In bulk heterojunction organic photovoltaics, electron-donating and electron-accepting materials form a distributed network of heterointerfaces in the photoactive layer, where critical photo-physical processes occur. However, little is known about the structural properties of these interfaces due to their complex three-dimensional arrangement and the lack of techniques to measure local order. Here, we report that molecular orientation relative to donor/acceptor heterojunctions is an important parameter in realizing high-performance fullerene-based, bulk heterojunction solar cells. Using resonant soft X-ray scattering, we characterize the degree of molecular orientation, an order parameter that describes face-on (+1) or edge-on (−1) orientations relative to these heterointerfaces. By manipulating the degree of molecular orientation through the choice of molecular chemistry and the characteristics of the processing solvent, we are able to show the importance of this structural parameter on the performance of bulk heterojunction organic photovoltaic devices featuring the electron-donating polymers PNDT–DTBT, PBnDT–DTBT or PBnDT–TAZ.
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The authors acknowledge the following support for this collaborative research. Characterization and analysis by J.R.T., B.A.C., E.G., W.M. and H.A. was supported by the US Department of Energy (DOE), Office of Science, Basic Energy Science, Division of Materials Science and Engineering under contract DE-FG02-98ER45737. X-ray data were acquired at the ALS, which is supported by the DOE (DE-AC02-05CH1123). W.Y., L.Y. and A.C.S. are supported by the Office of Naval Research (N000141110235) and an NSF CAREER award (DMR-0954280). W.Y. is a Camille Dreyfus Teacher–Scholar. The authors thank D. Kilcoyne at ALS beamline 18.104.22.168, A. Hexemer and S. Alvarez at beamline 7.3.3 and C. Wang and A. Young at beamline 22.214.171.124 for assistance with data acquisition and helpful discussions. F. Liu is thanked for TEM measurements and useful discussions. Insightful discussions with M. Toney (SSRL) are also acknowledged.
The authors declare no competing financial interests.
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Tumbleston, J., Collins, B., Yang, L. et al. The influence of molecular orientation on organic bulk heterojunction solar cells. Nature Photon 8, 385–391 (2014). https://doi.org/10.1038/nphoton.2014.55
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