Functionality-driven architecture of and photovoltaic conversion in hierarchical tree-like semiconductors
Date
2019
Authors
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Publisher
University of Delaware
Abstract
Vertically-alined tree-like semiconductor structures have been studied extensively for solar cell construction and optical device fabrication. The inspiration in architecture comes from mimicking natural trees by collecting photons with branches, transferring electrons through stems, and rendering eective charge carriers separation. ☐ ZnO is a promising n-type semiconductor with a wide band gap. It is considered as an alternative material to pristine TiO2 since ZnO facilitates high electron binding energy, fast electron mobility, morphological diversity, etc. However, ZnO can only absorb UV light, which contributes 4% of the solar spectrum, and high electron-holerecombination rate is also challenging for the efficiency of photovoltaic conversion. These drawbacks limit its application in photocatalysis and solar energy conversion at an industrial scale. Therefore, modification, which could improve the solar energy utilization and photoelectrochemical conversion efficiency, is the key topic regarding the application of ZnO in solar cells and optical devices. ☐ One solution is to modify ZnO with an appropriate semiconductor. CdSSe is a n-type alloy semiconductor made of CdS and CdSe. Its band gap structure can be simply tailored by adjusting the ratio between S and Se. CdSSe shows excellent light emitting performance. CuO and Cu2O are p-type semiconductors with narrow band gaps. These three semiconductors have narrower band gaps, so this modification would lead to broader solar light absorption range and facilitate separation between electrons and holes. ☐ In this work, design, synthesis methods, characteristics, and applications in photoelectrochemical catalysis of vertically-aligned tree-like semiconductors with ZnO nanorods as stems were demonstrated. ZnO/CdSSe nanotrees with n-n heterojunction, were prepared via chemical vapor deposition technique and unique Z-scheme electron transfer mode was confirmed via time-correlated single photon counting technique. Next, ZnO/CuO nanotrees were synthesized via two-step hydrothermal technique. The n-p heterojunction between ZnO and CuO was verified to have high photocatalysis performance in ascorbic acid or toluene sensing. ZnO/Au/CuxO nanoforest was fabricated with hydrothermal-prepared ZnO, followed by gold coating and electro-deposited CuxO growth. With n-m-p heterojunction, ZnO/Au/CuxO nanoforest shows not only fast electron transport, but also broad light absorption range.