Hole Injection and Transport in Organic Hole-Transport Materials

Abstract

ABSTRACT

Compare to their silicon counterparts, organic semiconductors have a larger band gap, creating a large injection barrier between hole-transport material and the metal contacts. This project aims to try and overcome this barrier by depositing a very thin (5-8 nm) highly doped (8%) layer of the same hole-transport material in between the interface of the metal contacts and undoped or lightly doped organic bulk polymer. Comparing the J-V characteristics from a semiconductor with only a thick layer of undoped α-NPD on gold contacts and one with a thin heavily-doped layer of α-NPD injection layer in between the undoped layer and metal shows higher current density as well as a bulk-limited transport characteristics for the latter. After determining that the injection layer had an impact on the transport of holes through the material, the bulk organic layer was then doped to .4% and 1.5% to see how injection and transport would change. I-V sweeps from -100 V to 120V were performed at various temperatures ranging from 300K – 230K. From these measurements, the conductivity was calculated using the ohmic region of the J vs F plots as 1.17E-8 S/cm to 2.15E-7 S/cm as doping in the bulk increased. In addition, activation energy found using an Arrhenius equation showed a decreasing activation energy from .44 eV to .38 eV as doping concentration increased. Finally additional measurements were performed to verify that the injection layer itself was stable as well as no dopants defused from the injection layer into the bulk.