Effects of arylene diimide thin film growth conditions on n-channel OFET performance

A series of eight perylene diimide (PDI)- and naphthalene diimide (NDI)-based organic semiconductors was used to fabricate organic field-effect transistors (OFETs) on bare SiO2 substrates, with the substrate temperature during film deposition (T-d) varied from 70-130 degrees C. For the N,N'-n-octyl materials that form highly ordered films, the mobility (mu) and current on-off ratio (I-on/I-off) increase slightly from 70 to 90 degrees C, and remain relatively constant between 90 and 130 degrees C. I-on/I-off and mu of dibromo-PDI-based OFETs decrease with increasing T-d, while films of N,N'-1H,1H-perfluorobutyl dicyanoperylenediimide (PDI-FCN2) exhibit dramatic I-on/I-off and mu enhancements with increasing T-d. Increased OFET mobility can be correlated with higher levels of molecular ordering and minimization of film morphology surface irregularities. Additionally, the effects of SiO2 surface modification with trimethylsilyl and octadecyltrichlorosilyl monolayers, as well as with polystyrene, are investigated for N,N'-n-octyl dicyanoperylenediimide (PDI-8CN(2)) and PDI-FCN2 films deposited at T-d=130 degrees C. The SiO2 surface treatments have modest effects on PDI-8CN2 OFET mobilities, but modulate the mobility and morphology of PDI-FCN2 films substantially. Most importantly, the surface treatments result in substantially increased V-th and decreased I-off values for the dicyanoperylenediimide films relative to those grown on SiO2, resulting in V-th > 0.0V and I-on/I-off ratios as high as 10(8). Enhancements in current modulation for these high-mobility, air-stable, and solution-processable n-type semiconductors, should prove useful in noise-margin enhancement and further improvements in organic electronics.