Super‐Efficient Exciton Funneling in Layer‐by‐Layer Semiconductor Nanocrystal Structures

Abstract

In semiconductor nanocrystals the electronic energy gap is determined not only by the material but also by the size of the nanocrystals. This allows the construction of an energy‐gap gradient normal to multiple layers of nanocrystals where the diameters of the nanocrystals are monotonically increasing or decreasing in subsequent layers. In such devices we observe a highly efficient funneling of excitation energy from layers comprising smaller nanocrystals towards the layer with the largest nanocrystals in the center of the funnel. Most importantly, not only are excitons in radiative states transferred, but also excitons from trapped states, usually lost for luminescence, can be effectively recycled, hence increasing the overall luminescence yield.

Resonant nonradiative energy transfer in CdSe/ZnS core/shell nanocrystal solids enhances hybrid white light emitting diodes

“…They further showed a cascaded energy transfer having a funnel-like band gap profile structure by using distinctly sized CdTe nanocrystals [24]. Also, Klar et al showed super efficient exciton funneling structure using layer-by-layer nanocrystals [17]. In that study it was demonstrated that using energy gradient structure not only excitons in radiative states are transferred, but also the trapped excitons that are generally nonradiatively recombined are recycled, resulting in an overall emission increase.…”

“…It is well known that excitons are more likely to be captured in surface traps leading to non-radiative recombination in defectrich NCs, while defect-poor NCs exhibit a higher probability for radiative recombination. Using energy transfer, it is possible to transfer the excitation energy of trapped excitons from defect-rich NCs in film to proximal defect-poor NCs, as extensively discussed in [17]. Therefore, the difference in the measured and predicted efficiencies is attributed to the portion of trapped excitons recycled by the energy transfer to additionally contribute to the emission.…”

“…The Förster energy transfer (ET) is the nonradiative transfer of excitation energy from an excited molecule (donor) to a ground-state molecule (acceptor) [17][18][19][20][21][22][23][24][25]. Kagan et al first demonstrated ET in spin-cast films of closely packed CdSe core nanocrystals with different sizes [18,19].…”

Hybrid Polyfluorene-Based Optoelectronic Devices

“…Recently, exciting reports such as the observation of superb multiple-exciton generation efficiencies (Sargent, 2009;Sukhovatkin et al, 2009), highly-efficient hot-electron injection (Tisdale et al, 2010), and cold-exciton recycling (Klar et al, 2009), have propelled nanocrystalline lead-chalcogenide film structures to the forefront of cutting-edge research (M. S. Kang et al, 2009;W. Ma et al, 2009;Sambur et al, 2010;Steckel et al, 2003).…”

Quantum efficiency enhancement in nanocrystals using nonradiative energy transfer with optimized donor-acceptor ratio for hybrid LEDs

“…9 To address this problem, the recycling of trapped excitons into the quantum dots by using nonradiative energy transfer ͑ET͒, which is also known as Förster resonance energy transfer, provides a possible solution to enhance the quantum efficiency of these NC solid films. Although Förster ET for colloidal NC emitters are investigated in various studies, [10][11][12][13][14][15][16][17][18] the dependence of quantum efficiency enhancement on the donor-acceptor variation in the solid film and its optimization have not been investigated for hybrid LEDs to date.…”