In-Situ Probe of Gate Dielectric-Semiconductor Interfacial Order in Organic Transistors: Origin and Control of Large Performance Sensitivities

Abstract

Organic thin film transistor (OTFT) performance is highly materials interface-dependent, and dramatic performance enhancements can be achieved by properly modifying the semiconductor/gate dielectric interface. However, the origin of these effects is not well understood, as this is a classic "buried interface" problem that has traditionally been difficult to address. Here we address the question of how n-octadecylsilane (OTS)-derived self-assembled monolayers (SAMs) on Si/SiO(2) gate dielectrics affect the OTFT performance of the archetypical small-molecule p-type semiconductors P-BTDT (phenylbenzo[d,d]thieno[3,2-b;4,5-b]dithiophene) and pentacene using combined in situ sum frequency generation spectroscopy, atomic force microscopy, and grazing incidence and reflectance X-ray scattering. The molecular order and orientation of the OTFT components at the dielectric/semiconductor interface is probed as a function of SAM growth mode in order to understand how this impacts the overlying semiconductor growth mode, packing, crystallinity, and carrier mobility, and hence, transistor performance. This understanding, using a new, humidity-specific growth procedure, leads to a reproducible, scalable process for highly ordered OTS SAMs, which in turn nucleates highly ordered p-type semiconductor film growth, and optimizes OTFT performance. Surprisingly, the combined data reveal that while SAM molecular order dramatically impacts semiconductor crystalline domain size and carrier mobility, it does not significantly influence the local orientation of the overlying organic semiconductor molecules.

Direct Patterning of Self‐Assembled Monolayers by Stamp Printing Method and Applications in High Performance Organic Field‐Effect Transistors and Complementary Inverters

“…C8-PFTS was printed onto SiO2, and C8-PFPA was used onto AlO x respectively. Figure 5 shows the transfer curves of the n-type F 16 CuPc OFETs on both substrates and these devices have a similar mobility around 0.0125 cm 2 V −1 s −1 with a positive threshold voltage of 19 and 2.1 V, respectively. By showing these highly comparable transfer curves with one that has been reported, [ 10 ] it is obvious that the current SAM deposition method could be used for different kinds of SAMs with different terminal groups, and a very competitive performance can be achieved by such a time saving direct patterning method of SAMs.…”

“…The schematics and transfer characteristics of the complementary circuits on an AlO x dielectric are shown in Figure 6 . We utilize DNTT and F 16 CuPc as the active layer materials and ODPA and C8-PFPA are separately printed on the dielectric. Both OFETs in a complementary inverter have a channel length of 100 µm, while the channel width for the p-type transistor is 500 µm and for the n-type transistor is 7500 µm.…”

“…Silicon wafers with 300 nm of thermally grown silicon oxide came from Silicon Quest International. DNTT and F 16 CuPc were obtained from Luminescence Technology Corporation and both were used as received.…”

“…Although post-processing treatments on the SAM and dielectric interface are needed, generally the required time is still shorter than immersion method. SAMs with end groups other than alkyl chains can also be deposited by this method by modifying the chemical components of the paraffi n. Here, we apply this printing method by using a paraffi n stamp to deposit OTS and trichloro (1H,1H,2H,2H-perfl uorooctyl) silane (C8-PFTS) onto SiO 2 , OPDA, and 1H,1H,2H,2H-perfl uorooctanephosphonic acid (C8-PFPA) onto aluminum oxide (AlO x ) to modify the dielectric surface energy for the growth of dinaphtho [2,3- 16 CuPc transistors show a maximum carrier mobility of 3.02 and 0.018 cm 2 V −1 s −1 respectively, which is comparable with the state-of-art values of these two types of OSCs. We increase the scale of the approach further in terms of a roller imprint for a larger area of the Si/SiO 2 substrate and test the uniformity of an array of 16 × 16 transistors.…”

Structural stability of naphthyl end-capped oligothiophenes in organic field-effect transistors measured by grazing-incidence X-ray diffraction in operando

“…Typical oligothiophenes are either fused systems [15] or end-capped by alkyl [16] or aromatic or polyaromatic groups in a symmetric [17] or asymmetric manner [18] manifesting vertically aligned face-to-face structure on the OFET surface. This structure is further modified by an additional molecular layer between electrodes and thiophene layer [19] or altered by external parameters such as externally induced uniaxial strain [20]. The structure of various oligothiophenes in OFETs has been resolved ex situ even in the monolayer level [16] but the effect of electric field in running OFETs, corresponding those observed for pentance [13], are not well reported.…”