Affordable light-trapping metamaterials for thin-film photovoltaic cells

The thesis reports on my doctoral studies in the field of light-trapping structure for planar multilayer thin-film photovoltaic diodes. In this report we consequently pass from plasmonic light-trapping structures enhancing the PV absorption of photodiodes in a narrow band to broadband all-dielectric light-trapping structures which were fabricated and successfully tested experimentally.  The first part of the thesis is focused on a novel regime of perfect absorption in a thin plasmonic layer that corresponds to a collective mode of a plasmonic nanosperes array. In the theoretical study we show that the absorption of the incident light occurs mainly in the semiconductor material hosting plasmonic nanospheres, whereas the absorption in the metal is negligible. The regime remains the same when the uniform host layer is replaced by a practical photovoltaic cell. Trapping the light allows the thickness of the doped semiconductor to be reduced to such values that the degradation under light exposure becomes insufficient. The light-trapping regime is compatible with both variants: the metal-backed photovoltaic cell and its semitransparent counterpart when both electrodes are made of a conductive oxide. Negligible parasitic losses, a variety of design solutions and a reasonable operational band make our perfect plasmonic absorbers promising for photovoltaic applications.  The second part of the thesis is devoted to synthetic perovskites with photovoltaic properties that opens a new era in solar photovoltaics. Due to high optical absorption perovskite-based thin-film solar cells are usually considered as absorbing solar radiation fully under conditions of ideal blooming. However, actually this assumption does not hold. In this part of the thesis we show that it is possible to cure this shortage by complementing the basic structure with an inexpensive plasmonic array of nanospheres.  The last research problem studied in the thesis is the dielectric metamaterial as an efficient light-trapping structure. It is shown theoretically that this metamaterial can decrease the reflection and simultaneously suppress the transmission through the photovoltaic layer because it transforms the incident plane wave into a set of focused light beams. This theoretical concept has been strongly developed and experimentally confirmed in the experimental work. Also the experiments show that a submicron layer of a transparent conducting oxide may act as a top electrode of a photovoltaic cell based on amorphous silicon when properly patterned by notches becomes an efficient light-trapping structure. The nanopatterning is achievable in a rather easy and affordable way that makes developed method of the solar cell enhancement attractive for industrial adaptation.