Colloidal quantum dots (CQDs) could be ideal as the light-absorbing component in inexpensive highly efficient solar cells. Photons having energies equal to or greater than the band gap of the photovoltaic material can produce excited charge carriers (electrons and holes) that can then be harvested to generate power. CQDs can absorb light over a wide spectrum of wavelengths thanks to the fact that the bandgaps in quantum dots can be tuned over a large energy range simply by changing the size of the dots during wet chemical synthesis – something that is not possible for bulk semiconductors. This is useful for building multiple junction solar cells to maximize power conversion efficiency.

The problem is, however, that only a limited number of CQD materials have been studied in any great detail for use in solar cells – with two common examples being lead or cadmium-based crystals. Charge carriers in these compounds have modest diffusion lengths (that is, the distance that carriers can travel before recombining). A long diffusion length is important for solar cells because it allows photogenerated electrons and holes to be collected by the device and produce useful current before they recombine.

To better understand charge transport in CQDs, researchers have been busy trying to actually measure the diffusion length in these materials, but most techniques to do so have only been indirect until now. For example, diffusion length can be estimated by combining mobility measurements (in field-effect transistor test structures) with separately measured charge carrier lifetimes. Unfortunately, such techniques invariably produce values that are too high.

Now, a team led by Edward Sargent has put forward not one, but two new methods to directly measure the diffusion length in CQDs. “We employ what we refer to as the ‘donor-acceptor’ scheme where charges preferentially flow from a large bandgap CQD layer (the donor) into an adjacent smaller bandgap layer (the acceptor), where they ultimately recombine and produce bright photoluminescence (PL) that we can measure,” explains Sargent.

1D and 3D methods

In their first, “1D” method, the Toronto researchers use a donor layer whose thickness can be varied, capped with a thin acceptor layer. By measuring the PL signal from the acceptor as a function of donor thickness, they are able to directly extract the diffusion length. In the second, “3D” method, they use a mix of donor and acceptors, where acceptor CQDs are uniformly mixed into a donor matrix. “With this technique, we can deconstruct diffusion length into two components – charge mobility and lifetime,” Sargent told nanotechweb.org.

The researchers have succeeded in measuring a diffusion length of as high as 80 nm for lead sulphide CQDs using both methods.

The new techniques, which are detailed in the journal ACS Nano, are an easy way to measure the diffusion length in CQDs without having to make full photovoltaic devices first, says Sargent. And, they will allow researchers to make such measurements quickly and simply in a host of solid CQD materials.

Spurred on by its results, the team says that it will now be looking at improving the critical parameters (such as charge carrier lifetimes) that the diffusion length hinges on. “This will ultimately lead to improvements in CQD photovoltaic device efficiency,” adds Sargent.