Accuracy control for roll and sheet processed printed electronics on flexible plastic substrates

The aim of the pilot printing experiment was to define how to control the stability of a PET substrate in the R2R process and to improve the dimensional accuracy in multilayer printing. The dimensional accuracy of HS-PET substrates was evaluated by exposing the substrate materials to the heat and tension of a R2R cycle, and measuring the substrate MD dimensions before and after the processing at ambient temperature and zero tension. With directional (MD or TD) accuracy defined as the dimensional offset in the MD or TD direction in relation to the initial dimensions of the substrate (large deviation results in low accuracy). Figure 1 presents the schematic image of the VTT’s pilot printing line (MAXI). In the web process, the substrate roll goes under controlled tension from the unwinder through the web guides and guide rolls, printing and hot air drying units to the rewinder after processing.

During each R2R cycle, DuPont Teijin Films’ 300-mm wide and 125-µm-thick Melinex ST506 (ST506), 100-µm Melinex PCS (PCS), 125-µm Melinex ST504 (ST504) and 125-µm ITO-coated Melinex ST504 substrate (ITO-ST504) were exposed to hot air at 140 °C and an unwinder–rewinder tension of 80 N (267 N/m). The 140 °C temperature was selected as this is the maximum processing temperature of the MAXI pilot machine (hot air ovens), and 80 N is the optimized web tension for the web handling (i.e. not too loose or tight, and avoiding slipping) and processing of 100–125-µm-thick PET substrate (tension being a substrate dependent condition) in register. With commonly used temperatures for annealing printed inks and pastes for electronic components and systems (i.e. flexible hybrid electronics), it ranges from 100–140 °C; choosing the upper limit provides the clearest guidance for manufacturing exploitation [2, 4, 9, 11]. To ensure reproducibility, the experiment was repeated with two PET ST506 and PCS substrate materials (Series II in Fig. 2).

The obtained results show consistently the PET and ITO-PET substrate MD elongation after being thermally treated in the first cycle, which challenges the layer-to-layer alignment. During the second cycle, MD inaccuracy of the substrate was reduced significantly. Furthermore, ST506 and PCS had the lowest dimensional change in MD resulting in the highest MD accuracy, whereas the ITO-PET showed the largest deviation in MD direction. Table 3 presents the MD accuracy of R2R processed PET substrates.

MD elongation was reduced from 0.357% to 0.002% with PET and from 0.206% to 0.042% with ITO-PET after the second R2R cycle. This implies that in a 100-mm length of substrate, the MD elongation of PET is decreased from 357 µm to 2 µm and in the context of a circumference of a printing screen and cylinder (409.570 mm) from 1460 µm to 8 µm, respectively. This value of 8 µm is an acceptable/minor error and easily corrected in automated registration systems. Furthermore, Series II supports high substrate accuracy and processing reproducibility after the second cycle. With the results clearly showing that the preliminary thermal treatment in the R2R cycle prior to R2R printing improves the dimensional accuracy of the PET and ITO-PET substrate.

In laboratory-scale experiments, the MD and transverse direction (TD, substrate width perpendicular to MD) accuracy of ST506 was assessed by thermally treating the substrates in unrestrained conditions and by performing measurements at ambient temperature and zero tension. The sheets of unprocessed and previously R2R processed substrates were exposed to thermal treatment under hot air at 140 °C, 150 °C and 85 °C temperatures in a laboratory-scale oven for 30 min. 140 °C was selected in order to have the same temperature as in the R2R process, 150 °C to have the same conditions as in products’ technical datasheets, and 85 °C as this is the same temperature used in 85/85 reliability test standards for electronics, such as JESD22-A101D, where the accelerated ageing conditions are comprised of a test temperature of 85 °C and relative humidity of 85% [27, 28]. The 150 °C hot air treatment was conducted twice, with the substrate elongation/shrinkage being measured after each annealing to detect if the magnitude changes after the first 150 °C hot air treatment. Table presents the MD and TD accuracy of the untreated PET substrates after S2S processing.

