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Manipulating Energy and Spin in Molecular Semiconductors

Event Details

Event Dates: 

Friday, April 27, 2018 - 4:00pm

Seminar Location: 

  • JILA Auditorium

Speaker Name(s): 

Sean Roberts

Speaker Affiliation(s): 

University of Texas at Austin
Seminar Type/Subject

Scientific Seminar Type: 

  • Phys Chem/Chem Phys Seminar

Event Details & Abstract: 

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 hybrid structures that upconvert infrared light into the visible range. However, integrating materials that undergo SF or TF with existing electronics remains challenging as the efficacy of these processes depends strongly on how molecules order in the solid state. I will summarize work aimed at identifying critical structure‐function relationships that guide SF within perylenediimide (PDI) films. By adding functional groups at key locations along the PDI backbone, we can force these molecules to adopt specific structures in the solid state. Guided by electronic structure calculations, we have used this approach to optimize the electronic coupling between PDIs such that they undergo SF with near quantitative efficiency. In addition, I will discuss the process of transferring triplet exciton pairs from PDI films to silicon. Silicon is an ideal triplet acceptor due to its dominance within the solar marketplace and the energetic matching of its bandgap (1.1 eV) with the PDI triplet energy (~1.2 eV). However, band misalignment between these materials can produce a type II heterojunction that facilitates charge transfer rather than exciton transfer. In this scenario, separated carriers can experience a binding interaction at the silicon:PDI interface that drives them to recombine. Using electronic sum frequency generation (ESFG), an interface selective technique, we map how the electronic structure of both silicon and PDIs are modified at their junction. Preliminary data suggests PDI crystallites experience significant strain when deposited on solid substrates such as silicon, which can narrow their bandgap. Implications for exciton extraction will be discussed. 

Research Site: https://robertslabut.weebly.com/group-members.html