Triplet generation at a hybrid inorganic/organic semiconductor interface is a very promising approach to increase the (photo-)excited state recombination lifetime, and thus, facilitate solar energy harvesting. Possible applications using the generated spin-triplet excitons are photon upconversion and photocatalysis.
Photon upconversion describes the process of shortening the wavelength of the light emitted after irradiation, resulting in a net gain in photon energy. Here, upconversion occurs by combining multiple low energy photons to a single high energy photon through a process called triplet-triplet annihilation. Since direct optical excitation of triplet states is ‘spin-forbidden’, so-called sensitizers are required to indirectly populate the triplet state by energy or charge transfer.
Currently, triplet sensitizers span a broad range of material classes including metal-organic complexes, nanomaterials, and bulk perovskite films. Understanding the fundamental energy transfer mechanism is crucial for the advancement of optoelectronic devices based on this process. The exact triplet sensitization mechanism varies depending on several factors including: (i) the absolute alignments of the sensitizer and acceptor energy levels. (ii) The exciton binding energy in the sensitizer, resulting in excited states in form of excitons or free carriers. (iii) The local trap density, which can impact doping levels and band bending. Here, I will present the current understanding of triplet generation at the bulk perovskite/organic interface.