Effects of thin film processing on pentacene/C60 bilayer solar cell performance

Solar cells based on pentacene/C-60 bilayer heterojunctions have been fabricated with a structure of ITO/poly(styrenesulfonate) (PEDOT:PSS)/pentacene (40 nm)/fullerene (C-60)(40 nm)/2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) (10 nm)/Al. The effect of pentacene crystalline domain size on performance was investigated by controlling the pentacene deposition rate. The devices show improved light- to -electricity conversion efficiencies from 0.49% to 1.12% under an AM 1.5 solar simulator (100 mW/cm(2)), when the pentacene evaporation rate is in a range of 5 angstrom/s-0.5 angstrom/s. Atomic force microscopy (AFM) measurements show that the pentacene films deposited by a slow evaporation rate have larger crystalline domains and a fewer amorphous domains, compared to films obtained by faster evaporation rates. Upon thermal annealing at 200 degrees C for 1 min, there is merging of pentacene crystalline domains. These changes in film morphology impact the charge separation at the donor/acceptor interface and the hole and electron mobilities, and hence, directly affect the device performance. (C) 2007 Elsevier B.V. All rights reserved.