Contact-dominated transport in carbon nanotube thin films: toward large-scale fabrication of high performance photovoltaic devices
Semiconducting single-walled carbon nanotubes (s-SWCNTs) have been widely regarded as potential channel materials for not only replacing silicon to extend Moore's law but also for building high performance optoelectronic devices. To realize these goals, high quality s-SWCNTs and contacts are needed to outperform devices based on traditional materials such as silicon. For a high quality conducting or active channel, the ideal CNTs consist of a pure s-SWCNTs array with a confined pitch of less than 10 nm via e.g., chemical vapor deposition (CVD) methods, although this has not been realized experimentally. On the other hand, significant progress has been made on solution-processed CNTs. However, only network and low performance optoelectronic devices have been realized. In this study, we systematically studied the performance of devices using solution-processed CNT films with different s-SWCNT purity, with particular emphasis being placed on disentangling those metallic-CNTs (m-CNTs)-dominated low performance and contacts-dominated high performance devices. We demonstrated that using high purity s-SWCNTs allowed for the construction of high performance diodes via a doping-free method. These diodes behave similarly to those based on individual CVD-grown s-CNTs, resulting in 250 mV photovoltage for a typical single diode and more than 4.35 V for cascading cells using the virtual contact technique and thus paving the way for large scale fabrication of higher performance photovoltaic devices using readily available solution processed CNTs.