The effect of p-type doping on the performance of organic thin-film photovoltaic devices— m-MTDATA/C₆₀ and 2-TNATA/C₆₀ systems

The effect of p-type doping of the donor layer with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) on the performance of planar pn-heterojunction organic photovoltaic devices using 4,4′,4″-tris[3-methylphenyl(phenyl)amino]triphenylamine (m-MTDATA) or 4,4′,4″-tris[2-naphthyl(phenyl)amino]triphenylamine (2-TNATA) as an electron donor and C60 as an electron acceptor was studied. It was found that doping of the donor layer with F4TCNQ increases both the short-circuit photocurrent and the fill factor by 1.7 to 2.0 times and 1.5 to 1.6 times, respectively, but reduces the open-circuit voltage, resulting in the enhancement of power conversion efficiency by 1.6 to 1.7 times. These features caused by the doping are attributed to the decrease in the bulk resistance of the electron donor layer as a result of the p-type doping. The decrease in the open-circuit voltage was partly compensated by incorporation of a thin layer of undoped m-MTDATA on the ITO electrode, and hence, the power conversion efficiency was further enhanced.