Benoît H. Lessard

Phthalocyanine-Based Organic Thin-Film Transistors: A Review of Recent Advances.

Metal phthalocyanines (MPcs) are versatile conjugated macrocycles that have attracted a great deal of interest as active components in modern organic electronic devices. In particular, the charge transport properties of MPcs, their chemical stability, and their synthetic versatility make them ideal candidate materials for use in organic thin-film transistors (OTFTs). This article reviews recent progress in both the material design and device engineering of MPc-based OTFTs, including the introduction of solubilizing groups on the MPcs and the surface modification of substrates to induce favorable MPc self-assembly. Finally, a discussion on emerging niche applications based on MPc OTFTs will be explored, in addition to a perspective and outlook on these promising materials in OTFTs. The scope of this review is focused primarily on the advances made in the field of MPc-based OTFTs since 2008.

Bis(tri-n-hexylsilyl oxide) Silicon Phthalocyanine: A Unique Additive in Ternary Bulk Heterojunction Organic Photovoltaic Devices

Previous studies have shown that the use of bis(tri-n-hexylsilyl oxide) silicon phthalocyanine ((3HS)2-SiPc) as an additive in a P3HT:PC61BM cascade ternary bulk heterojunction organic photovoltaic (BHJ OPV) device results in an increase in the short circuit current (J(SC)) and efficiency (η(eff)) of up to 25% and 20%, respectively. The previous studies have attributed the increase in performance to the presence of (3HS)2-SiPc at the BHJ interface. In this study, we explored the molecular characteristics of (3HS)2-SiPc which makes it so effective in increasing the OPV device J(SC) and η(eff. Initially, we synthesized phthalocyanine-based additives using different core elements such as germanium and boron instead of silicon, each having similar frontier orbital energies compared to (3HS)2-SiPc and tested their effect on BHJ OPV device performance. We observed that addition of bis(tri-n-hexylsilyl oxide) germanium phthalocyanine ((3HS)2-GePc) or tri-n-hexylsilyl oxide boron subphthalocyanine (3HS-BsubPc) resulted in a nonstatistically significant increase in JSC and η(eff). Secondly, we kept the silicon phthalocyanine core and substituted the tri-n-hexylsilyl solubilizing groups with pentadecyl phenoxy groups and tested the resulting dye in a BHJ OPV. While an increase in JSC and η(eff) was observed at low (PDP)2-SiPc loadings, the increase was not as significant as (3HS)2-SiPc; therefore, (3HS)2-SiPc is a unique additive. During our study, we observed that (3HS)2-SiPc had an extraordinary tendency to crystallize compared to the other compounds in this study and our general experience. On the basis of this observation, we have offered a hypothesis that when (3HS)2-SiPc migrates to the P3HT:PC61BM interface the reason for its unique performance is not solely due to its frontier orbital energies but also might be due to a high driving force for crystallization.

Nitroxide mediated controlled synthesis of glycidyl methacrylate-rich copolymers enabled by SG1-based alkoxyamines bearing succinimidyl ester groups

The controlled nitroxide-mediated copolymerization of glycidyl methacrylate (GMA) and styrene (S) with varying GMA molar feed fractions (fGMA,0 = 0.12–0.94) was accomplished by using a SG1-based alkoxyamine initiator bearing a N-succinimidyl ester group (NHS-BlocBuilder) in 50 wt% 1,4-dioxane solution at 90 °C. Copolymerizations indicated linear evolution of number average molecular weight Mn with respect to conversion up to approximately 50% and narrow molecular weight distributions with Mw/Mn = 1.22–1.44 and GMA incorporation into copolymer (FGMA) as high as 0.92. No additional SG1 free nitroxide was required to control polymerizations, even at high fGMA,0. Chain extensions of poly(GMA-ran-S) macroinitiators with S at 110 °C yielded a high fraction of block copolymer in most cases (except at the highest FGMA), as clear, monomodal shifts in Mn using gel permeation chromatography (GPC) were observed, thereby suggesting the poly(GMA-ran-S) macroinitiators were substantially “living”.

