A. J. Mäkinen

Molecular organic light-emitting diodes using highly conducting polymers as anodes

Films fabricated from commercially available poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) aqueous dispersions have been widely used in many electronic and optoelectronic applications. Previous attempts to utilize them as anodes in organic light-emitting diodes (OLEDs) were not satisfactory due to their low conductivity. In this letter we report on the fabrication and characterization of an OLED device made using a highly conductive form of PEDOT:PSS as anode and demonstrate its superior performance relative to that of a similar device using the commercial conducting polymer as an anode. An external electroluminescence quantum efficiency of ∼0.73% was measured at 100 A/m2.

Doped ZnO thin films as anode materials for organic light-emitting diodes

Zinc oxide thin films, doped with aluminum (AZO), gallium (GZO) and zirconium (ZZO), have been deposited on glass substrates by pulsed laser deposition. The electrical and optical properties of these films were investigated as a function of oxygen deposition pressure. These doped zinc oxide films were used as an anode contact to fabricate organic light-emitting diodes. Luminance efficiencies of 0.1 lm/W, 0.2 lm/W and 0.9 lm/W were measured at the light output of 100 cd/m2 for the AZO, GZO and ZZO films, respectively. Doped zinc oxides are attractive alternative materials as transparent conducting electrode because they are nontoxic and inexpensive compared with indium tin oxide (ITO).

Highly efficient molecular organic light-emitting diodes based on exciplex emission

Highly efficient exciplex emission is observed from molecular organic light-emitting diodes (MOLEDs) based on silole derivatives as emissive and electron transport materials, and a hole transporting amine derivative. A silole derivative, 2,5-di-(3-biphenyl)-1,1-dimethyl- 3,4-diphenylsilacyclopentadiene (PPSPP), which shows blue fluorescence (476 nm) with a high solid-state photoluminescence quantum yield of 85% was used as the emitter. Another silole derivative, 2,5-bis-(2′,2″-bipyridin-6-yl)-1,1-dimethyl-3,4-diphenylsilacyclopentadiene which exhibits high electron mobility, was used as the electron transport material. MOLEDs using these two siloles and N,N′-diphenyl-N,N′-(2-napthyl)-(1,1′-phenyl)-4,4′-diamine (NPB) as the hole transporter show electroluminescence (EL) emission centered at 495 nm. This red-shifted EL band relative to the blue fluorescence of PPSPP is assigned to a NPB:PPSPP exciplex. An operating voltage of 4.5 V was measured at 100 cd/m2 and an EL quantum efficiency of 3.4% was achieved ...

Initial stages of metal/organic semiconductor interface formation

We have used photoelectron spectroscopies to study the metal/organic semiconductor interfaces formed by depositing three different metal–quinolate derivatives on Ag, with the goal of better understanding the initial stages of interface formation. We find very consistent results at all three interfaces, which exhibit strong energy level shifts within the first molecular layer, followed by a nearly “flat-band” condition. These results were analyzed in the context of the interface dipole and “band-bending” models. We conclude that the interface dipole model, extended to account for the differences in polarization screening in the first molecular layer, most accurately describes our findings. In this article we present the most thorough description of the early stages of metal/molecular organic semiconductor interfaces to date.

Hole injection barriers at polymer anode/small molecule interfaces

A photoemission study of the interface between spin-cast films of a conducting polymer blend consisting of poly(3,4-ethylenedioxythiophene) (PEDOT), poly(4-styrenesulfonate) (PSS) and glycerol as an additive, and vacuum-evaporated hole transport layers (HTL) of 4,4′-bis(carbazol-9-yl)biphenyl, N,N′-diphenyl-N,N′-bis(1-naphthyl)-1-1′biphenyl-4,4′-diamine and N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′- biphenyl-4,4′-diamine reveals a hole injection barrier between 0.5 and 0.9 eV at the glycerol-modified PEDOT-PSS/HTL interface. The measured energy barriers imply a reasonable charge injection, which is very encouraging for further development of the novel anode structures based on a conducting polymer/small molecule interface to be utilized in electro-optic applications such as organic light-emitting devices.

