Polymeric conducting anode for small organic transporting molecules in dark injection experiments

Poly(3,4-ethylenedioxythiophene) doped with polystrenesulphonic acid (PEDOT:PSS) is used as a hole-injecting anode for small organic hole transporters in current-voltage (JV) and dark injection space-charge-limited current (DI-SCLC) experiments. The hole transporters under investigation are phenylamine-based 4,4′ ,4″-tris(N-3-methylphenyl-N-phenyl-amino) triphenylamine (MTDATA), N,N′-diphenyl-N,N′-bis(1-naphthyl) (1,1′-biphenyl)-4,4′diamine (NPB), and N,N′-diphenyl-N, N′-bis(3-methylphenyl)(1,1′-biphenyl)-4,4′diamine (TPD). Clear DI-SCLC transient peaks were observed over a wide range of electric fields in all cases. For MTDATA and NPB, hole mobilities evaluated by DI experiments are in excellent agreement with mobilties deduced from independent time-of-flight technique. It can be concluded that, for the purpose of JV and DI experiments, PEDOT:PSS forms an Ohmic contact with MTDATA and a quasi-Ohmic contact with NPB despite the relatively low-lying highest occupied molecular orbital of the latter. In the case of TPD, hole injection from PEDOT:PSS deviates substantially from Ohmic injection, leading to a lower than expected DI-extracted hole mobility. The performances of other hole-injecting anodes for DI experiments were also examined.