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Author: Hambsch, Mike × End date: 2022 × Start date: 2020 × DDC: 540 × Has files: Yes × Publisher: Weinheim : Wiley-VCH × Subject: charge transfer ×

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    Electronic Doping and Enhancement of n‐Channel Polycrystalline OFET Performance through Gate Oxide Modifications with Aminosilanes
    (Weinheim : Wiley-VCH, 2021) Shin, Nara; Schellhammer, Karl Sebastian; Lee, Min Ho; Zessin, Jakob; Hambsch, Mike; Salleo, Alberto; Ortmann, Frank; Mannsfeld, Stefan C.B.
    Self-assembled monolayers (SAMs) are widely employed in organic field-effect transistors to modify the surface energy, surface roughness, film growth kinetics, and electrical surface potential of the gate oxide to control the device's operating voltage. In this study, amino-functionalized SAM molecules are compared to pure alkylsilane SAMS in terms of their impact on the electrical properties of organic field-effect transistors, using the n-type polycrystalline small molecule semiconductor material N,N′-dioctyl-3,4,9,10-perylenedicarboximide (PTCDI-C8). In order to understand the electronic impact of the amino groups, the effect of both the number of amino-containing functional groups and the SAM molecular length are systematically studied. Though amino-functionalized SAM materials have been studied previously, this study is, for the first time, able to shed light on the nature of the doping effect that occurs when the gate oxide is treated with polar aminosilane materials. By a comprehensive theoretical study of the interface on the molecular level, it is shown that the observed shift in the threshold voltage is caused by free charges, which are attracted to the PTCDI-C8 and are stabilized there by protonated aminosilanes. This attraction and the voltage shift can be systematically tuned by varying the length of the neutral terminal chain of the aminosilane. © 2021 The Authors. Advanced Materials Interfaces published by Wiley-VCH GmbH
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    Short Excited-State Lifetimes Mediate Charge-Recombination Losses in Organic Solar Cell Blends with Low Charge-Transfer Driving Force
    (Weinheim : Wiley-VCH, 2021) Shivhare, Rishi; Moore, Gareth John; Hofacker, Andreas; Hutsch, Sebastian; Zhong, Yufei; Hambsch, Mike; Erdmann, Tim; Kiriy, Anton; Mannsfeld, Stefan C.B.; Ortmann, Frank; Banerji, Natalie
    A blend of a low-optical-gap diketopyrrolopyrrole polymer and a fullerene derivative, with near-zero driving force for electron transfer, is investigated. Using femtosecond transient absorption and electroabsorption spectroscopy, the charge transfer (CT) and recombination dynamics as well as the early-time transport are quantified. Electron transfer is ultrafast, consistent with a Marcus-Levich-Jortner description. However, significant charge recombination and unusually short excited (S1 ) and CT state lifetimes (≈14 ps) are observed. At low S1 -CT offset, a short S1 lifetime mediates charge recombination because: i) back-transfer from the CT to the S1 state followed by S1 recombination occurs and ii) additional S1 -CT hybridization decreases the CT lifetime. Both effects are confirmed by density functional theory calculations. In addition, relatively slow (tens of picoseconds) dissociation of charges from the CT state is observed, due to low local charge mobility. Simulations using a four-state kinetic model entailing the effects of energetic disorder reveal that the free charge yield can be increased from the observed 12% to 60% by increasing the S1 and CT lifetimes to 150 ps. Alternatively, decreasing the interfacial CT state disorder while increasing bulk disorder of free charges enhances the yield to 65% in spite of the short lifetimes.