Origin of Open Circuit Voltage in Solar Cells Based on Polymer Wrapped Carbon Nanotubes and Fullerenes
We report the effect of polymers on the performance of two single chirality SWCNT-C60 bilayer solar cells using: (a) poly(9,9-dioctylfluorene-2,7-diyl) (PFO) separated (7,5) tubes and (b) 9,9-di-octylfluorenyl-2,7-diyl and bipyridine copolymer (PFO-BPy) separated (6,5) tubes. We show that along with polymer-SWCNT weight ratio, the absolute weight of the polymer is also a critical parameter affecting the device operation. Performance of devices with and without SWCNTs both having the same polymer content is compared. In devices without SWCNTs, the polymer alone acts as interfacial layer between C60 and the anode. Polymer concentration dependence is also studied in these devices. It is found that the polymer layer, even at very low concentrations, increases the device efficiency by nearly an order of magnitude. While the reference cell with only C60 gave an efficiency of 0.07%, the devices with PFO interfacial layer had efficiencies up to 0.6% with an increase in all three efficiency determining parameters. The Voc of devices with PFO increased in the range of 0.2 -0.5 V over that of the C60 reference device, increasing with PFO layer thickness. In addition, while the PFO-SWCNT-C60 device gave a Voc of 0.40 V, PFO-C60 device gave a Voc of 0.56 V even though both devices contain equal concentration of PFO. These results indicate that the origin of Voc may be the PFO-C60 interface and that the presence of SWCNTs has a negative effect on achievable Voc. With PFO-SWCNT-C60 device working like a ternary system with PFO concentration dependent efficiency, optimization of this system should take into account the PFO-C60 interface as well.
 A. Dowgialo et al, ACS Nano, 8, 8573 (2014)
 M. Gong et al, Nano Lett., 14, 5308 (2014)
 M.J. Shea and M.S. Arnold, 102, 243101 (2013)
 M.P. Ramuz et al, ACS Nano, 6, 10384 (2012)
 D.J. Bindl et al, Chem. Phys., 413, 29 (2013)
 D.J. Bindl and M.S. Arnold, J. Phys. Chem. C, 117, 2390 (2013)