Photon Management Through Energy Transfer in Halide Perovskite Nanocrystal–Dye Hybrids: Singlet vs Triplet Tuning

ConspectusPhotoinduced energy and electron transfer processes offer a convenient way to convert light energy into electrical or chemical energy. These processes remain the basis of operation of thin film solar cells, light emitting and optoelectronic devices, and solar fuel generation. In many of these applications, semiconductor nanocrystals that absorb in the visible and near-infrared region are the building blocks that harvest photons and initiate energy or electron transfer to surface-bound chromophores. Such multifunctional aspects make it challenging to steer the energy transfer pathway selectively. Proper selection of the semiconductor nanocrystal donor requires consideration of the nanocrystal bandgap, along with the alignment of valence and conduction band energies relative to that of the acceptor, in order to achieve desired output of energy or electron transfer.In this Account, we focus on key aspects of managing energy flow from excited semiconductor nanocrystals to surface-bound chromophores. The singlet and triplet characteristics of the semiconductor nanoparticle enable tuning of energy transfer pathways through bandgap engineering. In addition to the alignment of energy levels between the semiconductor donor and the singlet/triplet energy levels of the acceptor dye, other parameters such as spectral overlap, surface binding through functional groups, and rate of competing energy transfer pathways all play integral roles in directing energy transfer. For example, in a prototypical halide perovskite nanocrystal-rhodamine dye assembly, singlet energy transfer is observed when the donor is a high-bandgap semiconductor (e.g., CsPbBr₃, <i>E</i>_g= 2.47 eV). However, when the donor is a low-bandgap semiconductor (e.g., CsPbI₃, <i>E</i>_g = 1.87 eV), one observes only triplet energy transfer. Tuning of the donor bandgap with mixed halide perovskites (e.g., CsPb(Br_<i>x</i>I_1-<i>x</i>)₃) allows for populations of both singlet and triplet excited states of the acceptor dye. Additionally, triplet characteristics of the donor semiconductor nanocrystal can be further enhanced through Mn doping which places low-energy triplet-active states within the nanocrystal donor.The ability to steer energy transfer pathways in a semiconductor nanocrystal-dye assembly finds its use in the design of semiconductor-multichromophoric films. Such hybrid films can <i>down-shift</i> or <i>up-convert</i> incident photons and deliver emission at desired wavelengths. By selecting high energy donor (e.g., CsPbBr₃) one can <i>down-shift</i> the incident photons through energy transfer cascade, as in the case of the CsPbBr₃-rubrene-tetraphenyldibenzoperiflanthene (DBP) system to populate singlet excited DBP (perylene derivative). On the other hand, when the donor energy is low as in the case of CsPbI₃-rubrene-DBP, one can populate singlet DBP via triplet-triplet annihilation. Thus, by steering energy transfer pathways, it is possible to manage the photon flow and obtain desired emission output. Fundamental understanding of excited state processes responsible for energy transfer will assist in designing light harvesting assemblies that can manage photon delivery effectively in display devices and other optoelectronic devices.