Photoluminescence efficiency of daytime luminescent materials determined under solar illumination

What is it about?

The materials efficiently luminescing under solar excitation (day-glo) are currently reaching high quantum yields and decent stability. This progress enables to broaden their application range from signalling and safety purposes to solar light harvesting (luminescent solar concentrators) or daytime radiative cooling of urban surfaces (mitigation of the urban heat island effect). Efficiency of emission is, obviously, the key parameter of day-glo materials In our research, we addressed the question of the appropriate characterization method of luminophores intended for applications under solar irradiation. The common laboratory testing cannot perfectly mimic the whole solar spectrum (solar simulators usually have bad characteristics in ultraviolet (UV) range and often very bad or missing infrared (IR) spectral range) and sometimes also the excitation power is much lower that the solar one (~1 kW/m2). Therefore, we developed and demonstrated a novel straightforward optical radiometry method for direct determination of power efficiency and quantum yield under the solar irradiation. The portable set-up is composed from two main parts: (a) the spectroscopy unit consisting of two fibre-coupled spectrometers for UV-visible and near-IR ranges and (b) the sample unit based on an integrating sphere and mounted on an equatorial astrophotography mount. The two units (called URSON – Universal Radiometer and Spectrometer for Outdoor applicatioN, LYRaE – Light Yield Radiometric Explorer) are connected by a bifurcated fibre bundle. The whole device is calibrated using a secondary standard of spectral irradiance. The sample is illuminated by Sun through a small hole and the efficiency is calculated simply from two spectral measurements under direct and indirect (diffused light) excitation.

Featured Image

Why is it important?

The spectral distribution of absorption and emission along with the absolute value of power efficiency and quantum yield are needed for developing the day-glo materials. In case of solar power harvesting or radiative-cooling applications these parameters are required for theoretical modelling and optimization. Recently, the potential application of daytime luminescent materials for radiative cooling of urban surfaces has been extensively investigated with the aim to mitigate the urban heat island effect. One may argue that the most efficient approach to reduce solar heating of surfaces is avoiding energy dissipation – i.e. making them highly reflective over the solar spectral range (~0.3–2.5 µm). However, such highly reflective layers would cause undesired visual disturbance (glare); therefore, the application of such materials is usually restricted to rooftop applications on tall buildings. In cases where we desire non-white, colourful surfaces, which inevitably must absorb part of the light in the visible range, the effective strategy to substantially reduce the dissipated energy is to transform absorbed photons to spectrally shifted luminescence photons. The design, fabrication and testing of such non-thermal luminescent radiative-cooling elements is the subject our collaboration between Charles University in Prague and University of New South Wales in Sydney.

Perspectives