Impact of Photoluminescence Imaging Methodology on Transport Parameters in Semiconductors.

Triplet-triplet annihilation-induced delayed emission provides a pathway for investigating triplets via emission spectroscopy. This bimolecular annihilation depends directly on the transport properties of triplet excitons in disordered organic semiconductors. Photoluminescence (PL) imaging is a direct method for studying exciton and charge-carrier diffusivity. However, most of these studies neglect dispersive transport. Early time scale measurements using this technique can lead to an overestimation of the diffusion coefficient (DT) or diffusion length (Ld). In this study, we investigated the time-dependent triplet DT using PL imaging. We observed an overestimation of Ld in classical delayed PL imaging, often 1 order of magnitude higher than the actual Ld value. We compared various thicknesses of polymeric thin films to study the dispersive nature of triplet excitons. Transient analysis of delayed PL imaging and steady state imaging reveals the importance of considering the time-dependent nature of DT for the triplet excitons in disordered electronic materials.

Growth of Hybrid Perovskite Films via Single-Source Perovskite Nanoparticle Evaporation.

Herein, we demonstrate a vacuum-based evaporation approach to fabricate organic-inorganic perovskite thin films by using phase-formed halide perovskite nanoparticles (NPs) as a precursor/source. We are able to consistently obtain MAPbX3 (MA=CH3 NH3 and X=Cl, Br or I) thin films at various substrates (e. g., glass, ITO, or plastic). The perovskite phase formation in thin film form is confirmed by x-ray diffraction (XRD) studies. Small micro-strain (tensile) values of 1.64×10-3 , 1.42×10-3 and 6.85×10-4 obtained for MAPbCl3 , MAPbBr3 and MAPbI3 films respectively from Williamson-Hall equation indicate low structural distortions in perovskite thin films. The absorption spectra of thin films show sharp band edge having direct band gap, which is followed by narrow full width at half maxima (FWHM ∼0.1 eV) of the emission peak. Thin films of MAPbCl3 , MAPbBr3 and MAPbI3 show direct band gap of 3.1 eV, 2.4 eV and 1.6 eV, respectively. Small Urbach energy values of 33 meV, 44 meV and 66 meV for MAPbCl3 , MAPbBr3 and MAPbI3 films respectively indicates low defect density in various perovskite films. Scanning electron microscopy (SEM) along with energy-dispersive X-ray spectroscopy (EDS) shows high surface coverage and uniform chemical composition of MAPbX3 (X=Cl, Br and I) thin films deposited by the present method. We have successfully controlled the film thickness from 250 nm to 1 µm by varying the nanoparticle precursor amount. The perovskite thin films deposited by the present method are highly stable against the degradation under ambient conditions. Systematic XRD studies along with absorption data demonstrate that the MAPbCl3 and MAPbBr3 films stored under ambient conditions remained stable for more than 30 days and MAPbI3 films for more than 7 days.

Ambient Room Temperature Phosphorescence and Thermally Activated Delayed Fluorescence from a Core-Substituted Pyromellitic Diimide Derivative.

Triplet harvesting under ambient conditions plays a crucial role in improving the luminescence efficiency of purely organic molecular systems. This requires elegant molecular designs that can harvest triplets either via room temperature phosphorescence (RTP) or by thermally activated delayed fluorescence (TADF). In this context, here we report a donor core-substituted pyromellitic diimide (acceptor) derivative as an efficient charge-transfer molecular design from the arylene diimide family as a triplet emitter. Solution-processed thin films of carbazole-substituted CzPhPmDI display both RTP- and TADF-mediated twin emission with a long lifetime and high efficiency under ambient conditions. The present study not only sheds light on the fundamental photophysical process involved in the triplet harvesting of donor-acceptor organic systems, but also opens new avenues in exploring an arylene diimide class of molecules as potential organic light-emitting materials.

Anthracene-Resorcinol Derived Covalent Organic Framework as Flexible White Light Emitter.

The ordered modular structure of a covalent organic framework (COF) facilitates the selective incorporation of electronically active segments that can be tuned to function cooperatively. This designability inspires developing COF-based single-source white light emitters, required in next-generation solid-state lighting. Here, we present a new anthracene-resorcinol-based COF exhibiting white light emission. The keto-enol tautomers present in the COF give rise to dual emission, which can be tuned by the O-donor and N-donor solvents. Importantly, when suspended in a solid polymer matrix, this dual emission is retained as both tautomers coexist. A mere 0.32 wt % loading of the COF in poly(methyl methacrylate) (PMMA) gives a solvent-free film with intense white light emission (CIE coordinates (0.35, 0.36)). From steady-state and time-resolved studies, the mechanism of the white light emission has been unambiguously assigned to fluorescence, with the blue emission originating from the π-stacked columns of anthracene, and the mixture of red and green from the keto-enol tautomerized resorcinol units. The study introduces the COF as a new class of readily processable, single-source white light emitter.