R2R coating and printing is an emerging processing method within the field of organic/polymer thin film device fabrication. “True” R2R equipment is very costly to acquire, and as a consequence reports of “true” R2R processing are limited to very few groups. To give a broader picture of how the above described processing methods have been used so far, we have chosen to include R2R-“compatible” processing techniques (sheet-to-sheet processing and the use of nonflexible substrates) in our overview of the current status of the field. It is important to stress though that there are huge differences between a process being claimed as R2R-compatible and true R2R processing, and the results obtained using the compatible processing will not be directly transferable to R2R! Table 1 gives a summary over various processing of layers in polymer-based devices including electrodes, blocking layers, and active layers and other layers in organic photovoltaic (OPV), organic thin film transistor (OTFT), polymer light-emitting diode (PLED), and electrochromic (EC) devices. The table also illustrates the fact that different coating or printing methods might be suited for processing of a specific layer, while proving less useful for the processing of others. Fixing the eye on a single technique for the processing of a whole device thus might prove less efficient than using several different techniques optimized for each specific layer. Figure 5 shows typical device architectures of OPVs, OTFTs, PLEDs, and EC devices.
Large Area Organic Solar Cells
Organic solar cells are as indicated in Figure 5 multilayer structures of typically a transparent electrode, a hole or electron blocking layer, the active layer (comprising a bulk heterojunction of a polymeric absorber, which acts as a donor material and an acceptor material typically a fullerene), a hole or electron blocking layer (the opposite of the one placed on top of the transparent electrode), and finally a back electrode.
A large palette of R2R techniques have been used in the preparation of organic solar cells, with slot-die coating at present being the most abundantly used for processing of hole/electron blocking layers and the active layer. On the other hand, screen printing is by far the most widely used for processing of the back electrode, and is a good example of how different processing procedures can be more suitable than others for the processing of a specific layer.
Krebs et al. introduced a new fabrication method for all R2R-processed OPVs on indium tin oxide (ITO)-coated PET substrates in 2009,18 with an architecture of PET|ITO|ZnO|active layer|PEDOT:PSS|silver, which has since been referred to as “ProcessOne.” The first three layers (ZnO, active layer, and PEDOT:PSS) were processed using slot-die coating, and finally a silver back electrode was slot-die coated or flat bed screen printed to finalize the stack before it was laminated. Since then, numerous productions of solar cells and modules based on this method have been published,20–26 and solar modules produced according to this have been used in interlaboratory studies,86, 87 round robins,88, 89 and in demonstrators of various kinds that have mostly involved sound, light, or in one case a laser (see Fig. 6).19, 23, 28, 90 The ProcessOne solar modules based on P3HT:PCBM are presently at a stage with a lifetime of 1 year (T80) when exposed to outdoor weather conditions.
Figure 6. Examples of ProessOne fabricated demonstrators. Left: picture of a very simple integrated lamp for the “Lighting Africa” project. Right: picture of a module powered flashlight. Reprinted from Ref.90 and Ref.19, with permission from The Royal Society of Chemistry 2010 and The Royal Society of Chemistry 2011, respectively.
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Various examples of the use of R2R-processed low-bandgap (LBG) polymers have been reported21, 22, 24, 25 with efficiencies approaching what can be achieved with P3HT:PCBM. These are, however, still generally inferior in performance, which can be ascribed to the fact that it takes a large effort (and a lot of material) to fully explore a new polymer. This goes to show that direct transfer of small-scale technology cannot always be expected, as the use of several different LBG polymers in small-scale devices has proven to be more efficient than P3HT:PCBM, when prepared in a glove box and with evaporation of electrodes. In a more special case, Helgesen et al. used a R2R photonic flash lamp to remove the side chains of the used polymer by selective heating of the active layer to high temperatures without heat damaging the flexible plastic substrate. The reason for removing solubilizing side chains in this extra step was to improve device stability as the solubilizing side chains are known to induce instability to the polymer.24
Another of the more exotic examples of R2R-processed solar cells was presented by Hübler et al. who have prepared fully R2R-processed solar cells on paper using a combination of gravure and flexo printing.17 The processing starts with gravure printing of a glue onto a paper substrate. The glue patterned paper is then brought into contact with a zinc-coated transfer foil, transferring the zinc only onto the patterned glue, and the surface of the zinc is then oxidized creating a thin hole blocking layer. The active layer (P3HT:PCBM) is now coated on top of the ZnO by use of gravure printing, and the device is finalized by flexo printing a thin layer of PEDOT:PSS as a transparent back electrode. Small devices (0.09 cm2) showed maximum efficiencies of 1.31% at 600 W/m2. Figure 7 shows a picture of the final device.
