Solution-Processed Small-Molecule Solar Cells with 6.7% Efficiency

1. Direct Imaging of Crystallite Morphology in High Efficiency Organic Solar CellsAlan J. Heeger, University of California-Santa Barbara, DMR 0856060 Solution-Processed Small-Molecule Solar Cells with 6.7% Efficiency We have recently demonstrated efficient small molecule bulk heterojunction solar cells based on DTS(PTTh2)2. Organic solar cells fabricated from DTS(PTTh2)2/PC70BM blend solutions (composition 70:30 w/w) show power conversion efficiencies of 6.7 % with short circuit current (Jsc) of 14.4 mA cm–2, open circuit voltage (Voc) of 0.78 V, fill factor (FF) of 59.3% and PCE = 6.7% under AM 1.5 G irradiation (100 mW cm–2). These are the highest reported values for solution-processed small-molecule solar cells. The molecular structures of DTS(PTTh2)2 (left) and PC70BM (right) are shown in the Figure below. NSF funding enabled us to study the morphology of the bulk heterojunction material at the molecular scale. The top figure on the right shows a real space image of the crystallites of DTS(PTTh2)2. Phase contrast electron microscopy (TEM) was used to investigate the nano-morphology. We focused on the strong in-plane stacking displayed by the DTS(PTTh2)2 phase at 0.31 Å-1, corresponding to a periodicity of approximately 2 nm.We speculate that this d-spacing is due to stacking of the DTS(PTTh2)2 long axis parallel to the substrate. As shown on the top figure on the right, with proper microscope defocus, the lattice planes of the DTS(PTTh2)2 crystallites can be directly imaged. Note that the crystallites are randomly oriented within the plane. To convey the differences in the size distribution, a false color map encoding the size and dominant local orientation of the crystalline regions is shown in the bottom figure on the right; different colors imply different crystallite orientation. Real space image of the crystallites of DTS(PTTh2)2 within an area of 170 nm by 170 nm False color map encoding the size and dominant local orientation of the crystalline regions (650 nm by 650 nm) TEM Imaging carried out by C.J. Takacs, graduate student in Physics

2. Direct Imaging of Crystallite Morphology in High Efficiency Organic Solar CellsAlan J. Heeger, University of California-Santa Barbara, DMR 0856060 Education The study of organic solar cells is a classic example of interdisciplinary science involving a combination of physics, chemistry, materials science and device science. The materials were synthesized by our colleagues in the Chemistry Department (under the direction of Prof. G. Bazan). The device fabrication, optimization and measurements were carried out by post-doctoral researchers in the Heeger group. The specific methodology for phase contrast electron microscopy (TEM) used to investigate the nano-morphology of the solution processed small molecule solar cells is novel and was developed by C. J. Takacs, a physics graduate student. He used defocused Transmission Electron Microscopy (Phase Contrast TEM) in order to obtain the contrast necessary to display the image shown above. The analysis of the local crystallinity (and separating these regions from the surrounding amorphous material) required the creation of a novel algorithm for displaying the false color figure. • Outreach • Organic solar cells fabricated by low-cost printing technology continue to be of interest. • They offer unique semi-transparent PV modules for building integrated products (for example. windows, Greenhouses, etc); • They enable thin, flexible, light-weight and rugged products; • They can be manufactured by high throughput roll to roll processes. • They have a low carbon footprint compared to inorganic solar cells. • The manuscript describing the solution processed small molecule solar cells with 6.7% efficiency (with a focus on the phase contrast imaging of the crystallites which characterize the morphology) has been submitted for publication. • Professor Heeger regularly gives invited (Keynote and Plenary) lectures at conferences in the United States, Europe and Asia.