The dimensions of the untreated ST506 substrates changed -0.097% in MD and -0.134% in TD after being S2S thermally treated for 30 min at 150 °C while unrestrained (Table 4). The results corresponded with the product datasheet [25]. Notably, the changes of the second S2S cycle at 150 °C were only 0.024% in MD and -0.013% in TD. In S2S processing, the thermal pre-treatment of the unstrained substrate improves the dimensional accuracy reducing the changes. Furthermore, the processing temperature decreases from 150 to 140 °C reduced the substrate shrinking under S2S processing conditions to -0.038% (MD) and -0.006% (TD) already in the first cycle. The processing at 85 °C led to dimensional changes that are somewhat similar to the changes obtained at 140 °C, namely 0.038% (MD) and -0.005% (TD).

Table 5 presents the MD and TD accuracy of the previously R2R processed PET substrates after S2S processing.

The dimensions of R2R processed ST506 substrates changed -0.435% in MD and 0.206% in TD after being S2S thermally treated for 30 min at 150 °C while unrestrained (Table 5). The MD shrinkage of R2R processed substrate after the S2S cycle refers to the substrate’s recovery towards its original dimensions. However, the main dimensional changes were observed after the first S2S cycle, and the changes of the second S2S cycle at 150 °C were only -0.026% in MD and -0.020% in TD. The dimensional changes of the previously R2R processed PET substrate were more dependent on temperature while unstrained.

The MD accuracy of the substrate in the R2R process after the first R2R cycle, and the recovery towards initial dimensions of unrestrained substrate after the S2S cycle can be explained by the polymer’s intrinsic behaviour. If an unrestrained PET film is heated to a specific temperature (>Tg), then once it has cooled, it will have reduced unrestrained ambient lengths that are dependent upon the temperature that it had been raised to. Essentially, there is an equilibrium length (in each direction) that is a function of that specific temperature. The length change upon heating is a release of “locked-in” or “frozen-in” strain (stretch) [18]. A key factor is that if this same film is raised again to the same elevated temperature, then after re-cooling and release of tension, no further (or only minor) dimensional changes will take place, as the film was already at the equilibrium dimensions (length) for that elevated temperature. However, when using the PET film to fabricate printed electronics, manufacturers often need to apply heat while under tension (e.g. when coating and printing). During such processes, a small degree of stretch of the PET film will occur in the machine which is essentially the opposite of shrinkage. If the film is then cooled down while still under this tension, some of this additional stretch will be locked into the structure of the film, such that the subsequent film will have slightly longer dimensions in the direction of this stretch. In other words, some of the stretch has been “locked into” or “frozen into” the structure of the film. In this way, even a film that has been previously stabilized would now, upon re-heating, be subject once more to the length reduction (shrinkage) that a standard, non-stabilized film would exhibit.

Essentially, if a film is heated to a given temperature, and at a specified tension (in each direction), there will be corresponding equilibrium ambient dimensions that the film will settle to, once re-cooled. The process that occurs when a film is heated under nonzero tension is a change in the strain that is locked-in but under tension, it can be either be a decrease (i.e. length reduction or shrinkage) or an increase (i.e. length increase or stretch) in the locked-in strain. The change will depend upon the relative size of the shrinkage force at the elevated temperature vs the tension used [29]. For that reason, the dimensional change upon reheating under no tension is expected to show an increased shrinkage equivalent to the increase in frozen-in strain due to the pre-processing treatment. Figure 3 graphically depicts the dimensional stability of ST506 substrate after S2S temperature treatment.

In S2S processing, the preliminary thermal treatment prior to S2S printing improves significantly the dimensional accuracy of the ST506 substrate. It also should be noted that the combination of R2R and S2S printing would decrease the dimensional accuracy due to the different thermal annealing conditions and applied tensions.

