Organic dyes have been used as gain medium for lasers since the 1960s, long before the advent of today’s organic electronic devices. Organic gain materials are highly attractive for lasing due to their chemical tunability and large stimulated emission cross section. While the traditional dye laser has been largely replaced by solid-state lasers, a number of new and miniaturized organic lasers have emerged that hold great potential for lab-on-chip applications, biointegration, low-cost sensing and related areas, which benefit from the unique properties of organic gain materials. On the fundamental level, these include high exciton binding energy, low refractive index (compared to inorganic semiconductors), and ease of spectral and chemical tuning. On a technological level, mechanical flexibility and compatibility with simple processing techniques such as printing, roll-to-roll, self-assembly, and soft-lithography are most relevant. Here, the authors provide a comprehensive review of the developments in the...

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Organic Lasers: Recent Developments on Materials, Device Geometries, and Fabrication Techniques

Lasing applications have spread over various aspects of human life. To meet the developing trends of the laser industry towards being miniature, portable, and highly integrated, new laser technologies are in urgent demand. Organic semiconductors are promising gain medium candidates for novel laser devices, due to their convenient processing techniques, ease of spectral and chemical tuning, low refractive indexes, mechanical flexibilities, and low thresholds, etc. organic solid-state lasers (OSSLs) open up a new horizon of simple, low-cost, time-saving, versatile and environmental-friendly manufacturing technologies for new and desirable laser structures (micro-, asymmetric, flexible, etc.) to unleash the full potential of semiconductor lasers for future electronics. Besides the development of optical feedback structures, the design and synthesis of robust organic gain media is critical as a vigorous aspect of OSSLs. Herein, we provide a comprehensive review of recent advances in organic gain materials, mainly focused on organic semiconductors for OSSLs. The significant breakthroughs toward electrical pumping of OSSLs are emphasized. Opportunities, challenges and future research directions for the design of organic gain media are also discussed.

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

Rational molecular design allows for manipulating the chain conformations of polymer semiconductors by cooperative arrangement of bulky groups with steric hindrance effect and supramolecular groups with noncovalent attractions. Herein, a model polyfluorene with β-phase, poly[4-(octyloxy)-9,9-diphenylfluoren-2,7-diyl]-co-[5-(octyloxy)-9,9-diphenylfluoren-2,7-diyl] (PODPF), has been synthesized successfully via key Baeyer–Villiger rearrangement reaction. Its thin film exhibited excellent spectral stability without green band emission after thermal annealing at 200 °C under air and nitrogen ambients. The β-phases of PODPF in the concentrated toluene solution, organogels, and films have been characterized and confirmed by UV absorption and PL spectra as well as grazing-incidence X-ray scattering. The results suggest that the octyloxy substituents enable backbone planarization via van der Waals forces of the in-plane alkyl chains to overcome intrachain repulsion between fluorene monomers. Organic lasers using ...

A Rational Molecular Design of β-Phase Polydiarylfluorenes: Synthesis, Morphology, and Organic Lasers

Poly(9,9-dioctylfluorene) (PFO) is a widely studied blue-emitting conjugated polymer, the optoelectronic properties of which are strongly affected by the presence of a well-defined chain-extended "β-phase" conformational isomer. In this study, optical and Raman spectroscopy are used to systematically investigate the properties of PFO thin films featuring a varied fraction of β-phase chain segments. Results show that the photoluminescence quantum efficiency (PLQE) of PFO films is highly sensitive to both the β-phase fraction and the method by which it was induced. Notably, a PLQE of ∼69% is measured for PFO films possessing a ∼6% β-phase fraction induced by immersion in solvent/nonsolvent mixtures; this value is substantially higher than the average PLQE of ∼55% recorded for other β-phase films. Furthermore, a linear relationship is observed between the intensity ratios of selected Raman peaks and the β-phase fraction determined by commonly used absorption calibrations, suggesting that Raman spectroscopy can be used as an alternative means to quantify the β-phase fraction. As a specific example, spatial Raman mapping is used to image a mm-scale β-phase stripe patterned in a glassy PFO film, with the extracted β-phase fraction showing excellent agreement with the results of optical spectroscopy. © 2016 The Authors. Journal of Polymer Science Part B: Polymer Physics Published by Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 1995-2006.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/polb.24106

Spectroscopic properties of poly(9,9-dioctylfluorene) thin films possessing varied fractions of β-phase chain segments: enhanced photoluminescence efficiency via conformation structuring.

The fused, bicyclic molecule, 2,1,3-Benzothiadiazole (BT), has become a key ingredient in the design of new organic semiconductors for light emission and energy harvesting applications. Here, the synthesis is reported of a series of trigonal, star-shaped compounds comprising a truxene core and three quater-dialkylfluorene arms into each of which a BT unit is inserted sequentially at each possible position (T4BT-A to T4BT-E). Analysis of the resulting electronic properties shows that as a consequence of conjugative coupling to the core and the resulting symmetry there are three distinct locations for the BT unit and the influence that these locations have on light emission and other spectroscopic characteristics is discussed. The systematic variation in photophysical properties for the different structural isomers helps to clarify the influence of BT unit addition to 9,9-dialkylfluorene chains. It also helps to establish a design template for the construction of donor-acceptor conjugated materials with targeted properties. For T4BT-E with a BT unit at the terminal position of each arm, the photoluminescence quantum efficiency is significantly reduced and no amplified spontaneous emission is observed under typical pumping conditions. Theoretical calculations assist in understanding the variation in behaviors among the T4BT-X family of compounds, especially in relation to their photoluminescence decay times and the Raman scattering intensities of their dominant BT-unit-centred molecular vibrations.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/adfm.201202644

Location, Location, Location - Strategic Positioning of 2,1,3-Benzothiadiazole Units within Trigonal Quaterfluorene-Truxene Star-Shaped Structures

The focus of this work is to study the refractive index in thin films of blue light-emitting poly(9,9-dioctylfluorene) (PFO) that contain extended rigid chains (β-phase). Using a post-deposition exposure to toluene vapour, β-phase chains were induced in glassy PFO films that had been spin coated from a range of solution concentrations to provide thicknesses from 50–200 nm. With the aid of absorption spectroscopy, a semi-empirical calculation of the change in refractive index due to the presence of β-phase chains is undertaken. Over the spectral region where optical gain occurs, the refractive index of β-phase PFO is found to be appreciably larger than for as-spin-coated glassy PFO. Further development and explicit evaluation of the Kramers–Kronig relations lead to analytic and closed form expressions describing the change in index Δn which are used to highlight the key contributions to index change. We find that the spectral response of Δn is largely governed by the emergence of a characteristic absorption band associated with the presence of β-phase chains. The changes in refractive index are experimentally studied by tracking the spectral shifts of optical resonances observed from films spin coated directly onto sub-wavelength 1D grating structures. These measurements are supported and compared with rigorous calculations of the spectra, which incorporate the full optical descriptions for glassy PFO and β-phase PFO. The excellent agreement confirms the accuracy of our semi-empirical extraction approach. Similar polymer-coated grating structures are examined under lasing conditions and can also be used to support the expected spectral response and the decline of Δn for longer wavelengths.

https://iopscience.iop.org/article/10.1088/0953-8984/19/46/466107/pdf

The change in refractive index of poly(9,9-dioctylfluorene) due to the adoption of the β-phase chain conformation

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The change in refractive index of poly(9,9-dioctylfluorene) due to the adoption of the β-phase chain conformation