Nanocarbons as charge carriers in organic solar cells

Organic photovoltaics (OPV) are an alternate to conventional silicon and thin film solar cells. They have several advantages such as simple fabrication, low material cost, light weight, and the ability to cover large areas. Typical OPV are based on bulk heterojunction structure consisting of a blend of an electron-donating semiconductor polymer and an electron-accepting molecule. Various approaches have been pursued to improve the performance of OPV: low bandgap polymers, new electron acceptors, processing additives, and tandem cell architecture. Nanocarbons possess unique physical and chemical properties that can contribute to the enhancement of OPV performance, such as charge carrier mobility, thermal conductivity, mechanical strength, increased optical absorption and solubility in organic solvents. Nanocarbons also can improve the morphology of the active layer, which is critical to the optimization of OPV performance. The objective of this study is to utilize nanocarbons namely carbon nanotubes, nanodiamond and carbon black as additives and to synthesize composites with fullerenes (C60, C70) as charge carriers in OPV to improve power conversion efficiency (PCE). Functionalized multi-walled carbon nanotubes (MWNTs) are used as additives to C70 to form a weak bonded composite, which result in better absorption and better phase separation. Consequently, PCE increases 70%. In another set of experiments, MWNTs are size sorted and MWNTs of different sizes are tested as an additive to the OPVs to improve PCE. It has been found that the shorted MWNTs provide larger surface area for exciton dissociation and better phase separation and probably lead to less charge recombination. Therefore, they show higher PCE. Functionalized nanodiamonds (ND) are also introduced to C60 to synthesize C60-ND-COOH composite by taking the advantage of their small size distribution (4-5 nm) and modest charge mobility. The NDs have shown the ability to enhance short circuit current density and PCE. Finally carbon black (CB) is implemented into the P3HT/C60 OPV system and the effect of concentration is investigated. The addition of low concentration (12.5 ppm) of CB results in 35% improvement in short circuit current density, and 79% improvement in PCE.In conclusion, the nanocarbons (MWNT, ND, CB) are promising additives for the performance enhancement of polymer photovoltaic cells and may work with diverse donor-acceptor systems