π-Extended Porphyrin–Phthalocyanine Heterojunction Devices Exhibiting High Ammonia Sensitivity with a Remarkable Light Effect

Abstract

π-Extended porphyrins represent an attractive class of organic compounds because of their unique photophysical, optoelectronic, and physicochemical properties. Herein, crossconjugated (Ace-PQ-Ni) and linear-conjugated (AM6) porphyrins are used to build double-layer heterojunction devices by combining them with a lutetium bisphthalocyanine complex (LuPc 2 ). The heterojunction effect at the porphyrin−phthalocyanine interface plays a key role in the charge transport properties. Both devices exhibit exceptionally high ammonia sensitivity at room temperature and under ambient relative humidity, with limit of detection values of 156 and 115 ppb for Ace-PQ-Ni/LuPc 2 and AM6/LuPc 2 sensors, respectively. Interestingly, the Ace-PQ-Ni/LuPc 2 and AM6/LuPc 2 sensors display opposite effects upon light illumination. While the former sensors show largely decreased ammonia sensitivity under light illumination, the current variation of the latter under ammonia is remarkably enhanced with a multiplication factor of 13 and a limit of detection (LOD) of 83 ppb. The striking difference in their sensing properties upon light illumination is attributed to their different π-conjugation pathways (cross-conjugation versus linear conjugation).

Ambipolar Heterojunction Sensors: Another Way to Detect Polluting Gases

“…Hence, the effective capacitance ( C eff ) is calculated using Brug’s relationship (eq ) C eff i = Q i 1 / α R i ( 1 − α ) / α The variations in bulk charge transport ( R 1 , α 1 , and C eff1 ) and interfacial charge transport ( R 2 , α 2 , and C eff2 ) with DC bias are depicted in Figure S5. Similar to our previous studies, ,, the bulk resistance (R1) demonstrates minimal dependency on the applied bias, whereas the interfacial resistance ( R 2 ) exhibits an exponential decline as the DC bias increases. This indicates that the interfacial charge transport mobility increases as the DC bias increases, while bulk charge transport remains relatively stable.…”

“…To investigate the electrical properties, I – V measurements were performed on both devices at applied bias in the range from −10 to +10 V. As expected for bilayer organic heterojunction devices, ZnF 8 Pc/LuPc 2 and CoF 8 Pc/LuPc 2 devices exhibit symmetric and nonlinear I – V curves (Figure ). The nonlinearity in I – V curves is due to the difference in work function, which leads to accumulation of mobile charges at the interface between the two layers. , Considering the distance between interdigitated electrodes (75 μm) and the thickness of the layers (50 nm), the horizontal resistance of the sublayer is exceptionally enormous compared to the vertical resistance. Due to the geometry of the IDE ITO electrodes, charges predominantly flow through the interface between materials to access the most conductive pathway.…”

“…Molecular semiconductor-based organic heterojunction devices represent a fascinating frontier in the field of organic electronics, enabling applications ranging from energy harvesting in solar cells to light emission in organic light-emitting diodes (OLEDs) and the development of OFETs, which hold promise for flexible and wearable electronics . Unlike silicon-based counterparts, organic semiconductors offer mechanical flexibility, solution processability, and potentially lower manufacturing costs. , These materials exhibit semiconducting behavior through the delocalization of π-electrons across conjugated molecular structures and their electrical properties are highly tunable by modifying the molecular structure, such as varying the size and arrangement of aromatic rings, substituents, or conjugation length. , Most interestingly, molecular semiconductors exhibiting ambipolar charge transport properties have become a focal point in recent years within the field of organic electronics . Optimizing the frontier molecular orbitals, the highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO), relative to the Fermi energy level of electrodes, can influence the polarity of the molecular material and also facilitate ambipolar charge transport. − Ambipolarity is often referred to as a physical balance, characterized by an infinite number of unbalanced positions but only one stable state.…”

Oxy Phosphorus Triazatetrabenzocorrole as a p-Type Organic Semiconductor in Organic Thin Film Transistors

“…Metal and main group phthalocyanines (Pcs) and their analogues are part of a family of materials that are continuously evolving in the field of organic electronics. − In addition to their highly conjugated system, the incorporation of certain metal and metalloid elements at the center of the framework allows fine-tuning of its unique electronic behavior, thus improving the overall semiconducting abilities of the material. ,− The added advantage of having higher oxidation state (+3 or higher) elements occupy the central position of Pcs is the inclusion of axial components to the macrocycle that can be exploited to tailor the chemical and physical properties of the material. − Additional derivatization can also be probed at the periphery of the conjugated framework, introducing other mechanisms in which to optimize Pcs to enhance performance in the desired application. , As such, phthalocyanine and its derivatives have been exploited as active materials in a plethora of applications including, but not limited to, organic photovoltaics (OPVs), − organic thin film transistors (OTFTs), ,, and sensors. − …”