R2R Process Research Articles

Web tension AI modeling and reconstruction for digital twin of roll-to-roll system

AbstractDigital twins (DT) are gaining attention as an emerging technology in Smart manufacturing systems. These DTs comprise various units that enable simulation, monitoring, and prediction of the manufacturing process. This study introduces a predictive model for web tension and a tension reconstruction algorithm for the DT of the roll-to-roll (R2R) system. The observed web tension signals from tension sensors decomposed into a mean component, a sinusoidal wave, and a random noise. Utilizing deep neural networks, the predictive model integrated various sub-models to forecast statistical (mean, standard deviation) and frequency domain (main frequency, signal-to-noise ratio) features of the web tension signal. Through fivefold cross-validation, 23 model architectures were optimized, with selected architectures ranging from 16-32-32-1 to 16-32-64-32-1 nodes per layer. Overall, R2 scores on the test set ranged from approximately 52 to 100%. The proposed reconstruction algorithm generated tension signals from the model’s predictions that closely resemble the original tension signals, indicating credible reconstructions. The proposed predictive model and reconstruction algorithm were integrated into the DT of the R2R system, offering a valuable tool for monitoring and optimizing the R2R process.

Recent Advancements on Slot-Die Coating of Perovskite Solar Cells: The Lab-to-Fab Optimisation Process

Perovskite solar cells (PSCs) are the most rapidly advancing photovoltaic technology in terms of power conversion efficiency. An efficiency of 26.1% was achieved in a decade, which is on par with the efficiency of very mature silicon panels. However, PSC commercialisation is partly hindered by the difficulty of scaling these devices without efficiency loss, mostly due to the increasing sheet resistance of the transparent conductive layer substrates and the nonuniformity of the layers when deposited across large areas. Therefore, it is crucial for the commercialisation of PSCs to implement easily scalable deposition processes with low material waste and compatibility with roll-to-roll (R2R) processes to reduce manufacturing costs. Slot-die coating can meet all these requirements, allowing for great uniformity over large areas. The most recent developments in PSC upscaling using slot-die coating as the main deposition process, along with its extension to the R2R process, are reviewed, including a thorough discussion of the slot-die coating process and the theory behind its operating limits. In fact, R2R coating is a very promising strategy for PSC industrialisation, since all processing steps use low-cost materials and scalable processes at temperatures lower than 120 °C, allowing the cost-effective and high-throughput production of PSC devices.

Control of Meniscus Formation Using an Electrohydrodynamics Module in Roll-to-Roll Systems for the Stable Coating of Functional Layers.

In fabricating functional layers, including thin-film transistors and conductive electrodes, using roll-to-roll (R2R) processing on polymer-based PET film, the instability of the slot-die coating meniscus under a high-speed web impedes functional layer formation with the desired thickness and width. The thickness profiles of the functional layers significantly impact the performance of the final products. In this study, we introduce an electrohydrodynamic (EHD)-based voltage application module to a slot-die coater to ensure the uniformity of the cross-machine direction (CMD) thickness profile within the functional layer and enable a stable, high-speed R2R process. The module can effectively control the spreadability of the meniscus by utilizing variations in the surface tension of the ink. The effectiveness of the EHD module was experimentally verified by applying a high voltage to a slot-die coater while keeping other process variables constant. As the applied voltage increases, the CMD thickness deviation reduces by 64.5%, and the production rate significantly increases (up to 300%), owing to the formation of a stable coated layer. The introduction of the EHD-based application module to the slot-die coater effectively controlled the spreadability of the meniscus, producing large-area functional layers.

Printable Flexible Organic Photovoltaic by R2R Process

Printable Flexible Organic Photovoltaic by R2R Process

A Real-Time Supervisory Control Strategy for Roll-to-Roll Dry Transfer of 2D Materials and Printed Electronics

With the advancement of manufacturing technology in transfer printing and two-dimensional materials fabrication, roll-to-roll (R2R) dry transfer by mechanical peeling is becoming a promising process for efficient transfer of thin film materials from a donor substrate to a target substrate for device fabrication. As a key process variable, the peeling angle determines the loading condition of the R2R dry transfer process and thus the quality of the transferred material. Controlling the peeling angle is challenging in a R2R process because of the adhesion energy and peeling front velocity in addition to the flexible substrates involved in the peeling process. In this article, a real-time supervisory control strategy is developed for the R2R mechanical peeling process. A strain energy-based peeling process model is employed to estimate the peeling front velocity and the adhesion energy. The real-time estimation is then used to generate reference signals for web tension controllers to track the desired peeling angles. The proposed control strategy is demonstrated with both simulation and experimental results.

