Mi Young Jo

Nonconjugated anionic polyelectrolyte as an interfacial layer for the organic optoelectronic devices.

A nonconjugated anionic polyelectrolyte, poly(sodium 4-styrenesulfonate) (PSS-Na), was applied to the optoelectronic devices as an interfacial layer (IFL) at the semiconducting layer/cathode interface. The ultraviolet photoelectron spectroscopy and the Kelvin probe microscopy studies support the formation of a favorable interface dipole at the organic/cathode interface. For polymer light-emitting diodes (PLEDs), the maximum luminance efficiency (LEmax) and the turn-on voltage (Von) of the device with a layer of PSS-Na spin-coated from the concentration of 0.5 mg/mL were 3.00 cd/A and 5.5 V, which are dramatically improved than those of the device without an IFL (LEmax = 0.316 cd/A, Von = 9.5 V). This suggests that the PSS-Na film at the emissive layer/cathode interface improves the electron injection ability. As for polymer solar cells (PSCs), the power conversion efficiency (PCE) of the device with a layer of PSS-Na spin-coated from the concentration of 0.5 mg/mL was 2.83%, which is a 16% increase compared to that of the PSC without PSS-Na. The PCE improvement is mainly due to the enhancement of the short-circuit current (12% increase). The results support that the electron collection and transporting increase by the introduction of the PSS-Na film at the photoactive layer/cathode interface. The improvement of the efficiency of the PLED and PSC is due to the reduction of the Schottky barrier by the formation of a favorable interface as well as the better Ohmic contact at the cathode interface.

Enhancing the efficiency of opto-electronic devices by the cathode modification

A non-conjugated polymer with a polar hydroxyl group as a side group, poly(vinyl alcohol) (PVA), is applied to polymer solar cells (PSCs) and polymer light-emitting diodes (PLEDs) as a buffer layer at the active layer/cathode interface. The best power conversion efficiency (PCE) of PSCs with the PVA film as a cathode buffer layer is 3.27%, which is a 27% increase compared to that of PSCs without the PVA film (2.58%). The PCE improvement is due to enhancement of the short circuit current, the fill factor, and the open circuit voltage, simultaneously. Also, the performances of polymer light-emitting diodes with the PVA film as a cathode buffer layer are better than those of the devices without PVA. Improvement of the performances of the devices is due to the fact that the PVA film reduces a Schottky barrier by the formation of favorable interface dipoles and improves the interface properties.

Synthesis and characterization of thermally cross-linkable trimer based on triphenylamine

AbstractA trimer with thermally crosslinkable vinyl groups, (4-butyl-phenyl)-bis-[4-((4-vinyl-phenyl)-(4-butyl-phenyl)-phenyl-amine)-phenyl]-amine (3-TPA), was synthesized successfully. Differential scanning calorimetry (DSC) thermogram of 3-TPA showed two endothermic processes at 122 and 218 °C at the first heating scan. The endothermic peak at 122 °C corresponds to the melting behavior of 3-TPA and the other at 218 °C seems to be from thermal crosslinking. Thermally cured 3-TPA film at 180 °C for 1 h showed very good solvent resistance and was electrochemically stable. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of cured 3-TPA film was −5.09 and −2.10 eV, respectively. The turn on electric field of PLEDs with cured 3-TPA was 1.37–1.80 MV/cm, which was smaller than that of the device without 3-TPA (2.51 MV/cm). The luminance efficiency and brightness of polymer light-emitting diodes (PLEDs) based on 3-TPA were much higher than that of the PLED without 3-TPA. This is due to that cured 3-TPA has hole injection and transporting property. Among the PLEDs, the device with a 20 nm-thick cured 3-TPA showed the best performance with a maximum efficiency of 1.29 cd/A and a maximum brightnes of 2,500 cd/m2.

Diketopyrrolopyrrole-based Small Molecule for Application in Solution Processed Organic Solar Cells

A novel donor-π bridge - acceptor (A)-π bridge - donor type molecule (PCTDPP20) with planar core based on diketopyrrolopyrrole (DPP) and carbazole is synthesized by the Suzuki coupling reaction. We introduce phenyl rings to extend the π-conjugation length as a π-bridge. The HOMO and LUMO energy levels of PCTDPP20 are −5.13 and −3.57 eV, respectively. From the UV-Vis spectra, we confirm that the maximum absorption wavelgth (λmax) of the PCTDPP20 annealed film at 100°C is red-shifted about 30 nm and the absorption intensity at 600 ∼ 650 nm increase compared to that of as-spun film. And the OFET based on PCTDPP20 annealed at 100°C exhibits a highest hole mobility of 1.6 × 10−4 cm2/Vs together with a Ion/Ioff ratio of 1.0 × 104 and a Vth of 14 V. Also, the photovoltaic properties are investigated in the device structure of ITO/PEDOT:PSS/PCTDPP20:PC60BM (with different blend ratios)/Al. Among the OSCs, PCTDPP20:PCBM weight ratio of 1:0.8 and termal annealing process at 100°C for 10 min are optimum belnd ratio and temperature, respectively. This device shows the best performance with a highest PCE of 0.95% a Jsc of −2.69 mA/cm2, a Voc of 0.79 V and a fill factor of 44.5%.

Synthesis and Characterization of Thermally Cross-Linkable Oligophenothiazine

A series of 10-butylphenothiazine trimer (3-PTVB) and pentamer (5-PTVB) with in-situ thermally cross-linkable vinyl benzene were synthesized successfully. The TGA thermograms revealed that 3-PTVB and 5-PTVB were stable up to 430 and 370 °C, respectively. In the first heating scan of DSC thermograms, 3-PTVB and 5-PTVB showed glass transition temperature (Tg) at 113 °C and 95.3 °C (143 °C). Both 3-PTVB and 5-PTVB showed broad endothermic process in the region of 185–220 °C, which is thermally cross-linking temperature. In the second heating process, Tg and endothermic process were not observed. Wavelength of UV-visible maximum absorption of thermally cured 3-PTVB is 340 nm, which is same as the UV-visible maximum absorption of 5-PTVB. The thermally cured films showed no damage nor any discernible change in the UV-visible spectrum after washing with organic solvents such as methylene chloride (MC), chloroform, and toluene. Thermally cured 3-PTVB and 5-PTVB films were electrochemically very stable. The HOMO energy level of 3-PTVB and 5-PTVB were −4.98 and −4.86 eV, respectively. Double layer structured polymer light-emitting diodes based on in-situ thermally cured 3-PTVB and 5-PTVB were fabricated. The maximum luminance efficiency of devices based on 3-PTVB and 5-PTVB were 1.004 cd/A at 14.0 V and 1.074 cd/A at 16.5 V, respectively, which are much higher than that of the device without thermally cured 3-PTVB or 5-PTVB (0.348 cd/A at 15.0 V).