Organic solid-state lasers: a materials view and future development

The Journey to Precious-Metal-Free Organic Phosphors from Single-Benzene-Cored Fluorophores

This article takes a look back over our research on the development of organic fluorophores that efficiently emit light in the solid state and precious-metal-free phosphors that emit light at room temperature. In particular, we place an emphasis on the prehistory of each project and the relationship between the established molecular designs. Our story starts from the serendipitous discovery of a luminophore with a single benzene core and follows the molecular design and characterization of 2,5-dipiperidyl-1,4-bis(acceptor-substituted ethenyl)benzenes that exhibit solid-state fluorescence with high-to-excellent quantum yields in the blue-to-red region. In addition, the design concepts, luminescence characteristics, and applications of eight novel classes of fluorophores are described, and the discovery, design, and luminescent properties of precious-metal-free compounds that show efficient room-temperature phosphorescence are presented.

Optical Gain in Semiconducting Polymer Nano and Mesoparticles

The presence of excited-states and charge-separated species was identified through UV and visible laser pump and visible/near-infrared probe femtosecond transient absorption spectroscopy in spin coated films of poly[N-9″-heptadecanyl-2,7-carbazole-alt-5,5-(4,7-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) nanoparticles and mesoparticles. Optical gain in the mesoparticle films is observed after excitation at both 400 and 610 nm. In the mesoparticle film, charge generation after UV excitation appears after around 50 ps, but little is observed after visible pump excitation. In the nanoparticle film, as for a uniform film of the pure polymer, charge formation was efficiently induced by UV excitation pump, while excitation of the low energetic absorption states (at 610 nm) induces in the nanoparticle film a large optical gain region reducing the charge formation efficiency. It is proposed that the different intermolecular interactions and molecular order within the nanoparticles and mesoparticles are responsible for their markedly different photophysical behavior. These results therefore demonstrate the possibility of a hitherto unexplored route to stimulated emission in a conjugated polymer that has relatively undemanding film preparation requirements.

Superior light-resistant dithieno[3,2-b:2',3'-d]arsole-based polymers exhibiting ultrastable amplified spontaneous emission

Herein, by amplified spontaneous emission measurements, dithieno[3,2-b:2',3'-d]arsole (DTA)-bithiophene (DTA-BT) and DTA-ethynylbenzene (DTA-EB) polymer films exhibited considerable photostability under continuous pulsed pumping excitation for at least 15 h at the pumping energies, 28.9 and 20.7 μJ, respectively, showing an outstanding high light-resistance among the polymer-based gain media.

Organic Semiconductor Micro/Nanocrystals for Laser Applications

Organic semiconductor micro/nanocrystals (OSMCs) have attracted great attention due to their numerous advantages such us free grain boundaries, minimal defects and traps, molecular diversity, low cost, flexibility and solution processability. Due to all these characteristics, they are strong candidates for the next generation of electronic and optoelectronic devices. In this review, we present a comprehensive overview of these OSMCs, discussing molecular packing, the methods to control crystallization and their applications to the area of organic solid-state lasers. Special emphasis is given to OSMC lasers which self-assemble into geometrically defined optical resonators owing to their attractive prospects for tuning/control of light emission properties through geometrical resonator design. The most recent developments together with novel strategies for light emission tuning and effective light extraction are presented.

Theoretical insights on tunable optoelectronics and charge mobilities in cyano-perylenediimides: interplays between -CN numbers and positions

Air-stable perylenediimide (PDI) and its derivatives, in particular the cyano-functionalized ones, have attracted great research attention for their potential use in flexible optoelectronics, organic field-effect-transistors (OFETs) as n-type transport materials and also as non-fullerene acceptors in organic photovoltaics (OPVs). Herein we provide a detailed theoretical study on the optical, electrochemical and charge-transport properties (electron and hole mobilities) in a few CN-substituted PDIs with varied number of -CN at different positions (both symmetric and asymmetric di- and tetra-CN derivatives) using density functional theory (DFT) and time-dependent DFT implementing optimally tuned screened range-separated hybrid (OT-SRSH) combining with kinetic rate theory. All cyano-PDIs studied here are energetically stable and form stable π-stacked structures similar to the pristine one, and also act as better electron acceptors. No significant changes in the PDI optical properties are found with the different ways of CN-functionalization, but, this strongly affects the π-stacked geometry, and thereby the electronic coupling, which greatly modulates the PDI intrinsic carrier mobility. Calculated room-temperature electron mobility for the pristine PDI is in excellent agreement with the reported OFET value (∼0.1 cm2 V-1 s-1). Interestingly, relatively large electronic couplings together with small reorganization energies of the symmetrically substituted tetra-CN PDI result in very large charge mobilities (0.4 cm2 V-1 s-1 for electrons and 5.6 cm2 V-1 s-1 for holes) among the systems studied. Therefore, this may serve as a potential ambipolar transport material and hence, naturally calls for experimental demonstration. This detailed and comprehensive study sheds light on the complex interplays between the -CN numbers and the positions for tailored optoelectronic and charge-transport in several functional PDIs, and also shows routes to molecularly design potential n-type materials.