Perovskite solar cell fabrication via scalable thermal‐assisted and meniscus‐guided bar‐coating method in open air

AbstractBackgroundPerovskite‐based solar cell is getting great attention as a promising candidate for future solar cell technology, since high power conversion efficiency above 25.5% have been demonstrated for small scale PSCs from films prepared mainly by spin‐coating method, which is not feasible for large area film preparation. Scalable and cost‐effective film fabrication techniques are urgently needed for the commercialization of perovskite solar cells.ObjectiveTo develop a process from which large‐area, high quality and unifoem perovskite films, as well as the hole‐transporting layer films, can be prepared in a simple and cheap way for the practical application and mass production of perovskite solar cells.MethodsLarge‐area electron‐transporting layer layers of mp‐TiO2 and c‐TiO2 were prepared on the FTO substrate by the solution‐shearing method. Then the 10 × 10 cm2 perovskite film was prepared by using a bar‐coater from perovskite precursor solution in ACN/MA solvent system under optimized conditions. The spiro‐OMeTAD film was also prepared by bar‐coating process. Standard‐sized pieces (1.5 × 1.8 cm2) were cut and the Ag metal electrode (80 nm) was deposited through a shadow mask in a thermal evaporation system for device evaluation.ResultsThe large crystalline grains and pinhole‐free perovskite film obtained were fully characterized to show better film properties compared to that from the spin‐coated film, and appeared less sensitive to moisture. A higher PCE of 19.72% was achieved for standard‐sized devices fabricated from the bar‐coated perovskite film with spiro‐OMeRAD film spin‐coated on top, or an excellent PCE of 18.10% was obtained with bar‐coated spiro‐OMeTAD layer on top.ConclusionsThis work demonstrates that the simple and scalable process of bar‐coating can offer high quality and homogeneous films of perovskite and spiro‐OMeTAD respectively. It is compatible with the R2R process and useful in realizing the commercialization of PSC technology.

Application of the Self-Made Flexible Three-in-One Microsen-Sor to the Laboratory Oven for Immediate Micro-Monitoring of the Roll-to-Roll Process of Polarizing Films

The main purpose of this study is to carry out the immediate micro-monitoring of the roll-to-roll (R2R) process of polarizing films. Therefore, a self-made flexible three-in-one (temperature, humidity, and flow) microsensor is developed. The temperature and flow sensing area are 585 μm × 450 μm, the humidity sensing area is 1065 μm × 1035 μm, and the minimum line width is 15 μm. The micro-electro-mechanical systems (MEMS) technology was applied to integrate temperature, humidity, and flow sensors on a 50 µm thick polyimide substrate. A 100 Å thick chromium (Chrome, Cr) section form the adhesion layer. A 1000 Å thick gold section forms the sensing layer. A self-made flexible three-in-one microsensor set up in a laboratory oven for immediate micro-monitoring of the R2R process of the polarizing film. Since it is not advisable to set up signal lines in a clean room, the analog signals of the sensor should be transmitted via wireless means. Thus, a monitoring module should be connected to the back end of the self-made flexible three-in-one microsensor to receive the analog signals of the sensor, convert them into digital signals, send them out in the form of wireless signals, and store the data on the server-side. Through these measures, both the R2R process and yield can be improved. Therefore, the focus of this study is the environmental monitoring of drying process ovens. However, commercially available all-in-one sensors cannot handle the temperature of high-temperature factory ovens, and commercial flow sensors are rarely used in high-temperature applications. Some are also expensive and cannot be widely distributed, so this study intends to develop an integrated sensor to measure the internal environment of the drying oven.

A method to estimate adhesion energy of as-grown graphene in a roll-to-roll dry transfer process

The interfacial adhesion energy of as-grown graphene on its metal growth substrate is an important variable in designing, monitoring, and controlling a roll-to-roll (R2R) graphene transfer process using mechanical peeling. In this study, we develop a novel method to estimate the adhesion energy of as-grown graphene during the R2R dry transfer process. An energy balance model is established to derive the adhesion energy based on web tension and bending curvature measurements. Experiments were conducted under various mechanical peeling conditions. The adhesion energy of as-grown graphene on copper foil was determined to be from 1.22 J/m2 to 2.58 J/m2 depending on the peeling front geometry. The developed adhesion energy estimation method is compatible with the R2R process and can be used to monitor and control the large-scale graphene transfer process with in-process measurements.

