LANL-engineered quantum dots boost carrier multiplication in solar cells

Core/shell PbSe/CdSe quantum dots (a) and a carrier multiplication (CM) pathway (b) in these nanostructures. In (a), a transmission electron microscopy image shows thick-shell PbSe/CdSe quantum dots developed for this study. A hot hole in (b) generated in the shell via absorption of a photon collides with a core-localized valence-band electron, promoting it across the energy-gap, which generates a second electron-hole pair. In thick-shell PbSe/CdSe quantum dots this process is enhanced due to slow relaxation of shell-localized holes into the core. (Image credit: LANL)

IMAGE: Core/shell PbSe/CdSe quantum dots (a) and a carrier multiplication (CM) pathway (b) in these nanostructures. In (a), a transmission electron microscopy image shows thick-shell PbSe/CdSe quantum dots developed for this study. A hot hole in (b) generated in the shell via absorption of a photon collides with a core-localized valence-band electron, promoting it across the energy-gap, which generates a second electron-hole pair. In thick-shell PbSe/CdSe quantum dots this process is enhanced due to slow relaxation of shell-localized holes into the core. (Image credit: LANL)

Researchers from Los Alamos National Laboratory (LANL; Los Alamos, NM) have demonstrated an almost four-fold boost of the carrier multiplication yield with nanoengineered quantum dots. Carrier multiplication is when a single photon can excite multiple electrons. LANL says that quantum dots are novel nanostructures that can become the basis of the next generation of solar cells, capable of squeezing additional electricity out of the extra energy of blue and ultraviolet photons.

RELATED ARTICLE: Colloidal quantum-dot solar cell reaches record 7% efficiency

"Typical solar cells absorb a wide portion of the solar spectrum, but because of the rapid cooling of energetic (or 'hot') charge carriers, the extra energy of blue and ultraviolet solar photons is wasted in producing heat," said Victor Klimov, director of the Center for Advanced Solar Photophysics (CASP) at LANL. "In principle, this lost energy can be recovered by converting it into additional photocurrent via carrier multiplication. In that case, collision of a hot carrier with a valence-band electron excites it across the energy gap," Klimov said. "In this way, absorption of a single photon from the high-energy end of the solar spectrum produces not just one but two electron-hole pairs, which in terms of power output means getting two for the price of one."

Carrier multiplication is inefficient in the bulk solids used in ordinary solar cells but is appreciably enhanced in ultrasmall semiconductor particles--quantum dots--as was first demonstrated by LANL researchers in 2004 (Schaller & Klimov, Phys. Rev. Lett. 92, 186601, 2004). In conventional quantum dots, however, carrier multiplication is not efficient enough to boost the power output of practical devices.

A new study conducted within the Center for Advanced Solar Photophysics demonstrates that appropriately engineered core/shell nanostructures made of lead selenide and cadmium selenide (PbSe and CdSe) can increase the carrier multiplication yield four-fold over simple PbSe quantum dots. Klimov explained, "This strong enhancement is derived primarily from the unusually slow phonon relaxation of hot holes that become trapped in high-energy states within the thick CdSe shell. The long lifetime of these energetic holes facilitates an alternative relaxation mechanism via collisions with core-localized valence band electron which leads to highly efficient carrier multiplication."

To realize the effect of slowed carrier cooling LANL researchers have fabricated PbSe quantum dots with an especially thick CdSe shell. Qianglu Lin, a CASP student working on the synthesis of these materials said, "A striking feature of the thick-shell PbSe/CdSe quantum dots is fairly bright visible emission, from the shell, observed simultaneously with the infrared emission from the core. This shows that intraband cooling is slowed down dramatically, so that holes reside in the shell long enough to produce emission."

While the present CASP work is based on PbSe/CdSe quantum dots, the concept of "carrier-multiplication engineering" through control of intraband cooling is general, and should be realizable with other combinations of materials and/or nanostructure geometries. Jeff Pietryga, lead CASP chemist says, "Further enhancement in carrier multiplication should be possible by combining this new approach with other demonstrated means for increasing multicarrier yields, such as by using shape-control (as in nanorods) and/or materials in which cooling is already naturally slower, like PbTe." Applied together, these strategies might provide a practical route to nanostructures exhibiting carrier multiplication performance approaching the limits imposed by energy conservation.

SOURCE: LANL; https://lanl.gov/newsroom/news-releases-by-date/2014/June/06.18-quantum-dot-solar-cells.php

 

Most Popular Articles

Webcasts

Multichannel Spectroscopy: Technology and Applications

This webcast, sponsored by Hamamatsu, highlights some of the photonic technology used in spectroscopy, and the resulting applications.

Handheld Spectrometers

Spectroscopy is a powerful and versatile tool that traditionally often required a large and bulky instrument. The combination of compact optics and modern pa...

Fracking, climate change, and lasers:  new tools to reduce fugitive methane emissions

This webcast, sponsored by Hamamatsu Corporation, covers recent developments and field deployments of compact quantum-cascade-laser (QCL)-based methane senso...

Opportunities in the Mid-IR

The technology for exploiting the mid-IR is developing rapidly:  it includes quantum-cascade lasers and other sources, spectroscopic instruments of many...
White Papers

Narrow-line fiber-coupled modules for DPAL pumping

A new series of fiber coupled diode laser modules optimized for DPAL pumping is presented, featur...

Accurate LED Source Modeling Using TracePro

Modern optical modeling programs allow product design engineers to create, analyze, and optimize ...

Optical Isolators Improve Engraving Performance of Pulsed Fiber Lasers

The deleterious effects of back reflections on pulsed fiber lasers used in marking and engraving ...
Technical Digests

WAVELENGTH-SWEPT LASERS: Dispersion-tuned fiber laser sweeps over a 140 nm range for OCT

By eliminating the use of mechanical tunable filters and instead tuning by intensity-modulation i...

Keeping pace with developments in Raman spectroscopy for molecular and nanoparticle research

For demanding or custom spectroscopy solutions, care must be taken in selecting and integrating a...

HIGH-POWER FIBER LASERS: Working in the kilowatt regime

High-power materials-processing fiber lasers are available in an increasing variety of forms, as ...

Click here to have your products listed in the Laser Focus World Buyers Guide.

RELATED PRODUCTS

P-series 1470nm to 1550 nm

Ultra-High Brightness Direct Diode Lasers 20W to 135W

T-Series 915nm, 940nm, or 976nm

Ultra-High Brightness Direct Diode Lasers 85W to 575W

PCB Laser Marking System

PCB Laser Marking Systems

RELATED COMPANIES

Lighthouse Photonics Inc

Provides sealed, turn-key, cost-effective, diode-pumped solid-state (DPSS) lasers for s...

Fibertek Inc

Specializes in the design, development, manufacture, and testing of advanced diode-pump...

Control Micro Systems Inc

Offers laser marking, laser cutting, laser drilling and laser welding systems for a wid...

Social Activity

  •  
  •  
  •  
  •  
  •  
Copyright © 2007-2014. PennWell Corporation, Tulsa, OK. All Rights Reserved.PRIVACY POLICY | TERMS AND CONDITIONS