Disentangling Bulk and Interface Phenomena in a Molecularly Doped Polymer Semiconductor

Doping the electron transport polymer poly [N,N amp; 8242; bis 2 octyldodecyl naphthalene 1,4,5,8 bis dicarboximide 2,6 diyl] alt 5,5 amp; 8242; 2,2 amp; 8242; bithiophene [P NDI2OD T2 ] with the bulky, strongly reducing metallocene 1,2,3,4,1 amp; 8242;,2 amp; 8242;,3 amp; 8242;,4 amp; 8242; octaphenylrhodocene OPR leads to an increased bulk conductivity and a decreased contact resistance. While the former arises from low level n doping of the intrinsic polymer and increased carrier mobility due to trap filling, the latter arises from a pronounced accumulation of dopant molecules at an indium tin oxide ITO substrate. Electron transfer from OPR to ITO leads to a work function reduction, which pins the Fermi level at the P NDI2OD T2 conduction band and thus minimizes the electron injection barrier and the contact resistance. The results demonstrate that disentangling the effects of electrode modification by the dopant and bulk doping is essential to comprehensively understand doped organic semiconductors