High-temperature-resistant and colorless polyimide: Preparations, properties, and applications

Solar Energy 195 (2020) 340–354

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High-temperature-resistant and colorless polyimide: Preparations, properties, and applications

T

Chenghan Yia,b, Weimin Lia, , Sheng Shia,b, Ke Hea,b, Pengchang Maa,c, Ming Chena, , ⁎ Chunlei Yanga, ⁎

⁎

a

Center for Information Photonics and Energy Materials, Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences, Shenzhen 510275, China Department of Nano Science and Technology Institute, University of Science and Technology of China, Suzhou 215123, China c Zhongshan Polytechnic, Zhongshan 528404, China b

ARTICLE INFO

ABSTRACT

Keywords: Colorless polyimide Synthesis Flexible Properties Applications

Colorless polyimide (CPI) is one of the most heat-resistant polymers. It has been widely used in many fields such as microelectronics and photoelectric fabrications, because it integrates the advantages of traditional aromatic polyimide (PI) films and common polymer optical films. This review summarizes the latest research and development of CPI films, including synthesis, properties and applications. In particular, the applications of CPI films in optoelectrical devices such as flexible thin film solar cells, advanced flexible display devices, flexible printed circuit boards, flexible terahertz sensors and smart windows are discussed.

1. Introduction Recent several years, due to the increasing demand for high integration, high signal transmission speed and high reliability in optoelectronic devices, optical transparent polymer films with high temperature resistance are highly desired. For example, in the process of the fabrication of active matrix organic light emitting display devices (AMOLEDs), the processing temperature of the flexible polymer film substrate may exceed 300 °C (Huang et al., 2011; Nakano et al., 2012; Wu et al., 2014). Most of the common optical polymer films, such as poly (methyl methacrylate) or polyethylene terephthalate (PET), will lose their mechanical and optical properties at such a high processing temperature and even cannot be processed. Therefore, the research and development of colorless and high-temperature resistance polymer films have attracted extensive attention of both academia and industry. Optical polymer films can be classified into three types according to their glass transition temperatures (Tg), including traditional optical films (Tg < 100 °C), common high temperature optical films (100 ≤ Tg ≤ 200 °C) and high temperature optical films (Tg > 200 °C), as shown in Fig. 1. The main physical and chemical properties of typical optical polymer films are shown in Table 1 (Ito et al., 2014; Sugimoto, 2002; Zardetto et al., 2011). Although traditional polymer optical films (PET, Tg: ~78 °C; PEN, Tg: ~123 °C; PC, Tg: ~145 °C, etc.) have excellent optical transparency, their limited glass transition temperature restrains their application in advanced optoelectronic engineering. On the

⁎

contrary, some kinds of high temperature optical polymer films (polyamideimide, polyetherimide, colorless polyimide, etc.) with high optical transmittance have a good compromise effect in the aspects of light color, good thermal stability, and excellent dielectric and mechanical properties. Table 2 provides a brief summary of the commercial available optical polymer films with relatively high optical transmittance and high-temperature resistance properties. CPI accounts for 90% of the consumption of the colorless and optically transparent polymer films. Aromatic polyimide was first produced in 1908 by Bogert and Renshaw (Ohya et al., 1997a). In 1955, high molecular weight aromatic polyimides were synthesized by a two-stage polycondensation of pyromellitic dianhydride with diamines (Mittal, 2007). Since then, this class of polymers has been attracted growing interests because of its excellent properties such as high heat resistance and chemical resistance, high mechanical properties and good dielectric features (Kreuz and Edman, 1998). PI is currently the most suitable polymer material for high temperature resistance application, which has been widely used in various fields, including aerospace, electronics, automotive, information recording and modern imaging technology, modern paper, solar cells and other green energy fields (Liaw et al., 2012; Lim et al., 2002; Min et al., 2010; Pakhuruddin et al., 2013; Sekitani et al., 2010). However, traditional all-aromatic PI films usually have lower optical transmittance due to the formation of intra- and inter-molecular chargetransfer complex (CTC) in their highly conjugated molecular structures

Corresponding authors. E-mail addresses: [email protected] (W. Li), [email protected] (M. Chen), [email protected] (C. Yang).

https://doi.org/10.1016/j.solener.2019.11.048 Received 6 August 2019; Received in revised form 24 October 2019; Accepted 13 November 2019 0038-092X/ © 2019 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved.

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low dielectric constant and low thermal expansion at high temperature, are basically achieved by the functionalizing the initial monomers. Besides, the cost of CPI films is determined by the monomer to a large extent. In fact, the development of the CPI industry is negatively affected by high costs and very limited commercially available monomers. Some typical commercial monomers for CPI films are listed in Table 3. To enhance their optical transmittance, intramolecular and/or intermolecular CTC formation in derived CPI films is reduced or inhibited by introducing flexible linkages, bulky substituents, non-coplanarity and pendant loop groups (cyclic side groups) in polymer backbone (Hu et al., 2010; Leu et al., 2003; Natansohn, 1999; Spiliopoulos and Mikroyannidis, 1996; Yu et al., 2016; Zhou et al., 2010). At the same time, the high chemical bond energy and large molecular volume of these groups will make the CPI films have good thermal stability. 2.1.1. Dianhydride synthesis CPI is usually synthesized from dianhydride and diamine monomers through different polymerization processes. However, the high cost and synthetic difficulties for functional dianhydrides have greatly limited the development and commercialization of CPI films (Matsumoto, 2000). Reaction conditions such as ozone, photoirradiation and highpressure oxidation with nitric acid are often used in the synthesis of dianhydrides. Moreover, the overall yield of these dianhydrides are very low. These two factors result into the high cost and limited kinds of dianhydrides. Therefore, people are eager to find a low-cost method for the synthesis of these dianhydrides to develop high performance CPI thin films. Guo et al. has developed a series of aliphatic anhydrides containing substituted tetrahydronaphthalene (Tetralin). The substituents in tetrahydronaphthalene anhydrides include alkyl group, chloromethyl group or fluorine, which may provide various functions for derivative CPI. The reaction of Diels-Alder and rearrangement was carried out by using cheap maleic anhydride and substituted styrene compounds as raw materials under the catalysis of nitric oxide (NO) gas. The obtained anhydrides have high yield and purity and can be directly used in polymerization (Guo et al., 2013).

