CHARGE-TRANSPORT IN MOLECULARLY DOPED POLYMERS AT LOW DOPANT CONCENTRATIONS - SIMULATION AND EXPERIMENT

A new computational technique has been developed to simulate charge transport in dilute, energetically random, media. The results predict that in materials containing 15-20% hopping sites, the transition from field-assisted to field-saturated drift occurs within a field range of 10(5)-10(6) V/cm, lower values referring to lower degrees of energetic disorder. Experimental results are reported for N,N'-diphenyl-N,N'-bis(3-methylphenyl)[1,1'-biphenyl]-4,4'-diamine (TPD) and 5'-[4-[bis(4-ethyl-phenyl)amino]-phenyl]-N,N,N',N'-tetrakis(4-ethylphenyl) [1,1': 3',1''-terphenyl]-4,4'-diamine (EFTP) doped in either a polycarbonate (PC) or poly(styrene) (PS). While TPD doped PS fulfills all criteria of disorder controlled hopping, a superposition of disorder and polaron effects has to be invoked for EFTP in both PC or PS, the latter resulting from coupling of the charge transfer process to a torsional mode of the dopant molecule. Criteria to separate disorder and polaron effects in experimental data are briefly outlined.