Transparent Indium Tin Oxide Electrodes on Muscovite Mica for High

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Letter

Transparent indium tin oxide electrodes on muscovite mica for high-temperature processed flexible opto-electronic devices Shanming Ke, Chang Chen, Nian-Qing Fu, Hua Zhou, Mao Ye, Peng Lin, Wen-Xiang Yuan, Xierong Zeng, Lang Chen, and Haitao Huang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b09166 • Publication Date (Web): 11 Oct 2016 Downloaded from http://pubs.acs.org on October 11, 2016

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ACS Applied Materials & Interfaces

Transparent indium tin oxide electrodes on muscovite mica for high-temperature processed flexible opto-electronic devices Shanming Ke,†,‡ Chang Chen,† Nianqing Fu,ǁ Hua Zhou,† Mao Ye, *,† Peng Lin,† Wenxiang Yuan¶, Xierong Zeng,† Lang Chen,§ and Haitao Huang*,┴ †

Shenzhen Key Laboratory of Special Functional Materials, College of Materials Science and

Engineering, Shenzhen University, Shenzhen 518060, PR China ‡

State Key Laboratory of Solidification Processing, Northwestern Polytechnical University,

Xi’an 710072, P. R. China ǁ

School of Materials Science and Engineering, South China University of Technology,

Guangzhou 510640, PR China ¶

College of Chemistry and Environmental Engineering, Shenzhen University, Shenzhen,

518060, PR China §

Department of Physics, South University of Science and Technology of China, Shenzhen,

518055, PR China ┴

Department of Applied Physics and Materials Research Center, The Hong Kong Polytechnic

University, Hung Hom, Kowloon, Hong Kong, PR China

ACS Paragon Plus Environment

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ABSTRACT

Sn-doped In2O3 (ITO) electrodes were deposited on transparent and flexible muscovite mica. The use of mica substrate makes a high-temperature annealing process (up to 500 °C) possible. ITO/mica retains its low electric resistivity even after continuous bending of 1000 times on account of the unique layered structure of mica. When used as a transparent flexible heater, ITO/mica shows an extremely fast ramping (< 15 s) up to a high temperature of over 438 °C. When used as a transparent electrode, ITO/mica permits a high temperature annealing (450 °C) approach to fabricate flexible perovskite solar cells (PSCs) with high efficiency.

Keywords: flexible electronics, mica, transparent heaters, flexible perovskite solar cells, high temperature process


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In recent years, advances in technologies for flexible optoelectronic devices, including various displays, sensors and photovoltaic devices, have laid more stringent requirement on highly flexible transparent substrates. For instance, heat-resistant substrate is imperative for polycrystalline Si thin-film transistors (poly-Si TFT) in flexible displays.1 To absorb light from both sides, wearable solar cells should be fabricated on highly lightweight, transparent and flexible substrates.2 Thin polymer materials, e.g. polyethylene terephthalate (PET) and polyimide (PI), are the leading candidates for flexible transparent substrates. However, there are some insurmountable limitations of the available polymer substrates, including low processing temperature, lack of dimensional stability during processing, and significant differences in the linear coefficient of thermal expansion (CTE) between the polymer substrate and the device films.3 Thin glass and metal foil can overcome the above limitations, but the former is fragile and difficult to handle while the latter is not transparent. Mica is a well-known natural transparent crystalline material, which possesses high flexibility due to the layered framework of aluminosilicates. It seems that mica satisfies all of the requirements for a flexible transparent substrate: low cost, optical transmittance of >90% in the visible, high temperature tolerance of up to 600 °C, a low CTE of ~1×106

/°C (which matches those of silicon device materials), impermeability against oxygen

and water, high dimensional stability, chemically inert surfaces parallel to the [001] planes and being flat at an atomic level, etc.4 It is then expected that mica could be used as an ideal substrate for flexible optoelectronic devices. More recently, muscovite mica has been used as the substrate for studying biological specimens,5 template-assisted selfassembling of biomolecules,6 as well as deposition of semiconductive ZnO4,7. In a recent


