Semiconductor nanoplatelets exhibit spectrally pure, directional fluorescence. To make polarized light emission accessible and the charge transport effective, nanoplatelets have to be collectively oriented in the solid state. We discovered that the collective nanoplatelets orientation in monolayers can be controlled kinetically by exploiting the solvent evaporation rate in self-assembly at liquid interfaces. Our method avoids insulating additives such as surfactants, making it ideally suited for optoelectronics. The monolayer films with controlled nanoplatelets orientation (edge-up or face-down) exhibit long-range ordering of transition dipole moments and macroscopically polarized light emission. Furthermore, we unveil that the substantial in-plane electronic coupling between nanoplatelets enables charge transport through a single nanoplatelets monolayer, with an efficiency that strongly depends on the orientation of the nanoplatelets. The ability to kinetically control the assembly of nanoplatelets into ordered monolayers with tunable optical and electronic properties paves the way for new applications in optoelectronic devices.

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We present measurements of rubrene single crystal field-effect transistors with textbooklike transfer characteristics, as one would expect for intrinsically trap-free semiconductor devices. Particularly, the high purity of the crystals and the defect-free interface to the gate dielectric are reflected in an unprecedentedly low subthreshold swing of 65 mV/decade, remarkably close to the fundamental limit of 58.5 mV/decade. From these measurements we quantify the residual density of traps by a detailed analysis of the subthreshold regime, including a full numerical simulation. An exceedingly low trap density of D bulk = 1 × 10 13 cm −3 eV −1 at an energy of ∼ 0.62 eV is found. This result corresponds to one trap per eV in 10 8 rubrene molecules. The equivalent density of traps located at the interface (Dit = 3 × 10 9 cm −2 eV −1 ) is as low as in the best crystalline SiO2/Si field-effect transistors. These results highlight the benefit of having van der Waals bonded semiconducting crystals without electronically active states due to broken bonds at the surface.

In light-emitting electrochemical cells (LECs), the position of the emission zone (EZ) is not predefined via a multilayer architecture design, but governed by a complex motion of electrical and ionic charges. As a result of the evolution of doped charge transport layers that enclose a dynamic intrinsic region until steady state is reached, the EZ is often dynamic during turn-on. For thick sandwich polymer LECs, a continuous change of the emission colour provides a direct visual indication of a moving EZ. Results from an optical and electrical analysis indicate that the intrinsic zone is narrow at early times, but starts to widen during operation, notably well before the electrical device optimum is reached. Results from numerical simulations demonstrate that the only precondition for this event to occur is that the mobilities of anions (μa) and cations (μc) are not equal, and the direction of the EZ shift dictates μc > μa. Quantitative ion profiles reveal that the displacement of ions stops when the intrinsic zone stabilizes, confirming the relation between ion movement and EZ shift. Finally, simulations indicate that the experimental current peak for constant-voltage operation is intrinsic and the subsequent decay does not result from degradation, as commonly stated.