Yasuo Nakayama

Identification of different origins for s-shaped current voltage characteristics in planar heterojunction organic solar cells

We investigate different parameters influencing the occurrence of s-shaped current voltage (j-V) characteristics in planar heterojunction organic solar cells. It is shown how substrate modification, purity of the active organic material as well as variation of the top contact can affect the shape of the j-V curves. The studies are performed on vacuum-evaporated planar heterojunction solar cells with diindenoperylene (DIP) as electron donor and fullerene C60 as acceptor. The focus is on the fill factor and forward current being the most direct indicators for s-shapes in j-V curves. We find that the main effect of substrate heating during film growth can be assigned to changes in energy barriers rather than to the modification of morphology and crystallinity, which is also influenced by elevated substrate temperatures. The decisive role of the barrier height between the anode work function and the HOMO (i.e., highest occupied molecular orbital) level of the donor is approved by comparing hole-injection laye...

Charge accumulation at organic semiconductor interfaces due to a permanent dipole moment and its orientational order in bilayer devices

Charge accumulation at the organic heterointerfaces in multilayer organic light-emitting diodes (OLEDs) is an important process for understanding their device operation, efficiency, and degradation properties. Charge accumulation behavior has typically been analyzed in terms of the energy barrier and difference of the charge carrier mobility across heterointerfaces. In this study, we demonstrate that permanent dipole moments and their orientational order also play a significant role in the charge behavior at organic semiconductor interfaces. The charge accumulation properties of bilayer devices composed of polar or nonpolar molecules deposited on a 4,4’-bis[N-(1-naphthyl)-N-phenylamino]-biphenyl layer between the anode and cathode were examined by displacement current measurement and impedance spectroscopy. In addition, Kelvin probe measurements for the corresponding bilayer structures excluding the cathode were performed to analyze the relationship between the potential profile and charge accumulation pr...

Direct observation of the electronic states of single crystalline rubrene under ambient condition by photoelectron yield spectroscopy

The electronic states of single crystalline (SC) rubrene were experimentally observed by photoelectron yield spectroscopy without the sample charging problem. The ionization energy (Is) in the SC phase was determined to be 4.85(±0.05) eV, which is reduced by 0.45 eV compared to that of the amorphous film. The changes in the electronic states during photo-oxidation reaction and under ambient air were also observed directly to reveal the further reduction in Is, which can be attributed to the generation of polar oxide molecules and reversible physisorption of H2O, respectively.

Threshold voltage shift and formation of charge traps induced by light irradiation during the fabrication of organic light-emitting diodes

We examined the effects of ambient light on the device properties of an organic light-emitting diode, indium tin oxide/4,4′-bis[N-(1-naphthyl)-N-phenylamino]-biphenyl (α-NPD)/tris-(8-hydroxyquinolate) aluminum (Alq3)∕Al, during fabrication using displacement current measurement. Light irradiation induces a shift in the threshold voltage for hole injection and results in the formation of charge traps in the Alq3 layer. The voltage shift implies a reduction in charge density at the α-NPD∕Alq3 interface. The origin of the interfacial charge can be attributed to dipole moment ordering in the Alq3 layer.

Photoemission measurement of extremely insulating materials: Capacitive photocurrent detection in photoelectron yield spectroscopy

The electronic structures of various materials have been investigated using photoemission measurements. The sample charge-up problem, however, limits the application of these measurements to insulating materials. In this study, we propose a capacitive photocurrent detection method that permits photoelectron yield spectroscopy measurements of extremely insulating materials in both vacuum and ambient pressure conditions. The mechanism of detection is discussed, and the application to gold and rubrene films on mica substrates is demonstrated.

