Photon Statistics as a Tool to (Dis)Prove Cooperative Energy Transfer Quantum Cutting in Near-Infrared Emitting Materials

Spectral conversion of sunlight can enhance solar cell efficiencies by shifting short-wavelength photons to longer wavelengths, where the photovoltaic response is stronger. This can be realized with near 100% energy efficiency by materials capable of quantum cutting. These emit two longer-wavelength photons, following the absorption of one shorter-wavelength photon. Although various quantum-cutting materials have been reported, follow-up research has often disproved initial claims. Here, we demonstrate that photon correlation analysis is a universal strategy to unambiguously identify quantum cutting. We investigate two materials, YPO4 codoped with Tb3+ and Yb3+ and yttrium aluminum garnet (YAG) codoped with Ce3+ and Yb3+. Both are reported to perform quantum cutting via blue light absorption followed by cooperative energy transfer to near-infrared-emitting Yb3+, but some studies have cast doubts for YAG:Ce3+,Yb3+. Our correlation analysis demonstrates bunched emission for YPO4:Tb3+,Yb3+, confirming quantum cutting. In contrast, YAG:Ce3+,Yb3+ shows Poissonian emission statistics, disproving its quantum-cutting capability, despite previous claims.