Photocathodes beyond NiO: charge transfer dynamics in a π-conjugated polymer functionalized with Ru photosensitizers

Abstract

A conductive polymer (poly(p-phenylenevinylene), PPV) was covalently modified with Ru^II complexes to develop an all-polymer photocathode as a conceptual alternative to dye-sensitized NiO, which is the current state-of-the-art photocathode in solar fuels research. Photocathodes require efficient light-induced charge-transfer processes and we investigated these processes within our photocathodes using spectroscopic and spectro-electrochemical techniques. Ultrafast hole-injection dynamics in the polymer were investigated by transient absorption spectroscopy and charge transfer at the electrode-electrolyte interface was examined with chopped-light chronoamperometry. Light-induced hole injection from the photosensitizers into the PPV backbone was observed within 10 ps and the resulting charge-separated state (CSS) recombined within ~ 5 ns. This is comparable to CSS lifetimes of conventional NiO-photocathodes. Chopped-light chronoamperometry indicates enhanced charge-transfer at the electrode-electrolyte interface upon sensitization of the PPV with the Ru^II complexes and p-type behavior of the photocathode. The results presented here show that the polymer backbone behaves like classical molecularly sensitized NiO photocathodes and operates as a hole accepting semiconductor. This in turn demonstrates the feasibility of all-polymer photocathodes for application in solar energy conversion.

Figure 1

Schematic representation of the molecular structures of π-conjugated PPV with Ru^II chromophores in the side chains, i.e. Ru1 and Ru2. Transient absorption spectroscopy is employed to unravel light-driven (1) hole injection (2) and charge recombination (3) dynamics in PPV-Ru. The red and blue shadings schematically indicate photogenerated holes residing on the PPV and the radical anion after hole injection from the photosensitizer to the PPV, respectively.

Figure 2

Normalized UV/Vis absorption and emission spectra of (a) PPV, (b) Ru1, PPV, and PPV-Ru1, and (c) Ru2, PPV, and PPV-Ru2 in CHCl₃. All emission spectra were obtained upon excitation at 480 nm. Emission decay of (d) Ru1 and PPV-Ru1, and (e) Ru2 and PPV-Ru2 in CHCl₃ upon excitation at 390 nm. The emission decay was measured using time-correlated single photon counting. The initial fast decay (< 1 ns) arises from the instrumental response function. (f) Cyclic voltammogram of 0.25 mM Ru1 and Ru2 in acetonitrile using 0.1 M TBABF₄ as supporting electrolyte.

Figure 3

Fs-transient absorption spectra of (a) PPV, (b) Ru1 and (c) PPV-Ru1 at different delay times measured in CHCl₃ solution with an excitation wavelength of 480 nm. The shaded areas at 360, 500 and 670 nm indicate the different spectral signatures among PPV, Ru1, and PPV-Ru1. The insets in figures (a) and (c) show the zero-crossing (ΔOD = 0) at 0.5 ps for PPV and PPV-Ru1, respectively. TBABF₄ as supporting electrolyte.

Figure 4

Differential fs-transient absorption spectra of (a) PPV-Ru1 and (b) PPV-Ru2 at different delay times. Normalized kinetic traces at different probe wavelength and decay-associated spectra (DAS) resulting from the global fit with three time constants for (c,e) PPV-Ru1 and (d,f) PPV-Ru2. The resulting time constants in the DAS are statistically taken from three independent measurements. The pump pulses are centred at 480 nm. The grey spectra in panel (e) and (f) depict the differential absorption spectra of Ru1 and Ru2, respectively, held at a potential of slightly more negative than that of the first reduction potential (Supplementary Figs. S7 and S8), which is arbitrarily scaled to fit the transient absorption data.

Figure 5

(a) Micromorphology of a PPV-Ru1 film dropcasted onto PET/ITO substrate. The inset depicts the cross-section image of PPV-Ru1 photocathodes. The black and white scale bar in panel (a) indicate 3 µm and 300 nm, respectively. (b) Fs-transient absorption spectra, (c) normalized kinetic traces recorded at different probe wavelength, and (d) decay-associated spectra (DAS) resulting from the global fit of the transient absorption data using a tri-exponential model. The grey spectra in panel (d) depict the absorption-difference spectra upon chemical reduction of Ru1 (at − 1.4 V vs. Ag|AgCl) in the PPV-Ru1 film. The absorption-difference spectra (Supplementary Fig. S10) are arbitrarily scaled to the figure.

Figure 6

Results of the photoelectrochemical measurements performed on drop-casted films of PPV, PPV-Ru1 and PPV-Ru2 in Co^III/Co^II electrolyte under chopped illumination (1000 nm > λ > 300 nm, 1000 W⋅m⁻²). (a) Linear scan voltammogram (scan speed 20 mV/s) and (b) chronoamperometric measurement at 0.5 V vs. Ag/AgCl applied potential. The obtained currents were divided by the film surface area to obtain current densities, and further divided by the maximum absorbance (A_max) of the films to account for film thickness. Shown are the average values of four individual measurements.