Hybrid Solar Cells and Hybrid LEDs

This project concerns itself with solar cells and light emitting diodes with the focus on their potential to be hybridised with colloidal nanocrystals. This has the ultimate goal of improving the solar conversion efficiency of the solar cells and to achieve efficient colour conversion in the light emitting diodes. The solar cells intended for hybridisation are based on crystalline silicon with the junction of the cell near the back of the cell. Interdigitated Back Contact (IBC) solar cells have their junction deep inside the device, which makes it necessary for the generated charge carriers to diffuse a relatively long distance in order for them to be collected by the contacts. Since a large portion of the spectrum of the light will be absorbed near the surface of the cell, IBC solar cells are particularly sensitive to the level of recombination that occurs near the front of the cell. This makes passivation of the front surface a key issue for the efficient conversion of light to electricity in these cells. Much of the work involved in this project was carried out to achieve a sufficient level of passivation after processing. Once the Quantum Dots (QDs) have been applied to the cell, the incident light will be absorbed by both the QDs and the bulk of the cell, with the generated charge carriers needing to be collected by the rear junction. QDs, when applied to a InGaN/GaN based blue LED will absorb some of the light emitted by the LED. Part of this absorbed energy will then be re-emitted by the QDs with the emission wavelength depending on the size and the specific type of QDs. One approach investigated during this project was the use of a wavelength dependent filter, which was to be placed on top of the layer of QDs on the LED. This would ideally achieve that the unabsorbed light gets blocked by the filter and reflected back onto the QD layer, thereby increasing the total amount of the absorbed light intensity. An important aspect of colour conversion materials for LEDs is the level of photoluminescence degradation under prolonged illumination. Different ways of mitigating this degradation process were investigated for the QDs employed in this project. This project aims to contribute to the knowledge of hybrid approaches to solar cells and LEDs and to provide information on the feasibility and potential of these devices.