Doping-induced decomposition of organic semiconductors: a caveat to the use of Lewis acid p-dopants

Abstract

Solution-processable molecular dopants are popular wet-lab mediators to engineer the electronic properties of organic semiconductors and to optimize the level performance of their corresponding devices. Nonetheless, the exact doping mechanism that is operative during the interaction of organic semiconductors with Lewis acid species is not fully elaborated. The products of the doping reactions between Lewis acids and organic semiconductors have not been studied in detail. Here we focus on the macromolecular poly[bis(4-phenyl)(2,4-dimethylphenyl)]amine (PTAA) and molecular fluorinated anthradithiophene (diF-TES-ADT) organic semiconductors for addressing their chemical integrity after p-doping by the tris(pentafluorophenyl) borane [B(C₆F₅)₃] Lewis acid agent. The PTAA and diF-TES-ADT organic substrates are studied in mixtures with B(C₆F₅)₃ at three discrete concentration regimes. In the dilute solution regime, UV-Vis absorption spectroscopy verifies the effectiveness of p-doping by the changes observed in the absorption spectra of the solutions at increased B(C₆F₅)₃ content. In the concentrated solution regime, the reactivity of B(C₆F₅)₃ with PTAA and diF-TES-ADT is monitored by proton nuclear magnetic resonance (¹H-NMR) and electrospray ionization mass spectroscopy (ES-MS), as well as thin-layer chromatography (TLC). Finally, in the solid-state the photophysical properties of spin-coated PTAA:B(C₆F₅)₃ and diF-TES-ADT:B(C₆F₅)₃ films are examined as a function of their B(C₆F₅)₃ content. Density functional theory (DFT) calculations corroborate the experimental findings. Both theoretical and experimental results exclude the formation of Lewis adduct species in the PTAA:B(C₆F₅)₃ and diF-TES-ADT:B(C₆F₅)₃ systems. In agreement with recent literature, the B(C₆F₅)₃ reactivity is attributed to the Brønsted-type acidity of the hydrated B(C₆F₅)₃–OH₂ complex that induces p-doping via the protonation of the organic substrates. The formation of the B(C₆F₅)₃–OH₂ acidic agent is identified experimentally by its characteristic ¹H-NMR signal at 4.7 ppm. All results for the three concentration regimes provide evidence for the occurrence of PTAA and diF-TES-ADT decomposition in the presence of B(C₆F₅)₃. At high B(C₆F₅)₃ loadings, ES-MS spectroscopy and TLC analysis suggest that B(C₆F₅)₃ remains unreacted, revealing the catalytic role in the decomposition process of PTAA and diF-TES-ADT. The results suggest that after interacting with Lewis acids, organic semiconductors may undergo detrimental decomposition reactions. This potentially undesired chemical reactivity should be considered for evaluating the operation stability of the p-doped electronic devices.