Interface-Centric Strategies in Kesterite Solar Cells: Addressing Challenges, solutions, and Future Directions for Efficient Solar-Harvesting Technologies

As reserves of non-renewable energy sources decline, the search for sustainable alternatives becomes increasingly critical. Next-generation energy materials play a key role in this quest by enabling the manipulation of properties for effective energy solutions and understanding interfaces to enhance energy yield. Studying these interfaces is essential for managing charge transport in optoelectronic devices, yet it presents significant challenges. This review emphasizes the critical role of interfaces in kesterite solar cells (KSCs), focusing on interfacial architecture, carrier losses, and non-radiative recombination. This review highlights the importance of addressing interface issues and utilizing advanced characterization tools to reveal interface properties. Current interface problems are addressed, recent advancements in interface engineering are summarized, and perspectives on future challenges and prospects are offered. The goal is to illuminate the nature of interfaces and tackle interface losses, which are crucial for improving device design and performance. Despite their pivotal role in device operation, comprehensive reviews on interfaces are lacking, underscoring the relevance of the work for researchers in material interfaces and device engineering. It is hoped that this article will spark interest and inspire further research into interface studies and the mitigation of interface losses.

Crystalline CdS/Amorphous Cd(OH)2 Composite for Electrochemical CO2 Reduction to CO in a Wide Potential Window

Electrochemical CO2 reduction is a promising method for converting atmospheric CO2 into valuable low-carbon chemicals. In this study, a crystalline cadmium sulfide/amorphous cadmium hydroxide composite was successfully deposited on the carbon paper substrate surface by in-situ chemical bath deposition (named as c-CdS/a-Cd(OH)2/CP electrodes) for the efficient electrochemical CO2 reduction to produce CO. The c-CdS/a-Cd(OH)2/CP electrode exhibited high CO Faradaic efficiencies (>90 %) under a wide potential window of 1.0 V, with the highest value reaching ~100 % at the applied potential ranging from -2.16 V to -2.46 V vs. ferrocene/ferrocenium (Fc/Fc+), superior to the crystalline counterpart c-CdS/CP and c-CdS/c-Cd(OH)2@CP electrodes. Meanwhile, the CO partial current density reached up to 154.7 mA cm-2 at -2.76 V vs. Fc/Fc+ on the c-CdS/a-Cd(OH)2/CP electrode. The excellent performance of this electrode was mainly ascribed to its special three-dimensional structure and the introduction of a-Cd(OH)2. These structures could provide more active sites, accelerate the charge transfer, and enhance adsorption of *COOH intermediates, thereby improving the CO selectivity. Moreover, the electrolytes consisting of 1-butyl-3-methylimidazolium tetrafluoroborate and acetonitrile also enhanced the reaction kinetics of electrochemical CO2 reduction to CO.

Advances and challenges in the electrochemical reduction of carbon dioxide

The electrocatalytic carbon dioxide reduction reaction (ECO2RR) is a promising way to realize the transformation of waste into valuable material, which can not only meet the environmental goal of reducing carbon emissions, but also obtain clean energy and valuable industrial products simultaneously. Herein, we first introduce the complex CO2RR mechanisms based on the number of carbons in the product. Since the coupling of C-C bonds is unanimously recognized as the key mechanism step in the ECO2RR for the generation of high-value products, the structural-activity relationship of electrocatalysts is systematically reviewed. Next, we comprehensively classify the latest developments, both experimental and theoretical, in different categories of cutting-edge electrocatalysts and provide theoretical insights on various aspects. Finally, challenges are discussed from the perspectives of both materials and devices to inspire researchers to promote the industrial application of the ECO2RR at the earliest.

Electronic Structure Design of Transition Metal-Based Catalysts for Electrochemical Carbon Dioxide Reduction

With the increasingly serious greenhouse effect, the electrochemical carbon dioxide reduction reaction (CO2RR) has garnered widespread attention as it is capable of leveraging renewable energy to convert CO2 into value-added chemicals and fuels. However, the performance of CO2RR can hardly meet expectations because of the diverse intermediates and complicated reaction processes, necessitating the exploitation of highly efficient catalysts. In recent years, with advanced characterization technologies and theoretical simulations, the exploration of catalytic mechanisms has gradually deepened into the electronic structure of catalysts and their interactions with intermediates, which serve as a bridge to facilitate the deeper comprehension of structure-performance relationships. Transition metal-based catalysts (TMCs), extensively applied in electrochemical CO2RR, demonstrate substantial potential for further electronic structure modulation, given their abundance of d electrons. Herein, we discuss the representative feasible strategies to modulate the electronic structure of catalysts, including doping, vacancy, alloying, heterostructure, strain, and phase engineering. These approaches profoundly alter the inherent properties of TMCs and their interaction with intermediates, thereby greatly affecting the reaction rate and pathway of CO2RR. It is believed that the rational electronic structure design and modulation can fundamentally provide viable directions and strategies for the development of advanced catalysts toward efficient electrochemical conversion of CO2 and many other small molecules.

