Tetsuya Taima

Glancing angle deposition of copper iodide nanocrystals for efficient organic photovoltaics.

We report a simple method to achieve efficient nanostructured organic photovoltaics via patterning copper iodide (CuI) nanocrystals on indium tin oxide by glancing angle deposition. The strong interfacial interaction between zinc phthalocyanine (ZnPc) and CuI leads to the formation of nanopillar arrays with lying-down molecular order, which greatly improve light absorption and surface roughness for exciton dissociation. Optimized ZnPc/C(60) bilayer cell has a power conversion efficiency of 4.0 ± 0.1%, which is about 3-fold larger than that of conventional planar cell.

Effects of intrinsic layer thickness on solar cell parameters of organic p‐i‐n heterojunction photovoltaic cells

We fabricated organic p‐i‐n heterojunction photovoltaic cells of a zinc phthalocyanine (ZnPc)∕1:1 codeposition (ZnPc:C60)∕C60 structure. We investigated the effects of the intrinsic (i-) layer thickness on the photovoltaic properties. The thickness was changed from 0 nm (=p‐n heterojunction) to 50 nm (=alli-layer) with the total thickness of 50 nm. While the short-circuit photocurrent increased with increasing the thickness, the fill factor showed the opposite tendency. Therefore, the power conversion efficiency showed a maximum (1.5%) at the thickness of 10 nm under air mass 1.5 global solar conditions. Device simulation based on idealized equivalent circuit of a solar cell demonstrates that the i-layer thickness is concerned in the series resistance of the cells.

Highly Efficient Organic Thin-Film Solar Cells Based on Poly(3-hexylthiophene) and Soluble C70 Fullerene Derivative

The C70 derivative [6,6]-phenyl-C71-butyric acid methyl ester ([70]PCBM) has wider and stronger absorption in the visible light region than the analogous C60 derivative ([60]PCBM). Using [70]PCBM as an n-type semiconductor in combination with poly(3-hexylthiophene) (P3HT), we successfully fabricated a highly efficient organic photovoltaic (PV) cell showing higher short-circuit current density (JSC) and incident photon-to-electron conversion efficiency (IPCE) values than a P3HT:[60]PCBM-blend cell, with a maximum power conversion efficiency (PCE) of 3.8% under standard simulated solar illumination (air mass 1.5 G, 100 mW/cm2). All of the PV characteristics of the P3HT:[70]PCBM and P3HT:[60]PCBM cells were identically improved by thermal annealing, suggesting that changes in the internal structure of the photovoltaic layer and in the contact between the photovoltaic layer and the top electrode of the P3HT:[70]PCBM cell occurred in a similar manner to those of the P3HT:[60]PCBM cell. Therefore, [70]PCBM is an easy-to-replace alternative n-type semiconducting molecule to [60]PCBM, which has a higher PCE.

Phase separation of co-evaporated ZnPc:C60 blend film for highly efficient organic photovoltaics

We demonstrate phase separation of co-evaporated zinc phthalocyanine (ZnPc) and fullerene (C60) for efficient organic photovoltaic cells. With introducing a poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) film and a crystalline copper iodide film on indium tin oxide, 20-nm-thick ZnPc film adopts a lying-down crystalline geometry with grain sizes of about 50 nm. This surface distributed with strong interaction areas and weak interaction areas enables the selective growth of ZnPc and C60 molecules during following co-evaporation, which not only results in a phase separation but also improve the crystalline growth of C60. This blend film greatly enhances the efficiencies in photocurrent generation and carrier transport, resulting in a high power conversion efficiency of 4.56% under 1 sun.

Realization of large open-circuit photovoltage in organic thin-film solar cells by controlling measurement environment

To obtain large open-circuit photovoltage (Voc), we introduced a p-type semiconductor 5,6,11,12-tetraphenylnaphthacene (rubrene), which has a highest occupied molecular orbital (HOMO) level of 5.4 eV. Owing to an adsorption of O2 and H2O into rubrene film, optical absorption peaks were markedly decreased after exposure in air. By controlling the measurement environment to prevent the adsorption of O2 and H2O into rubrene films, we could obtain a high-performance organic thin-film solar cell with a large Voc of 0.91 V.

Mg-doped C60 thin film as improved n-type organic semiconductor for a solar cell

Mg doping effect on a [2-methoxy, 5-(2′-ethylhexyloxy)-1,4-phenylene-vinylene] (MEH-PPV)/C60 bilayer solar cell has been investigated. We find that the Mg doping occurs during the deposition of the Mg top electrode onto the C60 film and that the solar cell property is markedly improved by the doping. The power conversion efficiency of the Mg-doped device under 100 mW/cm2 white light illumination is 0.54%, which is approximately 400 times larger than that of the nondoped device.

Electronic Structure of Bathocuproine on Metal Studied by Ultraviolet Photoemission Spectroscopy

The electronic structure at the interface between bathocuproine (BCP) and a metal was studied by ultraviolet photoemission spectroscopy (UPS). For Ag, Al, Mg, and Ca, interface states were observed near the Fermi level. On the other hand, no interface states were observed for Au. The intensities of the interface states increase with decreasing metal work function. It is suggested that the interface states play an important role in electron transport at the interface. It was found that the energy difference between the BCP vacuum level and the metal Fermi level is almost constant. This phenomenon is similar to the Schottky barrier pinning observed at a metal/semiconductor interface. The cause of this phenomenon is discussed.

Efficient Small‐Molecule Photovoltaic Cells Using a Crystalline Diindenoperylene Film as a Nanostructured Template

A cascade-type small-molecule organic photovoltaic cell using a crystalline diindenoperylene film as a nanostructured template is demonstrated. This cell architecture simultaneously realizes organic nanostructure and cascade energy concepts, which significantly improves the photocurrent generation and fill factor, leading to a power conversion efficiency of 5.2±0.3%.

Control of Measurement Environments for High-Efficiency Organic Photovoltaic Cells

To fabricate a high-performance of organic photovoltaic (PV) cells, not only short-circuit photocurrent density (Isc), but also fill factor (FF) should be improved. Here, we improved FF by controlling the measurement environment. Because of the prevention of electron trapping by oxygen in the organic semiconducting layer, FF in vacuum environment increased 40% compared with that in air under 80 mW/cm2 irradiation of air mass 1.5 global solar conditions at room temperature. Finally, we obtained a power conversion efficiency of 3.6% by controlling the measurement environment and the optimization of the organic PV cell structure, where FF was markedly improved up to 0.61.

Monocrystalline 2-Adamantylamino-5-Nitropyridine (AANP) – a Novel Organic Material for Laser Raman Converters in the Visible and Near-IR

We have open up new nonlinear-laser potential in polar C15H19N3O2 (AANP) single crystal. At room temperature under one-micron picosecond laser excitation high-order stimulated Raman scattering (SRS) and self-sum-frequency generation effects in this organic material were observed. All their registered lasing components were identified and put in relation with the SRS-active vibration modes (ωSRS ≈1280 cm-1). The investigated crystal is classified a as promising χ(2)+χ(3) medium for laser-frequency converters and Raman lasers in the visible and near-IR.