Abstract
The efficiency of crystalline silicon solar cells is limited to ~30%, mainly due to spectral mismatch losses: low energy photons are not absorbed (transmission losses), while high energy photons create a hot electron-hole pair that relaxes to the band edge (thermalization losses). To reduce these losses upconversion and downconversion are
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promising options. Downconversion is the process where two low energy photons are added to one higher energy photon that will be absorbed, and downconversion is the proces that cuts one high energy photon to two lower energy photons that still can be absorbed. It has been shown theoretically that with ideal up- or downconversion materials the efficiency of solar cells can be increased to 50% and 40%, respectively. Lanthanide doped materials are a promising class of materials for both upconversion and downconversion, due to their rich energy level structure and sharp atomic like optical transitions. In this thesis we present the results on downconversion studies on lanthanide ion couples in different host lattices. We found that efficient downconversion in the Er-Yb couple is possible. Previous research showed that downconversion in this couple is prevented by multi-phonon relaxation between closely spaced energy levels. By using a low phonon energy host material, multi-phonon relaxation becomes slow and downconversion is efficient. For the Nd-Yb couple, low conversion efficiencies were also attributed to multi-phonon relaxation, but using this couple in a low phonon energy host lattice shows that the efficiency is still low due to an energy mismatch between the transitions on the Nd and the Yb ions. For the Pr-Yb couple we studied the energy transfer mechanism responsible for downconversion in this couple. By comparison of experimental luminescence decay curves with modeled decay curves, we found that a first order two step energy transfer mechanism is responsible for downconversion. Furthermore we investigated if coupling to resonant plasmon modes can play a role in enhancing the conversion efficiency. We studied the optical properties of metal nanoparticles that were deformed from spheres to rods using MeV silver ions, and of monolayer protected gold clusters (Au38). The interaction between lanthanides and metal nanoparticles was studied using a modelsystem consisting of a gold-silica core-shell particle, with europium complexes incorporated in an amphiphilic layer around the particle. By varying the silica shell thickness, the distance between the europium and the gold particle was varied between 7 and 20 nm. We found that the decay rate was enhanced up to a factor of five for the shortest distance between the gold and europium
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