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.
Subscribe to Journal
Get full journal access for 1 year
only $4.92 per issue
All prices are NET prices.
VAT will be added later in the checkout.
Tax calculation will be finalised during checkout.
Rent or Buy article
Get time limited or full article access on ReadCube.
All prices are NET prices.
Halls, J. J. M. et al. Efficient photodiodes from interpenetrating polymer networks. Nature 376, 498–500 (1995).
Yu, G., Gao, J., Hummelen, J. C., Wudl, F. & Heeger, A. J. Polymer photovoltaic cells—enhanced efficiencies via a network of internal donor–acceptor heterojunctions. Science 270, 1789–1791 (1995).
He, Z. et al. Enhanced power-conversion efficiency in polymer solar cells using an inverted device structure. Nature Photon. 6, 591–595 10.1038/nphoton.2012.190(2012).
You, J. et al. A polymer tandem solar cell with 10.6% power conversion efficiency. Nature Commun. 4, 1446 (2013).
Hoppe, H. & Sariciftci, N. S. Morphology of polymer/fullerene bulk heterojunction solar cells. J. Mater. Chem. 16, 45–61 (2006).
Chen, L.-M., Hong, Z., Li, G. & Yang, Y. Recent progress in polymer solar cells: manipulation of polymer:fullerene morphology and the formation of efficient inverted polymer solar cells. Adv. Mater. 21, 1434–1449 (2009).
Kline, R. J., McGehee, M. D. & Toney, M. F. Highly oriented crystals at the buried interface in polythiophene thin-film transistors. Nature Mater. 5, 222–228 (2006).
Salleo, A., Kline, R. J., DeLongchamp, D. M. & Chabinyc, M. L. Microstructural characterization and charge transport in thin films of conjugated polymers. Adv. Mater. 22, 3812–3838 (2010).
Rivnay, J., Mannsfeld, S. C. B., Miller, C. E., Salleo, A. & Toney, M. F. Quantitative determination of organic semiconductor microstructure from the molecular to device scale. Chem. Rev. 112, 5488–5519 (2012).
Chabinyc, M. L. X-ray scattering from films of semiconducting polymers. Polym. Rev. 48, 463–492 (2008).
Graetzel, M., Janssen, R. A. J., Mitzi, D. B. & Sargent, E. H. Materials interface engineering for solution-processed photovoltaics. Nature 488, 304–312 (2012).
Collins, B. A. et al. Polarized X-ray scattering reveals non-crystalline orientational ordering in organic films. Nature Mater. 11, 536–543 (2012).
Brabec, C. J., Heeney, M., McCulloch, I. & Nelson, J. Influence of blend microstructure on bulk heterojunction organic photovoltaic performance. Chem. Soc. Rev. 40, 1185–1199 (2011).
Ma, W. et al. Domain purity, miscibility, and molecular orientation at donor/acceptor interfaces in high performance organic solar cells: paths to further improvement. Adv. Energy Mater. 3, 864–872 (2013).
Verlaak, S. et al. Electronic structure and geminate pair energetics at organic–organic interfaces: the case of pentacene/C60 heterojunctions. Adv. Funct. Mater. 19, 3809–3814 (2009).
Rand, B. P. et al. The impact of molecular orientation on the photovoltaic properties of a phthalocyanine/fullerene heterojunction. Adv. Funct. Mater. 22, 2987–2995 (2012).
Ojala, A. et al. Merocyanine/C60 planar heterojunction solar cells: effect of dye orientation on exciton dissociation and solar cell performance. Adv. Funct. Mater. 22, 86–96 (2012).
Yang, L., Tumbleston, J. R., Zhou, H., Ade, H. & You, W. Disentangling the impact of side chains and fluorine substituents of conjugated donor polymers on the performance of photovoltaic blends. Energy Environ. Sci. 6, 316–326 (2013).
