Mei-Hsin Chen

Electronic structures and electron-injection mechanisms of cesium-carbonate-incorporated cathode structures for organic light-emitting devices

In this letter, we investigate electronic structures and electron-injection mechanisms of the effective cathode structures for organic light-emitting devices incorporating cesium carbonate (Cs2CO3), either deposited as an individual thin injection layer or doped into the organic electron-transport layers. The electronic structures and the interface chemistry studied by ultraviolet and x-ray photoemission spectroscopy show that the enhanced electron injection is associated with strong n-doping effects and increase of electron concentrations in the electron-transport layer induced by Cs2CO3. Since such a reaction occurs without the presence of metals, cathode structures incorporating Cs2CO3 may be applied to a wide range of electrode materials.

The roles of thermally evaporated cesium carbonate to enhance the electron injection in organic light emitting devices

The properties of thermally evaporated cesium carbonate (Cs2CO3) and its role as electron injection layers in organic light emitting diodes were investigated. According to the ultraviolet photoemission spectra (UPS), the Fermi level of tris-(8-hydroxyquinoline)-aluminum (Alq3) after being doped with Cs2CO3 shifts toward or into the lowest unoccupied molecular orbital as a result of chemical reaction and charge transfer between Cs2CO3 and Alq3, which lowers the electron injection barrier and improves the current efficiency. As for whether Cs2CO3 being decomposed during the evaporation, we found that Cs2CO3 molecules were deposited on the substrates without decomposition, regardless of the evaporation rates, based on the signature features of carbonate groups and ionization energies measured in UPS spectra and the binding energy shifts of core level electrons. The reaction mechanisms between Cs2CO3 and Alq3 are also proposed. Since Cs2CO3 is not only used in the electron injection layer but also in converti...

Electronic and chemical properties of cathode structures using 4,7-diphenyl-1,10-phenanthroline doped with rubidium carbonate as electron injection layers

The electronic properties and chemical interactions of cathode structures using 4,7-diphenyl-1, 10-phenanthroline (Bphen) doped with rubidium carbonate (Rb2CO3) as electron injection layers were investigated. Current-voltage characteristics reveal that the devices with Bphen/Rb2CO3/Al as cathode structures possess better electron injection efficiency than those with cathode structures of Bphen/LiF/Al. Ultraviolet and x-ray photoemission spectroscopy shows that n-type doping effects resulting from Rb2CO3 and the gap states created by aluminum deposition are both keys to the improved carrier injection efficiency. Moreover, theoretical calculation indicates that the chemical reaction between aluminum and the nitrogen atoms in Bphen is the origin of the gap states.

Influences of evaporation temperature on electronic structures and electrical properties of molybdenum oxide in organic light emitting devices

The influence of evaporation temperatures on the electronic structures of molybdenum oxide (MoOx) films and the electrical properties of organic light emitting diodes were investigated. MoOx films evaporated at a high temperature and a high deposition rate are close to a stoichiometric phase, but become less effective when they are used as a hole injection layer. However, when MoOx is evaporated at a lower temperature and a slower rate, there are large amounts of defect-related states present in the forbidden gap, which make the films behave like a high work function conductor and an effective hole injection layer.

Enhancement of current injection in organic light emitting diodes with sputter treated molybdenum oxides as hole injection layers

The enhancement of current density and luminance in organic light emitting diodes is achieved by treating molybdenum oxide (MoO3) hole-injection-layers with slight argon ion sputtering. The sputter treated MoO3 layers provide improvement in current injection efficiency, resulting in better current density which is about ten times higher than that of the reference devices. Photoemission spectroscopy shows that molybdenum in MoO3 is reduced to lower oxidation states after sputter treatment due to the removal of oxygen. As a result, gap states are formed to enhance metallic characteristics of the sputter treated MoO3 surface and facilitate better hole injection efficiency.

