Chemical vapor growth of nitrogen doped carbon nanotube and graphene materials for application in organic photovoltaic devices.

[摘要] Application of carbon nanomaterials like fullerene, carbon nanotubes, and graphene in solarcells using solution processable methods presents a great potential to reduce the cost ofproducing electricity from solar energy. However, carbon nanotubes and graphene materialsare predominantly metallic and this limits their function in organic photovoltaic devices(OPVs) where semiconducting behavior is required. Doping of carbon nanomaterials is awell-known method for making them semiconducting. Doping of carbon nanomaterials withnitrogen and boron can tune their properties to suit the requirements for use in photovoltaicapplications as n-type and p-type semiconducting materials, respectively. Indeed, the use ofnitrogen doped and boron doped carbon nanotubes in organic solar cells together withfullerene acceptors can improve the current density of the OPV devices.Nitrogen doping of carbon nanotubes can be achieved by using nitrogen-containing precursormaterials during chemical vapor deposition. However the doping of carbon nanotubes withnitrogen does not automatically make them n-type materials; they remain metallic unless alarge amount of quaternary type nitrogen is incorporated in the carbon nanotubes. In thiswork we have developed a method to control the type of nitrogen that is incorporated inCNTs by using an appropriate synthesis temperature and use of oxygen-containing carbonprecursors during the chemical deposition of carbon nanotubes. Quaternary N wasincorporated in a CVD process when high temperatures and a high concentration of O in theprecursor materials were used. We also showed that the type and amount of N can bechanged from pyrrolic and pyridinic-N-oxide to pyridinic N and quaternary N by annealing Ndoped carbon nanotubes at temperatures above 400°C. At temperatures above 800°C most ofthe nitrogen is converted to quaternary nitrogen.N-CNT thin films were used in OPVs so as to modify the ITO electrode and transform it intoa 3D electrode. The resulting effect was an improved short circuit current density in thedevices containing an N-CNT thin film that was placed on top of the ITO electrode. Areduction in efficiency losses in OPVs at increasing light intensity was observed in the NCNTITO modified electrode OPVs. This is a remarkable finding when considering that oneof the main problems hindering commercialization of OPVs is the loss of efficiency at highlight intensities. We related these effects to the efficient charge collection by the modifiedITO electrode. Incorporation of N-CNTs in the bulk heterojunction layer of the OPV deviceresulted in poor performance when compared to an OPV device made without N-CNTs. Thiseffect is caused by shorting of the OPVs. We used a method of incorporating N-CNTs whilstminimizing shorting and this showed potential for better performance.A study on the attempted doping of graphene with B to make it a p-type material showed thatin the presence of a nitrogen carrier gas, BN instead of B was incorporated in graphene. Thisremarkable finding enabled us to grow a p-type graphene with a possible a band gap opening.This was corroborated by XPS and Raman spectroscopy studies of the material. This BNdoped graphene material showed potential as a possible replacement of PEDOT:PSS as ahole transport material in OPVs. The BN doped graphene material can match theperformance of PEDOT:PSS when the level of BN doping in graphene is increased.

[发布机构] University of the Witwatersrand