Elongated Nanodomains and Molecular Intermixing Induced Doping in Organic Photovoltaic Active Layers with Electric Field Treatment

The effects of the electric-field-assisted annealing on the bulk heterojunction nanomorphology in the P3HT/PCBM active layer of the organic photovoltaic cells (OPVCs) are presented here. It was widely accepted that the electric-field-assisted annealing will facilitate the P3HT, the polar polymer, to be better crystalline to enhance the charge mobility and hence the improvement of the OPVC performance. The influences of the electron donor and acceptor domains on the nanomorphology are not well understood. Here, using the cross-sectional scanning tunneling microscopy and spectroscopy (XSTM/S), the electric-field-assisted annealing treatment is found to influence the molecular domains to be elongated along the direction of the external electric field. The elongation of the molecular domains is believed to facilitate the domain percolation, which causes higher charge mobility and hence the higher short-circuit current density (Jsc). However, it was also observed that the electronic properties of the P3HT-rich and PCBM-rich domains in the electric-field-assisted annealed samples showed smaller energy band gaps and a smaller molecular orbital offset between the two domains, which is believed to decrease the open circuit voltage (Voc) and negatively impact the OPVC performance. On the basis of the X-ray diffraction (XRD) and small-angle X-ray scattering (SAXS) results, the altered electronic properties are argued to be due to the molecular-intermixing-induced doping effects. These results point out competing factors affecting the OPVC performance with the electric-field-assisted annealing treatment.