(579e) Bulk Heterojunction Polymer Solar Cells: Nanomorphology and Photovoltaic Properties

Much progress has been made in the development of polymer solar cells as low cost approaches to solar energy utilization. Bulk heterojunction (BHJ) polymer solar cells, which are composed of binary blends of a donor polymer semiconductor and an acceptor material such as fullerene, have emerged as the most efficient to date with power conversion efficiencies now approaching 8-10%. Further advances will depend on understanding and controlling the nanoscale morphology of the BHJ blend thin films. Towards this goal we have systematically investigated the nanoscale morphology of several BHJ polymer systems. In one example, we have found that the power conversion efficiency of a phthalimide-copolymer/fullerene BHJ solar cell system is improved by a factor of 10 by using a processing additive. The underlying mechanism for the 10-fold enhancement in photovoltaic efficiency is found to be inhibition of fullerene intercalation into the polymer side chains and regulation of the relative crystallization/aggregation rates of the polymer and fullerene. An optimal interconnected two-phase morphology with 15–20 nm domains is obtained when a processing additive is used compared with 100–300 nm domains without the additive. To overcome the limitations of current methods for characterizing the morphology of bulk heterojunction (BHJ) polymer solar cells, we have exploited energy dispersive X-ray spectroscopy (EDS) mapping and scanning transmission electron microscopy to investigate the nanoscale phase separation and distribution in all-polymer BHJ solar cells. The characteristic elements in the respective donor and acceptor polymers provided high Z-contrast and enabled EDS mapping of the phase-separated morphology of the BHJ devices. Our results demonstrate that the nanoscale morphology of BHJ polymer solar cells is critical to device performance and have also provided a new approach for imaging the BHJ photoactive layer with high spatial resolution and chemical specificity.