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Inks

Markus Hösel infinityPV ZnO ink

General ink properties

A very important issue in the fabrication of polymer solar cells, either for small-scale or R2R high throughput production, is finding suitable inks that match the processing method. Ideally, the solvents should be environmental friendly and nontoxic, with water being the cheapest and best case. Not all of the materials in polymer solar cells can currently fulfill this demand. PEDOT:PSS is being the only component generally processed from aqueous solution. Nevertheless, fully aqueous processed polymers solar cells were successfully produced by introducing thermo-cleavable polymers to make the active layer insoluble.DOI:10.1002/aenm.201000007

Multilayer devices such as polymer solar cells that are produced in an all-solution process generally require orthogonal solvents (polar and non-polar) for the deposition of the subsequent layers. It must be ensured that the deposition of one ink does not dissolve the functional layer underneath. Ink formulation can be divided into three categories: dissolved material (each molecule is fully solvated), emulsions (material is kept in solution by additives which allow for micelle formation), and particulate solutions or pastes (solid particles are suspended in solution). Careful exploitation of ink formulations and processes might be the answer to how to replace the use of orthogonal solvents.DOI:10.1016/S1369-7021(12)70019-6

The rheology (viscosity) of the ink or paste is important for the specific deposition process and can be measured with a rheometer or viscometer. Inkjet printing requires very low viscous inks with a viscosity <30 mPa$\cdot$s and typically contain a complex mixture of many solvents with high and low boiling points. This allows proper drop generation and avoids nozzle clogging. The viscosity in gravure printing and flexo printing is higher and generally in the range of 100-1000 mPa$\cdot$s. Typical screen printing pastes have very high viscosities >>1000 mPa$\cdot$s and should have shear thinning characteristics, also called thixotropy, to enable proper ink flow through the screen mesh once force is applied by the squeegee. Slot-die coating has a very broad range from less than 10 mPa$\cdot$s to >10000 mPa$\cdot$s. The slot-die head internals, ink pump, and coating window regime has to be chosen accordingly.

Conductive metal electrode

Silver is at present the only real candidate for printable electrodes. Back electrodes are typically processed by screen printing of highly viscous micro-flake pastes. These pastes are commercially available and can be cured thermally or with UV light. None of these are water based, but screen printable water based silver pastes can be prepared quite easily using an aqueous binder, silver flakes, and water.DOI:10.1002/aenm.201000007 The impact of the solvents and process conditions on the functionality of the OPV device has been studied using LBIC methods.DOI:10.1016/j.solmat.2010.11.007 Low viscosity silver inks containing nanoparticles, either aqueous or solvent based, are further commercially available, and can be processed by methods like gravure, flexography, and inkjet printing. Such inks and printing methods are typically used to fabricate front electrode grids for ITO-free devices.DOI:10.1002/ente.201200029DOI:10.1016/j.solmat.2013.06.033 DOI:10.1016/j.solmat.2010.08.011DOI:10.1002/aenm.201100552DOI:10.1039/c2nr31508d Silver nanowire inks are an upcoming class of commercially available conductive inks to fabricate front- or back electrodes using spray coating or blade coating methods. OPV devices with comparable parameters to their ITO based counterparts have successfully fabricated.DOI:10.1002/aenm.201300100DOI:10.1002/adfm.201202523 ITO-free conductive substrates are commercially available e.g. from infinityPV.com.

Charge selective layers

The most widely used hole transport layer is PEDOT:PSS, which is commercially available, e.g. from Hereaus Clevios and Agfa Orgacon, in a large variety of inks that have been tailored with respect to conductivity, viscosity, and surface tension for different application purposes and fabrication processes such as screen printing, inkjet, and slot-die coating. Typical PEDOT:PSS ink additives and solvents for ink customization are isopropanol, ethylene glycol, DMSO, and commercial wetting agents. PEDOT:PSS can also act as semi-transparent conductive layer and replace ITO.DOI:10.1002/ente.201200029DOI:10.1039/c2ee23244h

Zinc oxide (ZnO) is widely used as electron transport layer and can be fully R2R processed from acetone, water, and methanol.DOI:10.1021/am100505eDOI:10.1021/nn200933rDOI:10.3390/ma4010169DOI:10.1002/ente.201200029 Such inks are also commercially available, e.g. from infinityPV. Further metal oxides that can act as charge selective layer are nickel oxide (NiOx), molybdenum oxide (MoOx), vanadium oxide (VOx), and tungsten oxide (WOx) and can be processed from a variety of solutions.DOI:10.1039/c3tc30930d

Active layer

With respect to the processing of the active layers, chlorinated solvents such as chloroform, chlorobenzene, and 1,2-dichlorobenzene have been used in the majority of OPV reports and are also used for R2R processed polymer solar cells. Often they are used in blends including additional additives such as diiodooctance or chloronaphthalene to promote annealing and phase separation.

To achieve a more environmentally friendly and sustainable processing non-halagenated (chlorine free) solvents have been studied as well. Examples of solvents are m-xylene, o-xylene, toluene, and indane.DOI:10.1039/c4ra01783hDOI:10.1016/j.solmat.2011.09.059DOI:10.1039/c3ee41915k The main drawback is their poorer solubility of fullerene derivates and the tendency to produce less-than-ideal morphologies in the bulk heterojunction layer.DOI:10.1016/j.orgel.2012.08.026

Water as solvent is clearly the most environmental friendly and healthiest solution especially for large-scale processing. Direct solution of the most common active layer polymers in water are not possible and therefore emulsified solutions have shown promising results as it is possible to process active layer components.DOI:10.1016/j.solmat.2011.08.025DOI:10.1002/aenm.201000007DOI:10.1021/nn200933rDOI:10.1039/c3cp55037kDOI:10.1016/j.solmat.2012.03.016 The micelle stage only exists in solution and therefore the active layer is insoluble in aqueous solutions after drying of the solvent. Subsequent processing from water can be carried out without dissolving and damaging the layer.

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