Solar cell efficiency, self-assembly and dipole-dipole interactions of isomorphic narrow bandgap

1. Solar cell efficiency, self-assembly and dipole-dipole interactions of isomorphic narrow bandgap molecules Alan J. Heeger, University of California-Santa Barbara, DMR 08566060 We have examined the correlations of the dipole moment and conformational stability to the self-assembly and solar cell performance within a series of isomorphic, solution processable molecules. These charge transfer chromophores are described by a D1-A-D-A-D1 structure comprising electron rich 2-hexylbithiophene and 3,3’-di-2-ethylhexylsilylene-2,2’-bithiophene (DTS) moieties as the D1 and D units, respectively. The molecular structures are shown below. Using a combination of UV-visible spectroscopy, field effect transistors, solar cell devices, grazing incident wide angle X-ray scattering, and high-resolution transmission electron microscopy, three PT-containing compounds (1-3) with varying regiochemistry and symmetry, together with the BT-based compound 5,5’-bis{(4-(7-hexylthiophen-2-yl)thiophen-2-yl)-[1,2,5]thiadiazolobenzene}-3,3’-di-2-ethylhexylsilylene-2,2’-bithiophene (4) are compared and contrasted in solution, thin-films and as blends with the electron acceptor [6,6]-phenyl-C70-butyric acid methyl ester (PC70BM). Molecules with symmetric orientations of the PT acceptor, compounds 1 and 2, yield highly ordered thin films. The best films, processed with the solvent additive DIO show donor “crystallite” length scales on the order of 15-35 nm and photovoltaic power conversion efficiencies (PCEs) of 7.0% and 5.6%, respectively. Compound 3, with an unsymmetrical orientation of PT heterocycles, shows subtle differences in the crystallization behavior and a best PCE of 3.2%. In contrast, blends of the BT containing donor (4) are highly disordered and give PCEs below 0.2%. We speculate that the differences in self-assembly arise from the strong influence of the BT acceptor and its orientation on the net dipole moment and geometric description of the chromophore. HR-TEM Imaging carried out by C.J. Takacs, graduate student in Physics supported by NSF-DMR-0856060. TEM facilities were supported by the NSF- MRSEC (DMR05-20415). Manuscript submitted to J. Amer. Chem. Soc. for publication. HR-TEM images of BHJ films processed with 0.25% v/v DIO: (A) 1, (B) 2, (C) 3, and (D) 4 all blends with PC70BM (70:30 wt/wt). Lattice planes of ~2 nm are clearly resolvable in all films except for 4 and have been overlaid with colored lines to aid the reader. Of the four blends investigated, 4:PC70BM films exhibits the lowest degree of order. The decreased amount of ordering is evident the complete absence of crystalline material in the HR-TEM image (D). The lack of crystallinity and, presumably, phase separation within these thin films helps explain the low device performance. These images demonstrate that the acceptor component of the net dipole moment of the molecule is a strong contributor to the significant differences in the self-assembly of the BT- and PT-based compounds.

2. Solar cell efficiency, self-assembly and dipole-dipole interactions of isomorphic narrow bandgap Alan J. Heeger, University of California-Santa Barbara, DMR 08566060 Education This study of small molecule 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 analysis 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 role of dipole-dipole interactions in the self-assembly of the BT- and PT-based compounds (with a focus on the HR-TEM imaging of the crystallites which characterize the morphology) has been submitted to JACS for publication. • Professor Heeger regularly gives invited (Keynote and Plenary) lectures at conferences in the United States, Europe and Asia.