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November 2014

Synthetic Armor for Air-Stable Quantum Dots

Materials for cheap solar cells can handle oxygen for the first time

Ashley Marshall

New protective atoms on the surface of quantum dots have led to the assembly of the first, air-stable lead selenide (PbSe) quantum dot (QD) solar cell. Researchers show record efficiencies for this material with the added benefit of being made in air as opposed to a nitrogen-filled box. Reproduced with permission. Copyright 2014: American Chemical Society.

Solar cells have been showing up everywhere you look, from commercial buildings, to residential neighborhoods, to huge solar farms just outside of cities. Solar panels are often prohibitively expensive, partly because of their high manufacturing costs. Researchers at the Center for Advanced Solar Photophysics (CASP) are exploring a new class of materials that would allow solar cells to be manufactured from solution, like printing ink onto paper or painting a wall. This manufacturing process allows quicker and cheaper production of solar cells.

These materials are known as quantum dots (QDs), which are extremely small particles—so small that 20,000 particles lined up would just span the width of a human hair. Materials this small interact with light differently than a large piece of the same composition. At CASP, scientists work to improve the process of making these QDs and apply the new interactions with light to improve current solar cell technology.

One of the greatest challenges in using QDs is that certain types suffer greatly from degradation when exposed to oxygen. Many of the solar cells made from these small particles require completely air-free conditions during fabrication. Researchers at CASP developed a groundbreaking synthesis that includes protective agents for lead sulfide (PbS) and lead selenide (PbSe) QDs, shielding the materials from oxygen by protecting the surface. This new synthesis includes the protecting atoms as a part of the initial ingredients so that no extra steps or chemicals are required to make the air-stable version of the QDs. This lowers the overall cost and time of the material synthesis.

Using these protected QDs, the team fabricated the first-ever air-stable PbSe devices with the best devices converting 6.2 percent of sunlight into electricity, the highest reported efficiency. As an added benefit, the devices show good air stability even after being exposed to oxygen for multiple days.

Researchers also showed that the new synthesis can be scaled up—paving the way for large-scale industrialization of this material. While previous synthetic methods yielded ~1 gram of material (about the same weight as two Tic Tac® mints), CASP researchers successfully produced 47 grams of PbS in a single batch, without the use of inordinately harmful chemicals. Further advancements will lower the barrier to commercialization and bring the future of renewable energy closer.

Acknowledgments

This material is based upon work supported by the Center for Advanced Solar Photophysics, an Energy Frontier Research Center, funded by the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences. DOE funding was provided to NREL through contract DE-AC36-08G028308. In the Nano Letters article, the X-ray photoelectron spectroscopy work was conducted with support from a National Renewable Energy Laboratory (NREL) Director’s Postdoctoral Fellowship. In the ACS Nano article, EMM was supported with an NREL Director’s Postdoctoral Fellowship award.

More Information

Zhang J, J Gao, EM Miller, JM Luther, and MC Beard. 2014. "Diffusion-Controlled Synthesis of PbS and PbSe Quantum Dots with in Situ Halide Passivation for Quantum Dot Solar Cells." ACS Nano 8(1):614-622. DOI: 10.1021/nn405236k

Zhang J, J Gao, CP Church, EM Miller, JM Luther, VI Klimov, and MC Beard. 2014. "PbSe Quantum Dot Solar Cells with More than 6% Efficiency Fabricated in Ambient Atmosphere." Nano Letters 14(10):6010-6015. DOI: 10.1021/nl503085v

About the author(s):

Quantum Dots Suit Up

New method puts protective layer on tiny particles used in solar cells, could lower costs

By designing a new reaction that wraps a protective layer around tiny particles used in solar cells, scientists may have unlocked a key step to more widespread use of renewable energy.

Solar cells are a popular, but often expensive, choice for supplying energy. Quantum dots, extremely small particles with remarkable properties, could increase solar cells' efficiency. Unfortunately, most quantum dots can only be made and used in an oxygen-free environment, adding cost and complexity. Scientists at the Center for Advanced Solar Photophysics (CASP), led by Los Alamos National Laboratory, devised a process that forms a protective coating around lead sulfide and lead selenide quantum dots. The protective agents are added during the dots' formation. This process could lower the cost and time required to make devices from quantum dots, allowing wider use of solar cells.

More Information

Zhang J, J Gao, EM Miller, JM Luther, and MC Beard. 2014. "Diffusion-Controlled Synthesis of PbS and PbSe Quantum Dots with in Situ Halide Passivation for Quantum Dot Solar Cells." ACS Nano 8(1):614-622. DOI: 10.1021/nn405236k

Zhang J, J Gao, CP Church, EM Miller, JM Luther, VI Klimov, and MC Beard. 2014. "PbSe Quantum Dot Solar Cells with More than 6% Efficiency Fabricated in Ambient Atmosphere." Nano Letters 14(10):6010-6015. DOI: 10.1021/nl503085v

Disclaimer: The opinions in this newsletter are those of the individual authors and do not represent the views or position of the Department of Energy.