Enhanced Photocurrent of Transparent CuFeO₂ Photocathodes by Self-Light-Harvesting Architecture

Efficient sunlight-driven water-splitting devices can be achieved by using an optically and energetically well-matched pair of photoelectrodes in a tandem configuration. The key for maximizing the photoelectrochemical efficiency is the use of a highly transparent front photoelectrode with a band gap below 2.0 eV. Herein, we propose two-dimensional (2D) photonic crystal (PC) structures consisting of a CuFeO₂-decorated microsphere monolayer, which serve as self-light-harvesting architectures allowing for amplified light absorption and high transparency. The photocurrent densities are evaluated for three CuFeO₂ 2D PC-based photoelectrodes with microspheres of different sizes. The optical analysis confirmed the presence of a photonic stop band that generates <i>slow light</i> and at the same time amplifies the absorption of light. The 410 nm sized CuFeO₂-decorated microsphere 2D PC photocathode shows an exceptionally high visible light transmittance of 76.4% and a relatively high photocurrent of 0.2 mA cm^–2 at 0.6 V vs a reversible hydrogen electrode. The effect of the microsphere size on the carrier collection efficiency was analyzed by in situ conductive atomic force microscopy observation under illumination. Our novel synthetic method to produce self-light-harvesting nanostructures provides a promising approach for the effective use of solar energy by highly transparent photocathodes

Molecular Chemistry-Controlled Hybrid Ink-Derived Efficient Cu₂ZnSnS₄ Photocathodes for Photoelectrochemical Water Splitting

To realize economically competitive hydrogen production through photoelectrochemical (PEC) water splitting, it is essential to develop an efficient photoelectrode consisting of earth-abundant constituents in conjunction with low-cost solution processing. Cu₂ZnSnS₄ (CZTS) has received significant attention as a promising photocathode owing to its abundance and good absorption properties. However, the efficiency of the solution-processed CZTS photocathode is not yet comparable to its counterparts. Here, a hybrid ink, obtained by careful control of precursor mixing order, was used to produce a highly efficient CZTS photocathode. The molecular chemistry-controlled hybrid ink formulation, particularly the roles of thiourea–Sn²⁺ complexation, was elucidated by liquid Raman spectroscopy. The hybrid ink-derived CZTS thin films modified with conformal coating of an n-type TiO₂/CdS double layer and a Pt electrocatalyst achieved an exceptionally high photocurrent of 13 mA cm^–2 at −0.2 V versus a reversible hydrogen electrode under 1 sun illumination. The modified photocathodes showed relatively stable H₂ production with faradaic efficiency close to unity

Bandgap-Graded Cu₂Zn(Sn_1–<i>x</i>Ge_<i>x</i>)S₄ Thin-Film Solar Cells Derived from Metal Chalcogenide Complex Ligand Capped Nanocrystals

We demonstrate organic residue free, bandgap-graded Cu₂Zn­(Sn_1–<i>x</i>Ge_<i>x</i>)­S₄ (CZTGeS) thin-film solar cells based on metal chalcogenide complex (MCC) ligand capped nanocrystals (NCs). The bandgap of the CZTGeS films is tuned by varying the amount of Sn₂S₆^4– MCC ligand absorbed on the surface of the Cu₂ZnGeS₄ (CZGeS) NCs, without an undesirable postselenization process. Using CZGeS NCs inks with three different Sn/(Ge+Sn) ratios, bandgap-graded CZTGeS thin films are obtained via multicoating and annealing procedures. Compositional and spectroscopic analyses along the film thickness confirm that the band-graded CZTGeS absorber layer, with a gradually increasing bandgap from the back contact to the <i>p</i>–<i>n</i> junction, is successfully accomplished. Compared with an ungraded band structured CZTGeS cell, this normal grading structure facilitates both higher short circuit current and open-circuit voltage, facilitating a power conversion efficiency of 6.3%

Retarding Crystallization during Facile Single Coating of NaCl-Incorporated Precursor Solution for Efficient Large-Area Uniform Perovskite Solar Cells

We demonstrated crystallization retardation of CH₃NH₃PbI₃ thin film during single coating of precursor solution by simple addition of NaCl. NaCl was codissolved into a precursor mixture solution containing PbI₂ and methylammonium iodide (MAI). Dissolved NaCl interacted with the PbI₂ in solution and produced a stable intermediate phase, which was converted to a full-coverage uniform perovskite absorber layer via reaction with MAI during a single spin-coating. The resulting planar-structure perovskite solar cell made from NaCl-supplemented precursor solution showed a 48% improvement in power conversion efficiency (PCE) (maximum value 15.16%) over the device fabricated without the additive. Our NaCl-supplemented single coating represents an easy approach to effectively obtain highly reproducible uniform performance at an overall position in 5 cm × 5 cm sized cells (divided into 20 subcells with an active area of 0.06 cm²) with average PCEs of 12.00 ± 0.48%

Facile Sol–Gel-Derived Craterlike Dual-Functioning TiO₂ Electron Transport Layer for High-Efficiency Perovskite Solar Cells

Organic–inorganic hybrid perovskite solar cells (PSCs) are considered promising materials for low-cost solar energy harvesting technology. An electron transport layer (ETL), which facilitates the extraction of photogenerated electrons and their transport to the electrodes, is a key component in planar PSCs. In this study, a new strategy to concurrently manipulate the electrical and optical properties of ETLs to improve the performance of PSCs is demonstrated. A careful control over the Ti alkoxide-based sol–gel chemistry leads to a craterlike porous/blocking bilayer TiO₂ ETL with relatively uniform surface pores of 220 nm diameter. Additionally, the phase separation promoter added to the precursor solution enables nitrogen doping in the TiO₂ lattice, thus generating oxygen vacancies. The craterlike surface morphology allows for better light transmission because of reduced reflection, and the electrically conductive craterlike bilayer ETL enhances charge extraction and transport. Through these synergetic improvements in both optical and electrical properties, the power conversion efficiency of craterlike bilayer TiO₂ ETL-based PSCs could be increased from 13.7 to 16.0% as compared to conventional dense TiO₂-based PSCs

Role of Anions in Aqueous Sol–Gel Process Enabling Flexible Cu(In,Ga)S₂ Thin-Film Solar Cells

Recently, environmental-friendly, solution-processed, flexible Cu­(In,Ga)­(S,Se)₂ devices have gained significant interest, primarily because the solution deposition method enables large-scale and low-cost production of photovoltaics, and a flexible substrate can be implemented on uneven surfaces in various applications. Here, we suggest a novel green-chemistry aqueous ink that is readily achievable through the incorporation of molecular precursors in an aqueous medium. A copper formate precursor was introduced to lower the fabrication temperature, provide compatibility with a polyimide plastic substrate, and allow for high photovoltaic performance. Through a comparative spectroscopic study on temperature-dependent chemical/crystal structural evolution, the chemical role of copper formate was elucidated, which led to the chalcopyrite framework that was appropriate to low-temperature annealed Cu­(In,Ga)­S₂ absorber layers at 400 °C. This Cu­(In,Ga)­S₂ solar cell exhibited a power conversion efficiency of 7.04% on a rigid substrate and 5.60% on a polymeric substrate. Our cell on the polymeric substrate also demonstrated both acceptable mechanical flexibility and durability throughout a repeated bending test of 200 cycles