Phenanthroimidazole derivatives showing mild intramolecular charge transfer and high quantum yields and their applications in OLEDs

Phenanthroimidazole derivatives showing bipolar characters, strong emissions in the blue region and their applications in OLED is presented.

A "flexible" carborane-cored luminogen: variable emission behaviours in aggregates

The performance of tunable emissions in aggregates is highly desirable but challenging owing to the restricted molecular conformations of luminogens. Herein, we designed and synthesized a new "flexible" luminogen, a carborane-cored compound NAPH, which exhibits variable photophysical behaviours in aggregates, such as aggregation-induced emission, crystallization-induced emission, polymorph-dependent emission, and mechanochromic luminescence. Moreover, the two polymorphs with different emission colors show opposite mechano-responsive luminescence, which is rarely observed for single-component luminescent materials. Both theoretical calculations and photophysical experiments reveal that the carborane-cored luminogen could afford variable conformations. This endows the whole molecule with multiple conformations in aggregates, thus leading to variable emission behaviours. Therefore, the present work provides new access to the construction of multifunctional single-component solid-state luminescent materials.

3,6-Diamino-7,8-dihydroisoquinoline-4-carbonitrile derivatives: unexpected facile synthesis, full-color-tunable solid-state emissions and mechanofluorochromic activities

A facile synthesis of novel 3,6-diamino-7,8-dihydroisoquinoline-4-carbonitrile derivatives and their solid-state emissions are presented.

Fabrication of Thin Films from Powdered Cesium Lead Bromide (CsPbBr3) Perovskite Quantum Dots for Coherent Green Light Emission

High-quality thin films were obtained directly by spin-coating glass substrates with suspensions of powdered cesium lead bromide (CsPbBr₃) perovskite quantum dots (PQDs). The structural properties of the films were characterized via transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD) analysis, and atomic force microscopy (AFM). The crystal structure of the CsPbBr₃ PQDs was unique. The optical behavior of the CsPbBr₃ PQDs, including absorption and emission, was then investigated to determine the absorption coefficient and band gap of the material. The CsPbBr₃ PQDs were evaluated as active lasing media and irradiated with a pulsed laser under ambient conditions. The PQDs were laser-active when subjected to optical pumping for pulse durations of 70-80 ps at 15 Hz. Amplified spontaneous emission (ASE) by the CsPbBr₃ PQD thin films was observed, and a narrow ASE band (∼5 nm) was generated at a low threshold energy of 22.25 μJ cm^-2. The estimated ASE threshold carrier density (n _th) was ∼7.06 × 10¹⁸ cm^-3. Band-gap renormalization (BGR) was indicated by an ASE red shift and a BGR constant of ∼27.10 × 10^-8 eV. A large optical absorption coefficient, photoluminescence (PL), and a substantial optical gain indicated that the CsPbBr₃ PQD thin films could be embedded in a wide variety of cavity resonators to fabricate unique on-chip coherent light sources.

Fabrication and Microelectronic Properties of Hybrid Organic–Inorganic (poly(9,9, dioctylfluorene)/p-Si) Heterojunction for Electronic Applications

We report on the microelectronic characteristics of a novel hybrid heterojunction device based on a solution processable semiconducting polymer poly(9,9-dioctylfluorenyl-2,7-diyl)- co-(N,N0-diphenyl)-N,N′di(p-butyl-oxy-pheyl)-1,4-diamino-benzene) (PFB) and p-type silicon (p-Si). The PFB/p-Si heterojunction is prepared by spin coating 20 mg/mL solution of PFB in chloroform on the precleaned polished surface of p-Si substrate. Thermal evaporation of silver (Ag) electrode on top of PFB completes the fabrication of the Ag (90 nm)/PFB (180 nm)/p-Si heterojunction device. Morphology of PFB thin film is studied by using an atomic force microscope (AFM) and scanning electron microscope (SEM), which reveals grains are randomly distributed with slightly different grain sizes and shapes. It leads the film to form nonuniformity and some roughness in its topography that results in limiting the current (I) flow across the film/interface with p-Si. Ultraviolet (UV–vis) absorption and X-ray diffraction (XRD) spectra are measured for optical bandgap and crystal structure analysis of PFB. The key microelectronic parameters—rectification ratio (RR), ideality factor (n), barrier height (Φb), series resistance (Rs) and reverse saturation current (I0)—of the Ag/PFB/p-Si heterojunction are found from current–voltage (I–V) characteristics at room temperature (300 K) in dark conditions (≈0 lux). The Ag/PFB/p-Si heterojunction device exhibits improved microelectronic parameters when compared to those of earlier reported devices that were prepared in the same configuration. This improvement in the device parameters reveals enhancement in the microelectronic properties across the interface/depletion region of the Ag/PFB/p-Si device, which can be attributed to the remarkable electronic properties of PFB such as its relatively high hole mobility and better charge carriers’ conduction. The charge transport mechanisms through the device is also studied. Having the smaller values of I0 ≈ 7 × 10−10 A and n ≈ 3.23, as well as higher shunt resistance (Rsh) of 32 GΩ for the Ag/PFB/p-Si device suggest its potential for many electronic and optoelectronic applications.