One of the interesting things about the report from Hübler et al., besides the fact that they use paper as substrate, is that the architecture is ITO-free. ITO has for decades been the preferred transparent electrode material in many types of electronic devices, because of its high transparency combined with a low sheet resistance, but the scarcity of indium makes ITO quite expensive and the cost of indium accounts for the majority of the device cost. Much effort has been put into finding alternatives, and Galagan et al. showed that a silver collecting grid, screen printed on a PEN substrate and subsequently spin-coating of a high-conducting PEDOT:PSS solution on top, is a way of substituting ITO.32 Good efficiencies (1.93%, 4 cm2) were obtained using P3HT:PCBM for the active layer (spin-coated in glove box) and evaporative deposition of LiF/Al as the electrode, but the use of spin-coating, inert atmosphere, and low-pressure deposition makes the procedure difficult to transfer to a R2R setting.
Other examples where full R2R processing is actually achieved have recently been presented though.12–14 Larsen-Olsen et al. showed that the use of high-conductive PEDOT:PSS with or without a preprocessed silver collecting grid can be used as a substitute for ITO in inverted structure devices and modules with good results.12, 14 By appliance of a short pulse at high negative bias to the solar cell, it is possible to electrochemically “switch” (reduce) a thin layer of the top PEDOT causing a permanent conductivity change rendering it a rectifying junction. Further studies on the use of different types of front grids in the same process were carried out by Yu et al. showing that thermally embedded silver grids and flexo printed silver grids result in solar cells with similar efficiencies (slightly better for the embedded grid), whereas solar modules with inkjet printed silver front grid performed poorer, which was ascribed to the lower conductivity of the grid.13
Besides the previously mentioned example of use of paper as substrate by Hübler et al., the use of gravure printing for the preparation of solar cells has only been reported in very few cases.34–37 Kopola et al. reported the use of a desktop gravure printability tester (on PET, not R2R process) to process the hole transport layer (PEDOT:PSS) as well as the active layer (P3HT:PCBM) of single cells36 and of small modules.35 In both reported cases, the back electrode consisted of evaporated calcium and silver resulting in efficiencies of 2.8% for single cells (19 mm2) and 1.9% for small modules (five cells in series, 9.6 cm2) after optimization of the PEDOT:PSS ink with surfactants, wetting agents, and solvent mixtures.
Figure 8 shows an example of the necessity to optimize ink and processing conditions to obtain a smooth and homogeneous active layer and an example of a final flexible device. Voigt et al. recently reported the use of sheet-to-sheet gravure printing of inverted structured cells on PET after performing a systematic study of the wetting behavior of each layer.37 In this case, three of the layers (TiOx, P3HT:PCBM, and PEDOT:PSS) were processed by gravure, and the cells were finalized by evaporation of a back gold electrode (4.5 mm2, 0.6% PCE).
Figure 8. Left: Pictures of the optimization of ink and processing conditions for gravure processed P3HT:PCBM. Right: Picture of a gravure printed flexible organic solar cell module. Reprinted from Refs.35 and36, with permission from 2010 Elsevier B.V.
