3.1 From cellulose-based tissue paper to plastic-paper
Tissue paper is composed of cellulose microfibers with small pores, resulting in a high surface roughness and a strong light scattering surface in the air, which also results in the high optical haze and low transparency. The PVA/glycerol polymer or film has strong hydrogen bonding, excellent mechanics, easy to degrade and solvent resistance. The uniform structure of the film also makes it very tough and transparent, thus has a low optical haze while high transmittance. Herein, through a simple template penetration method, we combine the three materials perfectly to achieve a novel degradable plastic-paper with a "sandwich" and sidedness structure, which provides excellent mechanical properties, high optical transmittance (~ 88%) and high optical haze (> 90%) (Fig. 1).
We use commercially available single-layer tissue paper as the paper substrate. The cellulose micro-pore structure inside the single layer of tissue paper allows the polymer to permeate rapidly to form the plastic-paper hybrid substrate. Here, a mixture of polyvinyl alcohol and glycerin was used as the infiltration polymer because its refractive index of 1.55 is very close to that of cellulose (1.54), thus ensuring minimal light scattering at the plastic-paper interface. Moreover, the PVA is modified by glycerol as plasticizer and the polymer has excellent transparency, biocompatibility and gas barrier property. Through the interaction with the hydroxyl groups on the PVA molecular chain, the hydrogen bonds between the hydroxyl groups of PVA is weakened, which can effectively improve the mechanical properties of PVA(Jianget alLuoet alHouZhao 2016). The polymer solution prepared by the mixture of PVA and glycerol has superior fluidity and low viscosity, which makes it easier to penetrate into the fiber pores of tissue paper. After curing, the polymer has good mechanical properties and solvent stability, which can protect the cellulose paper from being easily decomposed or broken by solvent in the process of material preparation. The glass plate is used as the plate template to ensure super flat surface transferring.
Images of the plastic-paper are shown in Fig. S1. The clear pattern at the bottom of the plastic-paper (plastic-paper attached to the background) indicates the high transparency of the plastic-paper, and the fuzzy pattern at the top (plastic-paper away from the background) indicates the high haze of the plastic-paper (Fig. S1c). In addition to this unique optical property, compared with the tissue paper, plastic-paper hybrid substrate also has excellent mechanical properties and two-sided properties, which is attributed to the hydrogen bonds formed by polymer and single-layer tissue paper. Plastic-paper also has excellent folding resistance and solvent stability, benefits from the "sandwich" structure formed by polymer and cellulose. The polymer in the outer layer provides a good encapsulation for the plastic-paper, and the polymer in the inner layer and cellulose fiber is crosslinked to form an interpenetrating network structure. The preparation of plastic-paper is based on the mature and industrialized technology, which makes this new type of plastic-paper being very promising and attractive for the high-performance, low-cost optoelectronic substrates.
3.2 Surface topography of plastic-paper
Templated penetration is to impregnate a single layer of tissue paper in PVA/glycerol polymer solution and transfer to the surface of smooth glass plate, and then dry it at a certain temperature. Through the template transfer, the lower surface of the plastic-paper contacting with the glass plate becomes flat and smooth, and the upper surface contacting with the air has a certain roughness (Fig. 2). Very smooth surface is observed on the polymer film (Fig. S2a). Contrarily, the micro-sized cellulose fibers of single-layer tissue paper are extruded into a flat and loose shape, and interwoven together to form a certain surface roughness of the paper (Fig. S2b). When the polymer solution is infiltrated, the lower surface of the plastic-paper becomes super flat and smooth due to contact with the glass plate and the porous surface is replaced by the dense and flat polymer (Fig. 2d). The upper surface of the plastic-paper has a certain roughness due to the contact with the air, which leads to the incomplete polymer substitution (Fig. 2a).