Fig. 1. Classification of polymer optical films (Yang and Yuan, 2018).

(Ando et al., 1997; Hasegawa and Horie, 2001; Ke et al., 2013), as shown in Fig. 2. Therefore, the most challenging topic in the CPI is to balance their overall performance among high-temperature resistance, optical transmittance, and other properties. In this paper, we review the current situation and discuss the future development of CPI. The molecular structure design, synthetic chemistry, properties of CPI thin films are introduced. The applications of CPI thin films in several important optoelectronic fields, including flexible thin film solar cells, flexible display devices, flexible printing circuit board, transparent flexible terahertz sensors, smart windows are also discussed. 2. Synthesis To fabricate high overall performance CPI films, the most important issue is to balance high-temperature stability and optical transmittance property. Fig. 3 shows the molecular structure design for CPI films, including beneficial and unbeneficial designs. These functional groups or structure segments have been widely adopted to develop novel CPI films, endowing CPI films with both excellent thermal stability (Tg ≥ 300 °C) and good optical transmittance (> 85% in the visible light region).

2.1.2. Diamine synthesis In order to overcome the formation of intramolecular or intermolecular CTC, we need to modify the structure of diamines, such as the addition of flexible or asymmetric linkages in the backbone (Mittal, 2003; Ohya et al., 1997b), and the introduction of kink, spiro, bulky. cardo or pendant substitutes, while retaining the thermal resistance of polyimide, to improve the transmittance to the maximum extent to satisfy the need of experiment.

2.1. Monomer synthesis Monomer is the key component of the final CPI film because most of the functions of the CPI films, such as light color, high transmittance, Table 1 Typical properties of optical polymer films (Ito et al., 2006). Item1

Unit

PET

PC

PEN

PES

PEI

PPS

PI

CPI2

Density Transmittance Tm Tg WVTR OTR Water uptake σ Eb D.S. ε

g/cm3 % ℃ ℃ g/cm2/day cm3/cm2/day % Mpa % V/μm –

1.40 90 256 78 21 6 0.3 225 120 280 3.2

1.20 92 240 150 60 300 0.2 98 140 250 3.0

1.36 87 266 123 6.9 2 0.4 275 90 300 3.0

1.37 89 380 223 73 235 0.5 95 70 260 4.0

1.27 80 365 217 43.5 220 1 130 70 250 3.5

1.35 85 285 90 8 6 0.05 250 50 250 3.5

1.43 30–60 NA3 > 300 64 22 1.3 274 90 280 3.3

1.23 90 NA 303 93 NA 2.1 112 12 NA 2.9

1

Tm: melting point; Tg: glass transition temperature; WVTR: water vapor transmission rate; OTR: oxygen transmission rate; σ: tensile strength; Eb: elongation at break; D.S.: dielectric strength; ε: dielectric constant 2 Data from colorless PI film Neoplim® L-3430 developed by MGC, Japan. 3 Not available. 341

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Table 2 Several commercially available high-performance optical polymer films in the world (Liu et al., 2015a). Company

Country

Product name

Transmission (%)

Resin

Tg (℃)

Mitsubishi Gas Chemical DuPont-Toray Kolon Japan Synthetic Rubber Toyobo Nippon Steel Chemical Toray Sumitomo Bakelite Showa Electricity Tosoh Kurabo 1 not available

Japan USA South Korea Japan Japan Japan Japan Japan Japan Japan Japan

Neopulim® Colorless Kapton® NA1 Lucera® HM type Sillplus® Aramid® Sumilite® FS-1300 Shorayal® OPS film Examid®

89–90 87 89 88 91 91–92 NA 89 92 93 NA

PI PI PI NA Polymamideimide (PAI) Resin + glass Polyamide(PA) Polyethersulfone(PES) NA Polysulfone(PS) Polyamide(PA)

300–489 > 300 330–350 280 225 NA 315 223 250 220 220

Fig. 2. Formation of intra- and inter-molecular charge-transfer complex (Tapaswi and Ha, 2019).

2.1.3. Noncoplanar structures The noncoplanar structure includes kink, spiro and cardo and so on. The introduction of noncoplanar structure in polymer chain will prevent the chain from being arranged and destroy the formation of CTC, forming colorless polyimide film with high transmittance (Ohya et al., 1997a). The combination of substituted methylene and propylidene linkages lead to the kink structure. However, the poor chain flexibility of the propylidene groups decreases the thermal stability of the polyimide (Mittal, 2003, 2005). As shown in Fig. 4, two novel aromatic diamine monomers with pyridine structure were designed and synthesized to prepare the CPI by Yao et al. These results suggest that the

incorporating pyridine, kinky and bulky substituents to polymer backbone can improve the optical transmittance effectively without sacrificing the thermal properties (Yao et al., 2017). 2.1.3.1. Alicyclic units in main chain. Compared to aromatic polyimide, alicyclic polyimide has lower dielectric constant and higher transparency (Chung et al., 2006; Kudo et al., 2010; Mathews et al., 2006a; Mathews et al., 2006b; Matsumoto et al., 2006; Moore and Dasheff, 1989). These properties are ascribed to their low molecular density, low polarity and low probability of intermolecular or intramolecular charge transfer (Lemaire, 1976; M. H. Brink, 1994;

Fig. 3. Molecular structure design for colorless transparent polyimide films (Ni et al., 2015). 342

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excellent transparency and thermal properties. Adamantane (tricyclic [3.3.3.1.1]), a rigid alicyclic compound consisting of three cyclohexane rings, can be incorporated into the chair conformation without sacrificing the high transmittance, low dielectric constant and low coefficient of thermal expansion of CPI.

Table 3 Typical monomers for CPI synthesis. Monomers 1,4-Diaminocyclohexane

Chemical structure

Ref.