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Also, flexo printing has only had limited application in solar cells and has so far not yet been used for processing of the active layer. The potential of flexographic printing as an extremely fast processing method has so far proven best for the preparation of front grid silver electrodes that were prepared at high speeds (25 m/min).13
Among the more specialized coating procedures used for R2R OPV fabrications can be mentioned double-slot-die coating, introduced in organic solar cells by Larsen-Olsen et al. in order to approach a further increase in the production throughput by simultaneous deposition of several layers of the solar cell stack.91 The double slot-die method illustrated in Figure 9 was used to coat an aqueous suspension of P3HT:PCBM nanoparticles and aqueous PEDOT:PSS at the same time with a processing speed of 1 m/min. Although the solar cells showed a poor performance (PCE of 0.03%) because of the complex bilayer formation process, it demonstrates the potential as a future processing method to lower the energy payback time of organic solar cells.
Figure 9. Schematic illustration of the double slot-die setup for the simultaneous coating of the P3HT:PCBM active layer and PEDOT:PSS.
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Another specialized processing method especially for research and development purposes is the differentially pumped slot-die coating.92 Hereby two components of the functional ink are mixed together with the ability to generate changing material ratios over the length of the running web. The fast screening method was used to determine the optimum donor–acceptor ratio and film thickness for organic solar cells. The specially designed slot-die head with very low dead volume requires very little amount of material, which makes it an ideal tool to screen new materials in a wide parameter space compared to spin coating. Furthermore, it directly shows the R2R processability. The process has later been used on several occasions to optimize donor/acceptor blends.21, 22, 25
Electrochromic devices are based on materials exibiting electrochromic behavior (materials that present two discrete optical appearances when in a reduced or oxidized electrochemical state). Testing of the electrochromic properties is usually performed by coating the polymer on a substrate with a transparent electrode (typically ITO), followed by immersing into an electrolyte solution together with an expendable counter electrode. The electrochromic properties, such as color change, change in transmittance (ΔT), and switching times, can then be examined through electrochemical oxidation and reduction of the polymer. When wanting to build a thin device the use of a simple counter electrode is generally not preferable because of the slow deterioration of this, but as a substitute one can make use of a complementary redox compound (a material that is reduced when the electrochrome is oxidized and vice versa) deposited on a second substrate also with a transparent electrode. The final device then consists of the two coated substrates with the electrolyte sandwiched in between as shown in Figure 5.
The processing of polymeric ECs has been reported by use of spray coating,74, 76–80 inkjet printing,81, 82 screen printing,83, 84 and slot-die coating.74 Jensen et al. recently presented results showing that slot-die coating (or spray coating) of the electrochromic layers on flexible substrates can be achieved using a single roll coater with a roll having a diameter of 300 mm.74 By use of the EC commonly known as ECP-Magenta and a minimally color changing polymer (MCCP) as complementary redox compound, which were both slot-die coated on separate PET/ITO substrates, it was possible to make electrochromic devices >10 cm2, which could switch between magenta and a colorless state. Pictures of the coatings are shown in Figure 10. By connecting an electrochromic device (40 mm × 40 mm) directly with polymer solar cell modules or with batteries charged by polymer solar cells, they furthermore fabricated a demonstrator showing how different polymer thin film technologies can be combined to a final product. The small-scale approach was later upscaled to full R2R processing of ITO-free 18 cm × 18 cm electrochromic windows printed directly on barrier foil using flexographic printing of metal grids and slot-die coating of the electrochromic polymers (ECP-magenta and MCCP).74
Figure 10. Left: Picture of slot-die coating and spray coating of polymer electrochromes on a single roll system. Middle: Pictures of an integrated polymer electrochromic/polymer solar cell demonstrator. Right: Examples of inkjet printed electrochromic devices. Reprinted from Ref.74 and Ref.81, with permission from 2012 Wiley Periodicals, Inc and The Royal Society of Chemistry 2008, respectively.
