Photovoltaic Devices Based on Single Wall Carbon Nanotubes
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1210-Q04-06
Photovoltaic Devices Based on Single Wall Carbon Nanotubes Zhongrui Li,* Viney Saini, Shawn Bourdo, Liqiu Zheng, Enkeleda Dervishi, Alexandru S. Biris Nanotechnology Center and Applied Science, University of Arkansas at Little Rock, Arkansas, 72204
ABSTRACT Single-wall carbon nanotubes (SWNTs) are potentially an attractive material for photovoltaic applications due to their unique structural and electrical properties. SWNTs can be directly configured as energy conversion material with nanotubes serving as both photogeneration sites and charge carriers collecting/transport layers. SWNTs can be modified into either p-type conductor through chemical doping (like thionyl chloride, or just exposure to air) or n-type conductor through polymer (like polyethylene imine) functionalization. The solar cells consist of either a semitransparent thin film of p-type nanotubes deposited on an n-type silicon wafer or n-type SWNT film on p-type substrate to create high-density p-n heterojunctions between nanotubes and silicon substrate to favor charge separation and extract electrons and holes. The high aspect ratios and large surface area of nanotubes can be beneficial to exciton dissociation and charge carrier transport thus improving the power conversion efficiency. 1.
INTRODUCTION Single wall carbon nanotubes (SWNTs) are attracting more attention for photovoltaic applications thanks to their superior properties such as a wide range of direct bandgaps matching the solar spectrum,1,2 strong photoabsorption 3 , 4 from infrared to ultraviolet, and high carrier mobility 5 and reduced carrier transport scattering.6 Photovoltaic effect can be achieved in individual carbon nanotube diode with p-n junction.7 The SWNT diodes demonstrate significant power conversion efficiencies under illumination, but single device elements that contain an ensemble of defect-free and electronically isolated SWNTs are required for many real-world applications. A device consisting of a large number of tubes provides good overall uniformity, even if the properties of individual tubes are heterogeneous. More importantly, the current carrying capability of such a device increases with the tube density.8 A standard oxidative purification process is known to induce p-type charge-transfer doping of SWNTs,9 so it is easy to fabricate p-type SWNT solar cells.10 It is possible to fabricate n-type SWNTs by doping low work function metals (like Al) onto the tubes, 11 or through nitrogen doping of metallic SWNTs.12 In this work, we designed and tested simple photovoltaic devices based on the high density heterojunctions formed between either acid purified or polymer functionalized tubes and different type semiconductor substrates. The solar cells consist of a semi-transparent thin film of the p/n-type nanotubes conformally deposited on a n/p-type silicon substrate to create high-density p-n heterojunctions between nanotubes and Si to favor charge separation and extract electrons and holes. The SWNTs were serving as both photogeneration sites and a charge
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