Vacuum level alignment in organic guest-host systems

Using a combination of ultraviolet and x-ray photoelectron spectroscopies, we have studied the relative energy level alignment of two phosphorescent guest molecules, 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphine platinum (PtOEP) and tris(2-phenylpyridine)iridium (Ir(ppy)3), doped in an electron transport host, tris(8-hydroxyquinolinato) aluminum (III) (Alq3), and in a hole transport host, 4,4′-bis(carbazol-9-yl)biphenyl. In each of the guest–host systems, we find that the vacuum levels of the guest and the host molecules align, and that the position of the highest occupied molecular orbital of the phosphorescent guest is independent of the guest molecule concentration (0.8%–56% by mass). The vacuum level alignment implies free movement of the Fermi level within the gaps of the guest and the host molecules manifesting the intrinsic nature of these organic semiconductors. These results give insights into the role that the phosphorescent molecules play in trapping and transporting charges within the emissiv...

Band structure measurement of organic single crystal with angle-resolved photoemission

The electronic structure of bulk rubrene single crystal was studied with angle-resolved photoemission spectroscopy. Highly reproducible dispersive features were observed with nice symmetry about the Brillouin zone center and boundaries, representing the band structure measured for a bulk organic single crystal. The high quality of the surface was confirmed with scanning tunneling microscopy. The energy dispersion of the highest occupied molecular orbitals derived bands showed strong anisotropic behavior in the a-b plane of the unit cell. The measured band structure, however, differs unexpectedly from theoretical calculations in terms of the amount of the dispersion and the separation of the bands.

Photoemission study of frontier orbital alignment at a metal–organic interface as a function of conjugation length of oligothiophene derivatives

We report an ultraviolet photoemission spectroscopy (UPS) study of solid films of two molecules, 5,5′-bis(dimesitylboryl)-2,2′-bithiophene (BMB-2T) and 5,5′-bis(dimesitylboryl)-2,2′:5′2′terthiophene (BMB-3T) which have been characterized as potential electron transport materials in organic light-emitting devices. Using the frontier orbital positions at a metal-organic interface, determined from the UPS measurements, together with the optical band gaps of the molecules, the electron injection barriers for the two oligothiophene derivatives are found to be significantly different. This barrier is estimated to be ∼0.2-0.3 eV lower for BMB-2T than for BMB-3T at a metal–organic interface.

Electronic structure of a silole derivative-magnesium thin film interface

Photoemission spectroscopy of an interface between Mg and a silole derivative, 2,5-bis[6′-(2′,2″-bipyridyl)]-1,1-dimethyl-3,4-diphenyl silacyclopentadiene (PyPySPyPy) reveals the formation of two gap states both at metal-on-organic (Mg/PyPySPyPy) and organic-on-metal (PyPySPyPy/Mg) interfaces. The appearance of gap states is interpreted as the result of electron transfer from Mg to PyPySPyPy, leading to the formation of a charge transfer complex. A strong correlation found between the metal and organic core level shifts, and the changes in the Fermi level position at the Mg/PyPySPyPy interface further supports the model of charge transfer between the metal and the organic. Despite the similar chemical character of the two interfaces, the binding energy (BE) of the gap states at the PyPySPyPy/Mg interface is 0.2–0.3 eV higher than the gap state BE measured at the reversed interface.

Energy level evolution at a silole/magnesium thin-film interface

Photoemission spectroscopy of an interface formed through step-by-step deposition of Mg onto a film of a silole derivative, 2,5-bis[6′-(2′,2″-bipyridyl)]-1,1-dimethyl-3,4-diphenyl silacyclopentadiene (PyPySPyPy), reveals the formation of gap states arising from electron transfer from the metal to the organic. In addition, a 0.3-eV rigid shift of the molecular orbitals toward higher binding energy is observed, while the work function of the Mg-rich PyPySPyPy surface is reduced by 0.6 eV. The observed energy level evolution elucidates how the electronic structure of the Mg/PyPySPyPy interface depends on the chemical interaction between the metal and the organic. This is important for understanding and optimizing electron injection at the Mg/PyPySPyPy interface, which can be incorporated in electronic, electro-optic, and optoelectronic devices.