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study, van de Waals (vdW) epitaxy of VO2 films has been demonstrated on layered transparent and flexible muscovite mica.8 Mica is considered to be an excellent vdW epitaxy substrate for growing 2D materials (Graphene, MoS2, ReS2, etc.), because of its atomic flatness, surface inertness, and rather high thermal stability.9,10 Some transparent conductive electrode (TCE) materials have also been gown on mica recently.11,12 For instance, Liu et al.12 proposed a flexible TCE based on a few layers of topological insulator Bi2Se3 grown on mica substrates. Assuredly, the optoelectronic properties of these films have been improved significantly by a thermal treatment process in excess of 500 °C. In this letter, high-performance Sn-doped In2O3 (ITO) thin films with a thin muscovite mica layer were prepared by pulsed laser deposition (PLD), followed by a high-temperature annealing process. PET was also used as a reference substrate. Because of the capability of withstanding high temperature, ITO/mica shows a steady-state high temperature over 400 °C as a transparent heater. Prototype flexible perovskite solar cells (PSCs) based on ITO/mica were fabricated, demonstrating its potential for flexible optoelectronic devices that can withstand high temperature processing. Similar to graphite, muscovite mica has a layered structure as shown in Fig. 1a and b. The adjacent layers are attracted by van der Waals forces instead of chemical bonds, which makes mica be easily cleaved. The cleaved surface is free from any dangling bonds. Theoretically, a monolayer of mica (thickness = 1 nm), consisting of a potassium layer sandwiched between two hexagonal sheets of SiO4 tetrahedra, could be obtained by mechanical exfoliation.13 Mica flakes with a few (2-9) layers have been prepared successfully and reported as a gate dielectric layer for organic and carbon nanotube field-effect transistors.14 Herein, the typical scotch tape method was used to peel off thin mica plates. The produced mica plates exhibit ideal flexibility and extremely


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high transparency (Fig. 1c and d). For mica flakes with thicknesses below 15 µm, an average transmittance of 87% in the ranges of 400-800 nm can be achieved. The high transmittance of free-standing thin mica flake is comparable with those of polymer substrates (e.g. PET). A freshly-cleaved muscovite substrate is generally clean with negligible dust particles and has a surface roughness of 0.24 nm, as observed by atomic force microscopy (AFM, Fig. 1e). It is worth to note that the mechanical exfoliation is enough to ensure a clean and smooth substrate, which can be used directly for device film deposition. The crystal structure of ITO on mica was examined using XRD measurement. In Fig. 2a, the peaks denoted by asterisks correspond to the muscovite (001) planes that are parallel to the cleavage plane. Annealed (500 °C for 30 min) ITO thin films did not show preferred orientation growth on mica, in contrast to those on glass and polymer substrates,15 which could be attributed to the clean surface of muscovite mica without any dangling bonds. Fig. 2b and 2c display the surface and cross-sectional SEM images of annealed ITO/mica films, respectively. Well crystallized film with fine grains could be clearly observed. The surface of ITO/mica shows a beautiful chrysanthemum-like structure, which could be seen more clearly in an AFM image (inset of Fig. 2b). This unique pattern is most probably due to the defects of muscovite mica surface.16 The roughness of the surface calculated from a 5×5 µm2 image area is around 0.775 nm, indicating a very smooth surface. ITO with a very smooth surface is an ideal electrode for OLED and solar cell applications.17 The effect of annealing on transmittance and sheet resistance of ITO/mica and ITO/PET films was investigated and compared. Fig. 3a shows the sheet resistance and optical transmittance spectra of ITO/PET, ITO/mica, and annealed ITO/mica. The


