Photon Upconversion-Driven Charge Injection in CsPbI ₃ -Rubrene-TiO ₂ Films

Maximizing photoconversion efficiency in light-harvesting assemblies necessitates the effective capture of both visible and near-infrared photons. We integrated a CsPbI3-rubrene photon upconversion system to extend the spectral response of a dye-sensitization process. Excitation of the CsPbI3 film facilitates triplet energy transfer to rubrene, which subsequently generates excited singlet through triplet–triplet annihilation. Notably, charge injection into TiO2 is observed exclusively from the rubrene singlet excited state (ES = 2.23 eV), and not from the triplet state (ET = 1.14 eV) as it lacks sufficient energy to drive the injection process. In this architecture, rubrene serves a dual purpose: it acts as an annihilator for upconverting low-energy photons and participates directly in the dye-sensitization of TiO2. Mechanistic insights into the energy transfer steps and the kinetics of charge injection are elucidated using photoluminescence and transient absorption measurements. The ability to harvest low-energy photons is useful for enhancing the performance of solar cells and other light-energy conversion devices.