Assessing the Potential Roles of Silicon and Germanium Phthalocyanines in Planar Heterojunction Organic Photovoltaic Devices and How Pentafluoro Phenoxylation Can Enhance π–π Interactions and Device Performance

In this study, we have assessed the potential application of dichloro silicon phthalocyanine (Cl2-SiPc) and dichloro germanium phthalocyanine (Cl2-GePc) in modern planar heterojunction organic photovoltaic (PHJ OPV) devices. We have determined that Cl2-SiPc can act as an electron donating material when paired with C60 and that Cl2-SiPc or Cl2-GePc can also act as an electron acceptor material when paired with pentacene. These two materials enabled the harvesting of triplet energy resulting from the singlet fission process in pentacene. However, contributions to the generation of photocurrent were observed for Cl2-SiPc with no evidence of photocurrent contribution from Cl2-GePc. The result of our initial assessment established the potential for the application of SiPc and GePc in PHJ OPV devices. Thereafter, bis(pentafluoro phenoxy) silicon phthalocyanine (F10-SiPc) and bis(pentafluoro phenoxy) germanium phthalocyanine (F10-GePc) were synthesized and characterized. During thermal processing, it was discovered that F10-SiPc and F10-GePc underwent a reaction forming small amounts of difluoro SiPc (F2-SiPc) and difluoro GePc (F2-GePc). This undesirable reaction could be circumvented for F10-SiPc but not for F10-GePc. Using single crystal X-ray diffraction, it was determined that F10-SiPc has significantly enhanced π-π interactions compared with that of Cl2-SiPc, which had little to none. Unoptimized PHJ OPV devices based on F10-SiPc were fabricated and directly compared to those constructed from Cl2-SiPc, and in all cases, PHJ OPV devices based on F10-SiPc had significantly improved device characteristics compared to Cl2-SiPc.

The position and frequency of fluorine atoms changes the electron donor/acceptor properties of fluorophenoxy silicon phthalocyanines within organic photovoltaic devices

In a previous study we have shown the first example of silicon phthalocyanines (SiPcs) applied within organic photovoltaic (OPV) devices. In that study we showed the electronic performance of a SiPc is significantly increased by replacing the axial chloride groups with pentafluoro phenoxy moieties. It was further demonstrated that bis(pentafluoro phenoxy) SiPc (F10-SiPc) is best applied as an electron accepting material within a fullerene-free planar heterojunction (PHJ) OPV. Within this study we have synthesized a new series of fluorophenoxy silicon phthalocyanines ((XF)2-SiPc) whereby the frequency of the fluorine atoms on the fluorophenoxy groups was systematically reduced from 5. These relatively small changes resulted in small changes in UV-Vis absorption properties, thermal stability, electrochemical behavior, ultraviolet photoelectron spectroscopy characteristics and solid state arrangement in both single crystals and in thin films obtained by thermal evaporation. Single crystal X-ray diffraction determined that all (XF)2-SiPcs have significantly enhanced π–π interactions compared with dichlorosilicon phthalocyanine (Cl2-SiPc) and depending on the position and frequency of the fluorine atoms, the solid state arrangement varied significantly. The complete series of (XF)2-SiPcs were then introduced into PHJ OPVs as both electron accepting and electron donating materials when paired with pentacene, α-sexithiophene or C60. It was found that depending on the structure of the (XF)2-SiPcs the most favourable role for the material would either be as an electron donating or electron accepting material and in most cases the (XF)2-SiPcs outperformed Cl2-SiPc and F10-SiPc. One material, (246F)2-SiPc, did however emerge as one material that has dual functionality. Unoptimized PHJ OPV devices were generally characterized with open circuit voltages (VOC) as high as 0.94 V and power conversion efficiencies >2.0%. As only the second example of SiPcs being integrated into PHJ OPV devices, these results show that versatile phenoxylation chemistry can impact the application and performance and therefore also demonstrates great promise for this class of significantly understudied organic electronic materials. Finally these results give indications of the direction phenoxy-SiPc molecular design should take in order to potentially further improve their overall performance in PHJ OPV devices.

From chloro to fluoro, expanding the role of aluminum phthalocyanine in organic photovoltaic devices

Fluoro aluminum phthalocyanine (F-AlPc) was synthesized by simply heating a DMSO solution of chloro aluminum phthalocyanine (Cl-AlPc) in the presence of CsF, KF or NaF for less than an hour. The resulting F-AlPc has a significant blue shift in the absorbance of ≈8 nm in solution and ≈130 nm in a solid film compared to Cl-AlPc. Ultraviolet photoelectron spectroscopy (UPS) identified a change in work function and EHOMO of as much as 1 eV between Cl-AlPc and F-AlPc. Our observed change in UPS data, solid-state absorbance and sublimation temperature for F-AlPc further confirms the stacked fluorine bridge solid-state structure for F-AlPc previously described by Kenney et al. Preliminary planar heterojunction (PHJ) organic photovoltaic (OPV) devices were then fabricated using F-AlPc as an electron donating material paired with C60 and a ternary device including a Cl-AlPc interlayer. Additionally, an all AlPc device where F-AlPc functioned as the electron donor and Cl-AlPc as the electron acceptor was fabricated. The EQE plots of the resulting PHJ OPV devices illustrate that an exciton-rectifying layer is present between the Cl-AlPc and F-AlPc layers in the ternary devices as well as the all AlPc device. These results further exemplify that the seemingly minor change from chloride to fluoride in the AlPc structure has significant implications in optoelectronic properties and functionality of AlPc in PHJ OPV devices.