Price, S. C., Stuart, A. C., Yang, L., Zhou, H. & You, W. Fluorine substituted conjugated polymer of medium band gap yields 7% efficiency in polymer–fullerene solar cells. J. Am. Chem. Soc. 133, 4625–4631 (2011).
Zhou, H. et al. Development of fluorinated nenzothiadiazole as a structural unit for a polymer solar cell of 7% efficiency. Angew. Chem. Int. Ed. 50, 2995–2998 (2011).
Albrecht, S. et al. Fluorinated copolymer PCPDTBT with enhanced open-circuit voltage and reduced recombination for highly efficient polymer solar cells. J. Am. Chem. Soc. 134, 14932–14944 (2012).
Gann, E. et al. Soft X-ray scattering facility at the advanced light source with real-time data processing and analysis. Rev. Sci. Instrum. 83, 045110 (2012).
Collins, B. A. et al. Absolute measurement of domain composition and nanoscale size distribution explains performance in PTB7:PC71BM solar cells. Adv. Energy Mater. 3, 65–74 (2013).
Szarko, J. M. et al. When function follows form: effects of donor copolymer side chains on film morphology and BHJ solar cell performance. Adv. Mater. 22, 5468–5472 (2010).
Lyons, B. P., Clarke, N. & Groves, C. The relative importance of domain size, domain purity and domain interfaces to the performance of bulk-heterojunction organic photovoltaics. Energy Environ. Sci. 5, 7657–7663 (2012).
Stribeck, N. X-Ray Scattering of Soft Matter (Springer, 2007).
Shuttle, C. G., Hamilton, R., O'Regan, B. C., Nelson, J. & Durrant, J. R. Charge-density-based analysis of the current–voltage response of polythiophene/fullerene photovoltaic devices. Proc. Natl Acad. Sci. USA 107, 16448–16452 (2010).
Chen, W. et al. Molecular orientation dependent energy level alignment at organic−organic heterojunction interfaces. J. Phys. Chem. C 113, 12832–12839 (2009).
Beljonne, D. et al. Electronic processes at organic–organic interfaces: insight from modeling and implications for opto-electronic devices. Chem. Mater. 23, 591–609 (2010).
Vandewal, K. et al. Efficient charge generation by relaxed charge-transfer states at organic interfaces. Nature Mater. 13, 63–68 (2014).
Albrecht, S. et al. On the efficiency of charge transfer state splitting in polymer:fullerene solar cells. Adv. Mater. http://dx.doi.org/10.1002/adma.201305283 (2014).
Gélinas, S. et al. Ultrafast long-range charge separation in organic semiconductor photovoltaic diodes. Science 343, 512–516 (2014).
Jailaubekov, A. E. et al. Hot charge-transfer excitons set the time limit for charge separation at donor/acceptor interfaces in organic photovoltaics. Nature Mater. 12, 66–73 (2013).
Hexemer, A. et al. A SAXS/WAXS/GISAXS beamline with multilayer monochromator. J. Phys. Conf. Ser. 247, 012007 (2010).
Kilcoyne, A. L. D. et al. Interferometer-controlled scanning transmission X-ray microscopes at the Advanced Light Source. J. Synchrotron Radiat. 10, 125–136 (2003).
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 22.214.171.124, A. Hexemer and S. Alvarez at beamline 7.3.3 and C. Wang and A. Young at beamline 126.96.36.199 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.
About this article
Cite this article
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
Pure and Applied Chemistry (2021)
Binary Additive–Induced Performance Improvement of PM7:PC 71 BM Organic Solar Cells with High Open‐Circuit Voltage and Enhanced Current Intensity
Energy Technology (2021)
Advanced Energy Materials (2021)
The coupling and competition of crystallization and phase separation, correlating thermodynamics and kinetics in OPV morphology and performances
Nature Communications (2021)
Molecular orientation, anisotropic electron transport and photovoltaic properties of ladder-type heteroheptacene-based semiconductors
Chemical Engineering Journal (2021)