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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.

References

  1. Kovalenko, M. V. et al. Prospects of nanoscience with nanocrystals. ACS Nano 9, 1012–1057 (2015).

    Article  Google Scholar 

  2. Pietryga, J. I. et al. Spectroscopic and device aspects of nanocrystal quantum dots. Chem. Rev. 116, 10513–10622 (2016).

    Article  Google Scholar 

  3. Colvin, V., Schlamp, M. & Alivisatos, A. P. Light-emitting diodes made from cadmium selenide nanocrystals and a semiconducting polymer. Nature 370, 354–357 (1994).

    Article  ADS  Google Scholar 

  4. Coe, S., Woo, W.-K., Bawendi, M. & Bulovic, V. Electroluminescence from single monolayers of nanocrystals in molecular organic devices. Nature 420, 800–803 (2002).

    Article  ADS  Google Scholar 

  5. Dai, X. et al. Solution-processed, high-performance light-emitting diodes based on quantum dots. Nature 515, 96–99 (2014).

    Article  ADS  Google Scholar 

  6. Gur, I., Fromer, N. A., Geier, M. L. & Alivisatos, A. P. Air-stable all-inorganic nanocrystal solar cells processed from solution. Science 310, 462–465 (2005).

    Article  ADS  Google Scholar 

  7. Semonin, O. E. et al. Peak external photocurrent quantum efficiency exceeding 100% via MEG in a quantum dot solar cell. Science 334, 1530–1533 (2011).

    Article  ADS  Google Scholar 

  8. Sargent, E. H. Colloidal quantum dot solar cells. Nat. Photon. 6, 133–135 (2012).

    Article  ADS  Google Scholar 

  9. Konstantatos, G. et al. Ultrasensitive solution-cast quantum dot photodetectors. Nature 442, 180–183 (2006).

    Article  ADS  Google Scholar 

  10. Pal, B. N. et al. High-sensitivity p–n junction photodiodes based on PbS nanocrystal quantum dots. Adv. Funct. Mater. 22, 1741–1748 (2012).

    Article  Google Scholar 

  11. Harman, T., Taylor, P., Walsh, M. & LaForge, B. Quantum dot superlattice thermoelectric materials and devices. Science 297, 2229–2232 (2002).

    Article  ADS  Google Scholar 

  12. Talapin, D. V. & Murray, C. B. PbSe nanocrystal solids for n- and p-channel thin film field-effect transistors. Science 310, 86–89 (2005).

    Article  ADS  Google Scholar 

  13. Liu, Y. et al. Dependence of carrier mobility on nanocrystal size and ligand length in PbSe nanocrystal solids. Nano Lett. 10, 1960–1969 (2010).

    Article  ADS  Google Scholar 

  14. Choi, J.-H. et al. Bandlike transport in strongly coupled and doped quantum dot solids: a route to high-performance thin-film electronics. Nano Lett. 12, 2631–2638 (2012).

    Article  ADS  Google Scholar 

  15. Lee, J.-S., Kovalenko, M. V., Huang, J., Chung, D. S. & Talapin, D. V. Band-like transport, high electron mobility and high photoconductivity in all-inorganic nanocrystal arrays. Nat. Nanotech. 6, 348–352 (2011).

    Article  ADS  Google Scholar 

  16. Liu, Y. et al. Robust, functional nanocrystal solids by infilling with atomic layer deposition. Nano Lett. 11, 5349–5355 (2011).

    Article  ADS  Google Scholar 

  17. Guyot-Sionnest, P. Electrical transport in colloidal quantum dot films. J. Phys. Chem. Lett. 3, 1169–1175 (2012).

    Article  Google Scholar 

  18. Nagpal, P. & Klimov, V. I. Role of mid-gap states in charge transport and photoconductivity in semiconductor nanocrystal films. Nat. Commun. 2, 486 (2011).

    Article  ADS  Google Scholar 

  19. Bozyigit, D., Volk, S., Yarema, O. & Wood, V. Quantification of deep traps in nanocrystal solids, their electronic properties, and their influence on device behaviour. Nano Lett. 13, 5284–5288 (2013).

    Article  ADS  Google Scholar 

  20. Gao, J. & Johnson, J. C. Charge trapping in bright and dark states of coupled PbS quantum dot films. ACS Nano 6, 3292–3303 (2012).

    Article  Google Scholar 

  21. Hwang, G. W. et al. Identifying and eliminating emissive sub-bandgap states in thin films of PbS nanocrystals. Adv. Mater. 27, 4481–4486 (2015).

    Article  ADS  Google Scholar 

  22. Diaconescu, B., Padilha, L. A., Nagpal, P., Swartzentruber, B. S. & Klimov, V. I. Measurement of electronic states of PbS nanocrystal quantum dots using scanning tunnelling spectroscopy: the role of parity selection rules in optical absorption. Phys. Rev. Lett. 110, 127406 (2013).

    Article  ADS  Google Scholar 

  23. Rath, A. K. et al. Remote trap passivation in colloidal quantum dot bulk nano-heterojunctions and its effect in solution-processed solar cells. Adv. Mater. 26, 4741–4747 (2014).

    Article  Google Scholar 

  24. Yuan, M., Liu, M. & Sargent, E. H. Colloidal quantum dot solids for solution-processed solar cells. Nat. Energy 1, 16016 (2016).

