A photostable p-type NiO photocathode based on a bifunctional cyclometalated ruthenium sensitizer and a cobaloxime catalyst has been created for visible-light-driven water reduction to produce H2. The sensitizer is anchored firmly on the surface of NiO, and the binding is resistant to the hydrolytic cleavage. The bifunctional sensitizer can also immobilize the water reduction catalyst. The resultant photoelectrode exhibits superior stability in aqueous solutions. Stable photocurrents have been observed over a period of hours. This finding is useful for addressing the degradation issue in dye-sensitized photoelectrochemical cells caused by desorption of dyes and catalysts. The high stability of our photocathodes should be important for the practical application of these devices for solar fuel production.

Exploring new p-type semiconductor nanoparticles alternative to the commonly used NiO is crucial for p-type dye-sensitized solar cells (p-DSSCs) to achieve higher open-circuit voltages (Voc). Here we report the first application of delafossite CuGaO2 nanoplates for p-DSSCs with high photovoltages. In contrast to the dark color of NiO, our CuGaO2 nanoplates are white. Therefore, the porous films made of these nanoplates barely compete with the dye sensitizers for visible light absorption. This presents an attractive advantage over the NiO films commonly used in p-DSSCs. We have measured the dependence of Voc on the illumination intensity to estimate the maximum obtainable Voc from the CuGaO2-based p-DSSCs. Excitingly, a saturation photovoltage of 464 mV has been observed when a polypyridyl Co(3+/2+)(dtb-bpy) electrolyte was used. Under 1 Sun AM 1.5 illumination, a Voc of 357 mV has been achieved. These are among the highest values that have been reported for p-DSSCs.

We report the first application of cyclometalated ruthenium complexes of the type Ru[(N∧N)2(C∧N)]+ as sensitizers for p-type NiO dye-sensitized solar cells (NiO p-DSCs). These dyes exhibit broad absorption in the visible region. The carboxylic anchoring group is attached to the phenylpyridine ligand, which results in efficient hole injection. Moreover, the distance between the Ru[(N∧N)2(C∧N)]+ core and the carboxylic anchoring group is systematically varied by inserting rigid phenylene linkers. Femtosecond transient absorption (TA) studies reveal that the interfacial charge recombination rate between reduced sensitizers and holes in the valence band of NiO decreases as the number of phenylene linkers increases across the series. As a result, the solar cell made of the dye with the longest spacer (O12) exhibits the highest efficiency with both increased short-circuit current (J sc) and open-circuit voltage (V oc). The incident photon-to-current conversion efficiency (IPCE) spectra match well with the absorption spectra of sensitizers, suggesting the observed cathodic current is generated from the dye sensitization. In addition, the absorbed photon-to-current conversion efficiencies (APCEs) display an increment across the series. We further studied the interfacial charge recombination of our solar cells by electrochemical impedance spectroscopy (EIS). The results reveal an enhanced hole lifetime as the number of phenylene linkers increases. This study opens up opportunities of using cyclometalated Ru complexes for p-DSCs.

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p-Type dye-sensitized solar cells (p-DSCs) have attracted increasing attention recently, but they suffer from low fill factors (FFs) and unsatisfactory efficiencies. A full comprehension of the hole transport and recombination processes in the NiO p-DSC is of paramount importance for both the fundamental study and the practical device optimization. In this article, NiO p-DSCs were systematically probed under various bias and illumination conditions using electrochemical impedance spectroscopy (EIS), intensity modulated photocurrent spectroscopy (IMPS), and intensity modulated photovoltage spectroscopy (IMVS). Under the constant 1 sun illumination, the recombination resistance (R rec ) of the cell deviates from an exponential relationship with the potential and saturates at ∼130 Ω cm 2 under the short circuit condition, which is ascribed to the overwhelming recombination with the reduced dye anions. Such a small R rec results in the small dc resistance, which decreases the "flatness" of the J−V curve. The quantitative analysis demonstrates that the FF value is largely attenuated by the recombination of holes in NiO with the reduced dyes. Our analysis also shows that if this recombination can be eliminated, then an FF value of 0.6 can be reached, which agrees with the theoretical calculation with a V oc of 160 mV.