|M.Sc Thesis||Department of Electrical Engineering|
|Supervisor:||Prof. Orenstein Meir|
|Full Thesis text|
Plasmon excitations, i.e. the collective oscillations of the conduction electrons in metals exhibit some unique features which can be harnessed to increase photovoltaic cell efficiency. Plasmonics may allow the use of thinner absorption layers yielding better carrier collection efficiency and reduced cost while maintaining sufficient optical thickness to absorb most of the incoming photons. Plasmonic structures can be utilized in various ways. First metallic nano-particles can be used as efficient sub-wavelength scatterers. By placing the particles on the front or back of a solar cell we increase the fraction of light scattered into the absorbing layers and increase the effective path length through oblique scattering and light trapping. Second, by embedding the nano-particles inside the cell, they act as nano-antennas - concentrating the light - resulting in a high near field intensity. Third, a corrugated metallic back contact can couple free propagating plane waves to propagating plasmon modes at the metal semiconductor interface, thus trapping them inside the cell.
This research included both theoretical and experimental studies of different types of solar cells. We started by theoretically examining the various possible absorption enhancement mechanisms in a Silicon solar cell by a front plasmonic structure. We demonstrated that the main enhancement is achieved by properly tuning the more efficiently scattering dipole and quadruple plasmon resonances. The plasmon local field enhancement had a marginal effect on the absorption efficiency.
Subsequently we examined the impact of embedding gold nano-particles in organic photovoltaic cells. Our study was based on a highly controlled fabrication method using electron beam lithography enabling precise control of particle array parameters. First - simulations of the entire cell were performed to analyze and optimize the array parameters; subsequently the optimized parameters were implemented experimentally with good correlation to the predicted performance. Both the experimental and theoretical results showed enhancement of external quantum efficiency in the green-NIR spectrum. This enhanced efficiency was shown to stem from field enhancement originating from both localized plasmonic resonances and periodic nano-patch antennas configuration.
we analyzed the effect of a plasmonic intermediate reflection layer on a tandem
micromorph cell. In this work we showed that by embedding spherical Ag
nano-particles in a low index buffer layer, we significantly
the amount of photons absorbed in the a-Si layer. At wavelength of
where a-Si is poorly absorbing, we increase the power
absorbed at the top layer from 20% -70%.