Laboratory for Molecular Engineering of Optoelectronic Nanomaterials, École Polytechnique Fédérale de Lausanne (EPFL) , Station 6, 1015 Lausanne, Switzerland.
Nano Lett. 2018 Jan 10;18(1):215-222. doi: 10.1021/acs.nanolett.7b03948. Epub 2017 Dec 15.
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 WSe thin film photocathodes for solar H 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 WSe: solar photocurrents for H evolution up to 4.0 mA cm 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.
少层液相剥离的半导体过渡金属二卤族化合物 (TMD) 的纳米片有望实现大面积、高性能、低成本的太阳能转换，但它们的性能受到缺陷部位复合的限制。在此，我们通过应用两种分别针对片内和边缘缺陷的处理方法（预剥离退火和后沉积表面活性剂附着），考察了缺陷对 WSe 薄膜光阴极用于太阳能 H 生产性能的影响。TEM、XRD、XPS、光致发光和阻抗谱分析用于表征处理效果，光电化学（PEC）测量（使用优化的 Pt-Cu 共催化剂进行，与 Pt 相比发现其提供了更好的稳定性）用于量化由 100-1000nm 纳米片组成的光阴极（约 11nm 厚）的性能。表面活性剂处理导致光电流增加，这归因于边缘位的钝化。单独的预退火处理虽然明显改变了预剥离粉末的结晶度，但对光电流没有显著影响。然而，应用两种缺陷处理方法可显著提高性能，代表了溶液处理的 WSe 性能的新基准：用于 H 演化的光电流高达 4.0 mA cm，内部量子效率超过 60%（740nm 光照）。这些结果还表明，在裸 TMD 纳米片中，片边缘的电荷复合主导了性能，但当边缘缺陷被钝化时，内部缺陷变得重要，可以通过预退火来减少。
