Investigating the Morphology of Polymer/Fullerene Layers Coated Using Orthogonal Solvents

P3HT/PCBM bilayer samples were fabricated by spin coating PCBM dissolved in CH2Cl2 onto P3HT films. Dissolution of the P3HT does not occur because CH2Cl2 is a nonsolvent for P3HT. We show using steady-state spectroscopy, neutron reflectometry, and current-voltage measurements that substantial mixing occurs between the P3HT and the PCBM during the PCBM deposition. Penetration of the PCBM into the P3HT layer is assisted by the swelling of the P3HT by the dichloromethane solvent. We conclude that the PCBM mixes with amorphous P3HT and does not disrupt the existing crystalline domains. A PCBM loading of 25-30 wt % into the P3HT layer was determined using fluorescence quenching measurements and neutron reflectometry. This amount of mixing explains why reported photovoltaic performances of these solution-processed bilayer structures are comparable to that of bulk heterojunctions.