Khan, I. (2017). Light management in organic photovoltaic solar cells [Dissertation, Technische Universität Wien]. reposiTUm. http://hdl.handle.net/20.500.12708/79914
Organic Light Emitting Diodes; Organic Solar Cells; Light Management
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Abstract:
Sunlight is arguably the most abundant resource of renewable energy. Photovoltaic technology offers a sustainable and environmentally friendly way for the conversion of sunlight energy into electricity. Organic photovoltaic (OPV) solar cells represent next-generation photovoltaic technology that utilizes materials that can be synthesized at low cost and are compatible with mass production via large-area roll-to-roll type processing techniques on flexible substrates. To achieve high-efficiency OPV solar cells, one of the key issues is to reduce their optical lasses. ln particular, the incomplete absorption of light in the photoactive layer (which is inherently very thin due to the low charge-carrier mobility and small exciton diffusion length) and optical reflectance due to the abrupt change in the refractive index across the layer structure are the major contributors. This work mainly concerns the optical engineering at the OPV solar cell interface in order to circumvent these problems. Firstly, improved light trapping in a thin photoactive layer is pursued by using a novel absorber that is based on a phenomenon called surface plasmon resonance. This strategy enables the utilization of photoactive materials with short carrier diffusion length such as those that are used in common OPV solar cells architectures. Th is thesis presents a novel approach to plasmanie absorber, which offers the advantage of light harvesting over broad range of wavelengths and angles of incidence. lt takes advantage of a corrugated multi-diffractive metallic grating that can simultaneously serve as metallic electrode. The structure comprises multiple superimposed periodical modulations that can be prepared by facile means such as nanoimprint lithography. The developed multi-periodie grating enhances the light absorption in the investigated active layer P3HT: PCBM by a factor of 2.9 in the sp ectral window 600-750 nm where this material is inherently weakly absorbing. Over the whole visible!NIR part of the spectrum (400-750 nm) the number of absorbed photans in P3HT: PCBM photoactive layer is increased by 28 %. Secondly, reduction of the optical reflectance at OPV layer interfaces is investigated by using tailored antireflection (AR) periodic nanostructure s. ln this work AR nanostructures were deployed at transparent indium tin oxide (ITO) electrode by a combination of Iaser interference lithography, soft lithography and DC magnetron sputtering process. The structure was tailored to provide broadband AR characteristics in the visible and near-infrared region by supp ressing the Fresnel reflection with densely arranged nanostructures. The light transmission in the 450-850 nm range was enhanced by 8% (absolute) compared to flat ITO films, which is one of the largest performance improvements reported in the Iiterature for nanostructured transparent electrodes . Thirdly, alternative structures to ITO transparent electrodes with additional light harvesting features were explored based on multi-diffractive arrays of plasmanie nano-wires. Contrary to the majority of state-of-the art approaches that employ complex fabrication methods such as focused ion beam milling or electron beam lithography techniques , the presented work utilizes UV Iaser interference lithography with dry etching step that allows for facile structuring of Iarge areas. Detailed investigation of competing light trapping and transparency is carried out and arrays of gold nanowires with com parable transparency and resistance to regular ITO films were prepared. Finally, the prepared nanostructures were also exploited outside the OPV domain through other collaborative projects relating to biosensing. For example gold and aluminium nanostructures prepared through the course of this thesis were used in a smartphone label-free biosensor for the detection of lipopolysaccharide s, for surface enhanced Raman spectroscopy studies, and for UV surface plasmon spectroscopy sensing.