It has been evaluated that the pre-processing of the PET films does not significantly impact the film quality at these experimental temperatures. Furthermore, the flexibility of the BOPET film is a function of thickness and tensile strength properties which are not altered by the pre-heat treatments [23, 24]. For example, the mechanical and optical properties due to molecular orientation, chain length and level of crystallinity will not change significantly. A differential scanning calorimetry (DSC) assessment was performed on the substrates before and after treatment, which showed no significant differences in the low temperature peak, LTP peak (related to the temperature the film at which the film was crystallized after orientation) or the T_mp (melting temperature of the film changing from solid to liquid phase) (Table 6). DSC graphs are presented in Supplementary Information S2.

Even though no significant change in the PET morphology can be detected with the DSC assessment, a slight but statistically significant increase in haze and a change in transmittance can be observed as result of subsequent treatments (see Fig. 4 with one-way ANOVA charts below). This can be explained and is entirely consistent with an oligomer migration phenomenon (also called “blooming”). It is a physical process which happens as result of exposure of film to higher temperature for an extended time. The higher the temperature and the longer the exposure is, the worse the blooming and causes the film to have a milky appearance, yet it is possible to remove by washing the PET surface with methylethylketone solvent [18]. Changes are very small and when limited to a single extra processing step, unlikely to be noticed.

As mentioned in the Introduction, the registration accuracy of the printed layers is critical in printed electronics devices since the layer-to-layer alignment between the printed layers directly affects the electronic device performance. For example, misaligned multilayer films can negatively impact electronic device performance by decreasing the active area, causing open or incomplete circuits or creating short circuits. Examples of multilayer registration in printed electronics applications is shown in Fig. 5.

In R2R printing, automatic camera-based registration systems are typically used. These systems require camera-readable registration marks printed by every printing unit at predetermined distances from each other. Reference registration marks are printed with the first layer and all the following layers are registered to these reference marks. The registration system used in this work holds ±5 µm sensor accuracy and suits for processing of opaque, transparent, reflecting substrates and materials such as paper, film and metallized substrates. The registration camera is located directly after the printing unit and takes photos of the printed marks to control for accuracy. The registration system constantly measures the distance between the reference and actively printed registration marks and adjusts the layer-to-layer alignment. For the MD registration, the rotational speed of the printing rollers in the printing unit is varied, whereas for the TD registration, the physical location of the printing unit perpendicular to the substrate is controlled. The measured registration has either positive or negative values depending on its location relative to its optimal position. In MD, a negative value states that the overprinted layer is behind its optimum position, and a positive value implies that the overprinted layer is ahead of its optimum position. In TD, the overprinted layer can be offset either left (+) or right (-) from its correct position.

The registration accuracy of the utilized R2R printing system plays a major role in the final alignment quality of the printed electronics devices. The registration accuracy in both machine (MD) and transverse (TD) directions in a single R2R rotary screen printing run (with the cylinder circumference of 409.570 mm and 300-mm web width) using an automatic registration system is shown in Fig. 6. The registration fluctuates around the optimal registration value of 0 µm in both directions but the fluctuations are larger in MD than in TD. In MD, the registration varies between -169 µm and +145 µm resulting from the fluctuations in the substrate dimensions and web tension, changes in the rotational speed of the printing rollers, as well as oscillations created by the mechanical bearings, shafts and gears of the printing press. In TD, the registration varies between -25 µm and +111 µm. The registration accuracy in TD is mostly affected by the dimensional changes of the substrate since the adjustment of the position of the printing unit does not affect the web tension of the process much.

Table 7 presents the registration accuracy of the different R2R printing processes when the printing is done on untreated HS-PET Melinex ST506 substrate. The registration data of each printing method is calculated from two R2R print runs having different printing speeds. The utilized printing speeds were 2 m/min and 5 m/min in rotary screen printing and 5 m/min and 10 m/min in gravure and flexographic printing. As expected, the accuracy (MAX, MIN) and fluctuation (STD) of the registration is significantly better in TD than in MD due to the smaller effect of the web tension in the transverse direction of the substrate. The average (AVG) registration of the different print runs is rather close to 0 µm but some larger errors are seen with gravure printing in MD and with rotary screen printing in TD. However, the AVG value is significantly less than 50 µm, which is considered as good registration. The AVG registration should optimally be close to 0 µm since the automatic registration system tends to keep the layer-to-layer alignment as accurate as possible throughout the runs despite the process fluctuations.

