Tetracyanoborate Anion Research Articles

Anion-correlated conduction band edge shifts and charge transfer kinetics in dye-sensitized solar cells with ionic liquid electrolytes

In solvent-free ionic liquid electrolytes for efficient dye-sensitized solar cells, iodide and non-iodide melts are ordinarily blended in order to attain a high dynamic fluidity and thereby meet a fast mass transport of electroactive species. This common tactic could bring forth a prominent impact of various anions on cell efficiency by altering photocurrent and/or photovoltage. Herein we report evident effects of the dicyanamide versus tetracyanoborate anion on the energetics of titania conduction band edge and the kinetics of multi-channel charge-transfer reactions in cells employing a high absorption coefficient ruthenium sensitizer C106. A slightly shorter photoluminescence lifetime of C106 grafted on alumina is probed for the tetracyanoborate-based reference cell with respect to the dicyanamide counterpart. However, owing to a more favourable thermodynamic driving force of ∼90 meV, the tetracyanoborate anion prompts an almost 3-fold faster electron injection from the excited-state dye to titania than dicyanamide, leading to a higher charge separation yield, which is in good agreement with an almost indistinguishable ratio of external quantum efficiency enhancement in the whole spectral response region. Compared to tetracyanoborate, the presence of dicyanamide at the titania/electrolyte interface evokes a 27-fold smaller interfacial electron exchange rate (K) with triiodide, accounting for the open-circuit photovoltage variation observed in current-voltage measurements.

Anion Effects in Organic Dye-Sensitized Mesoscopic Solar Cells with Ionic Liquid Electrolytes: Tetracyanoborate vs Dicyanamide

We compare the impacts of tetracyanoborate and dicyanamide anions in solvent-free ionic liquid electrolytes upon the optoelectronic features of dye-sensitized solar cells based on an organic push−pull chromophore. With respect to dicyanamide, tetracyanoborate confers a bathochromic and higher photocurrent response upon a dye-coated mesoporous titania film. Numerical simulations on electrical impedance spectroscopies disclose that tetracyanoborate anions evoke a downward displacement of the conduction-band edge of an electrolyte-sinking nanocrystalline film in comparison with dicyanamide, generating a more favorable energy-offset at the titania/dye interface and therefore a better exciton dissociation yield, as proved via the transient emission measurements. An effective recombination reaction rate constant U0k is used to depict the charge-transfer behavior at the titania/electrolyte interface in dye-sensitized solar cells. The negative impact of an about 4 times larger U0k of the dicyanamide cell is overwhelmed by the noticeably higher conduction band edge, leading to the experimentally observed anion-dependent open-circuit photovoltage.