Lessard.Research.Group

Lessard Research Group - Next Generation Materials for Next Generation Applications

Stability of PBDB-T OTFTs! New Publication in J Mater Chem C!

Congrats Sam! The Influence of Air and Temperature on the Performance of PBDB-T and P3HT in Organic Thin Film Transistors

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Conjugated polymers such as poly(3-hexylthiophene) (P3HT) are commonly used as semiconducting components in organic photovoltaics (OPVs) and organic thin-film transistors (OTFTs). Such devices may be exposed to oxygen or moisture in air and increased temperature during operation, which can impart consequences on their charge transport properties. Therefore, we produced the first reported example of OTFTs using PBDB-T, a conjugated push-pull polymer used in high performance OPVs, and assessed their performance compared to P3HT under different environmental conditions. Drop casted and annealed bottom-gate, bottom-contact (BGBC) devices had an average mobility of 0.06 cm2/Vs, an on/off current ratio of 104 and a desirable threshold voltage around 0 V. These OTFTs showed distinct responses to characterization at increased temperature in vacuum (P < 0.1 Pa) and air, with PBDB-T devices retaining their performance better than P3HT over time. These findings suggest PBDB-T has higher stability to oxidation when exposed to air than P3HT, especially at high temperatures, and therefore represent a more stable alternative for use in OTFTs and OPVs.

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SiPc-based OTFTs! New Publication in J. Mater. Chem C!

Silicon Phthalocyanines as N-Type Semiconductors in Organic Thin Film Transistors

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Silicon phthalocyanines (SiPcs) represent a large class of molecules that have been studied as donors, acceptors and ternary additives in organic photovoltaics but not in organic thin-film transistors (OTFTs). We synthesized three novel SiPcs using axial substitution and examined their performance as the active layer in bottom-gate bottom-contact (BGBC) OTFTs. All three molecules exhibit N-type behaviour, with the dibenzoate substituted SiPc showing the greatest field-effect mobility of roughly 6 x 10-4 cm2 V-1 s-1 in vacuum. This performance improved to >0.01 cm2 V-1 s-1 when using a combination of dielectric modification with octadecyltrichlorosilane (ODTS) and a substrate temperature during deposition of 200 °C. These promising results point towards the possibility of high performance N-type SiPcs by exploring the wealth of available options in axial and peripheral substitution and careful process control during fabrication.

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To read the full article visit J. Mater. Chem C.

New Publication in Materials!

Organic Thin Film Transistors Incorporating Solution Processable Thieno[3,2-b]thiophene Thienoacenes

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Bottom-gate bottom-contact organic thin film transistors (OTFTs) were prepared with four novel star-shaped conjugated molecules containing a fused thieno[3,2-b]thiophene moiety incorporated either in the core and/or at the periphery of the molecular framework. The molecules were soluble in CS2, allowing for solution-processing techniques to be employed. OTFTs with different channel geometries were characterized in both air and vacuum in order to compare environmental effects on performance. Blending the small molecules with poly(styrene), an insulating polymer, facilitated the formation of an even semiconducting film, resulting in an order of magnitude increase in device mobility. The highest field-effect mobilities were in air and on the order of 10−3 cm2/Vs for three of the four molecules, with a maximum mobility of 9.2 × 10−3 cm2/Vs achieved for the most conjugated small molecule. This study explores the relationship between processing conditions and OTFT devices performance for four different molecules within this new family of materials, resulting in a deeper insight into their potential as solution-processable semiconductors.

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New publication in Dyes and Pigments!

Organic thin-film transistors incorporating a commercial pigment (Hostasol Red GG) as a low-cost semiconductor

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Inexpensive carbon-based semiconducting materials are of interest for the manufacturing of organic thin-film transistors (OTFTs) for use in radio-frequency tags, gas sensors, printed logic circuits and flexible screens. The molecular organization of these materials at the interface with the dielectric layer in an OTFT is critical to device performance. We incorporated an inexpensive commercial pigment, 14H-anthra[2,1,9-m,n,a]thioxanthen-14-one (Hostasol Red GG, Solvent Orange 63), which has not been characterized before as a semiconductor in OTFTs. We found that of the four surface treatments used to modify the silicon dioxide (SiO2) dielectric, octyltrichlorosilane (OTS) had the greatest effect on device operation. We also illustrate the importance of moisture in the surface chemistry and its direct implications to film forming and device performance. Furthermore, increasing substrate temperature during physical vapor deposition of Red GG generally decreased device performance, despite increasing molecular organization and apparent grain size as characterized with atomic force microscopy (AFM). Optimized OTFTs had a field-effect mobility of up to 5 × 10−5 cm2/V, an on/off current ratio the order of 103 and a threshold voltage around 10 V. These results demonstrate the important role of processing conditions for Red GG-based OTFTs and could be applied to optimizing devices made with synthetic derivatives of Red GG for improved performance in this family of relatively inexpensive and unexplored semiconducting compounds.

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New publication in J. Mater. Chem. C!

Silicon phthalocyanines as dopant red emitters for efficient solution processed OLEDs.

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The optical characterisation and device functionality for a series of axially modified silicon phthalocyanines (SiPcs) as dopant red emitters in solution-processed and vapour-deposited organic light-emitting diodes (OLEDs) is presented. Control over the axial group composition enables bandgap maintenance around 1.8 eV and photoluminescence quantum yield values of up to 66% in solution. We confirm efficient energy transfer between a polyfluorene host matrix and a SiPc dopant under photoexcitation, and demonstrate solution-processed OLED devices with narrow electroluminescence spectra (20 nm full-width at half-maximum) close to 700 nm. Vapour-deposited OLEDs based on TCTA as host exhibit characteristic SiPc emission around 715nm, but with greater spectral contamination from the host. An initial OLED optimisation exercise demonstrates external quantum efficiencies of up to 2.5% in solution-processed devices, indicating that such phthalocyanines may prove promising red dopant emitters for OLEDs with high colour purity.

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To read the full article visit J. Mater. Chem C.