Making Solar Cells more Efficient: Nanocrystals Expand the Range of Solar Cell Light Energy to Ultraviolet and Infrared Regions


Solar-spectrum1 042016Common solar cells made of crystalline silicon can only access roughly half of the total sunlight spectrum for conversion of light energy into electricity. Searching for more effective materials, Chinese scientists have now combined three semiconducting sulfide crystals to a ternary nanostructured photovoltaic system that absorbs irradiation from ultraviolet to near infrared regions.

 

As they report in the journal Angewandte Chemie, the nanorods effectively convert the full-spectrum light energy into electric current. This discovery marks a new level in the development of more efficient solar cells.

 

 
The photovoltaic material that is most commonly used today is crystalline silicon, but it absorbs sunlight effectively only in the visible region. Other semiconducting materials cover slightly different regions of the solar spectrum, but the most efficient photovoltaic materials would be clearly those that include every region from the ultraviolet to the infrared. Shu-Hong Yu and Jun Jiang and their collaborators at the University of Science and Technology of China in Hefei have now introduced a nanostructured system made of three sulfide crystals.

The ternary hybrid material of zinc, cadmium, and copper sulfides effectively absorbs the ultraviolet, visible, and near infrared light, and the segmented node-sheath structure of the tiny rods provides the ideal energy band alignment for an effective accumulation of charge carriers.

The basis of this photocollecting system is nanosized rods of zinc sulfide on which crystalline sheaths of cadmium sulfide are deposited like an arrangement of pearls. The zinc sulfide basis provides the UV absorption, while the covers the visible light region. As a third component for IR absorption, the scientists chose copper nanocrystals with copper deficiencies, as this material is known to feature a special type of absorption in the near-infrared region called surface plasmon resonance. “These heteronanorods absorb across nearly the full spectrum of solar energy,” the scientists report.

Nano Cystals for Solar Cells 042016 5715f7662555e

To test the functionality of the nanorods, the scientists measured their performance in a photoelectrochemical water-splitting cell. Upon full-spectrum illumination, the photocurrent response was pronounced, which was a first experimental evidence for the successful design of their photovoltaic material.

 

One of the crucial achievements of this work, however, was the correct adjusting of the sensitive heterojunctions that connect the different semiconducting structures to align the energy gaps of the semiconducting materials. “Such a staggered alignment enables the separation of the photogenerated electrons and holes in the ternary hybrid nanostructure,” say the authors. Although further experiments have to be performed, this ternary semiconducting system can be regarded as an important step toward a new generation of efficient covering the rainbow colors and beyond.

Light Ranges 042016 electromagnetic-spectrum

Explore further: Reshaping the solar spectrum to turn light to electricity

More information: Tao-Tao Zhuang et al. Integration of Semiconducting Sulfides for Full-Spectrum Solar Energy Absorption and Efficient Charge Separation, Angewandte Chemie International Edition (2016). DOI: 10.1002/anie.201601865

 

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Demystifying Nanocrystal Solar Cells: Engineering the Solar Cells of the Future


QD Solar Cell demystifyingETH researchers have developed a comprehensive model to explain how electrons flow inside new types of solar cells made of tiny crystals. The model allows for a better understanding of such cells and may help to increase their efficiency.

Scientists are focusing on nanometre-sized crystals for the next generation of solar cells. These have excellent optical properties. Compared with silicon in today’s solar cells, nanocrystals can be designed to absorb a larger fraction of the solar light spectrum.

However, the development of nanocrystal-based solar cells is challenging: “These solar cells contain layers of many individual nano-sized crystals, bound together by a molecular glue. Within this nanocrystal composite, the electrons do not flow as well as needed for commercial applications,” explains Vanessa Wood, Professor of Materials and Device Engineering at ETH Zurich. Until now, the physics of electron transport in this complex material system was not understood so it was impossible to systematically engineer better nanocrystal composites.