2,2′-Bis(trifluoromethyl)-4,4′diaminobiphenyl (TFMB)

(Jin et al., 1993) (Hasegawa et al., 2014)

1,2,4-Cyclohexanetricarboxylic anhydride (HTA)

(Hasegawa et al., 2012)

2,3,5-Tricarboxycyclopentylacetic dianhydride (TCA-AH)

(Tsuda et al., 1998)

1,2,3,4-Cyclobutane tetracarboxylic dianhydride (CBDA)

(Suzuki et al., 2000)

3,3′,4,4′-Bicyclohexyl tetracarboxylic dianhydride (HBPDA)

(Kaneya et al., 2014)

1R,2S,4S,5R-cyclohexane tetracarboxylic dianhydride (HPMDA)

(Uchida et al., 2012)

1S,2R,4S,5R-cyclohexane tetracarboxylic dianhydride (H''-PMDA)

(Hasegawa, Masatoshi et al., 2013)

2,2′-Bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA)

(Li and Hsu, 2007)

3,4-Dicarboxy-1,2,3,4-tetrahydro-1naphthalene succinic dianhydride (TDA) 3,4-Dicarboxy-1,2,3,4-tetrahydro-6fluoro-1-naphthalene succinic dianhydride (FTDA)

(Nishikawa et al., 1995)

3,4-Dicarboxy-1,2,3,4-tetrahydro-6chloro-methyl-1-naphthalene succinic dianhydride (CMTDA)

(Guo et al., 2012)

2.1.3.2. Fluorinated monomers. Fluorinated polymers have attracted much attention because of their low dielectric constant, low refractive index, high optical transmittance and low water absorption. The bulky trifluoromethyl group (-CF3) is also used to increase the free volume of the polymer, improving gas transmittance permeability and electrical insulation. McGrath et al. demonstrated a fluorinated diamine monomer based on trifluoroacetophenone was synthesized by two-step method (M. H. Brink, 1994). Trifluoroacetophenone was reacted with 4-nitrophenyl phenyl ether to yield 3F-dinitro compound, which was subsequently reduced to afford the fluorinated diamine, 1,1-bis [4- (4-aminophenoxy) phenyl)] phenyl-1-phenyl-2,2,2-trifluoro-ethane (3FEDAM), as shown in Fig. 5. 2.2. Film preparation Polycondensation is the most important polymerization reaction in the synthesis of CPI films. Polycondensation is a stepwise reaction between bifunctional or polyfunctional components with the release of small molecules, such as water, hydrogen halide, alcohol, etc. (Ni et al., 2015). Masatoshi et al. synthesized alkyl-substituted cyclobutanetetracarboxylic dianhydride (CBDA), from alkyl-substituted maleic anhydride. A new colorless polyimide (CPI) was obtained by two-step procedure and thermal imidization process, as shown in Fig. 6 (Hasegawa et al., 2014b). Toshihiko et al. reported the preparation of aliphatic polyimide from dianhydride with alkanone bis-spironorborane structure by twostep procedure and chemical imidization process, as shown in Fig. 7 (Matsumoto et al., 2014). A new unsymmetrical diamine monomer containing two trifluoromethyl (CF3) groups was synthesized by Sun et al. from 2-bromo5-nitro-1,3-bis (trifluoromethyl) benzene. The monomer was polymerized with 6FDA, using a one-step synthetic procedure to obtain CPI, as shown in Fig. 8. The prepared polyimide is soluble in a variety of organic solvents and can be solution-cast into colorless, flexible and tough films (Sun et al., 2013). Chang et al. prepared Poly(amic acid)(PAA) by the reaction of 4–4′hexafluoro-isopropylidene) diphthalic anhydride (6FDA) with 2, 2′-bis [4-(4-aminophenoxy) phenyl)] hexafluoropropane (BAPP) in N-N-

(Xiu-min et al., 2016)

Madhra et al., 2002; Susanta Banerjee et al., 2003; Yang et al., 2004; Yin et al., 2005). Therefore, the addition of alicyclic units in CPI is considered to be one of the effective ways to improve the transmittance and other desired properties. A number of partially alicyclic polyimides (Mathews et al., 2006a; Mathews et al., 2006b) and fully alicyclic polyimides have been studied (Chung et al., 2006; Kudo et al., 2010; Liu et al., 2004; Matsumoto et al., 2006). These polyimides perform

Fig. 4. Synthesis routes of BAPDBP and BAPDAP (Yao et al., 2017). 343

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Fig. 5. Chemical structures of 3FDAM and 3FEDAM (Brink et al., 1994).

O

O

O

O

O

CBDA

+

H2 N

NH2

3.2. Thermal dimensional stability

O

25 ,24h nitrogen

NMP

H N

O

the polymer maintain its mechanical properties at high temperature (Liaw et al., 2005; Liaw et al., 2007a, Liaw et al., 2007b; Wang et al., 2008a; Wang et al., 2008b; Zhang et al., 2005).

O

HOOC

O

H N

O

The curing sequence of CPI includes solvent evaporation, imidization of PAA precursors and cooling of the film. The curing rate and temperature have influence on the final film forming performance. If CPI is partially cured or rapidly solidified, the coefficient of thermal expansion (CTE) of the film will be higher due to the presence of a certain amount of unevaporated solvent (Chung et al., 2000). The coefficient of thermal expansion of CPI is related to the linearity of polymer molecule. When the conformation of chain is linear, the CTE of chain is lower (Numata et al., 1986). The difference in CTE of CPI, metal and the substrate leads to the thermal mismatch and thermal stress in the cured CPI film. And these stresses become more obvious with different curing periods. Therefore, the thermal expansion behavior of CPI plays an important role in the packaging structure. Thermal stress can be controlled at a lower level using CPI with a similar CTE to the substrate or metal (Lu and Zheng, 2018). For lowering the CTE values, CPI composites containing inorganic nanoparticles, such as titanium dioxide, silica, organoclay, via sol-gel route have been proven to be an effective approach (Choi et al., 2013; Hasegawa et al., 2014b; Kim et al., 2015; Komarov et al., 2009; Liu et al., 2011; Tseng et al., 2013).

n

COOH

PAA 25-300 O

O

N

N

O

O

nitrogen

O

n

CPI

Fig. 6. Two-step procedure with thermal imidization process.

