TABLE OF CONTENTS Photoconductivity refers to the induction of an electronic current when a material is illuminated. In general, simply illuminating a material will not lead to a net electronic current; the material symmetry has to be broken in order to create electron flow in a particular direction. Perhaps the simplest device that works in this manner is the p- n junction, which is at the heart of many opto-electronic devices such as photodetectors and solar cells. In the previous section, it was shown that the optical properties of carbon nanotubes are dominated by excitons, and that these excitons can be dissociated in the presence of strong enough electric fields, such as those present at a p- n junction. In that case, it is expected that the single-particle picture will become more accurate. Therefore, we first present in this section a single-particle description of photoconductivity in carbon nanotubes, which should be applicable to situations where the potential step at the junction is large. This theory will also serve as the basis for understanding the role of excitons in the future, as exciton dynamics becomes better understood.
Fig. 7.14 shows the calculated self-consistent band-bending for a nanotube p- n junction, as was discussed in Chapter 4. Clearly, the potential step at the junction breaks the spatial symmetry. Upon illumination of this device with photons of energy ? ?, electrons at energy E are excited to energy E + ? ? and are...
