Photobleaching of MEH-PPV thin films: Correlation between optical properties and the nanoscale surface photovoltage

The electro-optical properties of Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) thin films have been studied while the sample is illuminated with blue light. By combining Kelvin probe microscopy with classical optical techniques it has been possible to monitor the evolution of the nanoscale contact potential, the surface photovoltage and the optical properties of the material establishing a correlation between them. Contrary to what is usually accepted, it has been found that the nanoscale surface photovoltage is larger when the polymer is already photobleached. In addition, it has been possible to distinguish between the photophysical processes that govern the working principle of a polymer-based device and the photochemical reactions that degrade the material. Although complex charges trapping phenomena take place during the irradiation, the surface photovoltage effect depends only on the number of photons that have reached the polymer.