Nanostructured organic pn junctions towards 3D photovoltaics

The working principle of so-called organic bulk heterojunction solar cells prepared with blends of poly(2-methoxy-5-(3’,7’-dimethyl-octyloxy))-p-phenylene vinylene (MDMO-PPV), acting as an electron donor, and (6,6)-phenyl-C61-butyric-acid methyl ester (PCBM) (a soluble C60 derivative), acting as electron acceptor, is based on the presence of three-dimensional nanostructured pn junctions and percolation paths for charge transport. At high PCBM contents, spontaneous phase separation occurs giving rise to PCBM-rich spherical/ellipsoidal regions (electron transport) embedded in a MDMO-PPV-rich matrix (hole transport). With transmission electron microscopy and scanning probe microscopy techniques it has been demonstrated that the size of the PCBM-rich region depends strongly on the preparation conditions such as solvents and drying conditions. The morphology of the active films in high-performance bulk heterojunction solar cells is characterized by a significantly higher number and a smaller size (nanoscale) of the PCBM-rich regions than for the low-performance cells. This morphology yields both an increase of the useful photoactive volume and an increase of the percolation paths for charge transport. Towards mature and high-performance organic-based three-dimensional photovoltaics, it is clear that besides mastering the electro-optical properties of the constituting materials it also of key importance to control the nanomorphology of the solid-state blends in order to obtain efficient interpenetrating pn networks.