Majid Safdari

A quasi-liquid polymer-based cobalt redox mediator electrolyte for dye-sensitized solar cells

Recently, cobalt redox electrolyte mediators have emerged as a promising alternative to the commonly used iodide/triiodide redox shuttle in dye-sensitized solar cells (DSCs). Here, we report the successful use of a new quasi-liquid, polymer-based electrolyte containing the Co(3+)/Co(2+) redox mediator in 3-methoxy propionitrile solvent in order to overcome the limitations of high cell resistance, low diffusion coefficient and rapid recombination losses. The performance of the solar cells containing the polymer based electrolytes increased by a factor of 1.2 with respect to an analogous electrolyte without the polymer. The performances of the fabricated DSCs have been investigated in detail by photovoltaic, transient electron measurements, EIS, Raman and UV-vis spectroscopy. This approach offers an effective way to make high-performance and long-lasting DSCs.

Layered 2D alkyldiammonium lead iodide perovskites: synthesis, characterization, and use in solar cells

The synthetic route and properties of three 2D hybrid organic/inorganic lead iodide perovskite materials are reported. The 2D perovskites were synthesized from the reaction between PbI2 and the di-cations of 1,4-diaminobutane, 1,6-diaminohexane, and 1,8-diaminooctane. The resulting products were [NH3(CH2)4NH3]PbI4 (BdAPbI4), [NH3(CH2)6NH3]PbI4 (HdAPbI4), and [NH3(CH2)8NH3]PbI4 (OdAPbI4). Structural characterization shows that two dimensional perovskite structures were formed with inorganic structural planes separated by organic layers. Absorption spectra show band gaps of 2.37 eV (BdAPbI4), 2.44 eV (HdAPbI4), and 2.55 eV (OdAPbI4). The 2D perovskite materials were investigated as light absorbing materials in solid state solar cells. The best performing material under moist, ambient conditions was BdAPbI4 (1.08% efficiency), which was comparable to methylammonium Pb(II) iodide (MAPbI3) solar cells (2.1% efficiency) manufactured and studied under analogous conditions. When compared to MAPbI3, the 2D materials have larger band gaps and lower photoconductivity, while BdAPbI4 based solar cells shows a comparable absorbed photon-to-current efficiency as compared to MAPbI3 based ones.

Structure and function relationships in alkylammonium lead(II) iodide solar cells

Alkylammonium lead(II) iodide materials (APbI3), based on the general formula of CH3–(CH2)n–NH3PbI3, may lead to a monumental leap in developing affordable photovoltaics. Herein, we correlate the structure and function relationships of alkylammonium lead(II) iodide in solar cells. We investigated changes in the structure of APbI3 materials by varying the alkylammonium cations in their structure. As the size of the alkylammonium cation increased, the crystallographic unit cell increased in size and yielded lower symmetry crystals. High symmetry materials, those with cubic symmetry, showed the highest conductivity, the smallest bandgap, and produced the best performing solar cells. Structural changes were investigated by X-ray crystallography, X-ray powder diffraction, and Raman scattering.

Bis(1,1-bis(2-pyridyl)ethane)copper(I/II) as an efficient redox couple for liquid dye-sensitized solar cells

A new redox couple, [Cu(bpye)2]+/2+, has been synthesized, and applied in dye-sensitized solar cells (DSSCs). Overall efficiencies of 9.0% at 1 sun and 9.9% at 0.5 sun were obtained, which are considerably higher than those obtained for cells containing the reference redox couple, [Co(bpy)3]2+/3+. These results represent a record for copper-based complex redox systems in liquid DSSCs. Fast dye regeneration, sluggish recombination loss processes, faster electron self-exchange reactions and suitable redox potentials are the main reasons for the observed increase in efficiency. In particular, the main disadvantage of cobalt complex-based redox couples, charge-transport problems, appears to be resolved by a change to copper complex redox couples. The results make copper complex-based redox couples very promising for further development of highly efficient DSSCs.

Investigation of cobalt redox mediators and effects of TiO2 film topology in dye-sensitized solar cells

One-electron outer-sphere redox couples, such as cobalt metal-organic complexes, represent an interesting alternative as redox mediators in dye-sensitized solar cells since they show weak visible l ...

NMR longitudinal relaxation enhancement in metal halides by heteronuclear polarization exchange during magic-angle spinning

Orders of magnitude decrease of (207)Pb and (199)Hg NMR longitudinal relaxation times T1 upon magic-angle-spinning (MAS) are observed and systematically investigated in solid lead and mercury halides MeX2 (Me = Pb, Hg and X = Cl, Br, I). In lead(ii) halides, the most dramatic decrease of T1 relative to that in a static sample is in PbI2, while it is smaller but still significant in PbBr2, and not detectable in PbCl2. The effect is magnetic-field dependent but independent of the spinning speed in the range 200-15 000 Hz. The observed relaxation enhancement is explained by laboratory-frame heteronuclear polarization exchange due to crossing between energy levels of spin-1/2 metal nuclei and adjacent quadrupolar-spin halogen nuclei. The enhancement effect is also present in lead-containing organometal halide perovskites. Our results demonstrate that in affected samples, it is the relaxation data recorded under non-spinning conditions that characterize the local properties at the metal sites. A practical advantage of fast relaxation at slow MAS is that spectral shapes with orientational chemical shift anisotropy information well retained can be acquired within a shorter experimental time.

Impact of synthetic routes on the structural and physical properties of butyl-1,4-diammonium lead iodide semiconductors

We report the significant role of synthetic routes and the importance of solvents in the synthesis of organic-inorganic lead iodide materials. Through one route, the intercalation of dimethylformam ...

Correction: Structure and function relationships in alkylammonium lead(II) iodide solar cells

Correction for ‘Structure and function relationships in alkylammonium lead(II) iodide solar cells’ by Majid Safdari et al., J. Mater. Chem. A, 2015, DOI: 10.1039/c4ta06174h.

Light-Induced Interfacial Dynamics Dramatically Improve the Photocurrent in Dye-Sensitized Solar Cells: An Electrolyte Effect

A significant increase in the photocurrent generation during light soaking for solar cells sensitized by the triphenylamine-based D-π-A organic dyes (PD2 and LEG1) and mediated by cobalt bipyridine redox complexes has been observed and investigated. The crucial role of the electrolyte has been identified in the performance improvement. Control experiments based on a pre-treatment strategy reveals TBP as the origin. The increase in the current and IPCE has been interpreted by the interfacial charge-transfer kinetics studies. A slow component in the injection kinetics was exposed for this system. This change explains the increase in the electron lifetime and collection efficiency. Photoelectron spectroscopic measurements show energy shifts at the dye/TiO2 interface, leading us to formulate a hypothesis with respect to an electrolyte-induced dye reorganization at the surface.

Energetic Barriers to Interfacial Charge Transfer and Ion Movement in Perovskite Solar Cells

Highly efficient perovskite solar cells have been characterized by current-density/voltage measurements in the dark at varied scan rates. The results were compared to the solar cells without a hole-transporting layer to investigate the role of ultrathin hole-transporting layers in solar-cell function. The parameters of internal voltage, diode ideality factor, capacitive current, and capacitance were calculated from the current-density/voltage response of the cells in the dark. The results show that the absence of the hole-transporting layer can cause a large recombination current within the depletion region at the gold contact/perovskite interface, and thus affects the cell performance.