그라비어 인쇄기술을 이용한 페로브스카이트 태양전지 Roll-to-Roll 제작 공정

Perovskite solar cells (PSCs) are attracting attention as a next-generation thin-film solar cell based on their high efficiency. Technologies to commercialize PSCs for power generation and special purposes have been being developed. Since the PSCs are manufactured using a solution process, the development of a roll-to-roll (R2R) process to mass-produce flexible solar cells at low cost can be a stepping stone for commercialization. In order to manufacture high-quality perovskite solar cells through the R2R process, the perovskite material and charge transport materials must be formed in a roll-based continuous and scalable method. Gravure printing has the advantage of being able to directly form patterns on a substrate. The R2R process has limitations that were not considered in the existing sheet-to-sheet process to fabricate unit cells in terms of process temperature, process time, and environmental management. In this paper, we share the current status of R2R gravure printing process development for perovskite solar cells and discuss issues to be considered and problems to be solved during R2R process design.

Application of Monitoring Module Three-in-One Microsensor to Real-Time Microscopic Monitoring of Polarizer Sheet in Roll-to-Roll Process

The Roll-to-Roll (R2R) process refers to a high-efficiency, low-cost, continuous production method. The roll material used for processing is a flexible plastic or metal film. In many R2R processes, polarizing films are high-precision products with a high output value. In the production of conventional polarizers, product inspection will only be carried out after the production of the polarizing film is completed. The principal raw material of a polarizer sheet is a hydrophilic polymer, the properties of which may be influenced by water vapor, which degrades its quality. Whether or not the product is impacted can be ascertained by means of a quality inspection, but it must be performed after the process is finished. However, it is already too late when a defective product is detected: the production cost is increased, the schedule is influenced and the delivery date is delayed. The focus of this research was on environmental monitoring of the production drying process oven, but the commercial all-in-one sensors currently on the market cannot tolerate the temperature of the factory’s high-temperature oven. In particular, a commercial flow sensor is rarely suited to high-temperature applications. Some are expensive and cannot be widely distributed. Therefore, this study aimed to develop an integrated sensor to measure the internal environment of the drying process oven for the real-time monitoring of a polarizer sheet in the Roll-to-Roll (R2R) process. This study used micro-electro-mechanical systems (MEMS) technology to make a flexible three-in-one (temperature, humidity and flow) micro-sensor. We monitored the temperature, humidity and flow uniformity in a laboratory oven to simulate the environment of the actual factory oven, with the aim to provide data to confirm whether or not a polarizer sheet has dried. Our system can be monitored instantly by Arduino, or even Raspberry Pi 3, to realize the flexible micro-sensor layout and field verification, in order to optimize the R2R process and to enhance the yield and performance of the polarizer sheet process.

Characterization of silver nanoparticle inks toward stable roll-to-roll gravure printing

The rheological properties of silver inks are analyzed, and the printing results are presented based on the inks and roll-to-roll (R2R) printing speed. The shear viscosity, shear modulus, and extensional viscosity of the inks are measured using rotational and extensional rheometers. The inks exhibit the shear thinning power law fluids because the concentration of dispersed nanoparticles in the solvent is sufficiently low, which minimizes elasticity. After the inks are printed on a flexible substrate through gravure printing, the optical images, surface profiles, and electric resistances of the printed pattern are obtained. The width and height of the printed pattern change depending on the ink viscosity, whereas the printing speed does not significantly affect the widening. The drag-out tail is reduced at high ink viscosities and fast printing speeds, thereby improving the printed pattern quality in the R2R process. Based on the results obtained, we suggest ink and printing conditions that result in high printing quality for complicated printings, such as overlay printing registration accuracy, which imposes pattern widening and drag-out tails in printed patterns.

