Toshihiro Yamanari

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.

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.

Pentacene:Fullerene Multilayer-Heterojunction Organic Photovoltaic Cells Fabricated by Alternating Evaporation Method

Novel small-molecule organic photovoltaic cell structures with pentacene and fullerene multilayers are reported. Pentacene is an attractive material for photovoltaic cells, but it tends to crystallize with molecular orientation during vacuum deposition. Therefore, thus far, it has been difficult to fabricate a bulk-heterojunction active layer with fullerenes by coevaporation, despite the demand for pentacene/fullerene bilayer heterojunction cells owing to their good photovoltaic performance. Here, instead of preparing an active layer by coevaporation, we prepare a pentacene and fullerene multilayer-heterojunction active layer by alternating evaporation. It was found that multilayer cells show superior photovoltaic performance to bilayer cells owing to the improvements in their exciton dissociation efficiency and charge carrier transport properties.

Templating effects in molecular growth of blended films for efficient small-molecule photovoltaics.

A strategy to control the molecular growth of coevaporated zinc phthalocyanine (ZnPc) and fullerene (C60) blended films for efficient organic photovoltaic (OPV) cells was demonstrated. Introduction of a 2,5-bis(4-biphenylyl)-bithiophene (BP2T) film or a ZnPc film on BP2T as nanostructured templates not only results in phase-separated domains in blended films with clear interpenetrating networks but also improves the crystallinity of ZnPc domains, both of which enhance photocurrent generation and charge carrier transport. Such morphology is strongly associated with the molecular growth of the templating layers. Roughness and adhesion of the templating layers are of great importance for the molecular growth of the blended films and in turn for cell characteristics. By carefully regulating the molecular growth of the blended films, the power conversion efficiency was improved by 125%, from 1.85 to 4.15% under 1 sun.

Structural modifications of zinc phthalocyanine thin films for organic photovoltaic applications

Zinc phthalocyanine (ZnPc) thin films are vacuum-evaporated on bare indium-tin-oxide (ITO) coated glass by varying substrate temperature and growth rate. The samples are characterized by atomic force microscopy, x-ray diffraction, and infrared spectroscopy. The temperature does not play a clear role in the crystalline growth of ZnPc possibly due to the significant structural defects on ITO surface, while it strongly influences the surface morphology and molecular alignment. The relationships between growth characteristics and performances of photovoltaics with planar heterojunction are discussed in detail. Increasing temperature or growth rate leads to a rougher surface morphology, which enables more donor/accepter interface area for photocurrent generation. Moreover, at elevated temperature, more molecules adopt standing-up geometry, resulting in a reduction in overall efficiency. The results imply that low-temperature process in order to control the molecular alignment is preferred for efficient organic...

Structural influences on charge carrier dynamics for small-molecule organic photovoltaics

We investigated the structural influences on the charge carrier dynamics in zinc phthalocyanine/fullerene (ZnPc/C60) photovoltaic cells by introducing poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) and 2,5-bis(4-biphenylyl)-bithiophene (BP2T) between indium tin oxide and ZnPc layers. ZnPc films can be tuned to be round, long fiber-like, and short fiber-like structure, respectively. Time-resolved microwave conductivity measurements reveal that charge carrier lifetime in ZnPc/C60 bilayer films is considerably affected by the intra-grain properties. Transient photocurrent of ZnPc single films indicated that the charge carriers can transport for a longer distance in the long fiber-like grains than that in the round grains, due to the greatly lessened grain boundaries. By carefully controlling the structure of ZnPc films, the short-circuit current and fill factor of a ZnPc/C60 heterojunction solar cell with BP2T are significantly improved and the power conversion efficiency is increased to 2.6%, which...

Efficiency limit analysis of organic solar cells: model simulation based on vanadyl phthalocyanine/C60 planar junction cell

The power-conversion-efficiency (PCE) limit was simulated for a planar vanadyl phthalocyanine (VOPc)/C60 heterojunction model device. On the basis of the transfer matrix method and exciton diffusion analysis, short-circuit current density (Jsc) was simulated as a function of exciton diffusion length (Ld). The model device was simulated to be operated in a wide wavelength range (300?950 nm). A large Jsc up to 30 mA/cm2 was obtained at an Ld of approximately 10 ?m. On the basis of the estimated Voc values of 0.8?1.0 V, the PCE limit of the model device was calculated to be in the range of 17?22%, which is close to the reported results of thermodynamic analysis.

Dye-Sensitized Bulk Heterojunction Polymer Solar Cells

Organic solar cells based on poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-buteric acid methyl ester (PCBM) blend have been improved in their efficiencies and attracts most attention. The incident photon to converted electron (IPCE) spectra of P3HT:PCBM-blend solar cells reported so far range from UV to around 600 nm. Therefore, to utilize longer wavelengths (>600 nm) of sunlight is a potentially effective way to improve the performance. In the present study, we investigated dye-sensitizing effects of photosensitizing dyes to P3HT:PCBM-blend solar cells. We examined four kinds of dyes (cyanine, merocyanine, phthalocyanine and squarylium). The addition of squarylium caused an increase in the IPCE in the longer-wavelength region, while the addition of other dyes exhibited little increase