Evaluating Thiophene Electron‐Donor Layers for the Rapid Assessment of Boron Subphthalocyanines as Electron Acceptors in Organic Photovoltaics: Solution or Vacuum Deposition?

In this study, we consider the choice of a standard electron-donating material to be paired with boron subphthalocyanines (BsubPcs) to rapidly assess the viability of new BsubPc derivatives as electron-accepting materials within organic photovoltaic devices (OPVs). Specifically, we evaluate the effectiveness of solution-cast poly(3-hexylthiophene-2,5-diyl) (P3HT) as an electron donor paired with BsubPc derivatives relative to vacuum-deposited sexithiophene (α-6T). By using fullerene (C60 ), boron subphthalocyanine chloride (Cl-BsubPc), and hexachloro boron subphthalocyanine chloride (Cl-Cl6 BsubPc) as electron acceptors, we find that devices made with α-6T outperform those with P3HT. However, the two thiophene-based materials show the same performance trends. Given the preservation of these trends, we can recommend either option for assessing the potential of new BsubPc derivatives; P3HT as a solution-cast electron-donor layer or α-6T as a vacuum-deposited alternative.

Multifunctional ternary additive in bulk heterojunction OPV: increased device performance and stability

Great improvements in the development of organic photovoltaic (OPV) devices have been reported over the years; however, the overall efficiency and operational lifetimes of the devices must be improved. Maintaining a stable power conversion efficiency (PCE) in bulk heterojunction OPV devices can be achieved by utilizing cross-linkable ternary additives that freeze the optimal morphology. However, these additives currently do not contribute to improving the PCE of the device therefore limiting their overall effectiveness. In this study we present a dual functional bis(6-azidohexanoate)silicon phthalocyanine ((HxN3)2-SiPc) with cross-linking groups and near IR absorption as a ternary additive in poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PC61BM) devices. As an additive at 10 wt% with respect to PC61BM, (HxN3)2-SiPc increased the short circuit current density (Jsc) by ≈9% due to increased photocurrent generation in the near IR region. In addition, devices utilizing (HxN3)2-SiPc exhibited a 97% PCE retention after thermal ageing at 150 °C for 23 h (compared to 47% retention for baseline devices) showing the compound is an effective cross-linker. These findings represent the first example of a multifunctional dye additive in an OPV device that simultaneously broadens the spectral coverage, resulting in added photogeneration, and stabilizes the active layer morphology, resulting in increased PCE retention.

Boron Subphthalocyanine Polymers by Facile Coupling to Poly(acrylic acid‐ran‐styrene) Copolymers Synthesized by Nitroxide‐Mediated Polymerization and the Associated Problems with Autoinitiation

Boron subphthalocyanines (BsubPcs) are macrocyclic aromatic small molecules containing a chelated boron atom. BsubPcs have interesting optoelectronic and physical properties, justifying their use in various organic electronic devices such as organic solar cells and organic light-emitting diodes. However, our group has only recently reported the first incorporation of a BsubPc moiety into a polymer using a two-step post-polymerization procedure. This communication outlines the use of acrylic acid as a method for obtaining carboxylic acid functional copolymers for the facile coupling to BsubPc post polymerization. In addition, the observations and the proposed mechanism of a side product unique to the copolymerization of acrylic acid and styrene due to autoinitiation are presented.

Assessing the potential of group 13 and 14 metal/metalloid phthalocyanines as hole transport layers in organic light emitting diodes

In this study, we have assessed the potential application of group 13 and 14 metal and metalloid phthalocyanines ((X)n-MPcs) and their axially substituted derivatives as hole-transporting layers in organic light emitting diodes (OLEDs). OLEDs studied herein have the generic structure of glass/ITO/(N,N′-di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPB) or (X)n-MPc)(50 nm)/Alq3 (60 nm)/LiF (1 nm)/Al (80 nm), where X is an axial substituent group. OLEDs using chloro aluminum phthalocyanine (Cl-AlPc) showed good peak luminance values of 2620 ± 113 cd/m2 at 11 V. To our knowledge, Cl-AlPc has not previously been shown to work as a hole transport material (HTL) in OLEDs. Conversely, the di-chlorides of silicon, germanium, and tin phthalocyanine (Cl2-SiPc, Cl2-GePc, and Cl2-SnPc, respectively) showed poor performance compared to Cl-AlPc, having peak luminances of only 38 ± 4 cd/m2 (12 V), 23 ± 1 cd/m2 (8.5 V), and 59 ± 5 cd/m2 (13.5 V), respectively. However, by performing a simple axial substituti...