Photocurrent enhancement mechanisms in bilayer nanofilm-based ultraviolet photodetectors made from ZnO and ZnS spherical nanoshells
© Peng et al.; licensee Springer. 2014
Received: 17 July 2014
Accepted: 1 August 2014
Published: 11 August 2014
Hollow-sphere bilayer nanofilm-based ultraviolet light photodetectors made from ZnO and ZnS spherical nanoshells show enhanced photocurrent, which are comparable to or even better than those of other semiconductor nanostructures with different shapes. In this work, the photocurrent enhancement mechanisms of these bilayer nanofilm-based ultraviolet light photodetectors are explained, which could be attributed to the strong light absorption based on the whispering gallery mode resonances, the separation of the photogenerated carriers through the internal electric field within the bilayer nanofilms, the hopping-like electrical transport, and the effective charge injection from Cr/Au contacts to the nanofilms.
KeywordsHollow-sphere bilayer nanofilm Ultraviolet light photodetector Whispering gallery mode resonances
Monodisperse spherical nanoshells (or called hollow spheres) have attracted considerable interest due to their well-defined morphology, uniform size, low density, high surface area, and potential applications such as protection of biologically active agents, waste removal, and so on [1–3]. On the other hand, some novel nanodevices with high performance have been constructed using semiconducting hollow spheres as the building blocks [4, 5]. For instance, dye-sensitized solar cells using electrodes consisting of nanoembossed TiO2 hollow spheres exhibit outstanding light-harvesting efficiency . Nanocrystalline silicon (nc-Si) solar cells based on the hollow-sphere nc-Si nanofilm are constructed, which exploit the low-quality-factor whispering gallery modes (WGMs) in hollow spheres to dramatically enhance broadband absorption . Most of the incoming light couples into the WGMs in the hollow spheres and circulates in the active material with a considerably longer path length than that of the same material in the form of a planar film. Such light-trapping structure is an essential design consideration for high-performance photodetectors (PDs), as well as other optical devices such as solar cells.
Recently, we have developed a self-assembly strategy at the immiscible oil-water interface to fabricate monolayer hollow-sphere nanofilm-based devices, such as ultraviolet (UV) light PDs and electrical resistive switching memory devices [6–9]. On the other hand, we also use the self-assembly strategy to construct hollow-sphere bilayer nanofilm-based UV PD devices, which show improved optoelectronic properties . Hollow-sphere bilayer nanofilm-based UV PDs using abundant wurtzite ZnO and ZnS hollow nanospheres as the building blocks were constructed by the oil-water interfacial self-assembly strategy. These hollow-sphere nanofilm-based UV PDs showed high sensitivity, good stability, and fast response times, which are comparable to or even better than those of other ZnO nanostructures with different shapes [10–17]. It is quite promising for applications such as optical communications, flame sensing, missile launch, and so forth. However, we need to explore the photocurrent enhancement mechanisms in the bilayer nanofilm-based UV PDs made from ZnO and ZnS hollow spheres in order to further improve their optoelectronic properties. We believe that the photogenerated charges are extracted from these devices to not simply produce the photocurrent but instead cause some new changes in these devices which impel further free carriers to be generated and transported through the devices. In this work, the photocurrent enhancement mechanisms of these bilayer nanofilm-based UV PDs are explained. Especially, we prove a concept for light trapping in the hollow-sphere nanofilm-based UV PDs through the use of wavelength-scale resonant hollow spheres that support WGMs to enhance absorption and photocurrent. We numerically demonstrate this enhancement using full-field finite element method (FEM) simulations of hollow-sphere nanofilm-based UV PDs. It is proved that the WGM is an important concept for the manufacturing of the hollow-sphere nanofilm-based UV PDs, which facilitates the coupling of light into the resonant modes and substantial enhancement of the light path in the active materials, thus dramatically enhancing absorption and photocurrent.
Results and discussion
The optical and electrical measurements provide insight into the photoconductive mechanism in ZnO/ZnS (or ZnS/ZnO) bilayer nanofilm devices, including the light absorption, the generation of free carriers, the charge transport, and the charge injection from metal contacts to the nanofilms. We note a remarkable enhancement in photocurrent for the bilayer nanofilm-based UV PDs, so we require a mechanism where the photogenerated charges are extracted from the devices not simply to produce the photocurrent but instead cause some new changes in these devices which impel further free carriers to be generated and transported through the devices.
In addition, in the UV PDs based on the hollow-sphere bilayer nanofilms, the charge transfer between two neighboring hollow spheres is hopping-like due to the existence of physical boundaries . In these devices where the current is space charge limited, it is easy to see that decreasing the trapping of free charges will lead to an increase in effective mobility and hence current. For the electrical transport through the interface between the Cr/Au electrode and the semiconductor, the formed ohmic or injection-type electric contacts in these UV PDs also contribute to the high photoresponsivity [8, 10, 22–24].
In conclusion, we have demonstrated that the UV PDs can be conveniently fabricated using the hollow-sphere bilayer nanofilms. The UV PDs show high performance, which could be attributed to the strong light absorption based on the WGM resonances, the separation of the photogenerated carriers through the internal electric field within the bilayer nanofilms, the hopping-like electrical transport, and the effective charge injection from gold contacts to the nanofilms. It is quite promising for applications such as optical communications, flame sensing, and missile launch.
This work is sponsored by the Natural Science Foundation of Shanghai (No. 13ZR1417600), the Innovation Program of Shanghai Municipal Education Commission (No. 14YZ132), the Postdoctoral Science Foundation of China (No. 2012M520825), the Startup Fund for Talented Scholars of Shanghai University of Electric Power (No. K2011-014), the National Natural Science Foundation of China (No. 11374204), and the Science and Technology Commission of Shanghai Municipality (Nos. 12JC1404400 and 11160500700).
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