Yuhei Ogomi

CH3NH3SnxPb(1–x)I3 Perovskite Solar Cells Covering up to 1060 nm

We report photovoltaic performances of all-solid state Sn/Pb halide-based perovskite solar cells. The cell has the following composition: F-doped SnO2 layered glass/compact titania layer/porous titania layer/CH3NH3SnxPb(1-x)I3/regioregular poly(3-hexylthiophene-2,5-diyl). Sn halide perovskite itself did not show photovoltaic properties. Photovoltaic properties were observed when PbI2 was added in SnI2. The best performance was obtained by using CH3NH3Sn0.5Pb0.5I3 perovskite. 4.18% efficiency with open circuit voltage 0.42 V, fill factor 0.50, and short circuit current 20.04 mA/cm(2) are reported. The edge of the incident photon to current efficiency curve reached 1060 nm, which was 260 nm red-shifted compared with that of CH3NH3PbI3 perovskite solar cells.

Improved understanding of the electronic and energetic landscapes of perovskite solar cells: high local charge carrier mobility, reduced recombination, and extremely shallow traps.

The intriguing photoactive features of organic-inorganic hybrid perovskites have enabled the preparation of a new class of highly efficient solar cells. However, the fundamental properties, upon which the performance of these devices is based, are currently under-explored, making their elucidation a vital issue. Herein, we have investigated the local mobility, recombination, and energetic landscape of charge carriers in a prototype CH3NH3PbI3 perovskite (PVK) using a laser-flash time-resolved microwave conductivity (TRMC) technique. PVK was prepared on mesoporous TiO2 and Al2O3 by one or two-step sequential deposition. PVK on mesoporous TiO2 exhibited a charge carrier mobility of 20 cm(2) V(-1) s(-1), which was predominantly attributed to holes. PVK on mesoporous Al2O3, on the other hand, exhibited a 50% lower mobility, which was resolved into balanced contributions from both holes and electrons. A general correlation between crystal size and mobility was revealed irrespective of the fabrication process and underlying layer. Modulating the microwave frequency from 9 toward 23 GHz allowed us to determine the intrinsic mobilities of each PVK sample (60-75 cm(2) V(-1) s(-1)), which were mostly independent of the mesoporous scaffold. Kinetic and frequency analysis of the transient complex conductivity strongly support the superiority of the perovskite, based on a significant suppression of charge recombination, an extremely shallow trap depth (10 meV), and a low concentration of these trapped states (less than 10%). The transport mechanism was further investigated by examining the temperature dependence of the TRMC maxima. Our study provides a basis for understanding perovskite solar cell operation, while highlighting the importance of the mesoporous layer and the perovskite fabrication process.

Dye-sensitized solar cells consisting of dye-bilayer structure stained with two dyes for harvesting light of wide range of wavelength

Dye-sensitized solar cells (DSCs) containing dye-bilayer structure of black dye and NK3705 (3-carboxymethyl-5-[3-(4-sulfobutyl)-2(3H)-bezothiazolylidene]-2-thioxo-4-thiazolidinone, sodium salt) in one TiO2 layer (2-TiO-BD-NK) are reported. The 2-TiO-BD-NK structure was fabricated by staining one TiO2 layer with these two dyes, step by step, under a pressurized CO2 condition. The dye-bilayer structure was observed by using a confocal laser scanning microscope. The short circuit current (Jsc) and the incident photon to current efficiency of the cell (DSC-2-TiO-BD-NK) was almost the sum of those of DSC stained with black dye only (DSC-1-TiO-BD) and DSC stained with NK3705 only (DSC-1-TiO-NK).

Control of charge dynamics through a charge-separation interface for all-solid perovskite-sensitized solar cells.

The relationship between the structure of the charge-separation interface and the photovoltaic performance of all-solid dye-sensitized solar cells is reported. This cell is composed of porous a TiO2/perovskite (CH3NH3PbI(x)Cl(3-x))/p-type organic conductor. The porous titania layer was passivated with Al2O3 or Y2O3 to remove surface traps of the porous titania layer. Both passivations were effective in increasing the efficiency of the solar cell. Especially, the effect of Y2O3 passivation was remarkable. After passivation, the efficiency increased from 6.59 to 7.5%. The increase in the efficiency was discussed in terms of the electron lifetime in TiO2, the thermally stimulated current, the measurement of the microwave refractive carrier lifetime, and transition absorption spectroscopy. It was proven that surface passivation resulted in retardation of charge recombination between the electrons in the porous titania layers and the holes in the p-type organic conductors.

Optical absorption, charge separation and recombination dynamics in Sn/Pb cocktail perovskite solar cells and their relationships to photovoltaic performances

The interest in organometal trihalide perovskite (CH3NH3PbI3)-based solid-state hybrid solar cells has increased in recent years due to the high efficiencies achieved, with a record of over 20%, and the simple low temperature preparation method. Further improvements in the photovoltaic performance can be obtained by increasing the light harvesting in the NIR region up to 1000 nm. Recently, successful energy harvesting up to a wavelength of 1060 nm using Sn/Pb cocktail halide based perovskite materials has been achieved. However, the energy conversion efficiency of such solar cells (less than 10%) is much lower than that of CH3NH3PbI3 based solar cells, which is due to their lower open circuit voltage (Voc) and fill factor (FF). In order to improve this, we need to have a good understanding of the key factors governing the photovoltaic performance of these solar cells, i.e., the optical absorption, the charge separation and the recombination dynamics. Therefore, for this study, we characterized the optical absorption properties, including the bandgap and the Urbach energy, clarified the photoexcited carrier recombination dynamics in Sn/Pb cocktail perovskite (CH3NH3Sn0.5Pb0.5I3) and the charge separation and recombination dynamics at each interface in TiO2/Sn/Pb perovskite/P3HT solar cells, and lastly investigated the relationships between these and the photovoltaic performance.