QD Solar Cell demystifying

Wood and her colleagues conducted an extensive study of nanocrystal solar cells, which they fabricated and characterized in their laboratories at ETH Zurich. They were able to describe the electron transport in these types of cells via a generally applicable physical model for the first time. “Our model is able to explain the impact of changing nanocrystal size, nanocrystal material, or binder molecules on electron transport,” says Wood. The model will give scientists in the research field a better understanding of the physical processes inside nanocrystal solar cells and enable them to improve .

Promising outlook thanks to quantum effects

The reason for the enthusiasm of many solar cell researchers for the tiny crystals is that at small dimensions effects of quantum physics come into play that are not observed in bulk semiconductors. One example is that the physical properties of the nanocrystals depend on their size. And because scientists can easily control nanocrystal size in the fabrication process, they are also able to influence the properties of nanocrystal semiconductors and optimize them for solar cells.

One such property that can be influenced by changing nanocrystal size is the amount of sun’s spectrum that can be absorbed by the nanocrystals and converted to electricity by the solar cell. Semiconductors do not absorb the entire sunlight spectrum, but rather only radiation below a certain wavelength, or – in other words – with an energy greater than the so-called energy of the semiconductor.

In most semiconductors, this threshold can only be changed by changing the material. However, for nanocrystal composites, the threshold can be changed simply by changing the size of the individual crystals. Thus scientists can select the size of nanocrystals in such a way that they absorb the maximum amount of light from a broad range of the sunlight spectrum.

An additional advantage of nanocrystal semiconductors is that they absorb much more sunlight than traditional semiconductors. For example, the absorption coefficient of lead sulfide nanocrystals, used by the ETH researchers in their experimental work, is several orders of magnitude greater than that of silicon semiconductors, used traditionally as solar cells. Thus, a relatively small amount of material is sufficient for the production of nanocrystal solar cells, making it possible to make very thin, flexible solar cells.

Need for greater efficiency

The new model put forth by the ETH researchers answers a series of previously unresolved questions related to electron transport in nanocrystal composites. For example, until now, no experimental evidence existed to prove that the band gap energy of a nanocrystal composite depends directly on the band gap energy of the individual nanocrystals. “For the first time, we have shown experimentally that this is the case,” says Wood.

Over the past five years, scientists have succeeded in greatly increasing the efficiency of nanocrystal solar cells, yet even in the best of these solar cells just 9 percent of the incident sunlight on the cell is converted into electrical energy. “For us to begin to consider commercial applications, we need to achieve an efficiency of at least 15 percent,” explains Wood. Her group’s work brings researchers one step closer to improving the and efficiency.

Explore further: Solar cells made from polar nanocrystal inks show promising early performance

More information: Bozyigit D, Lin WMM, Yazdani N, Yarema O, Wood V: A quantitative model for charge carrier transport, trapping and recombination in nanocrystal-based solar cells. Nature Communications, 27 January 2015, DOI: 10.1038/ncomms7180

University of Washington: Nanocrystals for Luminescent Solar Concentrators


Abstract

Abstract Image

Luminescent solar concentrators (LSCs) harvest sunlight over large areas and concentrate this energy onto photovoltaics or for other uses by transporting photons through macroscopic waveguides. Although attractive for lowering solar energy costs, LSCs remain severely limited by luminophore reabsorption losses.

Here, we report a quantitative comparison of four types of nanocrystal (NC) phosphors recently proposed to minimize reabsorption in large-scale LSCs: two nanocrystal heterostructures and two doped nanocrystals. Experimental and numerical analyses both show that even the small core absorption of the leading NC heterostructures causes major reabsorption losses at relatively short transport lengths.

Doped NCs outperform the heterostructures substantially in this critical property. A new LSC phosphor is introduced, nanocrystalline Cd1–xCuxSe, that outperforms all other leading NCs by a significant margin in both small- and large-scale LSCs under full-spectrum conditions.

Read Publication Here: http://pubs.acs.org/doi/abs/10.1021/nl504510t?src=recsys&

Copyright © 2015 American Chemical Society

Department of Chemistry, University of Washington, Seattle, Washington 98195-1700, United States
Mathematics Department, Western Washington University, 516 High Street, Bellingham, Washington 98225, United States