High-work-function metal/carbon nanotube/low-work-function metal hybrid junction photovoltaic device

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.