3.3. Mechanical property Table 5 shows mechanical property of some CPIs. The mechanical properties of CPIs are influenced by molecular structure, molecular weight, solvent, drying condition, imidization condition, synthesis method, processing condition, etc. Therefore, the uncertainty of the experiment is likely to cover up the differences in mechanical properties. In general, the modulus of CPIs is over 2 GPa, and the tensile strength is over 70 MPa. But the elongation at break is between 2% and 20%, depending on the chemical structure. CPI containing flexible bonding units in the main chain, such as ether bonds and isopropyl, have more elongation. In addition, noncoplanar, amorphous and asymmetric polyimide also usually perform high elongation. In general, the CPI for electronic applications should be strong enough, with an elongation of at least 10%, and tensile strength greater than 100 MPa to withstand stress induced during processing and thermal cycling.

dimethylacetamide (DMAc). Hybrid films were prepared by mixing the precursor polymer with Cloisite 15 A (organically modified MMT), as shown in Fig. 9. When the PAA and clay solutions are mixed, the polymer chains intercalate and displace the solvent from between the layers of the clay. Upon solvent removal, the intercalated structure remains, resulting in hybrids with nanoscale morphology. The results show that the addition of a small amount of Cloisite 15 A can improve the thermal and mechanical properties of CPI (Choi and Chang, 2011). 3. Main properties CPIs have been applied in many fields because of their excellent properties. As the substrate or cover plate of the device, the various properties of CPI will directly affect the processing and use of the device.

3.4. Optical property

3.1. Thermal stability

Because of the CTC between the donor group of diamine and the dianhydride receptor molecule, PI usually exhibits a certain color. The CPIs can be obtained by using the lower electron receptor dianhydride and the lower electron donor diamine. When these monomers are used, the intramolecular and intermolecular CT interactions will be weakened. Some soluble CPIs can be obtained by using alicyclic dianhydride or diamine monomer. However, these polymers are not very stable at high temperatures because their aliphatic segments are not very stable (Yang et al., 2006). In addition, the addition of a large number of CF3 groups in polyimide is another way to obtain light-colored polyimide films. Polyimide containing adamantane (tricyclic [3.3.3.1.1] decane) is a rigid alicyclic compound, which consists of

Td and Tg are two typical parameters used to indicate the thermal stability of the CPI films, which can be determined by thermogravimetric analysis (TGA) and Differential scanning calorimetry analysis (DSC). Table 4 shows thermal stability of some CPIs. The Td and Tg of most CPIs are measured by TGA and DSC, commonly in the regions of 305–550 °C and 220–400 °C, respectively. CPIs containing noncoplanar units, heteroaromatic units have higher Tg and higher heat resistance. However, CPIs containing flexible bonds, such as ether units, have low Tg due to their flexible polymer backbone. Pyridine ring increases the symmetry and aromaticity of the polymer and improves the chemical and thermal stability of the polymer. In addition, pyridine ring helps 344

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O

O

O

O

O

O

O

O

+ H2N

O

O

O

HOOC

NH2

60 ,12h nitrogen

GBL

H N

O

H N

n O

COOH

O

PAA Ac2 O pyridine O

120 ,4h nitrogen

O

O

N

N

O

O

O

O

n

CPI Fig. 7. Two-step procedure with chemical imidization process.

O

O

CF3

O O

+

O

CF3

N O

O

NH2

CF3

m-cresol

O

H2N

O

6FDA

candidates for low-k applications (Willi Volksen and Miller, 2010). The dielectric constant of most CPIs does not change significantly with different frequency, except for some fluorine-containing CPIs, which exhibit significant (of the order of 0.3–0.5) variations. The low dielectric constant of the polyimide backbone containing CF3 unit is due to the low polarizability of C-F bond and the increased free volume of the polymer. Some transparent PIs containing fluorine have a dielectric constant of up to 2.4. In addition, the introduction of bridging groups and the disconnection of conjugate structures can be used to destroy the coplanar between aromatic rings, resulting in low interaction between the chains and thus low the dielectric constant. Surface modification of nanomaterials as well as polymers adds flavor to the dielectric properties of the resulting nanocomposites. Generally, one-dimensional and two-dimensional nanofillers with large aspect ratios provide enhanced flexibility versus zero-dimensional fillers. Nowadays, three-phase nanocomposites with either combination of fillers or polymer matrix help in further improving the dielectric properties as compared to two-phase nanocomposites (Prateek et al., 2016).

CF3

Isoquinoline 180 , 6h nitrogen O

CF3

N

CF3

CPI

O

CF3 O CF3

n

Fig. 8. One-step procedure for the preparation of CPI.

three cyclohexane rings in the chair conformation to produce lightcolored films with high transmittance and good thermal stability (Mathews et al., 2006b; Mathews et al., 2010).

4. Applications The fabrication of flexible electronic devices has attracted increasing attentions. With the development of structural support and optical signal transmission pathway and medium, flexible substrate is playing an increasingly important role in the advanced development of optoelectronic display devices. There are several challenges existing in fabricating high performance CPI for optoelectronic display application. The first consideration is that the thermal stability of the transparent substrate should meet the processing conditions of the device. Colorless and optically transparent polymer substrate with good

3.5. Dielectric property The dielectric constant of CPI thin film is usually measured by the parallel-plate capacitance method, in which the frequency range is from 0.1 kHz to 1 MHz and the temperature variation can be varied in a wide range (Hougham et al., 1994). Most fully cured, aromatic polyimides have dielectric constants in the 3.0–3.5 range, making them legitimate 345

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O

O

CF3

O

O

CF3

O

O

CF3 CF3

HOOC

O

R.T.

DMAc

CF3

O

NH2

CF3

O

6FDA

H N

+ H2N

BAPP

O NH

CF3

O

O

n

CF3

COOH

PAA Cloisite 15A

PAA hybrid Thermal Imidization O

CF3

N

CF3

O N O

O

CF3

O

O

n

CF3

CPI hybrid

Fig. 9. Preparation of CPI hybrid film. Table 4 Thermal stability of some CPIs.

Table 5 Mechanical property of some CPIs.

CPI

Tg

Td5

Ref.

CPI

σb(MPa)

E(GPa)

ɛb(%)

Ref.

