Photogeneration and transport of mobile photocarriers in high-mobility crystalline organic semiconductors remain underexplored. The photo-Hall effect was used to address the fundamental charge transport properties of these functional molecular materials, without the need for fabricating complex transistor devices or chemical doping. Here we demonstrate a photo-Hall effect in organic semiconductors, using a benchmark molecular system rubrene as an experimental platform. We show that this technique can be used to directly measure the charge carrier mobility and photocarrier density, disentangle the surface and bulk transport phenomena, as well as deepen our understanding of the mechanism of photoconductivity in these high-performance materials.
KEYWORDS: Crystals, Ultrafast phenomena, System on a chip, Picosecond phenomena, Photovoltaics, Organic materials, Organic electronics, Near infrared, Crystallography, Americium
Singlet fission (SF), which allows one singlet state to be converted to 2 triplets, is one of the most perspective phenomena that may facilitate overcoming of the Shockley-Quiser limit in organic and hybrid photovoltaics.
Rubrene, mobility champion of organic electronics, is one of the most popular SF materials. Yet, despite its popularity, SF fundamentals in Rubrene remain strongly debated in the literature due to both experimental and computational limitations.
In this work we applied sub-10 fs transient absorption spectroscopy (TAS) to fully disentangle SF mechanism in low-defects high-quality Rubrene single crystals. We found that on 0.2 ps – 6 ns timescale, SF may be treated as 2 components process with half of the singlets to be converted into triplets at 10ps. Fascinatingly, at early times (<0.2 ps) we found additional component to be involved, which may be associated with hybrid state facilitating coherent SF. Based on our experimental findings, we have built a complete model of singlet fission in crystalline rubrene, which may help to resolve current debates on SF in the literature.
Thiophene-phenylene co-oligomers (TPCO) single crystals are promising materials for organic light-emitting devices, e.g., light-emitting transistors (OLETs), due to their ability to combine high luminescence and efficient charge transport. However, optical confinement in platy single crystals strongly decreases light emission from their top surface degrading the device performance. To avoid optical waveguiding, single crystals thinner than 100 nm would be beneficial. Herein, we report on solution-processed ultrathin single crystals of TPCO and study their charge transport properties. As materials we used 1,4-bis(5'-hexyl-2,2'-bithiophene-5-yl)benzene (DH-TTPTT) and 1,4-bis(5'-decyl-2,2'-bithiophene-5-yl)benzene (DD-TTPTT). The ultrathin single crystals were studied by optical polarization, atomic-force, and transmission electron microscopies, and as active layers in organic field effect transistors (OFET). The OFET hole mobility was increased tenfold for the oligomer with longer alkyl substituents (DD-TTPTT) reaching 0.2 cm2/Vs. Our studies of crystal growth indicate that if the substrate is wetted, it has no significant effect on the crystal growth. We conclude that solution-processed ultrathin TPCO single crystals are a promising platform for organic optoelectronic field-effect devices.
Оrganic field-effect transistors (OFET) can combine photodetection and light amplification and, for example, work as phototransistors. Such organic phototransistors can be used in light-controlled switches and amplifiers, detection circuits, and sensors of ultrasensitive images. In this work, we present photophysical characterization of well-defined ultrathin organic field-effect devices with a semiconductive channel based on Langmuir-Blodgett monolayer film. We observe clear generation of photocurrent under illumination with a modulated laser at 405 nm. The increase of photocurrent with the optical modulation frequency indicates the presence of defect states serving as traps for photogenerated carriers and/or the saturation of charge concentration in the thin active layer. We also propose a simple one-dimensional numerical model of a photosensitive OFET. The model is based on the Poisson, current continuity and drift-diffusion equations allows future evaluation of the photocurrent generation mechanism in the studied systems.
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