    Article  ADS  Google Scholar 

  25. Kang, I. & Wise, F. W. Electronic structure and optical properties of PbS and PbSe quantum dots. J. Opt. Soc. Am. B 14, 1632–1646 (1997).

    Article  ADS  Google Scholar 

  26. Nirmal, M. et al. Observation of the “dark exciton” in CdSe quantum dots. Phys. Rev. Lett. 75, 3728 (1995).

    Article  ADS  Google Scholar 

  27. An, J. M., Franceschetti, A. & Zunger, A. The excitonic exchange splitting and radiative lifetime in PbSe quantum dots. Nano Lett. 7, 2129–2135 (2007).

    Article  ADS  Google Scholar 

  28. Tischler, J. G. et al. Band-edge excitons in PbSe nanocrystals and nanorods. Phys. Rev. B 82, 245303 (2010).

    Article  ADS  Google Scholar 

  29. Auston, D. H. Impulse-response of photoconductors in transmission-lines. IEEE J. Quantum Electron. 19, 639–648 (1983).

    Article  ADS  Google Scholar 

  30. Gao, J., Fidler, A. F. & Klimov, V. I. Carrier multiplication detected through transient photocurrent in device-grade films of lead selenide quantum dots. Nat. Commun. 6, 8185 (2015).

    Article  ADS  Google Scholar 

  31. Nozik, A. J. Quantum dot solar cells. Physica E 14, 115–120 (2002).

    Article  ADS  Google Scholar 

  32. Schaller, R. D. & Klimov, V. I. High efficiency carrier multiplication in PbSe nanocrystals: implications for solar energy conversion. Phys. Rev. Lett. 92, 186601 (2004).

    Article  ADS  Google Scholar 

  33. Luther, J. M. et al. Multiple exciton generation in films of electronically coupled PbSe quantum dots. Nano Lett. 7, 1779–1784 (2007).

    Article  ADS  Google Scholar 

  34. Smith, C. & Binks, D. Multiple exciton generation in colloidal nanocrystals. Nanomaterials 4, 19–45 (2014).

    Article  ADS  Google Scholar 

  35. Klimov, V. I., Mikhailovsky, A. A., McBranch, D. W., Leatherdale, C. A. & Bawendi, M. G. Quantization of multiparticle Auger rates in semiconductor quantum dots. Science 287, 1011–1013 (2000).

    Article  ADS  Google Scholar 

  36. Robel, I., Gresback, R., Kortshagen, U., Schaller, R. D. & Klimov, V. I. Universal size-dependent trend in Auger recombination in direct-gap and indirect-gap semiconductor nanocrystals. Phys. Rev. Lett. 102, 177404 (2009).

    Article  ADS  Google Scholar 

  37. McGuire, J. A., Sykora, M., Joo, J., Pietryga, J. P. & Klimov, V. I. Apparent versus true carrier multiplication yields in semiconductor nanocrystals. Nano Lett. 10, 2049–2057 (2010).

    Article  ADS  Google Scholar 

  38. Kang, M. S., Lee, J., Norris, D. J. & Frisbie, C. D. High carrier densities achieved at low voltages in ambipolar PbSe nanocrystal thin-film transistors. Nano Lett. 9, 3848–3852 (2009).

    Article  ADS  Google Scholar 

  39. Ip, A. H. et al. Hybrid passivated colloidal quantum dot solids. Nat. Nanotech. 7, 577–582 (2012).

    Article  ADS  Google Scholar 

  40. Schaller, R. D. et al. Revealing the exciton fine structure of PbSe nanocrystal quantum dots using optical spectroscopy in high magnetic fields. Phys. Rev. Lett. 105, 067403 (2010).

    Article  ADS  Google Scholar 

  41. Neo, D. C. J. et al. Influence of shell thickness and surface passivation on PbS/CdS core/shell colloidal quantum dot solar cells. Chem. Mater. 26, 4004–4013 (2014).

    Article  Google Scholar 

  42. Kang, M. S., Sahu, A., Norris, D. J. & Frisbie, C. D. Size-and temperature-dependent charge transport in PbSe nanocrystal thin films. Nano Lett. 11, 3887–3892 (2011).

    Article  ADS  Google Scholar 

  43. Yu, D., Wang, C., Wehrenberg, B. L. & Guyot-Sionnest, P. Variable range hopping conduction in semiconductor nanocrystal solids. Phys. Rev. Lett. 92, 216802 (2004).

    Article  ADS  Google Scholar 

  44. Schaller, R. D. et al. Breaking the phonon bottleneck in semiconductor nanocrystals via multiphonon emission induced by intrinsic nonadiabatic interactions. Phys. Rev. Lett. 95, 196401 (2005).

    Article  ADS  Google Scholar 

  45. Merkulov, I. A., Efros, Al. L. & Rosen, M. Electron spin relaxation by nuclei in semiconductor quantum dots. Phys. Rev. B 65, 205309 (2002).

    Article  ADS  Google Scholar 

  46. Bae, W. K. et al. Highly effective surface passivation of PbSe quantum dots through reaction with molecular chlorine. J. Am. Chem. Soc. 134, 20160–20168 (2012).

    Article  Google Scholar 

  47. Gao, J. et al. Quantum dot size dependent J–V characteristics in heterojunction Zn/O/PbS quantum dot solar cells. Nano Lett. 11, 1002–1008 (2011).

    Article  ADS  Google Scholar 

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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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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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