Organic LEDs and solar cells united

Process Parameter Optimization of a Polymer Derived Ceramic Coatings for Producing Ultra-High Gas Barrier

Silica is one of the most efficient gas barrier materials, and hence is widely used as an encapsulating material for electronic devices. In general, the processing of silica is carried out at high temperatures, i.e., around 1000 °C. Recently, processing of silica has been carried out from a polymer called Perhydropolysilazane (PHPS). The PHPS reacts with environmental moisture or oxygen and yields pure silica. This material has attracted many researchers and has been widely used in many applications such as encapsulation of organic light-emitting diodes (OLED) displays, semiconductor industries, and organic solar cells. In this paper, we have demonstrated the process optimization of the conversion of the PHPS into silica in terms of curing methods as well as curing the environment. Various curing methods including exposure to dry heat, damp heat, deep UV, and their combination under different environments were used to cure PHPS. FTIR analysis suggested that the quickest conversion method is the irradiation of PHPS with deep UV and simultaneous heating at 100 °C. Curing with this method yields a water permeation rate of 10⁻³ g/(m²⋅day) and oxygen permeation rate of less than 10⁻¹ cm³/(m²·day·bar). Rapid curing at low-temperature processing along with barrier properties makes PHPS an ideal encapsulating material for organic solar cell devices and a variety of similar applications.

Spiral Donor Design Strategy for Blue Thermally Activated Delayed Fluorescence Emitters

Thermally activated delayed fluorescence (TADF) emitters with a spiral donor show tremendous potential toward high-level efficient blue organic light-emitting diodes (OLEDs). However, the underlying design strategy of the spiral donor used for blue TADF emitters remains unclear. As a consequence, researchers often do "try and error" work in the development of new functional spiral donor fragments, making it slow and inefficient. Herein, we demonstrate that the energy level relationships between the spiral donor and the luminophore lead to a significant effect on the photoluminescent quantum yields (PLQYs) of the target materials. In addition, a method involving quantum chemistry simulations that can accurately predict the aforementioned energy level relationships by simulating the spin density distributions of the triplet excited states of the spiral donor and corresponding TADF emitters and the triplet excited natural transition orbitals of the TADF emitters is established. Moreover, it also revealed that the steric hindrance in this series of molecules can form a nearly unchanged singlet (S₁) state geometry, leading to a reduced nonradiative decay and high PLQY, while a moderated donor-acceptor (D-A) torsion in the triplet (T₁) state can induce a strong vibronic coupling between the charge-transfer triplet (³CT) state and the local triplet (³LE) state, achieving an effective reverse intersystem crossing (RISC) process. Furthermore, an electric-magnetic coupling is formed between the high-lying ³LE state and the charge-transfer singlet (¹CT) state, which may open another RISC channel. Remarkably, in company with the optimized molecular structure and energy alignment, the pivotal TADF emitter DspiroS-TRZ achieved 99.9% PLQY, an external quantum efficiency (EQE) of 38.4%, which is the highest among all blue TADF emitters reported to date.

Charge Energetics and Electronic Level Changes At the Copper(II) Phthalocyanine/Fullerene Junction Upon Photoexcitation

Energy offset at the donor (D)/acceptor (A) interface plays an important role in charge separation in organic photovoltaics. Its magnitude determines the charge separation process under illumination. Extensive studies have been carried out for elucidating the charge transfer (CT) process at different D/A junctions. These works lead to two different views: upon photoexcitation, energies would be (1) consumed in molecular polarization and orientation such that those opposite charges would overcome mutual Coulombic attractive potential at the interface and (2) spent on promoting charges to high-lying delocalized energy states (i.e., hot states), which is necessarily important prior to charge separation. Under these two scheme of studies, the electronic structures and the charge behaviors at the D/A interface should be different under photoexcitation, yet there is so far no direct experimental approach for probing the changes in electronics structures (i.e., Fermi level, vacuum level, frontier molecular orbitals, etc.) upon photoexcitation. Herein, a modified photoelectron spectroscopy (PES) system with an additional solar simulator is used to study the charge distributions and electronic interactions for a standard D/A heterojunction (i.e., copper phthalocyanine (CuPc)/ fullerene (C₆₀)) under photoexcitation. CT states formed as a result of photon energy transfer at the CuPc/C₆₀ junction. Subsequent superpositions of charge transfer and electron polarization effects increase the D/A energy level offsets from 0.75 (ground state measured in the dark) to 1.07 eV (high-lying state measured upon illumination). We showed that there is excess energy consumed for a subtle change in the energy level alignment of the CuPc/C₆₀ junction under illumination, suggesting a new insight for the energy loss mechanism during the photocharge generation processes.