Thermally-activated delayed fluorescence (TADF) emitters-just like phosphorescent ones-can in principle allow for 100% internal quantum efficiency of organic light-emitting diodes (OLEDs), because the initially formed electron-hole pairs in the non-emissive triplet state can be efficiently converted into emissive singlets by reverse intersystem crossing. However, as compared to phosphorescent emitter complexes with their bulky-often close to spherical-molecular structures, TADF emitters offer the advantage to align them such that their optical transition dipole moments (TDMs) lie preferentially in the film plane. In this report, we address the question which factors control the orientation of TADF emitters. Specifically, we discuss how guest-host interactions may be used to influence this parameter and propose an interplay of different factors being responsible. We infer that emitter orientation is mainly governed by the molecular shape of the TADF molecule itself and by the physical properties of the host-foremost, its glass transition temperature T g and its tendency for alignment being expressed, e.g., as birefringence or the formation of a giant surface potential of the host. Electrostatic dipole-dipole interactions between host and emitter are not found to play an important role.

Heterostructures comprising two-dimensional (2D) semiconductors fabricated by individual stacking exhibit interesting characteristics owing to their 2D nature and atomically sharp interface. As an emerging 2D material, black phosphorus (BP) nanosheets have drawn much attention because of their small band gap semiconductor characteristics along with high mobility. Stacking structures composed of p-type BP and n-type transition metal dichalcogenides can produce an atomically sharp interface with van der Waals interaction which leads to p-n diode functionality. In this study, for the first time, we fabricated a heterojunction p-n diode composed of BP and WS. The rectification effects are examined for monolayer, bilayer, trilayer, and multilayer WS flakes in our BP/WS van der Waals heterojunction diodes and also verified by density function theory calculations. We report superior functionalities as compared to other van der Waals heterojunction, such as efficient gate-dependent static rectification of 2.6 × 10, temperature dependence, thickness dependence of rectification, and ideality factor of the device. The temperature dependence of Zener breakdown voltage and avalanche breakdown voltage were analyzed in the same device. Additionally, superior optoelectronic characteristics such as photoresponsivity of 500 mA/W and external quantum efficiency of 103% are achieved in the BP/WS van der Waals p-n diode, which is unprecedented for BP/transition metal dichalcogenides heterostructures. The BP/WS van der Waals p-n diodes have a profound potential to fabricate rectifiers, solar cells, and photovoltaic diodes in 2D semiconductor electronics and optoelectronics.

employing a purely organic TADF emitter, 4CzIPN. [3] TADF molecules have an energy gap between the S 1 and T 1 states, ΔE ST , sufficiently small to allow the triplet excitons to convert into singlet excitons through reverse intersystem crossing (RISC), making possible their radiative decay from the singlet excited state, evidenced by a delayed fluorescence. To minimize ΔE ST and enhance k RISC , the overlap between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of the molecule must likewise be small, which is achievable in compounds containing electron-donating and electron-accepting moieties that are poorly electronically coupled giving the excited state an intra-molecular charge-transfer (CT) character. [4] The most common emitter designs rely on a large twist angle between the donor and acceptor coupled with the incorporation of aromatic spacer bridging moieties. [5] The external quantum efficiency (EQE) of the OLED is a function not only of the IQE but also of the light out-coupling efficiency. As the stack of an OLED device is composed of several different layers of materials included between two electrodes, total internal reflection at the interface between two media, and coupling to surface plasmon polaritons (SPP) at the interface with Organic thermally activated delayed fluorescent (TADF) materials can harvest 100% of the electrically generated excitons as a result of their small singlet-triplet energy difference. However, maximizing the external quantum efficiency (EQE) of a device also requires enhancing the light out-coupling efficiency. This work presents a new acceptor-donor-acceptor (ADA) emitter employing an indolocarbazole donor and diphenyltriazine acceptors that show nearly-completely horizontal orientation regardless of the host matrix, leading to a sky-blue organic light-emitting diode (λ EL = 483 nm, CIE coordinates of 0.17, 0.32) with EQE MAX of 22.1%, a maximum luminance of 7800 cd m −2 , and blue emission.

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Here, we report the synthesis of a vertical MoSe 2 /WSe 2 p-n heterostructure using a sputtering-CVD method. Unlike the conventional CVD method, this method produced a continuous MoSe 2 /WSe 2 p-n heterostructure. WSe 2 and MoSe 2 back-gated field effect transistors (FETs) exhibited good gate modulation behavior, and high hole and electron mobilities of $2.2 and $15.1 cm 2 V À1 s À1 , respectively.The fabricated vertical MoSe 2 /WSe 2 p-n diode showed rectifying I-V behavior with back-gate tunability.The rectification ratio of the diode was increased with increasing gate voltage, and was increased from $18 to $1600 as the gate bias increased from À40 V to +40 V. This is attributed to the fact that the barrier height between p-WSe 2 and n-MoSe 2 is modulated due to the back-gate bias. The rectification ratio is higher than the previously reported values for the TMDC p-n heterostructure grown by CVD.