Lessard Research Group - Next Generation Materials for Next Generation Applications

New Publication in Materials!

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


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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To read the full article visit Materials.

New publication in Dyes and Pigments!

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


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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To read the full article visit Dyes and Pigments.

New publication in J. Mater. Chem. C!

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


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.

New Publication in J. Mater. Chem. A! Congrats Trevor!

Bis(tri-n-alkylsilyl oxide) silicon phthalocyanines: A Start to Establishing a Structure Property Relationship as both Ternary Additives and Non-Fullerene Electron Acceptors in Bulk Heterojunction Organic Photovoltaic Devices

Previous studies have shown that the use of bis(tri-n-hexylsilyl oxide) silicon phthalocyanine ((3HS)2-SiPc) as a solid ternary electroactive additive in poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester P3HT:PC61BM bulk heterojunction organic photovoltaic (BHJ OPV) devices resulted in an increased performance. It has been hypothesized that the increase in efficiency is partially due to the unique and odd combination of high solubility and strong driving force of crystallize previously observed for (3HS)2-SiPc. In this follow up study, two chemical variants of (3HS)2-SiPc, bis(tri-n-butylsilyl oxide) ((3BS)2-SiPc) and bis(tri-n-isopropylsilyl oxide) ((3TS)2-SiPc) were synthesized to determine how small changes in chemical structure would affect the properties of the material and its performance within BHJ OPV devices. We observed that the use of either (3XS)2-SiPc compound results in a further ~10% increase in Jsc compared to the use of (3HS)2-SiPc. We also did a preliminary assessment of the use of the three (3XS)2-SiPcs as replacements for PC61BM in straight binary P3HT-based BHJ OPV devices. Despite achieving only ~1% PCE efficiencies, observations including a ≈ 50% increase in VOC over a P3HT:PC61BM baseline and a decent fill factor indicate to us that (3XS)2-SiPcs do have potential as non-fullerene acceptors and advantageous alternatives due to their low embedded energy and therefore their inherent sustainability. X-ray diffraction of ternary and binary BHJ devices demonstrate that both (3BS)2-SiPc and (3TS)2-SiPc experienced similar increase in crystallite density (d-spacing) relative to (3HS)2-SiPc which we surmise plays a role in the improved device efficiency. Like (3HS)2-SiPc, for these two new additives, we also observed a high tendency to crystallize. The results from this study suggest that solubility and driving force to crystalize are important factors in determining the extent to which an additive will migrate to the donor/acceptor interface and thus affect its performance as a ternary additive in BHJ OPV device. Based on the three (3XS)2-SiPcs used in this study, the smaller tri-n-alkylsilyl oxide molecular fragments seem to work better. Therefore, moving forward, we will continue to consider smaller molecular fragments that still enable solubility and processability of (3XS)2-SiPcs.

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


Cleaning up at the uOttawa Engineering Poster competition!

The Faculty of Engineering Hosts the Annual 2017 Engineering and Computer Science Graduate Poster Competition.

This annual research competition among the graduate students of the Faculty of Engineering, not only demonstrates the cutting-edge research that the Faculty’s graduate students are conducting, it also gives Ottawa’s industry and community a sneak preview of the next generation of designers, innovators, and entrepreneurs. (Link to website)

This year Owen Melville and Trevor Grant, two PhD students from the Lessard Research group discussed their recent results resulting in 2nd and 3rd place, respectively. This is a repeat performance for Mr. Melville who received 2nd place for his poster at the 2016 competition.