In this work, we probe the relationship between perovskite material microstructure and the nature of the photogenerated polarons. We prepare formamidinium lead iodide quantum dot films in two forms with distinctive inter-QD interactions. Using a combination of time-resolved spectroscopies, we observe a competition between carrier thermalization into polaronic excitons and ultrafast hot-carrier transport between QDs, with the balance between channels set by the excess photon energy and connectivity between QDs. These results underscore the importance of microscopic structure in steering the ultrafast processes which determines the fate of photoexcitations in perovskite materials.
The hybrid light-matter character of exciton-polaritons enables strikingly long-range energy transport in organic materials. We use femtosecond transient absorption microscopy to probe this behavior in the initial coherent regime, where photon and exciton wavefunctions are inextricably mixed. We achieve rapid polaritonic transport in highly ordered, pure organic semiconductor films without any external cavity. In a disordered system, confinement within a Fabry-Perot cavity provides enables comparable coherent transport effects. In both cases, the polaritons don’t travel alone: they are accompanied by intracavity dark states, which reduce the transport velocity, extend the lifetime, and provide a new mechanism for external control.
We report the synthesis of a new series of oligomers derived from dibenzo[a,i]pyrene. The compounds were characterized by steady-state and transient absorption spectroscopies. The results indicate that the compounds fluoresce from a formally dark S1 electronic state that is enabled by intensity borrowing from a neighboring bright S2 state. While the monomer exhibits a relatively low photoluminescence quantum yield (PLQY), the dimer exhibits a significantly enhanced PLQY due to a greater S2 intensity borrowing. Moreover, symmetry-breaking charge transfer in the dimer was demonstrated by investigations in solvents of different polarity.
The interaction of organic semiconductors with confined light fields offers one of the easiest means to tune their material properties. In the regime of strong light-matter coupling, the semiconductor exciton and cavity photon mode hybridize to form new 'polariton' states. In organic systems these light-matter hybrids are tuneably separated by as much as 100’s of meV from the parent exciton, enabling radical alteration of the energetic landscape. The effects of strong coupling can be profound, including reports of long-range energy transfer, enhanced carrier mobility and altered chemical reactivity. Theoretical work is now increasingly focused on the potential of polariton to manipulate electronic dynamics in the excited state, but experimental realisation has proved challenging. Here, we demonstrate the ability to manipulate triplet photophysics in singlet exciton fission materials in the strong coupling regime. Within microcavities, we dramatically enhance the emission lifetime and increase delayed fluorescence by >100%, which we explain through a shift in the thermodynamic equilibrium between dark states in the exciton reservoir and the bright polaritons. Indeed, with this approach we can create entirely new radiative pathways, turning completely dark states bright and opening new scope for microcavity-controlled materials.
The standard view of singlet exciton fission in organic semiconductor is that one photon creates a singlet exciton which subsequently decays into a correlated triplet pair state (TT) multiexciton states. The triplet pair state then splits to form two free triplets. Although the theoretical description of (TT) is well developed since 1970, it has so far proved difficult to determine the role and nature of the (TT) state in solid films from experiment directly. Here, using a combination of highly sensitive broadband transient absorption and photoluminescence spectroscopies on a range of polyacene films, we demonstrate that the (TT) multiexciton states is bound and energetically stabilised with respect to free triplets in even the most efficient singlet fission materials, such as TIPS-pentacene and pentacene. The (TT) multiexciton state is emissive, and we find that charge-transfer from one (TT) state to the neighboring electron acceptors has a yield of >100%, i.e. more than one charge is transferred per charge-transfer event. Our findings suggest that the formation of spin-correlated (TT) states emits as one particle and generates 2 charges in organic solar cells and thus open a range of fascinating questions regarding the potential to use entanglement to enhance organic photovoltaic efficiency and the application of organic materials in quantum information
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