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transmittance spectrum of mica is similar to that of PET. The deposition of ITO thin films resulted in a significant decrease in transmittance in both PET and mica substrates. However, the transmittance of ITO/mica did recover to a relatively high value after annealing at 500 °C for 30 min, while the PET substrate cannot withstand such a high annealing temperature. The sheet resistance of the ITO/mica decreased by 98% after the annealing process due to increased crystallization and carrier concentration.18 Furthermore, the annealing temperature and thickness dependent sheet resistance and optical transmittance of ITO/mica are shown in Fig. 3b and 3c, respectively. The optical transmission of ITO/mica film in the visible region improves with increasing annealing temperature, showing the maximum transmittance at 800 nm being 68% and 88% for 300 °C and 400 °C annealed ITO/mica films. The sheet resistance of 100 nm-thick ITO annealed at 300 °C shows the minimum value of 50 Ω/ and after that, the resistance increases with annealing temperature. As expected, the sheet resistance of ITO decreases significantly with increasing thickness under the same annealing temperature of 500 °C (Fig. 3c). To compare the electrical and optical performances for different samples, figure of merit (F=To/Rs, To is optical transmittance) is considered and shown in Fig. S3. The figure of merit of these samples is mainly influenced by the sheet resistance. In addition, the conductivity and light transmittance could be further improved by careful structure design (e.g. ITO/Ag/ITO sandwiched structure), or by using other transparent conductive oxides (e.g. Al-doped ZnO, AZO). In order to verify the stability against mechanical bending of the ITO/mica films, a cyclic bending test was performed. Fig. 3d shows the optical transmittance and sheet resistance of 100 nm-thick ITO/mica after a cyclic bending of 1000 cycles with a bending


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radius of 5 mm. The transmittance and sheet resistance maintained almost constant during the 1000 bending cycles (compressive bending), which is superior to commercial ITO/PET films. The change of sheet resistance is negligible under compressive and tensile bending states, suggesting the robust cyclability of this heterostructure. From the result of retention test (Fig. S4), the sheet resistance of ITO/mica shows no time dependence under different bending states. Similar excellent flexibility has also been observed in VO2/mica system.8 The good performance of ITO/mica in flexibility and conductivity clearly demonstrate that mica is a very promising candidate substrate for flexible optoelectronics, such as transparent heaters and flexible solar cells. To demonstrate the above mentioned potentials, preliminary work was carried out. Fig. 4 depicts the time-dependent temperature of ITO/mica-based heater with Rs = 27 Ω/. The temperature responses of the heater were measured by a thermocouple. By applying different bias voltages (3-19 V), the surface temperature of the heater increased gradually from ambient temperature to certain saturated values until the thermal equilibrium was achieved. Interestingly, the ITO/mica heater reached a maximum temperature of 438 °C within 15 s by applying 19 V input voltage. Repeated heating tests display unchanged temperature-time curves, indicating excellent stability and reproducibility of this heater (Fig. S5). For comparison purpose, Table 1 lists the main parameters of some flexible transparent heaters based on various materials, such as graphene, CNT, and metal nanowires. Compared to the previously reported heaters,19-23 ITO/mica heater shows an extremely faster response, which is further exhibited by the heating rate curve (Fig. S6), calculated by the first-order derivative of the time-dependent temperature rising curve. It is observed that ITO/mica films display an extremely high heating rate of 97.3 °C/s at 19 V, which is 5-10 times faster than graphene-based heaters.23,24 A fast cooling rate of


