The excitonic nature of organic semiconductors (OSCs) facilitates unique spin conversion processes that are difficult to achieve with inorganic semiconductors. In particular, OSCs with low energy spin triplet exciton states can undergo singlet exciton fission (SF), a process where a high energy spin-singlet exciton divides its energy to prepare two triplet excitons. By producing two excited electrons from a single excitation, SF can effectively be used to downconvert high energy photons. Likewise, SF’s reverse process, triplet fusion (TF), can produce bright singlet excitations from nonemissive triplets. When properly employed, SF and TF can boost the efficiency of light harvesting applications. However, incorporating SF/TF-capable OSCs into devices requires knowledge of how the OSC’s packing structure impacts both exciton interconversion and energy exchange with other materials. Here, we report time-resolved photoluminescence and transient absorption experiments investigating perylenediimide (PDI) thin films. PDIs are highly absorbing, photostable molecules that possess the proper energetic structure to undergo SF. However, the electronic couplings that facilitate SF are strongly dependent on how these molecules pack in the solid state. While charge transfer interactions that aid SF can be maximized by slipping neighboring PDIs along their long axes, we find this induces an endothermic barrier to SF by precipitously lowering the PDI singlet exciton energy. Slipping PDI along their short axis, in addition to their long axis, mitigates this issue and permits SF to unfold over a few hundred picoseconds, sufficiently fast that it occurs with near quantitative efficiency. We also report measurements of hybrid inorganic-organic semiconductor constructs designed to produce triplet excitations from NIR radiation for TF applications. Specifically, we examine PbS quantum dots functionalized with covalently attached pentacene ligands. Prior work has shown photoexcited PbS quantum dots can transfer triplet excitons to surface bound molecules, but the transfer mechanism remains poorly understood. Here, we show localized PbS surface states act as a key intermediate that facilitates triplet exciton transfer. These states, formed during the ligand exchange process, are populated within a few tens of picoseconds following PbS photoexcitation. These surface states then decay over tens of nanoseconds to produce pentacene triplet excitons.

Manipulating energy and spin in hybrid organic-inorganic semiconductors

255th National Meeting (2018)

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