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.


UV
NMR, 13 C NMR and 31 P NMR spectra were recorded with a Bruker Avance 600 MHz NMR spectrometer. Electrochemical data were obtained by differential pulse voltammetry using a three-electrode cell and a BAS100B/W electrochemical analyzer. The counter electrode and the working electrode were platinum electrodes, and the reference electrode was a saturated calomel electrode (SCE), and the supporting electrolyte was 0.1 M tetrabutylammonium hexafluorophosphate.

Computation Methods
The geometry optimizations were performed by the Gaussian 09 program (1) using DFT with the M06 meta-hybrid exchange-correlation functional. The geometry optimizations were performed in ethanol solution using "Triple-ζ" quality basis sets were employed for the ligands (6-311G*) (2) and the Ru (LanL2DZ) (3) . A relativistic effective core potential (ECP) (3) on Ru replaced the inner core electrons leaving the outer core [(4s) 2 (4p) 6 ] electrons and the (4d) 6 valence electrons of Ru(II). The geometries were fully optimized without symmetry constraints. Solvation effects were included by means of the conductor-like polarizable continuum model (C-PCM) (4) with ethanol parameters.
TD-DFT (5) excited states calculations were performed by the Amsterdam density functional program package (6) (ADF2013.01c) based on the zero-order regular approximation (ZORA) (7) two-component relativistic Hamiltonian. In SOC-TDDFT, SOC part is fully included self-consistently during the Self-Consistent Field (SCF) and TDDFT calculations. In contrast, PSOC-TDDFT is included the SOC effect as a perturbation for SR Hamiltonian. SOC-TDDFT is a more theoretically reasonable method, however it is more computationally demanding than PSOC-TDDFT for the same conditions. PSOC-TDDFT gives the correlation of the SOC effects for each excited state qualitatively. In this study, we performed the SOC-TDDFT calculations which included 40 spin mixed excitations. In PSOC-TDDFT, 10 singlet + 10 triplet excitations were calculated using the TDDFT calculations with SR Hamiltonian, which are used as the basis for the perturbative expansion. Solvation effects were included via COSMO continuum model (8) using ethanol parameters.

Materials
All organic solvents used were of puriss grade from Wako Pure Chemical Industries, Ltd, Japan.
DMF was distilled with BaO in vacuum, and treatment with molecular sieves 3A for 1day. After deaeration the solvent was stored under Ar and used as soon as possible.

Synthesis and characterizations
Synthetic scheme for DX2 and DX3 and DMF (small quantity) under Ar atmosphere, and then oxalyl chloride (TCI) (3.2 ml, 37.14 mmol) was dropped slowly at 0°C in an ice bath. The reaction solution was stirred for 3h at room temperature, and then the solvent was removed under vacuum. The reaction residue was then added dry THF (10 ml) and dry CH 2 Cl 2 (20 ml), and the reaction solution was then added slowly tBuOK in THF solution (2.78 g, 24.76 mmol) at 0°C. The reaction mixture was then stirred overnight at room temperature.
The reaction solution was then quenched by addition of water in an ice bath, the reaction mixture was then extracted with water and CH 2 Cl 2, and the organic layer was then washed with NaHCO 3 (aq) and then the organic layer was dried with Na 2 SO 4 . After removal of Na 2 SO 4 , the solvent was evaporated, the reaction mixture was then purified using column chromatography on silica gel

Synthesis of DX2.
RuCl 3 (TCI) was dissolved in dehydrated ethanol, and 4,4'-di-tert-butoxycarbonyl-4''-methoxycarbonyl -2,2';6',2''-terpyridine (3) was then added. The reaction mixture was refluxed under argon for 6 h. The reaction mixture was cooled to room temperature, the solvent was then removed, and the residue was dissolved into dry CHCl 3 at 0˚C. NEt 3 and methyldiphenylphosphine were then added to the reaction solution, and the reaction mixture was heated at 70˚C for 10 min. After cooling the reaction mixture, most of the solvent was removed under vacuum. The reaction residue was purified on a silica gel column chromatography