The cross section of single-layer tissue paper shows a loose fiber structure with micro-sized fibers and many pores (Fig. S2c). Plastic-paper shows a typical "sandwich" packaging structure (Fig. S2d). Both sides of the plastic-paper are encapsulated by the polymer solution to form a thin surface layer. The internal dense cellulose fiber and polymer are cross-linked to form an interpenetrating network structure. It can be seen that the polymer solution has well penetrated into the single-layer tissue paper, and the "sandwich" structure can greatly improve the mechanical properties of plastic-paper. In order to further characterize the surface roughness of plastic-paper, ultra-depth three-dimensional microscope is used and it can intuitively show that the lower surface of the plastic paper presents uniform blue color, indicating of the smoother Surface topography than the upper surface (Fig. 2b and 2e). AFM results show that lower surface roughness is approximately ~3.5 nm while upper surface roughness is ~45 nm, indicating of the two-sidedness property of the plastic-paper (Fig. 2c and 2f).
3.3 Optical Properties
The optical transmission characteristics of polymer film, single-layer tissue paper and plastic-paper are compared in Fig. 3a and 3b. Significantly different optical transmission characteristics are observed. Polymer film has the highest optical transmittance of ~92%. The smooth side (lower surface) of plastic-paper presents a total transmittance of ~89% from 300 nm to 800nm (broadband). The single-layer tissue paper substrate shows the lowest optical transmittance of less than 75%. The high transmittance of plastic-paper is in consequence of the increased material density and the smooth surface after polymer infiltration inside the porous tissue paper(Limet alRakuTokiwa 2004). The rough side (upper surface) of plastic-paper is equal to overall transmittance of ~85% from 300 nm to 800 nm. The optical duality is caused by the different roughness of two sides of plastic-paper. The higher surface roughness will increase the reflection area of light, which will affect the transmittance(Dai et al. 2014).
The polymer film shows the extremely optical haze below 10% (Fig. 3c). The tissue paper and plastic-paper show a high optical haze of ~90% from 300 nm to 800 nm (broadband), indicating that a great amount of lights are reflected and scattered as it passed through a single layer of tissue paper or plastic-paper. The light scattering effect or optical haze characteristic is display intuitively in the illustration (Fig. 3d). When the green laser passes through the plastic-paper, it shows a high intensity and high scattering light pattern on the white wall, indicating the excellent optical transmittance and high optical haze. The high haze of plastic-paper is due to the rough surface, loose porous fiber structure and polymer fiber interface with single layer tissue paper. The polymer coated plastic-paper can effectively reduce the backscattering of lights. Meanwhile, cellulose has mesoporous structure and polymer-fiber interface can cause forward scattering of transmitted light so that the plastic-paper has a very high optical haze (Dai et al. 2014), which is also the reason why the optical transmission haze characteristics of the two sides of the plastic-paper are different slightly. Due to the different roughness, the haze of the rough side (upper surface) of plastic-paper is larger than that of smooth side (lower surface) as shown in Fig. 3c and 3d.
The new plastic-paper developed in this study shows the advantages of both plastic and single-layer tissue paper. It shows high optical transmittance and high optical haze in the visible light range, which has the potential for improving the photoelectric conversion efficiency of solar cells and OLED. Usually, it is difficult for traditional substrate materials such as glass, silicon and plastic to have such excellent optical properties. A traditional light management method is to add a coating layer on the original transparent and smooth substrate to improve the optical haze (D Chenet alLiangPei 2016). However, this traditional method requires additional manufacturing steps and has limited enhancement capabilities. The developed plastic-paper can make up for the incompatibility of traditional optical materials. Even increasing the cellulose contents in plastic-paper from 30wt% to 50wt%, the plastic-paper still presents high optical transmittance and optical haze (Fig. S3).
