Effect of Aluminum Deposition and Annealing on Polymer-Based Solar Cell Performance

Previously we characterized the active layer in polymer-based solar cells containing Poly(3-hexylthiophene) with the electron acceptor Phenyl-C-61-butyric acid methyl ester (PCBM) to find a thin, pure polymer layer at the air interface just after spin coating. In this study, we find that when the aluminum back electrode was thermally evaporated onto the active layer, at high enough rate, craters were found in the pure polymer layer. This was determined by dissolving the aluminum and characterizing the active layer with an atomic force microscope. Poor device performance was noted under this condition. However, if the aluminum was evaporated at a slower rate, resulting in a flat active layer surface and no crater formation, the efficiency more than doubled. A similar result is found if lithium fluoride (LiF) is deposited before aluminum evaporation and no craters were found even for the higher aluminum evaporation rate. So, it appears that LiF acts as a momentum shield to crater formation allowing superior device performance. If the active layer is annealed before deposition of the back electrode then, regardless of deposition rate, similar device performance is found. Again, in our previous study, it was found that annealing the active layer forced PCBM to the air interface which apparently also acts as a momentum shield. Annealing the device after aluminum deposition produces poorer performance than annealing before deposition. However, these devices have a better fill factor. Examining the active layer shows that it undergoes a buckling transition due to differences in the aluminum and active layers' thermal expansivities which reduces overall contact with the electrodes. However, whatever contact is made, is superior, accounting for the improved fill factor. If the buckling instability could be avoided then this processing procedure may be used in the future to manufacture even better devices than with any of the other annealing procedures. (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 49: 772-780, 2011