Our research group focuses on nanostructured surfaces that increase the optical absorption and thereby the efficiency of solar cells. An example is ‘black silicon’, which is simply nanostructures etched into silicon in a dry, 1-step process called ‘maskless reactive ion etching’. By tuning process parameters like temperature, pressure, gas flows, power and time we can produce a broad range of structures from 100 nm conical pillars to 10 µm pyramids and cylinders. These surfaces suppress the optical reflectance loss from solar cells to below 1% over the entire relevant solar spectrum and for a broad range of non-ideal incident angles, which is relevant in real operation (clouds, dust, daily variation etc.).
We are also focusing on the electrical properties of solar cells, primarily by using a laser to define local regions on the surface with higher conductivity, so-called laser doping. A laser can locally melt silicon and if dopant atoms such as phosphorus, aluminum and boron are in the vicinity of the molten region, the material can be highly doped, which improves the electrical properties of the solar cell. We are collaborating with the Department of Physics at AU and DTU Photonics on novel ways to laser-dope silicon and make improved solar cells using lasers
A third branch within solar cell research is the tandem or triple-junction solar cells, where two or three cells are serial-connected vertically in order for each sub-cell to absorb a dedicated part of the light and the total cell voltage becomes the sum of sub-cell voltages. This is a global research focus, but we focus specifically on all-silicon triple-junction cells, combining ultra-thin layers of amorphous silicon on top of a conventional (thick) Si bottom cell. Our ‘black silicon’ nanostructures may also have benefits for perovskite-Si tandem solar cells, which we are currently investigating.
A different branch of our research focuses on photovoltaic retinal implants for blind patients. Millions of patients worldwide are blind due to degenerated photoreceptors. This includes patients suffering from AMD (age-related macular degeneration) and retinitis pigmentosa. Since photoreceptors work somewhat like solar cells; by creating small electrical signals to the brain via the optical nerve from the incoming light, we aim to mimic the photoreceptor function in an implantable device with thousands of small solar cell units. We are currently fabricating the 2nd version of the chip – in close collaboration with DTU Nanolab – and continuously testing the functionality on fresh retinal tissue from pigs at Aarhus University Hospital. Currently, we are testing a simple version of the chip, made for electrical stimulation, on tissue at AUH. The aim is to quantify the required voltages in order to produce action potentials from the retinal neurons. This enables us to optimize the photovoltaic implant and ensure that it delivers the required voltage, while maximizing the number of “pixels” on the device.
The chip contains 3D electrodes made from biocompatible carbon and we are working on passivating the sidewalls of those carbon pillars in order to confine the stimulation current to the tip of each pillar electrode.
