Enhancing efficiency in polymer-blend solar cells: Structural insights through scattering

All-polymer solar cells that employ blends of semiconducting polymers are capable of harnessing a greater portion of the incident solar spectrum than singly sensitized devices. However, they invariably show poor performance when compared with small-molecule bulk heterojunction cells. Following our successful approach in adding very small quantities of pristine graphene to the active layer to boost performance in P3HT/PCBM cells, we have recently reported a three-fold enhancement in efficiency of all-polymer (a blend of P3HT and F8BT) photovoltaic devices. These new cells exhibit more balanced transport of electrons and holes, strong dependence of recombination behavior on graphene content, and up to two orders of magnitude increase in mobility, resulting in a peak improvement of over 200% in the short-circuit current. However, active layer morphology must also be considered in designing high performance organic photovoltaic devices: structures deviating from the optimized bulk heterojunction (BHJ) structure are expected to show decreased efficiency. We therefore investigate the blend morphology and its dependence on graphene content via small angle neutron scattering (SANS) and neutron reflectivity (NR). SANS reveals that the structure in the cell devolves to a dispersion of disk-like crystals in an amorphous blend matrix, with crystal volume fraction being influenced by filler content. Graphene addition resulted in increased P3HT crystallinity, larger crystallites and a higher degree of ordering. We extend our methodology to other all-polymer solar cells (for example, those involving P3HT and PCPDTBT), and demonstrate how scattering data can be used to correlate morphological features with efficiency. Results on cell characteristics and recombination mechanisms are also reported, and indicate means of addressing fundamental problems in organic photovoltaic systems.