Increasing the efficiency of charge extraction limited poly-(3-hexylthiophene):[6,6] phenyl C61 butyric acid methyl ester solar cells using single walled carbon nanotubes with metallic characteristics

The charge transport properties of bulk heterojunction solar cells formed by blending poly-(3-hexylthiophene) (P3HT) and [6,6] phenyl C61 butyric acid methyl ester (PCBM) were improved by doping with single walled carbon nanotubes (SWNTs). The SWNTs used were not functionalized, and contained both metallic and semiconducting tubes. Their work function was found to be 4.89 eV. Unlike P3HT:PCBM interface, the P3HT:SWNT interface has been inefficient for charge generation. Using SWNTs at concentrations below 1 wt.%, the solar cell efficiency increased from 2.86% to 3.52% for 80 nm devices and from 2% to 3% in 125 nm devices at low light intensities. In both cases, the increment is because of higher fill factor with no change in short circuit current density and open circuit voltage. At higher light intensities, a 43% increase in fill factor and a 37% increase in short circuit current density were obtained, which doubled the efficiency. These improvements were primarily because of reduced recombination through improved charge extraction by SWNTs.