Carbon Nanotube Hybrids for Renewable Energy

Abstract

This editorial presents a short summary and perspectives of recent explorations of light energy conversion and related photovoltaic (PV) devices constructed from carbon nanotubes (CNTs) interfaced with organic and inorganic materials. The first reports about CNT photoconductivity [1][2][3] ) as compared with conductive organic materials. In addition, carbon nanotubes exhibit an exceptional environmental stability and resistance to photo bleaching which is one of the major drawbacks of organic optoelectronics. Device fabrication is simple and cost effective as CNTs can be easily incorporated in the device's architecture by wet processing (coating, spraying, and printing). Finally, CNTs can be simply doped or functionalized by many covalent and non-covalent routes, forming nano-assemblies with other molecules and polymers to provide an efficient photoinduced charge transfer or tuning Fermi level to the favorable position at heterojunction with semiconductors. CNT light absorption is an initial step leading to a generation of bound excitons. In order to convert light energy into an electrical signal, excitons should be separated on free charge carriers (electrons and holes) by an external or internal built-in electric field, before they relax to the ground state. Finally, the resulting carriers should be transported to the electrodes minimizing the recombination and trapping processes. Such a scenario is realized for photovoltaics and photodetectors when the internal built-in field is required at the interface between carbon nanotubes and other materials. Distinct from pristine CNTs, interface related photoconversion processes are more complex but at the same time very intriguing, rich in novel phenomena and are extremely attractive for many optoelectronics applications. For instance, very recently, a surprisingly high photoconversion efficiency (PCE) of ~14% for CNT/Si hybrid solar cells has been reported [7], exceeding any PCE for organic and hybrid photovoltaics. Noteworthy, research in the field of CNT/semiconductor PV is very new (just past five years) and limited by few groups [7][8][9][10] as compared with substantial efforts and time (about 20 years) spent by PV community in other directions such as polymer based and dye-synthesized solar cells where the best PCE is still in the range of 10-12% [11][12][13]. Photoactive CNT/ counterpart nanohybrids include significant diversity in the CNT morphology (network and individual CNTs), their structure (SWNTs and MWNTs) and electronic properties (semiconducting and metallic). The choice of CNT counterpart can also be very different comprising small molecules, oligomers, polymers, quantum dots and semiconductors (bulk and nanostructured). The most studied structures utilized in solar cell design are CNTs/small molecules and CNTs/polymers, where CNTs act as an electron acceptor (with some exceptions) and light is absorbed through the CNT complimentary component. Interestingly, in majority PV studies of CNT hybrids, the role of CNTs in light harvesting was underesti...