Probing electronic state charging in organic electronic devices using electroabsorption spectroscopy

In metal/organic-film/metal device structures with different metal contacts there is a built-in electrostatic potential at equilibrium due to the asymmetric contacts. At thermal equilibrium the electrochemical potential is constant across the device structure. The electrochemical potential can be divided into the sum of two parts, the electrostatic potential and the chemical potential. By measuring the built-in electrostatic potential change across a structure at equilibrium, one can determine the change in chemical potential across the structure. Measuring this built-in electrostatic potential for devices with different contact metals provides a way of changing the chemical potential (mu) in the organic material and identifying the values of mu at which charged excitations are populated. Such measurements can be used to gain information on the energy spectrum of intrinsic charged excitations, charged trap states and charged interface states. We apply an electroabsorption technique to measure the built-in potentials of metal/organic-film/metal structures fabricated from poly [2-methoxy,5-(2'-ethyl-hexyloxy)-1,4 phenylene vinylene] (MEH-PPV), C-60-doped MEH-PPV, and 1,4,5,8-naphthalene-tetracarboxylic dianhydride (NTCDA) and discuss what is learned about the charged excitations in these materials from the results.