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Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX
Negligible Electronic Interaction between Photoexcited Electron− Hole Pairs and Free Electrons in Phosphorus−Boron Co-Doped Silicon Nanocrystals Rens Limpens,*,† Minoru Fujii,‡ Nathan R. Neale,† and Tom Gregorkiewicz§ †
National Renewable Energy Laboratory, 15013 Denver West Parkway, Golden, Colorado 80401, United States Department of Electrical and Electronic Engineering, Kobe University, Rokkodai, Nada, Kobe 657-8501, Japan § Van der Waals-Zeeman, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands ‡
S Supporting Information *
ABSTRACT: Phosphorus (P) and boron (B) co-doped Si nanocrystals (NCs) have raised interest in the optoelectronic industry due to their electronic tunability, optimal carrier multiplication properties, and straightforward dispersibility in polar solvents. Yet a basic understanding of the interaction of photoexcited electron−hole (e−h) pairs with new physical features that are introduced by the co-doping process (free carriers, defect states, and surface chemistry) is missing. Here, we present the first study of the ultrafast carrier dynamics in SiO2-embedded P−B co-doped Si NC ensembles using induced absorption spectroscopy through a two-step approach. First, the induced absorption data show that the large fraction of the dopants residing on the NC surface slows down carrier relaxation dynamics within the first 20 ps relative to intrinsic (undoped) Si NCs, which we interpret as enhanced surface passivation. On longer time-scales (picosecond to nanosecond regime), we observe a speeding up of the carrier relaxation dynamics and ascribe it to doping-induced trap states. This argument is deduced from the second part of the study, where we investigate multiexciton interactions. From a stochastic modeling approach we show that localized carriers, which are introduced by the P or B dopants, have minor electronic interactions with the photoexcited e−h pairs. This is understood in light of the strong localization of the introduced carriers on their original P- or B-dopant atoms, due to the strong quantum confinement regime in these relatively small NCs (500 ps), single exciton dynamics were removed by subtracting the traces in the nonlinear regime, and the resulting traces were fitted to a E
DOI: 10.1021/acs.jpcc.7b12313 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
the carrier concentration (whether free or localized), which agrees with the decreased optical activity in co-doped samples found in this work and prior studies. These results show that co-doped Si NCs in the strong quantum confinement regime (offering sub-1.1 eV energy transitions) could be leveraged for QD-based multiexciton optoelectronics applications similar to those of intrinsic Si NCs. Their optical activities are not limited by the presence of some remaining free carriers in uncompensated NCs, a scenario that would have been unsolvable since it is inherent to the NC formation procedure by precipitation in a glass matrix (being the only synthetic method currently available for the preparation of these interesting nanostructures). Hence, to exploit these materials for charge extraction in any device concept, experimentalists must limit the presence of interstitial doping sites, potentially by developing new synthesis techniques.
explanation for the observed quench. This model could explain the previous observed PL quenching behavior in P or B singly doped Si NCs as well, in contrast to the often considered nonradiative trion recombination.13,23,44 As such, it may well be that charge compensation in P−B co-doped NCs is of lesser importance to the PL activity of these ensembles than previously assumed. We do recall that the interplay between localized carriers and photoexcited e−h pairs should strongly depend on parameters such as the doping concentration and the NC size. Further research is therefore needed to elucidate the effect of localized carriers (and the resulting trion formation) on the optical activity for differently sized NCs with varying doping concentrations. Further Discussion Regarding the Sample Inhomogeneity. On the basis of the below-bandgap NIR linear absorption that is indicative of the presence of localized carriers (Figure 1b), we know that our co-doped Si NC ensemble sample is an inhomogeneous set consisting of compensated and uncompensated NCs. The quantification of the level of inhomogeneity is however extremely challenging since the carriers are (1) localized and (2) present in a small number. Standard methods to determine the concentration of carriers rely on the presence of “free” carriers and a large amount of them (e.g., >10 free carriers per NC have to be present to support a localized surface plasmon resonance).45 This withholds us from performing a more sophisticated modeling procedure that treats an ensemble as a distribution, instead of the average ensemble-driven approach applied in this work. In this light we refer to a recent study based on similar samples that showed that there is (1) no realistic chance of finding NCs that are undoped or solely B-doped, (2) a small percentage of NCs (∼2−3%) that are solely P-doped, and (3) a >97% chance of finding a P−B co-doped Si NC.29 Despite the high probability of finding P−B co-doped NCs and the reduced formation energy for compensated doping,46 individual NCs are not necessarily compensated (the resolution of the atom probe tomography experiment used to derive the doping concentration was insufficient to resolve the level of charge compensation). Thus, within these co-doped NC samples we find a distribution of compensated and uncompensated NCs which together accounts for the ultrafast dynamics observed here.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b12313. Derivation of the Lorentzian shape of the SiO2 signal in the XRD pattern and verification of the nonlinear excitation regime (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Telephone: +1 (303) 2754687. ORCID
Rens Limpens: 0000-0002-2417-9389 Minoru Fujii: 0000-0003-4869-7399 Nathan R. Neale: 0000-0001-5654-1664 Tom Gregorkiewicz: 0000-0003-2092-8378 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS R.L. acknowledges the National Renewable Energy Laboratory (NREL) LDRD program for the award of the Nozik Postdoctoral Fellowship to perform this research. N.R.N. was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences, Solar Photochemistry Program under Contract DE-AC36-08GO28308 to NREL. The authors thank Chung Xuan Nguyen for his help with the optical characterization, Arnon Lesage for his efforts regarding the sample production, and Noah Bronstein for building the Ocean Optics PL system.
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CONCLUSIONS Making use of induced absorption spectroscopy, we present the first detailed investigation on the ultrafast carrier dynamics of highly confined P−B co-doped Si NCs. We report on a decreased carrier recombination rate within the first 20 ps after photoexcitation and ascribe it to a surface passivation effect by the introduction P and B surface atoms. In contrast, for timescales of >20 ps we observe a new nonradiative recombination channel not present for the intrinsic Si NCs. To determine its origin, we investigated Auger interactions between multiple photoexcited e−h pairs and show that they follow a three-charge interaction model with rates similar to those found for intrinsic NC ensembles, apparently being unaffected by the presence of localized carriers in uncompensated NCs. This indicates that the localized carriers only weakly interact with the photoexcited e−h pairs and are most probably not responsible for the additional nonradiative recombination channel. We thus assign this new nonradiative recombination channel to defect states resulting from the unintentional introduction of interstitial impurities that do not contribute to
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