Takumi Kinoshita

Spectral splitting photovoltaics using perovskite and wideband dye-sensitized solar cells

The extension of the light absorption of photovoltaics into the near-infrared region is important to increase the energy conversion efficiency. Although the progress of the lead halide perovskite solar cells is remarkable, and high conversion efficiency of >20% has been reached, their absorption limit on the long-wavelength side is ∼800 nm. To further enhance the conversion efficiency of perovskite-based photovoltaics, a hybridized system with near-infrared photovoltaics is a useful approach. Here we report a panchromatic sensitizer, coded DX3, that exhibits a broad response into the near-infrared, up to ∼1100 nm, and a photocurrent density exceeding 30 mA cm−2 in simulated air mass 1.5 standard solar radiation. Using the DX3-based dye-sensitized solar cell in conjunction with a perovskite cell that harvests visible light, the hybridized mesoscopic photovoltaics achieved a conversion efficiency of 21.5% using a system of spectral splitting.

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.

Highly Efficient 17.6% Tin–Lead Mixed Perovskite Solar Cells Realized through Spike Structure

Frequently observed high Voc loss in tin-lead mixed perovskite solar cells is considered to be one of the serious bottle-necks in spite of the high attainable Jsc due to wide wavelength photon harvesting. An amicable solution to minimize the Voc loss up to 0.50 V has been demonstrated by introducing an n-type interface with spike structure between the absorber and electron transport layer inspired by highly efficient Cu(In,Ga)Se2 solar cells. Introduction of a conduction band offset of ∼0.15 eV with a thin phenyl-C61-butyric acid methyl ester layer (∼25 nm) on the top of perovskite absorber resulted into improved Voc of 0.75 V leading to best power conversion efficiency of 17.6%. This enhancement is attributed to the facile charge flow at the interface owing to the reduction of interfacial traps and carrier recombination with spike structure as evidenced by time-resolved photoluminescence, nanosecond transient absorption, and electrochemical impedance spectroscopy measurements.

Visible to Near-Infrared Photoelectric Conversion in a Dye-Sensitized Solar Cell Using Ru(II) Porphyrin with Azopyridine Axial Ligands

A novel Ru(II) porphyrin sensitizer, bis[4-(4-pyridylazo)-benzoic acid]tetrakis(3,5-di-tert-butyl-4-hydroxyphenyl)porphyrinato ruthenium(II), [RuTBP(azpyba)2] derived from tetrakis(3,5-di-tert-butyl-4-hydroxyphenyl) Ru(II) porphyrin with azopyridine axial ligands have been synthesized and characterized by UV–vis, emission, nuclear magnetic resonance (NMR), differential pulse voltammetry, density functional theory (DFT) calculation, and subpicosecond time-resolved transient absorption. The RuTBP(azpyba)2 sensitizer shows both the intra-porphyrin π–π* absorption in the visible region and broad charge-transfer absorption from Ru2+ to the axial ligands in the near-IR region. The RuTBP(azpyba)2-sensitized solar cell shows sensitivity in the near-IR region up to ~1100 nm owing to the charge transfer absorption band. It was indicated that the excited electrons are injected to TiO2 through the azopyridine axial ligand of RuTBP(azpyba)2.

Theoretical investigation of [Ru(tpy)2]2+, [Ru(tpy)(bpy)(H2O)]2+ and [Ru(tpy)(bpy)(Cl)]+ complexes in acetone revisited: Inclusion of strong spin–orbit couplings to quantum chemistry calculations

In the present paper, we theoretically reinvestigate structural properties, and photo-physical and chemical characteristics and electronic absorption spectra of three kinds of ruthenium polypyridyl complexes [Ru(tpy)2]2+, [Ru(tpy)(bpy)(H2O)]2+, and [Ru(tpy)(bpy)(Cl)]+ complexes in acetone (tpy=2,2′,2′′-terpyridine and bpy=2,2′-bipyridine). In particular, the experimental absorption spectra of these complexes are revisited theoretically in detail and are simulated using the first-order perturbation theory based on time-dependent density functional theory (TD-DFT) where the first-order perturbation term is the spin–orbit (SO) coupling Hamiltonian, and quantum chemistry calculations based on various functionals and basis sets. It was found that in general the theory including SO coupling can reproduce experimental data better than the simple quantum chemistry calculation neglecting SO coupling, which indicates that SO coupling is very important to understand the optical features of these complexes and that therefore the mixing between singlet and triplet states is strong due to the large SO coupling constant of Ru atom involved in these complexes. This suggests the fact that the disagreement between the experimental and calculated absorption spectra was found in TDB3LYP/(SDD with triple-ζ for Ru and 6-31G* for others) [Jakubikova EJ et al., Inorg Chem 48:10720, 2009] can be tracked down to the neglect of SO couplings. It was also found that the choice of the DFT functionals and basis sets is crucial for a good theoretical reproduction of experimental data.