The printing method has some effect on the registration accuracy, i.e. the largest absolute MAX or MIN value. In MD, the registration accuracy of rotary screen printing, gravure, and flexography is 169 µm, 146 µm, and 191 µm, respectively. In TD, the accuracy is 111 µm in rotary screen printing, 127 µm in gravure and 76 µm in flexography. Gravure printing gives the most accurate registration in MD, i.e. the lowest MAX-MIN value, but the poorest accuracy in TD. By contrast, the poorest accuracy in MD is exhibited with flexography printing. In gravure printing, the hard printing cylinder and higher nip pressure prevent the substrate slipping in the printing direction, thus providing good alignment accuracy in MD. However, the transverse-directional registration is poorer since the high nip pressure can delay or prevent the registration adjustment. On the other hand, flexography and rotary screen printing use flexible printing plates and screens with low nip pressure, thus giving better register adjustment in TD but higher substrate slip to MD.

The registration accuracy depends on the printing speed, as shown in Fig. 7. As the printing speed increases with all the tested R2R printing methods, the registration becomes less accurate, i.e. higher MAX–MIN value, in both TD and MD. This comes from the fact that the web tension fluctuates, and therefore the need for the adjustments, increase at higher printing speeds. In addition, the registration system must simultaneously tackle the adjustments faster, thus increasing the MAX–MIN value. The smallest changes of less than 20 µm are seen with rotary screen printing. However, the changes in the registration accuracy are less than 50 µm with the changing printing speed. Therefore, the effect of the printing speed on the registration accuracy is considered to be very small.

In this study, it has been shown that the preliminary R2R thermal treatment of the substrate will improve the registration accuracy through the improved substrate performance by decreasing the dimensional changes of the substrate during printing as the MD elongation reduced from 0.357% to 0.002% (PET) and from 0.206% to 0.042% (ITO-PET), thus improving the web tension controllability. However, the registration accuracy of the printing press should be taken into account when designing multilayer printed electronics devices to avoid the deterioration of the device performance. It is recommended that a tolerance is provided to account for misalignment of the printed layers in every direction within the limits of the utilized registration system without creating short circuits, decreasing active areas of the devices, or hindering the formation of electrical contacts. This means that the dimensions of the printed layers should be larger than in the optimum case where the perfect registration of 0 µm is achieved. Thus, the MAX, MIN and MAX–MIN values of the registration of the printing system aid in the design of the multilayer electronic devices. For example, considering the utilized R2R printing press above, the layers should have tolerance to move at least 333 µm (±167 µm) with rotary screen printing, 260 µm (±130 µm) in gravure, and 381 µm (±191 µm) in flexography without creating performance issues. Please note that many printed electronics manufacturing processes combine several different printing and deposition techniques. Therefore, for a multilayered thin film stacked architecture, the deposition method with the poorest registration accuracy determines the design limits for the whole device.

The effect of the preliminary S2S thermal treatment of the HS-PET substrate on the registration accuracy of the S2S printed multilayer structures in MD is presented in Table 8. The preliminary S2S thermal treatment of the substrate improves the average registration accuracy by 40% from 81 µm to 48 µm. In addition, the maximum and minimum registration values are 33–43% smaller with the pre-treated substrate sheets than with the un-treated sheets. The S2S treatment above the glass transition temperature (Tg) of the substrate stabilizes the substrate dimensions by relaxing the polymer chains and residual strains within the plastic films. Therefore, any biaxial shrinkage or expansion of the plastic substrate sheet should take place during the preliminary S2S cycle, thus giving a dimensionally stabilized substrate for multilayer printing and thermal annealing. In R2R printing, the web tension plays a role in the substrate stabilization and in the following printing processes. It is, thus, expected that the registration accuracy is slightly poorer in continuous R2R processes than in S2S processes.