Electrocatalytic activity of CO2 reduction to CO on cadmium sulfide enhanced by chloride anion doping

The electrocatalytic CO2 reduction (ECR) to produce valuable fuel is a promising process for addressing atmospheric CO2 emissions and energy shortages. In this study, Cl-anion doped cadmium sulfide structures were directly fabricated on a nickel foam surface (Cl/CdS-NF) using an in situ hydrothermal method. The Cl-anion doping could significantly improve ECR activity for CO production in ionic liquid and acetonitrile mixed solution, compared to pristine CdS. The highest Faradaic efficiency of CO is 98.1 % on a Cl/CdS-NF-2 cathode with an excellent current density of 137.0 mA cm-2 at -2.25 V versus ferrocene/ferrocenium (Fc/Fc+ , all potentials are versus Fc/Fc+ in this study). In particular, CO Faradaic efficiencies remained above 80 % in a wide potential range of -2.05 V to -2.45 V and a maximum partial current density (192.6 mA cm-2 ) was achieved at -2.35 V. The Cl/CdS-NF-2, with appropriate Cl anions, displayed abundant active sites and a suitable electronic structure, resulting in outstanding ECR activity. Density functional theory calculations further demonstrated that Cl/CdS is beneficial for increasing the adsorption capacities of *COOH and *H, which can enhance the activity of the ECR toward CO and suppress the hydrogen evolution reaction.

Defect chemistry of electrocatalysts for CO2 reduction

Electrocatalytic CO2 reduction is a promising strategy for converting the greenhouse gas CO2 into high value-added products and achieving carbon neutrality. The rational design of electrocatalysts for CO2 reduction is of great significance. Defect chemistry is an important category for enhancing the intrinsic catalytic performance of electrocatalysts. Defect engineering breaks the catalytic inertia inherent in perfect structures by imparting unique electronic structures and physicochemical properties to electrocatalysts, thereby improving catalytic activity. Recently, various defective nanomaterials have been studied and show great potential in electrocatalytic CO2 reduction. There is an urgent need to gain insight into the effect of defects on catalytic performance. Here, we summarized the recent research advances on the design of various types of defects, including carbon-based materials (intrinsic defects, heteroatom doping and single-metal-atom sites) and metal compounds (vacancies, grain boundaries, and lattice defects). The major challenges and prospects of defect chemistry in electrocatalytic CO2 reduction are also proposed. This review is expected to be instructive in the development of defect engineering for CO2 reduction catalysts.

Photocatalytic C–H activation for C–C/CN/C–S bond formation over CdS: effect of morphological regulation and S vacancies

CdS catalytic materials were utilized to fabricate C–C, CN and C–S bonds for drug intermediates or other value-added products through the high bond energy, low polarity and strong inertia C–H bonds activation.

Accelerating Electron-Transfer and Tuning Product Selectivity Through Surficial Vacancy Engineering on CZTS/CdS for Photoelectrochemical CO2 Reduction

Copper-based chalcogenides have been considered as potential photocathode materials for photoelectrochemical (PEC) CO₂ reduction due to their excellent photovoltaic performance and favorable conduction band alignment with the CO₂ reduction potential. However, they suffer from low PEC efficiency due to the sluggish charge transfer kinetics and poor selectivity, resulting from random CO₂ reduction reaction pathways. Herein, a facile heat treatment (HT) of a Cu₂ ZnSnS₄ (CZTS)/CdS photocathode is demonstrated to enable significant improvement in the photocurrent density (-0.75 mA cm^-2 at -0.6 V vs RHE), tripling that of pristine CZTS, as a result of the enhanced charge transfer and promoted band alignment originating from the elemental inter-diffusion at the CZTS/CdS interface. In addition, rationally regulated CO₂ reduction selectivity toward CO or alcohols can be obtained by tailoring the surficial sulfur vacancies by HT in different atmospheres (air and nitrogen). Sulfur vacancies replenished by O-doping is shown to favor CO adsorption and the CC coupling pathway, and thereby produce methanol and ethanol, whilst the CdS surface with more S vacancies promotes CO desorption capability with higher selectivity toward CO. The strategy in this work rationalizes the interface charge transfer optimization and surface vacancy engineering simultaneously, providing a new insight into PEC CO₂ reduction photocathode design.

A Unique Gas-Migration, Trapping, and Emitting Strategy for High-Loading Single Atomic Cd Sites for Carbon Dioxide Electroreduction

Single-atom catalysts (SACs) exhibit great potential in heterogeneous catalysis. However, the achievement of obtaining high-loading SACs remains a bottleneck. Herein, we first demonstrate a unique gas-migration, trapping, and emitting strategy for building a kind of Cd-based SAC for CO₂ reduction (CO₂RR). The gas-migration and trapping processes (≤750 °C) endows the material with an ultrahigh Cd loading amount of 30.3 wt %, while the emitting process can facilely modulate the loading amount from 30.3 to 1.4 wt %. For the CO₂RR, the Cd-NC SACs with a loading amount of 18.4 wt % exhibits the maximum Faraday efficiency of 91.4% for CO at -0.728 V. The operando infrared spectroscopy studies prove the presence of main intermediates *COO^-, *COOH, and *CO on Cd-NC-5M SACs during the catalytic process, indicating that the CO₂RR follows the proton-decoupled electron-transfer mechanism. Density functional theory simulations reveal that the Cd-N₄ structure reduces the Gibbs free energy of the rate-determining step (the hydrogenation step of *COOH).