Article pubs.acs.org/cm
Small-Sized PbSe/PbS Core/Shell Colloidal Quantum Dots Diana Yanover,† Richard K. Č apek,† Anna Rubin-Brusilovski,† Roman Vaxenburg,† Nathan Grumbach,† Georgy I. Maikov,† Olga Solomeshch,‡ Aldona Sashchiuk,† and Efrat Lifshitz*,† †
Schulich Faculty of Chemistry, Russell Berrie Nanotechnology Institute, Solid State Institute and ‡Electrical Engineering Department and Microelectronic Center, Technion-Israel Institute of Technology, Haifa 32000, Israel S Supporting Information *
ABSTRACT: The work focuses on the synthesis of smallsized PbSe/PbS core/shell colloidal quantum dots with the core diameter of 2−2.5 nm and the shell thickness of 0.5−1.0 nm. The PbSe/PbS core/shell CQDs are chemically stable under time-limited air exposure and have emission quantum efficiency of 60% at room temperature. The PbSe/PbS core/ shell CQDs have a tunable absorption edge around 1 μm, large exciton emission Stokes shift (∼150 meV), and small exchange interaction (∼1.5 meV). Theoretical calculations associate the mentioned parameters to the small-size regime as well as to a lift of band-edge degeneracy due to slight shape anisotropy. The specific parameters are of special interest in photovoltaic applications. KEYWORDS: small-sized nanocrystals synthesis, PbSe/PbS core/shell quantum dots, surface oxidation, band-edge temperature coefficient
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INTRODUCTION Lead chalcogenide (IV−VI) colloidal quantum dots (CQDs) are of great scientific interest because of the possibility of their implementation in many opto-electronic applications.1 Since CQDs are characterized by a size-tunable narrow band gap (0.3−1.7 eV) with the broad band absorption profile ranging from near IR to UV,2−5 they are suitable for being used as light harvesters in photovoltaic cells (PVCs). Furthermore, the electron and hole effective masses of these CQDs are very small (me,h ≤ 0.1m0)6 and thus both carriers have very similar transport properties and high degeneracy of electronic states.2 This results in a large carrier population, which is advantageous for PVCs7 and gain8 devices. Besides, such CQDs have relatively long excited-state lifetime (∼μs),9,10 which permits efficient charge extraction in PVCs11 and population saturation in optical switches.12 IV−VI CQDs have been recently discussed in relation to the concept of multiple exciton generation (MEG). The effect of MEG is produced by two or more electron−hole pairs of a single absorbed photon with energy >2.7Eg. Although the issue is still controversial,13,14 MEG is presumed to occur in PbSe-based CQDs,13,15−22 which may provide the opportunity of increasing the PVC power conversion efficiency beyond the Shockley− Queisser thermodynamic limit.23,24 There is some recent practical evidence for the effect of MEG in PbSe-based optoelectronic prototype devices.25−29 Several studies have reported the integration of IV−VI CQDs into PVCs prototype devices in different configurations, including Schottky30,31 or CQDssensitized32,33 solar cells. The PbSe-based cells exhibited high © 2012 American Chemical Society
short-circuit current (JSC), while PbS-based devices exhibited high open-circuit voltage (VOC).34 Moreover, ultrasmall PbSe CQDs with the band gap energy of 1.3−2.3 eV show the power conversion efficiency of 4−5%32 and are favorable for being used in PVCs.32,35 Therefore, the current research should focus on the high chemical yield synthesis of small-sized IV−VI CQDs that would have high chemical and photochemical stability and a small number of carrier trapping sites despite their large surface-to-volume ratio. This study describes, for the first time, the synthesis and characterization of small-sized PbSe/PbS core/shell CQDs with the band-edge energy in the range of 1.1−1.4 eV. Previous studies have shown the benefit of PbS-shell coating on larger PbSe cores with band-edge energies 0)), similar to other narrow band gap materials.58,59 The band gap energy in IV−VI CQDs was shown to increase with temperature for CQDs more than 2 nm in diameter, while for diameters 10 μs. In contrast to this, at elevated temperatures, both the bright and the dark states are populated, which results in the typical lifetime value of 6 μs at room temperature. For example, An et al.57 calculated the room-temperature value of τrad to be 5 μs for 3 nm PbSe CQDs, and Moreels et al.65 measured the lifetime to be 2 μs for 3.5 nm PbSe CQDs dispersed in TCE and 2.5 μs for 3 nm PbSe CQDs dispersed in chloroform. This exceptionally long lifetime can be associated with the relatively high dielectric constant (ε∞ = 24) of the PbSe semiconductor or with possible contribution of intervalley mixing at the L-point of the Brillouin zone.57 The values of the bright-dark energy gap in spherical52 and in elongated56 CQD structures were found to be 10−30 meV and
