Improving the Efficiency of Polymer:Fullerene Bulk Heterojunction Solar Cells by Varying the Material Concentration in the Photoactive Layer

Abstract

Planar structure organic solar cells (ITO/PEDOT:PSS/P3HT:PCBM/Al) were studied to determine the optimal concentration of polymer and fullerene in the photoactive layer blend. The bulk heterojunction of these devices consisted of the electron donating polymer poly-3(hexylthiophene-2,5-diyl) (P3HT), and the electron accepting fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM). Four unique photoactive layer solutions were prepared by dissolving various amounts of the organics in 2 mL chlorobenzene, while keeping the weight ratio of P3HT:PC61BM at 1:0.8. The fabricated solar cells were characterized electrically to determine power conversion efficiency, fill factor, and current density. Subsequent characterization techniques including atomic force microscopy, ultraviolet-visible spectrophotometry, and variable angle spectroscopic ellipsometry were performed on P3HT:PC61BM films to determine surface roughness, optical absorption, and extinction coefficient. Through this systematic study, it was determined that an appropriate selection of P3HT:PC61BM concentration allows for significant increases in the overall device performance of organic solar cells.