Absorption cross-sections of hole polarons in glassy and β-phase polyfluorene

Francisco Montilla,Arvydas Ruseckas,Ifor D.W Samuel

doi:10.1016/j.cplett.2013.08.098

Francisco Montilla, Arvydas Ruseckas + Show 1 more

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https://doi.org/10.1016/j.cplett.2013.08.098

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Abstract

Absorption induced by electrochemically injected holes is studied in poly-9,9-dioctylfluorene (PFO) films. Injected charges form positive polarons which are delocalised over four fluorene units in the glassy phase and about seven fluorene units in its β-phase. Polaron absorption cross-sections at the 640nm peak are similar to the published values of chemically reduced oligofluorenes in solution. The absorption cross-section of polaron in the β-phase at 470nm is about eight times smaller than the stimulated emission cross-section derived from published data. This indicates that β-phase-rich PFO is an attractive candidate for a light-emitting layer in double-heterostructure organic laser diodes.

Highlights

The optical properties of charged excitations are important for understanding organic semiconductor photophysics
We find that the absorption cross-section of hole polarons in b-phase PFO at 470 nm is about eight times smaller than the stimulated emission cross-section of singlet excitons
The Indium-tin oxide (ITO) glass was cleaned by sonication in an acetone bath

Summary

Introduction

The optical properties of charged excitations are important for understanding organic semiconductor photophysics. Absorption of light and fluorescence quenching by polarons are important issues in the operation of organic optoelectronic devices. With recent advances in materials properties and optical design the lasing threshold of organic structures under optical pumping is low enough to enable pumping by inorganic laser diodes [1,2,3] and LEDs [4] which is promising for fabrication of very sensitive low-cost devices for biosensing and chemosensing [5,6]. Light absorption by injected charges has been reported to be the major obstacle to electrically pumped lasing [7]. Injected charges can quench luminescence as they accept energy from excitons by resonant dipole–dipole interactions and this is an important loss mechanism in organic LEDs as well as in lasers. Previous studies used controlled electrical injection of charges in unipolar devices through contacting electrodes and field-dependent charge mobility measurements to estimate the charge densities which were compared with the values obtained by capacitance–voltage analysis and the two results differed by a factor of three [8,9]

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