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Hybrid organic–inorganic inks flatten the energy landscape in colloidal quantum dot solids

Nature Materials volume 16, pages 258–263 (2017)Cite this article

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Abstract

Bandtail states in disordered semiconductor materials result in losses in open-circuit voltage (Voc) and inhibit carrier transport in photovoltaics. For colloidal quantum dot (CQD) films that promise low-cost, large-area, air-stable photovoltaics, bandtails are determined by CQD synthetic polydispersity and inhomogeneous aggregation during the ligand-exchange process. Here we introduce a new method for the synthesis of solution-phase ligand-exchanged CQD inks that enable a flat energy landscape and an advantageously high packing density. In the solid state, these materials exhibit a sharper bandtail and reduced energy funnelling compared with the previous best CQD thin films for photovoltaics. Consequently, we demonstrate solar cells with higher Voc and more efficient charge injection into the electron acceptor, allowing the use of a closer-to-optimum bandgap to absorb more light. These enable the fabrication of CQD solar cells made via a solution-phase ligand exchange, with a certified power conversion efficiency of 11.28%. The devices are stable when stored in air, unencapsulated, for over 1,000 h.

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Figure 1: Solution-phase ligand exchange with metal halide precursors and ammonium acetate.
Figure 2: PbS CQDs exchanged by lead halide with the aid of ammonium acetate suggest improved CQD packing density and sharper bandtail.
Figure 3: Energy funnelling in the exchanged CQD films.
Figure 4: The effect of flat energy landscape on CQD solar cell performance.
Figure 5: Certified solar cell performance.

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Acknowledgements

This publication is based in part on work supported by Award KUS-11-009-21, made by King Abdullah University of Science and Technology (KAUST), by the Ontario Research Fund Research Excellence Program, and by the Natural Sciences and Engineering Research Council (NSERC) of Canada. F.P.G.d.A. acknowledges financial support from the Connaught fund. A.H.B. and F.L. thank K. Vandewal for his contribution to the photothermal deflection spectroscopy set-up and M. Baier for help with the experiments. The authors thank E. Palmiano, L. Levina, R. Wolowiec, D. Kopilovic, G. Kim and F. Fan for their help during the course of study.

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Authors and Affiliations

  1. Department of Electrical and Computer Engineering, University of Toronto, 10 King’s College Road, Toronto, Ontario M5S 3G4, Canada

    Mengxia Liu, Oleksandr Voznyy, Randy Sabatini, F. Pelayo García de Arquer, Xinzheng Lan, Fengjia Fan, Grant Walters, Sjoerd Hoogland & Edward H. Sargent

  2. and Physical Sciences and Engineering Division, King Abdullah University of Science and Technology (KAUST), KAUST Solar Center (KSC), 4700 KAUST, Thuwal 23955-6900, Saudi Arabia

    Rahim Munir, Ahmed Hesham Balawi, Ahmad R. Kirmani, Frédéric Laquai & Aram Amassian

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  1. Mengxia Liu
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Contributions

M.L. conceived the idea and contributed to most experimental work. O.V., S.H. and E.H.S. supervised the project. O.V. carried out XPS measurements. R.S. performed transient absorption spectroscopy measurements. F.P.G.d.A. assisted in EQE measurements. R.M., A.R.K. and A.A. performed GISAXS measurements. A.H.B. and F.L. performed photothermal deflection spectroscopy measurements. X.L. assisted in device fabrication. F.F. performed TEM measurements. G.W. carried out PL studies. M.L., O.V. and E.H.S. wrote the manuscript. All the authors provided comments on the text.

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Correspondence to Edward H. Sargent.

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Liu, M., Voznyy, O., Sabatini, R. et al. Hybrid organic–inorganic inks flatten the energy landscape in colloidal quantum dot solids. Nature Mater 16, 258–263 (2017). https://doi.org/10.1038/nmat4800

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