Abstract
Unique one-dimensional structure and excellent diameter-depended optoelectrical properties make CNTs have great potential for novel high-performance photovoltaic applications. Among them, van der Waals heterojunction (vdW) solar cells have been demonstrated by combining the excellent optical and electrical properties of CNTs with the photoelectric properties of bulk semiconductors since 2007. Owing to simple device structure and easy fabrication process, these vdW heterojunction solar cells have received more and more attentions in the past ten years with the development of high-quality CNTs (conducting type, chirality and purity, etc.). Especially, the sharply atomic interface can remarkably reduce the recombination probability of charge carriers, and the carrier separation efficiency can be improved by changing the diameter and doping states of CNTs. Meanwhile, High transparency and high carrier mobility of thin CNT film also make it to be beneficial to collect the photo-generated carriers. In this chapter, the work mechanism of CNT/semiconductor vdW heterojunctions are firstly introduced when semiconducting or metallic CNTs are used, and some key parameters of solar cells are also introduced. Then, we mainly introduce the state-of-the-art research progress about the vdW heterojunction solar cells, which consist of bulk semiconductor (Si and GaAs) and CNTs with different wall number. Several key technologies have been optimized to improve the photovoltaic performance of CNT/Si vdW heterojunction solar cells, such as high-quality CNT film, Fermi level controlling, and interface engineering, and so on. Finally, the current challenges and future perspectives of highly efficient vdW heterojunction solar cells have also been discussed.