H∞ Optimal Control for Maintaining the R2R Peeling Front

Dry transfer using Roll-to-Roll (R2R) mechanical peeling could significantly increase the throughput and efficiency of the production of 2D materials such as graphene and flexible electronics. Currently, such a R2R process does not exist in industry. For this dry transfer R2R process to be practical for industrial applications, the peeling angle between the growth substrate and the functional material needs to be precisely controlled. In this paper, a nonlinear state space representation of the R2R dry peeling process is formulated with the peeling front velocity variation as a disturbance input. This state space model is used to construct a linear parameter varying (LPV) representation of the system, and a methodology on how to bound the LPV representation within a convex polytopic linear differential inclusion (PLDI) set is presented. This PLDI representation is then used in a linear matrix inequality (LMI) optimization framework to design a full state feedback controller that minimizes the H∞ gain of the connection between the adhesion energy variation and the peeling front geometry. Simulation results demonstrate that this controller improves the precision of the R2R peeling angle, and this increase in precision enables higher web speed. Thus, this technique can be an enabling tool for making R2R mechanical peeling dry transfer of 2D materials a reality in industrial settings.

Development of Roll-to-Roll Manufacture Process for Stretchable Device Substrate

As the next generation of smart devices, many stretchable devices with have been developed. In this process, it is necessary to fabricate stretchable substrates and electrodes. However, in many studies, stretchable devices are currently manufactured by hand. Considering future social implementation, it is important to realize a mass production process for substrates and wiring. In this study, we conducted a PoC of a mass production process for stretchable devices by combining a substrate fabrication process by Roll-to-Roll (R2R) and a liquid metal wiring fabrication process by dispensing. To realize this process, we assembled a new R2R system by incorporating a slot die coater and applicator, which is the R2R coating process, and a corona treatment system to improve the wettability of polymeric materials into the R2R process equipment. Using this equipment, we evaluated the device at 50% elongation and fabricated a large scale stretch device.

Modelling, Simulation and Control of Roll-to-Roll Physical Vapor Deposition Processes

Roll-to-roll (R2R) nano-manufacturing processes are critical in the manufacturing of electronic components for consumer products such as displays and flexible electronics. However, there are significant quality and process control challenges inherent to the control of R2R processes at nanometer scales. These challenges arise from the strong nonlinearities in the underlying phenomena, high levels of noise, and large measurement delays. This paper analyzes and evaluates the effectiveness of several control strategies in mitigating the challenges with control of R2R process steps. The proposed control strategies were tested on an experimentally derived model of an R2R physical vapor deposition (PVD) process, which simulates forward and double-pass R2R sputtering deposition procedures.

Investigation on graphene growth by roll-to-roll chemical vapor deposition

Both batch-to-batch (B2B) and roll-to-roll (R2R) processes can be used for mass production of graphene films. Because of the simplicity of equipment, the B2B process is more commonly used in laboratory research. In contrast, the R2R process is more industrially preferred because it is easier to automate and more compatible with the subsequent transfer process. Up to now, the research on R2R process has mainly focused on the design of the reactor or the loading configuration, but the in-depth understanding on the growth kinetics is lack. In this paper, we investigated the evolution of graphene growth and revealed that the graphene domain density grown by the R2R process was larger than that by the B2B process with the same reaction parameters, which was attributed to different reaction profiles between the two processes. Furthermore, the defective graphene could be healed with elongated reaction time. With the optimized R2R process, graphene films with quality comparable to those grown by conventional B2B process could be achieved. Our research will promote the development of the mass-production technique for graphene films.

Roll-to-roll manufacturing toward lab-to-fab-translation of perovskite solar cells

Perovskite solar cells (PSCs) are emerging next-generation photovoltaics, and some breakthroughs for the commercialization have been rapidly made. To develop the technologies for large-area modules, economically feasible fabrication using a roll-to-roll (R2R) solution process may be the ultimate target for the fabrication of PSCs. In order to achieve successful R2R production of PSCs, however, several issues still need to be addressed. Roll-based continuous and scalable deposition of perovskite and charge transporting layers on a flexible substrate needs to be developed to obtain high-quality R2R-processed PSCs. There are also critical factors involved in accomplishing R2R fabrication: heat treatment at low temperature and a short processing time over the whole process with industrial-compatible methods. We briefly discuss this perspective: scalable deposition of layers, considerations for the R2R process, and progress and challenges in the R2R fabrication of the PSCs.

Corona-Enabled Electrostatic Printing for Ultra-fast Manufacturing of Binder-Free Multifunctional E-Skins.

