Upconversion and Downconversion Utilizing Thin Film Colloidal Nanocrystals and Organic Materials

Technology

For either upconversion or downconversion, an organic material is combined with colloidal nanocrystals either as a deposited multilayer structure or in a blend of the two materials. The employed nanocrystals are lead (II) sulfide (PbS) colloidal nanocrystals coated with a thermally-evaporated layer of tetracene or rubrene. The PbS nanocrystals coupled to tetracene resulted in 90% triplet exciton transfer from tetracene to the nanocrystal, through the short-range Dexter process, for efficient downconversion. Those coupled to rubrene led to upconversion in the practical form of a thin film coating. The coating achieved a record-high anti-Stokes shift of about 1eV. This increase can shift light from NIR to the photovoltaic optimal spectrum (1.1-1.5eV), greatly increasing the harvestable spectrum and increasing overall efficiency.   

Problem Addressed

In current photovoltaics, about two-thirds of the incident spectrum is lost. Photons either do not have enough energy to move electrons to the conduction band or they have excess energy that is converted to heat. Tunable upconversion - converting two or more low energy photons to one higher energy photon - or downconversion - taking one high energy photon and converting it to two or more low energy photons - can reduce these spectrum losses by converting photons to the most useful energy.  In NIR photodetection, displays, medical imaging, and lighting, there are photons that are outside of the harvestable spectrum and photon conversion would reduce losses in emission or absorption resulting in better efficiency and increase detection range. For example, generating two red photons from a single UV photon instead of one could increase the efficiency of fluorescent lighting by up to 30%. Most state-of-the-art photon conversion devices (e.g. lanthanides and non-linear crystals) require high intensity, coherent light and the alternatives (e.g. triplet-triplet-annihilation) have limited bandwidth of operation, typically unable to make use of NIR spectrum. The proposed technology uses organic materials coupled to colloidal nanocrystals to allow the performance of up or down conversion from relatively low intensity, broadband, incoherent light.  

Advantages

• Increased external quantum efficiency
• Inclusion of more wavelengths into the useful spectrum
• Thin films for facile device integration