X-ray reflectivity and topography of the solvent-treated P3HT:PCBM thin films

P3HT:PCBM (poly(3-hexylthiophene):[6,6]-phenyl C₆₁ butyric acid methyl ester) thin films are widely used as an active layer in organic solar cells. The influence of solvent (1,8-diiodooctane) was studied on electron density and topography of thin spin-coated P3HT:PCBM films. The structural properties were studied by grazing incidence X-ray diffraction technique. It was found that the crystallinity of the investigated films was significantly increased after solvent treatment. Using X-ray reflectivity (XRR) analysis, data from P3HT:PCBM/air and P3HT:PCBM/glass interfaces were collected and compared with a theoretical model. The electron density, film thickness, and interface roughness were determined with a few angstroms accuracy by simulating the XRR curves. The XRR data were then analyzed using a scheme based on the distorted-wave Born approximation. It was found that the addition of diiodooctane did not change the absorption coefficient but changed electron density and roughness. The XRR simulation results were in good agreement with the experimental results obtained from AFM measurements.