Ye Eun Ha

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.

Effect of Polyelectrolyte Electron Collection Layer Counteranion on the Properties of Polymer Solar Cells

Polyviologen (PV) derivatives are known materials used for adjusting the work function (WF) of cathodes by reducing the electron injection/collection barrier at the cathode interface. To tune and improve device performance, we introduce different types of counteranions (CAs), such as bromide, tetrafluoroborate, and tetraphenylborate, to a PV derivative. The effective WF of the Al cathode is shown to depend on the size of the CA, indicating that a Schottky barrier can be modulated by the size of the CA. Through the increased size of the CA from bromide to tetraphenylborate, the effective WF of the Al cathode is gradually decreased, indicating a decreased Schottky barrier at the cathode interface. In addition, the change of the power conversion efficiency (PCE) and the short circuit current (Jsc) value show good correlation with the change of the WF of the cathode, signifying the typical transition from a Schottky to an Ohmic contact. The turn-on electric field of the electron-only device without PV was 0.21 MV/cm, which is dramatically higher than those of devices with PV-X (0.07 MV/cm for PV-Br, 0.06 MV/cm for PV-BF4, and 0.05 MV/cm for PV-BPh4) This is also coincident with a decrease in the Schottky barrier at the cathode interface. The device ITO/PEDOT/P3HT:PCBM/PV/Al, with a thin layer of PV derivative and tetraphenylborate CA as the cathode buffer layer, has the highest PCE of 4.02%, an open circuit voltage of 0.64 V, a Jsc of 11.6 mA/cm2, and a fill factor of 53.0%. Our results show that it is possible to improve the performance of polymer solar cells by choosing different types of CAs in PV derivatives without complicated synthesis and to refine the electron injection/collection barrier height 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.

Investigation of the property change of polymer solar cells by changing counter anions in polyviologen as a cathode buffer layer

Polyviologen (PV) derivatives are known as materials for lowering the work function of cathodes, thereby reducing the electron injection/collection barrier at the cathode interface of polymer solar cells (PSCs). In order to demonstrate the effect of the size of counter anions in PV derivative on the photovolatic properties, we introduce different types of counter anions such as bromide (Br), hexafluorophosphate (PF6), and p-toluene sulfonate (OTs), in PV derivative. The effective work function of the Al electrode is gradually increased by increasing the size of counter anion, indicating the larger counter anion leads to the larger reduction of a Schottky barrier. The power conversion efficiency (PCE) value of the devices is also improved by increasing the size of counter anion. The device (ITO/ PEDOT/P3HT:PCBM/PV/Al) with the thin layer of PV derivative bearing a counter anion of OTs as a cathode buffer layer demonstrates the PCE of 3.90%, with a open circuit voltage (Voc) of 0.64 V, a short circuit current density (Jsc) of 11.39 mA/cm2, and a fill factor (FF) of 53.5%, respectively. This is better than the device with PV derivative having a counter anion of PF6 (PCE=3.73%, Jsc=11.14 mA/cm2, Voc=0.64 V, FF=52.1%) or Br (PCE=3.62%, Jsc=10.95 mA/cm2, Voc=0.64 V, FF=51.6%). Here, our results show that it is possible to improve the performance of PSCs and to tune the electron injection/collection barrier height at the cathode interface by choosing different counter anions without complicated synthesis.

Effect of Phthalimide in 2,1,3-Benzooxadiazole Based Copolymer on the Performances of Solar Cells

Two newly designed donor-acceptor (D-A) conjugated polymers were prepared from the Stille coupling reactions by incorporating phthalimide derivative as a co-acceptor unit into 2,1,3-benzooxadiazole (BO) and thiophene with different ratios of 5 mol.% and 10 mol.% of TBO-I 5, TBO-I 10, respectively. Polymers possess moderate molecular weights and excellent thermal properties with a 5% weight loss temperatures (Td) around 300°C. Compared to poly{5,6-bis(decyloxy)-4-(thiophen-2-yl)benzo[c][1,2,5]oxadiazole} (TBO), adding phthalimide unit in TBO-I 5 and TBO-I 10 shifted UV-Vis absorption spectra and changed the HOMO and LUMO levels of polymers. Bulk hetero junction (BHJ) polymer solar cells (PSCs) based on polymers blended with [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) were fabricated to investigate photovoltaic properties. As a results, with increasing the composition of phthalimide, the Jsc of the devices based on polymers were reduced in the order of TBO (4.74 mA/cm2) > TBO-I 5 (3.94 mA/cm2) > TBO-I 10 (1.90 mA/cm2) due to the blue-shifted in absorption spectra and twisting in polymer main chain of TBO-I polymers. The decrease in Jsc led to low photovoltaic performances and the device based on TBO showed higher performances than those of TBO-I 5 and TBO-I 10 with the PCE of 1.66%, the Jsc of 4.74 mA/cm2, the Voc of 0.82 V and the FF of 42.4% under the illumination of AM 1.5G, 100 mW/cm2.