Flexible substrate

Many plastic substrates are available for use in flexible OPVs. As summarized in Table 13.3, high optical transmittance, heat resistance and solvent resistance are requirements for the flexible and scalable OPVs. Therefore, PET and PEN are most frequently used as the substrates in OPVs. 

As mentioned previously, the surface conditions of the substrate are important when solutions such as PEDOTrPSS or other inks are coated. Basically, it is not easy to coat aqueous based PEDOTrPSS onto the hydrophobic polymer substrate. Therefore, various techniques can be applied to modify the surface of the substrate, e.g. plasma treatment and treatment with various chemicals such as primers or coating promoters. The surface energy can be divided into the disperse energy and polar energy. The sum of the two terms is the total surface energy. Eor example, the surface energy of the substrate can be enhanced by O2 plasma. The surface energy is increased with respect to the treatment time with O2 plasma, as shown in Table 13.4. 

Flash points mainly from WolframAlpha [21]. Hildebrand values mainly from Ref. [22]. Other data from Refs. [23, 24]. Warning hexamethylphosphoramide and isophorone are confirmed animal carcinogens. Warning DMF and NMP are suspected embryotoxins. 

Flexible substrates for screen-printed electrodes must meet a range of demanding requirements, especially for display applications. Besides the most obvious requirement - low flexural rigidity - they also need high temperature stability. 

In general, however, the most important requirement is low flexural rigidity (low bending stiffness) D (N m ). From an engineering perspective, this is related to the first power of the tensile modulus and the third power of the substrate thickness as follows  

Widely used substrate materials include poly(ethylene naphthalate) and poly(ethylene terephthalate) of thickness 25-75 )im. A cheap commercial source of poly(ethylene terephthalate) is laser inkjet transparency film. Interestingly, both PEN and PET can withstand temperatures up to 180—220 C. PEN also has the advantage that it is about three times less permeable to water and oxygen than PET. Other widely used substrate materials include polycarbonate, polyimide, and PEEK. Some commercial plastic films are also obtainable with oxygen plasma-treated poly(urethane) primer layers on one side, to raise the surface energy and thereby improve the adhesion of inks. 

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Of practical interest are detailed studies to influence the magnetooptical properties of RE-TM materials by the substrate material and the substrate adhesion of RE-TM layers by the selected deposition technique (226). Accordingly, measurements have been performed on glass, BPA-polycarbonate, and poly(ethylene terephthalate) (as a flexible substrate). 

By varying molecular weight and functionaUty of the castor polyols and the type of isocyanate, a variety of clear and pigmented urethane coatings can be prepared. Copolymers of vinyl and castor-based urethane have also been reported for use as exterior coatings for plywood and flexible substrates (90) and for appHcation over steel, concrete, and wood substrates (91). 

It is noteworthy that an important industrial application is based on pure silicone network [9]. This is the organic PSA release technology where an uncured silicone is deposited as a thin coating to a flexible substrate. Strong adhesion develops at the silicone-substrate interface whilst the coating cures. 

Textiles, paper, and other flexible substrates such as fusible interlining, interlinking drapery and upholstered fabric, and carpets are examples of large volume applications. Another important market is with metals and other rigid materials. Included are plastics (pipes, tanks, screens, etc.) that can provide protective coatings using its variety of processing techniques. 

Electrodeposition has been attempted also on flexible substrates within the scope of fabricating flexible solar cells. Huang et al. [177] investigated the electrodeposition of CIS on Au-coated plastic substrate from aqueous acidic (pH 1.65) solutions of millimolar CuCh, InCb, Se02, containing triethanolamine and sodium citrate. Stoichiometric, semiconductive CIS films (Eg = 1.18 eV) were obtained after annealing at 150 °C in nitrogen. 

Huang CJ, Meen TH, Lai MY, Chen WR (2004) Formation of CulnSe2 thin films on flexible substrates by electrodeposition (ED) technique. Sol Energy Mater Sol Cells 82 553-565... 

An alternative method to position two electrodes at nanometer distances apart is the mechanically-controlled, break junction (MCBJ) technique. An ultra-thin, notched Au wire on a flexible substrate can be broken reliably by pushing on the Au with a piezoelectric piston, cracking the Au (Fig. 4). This produces a gap between the Au shards whose size can be finely varied to 1 A by a piston or control rod [46, 47]. When UE molecules with thiol groups on both ends are present in a surrounding solution, the gap can be adjusted until the molecules can span it. A dilute solution means the number of spanning molecules will be small, and the least-common-multiple of current flow among many junctions indicates those spanned by a single molecule [47]. 

Lee, K. J. et al. 2005. Large-area, selective transfer of microstructured silicon A printing-based approach to high-performance thin-film transistors supported on flexible substrates. Adv. Mater. 17 2332-2336. 

A.N. Krasnov, High-contrast organic light-emitting diodes on flexible substrates, Appl. Phys. Lett., 80 3853-3855, 2002. 

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