Influence of traps on top and bottom contact field-effect transistors based on modified poly(phenylene-vinylene)

We report on the investigations of the differences in the measured current characteristics of source/drain top contact (TOC) and bottom contact (BOC) organic field-effect transistors (OFETs). The active layer was made from poly[1,4-phenylene-(4-methylphenyl)imino-4,48-diphenylene-(4-methylphenyl)imino-1,4-phenylenevinylene-2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene-vinylene] and a highly doped silicon wafer with a thermal oxide was used for the gate and the insulator, respectively. Both transistors show a good subthreshold behavior characterized by a threshold voltage of V-th approximate to-5 V and an inverse subthreshold increase of S approximate to 0.25 V/dec. The estimated mobility is in the range of 10(-3) cm(2)/V s. The main difference between the TOC and the BOC transistors is a nonlinear increase of the drain current at small drain voltages in the output characteristics of the TOC OFETs. Numerical two-dimensional simulations show that a high concentration of donor-like traps with a Gaussian or exponential distribution are the reason for this peculiarity. The investigation of the influence of air confirms the presence of such traps. (c) 2007 American Institute of Physics.