document: The optical properties of charged excitations are important for understanding organic semiconductor photophysics. The injection of electric charge into organic materials polarizes the surroundings and changes the bond lengths around it, such an excitation is defined as a charged polaron. Absorption of light and fluorescence quenching by polarons are important issues in the operation of organic optoelectronic devices. It is particularly relevant to the development of electrically pumped lasers. With recent advances in materials properties and optical design the lasing threshold of organic structures under optical pumping is now low enough to enable pumping by inorganic laser diodes [1–3] and LEDs [4] which is promising for fabrication of very sensitive low-cost devices for biosensing and chemosensing [5,6]. However, light absorption by injected charges has been reported to be the major obstacle to electrically pumped lasing [7]. Injected charges can also 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. Absorption cross-sections of polarons are not known to the desired accuracy because of the difficulty of quantifying the charge density injected into the film. 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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