6FDA / TFMB H'-PMDA / ODA H'-PMDA / HFBAPP H'-PMDA / TFMB H-PMDA / p-CHDA H-PMDA / DCHM H″-PMDA / TFMB H″-PMDA / ODA CBDA / DCHM CBDA / TFMB DM-CBDA / p-CHDA HTA-HQ / TFMB HTA-44BCH / TFMB TDA / ODA MTDA / ODA

325 249 287 357 367 301 339 328 326 356 360 250 252 271 255

573 453 502 491 436 453 477 484 433 459 428 441 346 404 395

(Lan et al., 2019) (Hasegawa et al., 2014) (Hasegawa et al., 2014) (Hasegawa et al., 2014) (Hasegawa et al., 2014b) (Hasegawa et al., 2014b) (Hasegawa et al., 2013a) (Hasegawa et al., 2013a) (Hasegawa et al., 2014b) (Hasegawa et al., 2013b) (Hasegawa et al., 2014b) (Hasegawa et al., 2012) (Hasegawa et al., 2012) (Guo et al., 2012) (Guo et al., 2012)

H'-PMDA / ODA CHDA / BAPM BCDA / BAPM 6FDA / BAPP 6FDA / APS H″-PMDA / TFMB H″-PMDA / BA6F CBDA / TFMB

95 87 76 97 87 108 81 103

2.42 2.4 2.0 2.28 2.62 2.22 2.39 3.65

9.4 5.5 5.4 8 5 18 8 5

(Hasegawa et al., 2014a) (Liu et al., 2019) (Liu et al., 2019) (Choi and Chang, 2011) (Park and Chang, 2009) (Hasegawa et al., 2013a) (Hasegawa et al., 2013a) (Hasegawa et al., 2013b)

TS: tensile strength; TM:tensile modulus; Eb: elongation at break. CHDA: 1,2,4,5cyclohexane-tetracarboxylic dianhydride; BCDA: bicyclo[2.2.2]octa-7-ene2,3,5,6-tetracarboxylic dianhydride; BAPM: α,α-bis(4-amino-3,5-dimethylphenyl)-1-phenylmethane; BAPP: 2,2′-bis[4-(4-aminophenoxy)phenyl] hexafluoropropane; APS: bis-(3-aminophenyl) sulfone; BA6F: 2,2-Bis(4-aminophenyl)hexafluoropropane

Tg: glass transition temperature; Td5: the decomposition temperature at 5% weight loss.6FDA: 2,2-bis(3,4-dicarboxyphenyl)-hexafluoropropane; H'-PMDA: 1S,2S,4R,5R-Cyclohexanetetracarboxylic dianhydride; H″-PMDA: 1R,2S,4S,5Rcyclohexanetetracarboxylic dianhydride; H-PMDA: 1S,2R,4S,5R-cyclohexanetetracarboxylic dianhydride; CBDA: 1,2,3,4-cyclobutanetetracarboxylic dianhydride; DM-CBDA: 1,3-dimethyl-1,2,3,4-cyclobutanetetracarboxylic dianhydride; HTA-HQ: 5-isobenzofurancarboxylic acid-octahydro-1,3-dioxo-5,5′(1,4-phenylene) ester; HTA-44BCH: (3aR,5R,7aS)-rel-5-isobenzofurancarboxylic acidoctahydro-1,3-dioxo-5,5′-[1,1′-bicyclohexyl]- 4,4′-diyl ester; TDA: 3,4-dicarboxy-1,2,3,4-tetrahydro-1-naphthalene succinic dianhydride; MTDA: 3,4-dicarboxy-1,2,3,4-tetrahydro-6-methyl-1-naphthalene succinic dianhydride; p-CHDA:trans-1,4-cyclohexanediamine; DCHM:4,4′-methylenebis (cyclohexylamine); ODA: 4,4′-Oxydianiline; HFBAPP: 2,2-Bis[4-(4-aminophenoxy)phenyl] hexafluoropropane; TFMB: 2,20-Bis(trifluoromethyl)benzidine

thermal resistance above 300 °C is highly desirable in advanced flexible display engineering. Secondly, the polymer film substrate should have low oxygen permeability (OTR) and water vapor transmission rate (WVTR) characteristic. When exposing to environmental moisture, most of the high performance semiconductor organic compounds based on the substrate exhibit degraded performance (Pang et al., 2011b). For the AMOLED and organic solar cells, the OTR and WVTR of the flexible substrate are severely limited to be below 10−4 cm3/ m2 /day and 10−6 g/ m2/day, respectively (Park et al., 2011). Unlike glass, polymer film substrates usually do not provide adequate protection for the permeation. For example, the WVTR value of the general PI film is 10–100 g/m2/ day, depending on the aggregation structure of its molecular chains. 346

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Therefore, inorganic thin films with extremely high barrier properties have to be used on the substrate in practical applications. For CPI thin film substrates, specific additives such as graphene (Park and Chang, 2009; Tseng et al., 2012) or organic clay (Choi and Chang, 2011; Choi et al., 2008; Min et al., 2011) may be incorporated to some extent to improve their moisture barrier properties. Thirdly, the polymer film substrate should have a CTE comparable to that of the inorganic or metal components in the display devices. The CTE value of the common polymer film substrate is generally higher than 30 ppm/℃. However, the inorganic component such as SiNx gas barrier layer has a CTE value below 20 ppm/℃. The unmatched CTE values between polymer films and other materials are deemed to be one of the most important reasons for cracking, delamination and other failures in the devices (Dumont et al., 2007). The hybrid of the sol-gel pathway with an inorganic additive, such as titanium dioxide, silica has been proven to be an impactful way to reduce the CTE value of the CPI films (Choi et al., 2013; Wang and Chen, 2010).

Fig. 10. CdTe solar cells on CPI substrate (2012).