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With respect to spray coating especially the group of Reynolds has contributed to finding a series of polymers of different colors and optimizing the conditions for the use of this technique.76–80 As can be seen from Table 1, a multitude of different colored polymer are available, and recently Dyer et al. could announce that the color palette for spray-processable polymer electrochromics is complete.93
The use of inkjet printing of polymer electrochromes is highly useful when wanting to create patterned EC devices. This is well illustrated in a report by Shim et al., who showed that printing composite dispersions of polyaniline- or PEDOT-covered silica nanoparticles onto PET/ITO enabled preparation of electrochromic devices with good resolution (see Fig. 10).81
Screen printed flexible electrochromic devices were reported as early as in 1999 by Brotherston et al., who used a combination of PEDOT and V2O5 screen printed, respectively, onto separate ITO-coated Mylar substrates and finalized with an electrolyte sandwiched between the two substrates.84 The technique has not become widely used, as the only other example of the use of “screen printing” for electrochromic device preparation is the use of a precut barrier film and a screen printing squeegee for the processing of the electrolyte.83 As no actual screen was used in this latter example, the term screen printing should be taken with modification.
Thin Film Transistors
Although organic solar cells have so far been the most prominent technology to use R2R processing of functional polymer materials, printing is also one of the experimental techniques in a variety of fabrication methods for organic thin film transistors.94 In recent years, efforts have been made to R2R manufacture devices including organic thin film transistors to enable integrated circuitry such as inverters95 and ring oscillators.96 Still, most of the devices containing polymer materials are fabricated by sheet-to-sheet methods using R2R-compatible processes such as inkjet,63, 97 gravure,64, 65 screen printing,66 and spray coating.98 Kang et al., for example, utilize an optimized microgravure process to print silver patterns and poly(4-vinyl phenol) (PVP) dielectric layers.99 A transistor with record transition frequencies of >300 kHz was achieved with a spin-coated organic semiconductor poly(2,5-bis(3-alkythiophen-2-yl)thieno[3,2-b] thiophene) (pBTTT).
The transition from batch to R2R processing has partly been carried by Tobjörk et al. by using R2R reverse gravure coating for the deposition of a polymer semiconductor (P3HT) layer and a polymer dielectric PVP for the fabrication of low-voltage organic transistors with an on/off ratio of 100 and threshold voltage of 0.5 V.61 Silver source/drain electrodes and PEDOT:PSS polymer gate electrodes were sheet-to-sheet inkjet printed. Figure 11 (top) illustrates the processing methods used as well as the final device. The first integrated circuit fabricated completely by means of mass-printing technologies was reported by Huebler et al.62 The seven-stage ring oscillator contains organic field effect transistors in a top gate architecture with PEDOT:PSS source/electrodes prepared by offset lithographic printing. Gravure printing was used for the poly(9,9-dioctyl-fluorene-co-bithiophene) (F8T2) semiconductor layer and the first butadiene-styrene-copolymer low-k dielectric layer. The device was finalized with flexographic printing of a high-k BaTiO3 dielectric layer and silver gate electrodes. Although the frequency reached was only 3.9 Hz, it demonstrates the processability with mass-printing technologies with speeds in the order of 1 m/s. A picture of the seven-stage ring oscillator is show in Figure 11 (bottom).
Figure 11. Top: Process flow, cross section and optical microscope picture of an all-printed OTFT including inkjet printing R2R reverse gravure coating. Bottom: Photographic image of a fully printed seven-stage ring oscillator. Reprinted from Ref.61 and Ref.62, with permission from 2008 Elsevier B.V. and 2007 Elsevier B.V., respectively.
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A combined R2R flexo and gravure process was used to realize PEDOT:PSS source/drain electrodes with channel length down to 10 μm.100 A negative image of the electrodes was flexo printed from an amorphous perfluorinated poly(alkenyl vinyl ether). A secondary full layer gravure print of PEDOT:PSS leads to a self-formation of the electrodes. Full devices such as OFETs, inverters, and ring oscillators were fabricated with gravure printing of F8T2 semiconductor and dielectric material (butylene copolymer and PMMA). Flexo printing was utilized for the copper electrodes.
Full R2R gravure processing was used for the manufacturing of all-polymer field effect transistors with a yield of ∼75% out of a random selection of 50,000 produced transistors.60 The report highlights a special electrode layout to avoid longitudinal registration problems. The polymer materials used were PEDOT:PSS (source/drain, gate), PMMA and butylene copolymer (dielectric), and an amorphous poly(triphenylamine) (PTPA2) as semiconductor.
