1 Department of Energy Conversion and Storage, Technical University of Denmark2 Functional organic materials, Department of Energy Conversion and Storage, Technical University of Denmark
The global energy consumption is increasing steadily while natural energy sources are running out sooner or later. Solar electricity is one of many renewable energy sources that contributes to the world’s demand. Organic solar cells (OPV) are an attractive 3rd generation solar technology that can be produced cheaply and very fast from solution with printing processes. The current research all around the world is still focused on lab-scale sized devices « cm2, ITO-glass substrates, and spin coating as the main fabrication method. These OPV devices are far from any practical application although record efficiencies beyond 10% could be achieved. This dissertation describes process workflows and roll-to-roll (R2R) fabrication methods for upscaling the OPV technology to solar module sizes that enable real power production even at efficiencies <2 %. The fundamental cell technology was based on flexible plastic substrates and ITO-free transparent conductive electrodes made from special designed flexo printed silver grids, rotary screen printed PEDOT:PSS, and slot-die coated ZnO (= Flextrode). The organic solar cell was fabricated by slot-die coating a light absorbing photoactive layer (e. g. P3HT:PCBM) on top of the Flextrode substrate and completed by rotary screen printed PEDOT:PSS and silver electrodes. All layers were R2R printed and coated from solution under full ambient vacuum-free conditions with fabrication speeds reaching 25mmin−1 for some of the layers. Fabrication of modules with high power output requires intelligent connection of single cells that should involve as less as possible manual processes such as wiring or soldering. The problem was solved by serially connecting thousands of single cells entirely during the R2R processing by printing thin-film silver conductors. High voltage networks require only thin conductors to efficiently transport the relatively low current of the organic solar cells. The serial connection was possible through a special designed pattern layout that combined 1-dimensional coating and 2-dimensional printing processes. The so-called Infinity concept allowed the fabrication of virtually infinitely large module sizes without manual wiring. High voltage modules with 21000 cells, open circuit voltage >10 kV and power output >220Wpeak could be successfully manufactured while having only two terminal contacts. Real energy production from these modules was studied by setting up a whole solar park based on OPV modules. Infinity modules with a length of 100m (width 0.3 m) were rolled out and taped onto a wooden structure. The maximum power output of six parallel-connected modules with a total active area of 88.2m2 was beyond 1.3 kW while having energy payback times P1 year. Alternative installation concepts such as a balloon or special designed solar tubes on land or water were proved to be functional as well. Solar tubes with Infinity modules of around 200W generated 18 kWh in 5 weeks. The energy was fed back into the Danish power grid. The dissertation contains a brief introduction of organic solar cell technology and reviews important R2R compatible manufacturing methods including photonic sintering. The fabrication, design, and challenges of Flextrode and Infinity modules are described in detail. The potential future energy production is presented through large-scale OPV installation scenarios and performance analyses. Fatal failures such as fully burned cells are described while easy repair mechanisms are shown that avoid costly replacements of full modules. A conclusion and outlook finalizes the dissertation.