Investigating interfacial electron transfer in dye-sensitized NiO using vibrational spectroscopy

Abstract

Understanding what influences the formation and lifetime of charge-separated states is key to developing photoelectrochemical devices. This paper describes the use of time-resolved infrared absorption spectroscopy (TRIR) to determine the structure and lifetime of the intermediates formed on photoexcitation of two organic donor-π-acceptor dyes adsorbed to the surface of NiO. The donor and π-linker of both dyes is triphenylamine and thiophene but the acceptors differ, maleonitrile (1) and bodipy (2). Despite their structural similarities, dye 1 outperforms 2 significantly in devices. Strong transient bands in the fingerprint region (1 and 2) and nitrile region (2300-2000 cm) for 1 enabled us to monitor the structure of the excited states in solution or adsorbed on NiO (in the absence and presence of electrolyte) and the corresponding kinetics, which are on a ps-ns timescale. The results are consistent with rapid (<1 ps) charge-transfer from NiO to the excited dye (1) to give exclusively the charge-separated state on the timescale of our measurements. Conversely, the TRIR experiments revealed that multiple species are present shortly after excitation of the bodipy chromophore in 2, which is electronically decoupled from the thiophene linker. In solution, excitation first populates the bodipy singlet excited state, followed by charge transfer from the triphenylamine to the bodipy. The presence and short lifetime (τ ≈ 30 ps) of the charge-transfer excited state when 2 is adsorbed on NiO (2|NiO) suggests that charge separation is slower and/or less efficient in 2|NiO than in 1|NiO. This is consistent with the difference in performance between the two dyes in dye-sensitized solar cells and photoelectrochemical water splitting devices. Compared to n-type materials such as TiO, less is understood regarding electron transfer between dyes and p-type metal oxides such as NiO, but it is evident that fast charge-recombination presents a limit to the performance of photocathodes. This is also a major challenge to photocatalytic systems based on a "Z-scheme", where the catalysis takes place on a µs-s timescale.

Direct evidence of catalyst reduction on dye and catalyst co-sensitized NiO photocathodes by mid-infrared transient absorption spectroscopy

“…2 ). 21 , 22 For the C343-sensitized films, vibrational peaks at ca. 1612 cm –1 and 1310 cm –1 ascribed to the C343 dye attest the anchoring of C343 on the NiO films (Fig.…”

“…Gibson and co-workers reported similar spectral features in the transient IR absorption spectra of P1-sensitized NiO films. 21 Similarly to the E2 dye, P1 consists of a triphenylamine donor and two dicyanovinyl acceptor groups. Following irradiation, the electron density in E2 shifts away from the triphenylamine group and localizes on the cyano groups.…”

“…Recently, Gibson and co-workers showed that vibrations of the cyanovinyl group in the push–pull dye P1 – a dye that is analogous to E2 – are very sensitive to changes in electron density and hence could be used to monitor charge transfer processes in P1-sensitized NiO films. 21 As for the catalyst [1] , its IR spectra and the ones of its reduced states are readily distinguished in the carbonyl region (2100–1800 cm –1 ). 20 Thus, in the present study, femtosecond mid-infrared transient absorption spectroscopy is used, for the first time, to monitor reduction of catalyst upon dye excitation in co-sensitized NiO films.…”

A Comparative Investigation of the Role of the Anchoring Group on Perylene Monoimide Dyes in NiO‐Based Dye‐Sensitized Solar Cells

“…When the T 2 /T − electrolyte was added, the kinetics were slightly slower than in propylene carbonate only. There are previous studies in which the dye anion was more long‐lived in the presence of redox couple than without . This counterintuitive result must be attributed to changes in the fermi level (hole filling by the electrolyte), which slows down dye–NiO recombination.…”

“…When the T 2 /T À electrolyte was added, the kinetics were slightly slowert han in propylene carbonate only.T here are previous studies in which the dye anion was more long-lived in the presence of redox couple than without. [60] This counterintuitive result must be attributed to changes in the fermi level (hole filling by the electrolyte), which slows down dye-NiO recombination.T his makes it difficult to estimate the regeneration efficiency and may obscure differences in recombination and regeneration rates between dyes. Especially for PMI-acac, the fraction of the initial signal that remains (20 %) is smaller than the IPCE value (30 %, see below), indicating that (some) regeneration should take place on the sub-ns timescale.…”