Paper EM-ThA1
Dependence of Contact Resistance and Field-Effect Mobility on Morphology in Poly(3-hexylthiophene based Field-Effect Transistors

Thursday, October 23, 2008, 2:00 pm, Room 210

The field-effect mobility and the contact resistance (Rc) of field effect transistors (FETs) based on regioregular poly(3-hexylthiophene) (P3HT) were investigated as a function of the molecular weight (Mw). Bottom-contact FETs were fabricated having Pt or Au contacts with the channel lengths varying from 3 to 40 µm. Our electrical measurements show an initial increase (decrease) of the mobility (contact resistance) with Mw followed by a saturation of the values. For example, at a gate voltage of -80V and for Mw between 5.5 and 11 kDa in FETs with Pt contacts, the mobility increased from 0.05 to 0.16 cm²V^-1s^-1, whereas the contact resistance decreased from 1.7 to 0.5 MΩ. However, above 11 kDa, the values remained relatively constant. Devices with Au contacts displayed a similar trend, although the mobilities were lower and the contact resistances were higher, as expected due to the lower work function of Au. The inverse relationship between mobility and Rc is attributed to the dependence of Rc on bulk transport through a depletion region associated with defects near the contacts.¹ Atomic force microscopy (AFM) images indicate that P3HT self assembles in a nanofibrilar morphology.² It is also evident that an increase in Mw leads to an increase in the width of the self-assembled nanofibrils and, at the highest molecular weights, to a simultaneous disruption arising from folding of the polymer chains. An associated loss in conjugation within the polymer chains, along with a possible disruption in the π-π stacking that occurs concurrently with the increase in nanofibril size, would account for the saturation of the mobility and the contact resistance at higher molecular weights. In summary, our results show that mobility and contact resistance in P3HT-based FETs depend strongly on the polymer nanomorphology and display an inverse relationship with each other, supporting the theory that the contact resistance is affected by the bulk transport properties of P3HT. Future experiments including X-ray scattering studies are planned to further quantify the structure within the P3HT films.

¹ L. Bürgi, T. J. Richards, R. H. Friend, and H. Sirringhaus, J. Appl. Phys. 94[9], 6129(2003).
² R. Zhang, B. Li, M.C. Iovu, M. Jeffries-EL, G. Sauvé, J. Cooper, S. Jia, S. Tristram-Nagle, D.M. Smilgies, D.N. Lambeth, R.D. McCullough, and T. Kowalewski, J. Am. Chem. Soc. 128[11], 3480(2006).