Organic photovoltaics represent the third generation PV technology, and it is a technology that offers large cost reductions using inexpensive materials and low cost printing fabrication techniques such as roll-to-roll processing. The lower power conversion efficiencies of organic solar cells, however, limit their viability and sustainability for cost competitive commercial production. Plasmonic assisted organic photovoltaics have a potential to increase the performance of devices and thus reduce the cost/kW of generating capacity. This is the motivation of the work presented in this doctoral thesis.
Metallic nanostructures are used to enhance the light absorption within the semiconductor film in organic solar cells. A new lithography-free method for nanostructure formation from conjugated polymer is presented in this work. A highly concentrated polyethyleneglycol-capped gold nanoparticles suspension from ethanol is drop casted on corrugated silicon-based organic polymer stamps for arranging the nanoparticles. Surface-ordered gold nanoparticle arrangements are hereafter integrated at the bottom lectrode of organic solar cells. The resulting optical interference and absorption effects are experimentally and numerically investigated in bulk heterojunction solar cells. In addition, the light absorption effects are numerically investigated as a function of size and periodicity of the plasmonic arrangements. Our study reveals the light harvesting ability of template-assisted nanoparticle assemblies in organic solar cells. Although the integration of light-trapping features and exploitation of metal nanostructure plasmonic effects are promising approaches for enhancing the power conversion efficiency of organic solar cells, one has to also consider nanostructureinduced electrical effects on the device performance. In this work, we exemplarily model the electrical properties of organic solar cells with rectangular-grating structures, as compared to planar reference devices. Based on our numeric results, we demonstrate that, beyond an optical absorption enhancement, the device fill factor improves significantly by introducing the grating structures. From the simulations we conclude that enhanced carrier collection efficiency is the main reason for the increased solar cell fill factor. This work contributes towards a more fundamental understanding of the effect of nanostructured electrodes on the electrical properties of organic solar cells.
Finally, we investigate the effect of periodic gold nano-triangle arrays on the performance of the organic solar cells. We fabricate the large-area gold nano-triangle arrays on ITO substrate using nanosphere lithography. The device with periodic gold nanostructures exhibits a broadband light absorption enhancement. In this work, the correlation between light absorption enhancements and enhanced device performance is investigated.
Together with the prospect of low production cost and the ease of scalability, we believe that the presented nanostructures in this thesis will have the potential to increase the efficiency of the solar cells and hold prospect to be beneficial in various optical and optoelectronic devices.
The University of Southern Denmark
Investigate the effects of localized surface plasmon structures on organic thin-film solar cells in order to increase their power conversion efficiencies. Modelling and fabrication of the plasmonic structures are performed at the Universidad Autonoma de Madrid in close collaboration with the related industrial partner Plasmore.
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