Controlling Triplet Excitons in Organic Semiconductors with Lanthanide-Doped Nanoparticles

Organic semiconductors (OSCs) underpin a wide range of optoelectronic technologies, including light-emitting diodes (LEDs), photovoltaics, and sensing devices, owing to their chemical tunability and mechanical flexibility. The optoelectronic performance of these materials is governed by excitonic processes, in which electronically excited states mediate light absorption, emission, and charge generation. Triplet excitons play a central role in determining the excited-state dynamics of OSCs under both optical and electrical excitation. However, in most closed-shell OSCs, triplet excitons are dark states that can neither be directly photogenerated nor harvested luminescently, which fundamentally limits their practical utilisation. Overcoming these limitations requires strategies that enable the control of molecular triplet excitons. Traditional approaches to control triplets, such as heavy-metal-induced spin-orbit coupling (SOC) or engineering the singlet–triplet energy splitting, impose significant design constraints on OSCs. This thesis explores a new approach to molecular triplet exciton control by combining OSCs with lanthanide-doped nanoparticles (LnNPs). Using optical probes, we reveal the triplet exciton dynamics in a variety of organic–inorganic LnNP@OSC nanohybrid systems. We present the first direct distance-dependent energy transfer measurements in LnNP@OSC nanohybrids. Using transient absorption spectroscopy (TAS), we show triplet energy transfer (TET) to be governed by a concerted Dexter-type process. Contrary to previous beliefs, we find near-unity TET efficiencies that are independent of distance over the range of OSC–LnNP separations investigated. We show that close coupling between the OSC and LnNPs is primarily important to ensure efficient triplet exciton generation. Although singlet energy transfer (SET) is faster, we show SET efficiencies to be lower and more strongly distance-dependent than TET efficiencies, thereby establishing the advantage of using the triplet manifold for most efficient energy transfer in these systems. We carry out detailed optical studies of the triplet exciton dynamics in weakly-coupled solution-based LnNP@OSC nanohybrids and demonstrate the strong impact of molecular orientation of OSCs on the triplet exciton dynamics. Furthermore, we study the triplet exciton dynamics across the entire lanthanide series and show that triplet exciton generation is governed by hybridised electronic states that form at the OSC–LnNP interface rather than direct SOC or spin-exchange coupling. We show the resonance nature of both SET and TET in these systems and find near-unity TET efficiencies across the full lanthanide series. We use the established structure-function relationships to produce the first LnNP@OSC-based LEDs, operating in the biologically and technology relevant near-infrared (NIR) spectral region, where molecular triplet excitons mediate the function of these electrically driven devices. Finally, we investigate strongly-coupled LnNP–OSC systems. We demonstrate an alternative method to enhance spin conversion in OSCs: through spin-exchange interactions with unpaired lanthanide 4f electrons. We show that the radiative Tn ← S0 transition can be activated in these systems, enabling direct photogeneration of triplet excited states from the singlet ground state and allowing for a straightforward way to directly measure triplet energies of OSCs. We present the first TAS measurements directly exciting this transition and for the first time report Urbach energies associated with triplet excitons in OSCs. The findings in this thesis deepen our understanding of electronic and spin interactions in LnNP@OSC nanohybrids and provide a new paradigm for triplet exciton control in OSCs, enabling their informed design and engineering in optoelectronic devices.

Name dc:type.qualificationname

Doctor of Philosophy (PhD)

Level dc:type.qualificationlevel

Doctoral

Grantor dc:publisher.institution

University of Cambridge

Year dc:date.issued

2025

Author dc:creator

• van Turnhout, Lars

Advisor dc:contributor.advisor

• Rao, Akshay

• lanthanide-doped nanoparticles
• organic semiconductors
• photophysics
• time-resolved spectroscopy
• triplet excitons

Licence

• https://www.repository.cam.ac.uk/bitstreams/124d2c22-05c2-45ce-b4e1-6a3c07b391b1/download
• http://purl.org/NET/rdflicense/allrightsreserved

Language dc:language

eng

Author Identifier

0000-0002-5222-6250

OAI identifier oai:identifier

oai:www.repository.cam.ac.uk:1810/395201