New photovoltaic technologies based on organic and inorganic/organic bulk heterojunctions are commercially attractive because modules can be manufactured using low-cost and high volume roll-to-roll methods. The advantage of simple processing comes at the cost of control; most bulk heterojunction cells are made by applying a simple mixture of n- and p-type semiconductors, which are expected to phase segregate into nanoscale domains. The size, shape, and connectivity of these domains-as well as the microstructure within them-control the extent of charge separation and harvesting, and suboptimal morphology is typically blamed for failure to approach target efficiencies. This opportunity involves developing measurements to evaluate the buried architecture of bulk heterojunction photovoltaic films. Small angle scattering techniques are particularly well suited to this purpose. NIST has recently developed a unique small angle x-ray scattering (SAXS) tool to measure the shape, size, and location of domains in heterogeneous systems spanning length scales from microns to Angstroms. Grazing-incidence SAXS measurements will allow the evaluation of in-plane structure, while transmission will allow the evaluation of through-plane structure. Simultaneous x-ray diffraction at wide angles may provide complementary information about grain size and crystallinity within domains. The SAXS capability will be complemented by small angle neutron scattering, which provides a means to enhance scattering contrast in soft domains by selective deuteration. Bulk heterojunction architecture will be compared to cell performance to establish fundamental correlations that will guide the development and fabrication of new photovoltaic technologies.

 

key words

Microstructure; Morphology; Nanostructure; Organic electronics; Photovoltaic; Polymer; Scattering; Semiconductor;

Citizenship:  Open to U.S. citizens

Level:  Open to Postdoctoral applicants