Soichi Uchida

Efficient bulk heterojunction photovoltaic cells using small-molecular-weight organic thin films.

The power conversion efficiency of small-molecular-weight and polymer organic photovoltaic cells has increased steadily over the past decade. This progress is chiefly attributable to the introduction of the donor–acceptor heterojunction that functions as a dissociation site for the strongly bound photogenerated excitons. Further progress was realized in polymer devices through use of blends of the donor and acceptor materials: phase separation during spin-coating leads to a bulk heterojunction that removes the exciton diffusion bottleneck by creating an interpenetrating network of the donor and acceptor materials. The realization of bulk heterojunctions using mixtures of vacuum-deposited small-molecular-weight materials has, on the other hand, posed elusive: phase separation induced by elevating the substrate temperature inevitably leads to a significant roughening of the film surface and to short-circuited devices. Here, we demonstrate that the use of a metal cap to confine the organic materials during annealing prevents the formation of a rough surface morphology while allowing for the formation of an interpenetrating donor–acceptor network. This method results in a power conversion efficiency 50 per cent higher than the best values reported for comparable bilayer devices, suggesting that this strained annealing process could allow for the formation of low-cost and high-efficiency thin film organic solar cells based on vacuum-deposited small-molecular-weight organic materials.

Asymmetric tandem organic photovoltaic cells with hybrid planar-mixed molecular heterojunctions

We demonstrate high-efficiency organic photovoltaic cells by stacking two hybrid planar-mixed molecular heterojunction cells in series. Absorption of incident light is maximized by locating the subcell tuned to absorb long-wavelength light nearest to the transparent anode, and tuning the second subcell closest to the reflecting metal cathode to preferentially absorb short-wavelength solar energy. Using the donor, copper phthalocyanine, and the acceptor, C60, we achieve a maximum power conversion efficiency of ηP=(5.7±0.3)% under 1 sun simulated AM1.5G solar illumination. An open-circuit voltage of VOC⩽1.2V is obtained, doubling that of a single cell. Analytical models suggest that power conversion efficiencies exceeding 6.5% can be obtained by this architecture.

4.2% efficient organic photovoltaic cells with low series resistances

We demonstrate double-heterostructure copper phthalocyanine/C60 organic photovoltaic cells with series resistances as low as 0.1 Ω cm2. A high fill factor of ∼0.6 is achieved, which is only slightly reduced at very intense illumination. As a result, the power conversion efficiency increases with the incident optical power density, reaching a maximum of (4.2±0.2)% under 4–12 suns simulated AM1.5G illumination. The cell performance is accurately described employing an analysis based on conventional semiconductor p–n junction diodes. The dependence of the series resistance on the device area suggests the dominance of the bulk resistance of the indium-tin-oxide anode as a limiting factor in practical cell efficiencies.

Vortex-like excitations and the onset of superconducting phase fluctuationin underdoped La 2- x Sr x CuO 4

Two general features of a superconductor, which appear at the critical temperature, are the formation of an energy gap and the expulsion of magnetic flux (the Meissner effect). In underdoped copper oxides, there is strong evidence that an energy gap (the pseudogap) opens up at a temperature significantly higher than the critical temperature (by 100–220 K). Certain features of the pseudogap suggest that it is closely related to the gap that appears at the critical temperature (for example, the variation of the gap magnitudes around the Fermi surface and their maximum amplitudes are very similar). However, the Meissner effect is absent in the pseudogap state. The nature of the pseudogap state, and its relation (if any) to the superconducting state are central issues in understanding copper oxide superconductivity. Recent evidence suggests that, in the underdoped regime, the Meissner state is destroyed above the critical temperature by strong phase fluctuations (as opposed to a vanishing of the superfluid density). Here we report evidence for vortices (or vortex-like excitations) in La2-xSrxCuO4 at temperatures significantly above the critical temperature. A thermal gradient is applied to the sample in a magnetic field. Vortices are detected by the large transverse electric field produced as they diffuse down the gradient (the Nernst effect). We find that the Nernst signal is anomalously enhanced at temperatures as high as 150 K.

Organic small molecule solar cells with a homogeneously mixed copper phthalocyanine: C60 active layer

An efficient organic solar cell with a vacuum codeposited donor–acceptor copper phthalocyanine (CuPc):C60 mixed layer is described. A device with a structure of indium tin oxide/330 A CuPc:C60(1:1)/100 A C60/75 A 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline/Ag has a series resistance of only RS=0.25 Ω cm2, resulting in a current density of ∼1 A/cm2 at a forward bias of +1 V, and a rectification ratio of 106 at ±1 V. Under simulated solar illumination, the short circuit current density increases linearly with light intensity up to 2.4 suns. The maximum power conversion efficiency is ηP=(3.6±0.2)% at 0.3 suns (AM1.5G simulated solar spectrum) and ηP=(3.5±0.2)% at 1 sun. Although the fill factor decreases with increasing intensity, a power efficiency as high as ηP=(3.3±0.2)% is observed at 2.4 suns intensity.