3.4 Mechanical Properties.
In addition to its excellent optical properties, plastic-paper also possesses good mechanical properties and outstanding solvent stability, which is the key to direct device manufacturing. The stress-strain curves of single-layer tissue paper, polymer film and plastic-paper are given in Fig. 4a. The tensile strength (30 Mpa) and elongation at break (0.4 Mpa) of plastic-paper are the highest compared with that of tissue paper and polymer film. Plastic-paper can readily lift 1kg of reactor for more than 24h (Fig. S4b). Folding endurance is one of the basic mechanical properties of paper, which is used to express the ability of paper to resist reciprocating folding. Fig. 4b shows the excellent folding durability of plastic-paper. The results show that the longitudinal folding resistance of plastic-paper (3528 times) is far larger than that of tissue paper (15 times). Similarly, the transverse folding resistance (258 times) is also much higher than that of tissue paper (7 times). It is closely related to the "sandwich" structure of plastic-paper. Specifically, the polymer encapsulates the tissue paper on the outside. Polymer connects the cellulose in a single layer of tissue paper and fills the intervals between the cellulose fibers. The inner part of the plastic-paper is cross-linked with the cellulose fiber, and they interweave with each other to form an interpenetrating network structure. The fiber structure also enhances the composite effect of polymer matrix. It can significantly enhance the mechanical properties of plastic-paper, especially the tear resistance and folding resistance.
Compared to the original single-layer tissue paper, the plastic-paper significantly enhances the stability in solvents and water. As shown in Fig. 4c, after a three weeks water stability test, ordinary tissue paper is decomposed into cellulose fiber, while the plastic-paper keeps nice shape stability without any obvious changes. It is indicated that the polymer itself has good solvent resistance with relatively large contact angle (Fig. S4a) and binds the cellulose fibers together to prevent disintegration. Ordinary paper soaked in solvent and taken out to dry will expand and produce many wrinkles, which is a serious problem in the manufacture of microelectronic devices. We demonstrate the stability of plastic-paper in solvents required for the fabrication of optoelectronic devices, such as photoresist 1813 and acetone for 1 day (Fig. 4d). Plastic-paper has great shape retention and outstanding stability in these chemical reagents and organic solvent and therefore can be directly used for the purpose of flexible substrates requiring different solvents. The water and vapor permeation rate (WVPR) is critical for devices containing sensitive components such as conductive and semi-conductive polymers. Due to its good oil resistance and excellent gas barrier performance, PVA has unique advantages in food and drug packaging. By templated penetration, the plastic-paper substrate shows excellent WVPR of 0.5 g m−2 day−1.
3.5 Application in electronic devices
In order to prove the process compatibility and enhance the optical coupling of plastic-paper in optoelectronic devices, Electroluminescent (EL) devices are fabricated directly on plastic-paper. Ag NWs dispersion evenly coat in plastic rough surface (upper surface) to form a conductive network. The conductive film after coating is applied as the positive and negative pole of EL devices. EL light-emitting layer and insulating layer by means of silkscreen print on the conductive film. When the power supply is connected, the film shows blue fluorescence (Fig. 5a and 5b). Using the rough surface of plastic-paper as the substrate material of EL devices can make Ag NWs form a conductive network with strong adhesive force on the plastic-paper through a simple coating process. This application demonstrates the potential value of thin plastic-paper in the field of electronic devices.
Plastic-paper also has great application value in the field of solar cells due to its unique optical properties. The prepared plastic-paper is used as the anti-reflection layer on the solar cell, which contacts the smooth surface (lower surface) with the solar cell. The lower surface is flat and smooth, which can increase the effective contact area with the solar cell. The upper surface has a certain roughness, which can be used as the light trap of the solar cell, so that the light with a single small incidence angle can be scattered in all directions, thus increasing the light path in the solar cell and increasing the light absorption. This dual surface roughness can ultimately improve the photoelectric conversion efficiency of solar cells. The simple solar cells device is shown in Fig. 5c. The volt ampere characteristic curves of solar cells with or without plastic-paper are shown in Fig. 5d. After covering the solar cell with plastic-paper, the open circuit voltage increases from 7.01 V to 7.05 V, and the short circuit current increases from 130 mA to 134 mA. The filling factor increases from 64.23–64.66% with plastic-paper. It is indicated that plastic-paper can improve the light absorption efficiency of solar cells, not only improve the quantum efficiency, but also increase the short-circuit current and open circuit voltage of solar cells, leading to further improving the photoelectric conversion efficiency of solar cells.