Hydrogenated amorphous silicon germanium alloys (a-SiGe:H) have been prepared by rf glow discharge of silane, germane, and hydrogen gas mixture at substrate temperature of 200 and 250 °C. The structural properties of the films have been investigated by infrared, Raman, and secondary ion mass spectroscopy. It is found that there is a preferential incorporation of germanium into the film relative to silicon and the films with high germane gas phase composition Xg>0.4 tend to oxidize in atmosphere. Besides, polysilane is enhanced in the films with low germane gas phase composition. The electrical properties including dark, photo conductivities, and conduction activation energy are measured. As for the optical properties, optical transmission is adopted to determine the optical gap while photoluminescence spectra together with temperature variation are used to study the band tail states of the films. By applying Brodsky’s quantum well model, the various optical and electrical properties could be explained successfully.
Deep-blue thermally activated delayed fluorescence (TADF) molecules present promising potential in organic light-emitting diodes (OLEDs), especially for display applications. Here, an efficient molecular engineering approach to modifying the donor or acceptor features of the D−π−A-configured TADF molecules for deep-blue emission is reported. By introducing oxygen and sulfone as a bridge unit onto the macrocyclic donor, two emitters, c-ON-MeTRZ and c-NS-MeTRZ, are synthesized and characterized, respectively. The reduced donor strength of c-ON-MeTRZ and c-NS-MeTRZ as compared to that of the model molecule c-NN-MeTRZ leads to blue-shifted emissions with high photoluminescence quantum yields (PLQYs) and retains TADF characters, while the new emitter c-NN-MePym with the most blue-shifted emission only exhibits a pure fluorescent nature because of the electron-accepting feature of pyrimidine that is insufficient for inducing the TADF property. In the presence of macrocyclic donors, these new emitters show high horizontal dipole ratios (Θ // = 85−89%), which are beneficial for improving the light out-coupling efficiency. Deep-blue TADF OLEDs incorporating c-ON-MeTRZ as an emitter doped in the mCPCN host achieves a high maximum external quantum efficiency (EQE max ) of 30.2% together with 1931 Commission Internationale de I'Eclairage (CIE) coordinates of (0.14, 0.13), while the counter device employing c-NS-MeTRZ as a dopant gives EQE max of 15.4% and CIE coordinates of (0.14, 0.09). The EQE max of c-ON-MeTRZ-and c-NS-MeTRZ-based devices can be significantly improved to 34.4 and 29.3%, respectively, with a polar host DPEPO, which stabilizes the charge transfer (CT) S 1 state to give lower ΔE ST for improving the reverse intersystem crossing process. The efficient TADF character, high PLQYs, and high anisotropic emission dipole ratios work together to render the superior electroluminescence (EL) efficiencies. Based on the detailed characterizations of physical properties, theoretical analyses, and comprehensive study on the corresponding devices, a clear structure−property-performance relationship has been successfully established to verify the effective molecular design strategy of modulating the macrocyclic donor characters for efficient deep-blue TADF emitters.
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