Observation of Elongated Nano Domains in Organic Photovoltaic Active Layers with Electric Field Treatment using Cross-Sectional Scanning Tunneling Microscopy and Spectroscopy.*

The effects of the electric-field-assisted annealing on the bulk heterojunction nano-morphology in the P3HT/PCBM active layer of the organic photovoltaic cells (OPVCs) are presented here. It was widely accepted that the electric field will facilitate the P3HT, the polar polymer, to be better crystallized hence enhance the charge mobility and improve the OPVC performance. However, the effects on the nano-morphology in the active layer treated with the electric field are not well understood. Here, by utilizing the cross-sectional scanning tunneling microscopy and spectroscopy (XSTM/S), it is revealed that the electric-field-assisted annealing will facilitate the P3HT-rich domains formed elongated shape with the orientation close to the direction of the external electric field which is beneficial to the OPVC charge collection. On the other hand, it was also observed that the electric-field-assisted annealed samples showed smaller energy gaps and smaller energy off-set between the molecular domains which might negatively impact the charge separation efficiency. The XRD and SAXS results indicate that the smaller energy gaps might be caused by the molecular intermixing. These results point out competing factors affecting the OPVC performance.