Solution-Processed Polycrystalline Tetrabenzoporphyrin Organic Field-Effect Transistors With Nano-Structured Channels

Te t r a b e n z o p o rp h ryin (TBP) is an organic molecule widely studied in b i o c h e m i s t ry, and has a well-understood and controllable synthetic process. W h i l e TBP itself is insoluble in most organic solvents, the molecule can be synthesized in a precursor form that allows for solution-processing methods such as printing and spin coating. Upon t h e rmal annealing, the amorphous, insulating precursor film conve rts to a poly c ry s t a l l i n e , semiconducting film. We have shown free-base T B P o rganic fi e l d - e ffect transistors (OFETs) with fi e l d e ffect mobilities exceeding 0.01cm 2 / V-s, wh i l e maintaining low sub-threshold slopes (~1 V/dec), low OFF-state currents (~10 pA), and moderate threshold voltages (-20 V > V T > 0 V ) . While TBP OFET performance is susceptible to ambient exposure, we have demonstrated a novel packaging method to enhance device lifetime. To better understand the TBP electronic properties, we are studying the band-structure of TBP using various quantum mechanical models, and have found a close relation b e t ween experimental and computational results. While free-base TBP produces excellent OFET behav i o r, OFET performance is limited by crystallization and molecular packing. In collaboration with Ehime U n iversity (Japan), we have studied the thin-fi l m m o rp h o l ogy and electronic performance of metalsubstituted T B P. Significant variations in morp h o l og y are observe d, as well as several orders of magnitude enhancement in electrical performance. To date, we h ave observed fi e l d - e ffect mobilities as large as 0 . 6 c m 2 / V-s and crystals with diameters on the order of 1mm, all from OFETs using solution-processed fi l m s . We also show that thermal conversion of the precursor film to TBP can be enhanced by ordered TBP aggr egation in the pre-patterned nanometer-scale trenches, demonstrating precise control and placement of long- and short-range ordering of the organic semiconductor. T h i n - film transistor (TFT) with such nano-stru c t u r e d channels shows one order improved electrical performance in comparison with r egular TFTs. We envision the results of these studies leading to sensitive orga n i c biochemical sensors and easily fabricated organic circuits on plastic substrates. This project is part i a l ly supported by an NSF IGERT Fe l l ow s h i p .