Does Iodine or Thiocyanate Play a Role in p‐Type Dye‐Sensitized Solar Cells?

Abstract

Charge recombination at the NiO/electrolyte interface is one of the performance‐limiting factors that prevent p‐type dye‐sensitized solar cells (p‐DSCs) from matching the efficiency of conventional TiO2 devices. This article describes the role of redox mediator species I−, I3−, and I2 in the charge‐transfer processes at the NiO/electrolyte interface. NiO appears to catalyze the oxidation of iodide but the addition of guanidinium thiocyanate inhibits recombination by forming a complex with I2. This increases the voltage but decreases the current so the efficiency is only slightly improved. These results suggest that new redox electrolytes will be needed if tandem DSCs are to exceed the efficiency of state‐of‐the‐art TiO2 devices.

Dye-sensitized solar cells strike back

“…I À in the electrolyte has been shown to reduce the Ni 3+ states, which are thought to be responsible for rapid charge recombination, so a dual effect might be responsible for the increased charge-separated state lifetime in the presence of the redox electrolyte. 566,568,691 With electrolytes based on cobalt polypyridyl complexes, the hole lifetimes were shown to be -like with I À /I 3 À -strongly dependent on light intensity, whereas the hole transport times were largely independent of light intensity. Charge transport times have been found to be almost independent from the structure of the cobalt complexes, but charge lifetimes depend on the steric bulk of the cobalt polypyridyl complex.…”

“…643 The composition can be optimized for the p-type system by the choice of solvent, typically acetonitrile, and additives, for example using lithium salts to lower the valence band potential, promote charge transport, limit charge recombination and increase the V OC . 573,586,628,691,692 Ionicliquid iodide sources such as 1-butyl-3-methylimidazolium iodide (BMII), 1-ethyl-3-methylimidazolium (EMII) and dimethylpropylimidazolium (DMPII) have also been shown to give good performance. 586 Drawbacks to the I À /I 3 À redox mediator include strong light absorption in the blue region, its corrosivity and the small difference between the redox potential of this electrolyte (315 mV vs. NHE) and the Fermi level of NiO, which limits the V OC of these devices to 100-200 mV.…”

Charge-transfer dynamics at the dye–semiconductor interface of photocathodes for solar energy applications

“…I À reacts with Ni 3+ at the surface of the electrode. 24 This may slow the rate of charge recombination between Ni 3+ and D À . Alternatively, the ions in the electrolyte may stabilise the reduced dye.…”

p-Type dye-sensitized solar cells based on pseudorotaxane mediated charge-transfer

“…This self-assembly strategy is unknown for p-type cells based on the I À /I 3 À redox couple. 25 Pre-organization of the redox couple should result in improving the competitiveness of forward electron propagation in the device compared to charge recombination. This is in contrast to the operation of current liquid DSSCs where the electron transfer between the dye and redox mediator is dictated by diffusion (collisional) phenomena.…”