Takurou N. Murakami

Efficient Hybrid Solar Cells Based on Meso-Superstructured Organometal Halide Perovskites

Perovskite Photovoltaics For many types of low-cost solar cells, including those using dye-sensitized titania, performance is limited by low open-circuit voltages. Lee et al. (p. 643, published online 4 October; see the Perspective by Norris and Aydil) have developed a solid-state cell in which structured films of titania or alumina nanoparticles are solution coated with a lead-halide perovskite layer that acts as the absorber and n-type photoactive layer. These particles are coated with a spirobifluorene organic-hole conductor in a solar cell with transparent oxide and metal contacts. For the alumina particles, power conversion efficiencies of up to 10.9% were obtained. Mesostructured alumina acts as an insulating scaffold for the assembly of very thin films of n- and p-type semiconductors. The energy costs associated with separating tightly bound excitons (photoinduced electron-hole pairs) and extracting free charges from highly disordered low-mobility networks represent fundamental losses for many low-cost photovoltaic technologies. We report a low-cost, solution-processable solar cell, based on a highly crystalline perovskite absorber with intense visible to near-infrared absorptivity, that has a power conversion efficiency of 10.9% in a single-junction device under simulated full sunlight. This “meso-superstructured solar cell” exhibits exceptionally few fundamental energy losses; it can generate open-circuit photovoltages of more than 1.1 volts, despite the relatively narrow absorber band gap of 1.55 electron volts. The functionality arises from the use of mesoporous alumina as an inert scaffold that structures the absorber and forces electrons to reside in and be transported through the perovskite.

Highly Efficient Dye-Sensitized Solar Cells Based on Carbon Black Counter Electrodes

Carbon black was employed as the catalyst for triiodide reduction on fluorine-doped tin oxide glass substrates (FTO-glass) used as counter electrodes in platinum-free dye-sensitized solar cells. The fill factors were strongly dependent on the thickness of the carbon layer, and the light energy conversion efficiency also increased up to a thickness of 10 μm. The charge-transfer resistance (R ct ) of the carbon counter electrode decreased with the thickness of the carbon layer. The R ct for the thicker carbon layer is less than three times that for the platinized FTO-glass. The highest cell efficiency was 9.1% under 100 mW cm -2 light intensity (1 sun AM 1.5 light, J sc = 16.8 mA cm -2 , V oc = 789.8 mV, fill factor = 0.685).

The photocapacitor: An efficient self-charging capacitor for direct storage of solar energy

A light-driven self-charging capacitor was fabricated as an efficient solar energy storage device. The device, which we name the photocapacitor, achieves in situ storage of visible light energy as an electrical power at high quantum conversion efficiency. The photocapacitor was constructed on a multilayered photoelectrode comprising dye-sensitized semiconductor nanoparticles/hole-trapping layer/activated carbon particles in contact with an organic electrolyte solution, in which photogenerated charges are stored at the electric double layer. Repeated charge-discharge cycles with a charging voltage of >0.45V yielded a capacitance of 0.69Fcm−2.

Surface modification of polystyrene and poly(methyl methacrylate) by active oxygen treatment

Abstract We examined the possibility of the surface modification of hydrophobic polystyrene (PS) and poly(methyl methacrylate) (PMMA) by ozone aeration, UV irradiation and combination of ozone aeration and UV irradiation (ozone/UV) in distilled water. The surface states of treated films and particles were investigated by means of contact angle, atomic force microscopy (AFM) and FT-IR measurements. According to the contact angle measurements, the values of the contact angle of ultrapure water on treated PS films decreased with an increase in the elapsed time of the treatments. The most remarkable decrease was seen in ozone/UV treatment. On the other hand, the contact angle on treated PMMA films slightly increased to an equal extent after three types of treatments. The film and the particle surfaces of PS with aromatic rings were found to be well modified with hydroxyl (OH) and carbonyl (C=O) groups and to give the most remarkable effects in ozone/UV treatments, whereas those of PMMA with no aromatic ring were hardly modified, merely resulting in a slight disorder in their surface roughness. The experimental results on surface modification of PS and PMMA revealed that the ozone/UV treatment in distilled water is usable as one of the useful techniques for the surface modification of polymers with aromatic rings.

Crystallization Dynamics of Organolead Halide Perovskite by Real-Time X-ray Diffraction

We analyzed the crystallization process of the CH3NH3PbI3 perovskite by observing real-time X-ray diffraction immediately after combining a PbI2 thin film with a CH3NH3I solution. A detailed analysis of the transformation kinetics demonstrated the fractal diffusion of the CH3NH3I solution into the PbI2 film. Moreover, the perovskite crystal was found to be initially oriented based on the PbI2 crystal orientation but to gradually transition to a random orientation. The fluctuating characteristics of the crystallization process of perovskites, such as fractal penetration and orientational transformation, should be controlled to allow the fabrication of high-quality perovskite crystals. The characteristic reaction dynamics observed in this study should assist in establishing reproducible fabrication processes for perovskite solar cells.

