Light‐Induced Charge Transfer in Pyrene/CdSe‐SWNT Hybrids

Abstract

On account of their nano-scale size, large aspect ratio and high conductivity, single-walled carbon nanotubes (SWNTs) have emerged as an attractive choice for conducting composite materials. Composites incorporating SWNTs show percolation dominated conductivity with a much lower volume threshold (volume fraction ≈ 10), compared to those with nanoparticles. Two-thirds of SWNTs are p-type semiconductors with holes as the charge carriers. Using the holeblocking nature of SWNTs, conducting polymers having SWNTs as dopants are effectively used as the hole buffering and electron transport layers in organic light-emitted diodes (OLEDs). For the active layer of OLEDs and organic photovoltaic (OPV) devices, incorporation of SWNTs enhances the charge separation and facilitates charge transport, hence improving the performance, i.e., the short circuit current, the filling factor and power conversion efficiency. However, since SWNTs are a mixture of metallic and semiconducting nanotubes with a small bandgap (≈ 0.6 eV), both electrons and holes in the composite matrix prefer to transfer onto and then be quenched on the SWNTs. Directly incorporating SWNTs into active layers of OLEDs and OPV devices does not facilitate the electron/hole separation, nor does it improve the performances of devices. On the other hand, semiconductor nanoparticles, possessing large and tunable bandgaps (1–2 eV), can be incorporated into conducting polymers to form hybrid solar cells. Holes are transported along the polymer chains and electrons hop along the nanoparticle network. However, nanoparticles with large volume fractions are needed because of their high percolation threshold (>≈30%). By contrast, the volume fraction for the conducting polymer is small, leading to low hole mobility and hence limiting the solar cell efficiency. Semiconductor nanorod-polymer hybrid solar cells benefit from the quasi-one-dimensional (1-D) electron transport along the rods, a mechanism which allows the use of a smaller volume fraction of nanorods and a larger volume fraction of polymer to conduct holes with higher mobilities. Although Alivisatos et al. have demonstrated that longer nanorods lead to higher energy conversion efficiency, nanorods of larger lengths and uniformly distributed diameters are difficult to fabricate. Semiconductor nanoparticle-SWNT hybrids have been the subject of recent interest as a consequence of the development of methods for the chemical modification of SWNTs. Such hybrids are well suited for use in optoelectronic devices, given the tunable bandgap of nanoparticles, quasione-dimensional (1-D) transport of SWNTs, and the ease of chemical fabrication. As part of a drive towards finding applications, an examination of the charge transfer (CT) between the various components of the hybrids is needed. By using such hybrid materials as photo-electrodes, efficient electron transfer from semiconductor nanoparticles, such as CdS, CdSe, and CdTe (donor) to SWNTs (acceptor) has been demonstrated by several groups to lead to increased photon generated current (Fig. 1). The CT also results in photolumiC O M M U N IC A IO N