FRET-Activated Delayed Fluorescence in Densely Packed PbS Quantum-Dot Ensembles Aleksandr P. Litvin, Peter S. Parfenov, Elena V. Ushakova, Tatiana A. Vorsina, Ana L. Simõ es Gamboa, Anatoly V. Fedorov, and Alexander V. Baranov* ITMO University, 49 Kronverksky pr., Saint-Petersburg 197101, Russia ABSTRACT: The nonradiative photoexcitation energy transfer in systems of densely packed PbS quantum dots (QDs) of different sizes is studied by steady- state and transient photoluminescence (PL) spectroscopy. It is shown that Fö rster resonance energy transfer dominates the process and dramatically changes the dynamics of the photoexcited charge carriers as compared to systems of noninteracting QDs. As a result, the initially quite different lifetimes of the lowest energy electronic states of the QDs approach each other and increase up to values characteristic for the long-lived luminescence of the QD donors. This slowdown effect in the PL decay in densely packed systems of PbS QDs with optical transitions in the near-infrared may find applications in solar energy harvesting. ■ INTRODUCTION The performance of most quantum dot (QD) based device applications depends not only on the properties of the individual QDs but also on the properties arising from interactions between QDs in densely packed assemblies. Fö rster resonance energy transfer (FRET) is an important manifestation of these interactions between nanoparticles 1 which has been extensively studied for QDs of different sizes in solutions 2−4 and on different substrates. 5,6 Structures composed of QDs of different sizes constitute a novel platform for the creation of solar cells. The electron and energy transfer processes between densely packed QDs can noticeably improve the power conversion efficiency. 7,8 The enhancement of photoluminescence (PL) emission by the appropriate selection of QD size has been shown for CdTe, 9 CdSe/ZnS, 10 and PbS 11 QD systems. Careful choice of the chemical composition of the QDs and optimization of their band gap widths and spatial distribution may further improve QD-based photovoltaic elements. PbS QDs are known to be a perspective material for solar energy conversion due to their high extinction coefficient, high mobility of charge carriers, and efficient multiple electron−hole pair generation. 12,13 FRET between PbS QDs has been demonstrated for quasi- monodispersed QDs, 6,14−16 QD pairs, 6,14,17 QD superlattices, 18 and energy cascade structures 11 consisting of multilayer systems with QDs of different sizes in each layer. PbS QDs exhibit a peculiar electronic energy structure. There is a lowest energy luminescent electronic state in the band gap, the so-called “in-gap” state (GS). 19,20 The lifetime of the GS is characterized by a strong dependence on QD size. For example, at room temperature, the GS lifetime has been found to decrease almost 1 order of magnitude (from 2.5 to 0.25 μs) in solutions 21 and in porous matrices (from 0.5 to 0.05 μs) 22 when the PbS QD size increases from ∼2.5 to 8.8 nm. The relaxation of photoexcited charge carriers inside PbS QDs is therefore defined in a large extent by their sizes. However, the role of this effect on energy transfer processes in systems of densely packed QDs of different sizes has not been clarified yet. In the present work we performed steady-state and transient PL studies in systems of randomly distributed densely packed PbS QDs of two and three different sizes, where the smallest QDs can act as donors, the largest QDs can act as acceptors, and the middle-size QDs can play the role of either donors or acceptors. Besides typical evidence for FRETdecrease of PL intensity and decay times of the QDs acting as donors (QD D ) and enhancement of the PL intensity of the QDs acting as acceptors (QD A )we found a drastic increase of the PL lifetimes of the QD acceptors up to values characteristic for the QD donors. The increase of the PL lifetime of the QD A has been attributed to FRET from the long-lived GS of the QD D , with this state playing a role analogous to that of the long-lived triplet state in the thermally activated delayed fluorescence observed in molecular systems. 23 The observed effect can therefore be interpreted as FRET-activated delayed fluores- cence in systems of densely packed PbS QDs of different sizes. ■ EXPERIMENTAL SECTION PbS QDs of mean diameter 3.5 (QD1), 4.6 (QD2), and 6.0 nm (QD3) and ∼10% size distribution and stabilized by oleic acid were synthesized via the standard procedure described in detail elsewhere. 21 QD size was determined using absorption spectroscopy and the equation for size-dependent position of Received: June 8, 2015 Revised: June 30, 2015 Article pubs.acs.org/JPCC © XXXX American Chemical Society A DOI: 10.1021/acs.jpcc.5b05447 J. Phys. Chem. C XXXX, XXX, XXX−XXX