This work reveals the role that dynamic disorder of the crystalline lattice of halide perovskite materials has on the electronic states involved in charge-carrier transport and recombination. We demonstrate that this dynamic disorder leads to localization of charge carriers into states known as large polarons, which reduces their mobility but greatly extends their lifetime. This work lays the foundation for designing perovskite materials with tunable transport versus recombination properties for high-performance photovoltaics and light-emitting devices.

Singlet fission is an exciton multiplication mechanism in organic materials whereby high energy singlet excitons can be converted into two triplet excitons with near unity quantum yields. As new singlet fission sensitizers are developed with properties tailored to specific applications, there is an increasing need for design rules to understand how the molecular structure and crystal packing arrangements influence the rate and yield with which spin-correlated intermediates known as correlated triplet pairs can be successfully separateda prerequisite for harvesting the multiplied triplets. Toward this end, we identify new electronic transitions in the mid-infrared spectral range that are distinct for both initially excited singlet states and correlated triplet pair intermediate states using ultrafast mid-infrared transient absorption spectroscopy of crystalline films of 6,13-bis­(triisopropylsilylethynyl) pentacene (TIPS-Pn). We show that the dissociation dynamics of the intermediates can be measured through the time evolution of the mid-infrared transitions. Combining the mid-infrared with visible transient absorption and photoluminescence methods, we track the dynamics of the relevant electronic states through their unique electronic signatures and find that complete dissociation of the intermediate states to form independent triplet excitons occurs on time scales ranging from 100 ps to 1 ns. Our findings reveal that relaxation processes competing with triplet harvesting or charge transfer may need to be controlled on time scales that are orders of magnitude longer than previously believed even in systems like TIPS-Pn where the primary singlet fission events occur on the sub-picosecond time scale.

Triplet population dynamics of solution cast films of isolated polymorphs of 6,13‐bis(triisopropylsilylethynyl) pentacene (TIPS‐Pn) provide quantitative experimental evidence that triplet excitation energy transfer is the dominant mechanism for correlated triplet pair (CTP) separation during singlet fission. Variations in CTP separation rates are compared for polymorphs of TIPS‐Pn with their triplet diffusion characteristics that are controlled by their crystal structures. Since triplet energy transfer is a spin‐forbidden process requiring direct wavefunction overlap, simple calculations of electron and hole transfer integrals are used to predict how molecular packing arrangements would influence triplet transfer rates. The transfer integrals reveal how differences in the packing arrangements affect electronic interactions between pairs of TIPS‐Pn molecules, which are correlated with the relative rates of CTP separation in the polymorphs. These findings suggest that relatively simple computations in conjunction with measurements of molecular packing structures may be used as screening tools to predict a priori whether new types of singlet fission sensitizers have the potential to undergo fast separation of CTP states to form multiplied triplets.

The ligand shell around colloidal quantum dots mediates the electron and energy transfer processes that underpin their use in optoelectronic and photocatalytic applications. Here, we show that the surface chemistry of carboxylate anchoring groups of oleate ligands passivating PbS quantum dots undergoes significant changes when the quantum dots are excited to their excitonic states. We directly probe the changes of surface chemistry using time-resolved mid-infrared spectroscopy that records the evolution of the vibrational frequencies of carboxylate groups following excitation of the electronic states. The data reveal a reduction of the Pb–O coordination of carboxylate anchoring groups to lead atoms at the quantum dot surfaces. The dynamic surface chemistry of the ligands may increase their surface mobility in the excited state and enhance the ability of molecular species to penetrate the ligand shell to undergo energy and charge transfer processes that depend sensitively on distance.

Ultrafast vibrational spectroscopy in the mid-infrared spectral range provides the opportunity to probe the dynamics of electronic states involved in all stages of the singlet fission reaction through their unique vibrational frequencies. This capability is demonstrated using a model singlet fission chromophore, 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-Pn). The alkyne groups of the TIPS side chains are coupled to the conjugated framework of the pentacene cores, enabling direct examination of the dynamics of triplet excitons that have successfully separated from correlated triplet pair states in crystalline films of TIPS-Pn. Relaxation processes during the separation of triplet excitons and triplet-triplet annihilation after their separation result in the formation of hot ground state molecules that also exhibit unique vibrational frequencies. Because all organic molecules possess native vibrational modes, ultrafast vibrational spectroscopy offers a new approach to examine the dynamics of electronic intermediates that may inform ongoing efforts to utilize singlet fission to overcome thermalization losses in photovoltaic applications.