One Small Change Makes Solar Cells More Efficient

The Science

The quest for more efficient solar cells has led to the search of new materials. For years, scientists have explored using tiny drops of designer materials, called quantum dots. Now, we know that adding small amounts of manganese decreases the ability of quantum dots to absorb light but increases the current produced by an average of 300%. Under certain conditions, the current produced increased by 700%. The enhancement is due to the faster rate that the electrons move from the quantum dot to the balance of the solar cell (what the scientists call the electron tunneling rate) in the presence of the manganese atoms at the interface. Importantly, this observation is confirmed by theory, opening up possibilities for applying this approach to other systems.

The Impact

The power conversion efficiency of quantum dot solar cells has reached about 12%. However, the overall efficiency of quantum dot solar cells is relatively low compared to photovoltaic systems in use today that are based on silicon. In addition, quantum dot solar cells are not as efficient as emerging next-generation solar cells. The results obtained in this work point to a surprisingly straightforward alternative route. Scientists can significantly improve the performance of this family of solar cells by adding small amounts of alternate metals.

Summary

In the quest to replace more traditional solar materials, such as silicon, with more efficient and high-performing options, scientists have been studying quantum dot solar cells as an alternative to harvest sunlight for conversion to electricity. In this solar cell design, quantum dots are used as the material that absorbs sunlight and converts it to electricity. Quantum dots are very small, nanometer-sized, particles, whose solar conversion properties, in this case a characteristic gap in the energy levels of the electrons called the “bandgap,” are tunable by changing the size or chemical composition. This is in contrast to bulk materials whose bandgap is fixed by the chemical composition or choice of material(s) alone. This size dependence of bandgap makes quantum dots attractive for multi-junction solar cells, whose efficiency is enhanced by using a variety of materials that absorb different parts of the “rainbow” of wavelengths of light found in the solar spectrum. This research team discovered that adding small amounts of the transition metal manganese (Mn), or “doping,” resulted in a huge enhancement in the efficiency rate of changing light to electricity for lead sulfide (PbS) quantum dot sensitized solar cells. Relatively small concentrations of Mn (4 atomic percent) cause the current to increase by an average of 300% with a maximum increase of up to 700%. Moreover, the mechanism by which this occurs cannot be explained by the light absorption alone because both the experimental and theoretical absorption spectra demonstrate a several times decrease in the absorption coefficient on the addition of Mn. The team proposes that the dramatic increase is due to a mechanism of increased electron tunneling through the atom pairs at the quantum dot interface with the next layer of the solar cell. The team used ab initio calculations, which is a computational approach that can describe new phenomena without the need to fit or extrapolate experimental data, to confirm this mechanism. While typical doping approaches focus on improving exciton lifetime and light absorption channels, results obtained in this study provide an alternative route for significant improvement on the efficiency of quantum dot sensitized solar cells.

Funding

This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, as part of the Experimental Program to Stimulate Competitive Research (EPSCoR) program.

Publications

G. Rimal, A.K. Pimachev, A.J. Yost, U. Poudyal, S. Maloney, W. Wang, T.Y. Chien, Y. Dahnovsky, and J. Tang, “Giant photocurrent enhancement by transition metal doping in quantum dot sensitized solar cells.” Applied Physics Letters 109, 103901 (2016). [DOI: 10.1063/1.4962331]