One of the most commonly used cathode interlayers for increasing the efficiency of electron injection/extraction in organic electronic devices is an ultrathin layer of LiF. Our capacitance measurements and electrical conductivity analysis show that thin films of fullerene derivatives and their mixtures with polymers are unintentionally doped upon deposition of LiF. The level of doping depends on the chemical structure of the fullerene derivatives. The doping effect on polymer/fullerene mixtures is significant only for blends in which the fullerene content is greater than the polymer content by weight. Our finding has profound implications for the development and characterization of organic photovoltaic devices, including a negative impact of doping on the stability of the device and erroneous estimations of properties such as charge carrier mobility and the dielectric constant.
Keywords: LiF; cathode interlayers; charge carrier mobility; dielectric constant; doping; electrical conductivity; fullerene derivatives; organic solar cells.