Work Function and Electron Paramagnetic Resonance

The work function of PEDOT PSS films with and without additives was examined by Huang et al. using a Kelvin probe. It was found that the work function is reduced when sorbitol or glycerol are added. 

Contrary to these findings, Lai et al. observed no changes in the work function using ultraviolet photoelectron spectroscopy (UPS). Also Crispin et al. found the same work funchon for films prepared with and without diethylene glycoL i hi both cases the density of states drops to zero at 5.0 eV. However, in the range between 5 and 6 eV the density of states is three times larger for PEDOTd SS films with diethylene glycol than for pristine films. 

Electron paramagnetic resonance (EPR) spectroscopy allows the measurement of spins in a sample. It was found that the treatment of PEDOTrPSS films with ethylene glycol leads to a reduction in the EPR signal of 50% and therefore a reduction in the spins in the sample.  

The aim of the following discussion is to take all findings described in Section 9.3.3 into account and to find a consistent explanation for the effect of secondary dopants in PEDOTPSS. 

Although the concentration of these additives is low in the initial mixture, their concentration increases in the film-forming process due to their low vapor pressure, as compared to the main solvent water. A minimum quantity is required for the initial mixture to achieve the effecD In the final film the presence of the conductivity enhancement agent is no longer necessary. Based on these findings it becomes clear that the film forming process is the critical step for the secondary dopants to come into effect. Changes are observed both for the surface and the bulk of the material. On the surface of a pristine PEDOTPSS sample PSS is foimd in excess. When a secondary dopant is used, the surface composition resembles the bulk composihon.i  

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