Zhiqiang Fang

Transparent paper: fabrications, properties, and device applications

Although paper electronics is a compelling concept, the large surface roughness and opaqueness of most paper substrates has hindered its development from a dormant idea to a thriving technology. A recent demonstration of transparent paper with nanoscale surface roughness has revived an interest in using renewable cellulose substrates for electronics and optoelectronics. In this short review, we will first summarize the recent progress of transparent paper electronics through structure engineering. We will also discuss the properties and functionalization of transparent paper, such as surface roughness, printability, thermal stability, etc. Finally, we will summarize the recent achievements on proof-of-concepts of transparent paper, which pave the way for next-generation green electronics fabricated with roll-to-roll printing methods. Advantages of transparent paper over traditional flexible plastic substrates and its challenges will also be discussed.

Novel Nanostructured Paper with Ultrahigh Transparency and Ultrahigh Haze for Solar Cells

Solar cell substrates require high optical transparency but also prefer high optical haze to increase the light scattering and consequently the absorption in the active materials. Unfortunately, there is a trade-off between these optical properties, which is exemplified by common transparent paper substrates exhibiting a transparency of about 90% yet a low optical haze (<20%). In this work, we introduce a novel transparent paper made of wood fibers that displays both ultrahigh optical transparency (∼ 96%) and ultrahigh haze (∼ 60%), thus delivering an optimal substrate design for solar cell devices. Compared to previously demonstrated nanopaper composed of wood-based cellulose nanofibers, our novel transparent paper has better dual performance in transmittance and haze but also is fabricated at a much lower cost. This high-performance, low-cost transparent paper is a potentially revolutionary material that may influence a new generation of environmentally friendly printed electronics.

Wood-Derived Materials for Green Electronics, Biological Devices, and Energy Applications

With the arising of global climate change and resource shortage, in recent years, increased attention has been paid to environmentally friendly materials. Trees are sustainable and renewable materials, which give us shelter and oxygen and remove carbon dioxide from the atmosphere. Trees are a primary resource that human society depends upon every day, for example, homes, heating, furniture, and aircraft. Wood from trees gives us paper, cardboard, and medical supplies, thus impacting our homes, school, work, and play. All of the above-mentioned applications have been well developed over the past thousands of years. However, trees and wood have much more to offer us as advanced materials, impacting emerging high-tech fields, such as bioengineering, flexible electronics, and clean energy. Wood naturally has a hierarchical structure, composed of well-oriented microfibers and tracheids for water, ion, and oxygen transportation during metabolism. At higher magnification, the walls of fiber cells have an interesting morphology-a distinctly mesoporous structure. Moreover, the walls of fiber cells are composed of thousands of fibers (or macrofibrils) oriented in a similar angle. Nanofibrils and nanocrystals can be further liberated from macrofibrils by mechanical, chemical, and enzymatic methods. The obtained nanocellulose has unique optical, mechanical, and barrier properties and is an excellent candidate for chemical modification and reconfiguration. Wood is naturally a composite material, comprised of cellulose, hemicellulose, and lignin. Wood is sustainable, earth abundant, strong, biodegradable, biocompatible, and chemically accessible for modification; more importantly, multiscale natural fibers from wood have unique optical properties applicable to different kinds of optoelectronics and photonic devices. Today, the materials derived from wood are ready to be explored for applications in new technology areas, such as electronics, biomedical devices, and energy. The goal of this study is to review the fundamental structures and chemistries of wood and wood-derived materials, which are essential for a wide range of existing and new enabling technologies. The scope of the review covers multiscale materials and assemblies of cellulose, hemicellulose, and lignin as well as other biomaterials derived from wood, in regard to their major emerging applications. Structure-properties-application relationships will be investigated in detail. Understanding the fundamental properties of these structures is crucial for designing and manufacturing products for emerging applications. Today, a more holistic understanding of the interplay between the structure, chemistry, and performance of wood and wood-derived materials is advancing historical applications of these materials. This new level of understanding also enables a myriad of new and exciting applications, which motivate this review. There are excellent reviews already on the classical topic of woody materials, and some recent reviews also cover new understanding of these materials as well as potential applications. This review will focus on the uniqueness of woody materials for three critical applications: green electronics, biological devices, and energy storage and bioenergy.

Highly Thermally Conductive Papers with Percolative Layered Boron Nitride Nanosheets

In this work, we report a dielectric nanocomposite paper with layered boron nitride (BN) nanosheets wired by one-dimensional (1D) nanofibrillated cellulose (NFC) that has superior thermal and mechanical properties. These nanocomposite papers are fabricated from a filtration of BN and NFC suspensions, in which NFC is used as a stabilizer to stabilize BN nanosheets. In these nanocomposite papers, two-dimensional (2D) nanosheets form a thermally conductive network, while 1D NFC provides mechanical strength. A high thermal conductivity has been achieved along the BN paper surface (up to 145.7 W/m K for 50 wt % of BN), which is an order of magnitude higher than that in randomly distributed BN nanosheet composites and is even comparable to the thermal conductivity of aluminum alloys. Such a high thermal conductivity is mainly attributed to the structural alignment within the BN nanosheet papers; the effects of the interfacial thermal contact resistance are minimized by the fact that the heat transfer is in the direction parallel to the interface between BN nanosheets and that a large contact area occurs between BN nanosheets.

