Enhancing the Charge Separation in Nanocrystalline Cu₂ZnSnS₄ Photocathodes for Photoelectrochemical Application: The Role of Surface Modifications

Cu₂ZnSnS₄ (CZTS) colloidal inks were employed to prepare thin-film photocathodes that served as a model system to interrogate the effect of different surface treatments, viz. CdS, CdSe, and ZnSe buffer layers along with methylviologen (MV) adsorption, on the photoelectrochemical (PEC) performance using aqueous Eu³⁺ redox electrolyte. PEC experiments revealed that ZnSe and CdSe overlayers outperform traditional CdS, and the additional surface modification with MV was found to further boost the charge extraction. By analyzing the photocurrent onset behavior and measuring the open circuit photopotentials, insights are gained into the nature of the observed improvements. While a more favorable conduction band offset rationalizes the improvement offered by CdSe, charge transfer through midgap states is invoked for ZnSe. Improvement offered by MV treatment is clearly caused by both the shifting of the flat-band potential and a charge-transfer mediation effect. Overall, this work suggests promising alternative surface treatments for CZTS photocathodes for PEC energy conversion

Defect Mitigation of Solution-Processed 2D WSe₂ Nanoflakes for Solar-to-Hydrogen Conversion

Few-atomic-layer nanoflakes of liquid-phase exfoliated semiconducting transition metal dichalcogenides (TMDs) hold promise for large-area, high-performance, low-cost solar energy conversion, but their performance is limited by recombination at defect sites. Herein, we examine the role of defects on the performance of WSe₂ thin film photocathodes for solar H₂ production by applying two separate treatments, a pre-exfoliation annealing and a post-deposition surfactant attachment, designed to target intraflake and edge defects, respectively. Analysis by TEM, XRD, XPS, photoluminescence, and impedance spectroscopy are used to characterize the effects of the treatments and photoelectrochemical (PEC) measurements using an optimized Pt–Cu cocatalyst (found to offer improved robustness compared to Pt) are used to quantify the performance of photocathodes (ca. 11 nm thick) consisting of 100–1000 nm nanoflakes. Surfactant treatment results in an increased photocurrent attributed to edge site passivation. The pre-annealing treatment alone, while clearly altering the crystallinity of pre-exfoliated powders, does not significantly affect the photocurrent. However, applying both defect treatments affords a considerable improvement that represents a new benchmark for the performance of solution-processed WSe₂: solar photocurrents for H₂ evolution up to 4.0 mA cm^–2 and internal quantum efficiency over 60% (740 nm illumination). These results also show that charge recombination at flake edges dominates performance in bare TMD nanoflakes, but when the edge defects are passivated, internal defects become important and can be reduced by pre-annealing

Sensitization of TiO₂ with PbSe Quantum Dots by SILAR: How Mercaptophenol Improves Charge Separation

The use of PbSe quantum dots (QDs) as sensitizers for TiO₂ samples has been primarily hampered by limitations on charge injection. Herein, a novel successive ionic layer adsorption and reaction (SILAR) method, allowing for an intimate TiO₂/PbSe contact and a strong quantum confinement, is described. Photoelectrochemical experiments and transient absorption measurements reveal that charge separation indeed occurs when using either aqueous sulfite or <i>spiro</i>-OMeTAD as a hole conductor and that it can be further enhanced by attaching <i>p</i>-mercaptophenol (MPH) to the QD surface. These results suggest that MPH can promote an efficient funneling of the photogenerated holes from the PbSe to the hole scavenging medium, thereby increasing the yield of electron injection into TiO₂. In a more general vein, this work paves the way for the fabrication of PbSe-sensitized solar cells, emphasizing the importance of controlling the QD/hole scavenger interface to further boost their conversion efficiency

Toward Antimony Selenide Sensitized Solar Cells: Efficient Charge Photogeneration at <i>spiro</i>-OMeTAD/Sb₂Se₃/Metal Oxide Heterojunctions

Photovoltaic devices comprising metal chalcogenide nanocrystals as light-harvesting components are emerging as a promising power-generation technology. Here, we report a strategy to evenly deposit Sb₂Se₃ nanoparticles on mesoporous TiO₂ as confirmed by Raman spectroscopy, energy-dispersive X-ray spectrometry, and transmission electron microscopy. Detailed study of the interfacial charge transfer dynamics by means of transient absorption spectroscopy provides evidence of electron injection across the Sb₂Se₃/TiO₂ interface upon illumination, which can be improved 3-fold by annealing at low temperatures. Following addition of the <i>spiro</i>-OMeTAD hole transporting material, regeneration yields exceeding 80% are achieved, and the lifetime of the charge separated species is found to be on the millisecond time scale (τ_50% ∼ 50 ms). These findings are discussed with respect to the design of solid-state Sb₂Se₃ sensitized solar cells