Silicon and tin­(IV) phthalocyanines, which have been demonstrated as simple-to-synthesize materials for n-type organic thin-film transistors (OTFTs), have relatively shallow lowest unoccupied molecular orbital (LUMO) levels that create a Schottky barrier with the gold source–drain contacts typically used in device fabrication. To reduce the contact resistance (R C) associated with this barrier and improve the OTFT performance, we fabricated bottom-gate top-contact (BGTC) devices using low-work-function metals (Mn/Cr) and an electron dopant material (bathocuproine, BCP) as contact interlayers. We characterized two tin phthalocyanines (SnPcs), tin bis­(pentafluorophenoxy)­phthalocyanine (F10-SnPc) and tin bis­(2,4,6-trifluorophenoxy)­phthalocyanine (246F-SnPc), as organic semiconductors (OSCs) and compared them to their silicon phthalocyanine (SiPc) analogues. We found that using Mn and Cr interlayers with SiPc OTFTs reduces R C to as low as 11.8 kΩ cm and reduces the threshold voltage (V T) to as low as 7.8 V while improving linear region characteristics compared to devices using silver or gold electrodes only. BCP interlayers appear to reduce V T in all SiPc and SnPc devices and increase the off-state conductivity of SnPc devices if covering the entire OSC. Overall, this work demonstrates the potential for metal interlayers and solid-state organic interlayers for improving electron transport in low-cost, n-type OTFTs using group 14 phthalocyanines.

Polymerized ionic liquids (PILs) are a potential solution to the large-scale production of low-power consuming organic thin-film transistors (OTFTs). When used as the device gating medium in OTFTs, PILs experience a double-layer capacitance that enables thickness independent, low-voltage operation. PIL microstructure, polymer composition, and choice of anion have all been reported to have an effect on device performance, but a better structure property relationship is still required. A library of 27 well-defined, poly(styrene) -b- poly(1-(4-vinylbenzyl)-3-butylimidazolium- random -poly(ethylene glycol) methyl ether methacrylate) (poly(S)- b -poly(VBBI + [X]- r -PEGMA)) block copolymers, with varying PEGMA/VBBI + ratios and three different mobile anions (where X = TFSI – , PF 6 – or BF 4 – ), were synthesized, characterized and integrated into OTFTs. The fraction of VBBI + in the poly(VBBI + [X]- r -PEGMA) block ranged from to 100 mol % and led to glass transition temperatures ( T g ) between −7 and 55 °C for that block. When VBBI + composition was equal or above 50 mol %, the block copolymer self-assembled into well-ordered domains with sizes between 22 and 52 nm, depending on the composition and choice of anion. The block copolymers double-layer capacitance ( C DL ) and ionic conductivity (σ) were found to correlate to the polymer self-assembly and the T g of the poly(VBBI + [X]- r -PEGMA) block. Finally, the block copolymers were integrated into OTFTs as the gating medium that led to n-type devices with threshold voltages of 0.5–1.5 V while maintaining good electron mobilities. We also found that the greater the σ of the PIL, the greater the OTFT operating frequency could reach. However, we also found that C DL is not strictly proportional to OTFT output currents.

A series of well-defined polymerized ionic liquid (PIL) statistical and block copolymers consisting of ionic liquid monomer, 1-(4-vinylbenzyl)-3-butyl­imidazolium bis­(trifluoro­methyl­sulfonyl)­imide, and a nonionic monomer, methyl methacrylate (MMA), were synthesized by nitroxide-mediated polymerization (NMP) with the goal of understanding the influence of polymer structure on the thin film capacitance. Copolymer compositions were varied from 8 to 54 wt % for both statistical and block copolymers and were characterized by predictable changes in glass transition temperature (75 °C > T g > 45 °C). When integrated into thin film capacitors, block copolymers exhibited the formation of electrical double layer (EDL) at lower frequencies compared to the statistical copolymers of similar comonomer compositions. The materials that formed an EDL all produced a similar maximum double layer capacitance value, with the only difference being the frequency at which the EDL was formed. Finally, the PIL-containing materials that were utilized in organic thin-film transistors (OTFTs) showed a significant reduction in operating voltage compared to the poly­(MMA) baseline. These results indicate that not only composition but also polymer architecture plays a vital role in the formation of an EDL and determines at which frequency the resulting OTFTs can be operated.

Silicon phthalocyanines (SiPcs) are a class of n-type or ambipolar organic semiconductors that have been incorporated into organic thin-film transistors (OTFTs), organic light-emitting diodes (OLEDs), and organic photovoltaics (OPVs). Despite a relatively large catalogue of previously reported SiPc materials, fabricated OTFTs with these materials typically have threshold voltages (V T ) above 10 V, limiting their usage in commercial devices due to exceedingly high power consumption. Recent studies have suggested that the V T can be reduced in OTFTs prepared from phenoxy-substituted SiPcs by introducing electronwithdrawing groups onto the phenoxy moieties. Herein, we report the synthesis and characterization of three SiPcs with phenoxy axial substituents containing nitrile and fluorine functional groups. These SiPcs, along with 3,5-difluorophenoxy SiPc were evaluated as candidate materials for n-type OTFTs. We found that further increasing the electron-withdrawing character of the pendant phenoxy groups of the SiPc resulted in a significant decrease in average V T with the lowest reported value being 4.8 V, the lowest V T reported for a phenoxy-SiPc-based OTFT exceeding the previous record low of 7.8 V attributed to F 10 -SiPc. This decrease in V T could be directly correlated to the Hammett parameter of the axial functional groups. Furthermore, it was noted that dewetting occurred when the phenoxy pendant group of the SiPc was substituted at the para position with a nitrile group combined with orthoor meta-substituted fluorines, which was attributed to interactions at the semiconductor/dielectric interface. Depositing these SiPcs on silane-terminated poly(styrene) brush modified substrates improved long-term stability, demonstrated by a minimal change in surface morphology according to atomic force microscopy (AFM) images.

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