Photovoltaic Effects in Asymmetrically Contacted CNT Barrier-Free Bipolar Diode

Semiconducting single-walled carbon nanotubes (SWCNTs) are one-dimensional (1D) and direct band gap materials with unique electric and optical properties and are promising candidates for future nanoelectronics and optoelectronics.1,2 The use of carbon nanotube (CNTs), nanowires, and other nanomaterials represents a typical approach to reduce both cost and size and to improve efficiency in photovoltaic application.3-5 In a typical optoelectronic device, a build-in field is essential for effectively separating photon excited electron-hole pairs and for observing photovoltaic effects.6 In a p-n junction, the maximum achievable photovoltage is determined by the energy level difference between that of the donors (in the n-region) and acceptors (in the p-region), and typically this is smaller than the energy band gap Eg of the corresponding intrinsic semiconductor. On the other hand, a Schottky barrier (SB) between a metal electrode and a semiconductor may also provide the required field for separating electrons and holes. But again the SB will reduce the maximum achievable photovoltage. Early reports on the photovoltage of SWCNT devices are mainly based on the CNT field-effect transistors (FETs)5,7 but there exist few reports on the CNT p-i-n diode and p-n junction diodes.8-12