Cobalt Redox Mediators for Ruthenium-Based Dye-Sensitized Solar Cells: A Combined Impedance Spectroscopy and Near-IR Transmittance Study

Abstract

Dye-sensitized solar cells with power conversion efficiencies of up to 6.5% have been fabricated using a cobalt tris-bipyridyl redox mediator with the cis-diisothiocyanato-(2,2′-bipyridyl-4,4′-dicarboxylic acid)-(2,2′-bipyridyl-4,4′-dinonyl) ruthenium(II) (Z907) sensitizer. This represents a significant improvement in efficiency compared with previous reports using ruthenium sensitizers. In situ near-IR transmittance measurements in conjunction with electrochemical impedance spectroscopy have been used to explain the difference in performance between DSCs using Z907 and another benchmark sensitizer cis-diisothiocyanato-bis(2,2′-bipyridyl-4,4′-dicarboxylic acid) ruthenium(II) bis(tetrabutylammonium) (N719). It is found that the small-perturbation electron diffusion length (L n ) is significantly longer in Z907 cells compared with that in N719 cells, which can explain most of the difference in performance. It is also shown that the longer L n in Z907 cells is caused by inhibited recombination, as opposed to faster transport, and possible reasons for this are discussed. Our methodological approach is especially useful for accurately determining L n when it is shorter than the TiO2 layer thickness, where standard dynamic techniques start to become unreliable.

Towards Compatibility between Ruthenium Sensitizers and Cobalt Electrolytes in Dye‐Sensitized Solar Cells

“…The three IPCEs have a similar shape in the range of 380-770 nm, but with significantly different intensities. Similar to other ruthenium(II) sensitizers, [18,19,22] the photocurrent action spectra is limited to ca. 50 % with 3 a.…”

“…Among the few examples reported in the literature, [11,[16][17][18][19][20] only limited performances have been achieved. The highest performing dye (Z907) yields efficiencies of up to 6.5 % with cobalt [18] vs. 8.5 % with iodine, [21] unless the coadsorbent is specifically engineered. [22] Molecular dynamic simulations suggest close contact interactions between the cobalt(III) species and the anchored sensitizer(s), which causes undesired recombination of the electrons injected into the TiO 2 to the redox mediator.…”

Diffusion–Recombination Impedance Model for Solar Cells with Disorder and Nonlinear Recombination

“…In Equation (42), R tr and R rec appear in R G as a product and cannot be separately determined. [16] In Section 5, it was shown that R DC follows R rec at low applied voltage and coincides with R G after the cross-over potential.…”

“…[2,[13][14][15] However, in other instances the diffusion length could become shorter than L, and in that case the general diffusion-recombination impedance formula converges to the Gerischer impedance, in which the number of parameters is reduced by one with respect to the former. This situation has been recognized by some authors, [16] but it is of considerable interest to clarify the interpretation of the remaining parameters from a theoretical point of view. We will derive explicit parameters of the Gerischer impedance and their connection to the lifetime and diffusion length.…”

A new terpyridine cobalt complex redox shuttle for dye-sensitized solar cells

“…The DSCs open circuit voltage represents the difference between the quasiFermi level of the semiconductor under illumination and the redox potential of the redox shuttle electrolyte, the latter being nearly constant under operation conditions due to the high concentration of redox species. The quasi-Fermi level (E F,n ) of the semiconductor depends "statically" on the semiconductor conduction band (CB) energy and "dynamically" on the charge density (n) accumulated in the semiconductor, [5] according to the following equation: E F,n =E CB +k B T ln[n/N c ] (2) where N c is the density of states in the semiconductor. A higher semiconductor CB and charge density accumulation in the semiconductor provide a raise of the quasi-Fermi level and thus of the V oc .…”