Controlling Morphology and Chain Aggregation in Semiconducting Conjugated Polymers: The Role of Solvent on Optical Gain in MEH-PPV

Abstract

We report the results of a detailed investigation that addresses the influence of polymer morphology and chain aggregation, as controlled by the chemical nature of the solvent, on the optical gain properties of the conjugated polymer poly[2-methoxy-5-(2'-ethylhexyloxy)-p-phenylene vinylene] (MEH-PPV). Using the variable stripe length technique in the picosecond regime, we have extensively studied the optical gain performance of asymmetric planar waveguides formed with thin MEH-PPV films spin-cast from concentrated chlorobenzene (CB) and tetrahydrofuran (THF) solutions onto thermally oxidized silicon substrates. CB and THF solvents were chosen based on their known ability to promote and effectively limit aggregate formation, respectively. Very large net gain coefficients are demonstrated, reaching values of 330 and 365 cm(-1), respectively, when optically pumping the waveguides with a maximum energy density of 85 μJ/cm(2). Our results clearly demonstrate that polymer morphology, and hence, the chain conformation dependence of the degree of aggregation in the films as controlled by the solvent, has minimal impact on the net gain. Moreover, the waveguides exhibit low loss coefficients of 10-20 cm(-1) at the ASE wavelength. These results question the importance of polymer morphology and aggregate formation in polymer-based optical devices operating at high excitation densities in the stimulated emission regime as would be characteristic of lasers and optical amplifiers.

Amplified Spontaneous Emission and Optical Gain in Organic Single Crystal Quinquethiophene

“…This particular pumping intensity is known as the "threshold energy (Eth)" [34]. The value of Eth can also be obtained from the curves showing the emission line width of FWHM versus pumping intensity [35]. The ASE threshold energy "Eth" is extracted from Figure 5 and is found to be 52.64 µJ/cm 2 for the P5T crystal, which is smaller (better) than many organic semiconductors [29].…”

“…If ASE was achieved at short stripe-lengths, the spectrum would have been broad, and as the excitation length increased the spectrum would have got narrow. The output intensity depends exponentially on stripe-length which is given by equation (1) [35]. By the VSL method, stripe-length vs. output intensity curves of P5T is shown in Figure 6,…”

Optical Gain in the Near Infrared by Light‐Emitting Electrospun Fibers

“…Both the solution concentrations and the PMMA/dye relative content are optimized in order to obtain uniform fibers with good net gain. In particular, the relative content of the dopants is crucial for gain, the latter being disfavored by the possible aggregation of the active molecules and at too high dye concentrations . The solutions are then loaded into a 1 mL syringe and delivered at a constant flow rate (1 mL/h) through a metal needle (27 gauge) connected to a high‐voltage power supply (EL60R0.6–22, Glassman High Voltage, High Bridge, NJ).…”

Precipitation and sol formation from poly(p-phenylene)s solutions by spectroscopic study

“…Because of the similar structures, the aromatic solvent can result in a more planar (more conjugated conformation) aromatic polymer backbone [40]. However, the coiled conformation of side chains in aromatic solvents can be supported from the infrared spectra analysis (Supporting Information).…”