Solid-State Upconversion

Solid-State Infrared-to-Visible Upconversion Sensitized by Colloidal Nanocrystals

ONE Lab: Mengfei Wu, Joel Jean, Vladimir Bulović
Collaborators: Daniel N. Congreve, Mark W.B. Wilson, Nadav Geva, Matthew Welborn, Troy Van Voorhis, Moungi G. Bawendi, Marc A. Baldo
Sponsorship: Energy Frontier Research Center for Excitonics by the U.S. Department of Energy

 
Optical upconversion is a process that converts two or more low-energy photons into a single high-energy photon. Upconversion from the infrared to the visible is useful in photovoltaics, photodetection, bioimaging, and three-dimensional displays. In photovoltaic applications specifically, an optical upconversion layer can capture sub-bandgap photons, increasing the efficiency of a conventional single-junction solar cell beyond the Shockley-Queisser limit.
 
To upconvert light at relatively low intensities, a promising approach is the sensitized triplet-triplet annihilation (TTA). It utilizes a sensitizer and an annihilator. The sensitizer absorbs incident light, and transfers the energy as spin-triplet excitons to the annihilator. When two triplets meet in the annihilator, they form a single higher-energy spin-singlet exciton via TTA. Blue-shifted light is emitted when the singlets relax. It has been, however, difficult to identify effective molecular sensitizers that absorb in the infrared. Efficient demonstrations to date have been mostly conversion among visible wavelengths. Furthermore, the majority of them are in solution, while a solid-state architecture is necessary for solar and detection applications.
 
Here, we demonstrate a solid-state thin film for infrared-to-visible upconversion that employs lead sulfide colloidal nanocrystals as a sensitizer. Upconverison 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%. Upconversion efficiency reaches the maximum at an absorbed intensity equivalent to less than one sun. We show 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 observation of efficient energy transfer. This solid-state architecture for upconversion may prove useful for enhancing the capabilities of solar cells and photodetectors.

 

Solid-state infrared-to-visible upconverter: Nanocrystal absorption labeled by first excitonic peaks, emitter photoluminescence (red) and excitation spectra (purple crosses) for λ = 1010 nm nanocrystals. Inset: Photograph of a device under λ = 808 nm excitation.

 

Related publications:

  • Wu, M. et al., Solid-State Infrared-to-Visible Upconversion Sensitized by Colloidal Nanocrystals. Nature Photonics 10, 31-34 (2016).