Polymer solar cells with enhanced fill factors

Abstract

Recent advances in polymer solar cell (PSC) performance have resulted from compressing the bandgap to enhance the short-circuit current while lowering the highest occupied molecular orbital to increase the open-circuit voltage. Nevertheless, PSC power conversion efficiencies are still constrained by low fill factors, typically below 70%. Here, we report PSCs with exceptionally high fill factors by combining complementary materials design, synthesis, processing and device engineering strategies. The donor polymers, PTPD3T and PBTI3T, when incorporated into inverted bulk-heterojunction PSCs with a PC71BM acceptor, result in PSCs with fill factors of 76–80%. The enhanced performance is attributed to highly ordered, closely packed and properly oriented active-layer microstructures with optimal horizontal phase separation and vertical phase gradation. The result is efficient charge extraction and suppressed bulk and interfacial bimolecular recombination. The high fill factors yield power conversion efficiencies of up to 8.7% from polymers with suboptimal bandgaps, suggesting that efficiencies above 10% should be realizable by bandgap modification.

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Figure 1: Macromolecular building blocks, structures, optical absorption spectra, DSC thermograms and X-ray scattering patterns of the semiconducting polymers.
Figure 2: Device architecture and performance of inverted BHJ polymer solar cells.
Figure 3: GIWAXS data for neat and BHJ blend polymer films.
Figure 4: Morphology of a PBTI3T:PC71BM film yielding optimal solar cell performance.
Figure 5: TPC and TPV analysis of BHJ inverted solar cells fabricated using CF as the solvent with or without DIO as the processing additive.

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Acknowledgements

This research was supported as part of the ANSER Center, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, and Office of Basic Energy Sciences (award no. DE-SC0001059), by Polyera Corporation, and by AFOSR (FA9550-08-1-0331). The authors acknowledge the NSF-MRSEC programme of the Northwestern University Materials Research Science and Engineering Center for characterization facilities (DMR-1121262) and the Institute for Sustainability and Energy at Northwestern (ISEN) for partial funding for equipment. The microscopy work was performed in the EPIC facility of the NUANCE Center at Northwestern University, which is supported by NSF-NSEC, NSF-MRSEC, the Keck Foundation, the State of Illinois and Northwestern University. X.G. acknowledges financial support from an SUSTC start-up fund. Use of the Advanced Photon Source was supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences (contract no. DE-AC02-06CH11357). R.P.O. acknowledges the MICINN of Spain for a Ramòn y Cajal research contract. J.T.L.N. acknowledges financial support from the MICINN (project no. CTQ2012-33733) and the Junta de Andalucía (project no. PO9-4708). D.B.T. is funded by the NSF-IGERT Program.

Author information

X.G. designed and performed materials synthesis and characterization. N.Z. fabricated the solar cell device and characterized the film morphology using XRD, AFM, TEM and XPS. S.J.L. and J.St. performed two-dimensional GIWAXS film characterization. J.W.H. (OFET) and J.Sm. (SCLC) characterized charge carrier mobility. R.P.O. and J.T.L.N. performed DFT calculations. N.Z. and S.L. characterized the film morphology by cross-sectional TEM. N.Z., J.Sm. and D.B.T. carried out transient photovoltage and photocurrent measurements. X.G., N.Z. and T.J.M. prepared the manuscript. All authors discussed the results and commented on the manuscript. L.X.C., R.P.H.C., A.F. and T.J.M. supervised the project.

Correspondence to Lin X. Chen or Robert P. H. Chang or Antonio Facchetti or Tobin J. Marks.

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Guo, X., Zhou, N., Lou, S. et al. Polymer solar cells with enhanced fill factors. Nature Photon 7, 825–833 (2013). https://doi.org/10.1038/nphoton.2013.207

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