We report on the fabrication and characterization of hybrid polymer light emitting device (HPLEDs) with high
brightness and simplicity in design with improved robustness than the conventional polymer light-emitting diodes. We
demonstrate the incorporation of Au capped inorganic titanium oxide TiO2 nanocomposite in electroluminescent polymer
and fabricated HPLED. We achieved enhanced optical properties of the device and the increased performance of the
HPLED is attributed from the electronic charge transport properties of Au capped metal oxide particles in the
electroluminescence polymer. The interfacial contact area of electroluminescence polymer and cathode increased by the
incorporated nanoparticles in the organic polymer phase thereby improved luminescence properties.
New electroluminescent polymer with tetrakis-alkoxy group, poly[2,3,5,6-tetrakis(2-ethylhexyloxy)-1,4-
phenylenevinylene] (TEH-PPV), has been synthesized by the Gilch polymerization. TEH-PPV with tetrakisethylhexyloxy
groups in phenylene unit can get shorter conjugation length than MEH-PPV, and had more blue shifted
absorption and emission peaks due to steric hindrance. Tetrakis-ethylhexyloxy groups induced very conjugated
backbone is twisted. And, they can enhance the internal efficiency of the conjugated polymer as emissive layer in
PLED because of the restraint of inter-chain interaction by the avoidance of close packing to give decent device
performance.
By introducing a titanium oxide (TiOx) layer between the active layer and the aluminum cathode in polymer based
electronic devices, we have demonstrated devices with excellent air stability and with enhanced performance. The TiOx
layer acts as a shielding and scavenging layer which prevents the intrusion of oxygen and humidity into the
electronically active polymers, thereby improving the lifetime of unpackaged devices exposed to air by nearly two orders
of magnitude. We have also fabricated polymer tandem solar cells with a power conversion efficiency of 6.5%, with
each layer processed from solution. A transparent TiOx layer is used to separate and connect the front cell and the back
cell. The TiOx layer serves as an electron transport and collecting layer for the first cell and as a stable foundation that
enables the fabrication of the second cell to complete the tandem cell architecture. We use an inverted structure with the
low band-gap polymer/fullerene composite as the charge separating layer in the front cell and the high band-gap polymer
composite as the charge separating layer in the back cell.
Fluorescent conjugated polymers have attracted much attention due to their potential applications in flat panel displays.
There are few studies on the degradation of the PPV film in air when irradiated. The photodegradation reaction is a chain
scission process involving oxygen in air to yield terminal 4-vinylbenzoic acid groups. The photodegradation of
conducting polymer may seriously effect the performance of electroluminescence devices.
In order to reduce oxidation of the vinylene group, the vinylic group was cyclized using carbon-containing 5-membered
rings. In case of PININE, it is possible to introduce four alkyl groups in the sp3 carbons in the bicycle, which will
increase the solubility of the polymer. PININE was used as the electroluminescence layer for the light-emitting diode.
PININE shows turn-on voltage of 6.5 V, and EL with maximum peak at 477 nm, maximum brightness of 2187 cd/m2 at
12 V, and efficiency of 0.34 cd/A at 162 mA/cm2. The change in luminescence following irradiation with white light on
the PININE was not observed. When irradiated with white light, the films of MEH-PPV showed significantly decreased
peaks of UV and PL. As compared to this, the films of PININE showed stable spectra when irradiated over same period of time.
By introducing a solution-based titanium oxide (TiOx) layer between the polymer and Al electrode in polymer lightemitting
diodes, we have demonstrated that the devices exhibit an enhanced efficiency. The TiOx layer reduces the
barrier height between the polymer and Al cathode, thereby facilitating the electron injection in the devices and
enhancing the device performance by achieving a balance of charge injection and transport between the electrons and
holes. Moreover, we also believe that the TiOx layers prevent the diffusion of metal ions into the emitting polymers
during the Al deposition process, and reduce the degree of quenching centers in the active polymers.
One of the problems limiting the device efficiency of polymer light-emitting diodes is the imbalance of charge injection and transport between the electrons and holes. This problem is particularly serious for the case of aluminum (Al) electrode. By introducing solution-based titanium oxide (TiOx) layer between the polymer and Al electrode, we have demonstrated that the devices exhibit an enhanced efficiency. The TiOx layer reduces the barrier height between the polymer and Al cathode, thereby facilitating the electron injection in the devices and enhancing the device performance. Moreover, we also believe that the TiOx layers prevent the diffusion of metal ions into the emitting polymers during the Al deposition process, reducing the degree of quenching centers in the active polymers.
