Engineering carrier transport across organic heterojunctions by interface doping

Two common hole transporting materials, $4,4′,4″$-tris(⁠$N$-3-methylphenyl-$N$-phenyl-amino) triphenylamine (MTDATA) and $N,N′$-diphenyl-$N,N′$-bis(1-naphthyl) (⁠$1,1′$-biphenyl)-$4,4′$diamine (NPB), and a $p$-type dopant tetrafluorotetracyanoquinodimethane $(F4-TCNQ)$ were used to build various hole-only heterojunction devices. Both experimental results and theoretical modeling show that the current flow in such devices is limited by the heterojunction potential barrier or band offset at the MTDATA/NPB interface. It was found that the device current flow can be modulated to increase or decrease dramatically by introducing a $2nm$ $p$-doped $NPB:F4-TCNQ$ or $MTDATA:F4-TCNQ$ interlayer, respectively. The observed phenomena were discussed using quasi-Fermi energy level realignment at the doped/undoped organic-organic interface.