Due to its ∼2.4 eV band gap, BiVO₄ is a very promising photoanode material for harvesting the blue portion of the solar light for photoelectrochemical (PEC) water splitting applications. In WO₃/BiVO₄ heterojunction films, the electrons photoexcited in BiVO₄ are injected into WO₃, overcoming the lower charge carriers’ diffusion properties limiting the PEC performance of BiVO₄ photoanodes. Here, we investigate by ultrafast transient absorption spectroscopy the charge carrier interactions occurring at the interface between the two oxides in heterojunction systems to directly unveil their wavelength dependence. Under selective BiVO₄ excitation, a favorable electron transfer from photoexcited BiVO₄ to WO₃ occurs immediately after excitation and leads to an increase of the trapped holes’ lifetime in BiVO₄. However, a recombination channel opens when both oxides are simultaneously excited, evidenced by a shorter lifetime of trapped holes in BiVO₄. PEC measurements reveal the implication of these wavelength-dependent ultrafast interactions on the performances of the WO₃/BiVO₄ heterojunction.

Quantum dot-sensitized solar cells (QDSSCs) offer new opportunities to address the clean energy challenge, being one of the top candidates for third generation photovoltaics. Like dye-sensitized solar cells (DSSCs), QDSSCs normally use liquid electrolytes that suffer from issues such as evaporation or leakage. In this study a gel polysulfide electrolyte was prepared containing a natural polymer, agar, and was used as a quasi-solid-state electrolyte in solar cells to replace the conventional liquid electrolytes. This gel electrolyte shows almost the same conductivity as the liquid one. The solar cells were fabricated using CuInS₂ quantum dots (QDs), previously synthesized, deposited on TiO₂ photoanodes by electrophoretic deposition (EPD). CdS was deposited on TiO₂ by successive ionic layer adsorption and reaction (SILAR). Reduced graphene oxide (RGO)–Cu₂S, brass, and thin film Cu_xS were used as counter electrodes. Compared to a liquid polysulfide water based electrolyte, solar cells based on CuInS₂ and CdS using gel polymer electrolyte (GPE) exhibit greater incident photon to current conversion efficiency (IPCE = 51.7% at 520 nm and 72.7% at 440 nm), photocurrent density (J_sc = 10.75 and 13.51 mA cm⁻²), and power conversion efficiency (η = 2.97 and 2.98%) while exhibiting significantly enhanced stability. The solar cells employing the agar-based gel polymeric electrolyte are about a factor of 0.20 more stable than using a liquid electrolyte. The higher photovoltaic performance is due to the good conductivity and high wettability as well as the superior permeation capability of the gel electrolyte into the mesoporous matrix of a TiO₂ film.

Photoelectrochemical (PEC) water splitting converts solar light and water into oxygen and energy-rich hydrogen. WO₃/BiVO₄ heterojunction photoanodes perform much better than the separate oxide components, though internal charge recombination undermines their PEC performance when both oxides absorb light. Here we exploit the BiVO₄ layer to sensitize WO₃ to visible light and shield it from direct photoexcitation to overcome this efficiency loss. PEC experiments and ultrafast transient absorption spectroscopy performed by frontside (through BiVO₄) or backside (through WO₃) irradiating photoanodes with different BiVO₄ layer thickness demonstrate that irradiation through BiVO₄ is beneficial for charge separation. Optimized electrodes irradiated through BiVO₄ show 40% higher photocurrent density compared to backside irradiation.