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51.3 °C/s was also observed. Although a clear understanding of the thermal resistance between the substrate and the heating element is still lacking till now, the ultrathin and thermally conducting mica with ideal flat surface can result in a better heating performance in terms of response time, applied power and temperature attained than that of glass and PET substrates (As shown in Fig. S7). The obtainable maximum temperature is higher than any other heaters using polymer substrates, indicating that the transparent heaters based on mica have a unique advantage in the field of high temperature applications. In a typical perovskite solar cell (PSC), a perovskite absorber layer is sandwiched between the electron and hole transport layers (ETLs and HTLs, as shown in the inset of Fig. 5). Till now, TiO2 is the most commonly used ETL material in PSCs. Generally, a high temperature annealing at around 450 °C is indispensable for removing the binder and achieving better electric contact between TiO2 particles for PSCs.25 However, for polymer flexible substrates, the thermal treatment should be restricted to be below 150 °C. Fig. 5 and Fig. S9 illustrate the photovoltaic parameters of the ITO/mica based PSCs. The TiO2 ETL layer was annealed at 150 °C, 300 °C, and 450 °C, respectively. The primary results demonstrate that the ITO/mica based PSC with the high-temperature process yields a power conversion efficiency (PCE) of 9.67%, much higher than our previous flexible dye-sensitized solar cells (DSSCs) and low temperature processed PSCs,26 and is comparable to the best reported values of PSCs on polymer substrates (6-12%).27-30 The Jsc (short circuit current density), Voc (open circuit voltage), and FF (fill factor) are 19.05 mA·cm-2, 995 mV, and 0.51, respectively. The enhanced performance of the ITO/mica based PSC is thought to be resulted from the better crystallinity and electric contact of these device layers under high temperature (450 °C) treatment, and the smooth interface


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between ETLs and ITO electrodes. The preliminary results presented here demonstrate the potential application of the ITO/mica conductive substrate in flexible PSCs. It should be emphasized here that the sheet resistance of ITO films used in our PSC device is still too high (~130 Ω/). So the performance of PSCs could be further improved by using ITO/mica (or other transparent conductive oxides on mica with better performance) with optimized transparency and conductivity. In summary, high performance ITO thin films were deposited on muscovite mica by PLD technique with a high temperature annealing process. Repeated bending test shows that mechanically robust transparent conductive oxide electrodes could be obtained on flexible mica substrate. ITO/mica shows a fast ramping time (< 15 s) to achieve a very high temperature of over 438 °C when used as a transparent heater, which is a record temperature among flexible transparent heaters. By using the ITO/mica substrate as the flexible transparent electrode, a highly efficient perovskite solar cell can be achieved, and could be improved if the transparency and conductivity of the ITO/mica can be further optimized. Our work strongly suggests that muscovite mica is a very promising candidate substrate for flexible optoelectronics, which deserves to be investigated in depth.


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ASSOCIATED CONTENT Supporting Information. This Supporting Information is available free of charge via the Internet at http://pubs.acs.org. Experiment details for the preparation of ITO thin films and perovskite solar cells. Photographs of free-standing mica with different thickness. Heating and cooling cycles of an ITO/mica heater. Heating and cooling rate of an ITO/mica heater. The temperature profiles of ITO/mica, ITO/glass and ITO/PET heaters. Photograph of a PSC device. AUTHOR INFORMATION Corresponding Author *(M.Y.) E-mail: [email protected]. *(H.H.) E-mail: [email protected] Author Contributions S.K., M.Y. and H.H. conceived the idea, designed the experiments and interpreted the results. S.K. and C.C. performed the ITO and mica fabrications. N.F. performed the perovskite solar cell fabrications. H.Z., P.L., and M.Y. carried out the XRD, SEM and AFM characterizations. W.Y. and L.C. carried out the electrical and optical measurements. S.K., X.Z. and H.H. prepared the paper. All authors discussed the results and have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundations of China (Nos.51302172, 21405106, and 51272161), the Hong Kong, Macao and Taiwan Science & Technology Cooperation Program of China (No.2015DFH10200), the fund of the State Key Laboratory of Solidification Processing in NWPU (No.SKLSP201615), and the Science and Technology Research Items of Shenzhen (No.JCYJ20160422102802301 and KQJSCX2016022619562452). This work was also partially supported by the Hong Kong Polytechnic University (Project No.GUC71, G-YBB8 and G-YBFS).


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Table 1 Comparison of flexible transparent heaters Materials

Substrates

Max. T (°C)

Response time (s)

Ref.

