Charging and discharging of a planar polymer light-emitting electrochemical cell

Auroralike Light from a Polymer p-n Junction Emitting Free Electrons

Luminescent conjugated polymers are organic semiconductors possessing unique electrical and optical properties. Luminescent polymers have been extensively investigated because of their potential applications in light sources and display devices. Here, we provide the first evidence that a luminescent polymer is also a strong emitter of free electrons and a source of new light. Vivid green light flashes have been observed from a red-emitting polymer p-n junction under a large reverse bias. The p-n junctions have a planar configuration, revealing that the green light flashes are emitted into free space, far beyond the confines of electrodes. Moreover, the green flashes can be strongly distorted by a permanent magnet placed nearby, generating a light show resembling an aurora. Like the aurora, the green light flashes are caused by charged particles. By applying a known transverse magnetic field to bend the flashes into circular arcs, a charge to mass ratio is determined for the charged particles. The auroralike green light coincides with the formation of electrical trees in the n-doped region of the junction. We postulate that the green light is caused by field-emitted electrons exciting an unknown organic vapor released in the treeing process.

Toward Efficient and Metal-Free Emissive Devices: A Solution-Processed Host-Guest Light-Emitting Electrochemical Cell Featuring Thermally Activated Delayed Fluorescence

The next generation of emissive devices should preferably be efficient, low-cost, and environmentally sustainable, and as such utilize all electrically generated excitons (both singlets and triplets) for the light emission, while being free from rare metals such as iridium. Here, we report on a step toward this vision through the design, fabrication, and operation of a host-guest light-emitting electrochemical cell (LEC) featuring an organic thermally activated delayed fluorescence (TADF) guest that harvests both singlet and triplet excitons for the emission. The rare-metal-free active material also consists of a polymeric electrolyte and a polymeric compatibilizer for the facilitation of a cost-efficient and scalable solution-based fabrication, and for the use of air-stable electrodes. We report that such TADF-LEC devices can deliver uniform green light emission with a maximum luminance of 228 cd m^-2 when driven by a constant-current density of 770 A m^-2, and 760 cd m^-2 during a voltage ramp, which represents a one-order-of-magnitude improvement in comparison to previous TADF-emitting LECs.

Optical Properties and Amplified Spontaneous Emission of Novel MDMO-PPV/C500 Hybrid

The influence of the solvent nature on optical properties of poly[2-methoxy-5-3,7-dimethyloctyloxy-1,4-phenylenevinylene] (MDMO-PPV)/Coumarine 500 (C500) have been investigated. In addition, the amplified spontaneous emission (ASE) from MDMO-PPV and efficient energy transfer between the MDMO-PPV and C500 has been verified. The MDMO-PPV was dissolved in aromatic and nonaromatic solvents, while the solution blending method was employed to prepare the MDMO-PPV:C500 hybrid. The quantum yield of the MDMO-PPV was found to increase with the reduction of a few factors such as polarity index of the solvent, absorption cross section (σa), emission cross section (σe), and extinction coefficient (εmax). The fluorescence spectra of the MDMO-PPV appears from two vibronic band transitions (0-0, 0-1) and the ASE occurs at 0-1 transition, which was verified by the ASE from MDMO-PPV. The MDMO-PPV in toluene exhibited the best ASE efficiency due to its high quantum yield compared with other solvents. Strong overlap between the absorption spectrum of MDMO-PPV and emission spectrum of C500 confirmed the efficient energy transfer between them. Moreover, the ASE for energy transfer of the MDMO-PPV:C500 hybrid was proved.