Enhanced Performance from Acid Functionalised Multiwall Carbon Nanotubes in the Active Layer of Organic Bulk Heterojunction Solar Cells

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AbstractSolution-processable organic bulk-heterojunction photovoltaic devices have made great advances over the past decade. The concept, ultrafast photo induced electron transfer from a conjugated polymer to fullerene derivative molecules in bulk-heterojunction systems, leads to device efficiencies as high as 6%. Light absorption, charge separation and charge transport to electrodes are the most important steps in organic photovoltaic devices. The enhanced light absorption through thicker active layers results in more exciton creation, however, leads to increased recombination due to the relatively short exciton diffusion length. We fabricated poly(3-hexylthiophene)/ [6,6]-phenyl C61 butyric acid methyl ester bulk-heterojunction devices with multiwall carbon nanotubes in the active layer in a bid to address this deficiency. Functionalization of carbon nanotubes allows better dispersion in aromatic solvents, 1,2-dichlorobenzene in this study, and pristine multiwall nanotubes result in poorer dispersions. Organic photovoltaic devices fabricated with pristine multiwall carbon nanotubes in the active layer result in power conversion efficiencies ˜1.4%, which show localized nanotube-rich areas in the active layer. Alternatively, acid functionalized nanotubes in the active layer results in efficiencies as high as 2.2 % with no distinct nanotube-rich sectors. The open circuit voltages of the devices show a dependency on the loading of nanotubes in the active layer. Further, the shunt resistances of the devices with carbon nanotubes decrease, which needs careful selection of the tubes depending on active layer thickness. This work compares the device performances in detail and identifies further improvements to conjugated polymer/fullerene derivative/multiwall carbon nanotubes hybrid photovoltaic systems.