Abstract
Photovoltaic devices based on nanotechnology have attracted much attention because of their great potential for application in electronic and energy fields. Here, a photovoltaic device based on a high-work-function metal/single-walled carbon nanotube (SWNT)/low-work-function metal hybrid junction was investigated. In the device, asymmetric metal electrodes (palladium and aluminum) were fabricated on opposite ends of a single semiconducting SWNT, which was used as the photosensitive material. This structure allowed a strong built-in electric field to be generated in the SWNT to efficiently separate photogenerated electron-hole pairs and achieve good photovoltaic effect. In the dark, the device behaved as a gate-dependent Schottky diode and exhibited the electrical characteristics of a rectifier. The SWNT diameter (band gap) was found to have a significant effect on the device characteristics. For the device fabricated with a 1.4-nm-diameter SWNT, a high rectification ratio (Iforward/Ireverse) of >103 could be achieved in the dark. Under monochromatic illumination, this device had an open-circuit voltage of 0.15 V and a high quantum efficiency of ~75%. Carbon nanotubes have been turned into efficient solar harvesters by using metals with different work functions. Single-walled carbon nanotubes (SWNTs) have excited much interest in the photovoltaic industry because their response to different light wavelengths can be tuned by varying the nanotube diameter. However, most SWNT-based solar cells have poor power conversion efficiencies because of the difficulty of creating a strong, built-in electric field for photocarrier in these highly uniform nanomaterials. Changxin Chen and co-workers at Shanghai Jiao Tong University in China have overcome this challenge with a device that suspends an SWNT between separate palladium and aluminum electrodes. The contacts with these metals, which hold on to their electrons with different strengths, transformed the SWNT into a diode that converted light into electricity with an impressive quantum efficiency of 75%. A photovoltaic device based on a high-work-function metal/single-walled carbon nanotube (SWNT)/low-work-function metal hybrid junction was constructed to generate a strong built-in electric field in the SWNT for efficiently separating photogenerated electron-hole pairs. In the dark, the device behaved as a gate-dependent Schottky diode, with a high rectification ratio (Iforward/Ireverse) of >103 achieved for the device fabricated with an 1.4-nm-diameter SWNT. Under monochromatic illumination, this device exhibited an open-circuit voltage of 0.15 V and a high quantum efficiency of ~75%. It was found that the SWNT diameter had an important effect on the device characteristics.
Highlights
Photovoltaic devices have attracted much attention owing to their numerous applications in electronic and energy fields
The purity of the semiconducting SWNTs in the solution was estimated to be ~ 98% by comparing their absorption spectrum with that of the nanotubes used in the study by Engel et al 13 The SWNTs were estimated to have diameters in the range of 1.2–2.1 nm
The range of diameters calculated from the radial breathing mode (1.2–2.1 nm) was similar to that obtained from the light absorption measurement
Summary
Photovoltaic devices have attracted much attention owing to their numerous applications in electronic and energy fields. The unique and excellent physical and mechanical properties of single-walled carbon nanotubes (SWNTs) make them good building blocks for photovoltaic devices.[1,2] SWNTs have a direct band gap that increases with decreasing diameter, and they exhibit strong photoabsorption in the spectral range from ultraviolet to infrared. They have a separated electronic sub-band structure, which can prevent the rapid relaxation of hot carriers, allowing more photogenerated carriers to be collected by the electrodes. The power conversion efficiency of these cells was low (~0.11%)[3] or was only slightly enhanced compared with that of the traditional cells without SWNTs.[5,6] To fully exploit the properties of SWNTs and produce high-performance photovoltaic devices, it is desirable to use SWNTs as the photosensitive material in the device
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