Mixed donor-acceptor molecular heterojunctions for photovoltaic applications. I. Material properties

In this and the following paper (Parts I and II, respectively), we discuss the properties of mixed donor-acceptor organic thin films and their application to organic solar cells. In Part I, we present a study of the material properties of mixed donor-acceptor thin films. Through optical absorption, x-ray diffraction, microscopy, and charge transport measurements, we determine the relationships among film microstructure, mixing ratio, and charge conduction in mixtures of two organic molecular species. We find that mixed layers of the molecular pair of 1:1 (by weight) copper phthalocyanine in C60 have electron and hole mobilities reduced by more than one order of magnitude compared to corresponding films of pure composition. In Part II, we demonstrate that the performance of organic hybrid planar-mixed heterojunction photovoltaic cells based on a mixed donor-acceptor molecular layer sandwiched between the donor and acceptor layers of homogeneous composition can have improved performance over conventional pl...

Mixed donor-acceptor molecular heterojunctions for photovoltaic applications. II. Device performance

We demonstrate efficient organic photovoltaic cells employing a photoactive region composed of a mixed donor-acceptor molecular layer, the properties of which were introduced in the preceding paper (Part I) [Rand et al., J. Appl. Phys. 98, 124902 (2005)]. The hybrid planar-mixed heterojunction (PM-HJ) device architecture consists of a film mixture of donor and acceptor molecules inserted between layers of pure donor and acceptor composition. Using the donor, copper phthalocyanine, and the acceptor, C60, we demonstrate a hybrid PM-HJ cell with a maximum power conversion efficiency of (5.0±0.3)% under 1–4suns simulated AM1.5 solar illumination. The current-voltage characteristics of the PM-HJ cell are described using a model based on the field-dependent charge collection length.

Electrochemical properties of non-conjugated electrochromic polymers derived from aromatic amine derivatives

Abstract The electrochemical and optical properties of two types of non-conjugated electrochromic polymers derived from aromatic amine derivatives (DDP-A, DDB-P) are presented. DDP-A is synthesized by the polymerization of N , N ′-dimethyl- N , N ′-diphenyl-1,4-phenylenediamine (DDP) and acetaldehyde (A), and DDB-P is polymerized with N , N ′-dimethyl- N , N ′-diphenylbenzidine (DDB) and propionaldehyde (P). Both DDP-A and DDB-P have a band gap in the ultraviolet region, and are colorless and transparent in neutral states. At one-electron oxidation states, DDP-A + absorbs light mainly in a visible region, whereas DDB-P + absorbs light mainly in a near-infrared (NIR) region. DDP-A + and DDB-P + are nitrogen-centered π-bridged mixed-valence compounds showing intervalence-charge transfer bands. The electrochemical and optical properties of DDP-A and DDB-P depend on the nitrogen–nitrogen distance correlating the electronic coupling of nitrogen redox centers. From the standpoint of energy saving, DDB-P is very interesting because it absorbs light in an NIR region. Finally, focusing on the smart window application, the optical properties of the solid-state electrochromic cell fabricated with DDP-A and heptyl viologen are examined. The cell was confirmed to be colored blue by applying a potential of about 1.0 V, and bleached at 0 V.

Thermal and optical behavior of electrochromic windows fabricated with carbon-based counterelectrode

We proposed a carbon-based counterelectrode for electrochromic windows (ECWs) and fabricated a new solid-state ECW consisting of an indium tin oxide electrode (ITO, IN2O3:SN)/a WO3 film/a polymeric solid electrolyte (PSE)/a carbon-based counterelectrode. The carbon-based counterelectrode is a series of arrays of carbon material dots formed on an ITO substrate and is virtually transparent in a visible region, Those carbon dots play a part in the formation of an electric double layer in an electrochromic reaction of the ECW. The electric double layer capacitance of the counterelectrode increases linearly as a function of carbon-dot covering percentage on the ITO substrate. Maximum differential optical density of the ECW increases with the covering percentage of the carbon dots up to a point and levels off for further increase in the covering percentage. The response time of coloration decreases with temperatures, which is caused chiefly by the temperature dependence of an ionic conductivity of the PSE. The behavior of ECWs is explained well with a simple equivalent-circuit comprising two capacitors corresponding to the WO3, film and the carbon-based counterelectrode, an electric resistor of the PSE and a power source connected in series.

White polymer light-emitting electrochemical cells using emission from exciplexes with long intermolecular distances formed between polyfluorene and π-conjugated amine molecules

The authors present white polymer light-emitting electrochemical cells (PLECs) fabricated with polymer blend films of poly(9,9-di-n-dodecylfluorenyl-2,7-diyl) (PFD) and π-conjugated triphenylamine molecules. The PLECs have bulk heterojunction structures composed of van der Waals interfaces between the PFD segments and the amine molecules. White-light electroluminescence (EL) can be achieved via light-mixing of the blue exciton emission from PFD and long-wavelength exciplex emission from excited complexes consisting of PFD segments (acceptors (As)) and the amine molecules (donors (Ds)). Precise control of the distances between the PFD and the amine molecules, affected through proper choice of the concentrations of PFD, amine molecules, and polymeric solid electrolytes, is critical to realizing white emission. White PLECs can be fabricated with PFD and amine molecules whose highest occupied molecular orbital (HOMO) levels range from −5.3 eV to −5.0 eV. Meanwhile, PLECs fabricated with amine molecules whose ...