Researchers Propose New Model for Development of Nanocrystal Solar Cells

Researchers at ETH Zurich are studying nanocrystals in order to develop advanced solar cells which deliver greater efficiency.

These nanometre-sized crystals have remarkable optical properties and when compared to silicon which used in currently produced solar cells, it can be fabricated to absorb a larger part of the solar light spectrum.

However, it is a very difficult to develop nanocrystal solar cells. Until now, there was a lack of understanding about the physics of transport of electrons in this complicated material system, and hence it was not possible to design better nanocrystal composites.

"These solar cells contain layers of many individual nano-sized crystals, bound together by a molecular glue. Within this nanocrystal composite, the electrons do not flow as well as needed for commercial applications. Our model is able to explain the impact of changing nanocrystal size, nanocrystal material, or binder molecules on electron transport," said Vanessa Wood, Professor of Materials and Device Engineering at ETH Zurich.

Wood and the research team carried out a detailed study of nanocrystal-based solar cells, which were developed and characterized in the lab at ETH Zurich. For the first time, the scientists were able to portray the electron transport in these cells through a generally applicable physical model.

With the help of this model, scientists can gain a better insight into the physical processes that take place within the nanocrystal solar cells and thus enhance the efficiency of solar cells.

The nanocrystals crystals in small dimensions are able to produce quantum effects that are otherwise not seen in large semiconductors. This feature holds significance for many solar cell scientists.

One typical example is that the physical characteristics of the nanocrystals rely on their size and since researchers can easily control the size of nanocrystals during the development process, they can manipulate the characteristics of nanocrystal semiconductors and thus customize them for solar cells. For instance, by changing the size of nanocrystals, the quantity of the sun's spectrum absorbed by the nanocrystals can be changed to electricity by the solar cell.

Semiconductors are not capable of absorbing the entire sunlight spectrum, but absorb only radiation below a specific wavelength, or with an energy that is higher than the semiconductor’s band gap energy.

In a large number of semiconductors, it is possible to change this threshold only when the material is also changed. In case of nanocrystal composites, the threshold can be changed easily by merely changing the individual crystal size. Therefore, the size of nanocrystals can be selected in such a way that they are able to absorb the largest amount of light from a wide range of the sunlight spectrum.

Another benefit of nanocrystal semiconductors is that they are capable of absorbing more amount of sunlight when compared to conventional semiconductors. For instance, the ETH researchers used lead sulfide nanocrystals whose absorption coefficient was several orders of magnitude higher when compared to silicon semiconductors, which are traditionally utilized as solar cells. Hence, a small amount of material is adequate to develop nanocrystal solar cells, thereby making it possible to systematically engineer thin and flexible solar cells.

The latest model proposed by the ETH scientists addresses a set of unresolved questions pertaining to transport of electrons in nanocrystal composites. Till date, there was no empirical proof to establish that a nanocrystal composite’s band gap energy relies on the band gap energy of nanocrystals.

For the first time, we have shown experimentally that this is the case. For us to begin to consider commercial applications, we need to achieve an efficiency of at least 15 percent, explained Wood.

Over the past few years, researchers were able to improve the efficiency of nanocrystal-based solar cells, but even in premium solar cells only 9% of the sunlight incident on the cell was transformed to electrical energy. The latest study not only paves the way for enhancing the electron transport but also aids in improving the efficiency of solar cells.

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