Nanocomposite solar cells: influence of particle concentration, size, and shape on the device performance

Solar cells based on poly(3-hexylthiophene) (P3HT) :TiO 2 nanocomposite films were investigated. We studied the influence of the nanoparticle concentrations and different nanostructures (spherical particles with size ~5 nm and ~20-40 nm, and rods with diameter ~10 nm and length ~40 nm) on the performance of the nanocomposite solar cells. PL quenching and improved external quantum efficiency (EQE) was observed for all the nanocomposite devices compared to that of pristine P3HT solar cells. However, TiO 2 (~5 nm spheres) and TiO 2 rods showed only small improvement in EQE. The small improvement for the 5 nm TiO 2 spheres was attributed to the lack of connectivity of nanoparticles for electron conduction. Therefore, the charge collection efficiency was limited. For TiO 2 rods, the tendency of the rods to lie in the plane of substrates also limited the charge conduction and collection in the direction perpendicular to the substrates. Therefore, the improvement of the devices made by these nanoparticles was limited. For TiO 2 (20-40 nm spheres) with optimal concentration, external quantum efficiency up to 15% and AM1 power conversion efficiency of 0.42% were obtained. The improvement in the efficiency was related to the improved morphology of the film and was attributed to the formation of percolation paths of TiO 2 for electron conduction.