Biodegradable transparent substrates for flexible organic-light-emitting diodes

Electronics on flexible and transparent substrates have received muchinterest duetotheirnew functionalitiesand high-speedroll-toroll manufacturing processes. The properties of substrates are crucial, including flexibility, surface roughness, optical transmittance, mechanical strength, maximum processing temperature, etc. Although plastic substrates have been used widely in flexible macroelectronics, there is still a need for next-generation sustainable, high-performance substrates which are thermally stable with tunable optical properties and a higher handling temperature. In this communication, we focus on cellulose-based transparent, biodegradablesubstrates incorporatingeither nanopaperora regenerated cellulose film (RCF). We found that both their optical and mechanical properties are dramatically different due to the difference of their buildingblocks. Highly flexibleorganic-light-emitting diodes (OLEDs) are also demonstrated on the biodegradable substrates, paving the way for next-generation green and flexible electronics.

Silver nanowire transparent conducting paper-based electrode with high optical haze†

In this study we report a novel, rationally designed, solution based silver nanowire (Ag NW) paper hybrid that demonstrates a flexible, low cost, and scalable device ready transparent conducting electrode (TCE) with exceptional and stable optoelectronic properties. Its high transmittance (91%) and low sheet resistance (13 Ω sq−1) represent the highest reported figure of merit value for solution based TCEs according to conventional models. We also thoroughly investigate the diffuse light scattering properties of our Ag NW paper with various techniques that elucidate the total optical haze as well as the diffuse scattering angle distribution for this TCE. Through a simulation of the impact the optical properties of TCEs have on the light absorption in the conversion layers for various thin film solar cells, we demonstrate that our Ag NW paper induces greater light absorption than ITO for each simulated thin film solar cell.

Strong transparent magnetic nanopaper prepared by immobilization of Fe3O4 nanoparticles in a nanofibrillated cellulose network

Nanofibrillated cellulose (NFC) is highly regarded as a popular new material due to its impressive mechanical properties, great potential for functionalization, easy accessibility, and environmental sustainability as precursors. The immobilization of nanoparticles in an NFC network is an effective way to fabricate transparent functionalized nanopaper. In this work, a uniform, flexible, magnetic nanopaper is prepared by the immobilization of Fe3O4 nanoparticles in an NFC network in an aqueous medium. The resulting transparent magnetic nanopaper (TMNP) possesses excellent transparency and magnetic properties combined with outstanding mechanical performance and flexibility. The combination of these characteristics makes TMNP an excellent candidate for magneto-optical applications.

Highly transparent and writable wood all-cellulose hybrid nanostructured paper

Paper, as an inexpensive substrate for flexible electronics and energy devices, has garnered great attention because of its abundance, biodegradability, renewability and sustainability. However, the intrinsic opacity and higher roughness of regular paper greatly restricts further applications. One promising method is to use cellulose nanofibers (CNs) to fabricate nanopaper with a high optical transmittance and excellent smoothness, but there are still some challenges facing nanopaper substrates, such as high-energy consumption to extract nanofibers and the time-consuming process to prepare nanopaper. We design a bilayer hybrid paper using unbeaten wood fibers and CNs with a papermaking technique, which achieves a high optical transmittance and superior smoothness while remaining less expensive than nanopaper and useful as a writable surface. The first transparent paper touchscreen with an excellent anti-glare effect in bright environments is demonstrated using our novel transparent and conductive hybrid paper as the flexible electrode.

Extreme Light Management in Mesoporous Wood Cellulose Paper for Optoelectronics

Wood fibers possess natural unique hierarchical and mesoporous structures that enable a variety of new applications beyond their traditional use. We dramatically modulate the propagation of light through random network of wood fibers. A highly transparent and clear paper with transmittance >90% and haze <1.0% applicable for high-definition displays is achieved. By altering the morphology of the same wood fibers that form the paper, highly transparent and hazy paper targeted for other applications such as solar cell and antiglare coating with transmittance >90% and haze >90% is also achieved. A thorough investigation of the relation between the mesoporous structure and the optical properties in transparent paper was conducted, including full-spectrum optical simulations. We demonstrate commercially competitive multitouch touch screen with clear paper as a replacement for plastic substrates, which shows excellent process compatibility and comparable device performance for commercial applications. Transparent cellulose paper with tunable optical properties is an emerging photonic material that will realize a range of much improved flexible electronics, photonics, and optoelectronics.

Highly transparent paper with tunable haze for green electronics

The ability to manage the light scattering effect of transparent paper without sacrificing its original high transmittance is critical for the application in optoelectronics since different devices have different requirements for the optical properties. In this paper, we study highly transparent paper with a tunable transmission haze by rationally managing the ratio of nanoscale cellulose fibers to macroscopic cellulose fibers. The transparent papers present a largely modulated light scattering behavior while retaining a transparency of over 90%. Various measurements are then used to characterize the optical properties of the different transparent papers in detail. To demonstrate the device applications in green electronics, we fabricated a top gated transistor with MoS2 on the transparent paper containing 100% NFC that leads to an excellent on/off ratio. The highly transparent paper with a controllable light scattering behavior has an unprecedented potential for applications in optoelectronic devices as a substrate or a functional component.