Graphene Anodes and Cathodes: Tuning the Work Function of Graphene by Nearly 2 eV with an Aqueous Intercalation Process

To expand the applications of graphene in optoelectronics and microelectronics, simple and effective doping processes need to be developed. In this paper, we demonstrate an aqueous process that can simultaneously transfer chemical vapor deposition grown graphene from Cu to other substrates and produce stacked graphene/dopant intercalation films with tunable work functions, which differs significantly from conventional doping methods using vacuum evaporation or spin-coating processes. The work function of graphene layers can be tuned from 3.25 to 5.10 eV, which practically covers the wide range of the anode and cathode applications. Doped graphene films in intercalation structures also exhibit excellent transparency and low resistance. The polymer-based solar cells with either low work function graphene as cathodes or high work function graphene as anodes are demonstrated.

Enhancing the incorporation compatibility of molybdenum oxides in organic light emitting diodes with gap state formations

The enhancement of injection current and luminance in organic light emitting diodes is achieved by annealing molybdenum oxide (MoO3) hole injecting layers prior to the deposition of hole transport layers. While there is no benefit by the incorporation of non-annealed MoO3 in devices using 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) as the hole transport layers, the annealed MoO3 layers exhibit a significant improvement in hole injection from indium tin oxide anodes to TAPC. X-ray photoemission spectroscopy reveals the change of oxidation states of Mo atoms in MoO3 films due to the annealing process. The gap state formation is verified by ultra-violet photoemission spectroscopy. A more energetically favorable band alignment is obtained at the interface between the annealed MoO3 and TAPC, resulting in improved hole injection efficiency. The overall performance of OLEDs can be enhanced by adopting annealed MoO3 in most of the hole transport layers.

Use of Ultrafast Time-Resolved Spectroscopy to Demonstrate the Effect of Annealing on the Performance of P3HT:PCBM Solar Cells

The organic solar cells of heterojunction system, ITO/PEDOT:PSS/P3HT:PCBM/Al, with a thermal annealing after deposition of Al exhibit better performance than those with an annealing process before deposition of Al. In this study, ultrafast time-resolved spectroscopy is employed to reveal the underlying mechanism of annealing effects on the performance of P3HT:PCBM solar cell devices. The analyses of all decomposed relaxation processes show that the postannealed devices exhibit an increase in charge transfer, in the number of separated polarons and a reduction in the amount of recombination between excited carriers. Moreover, the longer lifetime for the excited carriers in postannealed devices indicates it is more likely to be dissociated into photocarriers and result in a larger value for photocurrent, which demonstrates the physical mechanism for increased device performance.

Metal-induced molecular diffusion in [6,6]-phenyl-C61-butyric acid methyl ester poly(3-hexylthiophene) based bulk-heterojunction solar cells

We demonstrate the direct evidence of metal-induced molecular diffusion in bulk-heterojunction solar cells and its correlations to the device performance are investigated via ultraviolet and x-ray photoemission spectroscopy (UPS and XPS). Both UPS and XPS results indicate that the post-anneal after cathode deposition induces the out-diffusion of [6,6]-phenyl C61-butyric acid methyl ester toward the cathode, which can provide better hetero-structures and thus improved device performance. However, with aluminum and calcium deposition onto the active layers, the highest occupied molecular orbital of poly(3-hexylthiophene) exhibits opposite shifts after annealing, resulting in different device enhancements of solar cells.

Origins of vertical phase separation in P3HT:PCBM mixed films

The origins of vertical phase separation and their implication on the device efficiency of poly(3-hexylthiophene):[6,6]-phenyl C61-butyric acid methyl ester (P3HT:PCBM) based solar cells, with both regular and inverted structures, were investigated. We found that the light irradiation and the filtration processes during the device fabrications are two key steps that induce the vertical phase separation in the active layers. Upon light irradiation, the devices with inverted structures exhibit improved power conversion efficiency, whereas the regular devices show degradation. The inverted devices spun cast with filtered P3HT:PCBM solution also present a better improvement as compared to regular devices. X-ray and ultraviolet photoemission spectroscopies indicate that both illumination and filtration enhance the vertical phase separation of the blend film with additional PCBM segregated to the bottom interface.