As essential components in intelligent systems, printed soft electronics (PSEs) are playing crucial roles in public health, national security, and economics. Innovations in printing technologies are required to promote the broad application of high-performance PSEs at a low cost. However, current printing techniques are still facing long-lasting challenges in addressing the conflict between printing speed and performance. To overcome this challenge, we developed a new corona-enabled electrostatic printing (CEP) technique for ultra-fast (milliseconds) roll-to-roll (R2R) manufacturing of binder-free multifunctional e-skins. The printing capability and controllability of CEP were investigated through parametric studies and microstructure observation. The electric field generation, material transfer, and particle amount and size selecting mechanisms were numerically and experimentally studied. CEP-printed graphene e-skins were demonstrated to possess an outstanding strain sensing performance. The binder-free feature of the CEP-assembled networks enables them to provide pressure sensitivity as low as 2.5 Pa and capability to detect acoustic signals of hundreds of hertz in frequency. Furthermore, the CEP technique was utilized to pattern different types of functional materials (e.g., graphene and thermochromic polymers) onto different substrates (e.g., tape and textile). Overall, this study demonstrated that CEP can be a novel contactless and ultra-fast manufacturing platform compatible with the R2R process for fabricating high-performance, scalable, and low-cost soft electronics.

Roll‐to‐Roll Processes for the Fabrication of Perovskite Solar Cells under Ambient Conditions

Recent progress in the engineering and fabrication optimization of hybrid perovskite solar cells (PSCs) has led to power conversion efficiencies (PCEs) of up to 25.2%. The properties of perovskite solutions enable low‐temperature, scalable coating methods that are compatible with the roll‐to‐roll (R2R) fabrication process. Despite this, there is still a significant lag in efficiency in large‐area devices fabricated using the R2R process, which achieves PCEs of 50% of the record value. Direct translation of research results to large‐scale commercially acceptable levels is crucial for commercialization, along with long‐term durability and processing reproducibility of the different device layers. Herein, the recent progress of several coating and R2R processes for the formation of PSC layers, detailing the key variables involved in the process, with an emphasis on the nucleation and crystallization of the perovskite film, is considered. The barriers and challenges in producing high‐quality perovskite films and improving the stability of devices are discussed. In addition, stability protocols for PSCs are discussed as they are key to unifying PSC stability and performance assessment, which is required to gain a better understanding of the different compounds and layers used in cell fabrication and the approaches necessary for their optimization.

Realizing Double Graded CIGSe Absorbers With the R2R Hybrid-CIGSe-Process

This article presents an approach for industrial roll-to-roll (R2R) deposition of Cu(In,Ga)Se2 (CIGSe) thin films on stainless steel foil for use as absorber layers in flexible thin-film solar cells. The hybrid-CIGSe-deposition-process combines magnetron sputtering with evaporation techniques and allows the realization of a double graded CIGSe layer, reaching a conversion efficiency of currently up to 14.0% (w/o antireflective coating). The influence of different power settings of the individual magnetrons on the gallium profile is described. For a deeper understanding of differences between a static and an R2R process, a calculated flux diagram is presented. In the second part of this contribution, the homogeneity of the industrial scale samples is discussed by comparing the change in CGI and GGI over the foil width of 30 cm. The hybrid-CIGSe-deposition-process excels in a straightforward scalability for industrial usage due to the application of planar magnetron sputtering sources as metal vapor supply.

Web Unevenness Due to Thermal Deformation in the Roll-to-Roll Manufacturing Process

In roll-to-roll (R2R) processing, web uniformity is a crucial factor that can guarantee high coating quality. To understand web defects due to thermal deformation, we analyzed the effect of web unevenness on the coating quality of an yttria-stabilized zirconia (YSZ) layer, a brittle electrolyte of solid oxide fuel cells (SOFCs). We used finite-element analysis to study thermal and mechanical deformations at different drying temperature levels. A YSZ layer was also coated using R2R slot-die coating to observe the effect of web unevenness on coating quality. Web unevenness was generated by thermal deformation due to conduction and convection heat from the dryer. Because of varying web unevenness with time, the YSZ layer developed cracks. At higher drying temperatures, more coating defects with larger widths were generated. Results indicated that web unevenness at the coating section led to coating defects that could damage the SOFCs and decrease yield in the R2R process. Coating defects generated by web unevenness caused by convection and conduction heat should be considered in the high-volume production of brittle electrolytes using the R2R process.