Pristine SWCNT and MWCNT were individualized and debundled in an N-N dimethyl tetraformamide solvent by a combination of ultrasonication and centrifugation. The wt% (mg) of the pristine CNTs loading were optimized with respect to quantity of solvent (ml). Choice of solvent, ultracentrifugation speed and ultrasonication time were essential parameters for obtaining good individualization of pristine CNTs. By using such CNTs, a thin nanotubes layer (similar to 15 nm) were inserted at the different hierarchical positions into bulk heterojunction solar cell devices made of poly(3-hexylthiophene-2,5-diyl):[6,6]-phenyl-C61-butyric acid methyl ester with weight ratio of 1:1. By inserting such a pristine SWCNTs between PEDOT:PSS and P3HT:PCBM layers, an increase in the solar cell efficiency from 1.51% ((J(sc) of 6.68 mA/cm(2),V-oc of 0.60, FF of 37) to 2.65% (J(sc) of 11 mA/cm(2),V-oc of 0.58, FF of 42) were observed. However, functionalized CNTs shows a degrading efficiency of 0.25% which can be attributed to degradation of corrugated tubular surface side walls leading to potential loss of their optoelectronic properties. The enhanced efficiency of devices with pristine SWCNTs can be conjectured to better opto-electrical properties, enhanced transition, improved local structure and the undamaged tubes. The microstructures of the heterojunction active layer were examined by using AFM, TEM, UV-Vis spectra, IV curve and EQE techniques. (C) 2014 Elsevier Ltd. All rights reserved.