A little explored technique in the context of polymer solar cells is doctor blading that allow for the formation of films with a well-defined thickness.In contrast to spin coating the technique is quite parsimonious and with some practice the loss of coating solution can be minimised such that less than about 5% is lost.The technique works by placing a sharp blade at a fixed distance from the substrate surface that is to be coated (typically 10–500um).The coating solution is then placed in front of the blade that is then moved linearly across the substrate leaving a thin wet film after the blade.The final wet thickness of the film is ideally half the gap width but may vary due to the surface energy of the substrate,the surface tension of the coating solution and the viscosity of the coating solution. It also depends on the meniscus formed between the blade and the wet film on the trailing edge of the blade,which is related to the shear field (proportional to the speed of the blade/knife).The final dry thickness of the coated film, d, can be calculated from the empirical relationship
where g is the gap distance between the blade and the substrate in cm, c is the concentration of the solid material in the ink in g*cm-3 and, ρ is the density of the material in the final film in g*cm-3. Since the reisessentially no loss of coating solution it should ideally be possible to arrive at functional films when very little material is available. inpractical terms, however, and for prototyping work it takes some time finding the right conditions for coating and the initial loss of coating solution is large and thus gives no advantage over spincoating. Further,compared to spincoating where the wet film formation is fast,doctor blading is relatively slow and if the dissolved material has a propensity to aggregate or crystallise at high concentration this often happens during doctor blading.
It is likely that this is the reason for spincoating being the preferred technique over doctor blading as they are at about the same level in terms of instrumental cost and operational complexity. Whereas spincoating has no R2R compatible pendent, doctor blading is easily transferable to a R2R coating environment and is in this context known as knife-over-edgecoating (vide supra). Of the few well documented studies where doctor blading, is employed as a film-forming technique in the context of polymer solar cells one has focussed on the ratio between crystalline (crystallised) and amorphously (intermixed) PCBM in bulk heterojunctions of MDMOPPV and PCBM. The study employed solid-state NMR techniques and highlighted that there is a higher degree of crystallised PCBM in doctor-bladed films as compared to spin-coated films using the same coating solutions and concentrations. This was ascribed to the slow speed of solvent evaporation during doctor blading as compared to spincoating. This also implies that spincoating give access to films that are perhaps further from thermodynamic equilibrium than doctorbladed films.
Kaivo's coaters can be used for Doctor blading
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