Recombination inhibitive structure of organic dyes for cobalt complex redox electrolytes in dye-sensitised solar cells

A very high open-circuit voltage (Voc) has recently been achieved in dye-sensitised solar cells (DSSCs) using cobalt complex redox couple electrolytes because of the high positive redox potential of the electrolyte. However, the obtained Voc is lower than the expected value owing to fast back electron transfer reactions from the TiO2 surface to the redox species. Recombination may be retarded by introduction of steric barrier groups into the dye structure to block the approach of the cobalt complex towards the TiO2 surface. Herein, carbazole dyes with various structural features in each dye component, i.e., number of n-hexylthiophene units in the linker, bulky appendage in the donor group, and alkyl chain length in the thiophene linker moiety, were studied in order to retard the fast back electron transfer reactions. There was a positive relationship between Voc and the number of n-hexylthiophene units, bulkiness of the appendage group in the donor segment, and shorter alkyl chains in the thiophene linker moiety. The increased Voc was attributed to the retardation of charge recombination, demonstrating that longer and larger molecules exert better blocking function.

Structural Effect of Donor in Organic Dye on Recombination in Dye-Sensitized Solar Cells with Cobalt Complex Electrolyte

The effect of the donor in an organic dye on the electron lifetime of dye-sensitized solar cells (DSSCs) employing a cobalt redox electrolyte was investigated. We synthesized organic dyes with donor moieties of carbazole, coumarin, triphenylamine, and N-phenyl-carbazole and measured the current-voltage characteristics and electron lifetimes of the DSSCs with these dyes. The cell with the triphenylamine donor dye produced the highest open circuit voltage and longest electron lifetime. On the other hand, the lowest open circuit voltage and shortest electron lifetime was obtained with coumarin donor dye, suggesting that the coumarin attracted the cobalt redox couples to the surface of the TiO2 layer, thus increasing the concentration of cobalt complex. On the other hand, the longest electron lifetime with triphenylamine was attributed to the blocking effect by steric hindrance of the nonplanar structure of the donor.

Improvement of TiO2/Dye/Electrolyte Interface Conditions by Positional Change of Alkyl Chains in Modified Panchromatic Ru Complex Dyes

A series of panchromatic ruthenium sensitizers (MJ sensitizers) with attached thiophene and phenyl units bearing alkyl chains was synthesized. A new synthetic route was used to examine all possible positions for the alkyl chains. The absorption spectra showed the sum of a ruthenium complex and peripheral organic chromophore units. The hypochromic effect and blueshift of the metal-to-ligand charge-transfer band observed in the modified ruthenium sensitizers were suppressed by changing the positions of the alkyl chains on the attached thiophene ring. Changing only one alkyl chain also influenced the performance of dye-sensitized solar cells. Ruthenium sensitizer MJ-10 with bulky substituent harvests visible and near-infrared light, and solar cells sensitized by MJ-10 exhibit an efficiency of 9.1% under 1 sun irradiation.

A high-voltage dye-sensitized photocapacitor of a three-electrode system

A high-voltage photo-rechargeable capacitor (photocapacitor) of three-electrode configuration, comprising a dye-sensitized mesoporous TiO2 electrode, two carbon-coated electrodes, and two liquid electrolytes, attained a charge-state voltage of 0.8 V and high energy density per area of 47 microW h cm(-2) which is five times larger than the previous two-electrode photocapacitor.

Hysteresis-free perovskite solar cells made of potassium-doped organometal halide perovskite

Potassium-doped organometal halide perovskite solar cells (PSCs) of more than 20% power conversion efficiency (PCE) without I-V hysteresis were constructed. The crystal lattice of the organometal halide perovskite was expanded with increasing of the potassium ratio, where both absorption and photoluminescence spectra shifted to the longer wavelength, suggesting that the optical band gap decreased. In the case of the perovskite with the 5% K+, the conduction band minimum (CBM) became similar to the CBM level of the TiO2-Li. In this situation, the electron transfer barrier at the interface between TiO2-Li and the perovskite was minimised. In fact, the transient current rise at the maximum power voltages of PSCs with 5% K+ was faster than that without K+. It is concluded that stagnation-less carrier transportation could minimise the I-V hysteresis of PSCs.