A low-cost alternative to conventional solar cells are devices made of quantum dots, tiny semiconductor nanocrystals, which are limited to harvesting visible light. Now scientists at KAUST have found a way to make quantum dot solar cells into efficient light absorbers for infrared radiation to improve the options for solar technology.
Solar cells based on quantum dots are cheap to manufacture and have light absorption properties that can be optimized for specific applications. Although the semiconductors used for the quantum dots can absorb infrared light, the electrical charges generated from this wavelength are very difficult to extract from solar cell devices.
Omar Mohammed, together with colleagues from the KAUST Solar and Photovoltaic Engineering Research Center, studied the transport of electrical charges within quantum dot solar cells to find the reason for the poor performance in the infrared range1.
“Controlling the charge transport process in the cells is crucial for optimizing their performance,” says Mohammed, “and now we can add a piece to the puzzle of how to tune this process from zero to very efficient.”
Important in this process is an organic molecule, a fullerene derivative called PCBM — or phenyl‑C61-butyric acid methyl ester — which is mixed with the quantum dots to form the active region of the solar cells. PCBM acts to transport the electric charges away from the dots, which means it is critical to have an efficient coupling between the quantum dots and PCBM.
To study this coupling, Mohammed and team used a cutting-edge femtosecond laser spectroscopy setup across a broad spectrum of light to chart how changes in the size of quantum dots influenced the transfer of electrons. They could show that the size of the quantum dots plays a pivotal role in the efficiency of the charge transport.
For small quantum dots, which are those that absorb light in the visible part of the spectrum, the transfer of electrons between the quantum dots and PCBM can occur very quickly. For larger quantum dots that absorb light in the infrared, an energy barrier between the dots and PCBM prevents an efficient transfer. The transfer can be slowed by as much as several orders of magnitude.
The researchers explain that these findings provide an important contribution into the design of quantum dot solar cells. “The size of the quantum dots makes a dramatic difference,” says Mohammed. ”We suggest that it should be considered when designing quantum dot solar cell devices. To make use of the near infrared region, researchers must search for an alternative charge transport layer instead of PCBM.”