CNT

PET

200

60

19

Ag NW

PDMS

150

10

20

Graphene

PET

100

75

21

Ag wire

PET

100

20

22

CuZr

PDMS

180

90

23

ITO

Mica

438

13

This work

Note: The response time is defined as the time required to reaching 90% of the steady-state temperature.


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Figure captions: Fig.1 (a) atomic stack of muscovite mica with layered structure; (b) optical image of a muscovite mica plate; (c) optical transmittance of muscovite mica with different thickness; (d) flexibility of muscovite mica; (e) AFM image of freshly cleaved surface of a muscovite mica plate. The surface roughness calculated from the 5×5 µm2 image is around 0.243 nm. Fig. 2 (a) XRD patterns of mica, ITO/mica (fabricated at room temperature), and annealed ITO/mica (500 °C for 30 min); (b) SEM and AFM images for surfaces of annealed ITO. The surface roughness of ITO is 0.775 nm; (c) cross-sectional SEM image of annealed ITO/mica. Fig. 3 Electrical and optical properties of ITO/mica films. (a) Sheet resistance and optical transmittance of ITO/PET, ITO/mica, and annealed ITO/mica. The thickness of ITO is fixed to 500 nm and the annealing temperature is 500 °C; (b) Sheet resistance and optical transmittance vs. annealing temperature for 100 nm-thick ITO on mica; (c) Sheet resistance and optical transmittance vs. thickness of ITO on mica annealed at 500 °C; (d) Sheet resistance and optical transmittance vs. bending cycles with a bending radius of 5 mm for 100 nm-thick ITO on mica annealed at 300 °C. The annealing time is fixed to 30 min. There is no clearly change between compressive and tensile bending. Fig. 4 The temperature profile of ITO/mica heaters measured by a thermocouple in ambient. The sheet resistance of the used 500 nm-thick ITO is 27 Ω/. Fig. 5 Typical current-voltage curve of perovskite solar cell based on ITO/mica photoanode, measured under 100 mW cm-2. The power conversion efficiency of this PSC is 9.67%. The sheet resistance of the used ITO/mica is about 130 Ω/.


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Fig.1 (a) atomic stack of muscovite mica with layered structure; (b) optical image of a muscovite mica plate; (c) optical transmittance of muscovite mica with different thickness; (d) flexibility of muscovite mica; (e) AFM image of freshly cleaved surface of a muscovite mica plate. The surface roughness calculated from the 5×5 µm2 image is around 0.243 nm.


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Fig. 2 (a) XRD patterns of mica, ITO/mica (fabricated at room temperature), and annealed ITO/mica (500 °C for 30 min); (b) SEM and AFM images for surfaces of annealed ITO. The surface roughness of ITO is 0.775 nm; (c) cross-sectional SEM image of annealed ITO/mica.


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Fig. 3 Electrical and optical properties of ITO/mica films. (a) Sheet resistance and optical transmittance of ITO/PET, ITO/mica, and annealed ITO/mica. The thickness of ITO is fixed to 500 nm and the annealing temperature is 500 °C; (b) Sheet resistance and optical transmittance vs. annealing temperature for 100 nm-thick ITO on mica; (c) Sheet resistance and optical transmittance vs. thickness of ITO on mica annealed at 500 °C; (d) Sheet resistance and optical transmittance vs. bending cycles with a bending radius of 5 mm for 100 nm-thick ITO on mica annealed at 300 °C. The annealing time is fixed to 30 min. There is no clearly change between compressive and tensile bending.


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Fig. 4 The temperature profile of ITO/mica heaters measured by a thermocouple in ambient. The sheet resistance of the used 500 nm-thick ITO is 27 Ω/.


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Fig. 5 Typical current-voltage curve of perovskite solar cell based on ITO/mica photoanode, measured under 100 mW cm-2. The power conversion efficiency of this PSC is 9.67%. The sheet resistance of the used ITO/mica is about 130 Ω/.


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Transparent Indium Tin Oxide Electrodes on Muscovite Mica for High

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