Through order comes efficiency

Tiny pieces of semiconductors, known as quantum dots, have attracted great interest as materials for photovoltaic cells, because it is possible to choose which wavelengths of light are absorbed simply by changing the size of the dots. However, the thickness of devices made with quantum dots has so far been limited to just 300 nanometres, because the electrons excited by the light can travel only short distances before their energy is lost.

Now, an international team including Aram Amassian at King Abdullah University of Science and Technology (KAUST), Saudi Arabia and Edward Sargent at the University of Toronto, Canada have developed record-breaking solar cells by embedding quantum dots in a new type of matrix, which holds the dots extremely close together in a well-ordered structure¹.

The researchers experimented by embedding lead sulfide quantum dots in three different types of chemical matrix. They found that the samples made with a new hybrid matrix, made from N-butylamine and hexylamine, showed a very strong excited peak in absorption of cyan coloured light. 

The team attribute this finding to the quantum dots taking a layered structure similar to that of the mineral perovskite. 

The perovskite structure improves the ordering of the quantum dots, which facilitates electron transfer. As well, it increases the density of the overall material. 

This means that, within this new hybrid matrix, the excited electrons can travel almost double the distance, and devices can have an active layer of up to 600 nanometres. The researchers proved the potential of their samples by building them into solar cells that showed a record efficiency of 12%.

In their paper, published in Nature Nanotechnology, the researchers claim that the enhanced structural order of their hybrid matrix approach will provide "a means to tune material properties within the wide scope of colloidal quantum dot applications, such as printable devices, tandem cells, photodetectors and light-emitting diodes.”

Sargent says that “the next steps for this project are to continue to pursue solar cells that considerably augment silicon – the workhorse of the fast-growing photovoltaics sector – by adding appreciable absolute power points."

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