Utilizing carbon nanotube electrodes to improve charge injection and transport in bis(trifluoromethyl)-dimethyl-rubrene ambipolar single crystal transistors

We have examined the significant enhancement of ambipolar charge injection and transport properties of bottom-contact single crystal field-effect transistors (SC-FETs) based on a new rubrene derivative, bis(trifluoromethyl)-dimethyl-rubrene (fm-rubrene), by employing carbon nanotube (CNT) electrodes. The fundamental challenge associated with fm-rubrene crystals is their deep-lying HOMO and LUMO energy levels, resulting in inefficient hole injection and suboptimal electron injection from conventional Au electrodes due to large Schottky barriers. Applying thin layers of CNT network at the charge injection interface of fm-rubrene crystals substantially reduces the contact resistance for both holes and electrons; consequently, benchmark ambipolar mobilities have been achieved, reaching 4.8 cm(2) V(-1) s(-1) for hole transport and 4.2 cm(2) V(-1) s(-1) for electron transport. We find that such improved injection efficiency in fm-rubrene is beneficial for ultimately unveiling its intrinsic charge transport properties so as to exceed those of its parent molecule, rubrene, in the current device architecture. Our studies suggest that CNT electrodes may provide a universal approach to ameliorate the charge injection obstacles in organic electronic devices regardless of charge carrier type, likely due to the electric field enhancement along the nanotube located at the crystal/electrode interface.

Influence of the direction of spontaneous orientation polarization on the charge injection properties of organic light-emitting diodes

A tris(7-propyl-8-hydroxyquinolinato) aluminum [Al(7-Prq)3] film shows negative giant surface potential (GSP) because of spontaneous orientation polarization. The polarity of this film is opposite to those of tris-(8-hydroxyquinolate) aluminum films. In Al(7-Prq)3-based organic light-emitting diodes, negative GSP leads to the positive interface charge and governs the electron injection and accumulation properties. In addition, a high resistance to the electron injection at the Al(7-Prq)3/Ca interface is suggested possibly because of the negative polarization charge at the interface. These results show the importance of orientation polarization in controlling the charge injection and accumulation properties and potential profile of the resultant devices.

Quantum regulation of Ge nanodot state by controlling barrier of the interface layer

Quantized energy in Ge nanodots aligned over oxidized Si surfaces could be regulated by modifying an interface atomic layer. The confining potential was evaluated from dot-size dependent energy shift of the ground state of confined holes, which revealed that epitaxial nanodots showed a lower confining potential barrier than nonepitaxial ones. The present results provide a new way to tune quantized energy levels of Ge nanodots not only by their size but also by interface condition.

Full Picture of Valence Band Structure of Rubrene Single Crystals Probed by Angle-Resolved and Excitation-Energy-Dependent Photoelectron Spectroscopy

The valence band structure of rubrene single crystals was experimentally determined by high-resolution angle-resolved and excitation-energy-dependent photoelectron spectroscopy at room temperature. The energy position of the peak derived from the highest occupied molecular orbital did not depend on the excitation energy, reflecting an absence of energy dispersion along the surface normal direction. A two-dimensional valence band dispersion relation over the surface Brillouin zone obtained by angle-resolved photoemission to three critical points was reproduced excellently by a two-dimensional tight binding approximation. Highly anisotropic values of intermolecular transfer integrals to four adjacent molecules were obtained from the present results.

Epitaxial Growth of an Organic p–n Heterojunction: C60 on Single-Crystal Pentacene

Designing molecular p-n heterojunction structures, i.e., electron donor-acceptor contacts, is one of the central challenges for further development of organic electronic devices. In the present study, a well-defined p-n heterojunction of two representative molecular semiconductors, pentacene and C60, formed on the single-crystal surface of pentacene is precisely investigated in terms of its growth behavior and crystallographic structure. C60 assembles into a (111)-oriented face-centered-cubic crystal structure with a specific epitaxial orientation on the (001) surface of the pentacene single crystal. The present experimental findings provide molecular scale insights into the formation mechanisms of the organic p-n heterojunction through an accurate structural analysis of the single-crystalline molecular contact.