Intermediate Band Solar Cells

s.j.farrell@hw.ac.uk (Super User) — Wed, 19 Sep 2012 13:41:33 +0000

What are Intermediate Band Solar Cells?

Intermediate band solar cells are formed through the addition of a narrow band of states in the middle of the bandgap of a photovoltaic material. The intermediate band of states is often achieved through the addition of quantum dots. The additional band allows for the absorption of low energy photons that would otherwise be transmitted through the photovoltaic material. Theoretical calculations have shown that efficiencies of 63% for intermediate band solar cells can be achieved under concentrated sunlight, which is a significant improvement on the corresponding maximum single junction efficiency of 41%.

Quantum dots are “nanoparticles” of semiconductor material, usually in the range of 2-10 nm in diameter. Due to their small size, they exhibit unique optical and electronic properties that are different to their corresponding bulk material. These include the emission of photons of wavelengths that are dependant on the size of the quantum dot. Precise control of the size of the quantum dots allows for control over the wavelength at which they emit. Material combinations under current research include the use of InAs quantum dots, within GaAs solar cells. Efficiencies of Quantum dot solar cells are now greater than 18%.

Highly mismatched alloys have also been shown to achieve the required intermediate band. An advantage of this type of material over quantum dots is that higher densities of states in the intermediate band can be achieved.

What are the challenges for Intermediate Band Solar Cells?

Quantum dots are very small and so do not absorb much light. Therefore it is necessary to have several layers of quantum dots. Each subsequent layer results in additional strain, causing damage to the photovoltaic material and reducing performance. Therefore the number of quantum dot layers is limited.

The addition of intermediate states introduces mechanisms for non-radiative recombination. This could severely hinder the efficiency of an intermediate band solar cell, cancelling out the benefits of the intermediate band states. Research into the mechanisms and solutions for this problem are required before intermediate band solar cells can reach their full potential.

How are these challenges being addressed by SISER researchers?

The University of Glasgow is carrying out research into quantum dot intermediate band solar cells. The specific intermediate band materials under investigation are self-assembled InAs quantum dots grown by molecular beam epitaxy and embedded in (Al,Ga)As. Furthermore, single junction (Al,Ga)As PV cells with one sun efficiencies in excess of 20% are being developed to benchmark the overall technology.

SISER researchers with interest in this area are: Prof. Colin Stanley.

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

Publications

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