Vapor Doped Spiro-OMeTAD as a Model System for Disordered Charge Transport

Amorphous small molecule semiconductor films are widely used in organic light emitting displays and have promising applications in solar cells and thermoelectric devices. Dopants can be used to increase the electrical conductivity of molecular films. Understanding how the resulting conductivity depends on the concentration of dopants is complex because they simultaneously add carriers to and alter the structure of the host film.

To study charge transport in amorphous small molecule films, we electrically doped spiro-OMeTAD films with F₄TCNQ vapor, increasing their conductivity to 0.02 S/cm. Spiro-OMeTAD provides a unique model system for doping because UV-Vis spectra show the presence of two charge states of the molecule, spiro-OMeTAD⁺ and spiro-OMeTAD²⁺. Structural measurements showed that the dopant molecules were dispersed throughout the film and did not aggregate. To examine the transport mechanism at low and high doping levels, we measured the temperature-dependent conductivity and Seebeck coefficient of the films at varying dopant concentrations. The relationship between the thermopower and conductivity in this amorphous system will be discussed in relation to recent results on ordered polymers and molecular materials.