Process Development for Inkjet Printing of Organic Photovoltaics

Due to the exceptional properties, such as flexibility and semitransparency, organic photovoltaics (OPV) is a topic of high interest in science and industry. These stand-alone characteristics enable applications like the integration in cloths, wearables or even buildings, which are out of reach for the market dominating silicon photovoltaics. As many of the OPV technology milestones, such as efficiencies > 15 %, have already been reached, industrially relevant production processes which perfectly suit the particular applications have to be developed. Requirements for such process are (i) a high throughput, (ii) low production costs, (iii) efficient use of material, (iv) free choice of colour and (v) form, (vi) the option for fast and easy layout changes and (vii) the possibility to coat on almost any substrate surface. As none of the established OPV fabrication processes is able to fulfil these conditions, new processing techniques have to be developed. In this context, Drop-on-Demand (DOD) inkjet printing seems to be a very promising approach because it completely satisfies the above specified processing requirements. Modern printing machines reach throughput speeds of more than 1000 m²/h. At the same time, the deposition of every single picoliter-sized droplet allows extraordinarily efficient use of raw materials as well as direct patterning of the layers in any arbitrary shape. Therefore, the aim of this thesis is to develop a sophisticated inkjet process for manufacturing of OPV on an industrially relevant scale. This comprises the ink formulation, inkjet deposition of functional layers and the demonstration of fully printed large area free-form solar modules. In the first part of the work, the necessary foundation for digital printing of OPV is provided. This includes the development of electronic inks and printing studies on manufacturing of homogeneous layers. By applying the Ohnesorge theory, stable inks are designed, which offer satellite-free drop formation. This comprises an alcohol based silver nanowire (AgNW) ink, which is applied for manufacturing of semitransparent electrodes. The successful printing is the first time proof that nanowires with a length of ~30 µm can be ejected through inkjet nozzles in the same size range. Furthermore, the alcohol based inks are environmentally non-critical and perfectly suited for the mass production of OPV. The water-free nature of the inks also enables the application in perovskite solar cells. Another highly important aspect of OPV fabrication is the layer homogeneity. To be able to print defect-free wet films also on low energy surfaces, such as the highly hydrophobic polymer active layers, the wetting properties of inks and substrates are investigated and modified with surface active agents. However, as shown by experiments, this strategy does not always result in defect-free layers and well working solar cells. With the aim to provide a process that allows printing of homogeneous wet films on almost every surface, an alternative strategy, relying on inkjet printed anchoring points, is developed. These so-called ‚pinning centres‘ fix the wet film at the desired position on the substrate, thus preventing contraction or rupturing. Local modification of the substrate surface with pinning centres results in a precise spatial definition of wetting as well as dewetting areas. This allows convenient layer patterning with a resolution of ~6 µm. The second part of the work describes the development of stable and scalable printing processes for every single layer of the OPV structure. Starting with a well working reference device, stepwise replacement of the single blade coated layers with inkjet printed equivalents is demonstrated. Inkjet printed silver grids or AgNW layers are applied as semitransparent electrodes. Solar cells with an optimised silver grid base electrode reach efficiencies up to 5.66 %. This value is significantly higher than literature reports of similar structures. Even better results are achieved by applying digitally printed AgNW mesh electrodes. The visually homogeneous layers have an excellent balance of sheet resistance and transmittance (Rsq 90 %). Devices with AgNW-cathode and -anode reach efficiencies of 4.3 %, which is up to now the record for fully inkjet printed OPV. The third part of the work describes the transition from small scale solar cells to large area fully inkjet printed solar modules. For the first time, solar modules with four inkjet printed layers are demonstrated, reaching a geometrical fill factor (GFF: ratio of active module area to total module area) of 85 % and efficiency of 4.3 %. Furthermore, a novel strategy to realise the monolithical cell-to-cell interconnection without visually obstructive gaps in the active layer is introduced. This is achieved by inkjet printing of ‘silver bridges’, which penetrate subsequently applied layers, thereby forming a visually inconspicuous contact between top and base electrodes of adjacent cells. Applying this technology in combination with inkjet printed AgNWs and differently coloured active materials, solar modules of unique design with sizes up to 150 cm² are demonstrated for the first time. The last subchapter of the work describes the transfer of the developed processes from laboratory scale to a high-throughput single pass inkjet printer. The machine is equipped with four print stations, which contain four printheads each, ink circulation, hot air- as well as infrared (IR) drying stations and enables the deposition of all solar cell layers at throughput speeds of up to 5 m²/min. Fully inkjet printed solar modules with an active area of 10 cm² reach efficiencies of up to 4.7 %, which is even higher than the single cells with AgNW electrodes (4.3 %) that held up to now the efficiency record for fully inkjet printed devices.