Polymer Light-Emitting Diodes
PLED is a class of organic light-emitting diodes (OLED) where the light-emitting layer is based on polymers opposed to small molecules, which are typically deposited by evaporation processes.101–104 OLEDs in general are widely explored and are already in use for display and lighting applications. The layer structure of PLEDs is almost identical to organic solar cells with a light-emitting layer instead of a light-absorbing layer as seen in Figure 5. Common conjugated polymer materials used as emitting layer are polyphenylene vinylenes such as poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV).101 Another approach is the dispersion of small-molecule emitters in a nonconjugated polymer matrix, such as poly(methyl methacrylate) (PMMA) or polyvinylcarbazole (PVK).102 One advantage of using polymers is their solubility that enables the solution-based manufacturing similar to organic solar cells with a variety of potential printing and coating processes. Although the processing advantage is present, reports on R2R manufacturing are rare.
Various studies with R2R-compatible methods have been made and show the applicability of slot-die coating,67 screen printing,73, 105, 106 blade coating,68, 72, 107 gravure printing,69, 71, 108, 109 and inkjet printing,70, 110, 111 which has its advantages for pixel-based display applications. The following reports are highlighted for their closest potential in R2R up scaling. A sample device is shown in Figure 12.
Figure 12. Photograph of a flexible PLED device (30 mm × 15 mm) with a slot-die-coated PEDOT:PSS layer and a gravure printed light-emitting polymer layer. Reprinted from Ref.108, with permission from 2009 Elsevier B.V.
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Youn et al. fabricated a PLED based on the yellow-emitting phenyl-substituted poly(para-phenylene vinylene) (Super Yellow).68 All layers including PEDOT:PSS, ZnO, and an ionic solution containing tetra-n-butylammonium tetrafluroborate were blade-slit coated with a high layer uniformity on ITO-coated glass substrates. The luminous efficiency of the in-air processed samples reached 5.26 cd/A.
On another occasion standard multilayer blade coating has been used to coat several small-molecule materials and PVK host solutions to build multicolored light-emitting devices.72 The conducting polymer PEDOT:PSS was blade coated as well. The applied layer thicknesses were below 100 nm with uniformities within 10%. The report covers challenges in uniform layer formation, drying and interaction of solvents in the multilayer approach to prevent dissolution. Large-area devices with 6 cm2 active area and efficiencies up to 25 cd/A for green phosphorescent OLED materials were fabricated.
Gravure printing was used by Kopola et al. to prove the feasibility of large-scale fabrication of PLEDs for lighting applications.69 PEDOT:PSS and a blue-emitting polymer dissolved in o-xylene was gravure printed on ITO-coated glass with an active area of up to 30 cm2. Brightness levels of up to 1000 cd/m2 at 5.4 V were achieved. Ink modification and printing form optimization played an important role to achieve homogenous layers for a uniform light generation.
Finally, a fully slot-die-coated light-emitting device based on the light-emitting electrochemical cell (LEC) technology has been demonstrated by Sandström et al.67 For the first time, devices were fabricated fully under ambient conditions using roll coating on PET foil. This included air processing of the back electrode. The authors used the emissive conjugated polymer “Super Yellow” and an electrolyte consisting of potassium triflate (CF3SO3K) in poly(ethylene oxide) (PEO). Active areas of around 3 cm2 have been achieved with brightness levels of up to 150 cd/m2 at 10 V. The highest current efficiency was recorded with 0.6 cd/A at 50 cd/m2. The processing of the devices was shown to be very reliable because of a thick active layer and air-stable materials. Figure 13 shows pictures of the slot-die coating and the final device.
Figure 13. Left: Close-up photograph of the slot-die coating process of the light-emitting polymer layer “Super Yellow”. Right: Photograph of the final LEC device showing a bidirectional light emission and device conformability. Reprinted from Ref.67, with permission from 2012 Macmillan Publishers Limited.
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The achieved results for gravure, blade, and slot-die coating will enable an upcoming transition to full R2R processing of cost-friendly and vacuum-free manufacturing of large area light sources on flexible substrates. Inkjet printing methods will most likely be seen in display manufacturing.