Polymer field-effect transistors with a field-effect mobility of μ ≈0.3 cm2/V.s have been demonstrated using
regioregular poly(3-hexylthiophene) (rr-P3HT). Devices were fabricated by dip-coating the semiconducting polymer
followed by annealing at 150°C for 10 minutes. The heat annealed devices exhibit an increased field-effect mobility
compared with the as-prepared devices. Morphology studies and analysis of the channel resistance demonstrate that the
annealing process increases the crystallinity of rr-P3HT and improves the contact between the electrodes and the P3HT
films, thereby increasing the field effect mobility of the films. Based on the results obtained from unipolar FETs using rr-
P3HT, we have also applied postproduction heat treatment to ambipolar polymer FETs fabricated with rr-P3HT and C61-
butyric acid methyl ester (PCBM). Devices were fabricated using aluminum (Al) source and drain electrodes to achieve
an equivalent injection for the both holes and electrons. As the case of P3HT unipolar FETs, the thermal annealing
method also improves the film morphology, crystallinity, and the contact properties between Al and active layer, thereby
resulting in excellent ambipolar characteristics with the hole mobility of 1.7×10-3 cm2/V.s and the electron mobility of
2.0×10-3 cm2/V.s.
New architectural polymer photovoltaic cells approaching 5% power conversion efficiency have been fabricated using titanium oxide (TiOx) as an optical spacer. Solar cells with a TiOx layer (deposited by a sol-gel process) between the active layer and the electron collecting aluminum electrode exhibit approximately 50% enhancement in power conversion efficiency compared to similar devices without the optical spacer. The TiOx layer increases the efficiency by modifying the spatial distribution of the light intensity inside the device, thereby creating more photogenerated charge carriers in the bulk heterojunction layer.
One of the serious problems in polymer light-emitting diodes (PLEDs) is the difficulty of electron injection in the current PLEDs device of anode/polymer/cathode geometry. This is particularly true for the case of aluminum (Al) electrode. The work function of Al is too high to match with the Lowest Unoccupied Molecular Orbit (LUMO) level of the luminescent polymers, thereby lowering the device efficiency. In this work, by introducing solution-based titanium oxide (TiOx) thin film as an electron injection layer between the polymer and Al electrode, we demonstrate that the devices exhibit an enhanced efficiency. The TiOx layer reduces the barrier height between the polymer and aluminum (Al) cathode, thereby facilitating the electron injection in the devices.
Conjugated polymers with a stabilized blue emission are of importance for the realization of full-color displays using polymer light-emitting diodes. We report a new class of blue-emitting polymers utilizing a new back-bone, poly(2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta [def] phenanthrene)) (PCPP). This material emits a stabilized, efficient blueelectroluminescence(EL) without exhibiting any peak in the long wavelength region (green region) even after prolonged annealing for 18 hours at an elevated temperature of 150°C in air. This attributes to the chemical structure of this new polymer. The backbone of PCPP intrinsically inhibits the formation of the keto-defects mainly responsible for the degradation to green color in typical poly(fluorine) type materials, thereby stabilizing the blue EL emission in the devices.
By applying the specific fabrication conditions such as postproduction annealing at 150oC for 30 minutes, polymer solar cells with 5% power conversion efficiency are demonstrated. These devices exhibit remarkable thermal stability. We attribute the improved performance to changes in the bulk heterojunction material induced by thermal annealing. The improved nanoscale morphology, the increased crystallinity of the semiconducting polymer, and the improved contact to the electron collecting electrode facilitate charge generation, charge transport to, and charge collection at the electrodes, thereby enhancing the device efficiency by lowering the series resistance of the polymer solar cells. Also new architectural polymer solar cells with 5% power conversion efficiency have been fabricated using titanium oxide (TiOx) as an optical spacer. Solar cells with a TiOx layer (deposited by a sol-gel process) between the active layer and the electron collecting aluminum electrode exhibit approximately 50% enhancement in power conversion efficiency compared to similar devices without the optical spacer. The TiOx layer increases the efficiency by modifying the spatial distribution of the light intensity inside the device, thereby creating more photogenerated charge carriers in the bulk heterojunction layer.
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