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.