Department of Electrical and Computer Engineering, University of Toronto , 10 Kings College Road, Toronto, Ontario M5S 3G4, Canada.
ACS Appl Mater Interfaces. 2017 Feb 15;9(6):5556-5565. doi: 10.1021/acsami.6b13713. Epub 2017 Feb 3.
The engineering of broadband absorbers to harvest white light in thin-film semiconductors is a major challenge in developing renewable materials for energy harvesting. Many solution-processed materials with high manufacturability and low cost, such as semiconductor quantum dots, require the use of film structures with thicknesses on the order of 1 μm to absorb incoming photons completely. The electron transport lengths in these media, however, are 1 order of magnitude smaller than this length, hampering further progress with this platform. Herein, we show that, by engineering suitably disordered nanoplasmonic structures, we have created a new class of dispersionless epsilon-near-zero composite materials that efficiently harness white light. Our nanostructures localize light in the dielectric region outside the epsilon-near-zero material with characteristic lengths of 10-100 nm, resulting in an efficient system for harvesting broadband light when a thin absorptive film is deposited on top of the structure. By using a combination of theory and experiments, we demonstrate that ultrathin layers down to 50 nm of colloidal quantum dots deposited atop the epsilon-near-zero material show an increase in broadband absorption ranging from 200% to 500% compared to a planar structure of the same colloidal quantum-dot-absorber average thickness. When the epsilon-near-zero nanostructures were used in an energy-harvesting module, we observed a spectrally averaged 170% broadband increase in the external quantum efficiency of the device, measured at wavelengths between 400 and 1200 nm. Atomic force microscopy and photoluminescence excitation measurements demonstrate that the properties of these epsilon-near-zero structures apply to general metals and could be used to enhance the near-field absorption of semiconductor structures more widely. We have developed an inexpensive electrochemical deposition process that enables scaled-up production of this nanomaterial for large-scale energy-harvesting applications.
为了开发用于能量收集的可再生材料，将宽带吸收体应用于薄膜半导体中以收集白光，这是一个重大挑战。许多具有高可制造性和低成本的溶液处理材料，例如半导体量子点，需要使用厚度在 1μm 左右的薄膜结构来完全吸收入射光子。然而，这些介质中的电子输运长度比这一长度小一个数量级，这阻碍了该平台的进一步发展。在此，我们通过设计合适的无序纳米等离子体结构，展示了我们已经创造了一种新的无弥散类近零介电常数复合材料，这种材料能够高效地利用白光。我们的纳米结构将光局域在类近零介电常数材料的介电区域中，其特征长度为 10-100nm，当在结构顶部沉积一层薄的吸收膜时，这导致了一种高效的宽带光收集系统。通过理论和实验的结合，我们证明了沉积在类近零介电常数材料顶部的厚度低至 50nm 的胶体量子点，与具有相同胶体量子点吸收体平均厚度的平面结构相比，宽带吸收增加了 200%至 500%。当类近零纳米结构用于能量收集模块时，我们观察到器件的外部量子效率在 400nm 至 1200nm 波长范围内的宽带增加了 170%，这是光谱平均的。原子力显微镜和光致发光激发测量证明了这些类近零结构的性质适用于一般金属，并且可以更广泛地用于增强半导体结构的近场吸收。我们已经开发出一种廉价的电化学沉积工艺，该工艺可以扩大这种纳米材料的生产规模，以用于大规模的能量收集应用。
