Senku Tanaka

Monolithic parallel tandem organic photovoltaic cell with transparent carbon nanotube interlayer

We demonstrate an organic photovoltaic cell with a monolithic tandem structure in parallel connection. Transparent multiwalled carbon nanotube sheets are used as an interlayer anode electrode for this parallel tandem. The characteristics of front and back cells are measured independently. The short circuit current density of the parallel tandem cell is larger than the currents of each individual cell. The wavelength dependence of photocurrent for the parallel tandem cell shows the superposition spectrum of the two spectral sensitivities of the front and back cells. The monolithic three-electrode photovoltaic cell indeed operates as a parallel tandem with improved efficiency.

Doping Effect of Tetrathianaphthacene Molecule in Organic Semiconductors on Their Interfacial Electronic Structures Studied by UV Photoemission Spectroscopy

The n-type doping ability of the electron-donating organic molecule tetrathianaphthacene (TTN) was examined by ultraviolet photoemission spectroscopy (UPS) for two kinds of matrix organic semiconductors, hexadecafluorophthalocyaninatozinc (F16ZnPc) and tris(8-hydroxyquinoline)aluminum (Alq3). The observed dependence of the UPS spectra suggests that TTN acts as a good donor in F16ZnPc, while the effect was not so significant for Alq3. These results and the consideration of the energy parameters of these molecules indicate that a close match between the highest occupied molecular orbital of the dopant and the lowest unoccupied molecular orbital of the matrix is important for efficient n-type doping.

Ga-Doped ZnO Film as a Transparent Electrode for Phthalocyanine/Perylene Heterojunction Solar Cell

The photovoltaic properties of Zn–phthalocyanine (ZnPc)/3,4,9,10-perylene-tetracarboxyl-bis-benzimidazole (PTCBI) heterojunction solar cells using Ga-doped ZnO (GZO) film as a transparent electrode are studied. When GZO is the electrode in contact with the donor layer, i.e., ZnPc, the energy conversion efficiency η is only 1/4 of that for the cell using indium–tin-oxide (ITO) as the electrode. When GZO is the electrode in contact with the acceptor layer, i.e., PTCBI, on the other hand, the cell has a three times larger η than the cell using ITO. These results are explained by the work function of GZO being lower than that of ITO. When GZO, or ITO, is used as an electrode for PTCBI, an interesting aging effect is observed. For example, by keeping the GZO-based cell in open air for 6 days in the dark, the energy conversion efficiency and the short-circuit current are increased by factors of 1.67 and 1.36, respectively.

Surface photovoltage effect and its time dependence in GaAs–GaAsP superlattice studied with combination of synchrotron and laser radiation

The surface photovoltage (SPV) effect and its temporal profiles in a GaAs–GaAsP superlattice (SL) were measured by core-level photoelectron spectroscopy with the combination of synchrotron radiation and laser. It was found that the SPV effect in the SL is remarkably suppressed as compared with that in a bulk GaAs. The difference in the temporal profile of the SPV between SL and bulk samples was observed in microsecond range. The suppression of the SPV effect in the negative electron affinity surfaces of the SL was also observed. It is concluded that the SL with a high-doping surface layer is suitable for the spin-polarized electron source without the SPV effect.

Dynamics of Surface Photovoltage Effects on Clean and Negative Electron Affinity Surfaces of p-GaAs (100)

Core-level photoelectron spectroscopy with the combination of synchrotron radiation and laser light was used for exploring the dynamics of the surface photovoltage (SPV) effect on a p -GaAs (100). It was found that a temporal profile of the SPV is very different in microsecond range between room temperature and 90 K. The results can be explained with the recombination of photoexcited carriers via thermionic and tunneling processes. The SPV effects and its temporal profiles on the negative electron-affinity (NEA) surface were also studied. It was observed that the SPV effect is suppressed on the NEA surface, which may be due to the escape process of the photoexcited carriers.

SURFACE-PHOTOVOLTAGE EFFECT IN A GaAs–GaAsP SUPERLATTICE STUDIED WITH COMBINATION OF SYNCHROTRON RADIATION AND THE LASER

Core-level photoelectron spectroscopy with the combination of synchrotron radiation (SR) and a laser was used for exploring the surface-photovoltage (SPV) effect and its temporal profiles in a GaAs–GaAsP superlattice (SL). It was observed that the SPV value in the SL is suppressed as compared with a bulk GaAs. However, no significant difference was found in the temporal profile between the bulk and the SL. It is suggested that the suppression of the SPV in the SL is dominantly due to the small value of band bending under thermal equilibrium.

Adsorption of K on NbC(100): photoemission and thermal desorption study

Abstract The adsorption state of K on NbC(100) has been studied with core-level photoemission spectroscopy (CLPES) and thermal desorption spectroscopy (TDS). The adsorption mechanism of K on the NbC(100) surface is found to be very unique as compared with those of many alkali-metal surface systems. The TDS study shows that the desorption energy of K increases with the increase of K coverage ( θ K ). The work function measurements show that the polarization of the NbC(100) surface after K adsorption is small as compared with those for many alkali-metal surface systems. The lineshape analysis of the K 3p photoemission spectra shows that the valence electronic density of states for adsorbed K is nearly independent on the coverage. Considering these results, it is proposed that the K-adsorbed layer is grown via the island formation mechanism.

Photo-induced change of the semiconductor–vacuum interface of p-GaAs(100) studied by photoelectron spectroscopy

Abstract The photo-induced change in the semiconductor–vacuum interface on GaAs(100) and Cs/GaAs(100) has been investigated with core-level photoelectron spectroscopy using synchrotron radiation and a mode-locked Nd:YAG laser. Both Ga-3d and As-3d photoelectron peaks showed transient energy shifts under the laser irradiation without any spectral change. The amounts of energy shift were strongly dependent on the sample temperature and laser photon flux. It is shown that the experimental results can be fitted to a theoretical curve which was derived from the photo-induced band bending scheme in the surface layer of the semiconductor.

Improvement of power conversion efficiency of phthalocyanine/C60 heterojunction solar cells by inserting a lithium phthalocyanine layer at the indium-tin oxide/phthalocyanine interface

Improvement of power conversion efficiency of a zinc phthalocyanine (ZnPc)/C60 heterojunction solar cell was achieved by inserting a lithium phthalocyanine (LiPc) layer at the indium-tin oxide (ITO)/ZnPc interface. The results of photoelectron spectroscopy suggest that the barrier height for the hole transport at the ITO/ZnPc interface is reduced by the LiPc layer. A similar improvement of the power conversion efficiency by the insertion of a LiPc layer was also observed in M-phthalocyanine (M=H2, Cu, and TiO)/C60 cells.

Organic thin-film solar cells with a Cu anode: Improvement of the photovoltaic properties on aging in air

The photovoltaic properties of a Ga-doped ZnO (GZO)/3,4,9,10-perylene-tetracarboxyl-bis-benzimidazole (PTCBI)/Zn-phthalocyanine (ZnPc)/Cu heterojunction cell (cell A) and a GZO/ZnPc/Cu Schottky-barrier cell (cell B) were investigated. The energy conversion efficiency η of cell A was only 0.02% immediately after the device preparation but improved to 0.46% after aging for 24 days in air. To elucidate the mechanism of this aging effect, photocurrent action and electro-absorption spectra were measured for cell B. The results reveal that a Schottky barrier exists at the ZnPc/Cu interface which blocks the transport of photogenerated holes to the Cu electrode, and the barrier height is reduced by a white-light illumination of the device after aging. The change in barrier height is attributed to the formation of electron traps at the surface of the ZnPc layer on aging that trap photogenerated electrons.