Improvement of Dye-Sensitized Solar Cell Through TiCl4-Treated TiO2 Nanotube Arrays

Titania nanotube arrays have been fabricated on a fluoride-doped tin oxide substrate by liquid phase deposition using ZnO nanorod arrays as a template and have been applied as the electrode for dye-sensitized solar cells (DSCs). The performance of DSCs based on Ti0 2 nanotube electrodes can be improved by treating the TiO 2 nanotube arrays with titanium tetrachloride (TiCl 4 ). Both the short-circuit current density and the conversion efficiency increased almost 2 times after the TiCl 4 treatment. The TiCl 4 treatment not only increased the amount of absorbed dyes but also enhanced the electron transport in the TiO 2 films. TiCl 4 -treated TiO 2 nanotube arrays with 4 μm thickness showed a short-circuit current density of 8.37 mA/cm 2 , an open-circuit voltage of 0.80 V, a fill factor of 0.67, and an overall conversion efficiency (η) of 4.53%.

Elucidating the Structure–Property Relationships of Donor–π-Acceptor Dyes for Dye-Sensitized Solar Cells (DSSCs) through Rapid Library Synthesis by a One-Pot Procedure

The creation of organic dyes with excellent high power conversion efficiency (PCE) is important for the further improvement of dye-sensitized solar cells. We wish to describe the rapid synthesis of a 112-membered donor-π-acceptor dye library by a one-pot procedure, evaluation of PCEs, and elucidation of structure-property relationships. No obvious correlations between ε, and the η were observed, whereas the HOMO and LUMO levels of the dyes were critical for η. The dyes with a more positive E(HOMO), and with an E(LUMO)<-0.80 V, exerted higher PCEs. The proper driving forces were crucial for a high J(sc), and it was the most important parameter for a high η. The above criteria of E(HOMO) and E(LUMO) should be useful for creating high PCE dyes; nevertheless, that was not sufficient for identifying the best combination of donor, π, and acceptor blocks. Combinatorial synthesis and evaluation was important for identifying the best dye.

Preparation of Double Dye-Layer Structure of Dye-Sensitized Solar Cells from Cocktail Solutions for Harvesting Light in Wide Range of Wavelengths

Dye-sensitized solar cells (DSCs) consisting of a double dye-layer structure of Ru dye (Z907) and organic dye (NK3705) were prepared by dipping porous titania substrates into dye mixture solutions. Dye structures such as NK3705 were crucial for realizing the double layer structure. A porous titania layer adsorbed NK3705 predominantly in the dye mixture. The adsorbed NK3705 was then replaced by Z907 from the top to the bottom of the porous titania surface as a function of dipping time. The DSCs based on the double dye-layer structure were able to harvest light in a wide range of wavelengths.

Rapid Synthesis of Thiophene‐Based, Organic Dyes for Dye‐Sensitized Solar Cells (DSSCs) by a One‐Pot, Four‐Component Coupling Approach

This one-pot, four-component coupling approach (Suzuki-Miyaura coupling/C-H direct arylation/Knoevenagel condensation) was developed for the rapid synthesis of thiophene-based organic dyes for dye-sensitized solar cells (DSSCs). Seven thiophene-based, organic dyes of various donor structures with/without the use of a 3,4-ethylenedioxythiophene (EDOT) moiety were successfully synthesized in good yields based on a readily available thiophene boronic acid pinacol ester scaffold (one-pot, 3-step, 35-61%). Evaluation of the photovoltaic properties of the solar cells that were prepared using the synthesized dyes revealed that the introduction of an EDOT structure beside a cyanoacrylic acid moiety improved the short-circuit current (Jsc) while decreasing the fill factor (FF). The donor structure significantly influenced the open-circuit voltage (Voc), the FF, and the power conversion efficiency (PCE). The use of a n-hexyloxyphenyl amine donor, and our originally developed, rigid, and nonplanar donor, both promoted good cell performance (η=5.2-5.6%).

Tandem dye-sensitized solar cells with a back-contact bottom electrode without a transparent conductive oxide layer

We report on the architecture of tandem dye-sensitized solar cells (DSSCs) with a back-contact bottom electrode without a transparent conductive oxide layer (TCO-less tandem DSSCs). The bottom electrode consists of glass/stained porous TiO2/back-contact porous metal. As the structure has fewer TCO layers than simple mechanical stack tandem DSSCs, more light reaches the bottom electrode. Two model dyes (D131 and N719) were used to confirm the tandem performance of the cells. The open-circuit voltage (Voc) was the sum of the Voc of the top cell and the Voc of the TCO-less bottom cell, showing that the cell worked as a tandem cell. The power conversion efficiency of the TCO-less tandem DSSC (7.10%) was greater than that of the stack tandem DSSCs (6.28%).