Spectral Shaping by Radiative Energy Transfer in CsPbBr₃ Nanocrystal-Dye Mixtures

Spectral shaping is a technique to spectrally focus the broadband solar spectrum for diverse energy conversion applications from luminescent solar concentrators to horticulture. Fluorescent dyes have been used as optically active components due to their high photoluminescence quantum yield (PLQY), but their absorption range is limited. Nanocrystals offer broadband absorption but are typically limited in spectral shifting, causing photon recycling. Here, we investigate radiative energy transfer from CsPbBr₃ nanocrystals to Nile Red dye, combining the nanocrystals’ broadband absorption with the dye’s targeted emission. We use experimental transmittance and (time-resolved) photoluminescence (PL) spectroscopy together with photon random walk simulations to show that indeed radiative energy transfer occurs and that the resulting extended absorption range due to the nanocrystals can significantly enhance the spectral conversion efficiency. Experimentally, the energy transfer manifests in PL excitation spectra as strongly enhanced absorption and in time-resolved PL as a prolonged rise and decay time, reflecting the delay due to the extra absorption and emission processes. The photon random walk simulations account for the observed spectra quantitatively and allow prediction of conversion spectra for a wide range of nanocrystal and dye concentrations as well as their material parameters such as the PLQYs of the components. Specifically, we highlight the role of competitive absorption and the importance of taking the spectral intensity profile of excitation light into account when quantifying broadband energy transfer. These results open the door to tuning of absorption and emission spectra via the design of optimized compound mixtures for targeted spectral shaping applications.