Controlling the Energy-Level Alignment of Silicon Carbide Nanocrystals by Combining Surface Chemistry with Quantum Confinement

Silicon Carbide Photonic Crystal Photoelectrode

The immense challenge of large-scale implementation of photoelectrochemical (PEC) water splitting and carbon fixation lies in the need for a cheap, durable, and efficacious photocatalyst. Cubic silicon carbide (3C-SiC) holds compelling potential due to its auspicious band positions and high-volume, high-quality, single crystal industrial manufacturing, but is hindered by its inferior light absorptivity and anodic instability. A slanted parabolic pore photonic crystal (spbPore PC) architecture with graphitic carbon nitride (g-CN), nickel(II) oxide (NiO), or 6H silicon carbide protective coatings is proposed to overcome the drawbacks of 3C-SiC photoelectrodes. A 30 µm- and 62 µm-thick 3C-SiC spbPore PC of lattice constant 0.8 µm demonstrates maximum achievable photocurrent density (MAPD) of 9.95 and 11.53 mA cm-2 in the [280.5, 600] nm region, respectively, representing 75.7% and 87.7% of the total available solar photocurrent density in this spectral range. A 50 nm-thick g-CN or NiO coating forms type-II heterojunctions with the 3C-SiC spbPore PC, facilitating the charge transport and enhancing the corrosion resistivity, all together demonstrating the MAPD of 9.81 and 10.06 mA cm-2, respectively, for 30 µm-thick PC. The scheme advances the low-cost, sustainable, real-world deployment of PEC cells for green solar fuel production.

Colloidal 2D Layered SiC Quantum Dots from a Liquid Precursor: Surface Passivation, Bright Photoluminescence, and Planar Self-Assembly

We report the bottom-up synthesis of colloidal two-dimensional (2D) layered silicon carbide (SiC) quantum dots with a cubic structure, lateral size of 5-10 nm, ⟨110⟩ exfoliation to few atomic layers, and surface passivation with 1-dodecene. Samples shielded from oxygen and plasma-annealed for purity exhibit narrow blue photoluminescence (PL) with quantum yields (QYs) over 60% in exceptional cases, while unshielded nanocrystals (NCs) exhibit broad blue/green/white PL with 10-15% QY. The latter scenario is attributed to excess surface carbon and oxygen accrued during synthesis and processing, with size separation through ultracentrifugation revealing size-dependent impurity emission. In contrast, the shape of the bright narrow blue PL shows little variation with NC size, while in both scenarios, the maximum QY occurs near four atomic layers. When dried under heat, the disk-like NC suspensions are observed to aggregate into microscale domains, with further self-assembly into planar superlattice domains with common crystalline orientation. The results are compared with photophysical simulations and bring clarity to the broad emission commonly reported for top-down approaches, while inspiring bottom-up schemes directed at improved material quality.

Formamidinium lead iodide perovskite photovoltaics with MoS2 quantum dots

We present the formation of a composite film made out of formamidinium lead iodide (FAPI) and molybdenum disulphide quantum dots (MoS2 QDs) and propose a corresponding photovoltaic device architecture based on a 'type-I' alignment of the two materials' electronic energy levels. The introduction of the MoS2 QDs has not compromised the overall crystallinity of the FAPI film and the composite absorber has shown improved stability. We report on the benefits of this composite film and energy band arrangement as the photogenerated carriers in MoS2 QDs, both positive and negative, are injected into the FAPI host matrix, resulting in an increased current density of 24.19 mA cm-2 compared to a current density of 19.83 mA cm-2 for the control device with FAPI only. The corresponding photoconversion efficiency increases from 12.6 to 15.0%. We also show that inclusion of MoS2 QDs in FAPI films resulted in a notable improvement in the fill factor and open-circuit voltage of the solar cells. Most importantly, MoS2 QDs enhanced the film stability by reducing defect formation and acting as passivating agents that minimize recombination losses and improve charge carrier transport. Our results suggest that a composite film in a type-I device architecture can introduce benefits for both future developments in perovskite solar cells and effectively tackling the longstanding challenges of carrier transport in QDs solar cells.