Conventional luminescent solar concentrators suffer from high losses due to the large path length inside the device. Our system reduces these losses by using the free-space to collimate the light. We have developed an analytical model for the performance of those free-space luminescent solar concentrators. Our model takes all loss mechanisms into account and outputs the angle and wavelength dependent emission. Using realistic material parameters, we calculate the intensity emitted by the free space concentrator to be 1.5 times higher than that of a perfect diffuse reflector for all emission angles below 19 degrees.

We are presenting a new strategy to concentrate diffused light in free space based on luminophore doped waveguides with nanophotonic surface control. The efficiency of traditional luminescent solar concentrators (LSCs) has been limited due to loss mechanisms associated with every single component of the process, such as the luminophore quantum yield, reabsorption/emission rates, waveguide parasitic absorption, and unwanted escape. Here, we are proposing a paradigm shift to mitigate this issue: Instead of trapping light in waveguides, waveguides are designed to allow for escape under a specific escape cone.

Collimating and concentrating broad-band diffused light can increase the yield, decrease the cost, and open new opportunities for solar-generated electricity. Adherence to the second law of thermodynamics requires that collimation, and therefore the reduction of étendue or entropy, of diffused sunlight, i.e., light scattered by clouds or the atmosphere, can only occur if the photons lose energy during the process. This principle has been demonstrated in luminescent solar concentrators; solar photons are energetically down-shifted by a luminophore and the emitted photons are trapped within a transparent matrix and guided toward an edge lining solar cell. However, this process suffers from low efficiency as the photons are trapped within the waveguide for a long time, encountering many instances of accumulating loss mechanisms. Here, we theoretically describe and experimentally demonstrate the first free-space diffused light collimation system which overcomes these efficiency losses. The high photon energy solar spectrum is allowed to enter the system from all angles, whereas the re-emitted luminescent photons can only escape under a desired emission cone. We achieved this through doping a polymethylmetacrylate waveguide with Lumogen Red dye, which we cover on one side with a Lambertian reflector for photon recycling and induced randomization and on the top face with a complex multilayer dielectric nanophotonic coating stack. We experimentally found an angular concentration of 118% within the designed escape cone, where isotropic emission corresponds to 100%, thereby verifying the reduction of étendue in free space experimentally. Such free-space collimation systems will enable efficient redirection of sunlight toward solar panels, thereby increasing yield, decreasing heating through the emission of low energy photons, and expanding the range of available surfaces from which sunlight can be harvested.

In present study we investigate the effect of the external potential on a dye-sensitized NiO photocathode on the light-induced charge carrier dynamics by time-resolved photoluminescence and femtosecond transient absorption spectroscopy under operating conditions. Instead of the anticipated acceleration of photoinduced hole injection from dye into NiO at more negative applied potential, we observe that both hole injection and charge recombination are slowed down. We assign this effect to a variation in OH- ion concentration in the inner Helmholtz plane (IHP) of the electrochemical double layer with applied potential, showing that ion adsorption and desorption onto the NiO surface play an essential role as a relay in light-induced charge transfer and recombination. Our work highlights the key role of ions at the electrode surface and in the electrolyte in the realization of efficient solar to fuel devices.

We present free-space luminescent solar concentrators (LSCs), analytical performance calculations, and experiments. This will enable concentrating diffused light onto commercial solar modules mitigating performance challenges of conventional LSCs.