Semiconducting (Conjugated) Polymers as Materials for Solid-State Lasers

Direct optical probing of the doping progression and simultaneous recording of the current–time behavior allows the establishment of the position of the light‐emitting p–n junction, the doping concentrations in the p‐ and n‐type regions, and the turn‐on time for a number of planar light‐emitting electrochemical cells (LECs) with a 1 mm interelectrode gap. The position of the p–n junction in such LECs with Au electrodes contacting an active material mixture of poly(2‐methoxy‐5‐(2′‐ethylhexyloxy)‐p‐phenylene vinylene) (MEH‐PPV), poly(ethylene oxide), and a XCF3SO3 salt (X = Li, K, Rb) is dependent on the salt selection: for X = Li the p–n junction is positioned very close to the negative electrode, while for X = K, Rb it is significantly more centered in the interelectrode gap. Its is demonstrated that this results from that the p‐type doping concentration is independent of salt selection at ca. 2 × 1020 cm–3 (ca. 0.1 dopants/MEH‐PPV repeat unit), while the n‐type doping concentration exhibits a strong dependence: for X = K it is ca. 5 × 1020 cm–3 (ca. 0.2 dopants/repeat unit), for X = Rb it is ca. 9 × 1020 cm–3 (ca. 0.4 dopants/repeat unit), and for X = Li it is ca. 3 × 1021 cm–3 (ca. 1 dopants/repeat unit). Finally, it is shown that X = K, Rb devices exhibit significantly faster turn‐on times than X = Li devices, which is a consequence of a higher ionic conductivity in the former devices.

Section: Introductionmentioning

confidence: 67%

Polymer Light‐Emitting Electrochemical Cells: Doping Concentration, Emission‐Zone Position, and Turn‐On Time

Shin

Robinson

Xiao³

et al. 2007

“…[1] They are promising materials for use in electroluminescent displays, [2,3] solar cells, [4] sensors, [5,6] thin-film organic transistors, [7] lasers [8] and light-emitting electrochemical cells. [9] However, the stability of these materials under operating conditions needs improvement if they are to be widely used in real products.…”

Section: Introductionmentioning

confidence: 99%

Stabilization of Semiconducting Polymers with Silsesquioxane

Xiao¹,

Nguyên²,

Gong

et al. 2003

199

162

Polyhedral oligomeric silsesquioxanes (POSS) anchored to poly(2‐methoxy‐5‐(2‐ethylhexyloxy)‐1.4‐phenylenevinylene) (MEH‐PPV) (MEH‐PPV–POSS), and to poly(9,9‐dihexylfluorenyl‐2,7‐diyl) (PFO) (PFO–POSS) were synthesized. Compared with the corresponding parent polymers, MEH‐PPV and PFO, MEH‐PPV–POSS and PFO–POSS have better thermal stability. MEH‐PPV–POSS and MEH‐PPV have identical absorption and photoluminescent (PL) spectra, both in solution and as thin films. They also have identical electroluminescent (EL) spectra. Devices made from MEH‐PPV–POSS exhibit higher brightness (1320 cd m–2 at 3.5 V) and higher external quantum efficiency (ηext = 2.2 % photons per electron) compared to MEH‐PPV (230 cd m–2 at 3.5 V and ηext = 1.5 % ph el–1). Compared with PFO in the same device configuration, PFO–POSS has improved blue EL emission and higher ηext.

“…[19] The average refractive index n average for wave propagation perpendicular to the grating lines will be between the index of the nonilluminated polymer (n pristine ) and the index of a flood-illuminated film (n illuminated ), where n illuminated > n pristine as a result of the SCN-NCS photoisomerization. It has been demonstrated that changing the refractive index between these limits does not strongly alter the emission pattern (vide supra).…”

Section: Full Papermentioning

confidence: 99%

Modifying the Output Characteristics of an Organic Light‐Emitting Device by Refractive‐Index Modulation

Höfler

Weinberger

Kern

et al. 2006

In order to modify the output characteristics of organic light‐emitting devices (OLEDs), the optical properties of an active layer within the device are patterned without introducing any thickness modulation. For this purpose a new conjugated copolymer, which serves as a hole‐transporting material and at the same time can be index patterned using UV techniques, is synthesized. Poly(VC‐co‐VBT) (VC: N‐vinylcarbazole; VBT: 4‐vinylbenzyl thiocyanate) is prepared by free‐radical copolymerization of VC and VBT. The material contains photoreactive thiocyanate groups that enable altering of the material's refractive index under UV illumination. This copolymer is employed as a patternable hole‐transporting layer in multilayer OLEDs. Refractive‐index gratings in poly(VC‐co‐VBT) are inscribed using a holographic setup based upon a Lloyd mirror configuration. The fourth harmonic of a Nd:YAG (YAG: yttrium aluminum garnet) laser (266 nm) serves as the UV source. In this way 1D photonic structures are integrated in an OLED containing AlQ3 (tris(8‐hydroxyquinoline) aluminum) as the emitting species. It is assured that only a periodical change of the refractive index (Δn = 0.006 at λ = 540 nm) is generated in the active material but no surface‐relief gratings are generated. The patterned devices show more forward‐directed out‐coupling behavior than unstructured devices (increase in luminosity by a factor of five for a perpendicular viewing direction). This effect is most likely due to Bragg scattering. For these multilayer structures, optimum outcoupling was observed for grating periods Λ ∼ 390 nm.