4.1. Substrates for flexible thin film solar cells Thin film solar cells or flexible photovoltaics (PV) have been widely studied because of their potential to reduce the cost per watt of solar energy and improve the lifetime performance of solar cells (Partain and Fraas, 2010). Traditional thin film solar cells are usually fabricated on 3–5 mm thick soda-lime glass substrates coated with transparent conductive oxides (TCOs) and have no weight advantage or shape adaptability for curved surfaces. The fabrication of thin film solar cells on flexible polymer substrates seems to offer a number of advantages in practical applications, such as weight reduction, cost savings and easy manufacture. Polymer substrates for thin-film solar cells should be optically transparent and able to withstand the high processing temperatures. For example, for the current manufacturing technology of cadmium telluride (CdTe) cells, the processing temperatures are range of 450–500 °C. Most transparent polymers will degrade at such a high temperature. There is no doubt that the lack of stable transparent polymer at high processing temperature of solar cells is one of the biggest obstacles to the application of polymer substrates in flexible solar cells. Flexible Cu(In,Ga)Se2 (CIGS) thin film solar cell based on PI has many advantages than other substrates, such as lightweight, good folding, but there are still several challenges on PI substrate. One of the challenges is the thermal stability can’t meet the requirements of CIGS thin film crystal growth. The substrate temperature required for the growth of CIGS film crystals with continuous large grains should be above 550 °C, but the current temperature that PI substrate can withstand is generally below 450 °C, leading to smaller CIGS film crystals growing on PI substrate and increased crystal defects. This restriction of lower maximum substrate temperatures on PI has for a long time constrained the maximum demonstrated conversion efficiency of CIGS solar cells on PI to below 16% (Caballero et al., 2011; Niki et al., 2010; Rudmann et al., 2005). Another challenge is the growth of CIGS films on PI substrates requires an alkali metal post-treatment process to introduce alkali metal doping, because a large number of experiments have proved that alkali metal doping in CIGS films can improve the crystal quality of the films, reduce the recombination of carriers, and improve the open circuit voltage, filling factor and efficiency of the devices. Through continuous efforts, the performance of the device has been greatly improved in 2011, Empa achieved 18.7% conversion efficiency on PI substrate for CIGS by using modified three-stage process, indicating that flexible CIGS solar cell has a promising future (Adrian et al., 2011). In 2019, Empa utilized heavy alkali element Rb post deposition treatment (PDT), which is RbF-PDT, attaining the new record efficiency of 20.8% (Carron et al., 2019). All aromatic PI films, such as Kapton® (DuPont, USA) can withstand a high temperature of 450 °C. However, it shows deep color and strongly absorb visible light. Due to large optical absorption, CdTe solar

Fig. 11. Representative cross-sectional FESEM image of flexible MAPbI3 perovskite solar cell (Park et al., 2017).

cells on this PI substrate will produce only low current (Mathew et al., 2004). The development of CPI thin films with good high temperature stability makes it possible to produce high-efficient solar cells. In order to improve the performance of thin film solar cells and simplify the fabrication process, Empa has been working on the development of highly efficient thin film solar cells. Empa used CPI film developed by DuPont Company as flexible substrate for photovoltaic module of CdTe thin film solar cells in 2011, as shown in Fig. 10. The conversion efficiency of the new substrate reached 13.8%, which was a new record for such solar cells at that time (Aliyu et al., 2012). Kim et al. utilized a CPI as the substrate to deposit ITO electrode, which could achieve a low sheet resistance of 57.8 Ω/square and a high transmittance of 83.6% after a thermal annealing process at 300℃, which are better values than those of an ITO/PET sample. The highperformance flexible CH3NH3PbI3 (MAPbI3) perovskite solar cells with the structure of Au/PTAA/MAPbI3/ZnO/ITO/CPI shown in Fig. 11. The successful operation of these flexible perovskite solar cells on ITO/CPI substrate indicated that the ITO film on thermally stable CPI substrate is a promising of flexible substrate for high-temperature processing, a finding likely to advance the commercialization of cost-efficient flexible perovskite solar cells (Xie et al., 2019). 347

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Table 6 The development of the smartphone screen.

AMOLED technology Brand Characteristic Diagrammatic sketch

2007

2013

2017

2018

Rigid AMOLED production Samsung Any call Haptic Rigid, picture quality

The first production of flexible AMOLED Samsung Galaxy Round Not suitable for breaking, local bending

Apple adopts flexible AMOLED screen Apple iPhone X Full screen (grooves)

Flexible AMOLED foldable smartphone ROYOLE FlexPai Foldable screen

Fig. 12. Roadmap for flexible products and substrates (Janglin and Liu, 2013).

4.2. Substrates for advanced flexible display devices

have attractive features such as thinness, light weight and flexibility. The commercial roadmap for flexible display development is shown in Fig. 12. We have envisioned a flexible four-stage development roadmap that shows: (1) low power-displays are actually built on rigid glass and provide low power and thin shape coefficients; (2) bendable-displays are bendable and integrated. From this stage, displays begin to show real flexibility; (3) rollable-displays in this stage are extremely flexible in one dimension, so they can roll around cylinders with small diameters; And (4) free form-when the display can be made into any form, this is the final stage of flexibility, just like paper (Pang et al., 2011a). For AMOLED, typical coating / debonding or transfer methods can be used on flexible PI films, as shown in Fig. 13 and Fig. 14 (Hasegawa et al., 2014b). The basic process steps of the coating / debonding method are shown in Fig. 8: (1) coating the PI solution directly on the glass substrate, (2) fabricating the thin film transistor (TFT) device on the PI substrate, (3) the PI substrate is stripped from the glass carrier by mechanical delamination or laser delamination (Delmdahl et al., 2013; Hong et al., 2014; Lin et al., 2014). The coated PI substrate is basically a PAA solution. In the first step, PAA solution was spin-coated on the substrate of temporary glass carrier, and PI film was formed after thermal imidization. In the transfer method, it contributes to the top emitting OLED and the color filter (Kataish et al., 2015; Ryu Komatsu, 2014), the PI film is used as a flexible substrates. Fig. 9 shows the principle of the transmission method, in which the TFT/OLED substrate and the color filter are prepared and bonded together. After separating

