No 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 (I forward /I reverse ) of INTRODUCTIONPhotovoltaic 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 also 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. Moreover, ideal SWNTs have defect-free structures and can greatly inhibit the recombination of photogenerated electronhole pairs. In addition, they exhibit high carrier mobility and good mechanical strength. SWNTs have been used in some photovoltaic devices such as organic solar cells 3,4 and dye-sensitized solar cells 5,6 to improve the photoelectric conversion efficiency. However, in these devices, SWNTs only acted as the conductive material or as the transparent electrode that conducted or collected the photogenerated charge carriers. 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.To achieve a photovoltaic effect, a photovoltaic device must have a strong built-in electric field to separate photogenerated electron-hole

A p-i-n junction diode constructed by the locally doped network of single-walled carbon nanotubes (SWNTs) was investigated. In this diode, the two opposite ends of the SWNT-network channel were selectively doped by triethyloxonium hexachloroantimonate (OA) and polyethylenimine (PEI) to obtain the air-stable p- and n-type SWNTs respectively while the central area of the SWNT-network remained intrinsic state, resulting in the formation of a p-i-n junction with a strong built-in electronic field in the SWNTs. The results showed that the forward current and the rectification ratio of the diode increased as the doping degree increased. The forward current of the device could also be increased by decreasing the channel length. A high-performance p-i-n junction diode with a high rectification ratio (~104), large forward current (~12.2 μA) and low reverse saturated current (~1.8 nA) was achieved with the OA and PEI doping time of 5 h and 18 h for a channel length of ~6 μm.

A p-i-n junction diode based on the selectively doped single-walled carbon nanotube (SWCNT) had been investigated, in which two opposite ends of individual SWCNT channel were doped into the p- and n-type SWCNT respectively while the middle segment of SWCNT was kept as the intrinsic. The symmetric and asymmetric contacts were used to fabricate the p-i-n junction diodes respectively and studied the effect of the contact on the device characteristics. It was shown that a low reverse saturation current of ~20 pA could be achieved by these both diodes. We found that the use of the asymmetric contact can effectively improve the performance of the p-i-n diode, with the rectification ratio enhanced from ~102 for the device with the Au/Au symmetric contact to >103 for the one with the Pd/Al asymmetric contact. The improvement of the device performance by the asymmetric-contact structure was attributed to the decrease of the effective Schottky-barrier height at the contacts under forward bias, increasing the forward current of the diode. The p-i-n diode with asymmetric contact also had a higher rectification ratio than its counterpart before doping the SWCNT channel, which is because that the p-i-n junction in the device decreased the reverse saturated current.

No abstract