Impact of material parameters on strong exciton–photon coupling states formed in microcrystal resonators of p- and n-type thiophene/phenylene co-oligomers

Abstract

Microcrystals with different orientations of thiophene/phenylene co-oligomers (TPCOs) serving as Fabry–Pérot microresonators were prepared using a miniemulsion technique with modification. Reflecting molecular packings and crystal structures, p-type 5,5′-bis(4-biphenylyl)-2,2′-bithiophene (BP2T) and n-type 5,5′-bis(4′-cyanobiphenyl-4-yl)-2,2′-bithiophene (BP2T-CN) grow in square-shaped (microplatelet) and rod-shaped (microrod) microcrystals, respectively. Strong exciton–photon coupling was demonstrated for BP2T (Rabi splitting energy Ω: 1.05 eV) and BP2T-CN (Ω: 1.25 eV) microcrystals owing to their active microresonators providing effective photonic mode confinement. We find that the strength of the coupling between excitons and photons is correlated with the refractive index and absorption coefficient originating from the transition dipole moment and oscillator strength. For BP2T-CN, a larger value of Ω was achieved because of a higher absorption coefficient, larger refractive index, and smaller mode volume.