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Electron–hole exchange blockade and memory-less recombination in photoexcited films of colloidal quantum dots

Nature Physics volume 13, pages 604–610 (2017)Cite this article

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Abstract

Understanding charge transport and recombination dynamics in photoexcited colloidal quantum dot (QD) solids is key to their applications in optoelectronic devices. Towards this end, we conduct transient photocurrent studies of films of electronically coupled, device-grade PbSe QD films. We observe that the photocurrent amplitude detected following excitation with a short 100 fs pulse is virtually temperature independent down to 6 K, suggesting a tunnelling mechanism of early-time photoconductance. The later-time signal exhibits clear signatures of thermal activation with characteristic energies that are surprisingly robust and independent of the exact type of QD surface treatment. We attribute this behaviour to the involvement of intrinsic fine-structure states and specifically the electron–hole exchange interaction, which creates an energetic barrier to electron–hole separation between adjacent QDs. At room temperature, which is well above the largest activation energy, relaxation of photoconductivity is dominated by non-geminate recombination involving mobile band-edge carriers of one sign and low-mobility carriers of the opposite sign (pre-existing and photoexcited) residing in intragap states. This process leads to memory-less dynamics when the photocurrent relaxation time is directly linked to the instantaneous carrier density.

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Figure 1: Charge carrier dynamics in quantum dot films probed by transient photocurrent spectroscopy.
Figure 2: Room-temperature ‘memory-less’ non-geminate decay in QD films.
Figure 3: Non-geminate recombination dynamics and the effect of QD surface treatment on recombination parameters.
Figure 4: Temperature dependence of transient photocurrent reveals exchange blockade.

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Acknowledgements

J.G. and V.I.K. acknowledge the support of the Centre for Advanced Solar Photophysics (CASP), an Energy Frontier Research Centre funded by the US Department of Energy, Office of Science, Office of Basic Energy Sciences. A.F.F. is a CASP member supported by a LANL Director’s Postdoctoral Fellowship. We would like to thank our CASP collaborator A. Marshall for providing samples of MPA-treated PbSe QDs.

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Authors and Affiliations

  1. Chemistry Division, Center for Advanced Solar Photophysics, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA

    Andrew F. Fidler, Jianbo Gao & Victor I. Klimov

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  1. Andrew F. Fidler
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  2. Jianbo Gao
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  3. Victor I. Klimov
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Contributions

J.G. fabricated the devices. J.G. and A.F.F. performed the experiments. A.F.F. and V.I.K. analysed the data and developed the models. A.F.F. and V.I.K. wrote the manuscript.

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Correspondence to Victor I. Klimov.

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The authors declare no competing financial interests.

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Fidler, A., Gao, J. & Klimov, V. Electron–hole exchange blockade and memory-less recombination in photoexcited films of colloidal quantum dots. Nature Phys 13, 604–610 (2017). https://doi.org/10.1038/nphys4073

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