Harnessing spin and exchange interactions in organic semiconductors (08/08/23)

Speaker and Affliation:

Dr. Ashish Sharma,
Cavendish Laboratory, University of Cambridge, UK


08th August, 2023 (Tuesday), 04:30 PM (India Standard Time)


KPA Auditorium, Department of Materials Engineering, Indian Institute of Science (IISc)


Organic semiconductors (OSCs) based on molecular or polymeric π - conjugated systems are being explored for applications in a range of optoelectronic devices, ranging from transistors and light-emitting devices to solar cells. The electronic properties of OSCs derive from their π valence and π* conduction orbitals meaning that the parameter space to engineer semiconductor phenomena can be controlled through molecular design - this enables immense tunability of electronic and functional properties of OSCs. Of special relevance for optoelectronic devices is the spin exchange energy, that governs the energetic difference between the spin-singlet and spin-triplet excitons. Organic Iridium based complexes, thermally activated delayed fluorescence (TADF) materials, multi-resonant TADF systems and triplet fusion based blue emitters are some examples where the spin and exchange interactions are optimized through molecular design to achieve efficient organic light-emitting diodes (OLEDs), as used in smart phone and TV displays.

Recently there is an expanding interest in the process of interconversion between a singlet exciton and a pair (spin-zero) of triplet excitons, which is energetically possible when the exchange energy brings the triplet exciton to about half the singlet exciton energy. The forward process, termed as singlet fission (SF), has the potential to improve the efficiency of photovoltaics beyond the single-junction limit. While a number of SF-capable OSCs have been identified in the last decade, controlling the fate of triplet excitons generated from SF has proved challenging. In this talk, I will cover the fundamental questions related to triplet dynamics and lifetime in SF systems and discuss the strategies that mitigate triplet loss pathways. By experimentally investigating triplet dynamics in a series of structurally modified diphenylhexatrienes, a well-known SF chromophore, we demonstrate that losses can be avoided if the rate of SF is balanced with respect to the rate of the reverse process of triplet fusion. We analyse our results in the context of dipolar and exchange interactions between triplet excitons to highlight the molecular design principles that lead to efficient SF systems and spin control in OSCs in general and enable SF systems to operate close to the detailed balance limit.