Solid-state infrared-to-visible upconversion sensitized by colloidal nanocrystals


Optical upconversion via sensitized triplet–triplet exciton annihilation converts incoherent low-energy photons to shorter wavelengths under modest excitation intensities1,2,3. Here, we report a solid-state thin film for infrared-to-visible upconversion that employs lead sulphide colloidal nanocrystals as a sensitizer. Upconversion is achieved from pump wavelengths beyond λ = 1 μm to emission at λ = 612 nm. When excited at λ = 808 nm, two excitons in the sensitizer are converted to one higher-energy state in the emitter at a yield of 1.2 ± 0.2%. Peak efficiency is attained at an absorbed intensity equivalent to less than one sun. We demonstrate that colloidal nanocrystals are an attractive alternative to existing molecular sensitizers, given their small exchange splitting, wide wavelength tunability, broadband infrared absorption, and our transient observations of efficient energy transfer. This solid-state architecture for upconversion may prove useful for enhancing the capabilities of solar cells and photodetectors.

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Figure 1: Schematics of nanocrystal-sensitized upconversion via triplet-triplet annihilation.
Figure 2: Absorption, photoluminescence and excitation spectra of infrared upconverter devices.
Figure 3: Nanocrystal-sensitized upconverters reach peak efficiency at a sub-solar absorbed power density.
Figure 4: Photoluminescence dynamics show slow, yet efficient triplet transfer.


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This work was supported as part of the Center for Excitonics, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001088 (MIT). The authors thank P. Deotare for assistance with optical measurements, as well as J. M. Scherer, C-H. Chuang, P. R. Brown and M. Sponseller for assistance with nanocrystal synthesis.

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M.Wu and D.N.C. fabricated the samples. M.Wu measured absorption spectra and the intensity dependence. D.N.C. measured excitation spectra and the yield of upconversion. M.W.B.W. made the transient PL measurements and synthesized the nanocrystals. M.Wu and J.J. prepared nanocrystal solutions for sample fabrication and performed AFM measurements. N.G. and M.Welborn simulated the nanocrystal structure. The project was conceived by M.A.B. All authors discussed the results and commented on the manuscript.

Correspondence to Vladimir Bulović or Moungi G. Bawendi or Marc A. Baldo.

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MIT has filed an application for patent based on this technology that names D.N.C., M.Wu, M.W.B.W., V.B., M.G.B., and M.A.B. as inventors.

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Wu, M., Congreve, D., Wilson, M. et al. Solid-state infrared-to-visible upconversion sensitized by colloidal nanocrystals. Nature Photon 10, 31–34 (2016).

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