Luminescent Down Shifting involves the conversion of one photon of high energy light, which is inefficiently absorbed by the photovoltaic material, to one photon of with an energy that can be efficiently absorbed by the photovoltaic material. The luminescent down shifting layer is placed on top of the photovoltaic material. Therefore no change to the optimised electrical properties of the photovoltaic material is required. A common trend across current photovoltaic technologies is that the response of the photoactive material reduces for lower wavelengths (blue and UV). Therefore luminescent down shifting layers can be used to enhance a range of photovoltaic materials. 

Ideal luminescent down shifting material should exhibit; a wide absorption band in the range of enhancement, high absorption coefficient, narrow emission band in the peak conversion efficiency region of the photoactive material, good separation between absorption and emission bands, and low cost. Luminescent materials under research include quantum dots, organic dyes and rare-earth ions/complexes. The most frequently used host materials for the luminescent species are polymers such as PMMA and inorganic crystalline materials such as Al2O3. 

What are the challenges for Luminescent Down Shifting?

The addition of an extra layer in front of the photovoltaic material introduces additional loss mechanisms for the light. Therefore any potential gain made by shifting the spectrum will have to compensate for these losses before showing an additional improvement. These loss mechanisms include parasitic absorption from the host material, low conversion efficiency of the luminescent material, re-absorption from the luminescent species, increased reflection due to the additional surfaces and losses from the top surface of the luminescent layer. 

The cost of both the materials used and fabrication procedures should be minimised and the layer should be stable for the lifetime of the photoactive material for luminescent down shifting layers to become economically viable. 

How are these challenges being addressed by SISER researchers?

Heriot Watt University is carrying out research into the use of organic luminescent down conversion layers for both microcrystalline silicon (mc-Si) and cadmium telluride (CdTe) solar cells. Their work includes the encapsulation of organic dyes into the encapsulating layer that is present in all mc-Si solar cells: thereby removing the need for an extra layer of material. They are also investigating the use of the luminescent down shifting species to remove the need for UV stabilisers in the encapsulant material. 

SISER researchers with interest in this area are: Prof. Neil Robertson.

Full details of all SISER researchers are found on the SISER People Page. 

Publications

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