determination of the performance of the devices, which lead the relative lower PCE in this work. Figure 4a is the typical current-voltage (J–V) characteristic of PHJ-PSCs devices with PEIE layer when it was exposed in air for 2 h. The PCE of the PHJPSCs devices was 12.19% at the initial state, and possessed the Voc of 1.03 V, FF of 71, and Jsc of 16.78 mA cm2. After exposed in air for 1 h, the PCE of devices dropped to 8.42%, and it still maintained 5.17% even after exposed in air for 2 h. The T50 (time for performance to decay to 50% of initial measured value) of the devices was 90 min with PEIE layer, which was enhanced by 9 times than that of without any modification layer (T50 ¼ 10 min). Comparing with the rapid decay for the reference device (shown in Fig. 1d), the air stability of the devices with PEIE was dramatically enhanced. This improved stability may closely relate to the preventing of the interface degradation that was caused by the electrode bubbles. In order to estimate the effect of the PEIE layer in preventing the cathode bubbles, the morphology evolution of the Al cathode was analyzed by AFM characterization. Figure 5a and c were the initial Al cathode morphology of reference devices and PEIE layer based device, respectively. In Fig. 5a, the Al cathode shows the RMS of 7.90 nm. The morphology of the electrode was continuously changing during the AFM measurements, which shows that the electrode bubbles are forming immediately after the devices exposed in air. The lower part of Fig. 5a showed that some bubbles had formed. The Al cathodes became quite rough after it was exposed in air for 12 h, shown in Fig. 5b. For the PEIE modified devices, the morphology of the Al cathode was uniform and smooth with RMS of 3.51 nm, shown in Fig. 5c. The morphology of the cathode did not change even after the device was exposed in air for 12 h and the RMS can still maintain 3.73 nm. In order to further confirm that the PEIE interlayer can prevent the electrode bubble formation, we introduced the SEM characterization to analyze the cross profile of the devices which was shown in Fig. S3 (see Supporting Information). From the SEM images (Fig. S3a, see Supporting Information), the

Table 1 Key values of the J–V characteristics of hybrid organic/inorganic perovskite PHJ solar cell with and without interlayers. cathode configuration

PCE (%)

Voc (V)

FF (%)

Jsc (mA cm2)

PCBM/Ala PCBM/IPA/Al PCBM/PEIE/Al

11.19 1.70 (12.71) 11.41 0.76 (12.06) 12.10 0.98 (13.08)

0.92 0.12 0.94 0.04 1.00 0.06

66 4 66 2 69 5

18.49 2.77 18.37 1.31 17.51 1.57

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This statistical data from Ref. [34].

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Figure 4 (a) The evolution of typical current–voltage (J–V) characteristic of PHJPSCs devices with PEIE layer when it is exposed in air. (b) Normalized device lifetime for PHJ-PSCs devices with PEIE layer (black square) compared to the reference devices without PEIE layer (red circle) which the value from Ref. [34].

Figure 5 (a) and (c) are the initial Al cathode morphology of reference devices and PEIE layer-based device, respectively; (b) and (d) are the Al cathode morphology after the devices exposed in air for 12 h, respectively, for the reference device and PEIE layer-based devices.

Al cathode of the device without PEIE interlayer was totally peeled off from the active layer after the air exposure. In Fig. S3b (see Supporting Information), the Al cathode of the devices with PEIE interlayer showed no exfoliation and closely attached to the active layer. According to above statements, it can be concluded that the PEIE layer can efficiently prevent the electrode bubble’s formation and enhance the interface stability. The PEIE layer can be used to enhance the interface decay at short term due to its hydrophilicity. However, in the near future, finding or synthesizing more stable and hydrophobic materials to insert between cathode and PCBM/CH3NH3PbI3 layer will be significant. This may help to obtain more stable interface between Al and perovskite layer, and avoid water diffusion into perovskite layer as well, which may benefit to configure high air stable and efficient PHJ-PSCs devices. 4 Conclusions The p-i-n configuration PHJ-PSCs with CH3NH3PbI3/PCBM as the active layer was prepared ß 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

via a solvent engineering method. The decay of the planar p-i-n PHJ-PSCs dramatically takes place and the electrode bubble was the main reason for the exploration of this rapid degradation of the PHJ-PSCs. A preliminary physical mechanism was given for this interface degradation. In order to overcome this quick decay of the devices and prevent the electrode bubbles formation, the PEIE was inserted between the active layer and the cathode. The results showed that the the PCE of the device was enhanced from 11.19 to 12.10% with PEIE insertion and the highest PCE over 13% was also achieved. Meanwhile, the Voc was dramatically enhanced and the average Voc was up to 1.00 V. By inserting the PEIE layer, between the Al cathode and the PCBM/CH3NH3PbI3 layer, the electrode bubbles was efficiently suppressed and the short-term stability of the devices was apparently improved accordingly. The T50 of the devices with PEIE layer was 90 min, which was enhanced by 9 times than that of without any modification layer. These results may provide important information to understand the interface decay of the p-i-n configuration www.pss-a.com


PHJ-PSCs and the improvement of its interface contact and stability. Supporting Information Additional supporting information may be found in the online version of this article at the publisher’s web-site. Acknowledgements This work was supported by the National Natural Science Foundation of China (61604047, 61564003) and Guangxi Natural Science Foundation Program (2015GXNSFGA139002, 2016GXNSFBA380119). Jian Xiong acknowledges the support from the PhD research startup foundation of Guilin university of Electronic Technology (UF15016Y) and Free the Discovery Fund of Guangxi Key Laboratory of Information Materials (151010-Z).

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