Silicon Phthalocyanines for n-Type Organic Thin-Film Transistors: Development of Structure–Property Relationships

Abstract

Silicon phthalocyanines (SiPcs) have shown great potential as n-type or ambipolar organic semiconductors in organic thin-film transistors (OTFTs) and organic photovoltaics. Although properly designed SiPcs rival current state-of-the-art n-type organic semiconducting materials, relatively few structure–property relationships have been established to determine the impact of axial substituents on OTFT performance, hindering the intelligent design of the next generation of SiPcs. To address this omission, we have developed structure–property relationships for vapor-deposited SiPcs with phenoxy axial substituents. In addition to thorough electrical characterization of bottom-gate top-contact OTFTs, we extensively investigated SiPc thin films using X-ray diffraction, atomic force microscopy (AFM), grazing-incidence wide-angle X-ray scattering (GIWAXS), and density functional theory (DFT) modeling. OTFT performance, including relative electron mobility (μe) of materials, was in general agreement with values obtained through DFT modeling including reorganization energy. Another significant trend observed from device performance was that increasing the electron-withdrawing character of the axial pendant groups led to a reduction in threshold voltage (V T) from 47.9 to 21.1 V. This was corroborated by DFT modeling, which predicted that V T decreases with the square of the dipole induced at the interface between the SiPc pendant and substrate. Discrepancies between modeling predictions and experimental results can be explained through analysis of thin-film morphology and orientation by AFM and GIWAXS. Our results demonstrate that a combination of DFT modeling to select prospective candidate materials, combined with appropriate processing conditions to deposit molecules with a favorable thin-film morphology in an “edge-on” orientation relative to the substrate, yields high-performance n-type SiPc-based OTFTs.

Metal phthalocyanines: thin-film formation, microstructure, and physical properties

“…17iii). 130,132 GISAXS is used to analyze the nanoscale surface morphology of polymer and multi-component thin-lms with some use in quantifying domain size in singlecomponent small molecule lms as demonstrated in Fig. 17iv which characterizes CuPc thin-lm formation using different annealing temperatures on different surfaces.…”

“…13ii and iii ) show two distinct morphologies either consisting of small regular circular grains or more elongated rectangular grains depending on the structure of the phenoxy substituent. 130,131 Additionally, R 2 -SiPc molecules with alkyl axial substituents fabricated by solution processing ( Fig. 13iv ) highlight how the alkyl chain length, branching position and symmetry affect thin-film morphology; creating films with either small dense cylindrical grains, large interconnected grains, or very large plate-like features.…”

Changes in Optimal Ternary Additive Loading when Processing Large Area Organic Photovoltaics by Spin‐ versus Blade‐Coating Methods

“…[30] In fact, SiPc derivatives in general are mostly n-type semiconductors. [31,32] However, there are some examples of their hole-transport properties. [13] Based on our previous work, we surmise the hole mobility of (3HS) 2 -SiPc is lower than that of PCDTBT.…”

Variance-resistant PTB7 and axially-substituted silicon phthalocyanines as active materials for high-Voc organic photovoltaics

“…Axially substituted silicon phthalocyanines (R 2 -SiPc) are ideal candidates for low-cost, high- Voc acceptor materials 24 , 26 , 27 . While metal phthalocyanine (MPc) have been investigated in organic electronic applications for more than 50 year, R 2 -SiPcs are relatively understudied, having emerged in recent years 28 and successfully incorporated in multiple new application, including organic thin-film transistors (OTFTs) 29 – 34 , organic light-emitting diodes (OLEDs) 26 , 35 , 36 and in OPVs 24 , 26 , 37 – 40 . The synthetic complexity (SC) index 41 of R 2 -SiPcs have been calculated to be at least three times lower (SC = 12) 27 than that of several prominent OPV acceptors materials, such as PC 61 BM (SC = 36) 42 , Y6 (SC = 59) 43 and ITIC (SC = 67) 44 .…”