In recent years, great progress has been made in flexible activematrix organic light-emitting diode (AMOLED) displays and a large number of products have been displayed. Replacing hard glass with flexible substrates and thin film packages to make displays thinner, lighter, and less fragmented are the attractions of portable applications (Kane, 2018). The development of the smartphone screen is shown in Table 6. As the structural support and optical signal transmission pathway and medium, flexible substrates have become the key component of the advanced optoelectronic display devices in the near future. The characteristics and functionalities of flexible substrates have become an important factor that affect the quality of flexible devices. A typical AMOLED display is majorly composed of three parts: backplane, frontplane and packaging. Currently, there are three types of substrates for flexible displays: thin glass, transparent polymer, and metal foil. Ultra-thin glass has the advantages of Tg, low CTE, smooth surface, high transparency, low WVTR, and good chemical resistance. But its fragile features bring inconvenience to display manufacturing and end-user experience. The metal foil has a poor surface weakness and opaque features to limit its application in the display (Chiang et al., 2015). The transparent polymer substrate has good optical transmittance similar to thin glass and has good flexibility and toughness comparable to that of the metal foil. Therefore, they are an ideal choice for flexible display, which are believed to be one of the promising displays because they 348

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Fig. 13. Illustration of the coating / debonding method: (a) coating PI film, (b) fabricating TFT device, (c) de-bonding of glass substrate (Koden, 2017).

Fig. 14. Illustration of the transfer method: (a) bonding TFT/OLED substrate and color filter substrate together, (b) debonding the glass substrates from the passivation layers, (c) bonding CPI film to the AMOLED device (Furukawa and Koden, 2017).

the glass substrate from the passivation layer by physical force, the PI film is finally bonded to the AMOLED device. Currently, the properties of TFT based on polymer sheets are limited by the low temperature process of polymer substrates, usually less than 250 °C (Kane et al., 2012; Lee et al., 2012). Fabricating a TFT on a flexible substrate is one of the most important procedures for a flexible display device, such as an active matrix-driven organic light emitting diode (AMOLED) processing. Up to now, there are four manufacturing techniques for TFT fabrication, including amorphous silicon (a-Si) TFTs, oxide TFTs, low temperature polysilicon (LTPS) TFTs and organic TFTs (OTFTs). Recently, the indium gallium zinc oxide (IGZO) TFTS technique has been reported to process excellent field effect mobility (> 10 cm2/Vs), good uniformity and good electrical stability in a simple process (Nomura et al., 2004; Park et al., 2009). However, the

manufacturability of this technology is still under study. Table 7 summarizes the key features of the current TFT. It can be seen that LTPS TFT technology exhibits the highest field effect mobility and stable electrical properties (Ni et al., 2015). However, this process requires a high process temperature of about 500 °C during silicon crystallization. For a-Si TFT process, because of a large area of uniform electrical characteristics, a reasonable field effect mobility, a low temperature process (< 300 °C), and a low cost compared to other techniques, it has been widely used in AMOLED devices. Toshiba Corp. Japan has successfully developed a flexible 10.2-inch product, as shown in Fig. 15. The bottom-emission AMOLED display device of WUXGA (1920 × RGBW × 1200) was driven by an amorphous indium zinc oxide (IGZO) TFTs on a CPI thin film substrate. First, a CPI film was formed on a glass substrate, and then a barrier layer was

Table 7 Key features for a-Si TFTs, Oxide TFTs, LTPS TFTs. Item

a-Si TFT

Oxide TFT

LTPS TFT

Active layer material Active layer deposition Electron mobility Resistance Uniformity Process temperature Substrate size Advantage Target application

a-Si PECVD < 1cm2/Vs high low < 380 ℃ 11 G Smallest number of process All flat panel display

Additional process

Basic display process

IGZO Sputter 5–25 cm2/Vs medium medium < 350 ℃ 8G Fast response, high resolution Large OLED TV High resolution tablet, notebook panels Transparent display IGZO evaporation, ESL evaporation, mask process

Cost Yield Major company

low high All panel manufacturers

high Mass production level LG Display, Sharp

Poly-Si PECVD + ELA 50–120 cm2/Vs low high (but low for large panel display) < 450 ℃ 6G Fast response, high resolution Flexible display High resolution smartphone panel Automobile display ELA, annealing (hydrogenation /dehydrogenation), ion implantation, mask process increase highest Mass production level Samsung Display, LG Display, BOE, GVO etc.

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deposited to prevent water permeation. Then, the gate insulator, an IGZO thin film, source-drain metal, and passivation layer are deposited in sequence to provide an IGZO TFT. Second, a flexible LED panel was fabricated using an IGZO TFT, color filter, white OLED, and encapsulation layer. Finally, the OLED panel was debonded from the glass substrate to obtain the final AMOLED panel (Saito et al., 2013). 4.3. Substrates for flexible printing circuit boards (FPCBs) FPCB applications have been the largest market for high temperature polymer films for many years, such as PI, polyamideimide and polyetherimide films. The flexibility of FPCB makes it easy to use in compact electronic devices such as portable computers, watches, panel boards and digital cameras. In general, conventional FPCB is mainly prepared from flexible copper clad laminate (FCCL), as shown in Fig. 16. FCCL is composed of PI films bonded to copper foil (Zhang et al., 2017). Depending on the intended use of the laminate, copper can be applied to one (single-sided) or both sides (double-sided) of the PI film. PI films almost dominate the FCCL market, which requires heat resistance to withstand soldering temperatures. With the development of flexible displays, the need for transparent thin film substrates to replace glass substrates is increasing. However, most of the all-aromatic PI films currently used in FCCL exhibit color from yellow to deep

Fig. 15. Schematic cross-sectional view of flexible AMOLED display driven by a-IGZO TFT on a transparent polyimide film (Hajime Yamaguchi et al., 2012).

Fig. 16. FPCB industry chains from FCCL to final products.

Fig. 17. Application of polymer optical thin films in transparent FPCBs: (a) Fujikura, Japan (b) Toyobo, Japan, (c) Mektek, Japan (Ni et al., 2015).

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Fig. 18. Frequency and wavelength regions of the electromagnetic spectrum(Ghann and Uddin, 2017).

