New Insight into Morphology of High Performance BHJ Photovoltaics Using High Resolution AFM

Direct imaging of the bulk BHJ thin film morphology in OPV is essential to understand device function and optimize efficiency. While transmission electron tomography provides a 3D, real-space image of the morphology, quantifying the structure is not possible. Here we used high-resolution atomic force microscopy in the tapping and nano-mechanical modes to investigate the BHJ active layer morphology which, when combined with Ar ion etching, provided unique insights with unparalleled spatial resolution. PCBM was seen to form a network that interpenetrated into the fibrillar network of the hole-conducting polymer, both being imbedded in a mixture of the two components. The free surface was found to be enriched with polymer crystals having a face-on orientation, and the morphology at the anode interface was markedly different.