The multilayered OPV structure was chosen as a case study to evaluate the fabrication and performance of multilayer printed free-form optoelectronic devices. This is particularly important for electronic devices that pattern by printing (i.e. gravure, screen, flexographic) and all R2R deposited thin film layers providing incredible flexibility for design freedom. Nevertheless, there are a variety of methods to fabricate 2D designs for multilayered stacks, which includes R2R compatible processes such as slot-die coating and laser patterning [30].

The fabricated OPV structure comprised five R2R printed or R2R patterned layers with the configuration of PET/ITO/ZnO/P3HT:PCBM/PEDOT:PSS/Ag. First, ITO was patterned as a negative image in rotary screen printing process via wet etching. Next, ZnO was gravure printed as an electron transport layer and P3HT:PCBM as a photoactive layer. PEDOT:PSS was rotary screen-printed as a hole transport layer and silver as the hole contact. Figure 8 presents the processing equipment and the process flow that were used in the fabrication of all OPV designs presented in this paper.

The solar cells were designed with an overlap between the printed layers to obtain the serial connection between the cells through the ITO and silver layers. Triangle design comprised three modules with seven serially connected cells where the modules were connected with each other in parallel (Fig. 9a). Santa Claus and Camouflage designs comprised a module with eight cells in series (Fig. 9b). Calligraphy design comprised seven modules where each module comprised ten cells in series. The seven modules were connected with each other in parallel to form Fig. 9c Calligraphy layout. For the continuous large-area calligraphy pattern, the repetition lengths were connected with each other in parallel.

As already mentioned, for designing R2R printed electronics, the registration accuracy needs to be considered during the layout design by controlling the size of the printed forms. In practice, this means that the layers require some increase to their optimal dimensions to meet the limitations of the printing accuracy. With this is mind, the layers endure their desired overlap to avoid a decrease in performance or short circuits, even though the layer-to-layer registration may fluctuate during the R2R print run. Accordingly, the dimensions of the layers should be tuned according to the registration performance of the utilized printing press and registration system. Table 9 presents minimum layer-to-layer offset for each printed layer for free-form designs.

Each layer was designed to have an offset to the previous layer and a tolerance to cover the influences caused by printing inaccuracy and layer-to-layer misalignment. Based on earlier study of Välimäki et al. gravure printed P3HT:PCBM and screen-printed silver are relatively close to the nominal values of the layout, whereas gravure printed ZnO ink is prone to spreading, and screen-printed PEDOT:PSS paste prone to shrinking [2]. Here, the designed Santa Claus and Camouflage shapes comprise small features with the higher complexity of form factor thus, being more sensitive to critical misalignment, whereas Triangle design comprise the larger tolerances due to the significantly large size of a single cell. The tolerances of the calligraphy design are very close to or even below the limits of the registration system of the utilized R2R printing line, as stated in Section 3.4. Four OPV designs with varying degrees of complexity were prepared with the MAXI pilot printing line (Fig. 10).

Current–voltage characteristics show that the printed Santa Claus, Camouflage and Triangle modules reached the same or even higher average open-circuit voltage (V_oc) than the standard Rectangle-shaped module (Table 10). In comparison with the Rectangle-shaped module, the short-circuit current (I_sc) and fill factor (FF) of the Camouflage-shaped module was similar and with the Santa Claus-shaped module slightly lower. The changes in the active area of the solar cell caused by layer-to-layer inaccuracy result fluctuation in I_sc; however, the printed free-form shapes obtained functionality that was comparable with the reference Rectangle-shaped OPV.