4.5. Coating for smart windows The visible light accounts for 45% of the solar energy reaching the ground, and the infrared light accounts for 50%. At present, there are not many applications for this nearly 50% infrared light. With the modernization of architectural forms, windows and glass curtain walls account for more and more of the exterior area, due to their multiple functions in heat preservation, heat insulation, and absorption of sunlight. The intelligent window film material becomes the most important factor that affects the energy consumption of the building body. In 1958, Morin found that the vanadium oxide (VO2) thin film has the characteristic of temperature phase transition, its phase transition temperature is 68 °C, when the temperature is below 68 °C, it is a semiconductor state. When the temperature is higher than 68 °C, it is a metal state (Morin, 1959). After the phase transition, the optical properties will change obviously due to the sharp enhancement of the free electron conductivity, and the light transmittance will be influenced by the absorption of the free electron to the light. In particular, the transmittance of infrared band decreases sharply and the reflectivity increases, thus blocking the entry of infrared ray. When the temperature drops below 68 °C, the reverse effect will occur. That is the VO2 thin film will change from metal state to semiconductor state. The transmittance of infrared light is increased automatically. In this way, the vanadium oxide film can automatically regulate the outdoor solar radiation and the heat loss due to heat transfer, convection and radiation loss in the indoor, avoid the indoor supercooling or overheating, and realize the intelligent control of the indoor temperature. However, pure VO2 does not meet the requirement of smart window due to high critical temperature (τc, about 68 °C), low luminous transmittance (Tlum, lower than 40%) and poor solar modulation ability (ΔTsol, the difference between solar transmittance at low temperature and high temperature, less than 10%) (Li et al., 2012). Therefore, thermochromic performance of VO2 need to be improved to meet the requirement of smart window. There are two approaching for the performance enhancement, reduce τc via dopant and enhance both ΔTsol and Tlum via structure engineering. Tungsten (W), molybdenum (Mo), and rare-earth elements have been investigated to decrease τc by introducing distorting lattice and charge balance (Cao et al., 2009; Li et al., 2014). Because of the high transmittance of CPI in the visible range, coating vanadium dioxide thin film on the surface of the CPI film and then sticking it to the window glass will have a good energy saving effect and can be recycled, as shown in Fig. 20.

Fig. 19. The schematic diagram of terahertz detector structure.

brown. Therefore, they cannot be used for transparent FCCL. Recently, the development and commercialization of transparent FPCB products in the world has been very active. Various new products based on TCOs (In2O3 + SnO2, ITO) coated high-temperature optical polymer thin films, including PEN, PAI and PI films have been developed as shown in Fig. 17. 4.4. Substrates for transparent flexible terahertz sensors Terahertz is an electromagnetic wave with a frequency of 0.1–10 THz (1 THz = 1012 Hz), corresponding to a wavelength range of 3 mm ~ 30 μm, between millimeter wave and infrared wave (Pawar et al., 2013). The position of terahertz in the electromagnetic spectrum is shown in Fig. 18. In the last two decades, the stability and reliability of terahertz emitter and radiation detector have been improved significantly as a result of the rapid development of ultra-fast electronics, laser technology and low-scale semiconductor technology. Terahertz sensors are widely used in explosive and biological warfare agent detection, military communication, strategic missile and aerospace vehicle nondestructive detection, hidden weapon inspection, field medical treatment, etc. (Sharma et al., 2011). The traditional substrate used in terahertz sensors is silicon wafer, but with the development of new applications, new requirements for terahertz sensor are put forward, such as transparency, flexibility, portability, high efficiency and so on. The schematic diagram of flexible terahertz detector structure is shown in Fig. 19. The CPI substrate can not only meet the requirements of high temperature resistance in the process of device fabrication, but also meet the requirements of transparency and flexibility. 351

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Fig. 20. Coating of smart windows.

5. Conclusion and outlook In this paper, we have integrated the synthesis, properties and applications of various CPIs. Since high molecular weight PIs were firstly synthesized by a two-stage polycondensation of pyromellitic dianhydride with diamines in 1955, the interest in this class of polymers has been growing steadily. PIs present excellent comprehensive performance including very high thermal stability, chemical resistance, and good mechanical properties. However, their light or dark yellow color due to intra- and inter-chain CT interactions, high hydrophilicity and dielectric constants, as well as low hydrolysis-resistance and alkali-resistance are problems. These problems hinder their extended applications in electric, electronic, optical materials, and other advanced material fields. In recent years, PIs with high optical transparency and low dielectric constants have been in demand for optoelectronic and flexible display applications. CPIs are promising candidates for applications in optoelectronics and interlayer dielectrics as materials for soft-printed circuit boards, displays of smart phones, tablet PCs, and other types of mobile electronic devices, due to their high transparency and low dielectric constants compared to all-aromatic PIs. With their unique features, CPIs will continue to find new industrial applications in the future. Novel CPIs with various functions are possible and can be explored using new monomers, polymer design principles, and modifications. But so far, there are still many obstacles to the application of CPIs, limited to high-end optoelectronic products. In a word, there are two reasons: one is the high price of CPI monomer, leading to the increase of the cost of the product; the other one is that some properties of CPI need to be improved, such as water vapor transmittance rate and dielectric coefficient. By improving the design and synthesis of monomers, the performance of CPI will be improved remarkably, its price will be reduced gradually, and thus it will be applied more widely in the future. Declaration of Competing Interest We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work. Acknowledgements The work was supported by the National Key R&D Program of China Grant nos. 2018YFB15002, the National Natural Science Foundation of China under Grant nos. 61804159, 61574157 and 61774164, the Shenzhen Basic Research Grant nos. JCYJ20150925163313898 and JCYJ20160331193134437. References Adrian, C., Stephan, B., Fabian, P., Patrick, B., Christina, G., Uhl, A.R., Carolin, F., Lukas, K., Julian, P., Sieghard, S., 2011. Highly efficient Cu(In, Ga)Se2 solar cells grown on flexible polymer films. Nat. Mater. 10 (11), 857–861. Aliyu, M.M., Islam, M.A., Hamzah, N.R., Karim, M.R., Matin, M.A., Sopian, K., Amin, N., 2012. Recent Developments of flexible CdTe solar cells on metallic substrates: issues and prospects. Int. J. Photoenergy. Ando, S., Matsuura, T., Sasaki, S., 1997. Coloration of aromatic polyimides and electronic

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