Interfacial Energy and Charge Transfer for Photon Up- and Down-conversion

Seminar | October 2 | 4-5 p.m. | 120 Latimer Hall

 Sean Roberts, Materials Science & Engineering, University of Texas, Austin

 College of Chemistry

The negligible spin-orbit coupling in many organic molecules creates opportunities to alter the energy of excited electrons by manipulating their spin. In particular, molecules with a large exchange splitting have garnered interest due to their potential to undergo singlet fission (SF), a process where a molecule in a high-energy spin-singlet state shares its energy with a neighbor, placing both in a low-energy spin-triplet state. When incorporated into photovoltaic and photocatalytic systems, SF can offset losses from carrier thermalization, which account for ~50% of the energy dissipated by these technologies. Likewise, compounds that undergo SF’s inverse, triplet fusion (TF), can be paired with infrared absorbers to create structures that upconvert infrared into visible light. However, integrating materials that undergo SF or TF with existing electronics is challenging as this necessitates interfaces that can readily transfer spin-triplet excitations between these materials and bulk semiconductors, such as silicon. In this talk, I will summarize work investigating energy transfer dynamics in two key interfaces for SF and TF devices. First, I will describe work investigating how the local ordering of perylenediimide (PDI) molecules at a silicon surface impacts triplet energy transfer from PDI to silicon. We find small changes in the ordering of PDIs from their bulk structure due to strain at the silicon surface can shift their triplet energy by a few hundred meV, substantially impacting the driving force for triplet energy transfer. Strategies to control the structure of PDI films during growth will be discussed. Second, I will discuss measurements characterizing triplet energy transfer from near-infrared absorbing PbS nanocrystals to TF materials covalently bound to their surfaces. We find PbS nanocrystals are decorated with a number of surface states. While some of these states facilitate triplet energy transfer to surface-bound molecules, others hinder it by trapping excitations. Throughout the presentation, an important theme will be use of key spectroscopic techniques, such as broad-band electronic sum frequency generation (ESFG), to visualize interfacial states involved in energy transfer between organic and inorganic systems.

 Light refreshments will be served at 3:50 at The Coffee Lab, 510-643-0572