A key discovery at the TiO₂/dye/electrolyte interface: slow local charge compensation and a reversible electric field

Abstract

Dye-sensitized mesoporous TiO2 films have been widely applied in energy and environmental science related research fields. The interaction between accumulated electrons inside TiO2 and cations in the surrounding electrolyte at the TiO2/dye/electrolyte interface is, however, still poorly understood. This interaction is undoubtedly important for both device performance and fundamental understanding. In the present study, Stark effects of an organic dye, LEG4, adsorbed on TiO2 were well characterized and used as a probe to monitor the local electric field at the TiO2/dye/electrolyte interface. By using time-resolved photo- and potential-induced absorption techniques, we found evidence for a slow (t > 0.1 s) local charge compensation mechanism, which follows electron accumulation inside the mesoporous TiO2. This slow local compensation was attributed to the penetration of cations from the electrolyte into the adsorbed dye layer, leading to a more localized charge compensation of the electrons inside TiO2. Importantly, when the electrons inside TiO2 were extracted, a remarkable reversal of the surface electric field was observed for the first time, which is attributed to the penetrated and/or adsorbed cations now being charge compensated by anions in the bulk electrolyte. A cation electrosorption model is developed to account for the overall process. These findings give new insights into the mesoporous TiO2/dye/electrolyte interface and the electron-cation interaction mechanism. Electrosorbed cations are proposed to act as electrostatic trap states for electrons in the mesoporous TiO2 electrode.

A small electron donor in cobalt complex electrolyte significantly improves efficiency in dye-sensitized solar cells

“…The bleach peak at ∼600 nm was recently shown to be caused by a Stark effect on the dye ground-state absorption band, as a result of the change in electric field at TiO 2 /dye/electrolyte interface after electron injection3235. Significantly, in the case for the TPAA electrolyte, a new positive absorption peak at 720 nm appears, which is consistent with the characteristic absorption of TPAA ˙ + , as shown above by the spectroelectrochemistry measurements.…”

“…Two characteristic peaks were found in the PIA spectrum of cells with inert electrolyte at ∼670 and 770 nm, which has been previously demonstrated to be the characteristic absorption peaks of the oxidized LEG4 + (ref. 32). These peaks vanished for the cells with Co, TPAA and TPAA/Co electrolyte, which indicates an efficient dye regeneration of the oxidized LEG4 + in all these three electrolytes3334.…”

Supramolecular Hemicage Cobalt Mediators for Dye‐Sensitized Solar Cells

“…In the absence of ar edox mediator, for LEG4, ab leach of the main absorption band is found at 540 nm, and absorption peaks of oxidized dye at 680 and 780 nm. [42] The bleach occurs in the presence of the oxidized dye and is due to somee xtent of electrons in TiO 2 .T he bleach corresponds to the conversion of the ground-stated ye to the oxidized form with aS tarks hift caused by the photoinjected electrons that give al ocal change in the electrical field across the dye molecules. [43] When a[ Co(ttb)] 2 + /3 + or [Co(bpy) 3 ] 2 + /3 + based electrolyte is added, the bleach of ground-statedye andthe absorption peak of oxidized dye disappear because the ground state is rapidlyr egenerated from the oxidized dye by the redoxm ediator.Ab leach in the regionb etween 560 to 600 nm persists, caused by the Stark effect.I ta lso indicates that the difference in driving forces between the redox couples does not affect the regeneration yield (and the IPCE).…”

Cross‐Linked Sulfur–Selenium Polymers as Hole‐Transporting Materials in Dye‐Sensitized Solar Cells and Perovskite Solar Cells

“…As seen in Figure , after excitation of LEG4 on TiO 2 with blue light, a bleach is observed between 525–625 nm and new absorption bands emerge between 625–850 nm. These changes in absorption are attributable to the ground‐state bleach of LEG4 and absorption of oxidized LEG4, respectively . After deposition of the S:DIB and SeS 2 :S:DIB HTMs onto the LEG4‐sensitized TiO 2 , the PIA spectra show a decrease in the LEG4 ground‐state bleach, as well as a decrease in the absorption from oxidized LEG4.…”

“…These changes in absorption are attributable to the ground-state bleach of LEG4 and absorption of oxidized LEG4, respectively. [29,30] After deposition of the S:DIB and SeS 2 :S:DIB HTMs onto the LEG4-sensi-tized TiO 2 ,t he PIA spectra show ad ecrease in the LEG4 ground-state bleach, as well as ad ecrease in the absorption from oxidized LEG4. These changes represent clear indications of dye regeneration.…”