A team led by Edward Sargent studied solar cells made using colloidal quantum dots (CQDs) as the light-absorbing component. These materials are attractive for many reasons – for example, they can be processed in solution and their optoelectronic properties can be tuned by varying the size of the dots. Such size-effect tuning, as it is called, has already been exploited to control how the quantum dots respond to incoming light by altering the material's bandgap. Indeed, it has recently been used to build "tandem" solar cells that can harvest visible and infrared light from the Sun in two distinct light-absorbing layers.

Now, the Toronto team has gone a step further by using varying degrees of quantum confinement to enhance electron transport in CQD solar cells. "Since CQD cells are currently limited by poor electron transport, our work strikes at the heart of the major challenge in this field," lead author of the study Illan Kramer told nanotechweb.org. The technique is known as quantum funnelling because it directs electrons towards their exit (or collecting electrode), he explained.

So how does it actually work? Since the bandgap of the CQDs can easily be tuned (by reducing or increasing the dimensions of the CQDs) and since that tuning occurs primarily in the conduction band of the structures, the researchers are able to layer CQDs with ever increasing bandgaps on top of each other. This creates an energy profile that propels electrons from the largest bandgap structure to the smallest – hence the funnelling effect.

"The main advantage of the technique is that it enhances current extraction when the solar cell transfers power to an external device," said Sargent. "In solar cell vocabulary, the biggest benefit is on the 'fill factor' of the cell." This is a key parameter when it comes to evaluating the performance of solar cells and is defined as the percentage of the actual maximum power obtained with respect to the theoretically obtainable power.

The fill factor achieved in this work was 54%, a value that is much better than the 37% achieved in a previous device that did not use a quantum funnel. The efficiency of the device also increased from 1.5% without a funnel to 2.7%.

The technique is a new way to engineer higher efficiency CQD solar cells, added Sargent. "And, it opens the way to a more comprehensive use of one of CQDs' greatest features – namely the ability to tune its optical and electrical properties with ease."

The work also shows that innovative architecture is important for improving the light-absorbing efficiency of CQDs. "In this way, even 'imperfect' materials can be enhanced when incorporated into a suitably-tailored device," he said.

The team now plans to further explore how it can engineer the energy profile in CQD devices to selectively confine or direct carriers as needed and so improve overall device performance.

The results were published in Nano Letters.