Directed Energy Transfer in Films of CdSe Quantum Dots: Beyond the Point Dipole Approximation Kaibo Zheng, † Karel Z ̌ ídek, † Mohamed Abdellah, †,‡ Nan Zhu, § Pavel Cha ́ bera, † Nils Lenngren, † Qijin Chi, § and Tõ nu Pullerits* ,† † Department of Chemical Physics, Lund University, Box 124, 22100, Lund, Sweden § Department of Chemistry, Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark ‡ Department of Chemistry, Faculty of Science, South Valley University, Qena 83523, Egypt * S Supporting Information ABSTRACT: Understanding of Fö rster resonance energy transfer (FRET) in thin films composed of quantum dots (QDs) is of fundamental and technological significance in optimal design of QD based optoelectronic devices. The separation between QDs in the densely packed films is usually smaller than the size of QDs, so that the simple point-dipole approximation, widely used in the conventional approach, can no longer offer quantitative description of the FRET dynamics in such systems. Here, we report the investigations of the FRET dynamics in densely packed films composed of multisized CdSe QDs using ultrafast transient absorption spectroscopy and theoretical modeling. Pairwise interdot transfer time was determined in the range of 1.5 to 2 ns by spectral analyses which enable separation of the FRET contribution from intrinsic exciton decay. A rational model is suggested by taking into account the distribution of the electronic transition densities in the dots and using the film morphology revealed by AFM images. The FRET dynamics predicted by the model are in good quantitative agreement with experimental observations without adjustable parameters. Finally, we use our theoretical model to calculate dynamics of directed energy transfer in ordered multilayer QD films, which we also observe experimentally. The Monte Carlo simulations reveal that three ideal QD monolayers can provide exciton funneling efficiency above 80% from the most distant layer. Thereby, utilization of directed energy transfer can significantly improve light harvesting efficiency of QD devices. ■ INTRODUCTION Semiconductor quantum dots (QDs) hold promises for a broad spectrum of applications. For example, QDs are novel light harvesters for next generation solar cells due to their high extinction coefficient, ability to generate multiple excitons, size- dependent tunability of bandgap, and high stability. 1-8 They are also efficient light emitters, and therefore can be used in fabrication of light-emitting diodes and QD lasers. 9-11 In the development of such devices, QDs are often densely packed to form a thin film with strong absorption or emission. Fö rster resonant energy transfer (FRET) induced by electronic coupling between QDs is an essential process for function of such devices. 12 Since the process has a significant impact on optoelectronic properties of QD layers, detailed understanding of FRET, its dynamic features in particular, is of both fundamental and technological importance. For instance, in conventional Grä tzel-type QD sensitized solar cells, electron injection occurs from directly attached QDs to acceptors. Although this process can be fast (∼ps), the overall efficiency of the solar cells is likely limited by insufficient utilization of the sun light with monolayer QDs coverage. 13 The problem can be solved by using band gap-controlled multilayered QDs where energy is funneled toward electrodes via FRET. 14-16 FRET is a vital process in photosynthetic light harvesting 17 and has been widely used to describe excitation dynamics in conjugated polymers as well. 18 Time-resolved fluorescence studies of the films of QDs with different sizes have provided clear evidence for FRET from smaller to larger size QDs. 15 Time-constant of such transfer depends strongly on the system and has been reported to be from about a nanosecond down to few tens of picoseconds. 16,19-25 It is generally accepted that the usual point dipole approximation in FRET is valid if the excitation donor and acceptor separation is significantly larger than the size of them. In cases of closely lying molecular donor-acceptor systems, various refinements to the approximation have been suggested, like atom-centered transition monopoles, 26,27 continuous transition densities, 28 or partial dipoles. 29 Already, early studies of FRET in QD films pointed out that higher multipole interactions should probably be considered for the excitation transfer between closely packed QDs. 30 However, to the best of our knowledge no such attempts have been reported so far. Received: October 31, 2013 Published: March 31, 2014 Article pubs.acs.org/JACS © 2014 American Chemical Society 6259 dx.doi.org/10.1021/ja411127w | J. Am. Chem. Soc. 2014, 136, 6259-6268