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Hybrid passivated colloidal quantum dot solids

Nature Nanotechnology volume 7, pages 577–582 (2012)Cite this article

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Abstract

Colloidal quantum dot (CQD) films allow large-area solution processing and bandgap tuning through the quantum size effect1,2,3,4,5,6. However, the high ratio of surface area to volume makes CQD films prone to high trap state densities if surfaces are imperfectly passivated, promoting recombination of charge carriers that is detrimental to device performance7. Recent advances have replaced the long insulating ligands that enable colloidal stability following synthesis with shorter organic linkers or halide anions8,9,10,11,12, leading to improved passivation and higher packing densities. Although this substitution has been performed using solid-state ligand exchange, a solution-based approach is preferable because it enables increased control over the balance of charges on the surface of the quantum dot, which is essential for eliminating midgap trap states13,14. Furthermore, the solution-based approach leverages recent progress in metal:chalcogen chemistry in the liquid phase15,16,17,18,19. Here, we quantify the density of midgap trap states20,21,22 in CQD solids and show that the performance of CQD-based photovoltaics is now limited by electron–hole recombination due to these states. Next, using density functional theory and optoelectronic device modelling, we show that to improve this performance it is essential to bind a suitable ligand to each potential trap site on the surface of the quantum dot. We then develop a robust hybrid passivation scheme that involves introducing halide anions during the end stages of the synthesis process, which can passivate trap sites that are inaccessible to much larger organic ligands. An organic crosslinking strategy is then used to form the film. Finally, we use our hybrid passivated CQD solid to fabricate a solar cell with a certified efficiency of 7.0%, which is a record for a CQD photovoltaic device.

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Figure 1: Traps in colloidal quantum dot films.
Figure 2: Performance of CQD photovoltaics as a function of passivation.
Figure 3: Effect of matrix on film density.
Figure 4: Passivation of majority carrier traps via the hybrid method.

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Acknowledgements

This publication is based in part on work supported by an award (KUS-11-009-21) from the 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. The authors thank Angstrom Engineering and Innovative Technology for useful discussions regarding material deposition methods and control of the glovebox environment, respectively. The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the US Department of Energy (contract no. DE-AC02-05CH11231). The authors thank L. Goncharova, M. T. Greiner, E. Palmiano, R. Wolowiec and D. Kopilovic for their help during the course of the study. A.H.I. acknowledges support from the Queen Elizabeth II Graduate Scholarship in Science and Technology. D.Z. acknowledges support from the NSERC CGS D scholarship.

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  1. Alexander H. Ip and Susanna M. Thon: These authors contributed equally to this work

Authors and Affiliations

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

    Alexander H. Ip, Susanna M. Thon, Sjoerd Hoogland, Oleksandr Voznyy, David Zhitomirsky, Ratan Debnath, Larissa Levina, Lisa R. Rollny, Graham H. Carey, Armin Fischer, Kyle W. Kemp, Illan J. Kramer, Zhijun Ning, André J. Labelle & Edward H. Sargent

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

    Kang Wei Chou & Aram Amassian

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Contributions

A.H.I., S.M.T. and E.H.S. designed and directed this study and analysed the experimental results. A.H.I. and S.M.T. contributed to all the experimental work. S.H. and D.Z. carried out the photovoltage transient measurements. S.H. performed PLQE experiments. O.V. performed DFT, RBS and XPS analyses. D.Z. performed the optoelectronic simulations. R.D. assisted in conceptualization of the study and assisted in experimental work. L.L. synthesized the CQDs. L.R.R., K.W.C. and A.A. carried out the GISAXS measurements. G.H.C. performed the TOF measurements. A.F. performed XPS and CHN analyses. K.W.K. performed TAS and capacitance–voltage measurements. I.J.K. assisted with EQE measurements. Z.N. developed protocols for the separation of treatment constituents experiment and facilitated ICP AES measurements. A.J.L. assisted with device fabrication and testing. A.H.I., S.M.T. and E.H.S. wrote the manuscript. All authors commented on the paper.

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Correspondence to Edward H. Sargent.

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The authors declare no competing financial interests.

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Ip, A., Thon, S., Hoogland, S. et al. Hybrid passivated colloidal quantum dot solids. Nature Nanotech 7, 577–582 (2012). https://doi.org/10.1038/nnano.2012.127

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