Simultaneously Enhancing Photoluminescence Quantum Efficiency and Optical Gain of Polyfluorene via Backbone Intercalation of 2,5‐Dimethyl‐1,4‐Phenylene

Abstract

Simultaneous enhancement of photoluminescence quantum efficiency (PLQE) and optical gain in semiconducting polymer films is desirable for optically‐ or electrically‐pumped organic solid‐state lasers. In this work, a simple self‐dilution effect is achieved by introducing a small amount (≈10% by weight) of 2,5‐dimethyl‐1,4‐phenylene (DP) units in the backbone of poly(9,9‐dioctylfluorene) (PFO). The resulting copolymers, compared with PFO (PLQE 39%), exhibit a significantly increased PLQE (66%) while keeping similar absorption and photoluminescence profile, concomitant with an enhancement in optical gain properties. The radiative decay rate increases sharply along with a sustaining reduction in the non‐radiative decay rate in these copolymers, following similar principle of physical dilution of a luminescent compound in solution or in a polymer matrix. Among all the copolymers, the one containing 10% DP units exhibits the lowest amplified spontaneous emission/distributed feedback laser threshold (10.9 nJ/1.4 nJ, eightfold reduction), and the highest gain coefficient (54.4 cm−1). The results demonstrate that a moderate DP/fluorene ratio can maximize the beneficial self‐dilution effects. These investigations shed light on new design strategies to achieve conjugated polymers with concurrent high PLQE and large optical gain properties.

Toward Electrically Pumped Organic Lasers: A Review and Outlook on Material Developments and Resonator Architectures

“…[ 13 ] Broad fluorescence spectra allow, on the other hand, for broadband amplification. [ 14,15 ] In addition, the photoluminescence (PL) spectra can be tailored by backbone modification, e.g., by controlling the conjugation length [ 16 ] or by the introduction of electron‐donor or electron‐acceptor groups, [ 17 ] in contrast to inorganic semiconductors where the emitting wavelengths are restricted to lattice‐matching requirements. The efficiency of solid‐state light emission is quantified through the photoluminescence quantum efficiency (PLQE), which stands for the ratio between the number of emitted and absorbed photons.…”

“…The efficiency of solid‐state light emission is quantified through the photoluminescence quantum efficiency (PLQE), which stands for the ratio between the number of emitted and absorbed photons. [ 18 ] Considerable efforts have been devoted to further increase the PLQE of organic semiconductor thin films to achieve high gain and low‐threshold organic lasers [14a,19] . The major strategy to achieve this goal is to localize excitons and minimize the intermolecular interactions, so that dynamic and static luminescence quenching is reduced.…”

“…This phenomenon is caused by polaron photogeneration in picosecond timescales in PFO and the spectral location of their absorption overlapping with F8BT SE. Indeed, the substitution of PFO by other energy‐transfer matrices with reduced polaron yield has shown to restitute F8BT SE in the resulting blends [14b,40] . When dispersing F8BT in a short conjugated fluorene‐based host, namely, 3,6‐ bis (2,7‐di([1,1′‐biphenyl]‐4‐yl)‐9‐phenyl‐9H‐fluoren‐9‐yl)‐9‐octyl‐9H–carbazole (DBPhFCz), larger exciton confinement is manifested in a much slower polaron formation rate, enabling the observation of ASE and lasing in DBPhFCz:F8BT blends with the lowest thresholds of 2.8 and 9.79 μJ cm −2 , respectively (see in Figure ).…”

Organic Semiconductor Micro/Nanocrystals for Laser Applications

“…Organic π-conjugated materials exhibit very interesting features as active media in laser devices. They show high room-temperature photoluminescence quantum efficiencies (PLQE) [ 89 , 90 ] which translates into large stimulated emission (SE) cross-section values and their photoluminescence (PL) spectra can be tuned through chemical functionalization [ 91 ]. Moreover, organic lasers can be processed by cost-effective solution-based methods [ 92 ], they have capabilities to confine and guide light due to their elevated refractive indexes [ 93 ] and they possess unique mechanical properties (flexibility and light weight) leading to new potential market niches for organic laser devices.…”

Optical Gain in Semiconducting Polymer Nano and Mesoparticles

“…The separation of semiconducting polymers from each other (dilution) does not allow for good electronic transport, but other work [ 21 ] showed that, by mixing different polyfluorenes, it was possible to have films of semiconducting polymers with both optical gain and excellent charge transport properties. Further, alternative, approaches include mimicking the effects of dilution by adding a small amount of dimethyl phenylene into polyfluorene chains [ 22 ] or by the supramolecular separation of chains by threading cyclodextrin rings around the conjugated polymer to create polyrotaxanes [ 23 ].…”