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Self-nanostructuring Polymer Solar Cells

Final Report Summary - SOLARPAT (Self-nanostructuring polymer solar cells)

Light absorbed by photoactive organic material produces excitons. Here, we have theoretically shown that the radiative lifetime of excitons in a typical solar cell geometry can increase or decrease orders of magnitude depending on the orientation of the exciton or exciton molecule. The conducting electrodes that sandwich conventional solar cells form an asymmetric and non-ideal cavity, in which there is only a partial wave-guiding of light. It is well-known that the spontaneous-emission rates are changed for an atom within a metallic cavity, however until this project, there had not been a study of the spontaneous-emission rates between a perfect mirror and a semi-transparent thin-metal layer, which is the prototypical architecture of a solar device.

In addition, we have investigated new design considerations for controlling exciton recombination. Sunlight is incident on and partially absorbed within the semi-transparent electrode and the exciton population in the active layer is governed by diffusion, absorption, and recombination. We model the performance of thin-film solar cells incorporating the effects of diffusion, accounting for partial sunlight reflections, and accounting for the partial transparency and finite conductivity of the upper electrode. The opaque electrode is modelled as an ideal mirror. Since in Schottky and bilayer-junction organic photovoltaic devices, this nanometre-scaled diffusion length is one of the limitations for achieving higher efficiency organic solar cells, our findings predict that high-efficiency devices may be achieved by employing fluorescent polymer materials in photovoltaic designs.

Sunlight produces excitons in the fluorescent active material, which diffuse towards the lower opaque electrode. If excitons recombine, a photon is emitted. There are large differences in the exciton behaviour depending on exciton position and on the emission polarisation, and subsequently we observe appreciable changes in the exciton diffusion current. The lifetime associated with dipoles aligned in the plane of the solar device increases sharply near the opaque electrode. Moreover, the exciton lifetime varies orders of magnitude depending on its position within the thin film.

To experimentally-verify our calculations of the radiative lifetime, we have worked towards achieving a high-quantum yield fluorescent material by separating rhodamine dye molecules with a polymer background. This approach yields a factor of 20 increase in the fluorescence quantum yield fluorescence, accompanied by a decrease in the dimer absorption. Further experiments are underway to incorporate this novel polymer into a solar device.

In conclusion, we show that by controlling the spontaneous-emission rate for excitons generated between an ideal mirror and a semi-transparent thin semitransparent film, the exciton diffusivity can be enhanced by as much as a factor of five. In organic solar cells whose active material is fluorescent, there is significant potential for reducing recombination losses by employing nanostructures to inhibit spontaneous emission. This work provides an important calculation that may be used for characterising self-assembly and nanocrystal patterning in fluorescent polymer materials. By quantifying the primary influence of exciton or organic molecule alignment on solar devices, we provide a theoretical foundation for understanding self-assembled crystals in organic thin films. Moreover, our work may enable new diagnostic methods for studying the morphology of polymer films.

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