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 RuII 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 RuII 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.

Coupling of photoactive transition metal complexes to a functional polymer matrix

“…This indicates that PPV-Ru2 will have a faster charge recombination than PPV-Ru1. These findings are confirmed by the transient absorption spectroscopy measurement 15 which find that the hole injection process on PPV-Ru1 occurs within 49 ± 4 ps which is signficantly shorter that the hole injection time 86 ± 9 ps for PPV-Ru2. Furthermore, PPV- Still, such a photocathode system is usually not operated under gas-phase conditions but in the presence of an electrolyte.…”

“…3) and elsewhere. 15 Energies of the MLCT bands in the two compounds are also similar, in spite of the fact that oscillator strengths are higher for PPV-Ru1. The main differences lie in the low-lying dark states with charge transfer character.…”

“…Furthermore, PPV- Still, such a photocathode system is usually not operated under gas-phase conditions but in the presence of an electrolyte. 15 In fact, it is well-known that the presence of an electrolyte can significantly modify the electronic properties of photoactive complexes. 59,60 In a first step, we calculated the electronic properties of teh isolated PPV oligomers (monomer, dimer, and trimer) and the Ru2 dye separately in the presence of implicit solvents.…”

“…Experimental yield lifetimes of the charge-separated states of (1587±201 ps) for PPV-Ru1 and (994±203 ps) for PPV-Ru2. 15 At these timescales, it is most likely that the charge-separated states have triplet character, which delays the spin-forbidden transition back towards the singlet ground state.…”

“…14 In addition, the electron may also be transferred from the excited state of the photocathode to the dye, as the energy gap of this semiconductor is typically comparable to the energy gap of the dye. 15 The concept of p-DSSCs can also be extended to photochemical molecular devices (PMD) by combining a photosensitizer and a catalyst connected by an electron relay. [16][17][18] In such a PMD, the electron is directly transferred to the catalytic agent, e.g.…”