The performance of the free-form OPV is comparable to the performance of rectangle OPV. However, if the module size increases to a full repetition length of 407 mm (in the MD), as with the calligraphy design, the device performance is more challenging to maintain with 300–400 µm registration tolerance due to the fluctuation of registration. The effect of the registration accuracy on the performance of the printed multilayer OPV modules with ten cells in series using the Calligraphy design is shown in Fig. 11. The registration accuracy of these multilayer devices is calculated as a sum of registration errors of each printed layer and the cells are arranged in the order of the alignment accuracy. In total, four layers were deposited onto the patterned ITO-PET and the registration is measured in both MD and TD. As the registration accuracy improves, V_oc increases. This results from the fact that the good electrical contacts to the device are formed, and no short circuits are seen as the printed layers are properly aligned with each other. However, the performance remains rather constant after a certain threshold registration accuracy is reached. It is also seen that the registration in TD is much better and more even than in MD. When the registration sum is more than 2000 µm (cell numbers 170, 171, 172 and 158), the V_oc values are less than 3 V. As the sum of registration accuracies decreases below 600 µm (cell numbers 210 and 209), the V_oc increases above 5 V. Figure 11 also shows the V_oc of the OPV as a function of average registration and average maximum registration error. There is a strong correlation between the layer-to-layer registration and the device performance. The poorer the registration, the lower the open-circuit voltage, thus the poorer the device performance. When the average registration is less than 150 µm and the maximum average registration error is less than 230 µm, V_oc of OPV Calligraphy design with ten serially connected cells is 4.7-5.2 V. The poorest V_oc of less than 0.5 V is seen when the average and maximum average registration errors are more than 300 µm and 700 µm, respectively. It is, thus, evident that when the maximum registration error is less than 300 µm and the MAX–MIN error of the printing system has been taken into account in the OPV device design, the performance of the device can be optimized.

Calligraphy OPV modules were used to build a decorative large-area smart glass façade element, shown in Fig. 12. In the demonstrator, Calligraphy OPV modules were combined with both a flexographic printed graphic foil and a LED foil with screen-printed silver wirings. The whole stack was then laminated together between glass panels. It is shown that the fabrication of these kinds of decorative large-area devices requires not only a good layer-to-layer registration in the multilayer printing processes within a single foil but also good alignment accuracies when combining all the separately patterned foils together. Therefore, the thermal pre-treatment of the foils is important before any processing to stabilize the substrate dimensions [31].

The design of free-form electronics should take into account the limitations of the printing process and printing methods, as well as the printing ink and substrate materials. The minimum achievable line width depends on the printing method and on the ink properties. For example, the minimum achievable line width in rotary screen printing is approximately 100 µm, but in flexography, 20–40-µm wide lines can be easily reproduced. In addition, reduced ink viscosity typically increases the ink spreading on the substrate, thus increasing the achievable line width [32]. However, other ink properties, such as surface tension and rheology, also affect the ink transfer, substrate wetting, layer levelling on the substrate, and ink layer spreading. The surface energy and roughness of the substrate can also change the ink transfer and ink behaviour on the surface of the substrate. For example, ink can flow into the irregularities of the substrate surface, thus changing the dimensions of the patterns and creating irregularities in the film topology.

The registration accuracy of the printing system should also be taken into account in multilayer printings, as mentioned earlier. Each printed layer should provide tolerance in the layouts to allow for some misalignment without having an effect on the device performance. This requires that the deposited layers are designed for sufficient overlap which implies that one must have the knowledge of the limitations of the registration system in use. For example, with MAXI R2R printing machine with the automated registration system that was used, each deposited layer should be able to misalign by 300–400 µm without affecting the device performance. This applies for all the printing methods available in the MAXI line and for multilayer structures having more than two layers. To verify this, a demonstration was prepared by printing OPV devices into a form of a Santa Claus, Camouflage, Triangle and Calligraphy.

If a series of patterns of specific dimensions is printed onto a PET sheet, then only very small dimensional changes between processing stations can be tolerated, or there will be registration issues with subsequent printed patterns. The length of the film under tension at ambient temperature before and after each stage will change significantly only through the first processing station (0.357% in MD), and not subsequent ones (0.002% in MD). For that reason, substrate pre-treatment prior printing is recommended as demonstrated in the findings of this study.