Unraveling the Mechanisms of Beneficial Cu-Doping of NiO-Based Photocathodes

Rotaxane-Functionalized Dyes for Charge-Rectification in p-Type Photoelectrochemical Devices

A supramolecular photovoltaic strategy is applied to enhance power conversion efficiencies (PCE) of photoelectrochemical devices by suppressing electron-hole recombination after photoinduced electron transfer (PET). Here, the author exploit supramolecular localization of the redox mediator-in close proximity to the dye-through a rotaxane topology, reducing electron-hole recombination in p-type dye-sensitized solar cells (p-DSSCs). Dye PRotaxane features 1,5-dioxynaphthalene recognition sites (DNP-arms) with a mechanically-interlocked macrocyclic redox mediator naphthalene diimide macrocycle (3-NDI-ring), stoppering synthetically via click chemistry. The control molecule PStopper has stoppered DNP-arms, preventing rotaxane formation with the 3-NDI-ring. Transient absorption and time-resolved fluorescence spectroscopy studies show ultrafast (211 ± 7 fs and 2.92 ± 0.05 ps) PET from the dye-moiety of PRotaxane to its mechanically interlocked 3-NDI-ring-acceptor, slowing down the electron-hole recombination on NiO surfaces compared to the analogue . p-DSSCs employing PRotaxane (PCE = 0.07%) demonstrate a 30% PCE increase compared to PStopper (PCE = 0.05%) devices, combining enhancements in both open-circuit voltages (VOC = 0.43 vs 0.36 V) and short-circuit photocurrent density (JSC = -0.39 vs -0.34 mA cm-2 ). Electrochemical impedance spectroscopy shows that PRotaxane devices exhibit hole lifetimes (τh ) approaching 1 s, a 16-fold improvement compared to traditional I- /I3 - -based systems (τh = 50 ms), demonstrating the benefits obtained upon nanoengineering of interfacial dye-regeneration at the photocathode.

Photophysical Study on the Effect of the External Potential on NiO-Based Photocathodes

In the present study, we investigate the effects of the applied external potential on a dye-sensitized NiO photocathode by time-resolved photoluminescence and femtosecond transient absorption spectroscopy under operating conditions. Instead of the anticipated acceleration of photoinduced hole injection from dye into NiO at a more negative applied potential, we observe that both hole injection and charge recombination are slowed down. We cautiously assign this effect to a variation in OH- ion concentration in the inner Helmholtz plane of the electrochemical double layer with applied potential, warranting further investigation for the realization of efficient solar fuel devices.

CuBO2 : A Potential Alternative for NiO as a Hole Acceptor Layer

P-type metal oxides, and in particular NiO, are typically used as hole accepting layers in dye-sensitized photocathodes. Delafossites (CuMO2 ) with M=B, Al, Cr or Ga have recently been proposed as attractive substitutes for NiO, with theoretically a higher hole mobility than NiO, therefore allowing a higher efficiency when the photocathode is applied in solar to fuel devices. We have experimentally validated the photoelectrochemical performance of photocathodes consisting of nanoporous CuBO2 (CBO) on Fluorine-doped Tin Oxide substrates, photosensitized with a light absorbing P1 dye. Femtosecond transient absorption and time-resolved photoluminescence studies show that light-induced hole injection occurs from the P1 dye into the CBO in a few ps, comparable to the time constant observed for NiO-based photocathodes. Importantly, the CBO-based photocathode shows significantly slower charge recombination than the NiO-based analogue. These results illustrate the promise of CBO as a p-type semiconductor in solar energy conversion devices.

Charge Recombination Deceleration by Lateral Transfer of Electrons in Dye-Sensitized NiO Photocathode

Control of charge separation and recombination is critical for dye-sensitized solar cells and photoelectrochemical cells, and for p-type cells, the latter process limits their photovoltaic performance. We speculated that the lateral electron hopping between dyes on a p-type semiconductor surface can effectively separate electrons and holes in space and retard recombination. Thus, device designs where lateral electron hopping is promoted can lead to enhanced cell performance. Herein, we present an indirect proof by involving a second dye to monitor the effect of electron hopping after hole injection into the semiconductor. In mesoporous NiO films sensitized with peryleneimide (PMI) or naphthalene diimide (NDI) dyes, dye excitation led to ultrafast hole injection into NiO from either excited PMI* (τ < 200 fs) or NDI* (τ = 1.2 ps). In cosensitized films, surface electron transfer from PMI^- to NDI was rapid (τ = 24 ps). Interestingly, the subsequent charge recombination (ps-μs) with NiO holes was much slower when NDI^- was generated by electron transfer from PMI^- than when NDI was excited directly. We therefore indicate that the charge recombination is slowed down after the charge hopping from the original PMI sites to the NDI sites. The experimental results supported our hypothesis and revealed important information on the charge carrier kinetics for the dye-sensitized NiO photoelectrode system.

Using supramolecular machinery to engineer directional charge propagation in photoelectrochemical devices

Molecular photoelectrochemical devices are hampered by electron-hole recombination after photoinduced electron transfer, causing losses in power conversion efficiency. Inspired by natural photosynthesis, we demonstrate the use of supramolecular machinery as a strategy to inhibit recombination through an organization of molecular components that enables unbinding of the final electron acceptor upon reduction. We show that preorganization of a macrocyclic electron acceptor to a dye yields a pseudorotaxane that undergoes a fast (completed within ~50 ps) 'ring-launching' event upon electron transfer from the dye to the macrocycle, releasing the anionic macrocycle and thus reducing charge recombination. Implementing this system into p-type dye-sensitized solar cells yielded a 16-fold and 5-fold increase in power conversion efficiency compared to devices based on the two control dyes that are unable to facilitate pseudorotaxane formation. The active repulsion of the anionic macrocycle with concomitant reformation of a neutral pseudorotaxane complex circumvents recombination at both the semiconductor-electrolyte and semiconductor-dye interfaces, enabling a threefold enhancement in hole lifetime.

Dual Role of Surface Hydroxyl Groups in the Photodynamics and Performance of NiO-Based Photocathodes

Photoelectrochemical (PEC) cells containing photocathodes based on functionalized NiO show a promising solar-to-hydrogen conversion efficiency. Here, we present mechanistic understanding of the photoinduced charge transfer processes occurring at the photocathode/electrolyte interface. We demonstrate via advanced photophysical characterization that surface hydroxyl groups formed at the NiO/water interface not only promote photoinduced hole transfer from the dye into NiO, but also enhance the rate of charge recombination. Both processes are significantly slower when the photocathode is exposed to dry acetonitrile, while in air an intermediate behavior is observed. These data suggest that highly efficient devices can be developed by balancing the quantity of surface hydroxyl groups of NiO, and presumably of other p-type metal oxide semiconductors.